Ordered Organic Solids via Liquid

Crystalline Mesomorphism for

Optoelectronics

by

Simon Ku-Hsien Wei

Submitted in Partial Fulfillment

of the

Requirements for the Degree

Doctor of Philosophy

Supervised by

Professor Shaw H. Chen

Department of Chemical Engineering

Arts, Sciences and Engineering

Edmund A. Hajim School of Engineering and Applied Sciences

University of Rochester

Rochester, New York

2011

ii

To my family

iii

CURRICULUM VITAE

Simon Ku-Hsien Wei was born in 1980 in Kaohsiung, Taiwan. In 2002, he

received a Bachelors of Science degree in Chemical Engineering from the National

Taiwan University, Taipei, Taiwan. He joined the Department of Chemical

Engineering at the University of Rochester in September of 2004 and pursued his

doctorate in Chemical Engineering under the supervision of Professor Shaw H. Chen,

receiving a Master of Science degree in 2008. His field of specialization was in

ordered organic materials for optoelectronics. After receiving the PhD degree, he will

return to Taiwan and join Neoway Chemical Company.

Publications in Refereed Journals

1. Wei, S. K. H.; Chen, S. H.; Marshall, K. L.; Dorrer, C.; Jacobs, S. D.,

“Anchoring Energy and Nematic/Twisted-Nematic Pixel Resolution in a

Photopatterned Liquid Crystal Cell in Relation to Processing of Coumarin-

Based Photoalignment Layers”, to be submitted to Applied Physics Letters.

2. Dorrer, C.; Wei, S. K. H.; Leung, P.; Vargas, M.; Wegman, K.; Boulé, J.;

Zhao, Z.; Marshall, K.; Chen, S. H., "High-damage-threshold Static Laser

Beam Shaping Using Optically Patterned Liquid Crystal Devices", submitted

to Optics Letters.

3. Wei, S. K. H.; Zeng, L.; Marshall, K. L.; Chen, S. H., "Room-Temperature

Processing of π-Conjugated Oligomers into Monodomain Glassy-Nematic

iv

Films on Coumarin-Based Photoalignment Layers", Journal of Polymer

Science Part B: Polymer Physics 2011, 49, 725.

4. Wei, S. K. H.; Chen, S. H., "Spatially Resolved Lasers Using a Glassy-

Cholesteric Liquid Crystal Film with Lateral Pitch Gradient", Applied Physics

Letters 2011, 98, 111112.

5. Zeng, L. C.; Yan, F.; Wei, S. K. H.; Culligan, S. W.; Chen, S. H., "Synthesis

and Processing of Monodisperse Oligo(fluorene-co-bithiophene)s into

Oriented Films by Thermal and Solvent Annealing", Advanced Functional

Materials 2009, 19, 1978.

6. Wei, S. K. H.; Chen, S. H.; Dolgaleva, K.; Lukishova, S. G.; Boyd, R. W.,

"Robust Organic Lasers Comprising Glassy-Cholesteric Pentafluorene Doped

with a Red-Emitting Oligofluorene", Applied Physics Letters 2009, 94,

041111.

7. Dolgaleva, K.; Wei, S. K. H.; Lukishova, S. G.; Chen, S. H.; Schwertz, K.;

Boyd, R. W., "Enhanced Laser Performance of Cholesteric Liquid Crystals

Doped with Oligofluorene Dye." Journal of the Optical Society of America B-

Optical Physics 2008, 25, 1496.

8. Chen, A. C. A.; Wallace, J. U.; Wei, S. K. H.; Zeng, L. C.; Chen, S. H.,

"Light-Emitting Organic Materials with Variable Charge Injection and

Transport Properties", Chemistry of Materials 2006, 18, 204.

v

ACKNOWLEDGEMENTS

I want to express my foremost gratitude to my thesis advisor, Professor Shaw

H. Chen. His interdisciplinary and integral view of research has made this thesis

possible. His guidance, exceptional work ethics, and persistence will continue to be

an inspiration to my future professional career.

I would also like to thank Professor Matthew Z. Yates of the Department of

Chemical Engineering and Professor Stephen D. Jacobs of the Laboratory of Laser

Energetics (LLE), the Institute of Optics, and the Department of Chemical

Engineering for serving as my committee members.

I would like to thank Professor Stephen D. Jacobs and Mr. Kenneth L.

Marshall of LLE for their unflagging interest in my research, their professional advice

and technical discussions. I also want to thank Professor Robert W. Boyd of the

Institute of Optics for making his laser facility available for the characterization of

cholesteric liquid crystal lasers. I am grateful to the technical assistance of and helpful

discussions with Dr. Svetlana G. Lukishova, Dr. Ksenia Dolgaleva, and Mr. Andreas

Liapis of Professor Boyd’s group regarding laser experiments. I would also like to

thank Mr. Joseph Henderson and Mr. Richard Fellows of LLE for providing a great

deal of assistance in the construction of solvent-vapor annealing apparatus, and Dr.

Semyon Papernov and Mr. Brian McIntyre for teaching me how to use atomic force

microscopy and transmission electron microscopy, respectively.

vi

My gratitude also goes to Dr. Sean Culligan, Dr. Anita Trajkovska, Dr.

Andrew Chien-An Chen, Dr. Chunki Kim, and Dr. Jason U. Wallace for their

mentoring in my development of expertise ranging from organic synthesis to device

fabrication and characterization. My thesis research has benefited tremendously from

the red-emitting ologofluorene used in Chapters 2 and 3 as a dopant synthesized by

Dr. David Yanhou Geng, the liquid crystalline oligofluorenes used in Chapter 3 as

hosts synthesized by Dr. Lichang Zeng and Mr. Thomas Yung-Hsin Lee, the

oligo(fluorene-co-bithiophene) and the oligofluorene used in Chapter 4 synthesized

by Dr. Lichang Zeng and Dr. David Yanhou Geng, respectively, and the

photoalignment polymer used in Chapter 4 synthesized by Dr. Chunki Kim.

My gratitude is also due to my fellow postdoctoral and graduate students

throughout my graduate career: Dr. Feng Yan, Dr. Anita Trajkovska, Dr. Sean

Culligan, Dr. Andrew Chien-An Chen, Dr. Chunki Kim, Dr. Zhijian Chen, Dr. Jason

U. Wallace, Dr. Lichang Zeng, Dr. Qiang Wang, Mr. Yung-Hsin Thomas Lee, and

Mr. Kai Kao for technical discussions, extensive collaborations, and camaraderie on a

daily basis.

Through the years I have made many friends outside my research laboratory

that have enriched my personal life in Rochester. I would like to take this opportunity

to thank Chung-Yu Chen, Pin-Yi Wang, Poya Kan, Hank Lin, Tuan-Hsiu Hsieh,

Keen Chung, Melissa Wu, Hao-Fan Peng, Yi-Ming Lai, and Tim Huang for being

there when I needed to take a break.

vii

My family deserves the most credit for this work. None of this would have

been possible without the support and nurturing from my family members. Love and

thanks to my parents, A-Jen Wei and Shu-Chen Yu, for their unconditional support,

as well as my apologies for being away for these years. Thanks also go to my

youngest sister, Hsiu-Ru, for always bringing fun. I owe my deep gratitude to my first

younger sister Hsiu-Ying and her boyfriend, Ting-Hao Phan. Because of their

company, I have a caring and supportive “quasi-family” in a foreign county.

This thesis research was funded by the U.S. Department of Energy (DOE)

through the Laboratory for Laser Energetics (LLE) and the New York State Energy

Research and Development Authority. The Frank J. Horton Graduate Fellowship

provided by LLE and Sproull University Fellowship by the University of Rochester

are both gratefully acknowledged.

viii

ABSTRACT

Orientation of π-conjugated molecules across a large area without defects is

potentially useful to a host of optoelectronic devices. Solution cast films of active

materials have been processed into uniaxially oriented and helically stacked

conjugated molecules mediated by nematic and cholesteric liquid crystalline

mesomorphism, respectively. In this thesis, both glassy-nematic and glassy-

cholesteric films were prepared via thermal annealing of thermotropic liquid crystals

at an elevated temperature and solvent-vapor annealing of lyotropic liquid crystals at

room temperature on both rubbed polyimide alignment and noncontact

photoalignment layers followed by cooling through glass transition temperature and

thorough drying in vacuo, respectively. Major accomplishments are recapitulated as

follows.

Mixtures of pentafluorenes doped with a red-emitting oligofluorene were

prepared between rubbed polyimide alignment layers into constant-pitch glassy-

cholesteric liquid crystal films, yielding lasing thresholds and efficiencies comparable

to widely reported fluid cholesteric liquid crystal lasers. Robust glassy-cholesteric

liquid crystal films, however, produced temporally stable lasing output, an advantage

over their fluid counterparts whose lasing output diminished with time presumably

because of undesirable heating during optical pumping, light-induced pitch dilation,

and laser-induced fluid flow.

ix

Glassy-cholesteric liquid crystals were further processed into spatially

resolved lasers via thermally activated diffusion across the interface of two films

comprising disparate chiral contents. The observed lasing thresholds and efficiencies

are superior to those from spatially resolved fluid and glassy-cholesteric liquid crystal

lasers reported to date. The formation of Grandjean-Cano bands, each characterized

by a discrete pitch length, was explained by strong anchoring on rubbed polyimide

alignment layers and the lateral chiral concentration profile frozen in the solid state

after thermal processing.

A room-temperature photoalignment strategy was demonstrated for processing

oligo(fluorene-co-bithiophene) and heptafluorene into monodomain, uniaxially

oriented glassy-nematic films possessing orientational order parameter values

identical to those achieved on rubbed polyimide alignment layers. The difference in

time to achieve maximum uniaxial orientation with solvent-vapor annealing on

rubbed polyimide alignment layers and photoalignment layers, 5-10 s versus 6-8 min,

might have originated from π-π electronic interactions at the polyimide and

conjugated oligomer interface. Nevertheless, the feasibility of room-temperature

photoalignment methodology on plastic substrates, as demonstrated herein, represents

a crucial step forward in plastic optoelectronics.

x

TABLE OF CONTENTS

CURRICULUM VITAE iii ACKNOWLEDGEMENTS v ABSTRACT viii TABLE OF CONTENTS x LIST OF TABLES xiii LIST OF CHARTS xiv LIST OF FIGURES xv CHAPTER 1 BACKGROUND AND STATEMENT OF

RESEARCH OBJECTIVES

1 1.1. Lasers 1 1.2. Cholesteric Liquid Crystals 7

1.3. Glassy-Cholesteric Liquid Crystals Lasers 14 1.4. Spectrally Tunable and Spatially Resolved CLC Lasers 16 1.5. Uniaxial Orientation of π-Conjugated Polymers and

Oligomers

17

1.6. Statement of Research Objectives 21

xi

CHAPTER 2 ROBUST ORGANIC LASERS COMPRISING GLASSY-CHOLESTERIC PENTAFLUORENE DOPED WITH A RED-EMITTING OLIGOFLUORENE

25 2.1. Introduction 25 2.2. Experimental Section 28 2.3. Results and Discussion 31 2.4. Summary 35 CHAPTER 3 SPATIALLY RESOLVED LASERS USING A

GLASSY-CHOLESTERIC LIQUID CRYSTAL FILM WITH LATERAL PITCH GRADIENT 48

3.1. Introduction 48 3.2. Experimental Section 50 3.3. Results and Discussion 51 3.4. Summary 57 CHAPTER 4 ROOM-TEMPERATURE PROCESSING OF ππππ-

CONJUGATED OLIGOMERS INTO UNIAXIALLY ORIENTED MONODOMAIN FILMS ON COUMARIN-BASED PHOTOALIGNMENT LAYERS

68

4.1. Introduction 68 4.2. Experimental Section 70

xii

4.3. Results and Discussion 74 4.4. Summary 80 CHAPTER 5 CONCLUSIONS AND FUTURE RESEARCH 94 5.1. Conclusions 94 5.2. Future Research 97 REFERENCES 99

xiii

LIST OF TABLES Table 2.1. Dissymmetry Factor, ge, of Circularly Polarized Lasing from

Glassy CLC and Fluid CLC Films Pumped with 165 and 44 mJ/cm2, Respectively, with a Typical Error of ±0.05

47

Table 3.1. Dissymmetry Factor Values, ge, of Spatially-Resolved

Granjean-Bands from the Gradient-Ptich Glassy CLC Film Pumped with 165 mJ/cm2 with a Typical Error of ±0.05

67

Table 4.1. S Values of OF2T, OF, and PF2t Films after Annealing with

Chloroform Vapor at 22 oC on Polymer 1 and Rubbed Polyimide Alignment Layers

93

xiv

LIST OF CHARTS

Chart 4.1. π-Conjugated oligomers and coumarin-based Polymer 1 for photoalignment under present investigation 83

xv

LIST OF FIGURES

Figure 2.1. Molecular structures of light emitters, OF-r and DCM , as

well as F(MB)5-Ch and F(MB)5-N used for the preparation of glassy CLC host films. The phase transition temperatures of F(MB)5-Ch, F(MB)5-N, and OF-r were reported in References [116, 161, 164], and those of DCM in Reference [175]. 37

Figure 2.2. Schematic diagram of lasing experimental apparatus. 38 Figure 2.3. Glassy CLC film consisting of 1.5 wt% OF-r in F(MB)5-

Ch:F(MB)5-N at a 24.0:76.0 mass ratio: (a) Reflection spectrum (green solid curve), OF-r fluorescence spectrum from a nematic F(MB)5-N film (dashed curve), and the lasing peak at 635 nm (red solid curve) with a pump fluence of 121 mJ/cm2 at 10 Hz, and (b) Lasing output energy as a function of pump energy; monodomain character of the glassy CLC film verified with a polarizing optical micrograph included as the inset. 39

Figure 2.4. Polarized fluorescence spectrum of 22-µm-thick glassy-

nematic F(MB)5-N film doped with 1.5 wt% OF-r to arrive at Sem=0.54. 40

Figure 2.5. Transmission spectra of single (dotted), parallel (dashed),

and crossed (solid) HNP’B linear polarizers. 41 Figure 2.6. Fluid CLC film consisting of 2.0 wt% OF-r in CB-15:ZLI -

2244-000 at a 35.6:64.4 mass ratio: (a) Reflection spectrum (green solid curve), OF-r fluorescence spectrum from a ZLI-2244-000 film (dashed curve), and the lasing peak at 658 nm (red solid curve) with a pump fluence of 30 mJ/cm2 at 10 Hz, and (b) Lasing output energy as a function of pump energy; monodomain character of the glassy CLC film verified with a polarizing optical micrograph included as the inset. 42

Figure 2.7. Polarized fluorescence spectrum of 22-µm-thick fluid

nematic ZLI-2244-000 film doped with 2.0 wt% OF-r to arrive at Sem=0.60 43

xvi

Figure 2.8. Slope efficiency as a function of wt% OF-r in glassy (●) and fluid (▲) CLC lasers determined with the initial slopes of output-input relationships as illustrated in Figures 2.3b and 2.6b. For glassy CLC films with OF-r at 1.5, 2.0, 2.5, and 3.0 wt%, the F(MB)5-Ch:F(MB)5-N mass ratios are 24.0:76.0, 24.1:75.9, 24.2:75.8, and 24.3:75.7, respectively. For fluid CLC films with OF-r at 1.5, 2.0, 2.5, and 3.0 wt%, the CB-15:ZLI-2244-000 mass ratios are 35.5:64.5, 35.6:64.4, 35.7:64.3, and 35.9:64.1, respectively. All the efficiency data were gathered from the initial slopes of output-input relationships. 44

Figure 2.9. Time dependence of lasing output from (a) a glassy CLC

film and (b) a fluid CLC film, both containing OF-r at 2.0 wt%, at specified pump fluences and 10 Hz each on a pristine spot on the films. 45

Figure 2.10. Lasing output from the same spot on a fluid CLC film

containing OF-r at 2.0 wt% pumped with 550±12 mJ/cm2 at 10 Hz rested for 0.5 and 20 h time intervals between three runs. 46

Figure 3.1. Molecular structures of OF-r , F(MB)5-N, and F(MB)5-Ch

accompanied by their phase transition temperatures used for the preparation of constant-pitch and gradient-pitch GCLC films. Symbols: G, glassy; N, nematic; and Ch, cholesteric. 59

Figure 3.2. Polarizing optical micrograph of a 14-µm-thick GCLC film

with lateral pitch gradient selectively reflecting green to red, where Grandjean-Cano bands are located by white lines to enhance visibility. The film was prepared by thermally activated molecular diffusion at 220 oC for 62 h across the interface of films comprising F(MB)5-Ch:F(MB)5-N mixtures at 29.0:71.0 and 20.0:80.0 mass ratios, both doped with 2.5 wt% OF-r . At the conclusion of thermal annealing, the pitch gradient and the Grandjean-Cano bands were frozen in the glassy state by quenching to room temperature. The widths of the Grandjean-Cano bands from left to right are 1255, 584, 373, 323, 199, 180, 186, 161, 155, 161, 155, 143, 155, 143, 143, 149, 137, 137, 130, 130, 149, 143, 155, 230, 242, 298, 385, 528, 1168 µm. 60

Figure 3.3. (a) At three lateral positions identified as X’s in an arbitrary

Grandjean-Cano band of the gradient-pitch GCLC film, all 61

xvii

three stop-bands are centered at 624 nm with overlapping lasing peaks at 665 nm, as shown in (b). The diameters of the probe beam for selective reflection spectra measurements and the pump beam for lasing experiments are 30 and 28 µm (full width at half maximum), respectively, much less than the widths of all the Grandjean-Cano bands.

Figure 3.4. Lasing output at 622 nm as a function of pump energy of a

14-µm-thick gradient-pitch GCLC film doped with OF-r at 2.5 wt% to illustrate the determination of lasing threshold and slope efficiency. 62

Figure 3.5. (a) Selective reflection spectra (i.e. stop-bands) of every

other Grandjean-Cano band in the gradient-pitch GCLC film shown in Figure 3.2; (b) lasing from dopant OF-r using the gradient-pitch GCLC film shown in Figure 3.2, where optical pumping was furnished with a 532 nm laser for 35 ps at a 10 Hz repetition rate; the six lasing peaks from right to left are attributed to the 10th, 12th, 14th, 16th, 18th, and 20th Grandjean-Cano bands from the right shown in Figure 3.2. In addition, the dashed curve in (b) represents the fluorescence spectrum of a 14-µm-thick glassy-nematic F(MB)5-N film containing 2.5 wt% OF-r with unpolarized photoexcitation at 525 nm. 63

Figure 3.6. Polarized fluorescence spectrum of 14-µm-thick glassy-

nematic F(MB)5-N film doped with 2.5 wt% OF-r . 64 Figure 3.7. (a) Slope efficiency and (b) lasing threshold of OF-r at 2.5

wt% in 14-µm-thick GCLC film as functions of lasing wavelength. Typical errors for both efficiency and threshold are ±10%. 65

Figure 3.8. Lasing outputs at 636 nm as functions of pump energy for

OF-r at 2.5 wt% from a Grandjean-Cano band of the 14-µm-thick gradient-pitch (circles) GCLC film between F(MB)5-Ch and F(MB)5-N mixtures at 71.0:29.0 and 80.0:20.0 mass ratio and the corresponding constant-pitch (triangles) GCLC film of F(MB)5-Ch:F(MB)5-N at a 24.0:76.0 mass ratio. The lasing threshold and slope efficiency are 6.7 mJ/cm2, 1.1% and 6.3 mJ/cm2, 1.4% for the gradient-pitch and constant-pitch GCLC films, respectively. 66

xviii

Figure 4.1. A schematic diagram of the apparatus for solvent-vapor

annealing under controlled partial pressure. 84 Figure 4.2. UV-vis absorption spectra of 10-nm-thick pristine Polymer

1 film (blue curve) and the film exposed to linearly polarized UV-irradiation to a fluence of 1.0 J/cm2 (red curve) at room temperature. 85

Figure 4.3. (a) Polarized absorption spectrum of 90-nm-thick spin-cast

OF film on 10-nm-thick Polymer 1 photoalignment layer at X=0.31 annealed with chloroform vapor at Pr=0.90 for 8 min exposure; Symbols A|| and A⊥ represent absorbance parallel and perpendicular to rubbing direction, respectively. (b) S of 90-nm-thick spin-cast OF films on 10-nm-thick Polymer 1 photoalignment layers with X=0.31 as a function of exposure time to chloroform vapor at specified Pr values preceded by 25-min purge. The data points are accompanied by an error of ±0.03 overall. 86

Figure 4.4. Polarizing optical micrographs of 90-nm-thick OF films on

10-nm-thick Polymer 1 photoalignment layers at X=0.31 annealed with chloroform vapor at Pr=0.90 followed by vacuum drying, both conducted at room temperature. (a) pristine, (b) 1/2 min exposure to yield polydomain glassy-nematic film with S=0.23, and (c) 8 min exposure to yield uniaxially oriented monodomain film with S=0.74. 87

Figure 4.5. (a) AFM phase contrast, and (b) AFM topography with an

rms roughness of 0.45 nm of a polydomain glassy-nematic OF film on Polymer 1 after annealing with chloroform vapor at Pr=0.90 for 1/2 min exposure. (c) AFM phase contrast, and (d) AFM topography with an rms roughness of 0.35 nm of a monodomain glassy-nematic OF film on Polymer 1 after annealing with chloroform vapor at Pr=0.90 for 8 min exposure. 88

Figure 4.6. Electron diffraction patterns of (a) pristine OF film and (b)

OF film annealed with chloroform vapor at Pr=0.90 for 8 min exposure on Polymer 1 photoalignment layers. 89

Figure 4.7. (a) Polarized absorption spectra of 90-nm-thick spin-cast

OF2T film on 10-nm-thick Polymer 1 photoalignment layer at X=0.31 annealed with chloroform vapor at Pr=0.95 90

xix

for 6 min exposure; Symbols A|| and A⊥ are as defined in Figure 4.3(a). (b) S of 90-nm-thick OF2T films on 10-nm-thick Polymer 1 photoalignment layers with X=0.31 as a function of exposure time to chloroform vapor at specified Pr values followed by vacuum drying at room temperature for 3 h. At Pr=1.00 over ½ and 1 min exposure, S=0.28 and 0.00 (dewetting), respectively. The reported S values are accompanied by an experimental error of ±0.02 overall. The last two data points at Pr=0.95 correspond to uniaxially oriented monodomain films, and the rest to polydomain glassy-nematic films.

Figure 4.8. (a) S of 90-nm-thick OF films on 15-nm-thick rubbed PI as

a function of exposure time to chloroform vapor at Pr=0.90 followed by vacuum drying at room temperature for 3 h. The reported S values are accompanied by an experimental error of ±0.02 overall. (b) Polarized absorption spectra of 55-nm-thick spin-cast PF2T film on 15-nm-thick rubbed PI annealed with chloroform vapor at Pr=1.00 for 8 h exposure; Symbols A|| and A⊥ are as defined in Figure 4.3(a). 91

Figure 4.9. 90-nm-thick OF2T films on 10-nm-thick Polymer 1

photoalignment layers with X=0.31: (a) S of polydomain glassy-nematic films as a function of exposure time to chlorobenzene vapor at specified Pr values followed by vacuum drying at room temperature for 3 h; (b) In situ polarizing optical micrograph of lyotropic nematic mesomorphism upon exposure to saturated chlorobenzene vapor for 40 min. 92

1

CHAPTER 1

BACKGROUND AND STATEMENT OF RESEARCH

OBJECTIVES

1.1. LASERS

1.1.a. Principles of Lasers

The term "laser" is an acronym for “Light Amplification by Stimulated

Emission of Radiation.” First demonstrated in 1960 using a ruby crystal [1], lasers

have brought major advances in science and technology. All lasers are characterized

by a unique set of properties: monochromaticity, coherence, directionality, and

brightness. Physically, a laser consists of two parts: an active medium pumped by an

external source and a resonator providing optical feedback.

When an electron in an atom or a molecule is excited from a lower energy

level, E1, to a higher energy level, E2, it can return to E1 by emitting a photon in one

of the two distinct ways. It may undergo spontaneous or stimulated emission.

Stimulated emission is characterized by the excited electron being perturbed by the

electromagnetic field of an incoming photon, thus releasing a second photon with the

same wavelength, phase, polarization and direction of propagation as the stimulating

photon. Absorption and stimulated emission rates are expressed as follows [2]:

2

( ) 1121 / FNdtdN abs σ−= (1)

( ) 2212 / FNdtdN stim σ−= (2)

where N1 and N2 are the populations at E1 and E2 energy levels, respectively, F is the

incoming photon flux, and σ12 and σ21 are the absorption and stimulated emission

cross sections, respectively. For a laser transition involving energy levels 1 and 2, it

can be shown that σ12 = σ21 = σ holds for two- and multi-level systems.

Let us consider a plane wave propagating in the z direction with a photon flux

F. Considering both absorption (loss of photons) and stimulated emission (generation

of photons), an incremental change of photon flux, dF, over an incremental distance,

dz, is simply:

( )dzNNFdF 12 −= σ (3)

This equation shows that the medium can act as an optical amplifier (dF/dz

>0) if a population inversion is reached, i.e. N2>N1. With sufficiently high pump

energy, N2=N1 is the best that one can expect of a two-level system. In a three-level

system, electrons are excited into a higher-lying level and then rapidly decay to E2

followed by a relatively slow transition to E1, thereby attaining the required

population inversion. The four-level system is intended to moderate the high pump

energy demanded by the three-level system by installing one energy level each above

and below the two energy levels involved in the lasing transition. The material in

which a population inversion can be reached is referred to as an active medium.

3

Because the amplification per pass in the active medium is usually small, a

resonator is required to provide a positive feedback so that photons can bounce back

and forth through the active medium, thus increasing the effective length. The active

medium together with the resonator behaves as an optical oscillator. To overcome

losses due to transmission at the mirrors and internal losses in the active medium such

as absorption and scattering, a threshold must be exceeded to arrive at lasing action

by furnishing sufficient pump energy. Lasing can be identified by the following

common features: a clear indication of a lasing threshold, spatial coherence, spectral

narrowing, and existence of laser resonance modes [2,3].

1.1.b. Material Classes

Based on the physical states of the active media, lasers can be classified as

follows [4]:

(i) Gas Lasers. The active medium can be an atomic, an ionic or a molecular

species with an appropriate energy structure enclosed in a tube. Pumping is

furnished via an electrical discharge through the gas. Representative examples

are HeNe, CO2, argon ion, and krypton ion lasers.

(ii) Liquid Lasers. The active medium can be an organic light emitter, such as

rhodamine, coumarin, and stilbene in solution. The active medium can also be

a rare earth metal complex in an organic solvent.

4

(iii) Solid Lasers. The active medium can be a transition metal or a rare-earth

metal ion, an organic light emitter, or a color center in a crystalline or

amorphous matrix. Representative examples are neodymium-doped yttrium

aluminium garnet (Nd:YAG), rhodamine-doped polymer, and color center

lithium fluoride lasers. Inorganic semiconductors represent a subset of solid-

state lasers, of which single crystalline gallium arsenide (GaAs) is the most

prominent example.

(iv) Other Lasers. Included here are lasers with special operating configurations

and properties, such as X-ray lasers, free electron lasers, and nuclear pumped

lasers.

Since the advent of the first dye laser in 1966 [5,6], organic materials have

become an important class of active media for tunable lasers across the visible

spectrum. Although liquid dye lasers effectively prevent the problems of heat

generation and concentration quenching, they tend to be complex and bulky requiring

a large amount of organic solvent. Solid-state organic lasers have several distinct

advantages over their inorganic counterparts summarized as follows:

(i) Organic materials have a broad and continuous spectral emission range [7,8],

which is crucial to spectral tunability.

5

(ii) Organic materials are relatively low cost and readily processable by

photolithography, ink-jet printing, or micromolding on curved or flexible

substrates [9]. Device fabrication is further facilitated by the diversity of

resonator structures demonstrated for organic materials [10].

(iii) Organic solids can be fabricated into optical quality films across a large area

[11]. Because of their strong optical absorption, they can also be made

compact and lightweight [12].

(iv) Because of the relatively large exciton binding energies (~0.5 eV) [12], solid-

state organic lasers’ wavelength and threshold bear much weaker temperature

dependence than their inorganic counterparts [13,14].

The first solid-state organic laser involved embedding dyes in a polymer

matrix [15]. Lasing was also demonstrated with organic dyes doped in a single crystal

[16] and with single crystalline anthracene [17] in the 1970’s. The successes of small

molecules [18] and conjugated polymers [19] in organic light-emitting diodes in the

late 1980’s and early 90’s paved the way for organic materials as lasing media. In

1996 Hide et al. [20] reported the first laser utilizing a conjugated polymer doped into

a solid host to minimize concentration quenching. Later in the same year Tessler et al.

[21] reported the first lasing from a neat conjugated polymer in a microcavity

resonator. Small organic molecules doped in 8-hydroxyquinolinato aluminum (Alq)

were also demonstrated for lasing from a variety of resonator structures in the late

6

1990’s [22,23]. In a nutshell, solid organic materials have shown versatility not only

in the types of active medium, such as polymers [21,24] and small molecules [22,23],

but also in their feasibility with various resonator structures [10].

1.1.c. Resonator Structures

The conventional design of an optical resonator consists of enclosing an active

medium between a pair of mirrors. The mirrors can be planar, as in the Fabry-Perot

resonator, or curved. An alternative to mirrors is distributed Bragg reflectors

consisting of multiple layers of alternating refractive indices [2], as exemplified by

the vertical-cavity surface-emitting laser [25]. As another class of resonator

structures, ring resonators consist of a series of mirrors that define a circular optical

path.

In addition to the generic resonator structures identified above, there is a

distinct class of micro-sized lasers. An active medium with a dimension on the order

of the emission wavelength in a resonator structure constitutes a microcavity,

resulting in a modified emission spectrum and an enhanced efficiency [26,27].

Organic materials can be readily fabricated into a micro-ring structure [28,29],

through which light is wave-guided by total internal reflection. Microdisk [29] and

microsphere resonators [30] are similar to the micro-ring resonator in their design

concepts.

7

A microcavity can also be realized in a distributed feedback resonator, in

which feedback is provided through Bragg diffraction within a photonic crystal.

Photonic crystals are one-, two-, or three-dimensional periodic dielectric structures,

which can affect the propagation of photons depending on the wavelength. The

disallowed spectral range is called a photonic band gap. Only the light whose

wavelength satisfies the Bragg condition can be fed back to experience gain. Lasing is

expected at the band gap’s edges because of the enriched density of states, i.e. the

optical modes for spontaneous emission [31]. Such a laser requires no external

mirrors and is capable of a tunable output wavelength. Unlike a distributed Bragg

reflector, the active medium is evenly distributed throughout the photonic crystal. The

distributed feedback resonator structure has been implemented with organic dyes [32]

and semiconductors [33].

1.2. CHOLESTERIC LIQUID CRYSTALS

1.2.a. Liquid Crystals

As an intermediate phase between ordered crystals and isotropic liquids,

liquid crystals (LCs) are a class of self-organized fluids possessing long-range

orientational and/or positional orders [34]. Liquid crystals typically comprise

molecules with highly anisotropic shapes which enables self-assembly into uniaxial,

lamellar, helical, or columnar molecular arrangements, giving rise to nematic,

smectic, cholesteric, or discotic LC structures, respectively.

8

The nematic mesophase exhibits uniaxial orientational order manifested as the

long molecular axes statistically oriented along a preferential direction referred to as a

director. The orientational order parameter, S, characterizes the order in the nematic

mesophase [34]:

1cos32

1 2 −= θS (4)

where θ is the angle between the long molecular axis of an arbitrary molecule and the

director, and the angular bracket represents the ensemble average over all molecules.

Note that S=1 and −0.5 when all molecules are oriented parallel and perpendicular to

the director, respectively, and that S=0 when all the molecules are randomly

distributed relative to each other as in the isotropic phase.

Cholesteric (or chiral-nematic) liquid crystals are capable of self-organization

into helically stacked quasi-nematic layers, each of which comprises uniaxially

oriented molecules. The director is helically twisted along the axis perpendicular to

the quasi-nematic layers. The helical pitch length, p, is defined as the distance along

the helical axis over which the local nematic director completes a 360o rotation, and

handedness characterizes the sense of the nematic director’s rotation. Selective

wavelength reflection is a unique optical property of choleteric liquid crystal films

[35]. Bragg-like selective reflection is observed when the wavelength of normally

incident light on a cholesteric liquid crystal film satisfies the following condition:

pnR =λ (5)

9

where λR is the center wavelength of the selective reflection. The average refractive

index, n , is defined as

( ) 2/oe nnn += (6)

where ne and no are the extraordinary and ordinary refractive indices of the quasi-

nematic layer comprising a cholesteric liquid crystal film, respectively. The

bandwidth of selective reflection ∆λ is determined by the product of pitch length and

optical birefringence of the liquid crystal, ∆n = ne – no.

∆λ = p∆n = p( ne – no) (7)

Circularly polarized light of the same handedness as that of cholesteric liquid

crystal film with a wavelength in the selective reflection band will be completely

reflected without altering its polarization state. Circularly polarized light of opposite

handedness will be transmitted without experiencing any interaction with the

cholesteric liquid crystal film. Moreover, incident light outside the selective reflection

band will not be affected in any way regardless of its polarization state.

1.2.b. Cholesteric Liquid Crystal Lasers

With its refractive index undergoing a periodic variation along the helical

axis, the cholesteric stack can be considered a one-dimensional photonic band gap

structure [36,37]. According to de Vries’ calculation [38], the light propagating

10

through a CLC film is split into two modes. One of these two modes (i.e., m1) is

circularly polarized with the same handedness as that of the CLC film, while the other

(i.e., m2) is opposite. The propagation of mode m1 is disallowed within the band gap,

which is also known as the stop-band. Consequently, if a photoluminescent light-

emitting dopant with a proper emission spectrum is present, its emission of mode m1

is suppressed within the stop-band. At the edges of the stop-band, however, the

photon’s dwell time is significantly increased because of the high density of states. In

consequence, the spontaneous emission from a photoluminescent dopant is allowed

sufficient time to interact with pumping light to produce stimulated emission and

ultimately lasing [39].

Motivated by the success of distributed feedback lasers [32], Goldberg et al.

proposed lasing from a cholesteric liquid crystal film [40], which was complemented

by Kuhtarev’s theoretical explanation [41]. Il’chishin et al. experimentally

demonstrated the first CLC laser and studied its properties [42]. Kopp et al. renewed

intensive interest in CLC lasers by a clear demonstration of lasing at the stop-band’s

edge of a CLC film [37] on the basis of the photonic band gap laser concept [31].

Since then lasing has been reported using various CLC materials, including lyotropic

[43], ferroelectric [44], blue phase [45,46], glassy [47], and photopolymerized [48,49]

systems. The resulting laser is accompanied by an extremely high degree of circular

polarization [50,51]. In addition to a single dopant in a CLC film, lasing from a neat

film [52] or via Förster energy transfer between co-dopants [53] have also been

11

demonstrated. Defect-mode lasers with a phase jump [54] or an intermediate layer

[55] between two CLC films were proposed and successfully executed [56,57].

Cholesteric liquid crystal lasers are characterized by ease of fabrication, low

cost, compact size, and tunability from the ultraviolet through visible to the near

infrared regions [58]. Potential applications include spectroscopy, medical diagnostics

and skin treatment thanks to the broad lasing wavelength range [58]. Development of

cholesteric liquid crystal laser arrays [59] also create an opportunity for laser

projection displays that offer wide color gamut, high resolution and contrast ratio

[60]. Another unique feature is their inherent circular polarization, which may find

applications in quantum cryptography, optical interconnects, optical switches and

modulators [61].

1.2.c. Factors Conducive to CLC Laser Performance

Cholesteric liquid crystal laser performance can be appraised in terms of three

parameters, namely, the lasing threshold, efficiency, and spectral purity. Factors

known to affect laser performance are identified as follows:

(i) Molar extinction coefficient and fluorescence quantum yield of dopant. In

general, light-emitting dopants with high extinction coefficients and

fluorescence quantum yields are desirable for efficient absorption of pump

energy and emission. As shown by Mowatt et al. [62], pyrromethene 597-

12

doped CLC lasers exhibit higher slope efficiency and lower threshold at low

dopant concentration regime in comparison to CLC lasers doped with 4-

(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (DCM),

which is attributed to pyrromethene 597’s higher molar extinction coefficient

and fluorescence quantum yield. For varied dopants in a same CLC host at

identical absorbance, the lasing threshold decreases when the product of these

two factors in question increases [63].

(ii ) Light-emitting dopant’s orientational order in a CLC host film. For

dopants with emission dipoles aligned with the local quasi-nematic director,

lasing takes place preferentially at the stop-band’s long wavelength edge. The

emission rate at this edge depends upon the projection of the dopant’s

emission dipole moment on the electric field vector of propagating light,

which is parallel to the local quasi-nematic director; therefore, the emission

rate increases with increasing order parameter [39]. For a given dopant at an

increasing orientational order parameter, lasing threshold is expected to

decrease, accompanied by an increased slope efficiency and improved spectral

purity [64,65].

(iii) Optical birefringence of the CLC host. A theory developed by Kopp et al.

predicts that a highly birefringent cholesteric liquid crystal host would

contribute to a high density of states at the stop-band’s edges [37,66], which

in turn would increase the emission rate. It follows that a CLC laser with a

13

high birefringence is expected to exhibit a lower lasing threshold and a high

slope efficiency, as experimentally validated by Morris et al. [64]. To

maximize lasing efficiency, it is preferable to have the long wavelength edge

of the stop-band coincide with the light-emitting dopant’s fluorescence

maximum. A high birefringence is responsible for a broad stop-band, leading

to its short wavelength edge intersecting the fluorescence spectrum at a lower

intensity. Therefore, the spectral purity can be also improved by increasing the

CLC host’s optical birefringence.

(iv) Film thickness. With an increasing film thickness, the lasing threshold and

lasing efficiency traverse a minimum and a maximum, respectively [67].

These behaviors are accounted for by both the density of states at the stop-

band’s edges and absorption losses that increase with film thickness. An

observation to be concerned about is the deteriorating quality of helical

stacking with an increasing film thickness that tends to decrease orientational

order and increase scattering loss [68].

(v) Light-emitting dopant’s concentration in the CLC host. The optical gain

increases and hence the threshold decreases with an increasing dopant

concentration. Excimer formation above a critical concentration, however,

tends to elevate the threshold [67]. Moreover, concentration quenching at an

increasing concentration is likely to have adverse effects on lasing threshold

14

and efficiency. Therefore, there exists an optimum dopant concentration to

achieve maximum slope efficiency.

Since the definitive demonstration of CLC lasers more than two decades ago

[69], both the lasing threshold and slope efficiency have been greatly improved using

a dopant with high molar extinction coefficient, fluorescence quantum yield, and

superior orientational order of emission dipoles, together with a CLC host with high

optical birefringence, while minimizing absorptions by the dopant’s excited states

[62,63,70]. The hitherto lowest lasing threshold and highest slope efficiency for

cholesteric liquid crystal lasers are 0.13 mJ/cm2 [50] and 60% [71] , respectively.

1.3. GLASSY-CHOLESTERIC LIQUID CRYSTAL LASERS

Extrinsic factors that may disturb the host film’s helical stack or light-emitting

dopant’s orientational order would cause adverse effects on lasing behavior. Optical

pumping and/or lasing could result in heating, thus causing the stop-band to undergo

a red or blue shift depending on the type of cholesteric liquid crystal used as the host

[72]. A rise in temperature would also reduce the light emitter molecules’

orientational order parameter [73]. Optical torque on the cholesteric liquid crystal

structure [74,75] caused by circularly polarized photoexcitation could lead to a

deterioration of laser performance, as suggested by Morris et al. [76]. Yet another

15

potential problem is laser-induced fluid flow disturbing the stop-band [77]. All of the

above effects are accountable for the temporal instability of cholesteric liquid crystal

lasers [78].

Cholesteric liquid crystal fluids as the hosts are vulnerable to any or all of

these adverse effects, which can be avoided in principle by using solid films of

cholesteric liquid crystals. To ensure temporal stability of lasing, solidification of

CLCs has been accomplished through photopolymerization or vitrification by cooling

through the glass transition temperature, both producing glassy-cholesteric liquid

crystal (GCLC) films. As a solid host, Schmidtke et al. [48] used an in-situ

photopolymerized cholesteric liquid crystal film doped with DCM to show an

improved slope efficiency of 26% over a cholesteric liquid crystal fluid film with a

lasing threshold of 130 mJ/cm2. Instead of a single lasing peak, two peaks appeared at

the long wavelength edge of the stop-band, presumably because of the polydomain

character of the film. The lasing wavelength of photopolymerized cholesteric liquid

crystal films has been also shown to have negligible temperature dependence in

contrast to the unpolymerized counterpart [49]. Shibaev et al. [47] used cyclosiloxane

to prepare the first cholesteric glassy liquid crystal laser with a relatively high

threshold, 1.3 J/cm2, with a poor film quality as evidenced by the disclinations

observed under polarized optical microscopy and inferior selective reflection spectra.

16

1.4. SPECTRALLY TUNABLE AND SPATIALLY RESOLVED

CHOLESTERIC LIQUID CRYSTAL LASERS

One of the attractive features of cholesteric liquid crystal lasers is the potential

for modification of its helical structure using a variety of external stimuli.

Researchers have demonstrated spectral tunability of lasing in real time from CLC

films via the following means.

(i) Temperature modulation. The temperature dependence of helical pitch

length was used to tune the lasing wavelength discontinuously [79] and

continuously [80]. The latter was achieved by adding two chiral dopants with

opposing temperature dependencies on helical twisting power.

(ii) Electric field. Tunability can be accomplished by application of an electric

field on cholesterics [81] and ferroelectrics [82,83]. Spectral tunability has

also been shown for a defect-mode CLC laser by reorienting the nematic

liquid crystal in the defect layer under an applied electric field [84].

(iii) Mechanical stress. Biaxial [85] or uniaxial [86] stretching of a cholesteric

elastomer film perpendicular to its helical axis leads to a continuous pitch

contraction, corresponding to a spectral tunability range of up to 95 nm [86].

(iv) Photoinduced reaction. Spectral tunability has been demonstrated for

cholesteric liquid crystal lasers, both irreversibly [87-89] and reversibly [90-

92], through photoinduced reaction on the CLC host, offering a spectral

17

tunability range of up to 100 nm [91]. Tunability in defect-mode lasers was

also carried out via the cis-trans photoisomerization in the defect layer [93].

In addition to real-time spectral tunability, the potential of cholesteric liquid

crystal lasers has been further enhanced with the feasibility of spatially resolved

lasing at multiple wavelengths. The temperature-dependent stop-band allows for a

spatially resolved CLC laser along the direction of the pitch (or temperature) gradient

[94]. This approach was refined by preserving the thermally induced pitch gradient

via in-situ photopolymerization [95] or by supercooling into solid state [51]. The

desired pitch gradient can also be attained through molecular diffusion in fluid [96-

98] or polymerized CLC films [99]. Furthermore, doping with multiple light-emitting

dopants [97,98] is instrumental to a wide tunability range of 300 nm through Förster

energy transfer.

1.5. UNIAXIAL ORIENTATION OF ππππ-CONJUGATED

POLYMERS AND OLIGOMERS

Organic semiconductors have been intensively explored to exploit their

versatility in molecular design, relative ease in thin film processing across a large

area, mechanical flexibility, and competitive performance in optics, photonics, and

electronics with their inorganic counterparts. The organic semiconductors can be

18

categorized into small molecules, oligomers, and polymers depending on their

molecular weights. Amorphous, liquid crystalline, and crystalline organic

semiconductors each has found its own niche in a variety of optoelectronic devices.

These on-going studies have brought about important technologies [100-106], such as

organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs),

organic photodiode and photovoltaic cells (OPVs). The uniaxial orientation of

conjugated backbones is especially beneficial to organic semiconductor device

performance, viz. anisotropic charge transport in organic field-effect transistors to

suppress crosstalk in logic circuit and pixel switching elements [107], polarized

organic light-emitting diodes [108] as energy-saving backlights for liquid crystal

displays, and polarization-sensitive photodiodes [109-111].

Typically, the orientation of conjugated backbones is dictated by alignment

layers, on which physical or chemical anisotropy is generated to direct the overlying

molecules in an anisotropic fashion. Mechanical rubbing has been widely practiced

for the preparation of alignment layers. Uniaxial alignment of liquid crystalline π-

conjugated molecular or macromolecular systems has been accomplished on rubbed

polyimide (PI) [112-114], poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate)

(PEDOT:PSS) [115-117], and poly(p-phenylenevinylene) (PPV) [118] by thermal

annealing with long molecular axes oriented parallel to the rubbing direction. The

spin-cast films of PI and PPV precursor were heated at 250−280 oC and 165−180 oC,

respectively, to achieve complete conversion. Thermal annealing of spin-cast films of

oligofluorenes on rubbed PI or PEDOT:PSS [113-117], polyfluorene on rubbed PPV

19

[118], and poly(9,9’-dioctylfluorene-co-bithiophene) (PF2T) on rubbed PI [112], was

performed at 100−170 oC, 275−285 oC and 200 oC, respectively. Solvent-vapor

annealing of liquid crystalline oligo(fluorene-co-bithiophene)s on rubbed PI at room

temperature can be as effective as thermal annealing [119]. Without subsequent

annealing, an oligo(p-phenylenevinylene) film on a rubbed PEDOT:PSS alignment

layer was found to self-organize uniaxially with an orientational order parameter

value of 0.93 during the spin coating process at room temperature [111]. Uniaxially

oriented films of conjugated p-sexithiophene and p-sexiphenylene were prepared by

vacuum deposition onto rubbed alignment layers of the same compounds being

vacuum sublimed without further annealing [120]. The resultant films were

polycrystalline with the long molecular axes oriented along the rubbing direction.

Alignment layers can also be prepared by friction transfer, i.e. drawing under pressure

at an elevated temperature. For instance, an oriented polycrystalline oligo(p-

phenylenevinylene) film with an orientational order parameter value of 0.88 was

obtained upon annealing at 120 oC on a polytetrafluoroethylene alignment layer

prepared by friction transfer [121].

In contrast, photoalignment is a noncontact method without the adverse

effects arising from the rubbing process, e.g. dusts, electrostatic charges, and

mechanical damage to the alignment coating. Three distinct approaches to

photoalignment have been explored with linearly polarized irradiation on the basis of

anisotropic degradation of polyimides [122-126], cis-trans isomerization of

azobenzenes [127-129], and (2+2) cycloaddition of cinnamates [130-134] or

20

coumarins [135-142]. Orientational order parameter values emulating those on rubbed

polyimide layers have been reported via thermal annealing of films comprising

conjugated oligomers [139,140]. Photoalignment techniques have been demonstrated

for the fabrication of polarized light-emitting diodes and field-effect transistors.

Liquid crystalline polymer PF2T on a photoalignment layer can be aligned by thermal

annealing at 280 oC to achieve an absorption dichroic ratio of 5.5:1 and mobility

anisotropy ratio of 8.7:1 [143]. The first polarized OLED utilizing a photoalignment

layer is achieved with the polymerizable oligo(fluorene-co-thiophene) mesogens on

photoalignment layer doped with hole transport material by thermal annealing at

90−100 oC, yielding an anisotropic electroluminescence polarization ratio of 13:1

[144] Similar work using poly(fluorene) was also shown [145,146] with an EL ratio

of 14:1. In addition to physical blending of photoalignment and hole transport

materials, other approaches to improving charge transport through a photoalignment

layer involved the use of ultrathin photoalignment layers that facilitate tunneling

injection [147] or copolymers incorporating the hole transporting and coumarin

moieties [148].

Conjugated backbones can also be aligned by mechanical forces without

underlying alignment layers. Stretching [149,150], rubbing with a velvet cloth [151],

or friction transfer [152-154] have been explored to orient conjugated molecules.

Pellets of poly(3-alkylthiophene)s [152,153] and poly(9,9-dioctylfluorene) [155] were

pressed and drawn on a quartz or silicon substrate kept at 100−150 oC using the

friction transfer approach. The resultant polymers were aligned along the drawing

21

direction with an orientational order parameter of 0.97 and 0.74, respectively.

Nanoimprinting was reported to align liquid crystalline conjugated polymers [156]. A

thin film of PF2T was imprinted at 160 oC by a silicon mold with nanochannel arrays,

followed by cooling before the mold is released. Conjugated polymers can also be

aligned via the Langmuir-Blodgett technique [157,158], which provides a high degree

of precision in film thickness. Substituted poly(p-phenylene) was aligned with an

orientational order parameter of 0.67 [157]. Shearing the solutions of substituted

phenylene-dithiophene-phenylenes, phenylene-trithiophene-phenylene and

quarterthiophene in chloro-benzene was shown to be capable of producing uniaxially

aligned films [159]. This method involves a small volume of solution sandwiched

between two substrates, in which polycrystalline films with elongated grains along

the shearing direction were obtained.

1.6. STATEMENT OF RESEARCH OBJECTIVES

Since the first successful demonstration of cholesteric liquid crystal lasers

[42], numerous material systems have appeared with a common objective of pursuing

its potential applications. Fluid CLC films are susceptible to external perturbations,

such as heating via optical pumping, light-induced pitch dilation, and laser-induced

flow. To substantially improve device robustness and temporal stability, solidification

of CLC films have been carried out through photopolymerization or supercooling into

the glassy state while bypassing crystallization. These prior attempts, however,

22

suffered from spectral impurity of lasing, apparently because of the difficulty in

preparation of highly ordered, disclination-free films. A solid cholesteric liquid

crystal host material that can be prepared into monodomain (i.e. defect-free) films of

superior optical quality is highly desired. The glassy-cholesteric liquid crystals with

excellent morphological stability developed in our laboratory [160-164] appear to be

promising for GCLC lasers.

The potential of CLC lasers has been enhanced by introducing a lateral pitch

gradient to realize spatially resolved, variable wavelength lasing. The helical pitch

gradient induced by molecular diffusion is especially appealing to spatially resolved

lasing across a broad spectral range [96-98]. Photopolymerization is an effective

approach to ensuring temporal stability of lasing by preserving the desired pitch

gradient through crosslinking, but the quality of stop-bands has left much to be

desired [95]. The glassy-cholesteric liquid crystal of excellent optical quality holds

promise to overcome the afore-mentioned problem. Nevertheless, the origin of spatial

resolution of a CLC film into Grandjean-Cano bands has remained to be elucidated.

Uniaxial orientation of π-conjugated backbones is critical to a variety of

optoelectronic applications. Thermotropic nematic π-conjugated polymers, typically

with a high molecular weight, could be oriented on rubbed polyimide alignment

layers through high-temperature annealing. Monodisperse π-conjugated oligomers

with a relatively low molecular weight, however, could be oriented through thermal

annealing at substantially lower temperatures. In a recent paper [119], we have

23

reported the first demonstration of chlorobenzene-vapor annealing at room

temperature of oligo(fluorene-co-bithiophene) films on rubbed polyimide alignment

layers. The resultant monodomain glassy-nematic film yielded an orientational order

parameter value of 0.82, identical to that from thermal annealing at 120 oC.

Nonetheless, the preparation of polyimide alignment layers entails high-temperature

baking (> 200 oC), which is incompatible with commonly used plastic substrates, e.g.

poly(ethylene terephthalate), polyer(ethylene naphthalate), and poly(carbonate) with

glass transition temperatures up to 150 oC [165]. The rubbing process also causes

mechanical damage to alignment layers in addition to generating dust particles and

electrostatic charges. To pave the way for plastic electronics and roll-to-roll printing

of organic electronic devices, it is highly desirable to develop room-temperature

processing of oriented films, including the preparation of photoalignment layers

without contact.

Therefore, this thesis was motivated by the following objectives:

(i) To fabricate monodomain glassy-cholesteric liquid crystal films doped with a

light emitter, and demonstrate their superior temporal stability and more

sustainable lasing at increasing pumping fluence compared to that of a fluid

CLC counterpart.

(ii) To demonstrate variable lasing wavelengths using a spatially resolved glassy

liquid crystal film, furnish insight into the formation of resolved Granjean-

24

Cano bands, and interpret their monotonically increasing widths toward both

lateral edges.

(iii) To develop a new room-temperature process in which photoalignment layers

are prepared using linearly polarizaed UV irradiation for solvent-vapor

annealing of spin-cast conjugated oligomer films into uniaxially oriented

monodomain films with orientational order parameter values emulating those

achieved on rubbed polyimide alignment layers.

25

CHAPTER 2

ROBUST ORGANIC LASERS COMPRISING GLASSY-

CHOLESTERIC PENTAFLUORENE DOPED WITH A RED-

EMITTING OLIGOFLUORENE

2.1. INTRODUCTION

Mediated by molecular self-organization, cholesteric liquid crystals (CLCs)

can be readily processed into a helical stack of quasi-nematic layers. The quasi-

nematic layer is characterized by no and ne, the refractive indices parallel and

perpendicular to the local director, respectively. In addition to helical sense, a CLC

film is described by the average refractive index, navg = (no+ne)/2, and pitch length, p,

defined as the distance along the helical axis over which the local director completes

a 360o rotation [166]. Incident circularly polarized light of the same handedness as

that of the sufficiently thick (~10 µm) CLC film is completely reflected across a

spectral region centered at selective reflection wavelength, λR = pnavg, across a

bandwidth, p(ne–no), in the majority of cases where ne>no. The incident light is

transmitted elsewhere in the spectrum regardless of its polarization state. Therefore,

the CLC film constitutes a one-dimensional photonic band gap serving as a resonator

26

for the embedded light-emitting molecules to undergo lasing [37]. To exploit their

potential for practical application, CLC lasers have been actively pursued for both

fixed [37,42,47,48,53] and tunable [85,92,95-99] lasing wavelengths using fluid

CLCs in most cases. The helical stacking in fluid CLC films, however, is susceptible

to external perturbations such as heating via optical pumping, light-induced pitch

dilation, and laser-induced flow [74-78,167,168], all posing potentially adverse

effects on laser performance. To impart device robustness, solid films comprising

photopolymerized [48] and glassy cyclosiloxane [47] CLCs have also been explored.

Using liquid crystalline pentafluorenes, we have demonstrated the feasibility of

robust glassy CLC lasers with a sharply defined stop-band, producing temporally

stable energy output with otherwise comparable performance to conventional fluid

CLC lasers.

Of all the light emitters that have been explored, DCM, 4-

(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran, as depicted in

Figure 2.1 is by far the most popular for CLC lasers presumably for its solubility and

alignment with the local director. The light emitter's orientational order plays an

important role in lasing performance from both the theoretical [39] and experimental

[64,65] standpoints. Specifically, a high orientational order parameter is expected to

reduce threshold, to increase efficiency, and to deliver spectral purity. Nonetheless,

other factors such as fluorescence quantum yield, radiative lifetime, and excited-state

absorption will also influence CLC laser performance [169,170]. In a recent report

[171], a red-emitting oligofluorene OF-r , 4,7-bis[5-(9,9-bis(2-ethylhexyl)-

27

9’,9’,9’’,9’’, 9’’’,9’’’-hexakis(2-methylbutyl)-7,2’;7’,2’’;7’’,2’’’-tetrafluoren-2-yl)-

thien-2-yl]-2,1,3-benzothiadiazole, as depicted in Figure 2.1, was compared to DCM

in 22-µm-thick fluid CLC films prepared with a mixture of a cholesteric liquid

crystal, CB-15, and a nematic liquid crystal, ZLI-2244-000. As a measure of light-

emitting molecules’ alignment in the quasi-nematic layers comprising a CLC film, the

orientational order parameter, Sem = (I‖ – I⊥)/(I‖ + 2I⊥), was independently evaluated

by linearly polarized fluorescence using a nematic ZLI-2244-000 film. In this

formula, I‖ and I⊥ represent the fluorescence intensity parallel and perpendicular to

the local director, respectively. Despite the difference in Sem values, 0.60 for OF-r

and 0.36 for DCM , about the same threshold and slope efficiency were obtained for

the two light emitters in the transverse single-mode regime. However, OF-r exhibited

superior spatial and temporal stability and a sustained increase in laser output at

increasing pump energy, generating output energy five times that of DCM . In the

transverse multi-mode regime, OF-r was more than twice as efficient as DCM .

For the preparation of robust CLC lasers, non-emissive glassy CLCs with

absorption edges deep in the ultraviolet region are highly desirable to avoid

photoexcitation of the host. In addition, the ability to form monodomain CLC films

with a square-top stop-band is especially beneficial. At the same time, light emitters

must be soluble in the glassy CLC host in amount up to a few weight percent. Of all

the prospective glassy CLCs that we have developed in our laboratory [160-162,164],

glassy-cholesteric and glassy-nematic pentafluorenes, F(MB)5-Ch, penta[9,9-bis(2S-

28

methylbutyl)fluorene], and F(MB)5-N, penta[9,9-bis(2-methylbutyl)-fluorene], were

identified to meet the demands for accommodating OF-r . The molecular structures of

F(MB)5-Ch and F(MB)5-N depicted in Figure 2.1 are accompanied by identical

glass transition temperature, Tg, and clearing temperature, Tc, within an experimental

error of ± 2oC. The morphological stability against crystallization is also evidenced

by the absence of crystallization or crystalline melting on cooling or heating. These

material traits are conducive to the construction of robust glassy CLC lasers, as to be

demonstrated in what follows.

2.2. EXPERIMENTAL SECTION

2.2.a. Materials

The red-emitting oligofluorene, OF-r , glassy-nematic liquid crystal, F(MB)5-

N, and glassy-cholesteric liquid crystal, F(MB)5-Ch were synthesized and

characterized following literature procedures [161,163,164]. A nematic liquid crystal,

ZLI-2244-000, and a cholesteric liquid crystal, CB-15, were used as received from

EM Industries.

2.2.b. Film Preparation and Characterization

29

All the CLC films were sandwiched between two optically flat fused-silica

substrates coated with rubbed polyimide alignment layers (Nissan SUNEVER). The

film thickness was controlled with 22-µm glass fiber spacers (Bangs Laboratories).

Fluid CLC devices were assembled by capillary filling at 55°C, sheared at 50°C, and

thermally annealed at 45°C for 2 h, followed by slow cooling at 10oC/h to room

temperature. Glassy CLC devices were assembled by melting the powder at 230°C

followed by shearing at 170°C to induce alignment. The film was then annealed at

160°C for 48 h before quenching to room temperature. The resultant films were

characterized for selective reflection at 6° off normal incidence with unpolarized

incident light using a UV-vis-NIR spectrophotometer (Lambda-900, Perkin-Elmer).

Fresnel reflections from the air-glass interfaces were accounted for with a reference

cell containing an index-matching fluid between two surface-treated substrates. The

photomicrographs of glassy and fluid films were produced with a digital camera

(MicroPix C-1024) mounted on a polarizing optical microscope (Leitz Orthoplan-

Pol). Linearly polarized fluorescence spectroscopy was used to determine Sem for OF-

r and DCM uniaxially oriented in equivalent nematic liquid crystal films using a

spectrofluorimeter (Quanta Master C-60SE, Photon Technology International)

equipped with a linear polarizer (HNP’B, Polaroid).

2.2.c. Characterization of Orientational Order Parameter of OF-r Emission

Dipoles in a Helical Stack of a Constant-Pitch GCLC Film

30

Equivalent to quasi-nematic sublayers comprising a CLC films, an OF-r -

doped F(MB)5-N glassy-nematic film and OF-r -doped ZLI-2244-000 fluid nematic

film was characterized for linearly polarized fluorescence spectroscopy to determine

orientational order parameter values of OF-r emission dipoles, Sem, using a

spectrofluorimeter (Quanta Master C-60SE, Photon Technology International)

equipped with a linear polarizer (HNP’B, Polaroid). Sem is arrived at by (I || − − − − I⊥)/(I || +

2I⊥), in which I || and I⊥ are the emission intensities parallel and perpendicular to the

nematic director.

2.2.d. Characterization of Laser Performance

For glassy CLC films with OF-r at 1.5, 2.0, 2.5, and 3.0 wt%, the F(MB)5-

Ch:F(MB)5-N mass ratios are 24.0:76.0, 24.1:75.9, 24.2:75.8, and 24.3:75.7,

respectively. For fluid CLC films with OF-r at 1.5, 2.0, 2.5, and 3.0 wt%, the CB-

15:ZLI-2244-000 mass ratios are 35.5:64.5, 35.6:64.4, 35.7:64.3, and 35.9:64.1,

respectively.

The experimental apparatus depicted in Figure 2.2 was employed for the

characterization of CLC lasers in this study. Both glassy and fluid CLC films doped

with OF-r were optically pumped with the second harmonic of a EKSPLA Nd:YAG

laser (Altos, 532 nm; 35 ps pulse width; 10 Hz repetition rate). To avoid rejection of

input energy by CLC films, the pump beam was left-handed circularly polarized

using a linear polarizer as described above and a quarter waveplate (Tower Optical

31

Corporation). The pump beam was split into two parts: one for monitoring the input

energy, and the other for lasing experiments. It was focused at 45o incidence on the

sample film through a convex lens with a 20 cm focal length in a 34-µm diameter

(1/e), which is used to calculate the pump fluence. A (x-y-z) translation stage was

used to precisely position the sample film. A collection lens was used to direct

emission along the sample film’s surface normal to a fiber bundle equipped with a

spectrometer (Ocean Optics 2000) or an energy meter (LaserProbe Inc.) for

evaluating the output energy. Each data point for a specific time represents an average

over 30 consecutive measurements. The optical set-up for analyzing the right- and

left-handed circularly polarized components of lasing consisted of a quarter

waveplate (Tower Optical Corporation, 615−860 nm) and a linear polarizers (HNP'B,

Polaroid) to characterize the dissymmetry factor, ge =2( IL − IR ) /( IL +IR ) [172],

where IL and IR are the left- and right-handed circularly polarized emission intensity,

respectively.

2.3. RESULTS AND DISCUSSION

With 2S-methylbutyl bromide as the chiral precursor, the glassy CLC film of

F(MB)5-Ch was represented as a right-handed helical stack with a pitch length of

123 nm [161], corresponding to a stop-band in the deep ultraviolet region. Mixtures

of F(MB)5-Ch with an increasing amount of F(MB)5-N were prepared to tune λR

without altering handedness through the visible and near infrared to essentially

32

infinity in the limit of pure F(MB)5-N. The miscibility between F(MB)5-Ch and

F(MB)5-N is ensured by the identical molecular structure except chirality of the 2-

methylbutyl moieties. The F(MB)5-Ch:F(MB)5-N mass ratio was adjusted in the

presence of OF-r so that the stop-band’s low energy edge was aligned with the

fluorescence maximum of OF-r . As an example, the stop-band of a 22-µm-thick

glassy CLC film comprising 1.5 wt% OF-r in F(MB)5-Ch:F(MB)5-N at a mass ratio

of 24.0:76.0 is represented by the solid curve in Figure 2.3a. To avoid interaction with

the stop-band [39,173] and to simulate linearly polarized photoluminescence from

quasi-nematic layers within a cholesteric film, a nematic liquid crystal film should be

used to gather the fluorescence spectrum of OF-r . The fluorescence spectrum, shown

as the dashed curve in Figure 2.3a, was measured for OF-r at 1.5 wt% in a 22-µm-

thick glassy-nematic liquid crystal film of F(MB)5-N. This glassy-nematic liquid

crystal film was further characterized by polarized fluorescence spectroscopy as

displayed in Figure 2.4 to arrive at Sem = 0.54 using an HNP’B polarizer. The

transmission spectra for single, parallel, and crossed HNP’B polarizers were

illustrated in Figure 2.5, revealing the working range of 275 to 750 nm. This value

quantifies the orientational order of the OF-r molecules’ emission dipoles in the

quasi-nematic layers consisting of a F(MB)5-Ch:F(MB)5-N mixture at any mass

ratio. As an illustration of laser output, a sharp peak at 635 nm is recorded in Figure

2.3a at the pump fluence of 121 mJ/cm2. Plotted in Figure 2.3b is the output-input

relationship for the determination of threshold, Γ = 6.8 mJ/cm2, and slope efficiency,

33

η = 1.3 %. The absence of disclinations in the glassy CLC film was revealed by the

polarizing optical micrograph reproduced as the inset in Figure 2.3b.

Monodomain, right-handed fluid CLC films were prepared using mixtures of

ZLI-2244-000 with CB-15. The fluorescence spectrum of OF-r at 2.0 wt% in a 22-

µm-thick nematic ZLI-2244-000 film, shown as the dashed curve in Figure 2.6a, is

red-shifted from that in the glassy-nematic liquid crystal film, presumably because of

the difference in the dielectric environment furnished by the two host materials [174].

From the polarized fluorescence spectrum in Figure 2.7, the OF-r molecules in the

fluid nematic film were found to be better oriented, Sem = 0.60, than in the glassy-

nematic film. At a cholesteric-to-nematic mass ratio of 35.6:64.4 in the presence of

OF-r at 2.0 wt%, the fluid CLC film exhibited a stop-band with its low-energy edge

aligned with the fluorescence maximum. The monodomain character is displayed as

the inset in Figure 2.6b. With a pump fluence of 30 mJ/cm2, lasing occurred at 658

nm, as also shown in Figure 2.6a. The output-input relationship shown in Figure 2.6b

yielded Γ = 7.0 mJ/cm2 and η = 5.2 %. The highest pump fluences encountered with

glassy and fluid CLC films were 66 and 26 mJ/cm2, respectively, where photostability

is of no concern as to be demonstrated by stability tests below.

At an increasing concentration of OF-r , the improvement in laser gain appears

to have been overcome by concentration quenching. As shown in Figure 2.8, the

maximum slope efficiency is expected at an optimum concentration between 2.0 and

2.5 wt% of OF-r in both glassy and fluid CLC films. Across the OF-r concentration

34

range from 1.5 to 3.0 wt%, both Tg and Tc values remained the same as those reported

in Figure 2.1 for F(MB)5-Ch and F(MB)5-N, thereby facilitating thermal processing

of the glassy CLC films. Although the slope efficiency of the fluid CLC film is about

twice that of the glassy CLC film, with OF-r at 2.0 wt% the laser output from the

glassy CLC film was approximately 30 % higher than that from the fluid CLC film

before leveling off at a pump fluence up to 550 mJ/cm2. As shown in Table 2.1, the

dissymmetry factor values of lasing are independent of the OF-r concentration for

both types of CLC films.

A glassy CLC film comprising 2.0 wt% OF-r in F(MB)5-Ch:F(MB)5-N at a

mass ratio of 24.1:75.9 (with η = 2.7 %) was used to test the temporal stability of

lasing output with increasing pump fluences all at 10 Hz. The expectation that solid

CLC films would produce temporally stable lasing output was realized according to

Figure 2.9a. The observed stability of laser output also served to validate

photostability of all material components in the glassy CLC film. A fluid CLC film

containing 2.0 wt% OF-r in CB-15:ZLI-2244-000 at a mass ratio of 35.6:64.4 (with

η = 5.2 %) was also subjected to stability test. As shown in Figure 2.9b, the lasing

output decays with time, and the decay rate increases with pump fluence most likely

due to heating via optical pumping that might have caused a spectral shift of the stop-

band, light-induced pitch dilation, and laser-induced fluid flow, any or all of which

would disrupt the CLC structure and the orientational order of OF-r molecules.

35

The same spot on a fluid CLC film of the same composition as used in Figure

2.9b was pumped with a fluence of 550 mJ/cm2 at 10 Hz to further test the temporal

stability of laser output. The output energy from the first series of lasing experiments

diminished with time, as shown in Figure 2.10, which is consistent with the

observations presented in Figure 2.9b at a pump fluence of 186 or 553 mJ/cm2. The

second series of experiments was conducted after the CLC film had been left at room

temperature for 0.5 h. As also presented in Figure 2.10, the lasing output decreased by

35 to 40 % because fluid CLC film did not have sufficient time to recover from the

external perturbations incurred during the first series of experiments. The film was

then left at room temperature for 20 h before the third series of experiments was

performed. The pristine film’s output energy as a function of time was restored (see

Figure 2.10), indicating a full recovery of order in the fluid CLC film and the absence

of photodegradation of materials.

2.4. SUMMARY

Monodomain fluid and glassy CLC films containing a red-emitting

oligofluorene up to 3.0 wt% were prepared for an evaluation of laser threshold, slope

efficiency, and the temporal stability of laser output. The thresholds to lasing are 7.0

versus 6.8 mJ/cm2 for the fluid and glassy CLC films, respectively, and the maximum

slope efficiency of the fluid film is about twice that of the glassy film, 5.2 over 2.7 %.

Nevertheless, the glassy CLC film produced output energy approximately 30 %

36

higher than the fluid CLC film because of the more sustainable lasing at increasing

pump fluence with the solid film. Furthermore, the glassy CLC film produced

temporally stable laser output up to a pump fluence of 466 mJ/cm2 at a repetition rate

of 10 Hz. In contrast, the laser output from the fluid CLC film consistently

diminished with time, and the decay rate increased at increasing pump fluence from

74 to 553 mJ/cm2 all at 10 Hz. The diminishing output energy can be attributed to

perturbations on the fluid CLC film’s helical stacking and the light emitter’s

orientational order, including heating through optical pumping, light-induced pitch

dilation, and/or laser-induced fluid flow, all of which are substantially retarded or

inhibited in the glassy CLC film. At 550 mJ/cm2 pump fluence and 10 Hz, however,

the laser output from a fluid CLC film as a function of time was restored to that from

a pristine film provided that sufficient time was allowed for the helical structure to

fully recover from the external perturbations.

37

S

S

NN

S O

CNNC

H3C

N

F(MB)5-N: G 92 oC N 171oC I F(MB)5-Ch: G 91 oC Ch 173oC I

OF-r: G 104 oC N 304 oC I DCM: K 227 oC I

S

S

NN

S O

CNNC

H3C

N

F(MB)5-N: G 92 oC N 171oC I F(MB)5-Ch: G 91 oC Ch 173oC I

OF-r: G 104 oC N 304 oC I DCM: K 227 oC I

Figure 2.1. Molecular structures of light emitters, OF-r and DCM , as well as

F(MB)5-Ch and F(MB)5-N used for the preparation of glassy CLC host films. The

phase transition temperatures of F(MB)5-Ch, F(MB)5-N, and OF-r were reported in

References [116,161,164], respectively, and those of DCM in Reference [175].

38

Nd:YAG λλλλ = 532 nm, t = 35 ps, Rep. Rate = 10 Hz

Focusing lens

CLC laser

Lens condenser

Reference detector

Output detector

Computer

λ/λ/λ/λ/2wave-plate

Linear polarizer

λ/λ/λ/λ/4wave-plate

Figure 2.2. Schematic diagram of lasing experimental apparatus

39

0

25

50

75

0.0

0.5

1.0

1.5

450 500 550 600 650 700 750

Ref

lect

ance

, %

Fluorescence, N

ormalized

Wavelength, nm

0

2

4

6

8

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Out

put,

nJ

Input, µµµµJ

100 µm

Γ =6.8 mJ/cm2

η =1.3 %

(b)(a) ReflectanceFluorescence

Lasing

Figure 2.3. Glassy CLC film consisting of 1.5 wt% OF-r in F(MB)5-Ch:F(MB)5-N

at a 24.0:76.0 mass ratio: (a) Reflection spectrum (green solid curve), OF-r

fluorescence spectrum from a nematic F(MB)5-N film (dashed curve), and the lasing

peak at 635 nm (red solid curve) with a pump fluence of 121 mJ/cm2 at 10 Hz, and

(b) Lasing output energy as a function of pump energy; monodomain character of the

glassy CLC film verified with a polarizing optical micrograph included as the inset.

40

500 600 700 800

Flu

ores

cenc

e in

tens

ity, a

.u.

Wavelength, nm

I||

I⊥⊥⊥⊥

Sem=0.54

Figure 2.4. Polarized fluorescence spectrum of 22-µm-thick glassy-nematic F(MB)5-

N film doped with 1.5 wt% OF-r to arrive at Sem=0.54.

41

200 400 600 80010-4

10-3

10-2

10-1

100

101

102

Per

cent

Tra

nsm

issi

on

Wavelength, nm

Figure 2.5. Transmission spectra of single (dotted), parallel (dashed), and crossed

(solid) HNP’B linear polarizers.

42

0

2

4

6

8

10

0.0 0.1 0.2 0.3

Out

put,

nJ

Input, µµµµJ

100 µm

Γ =7.0 mJ/cm2

η =5.2 %

(b)(a) ReflectanceFluorescence

Lasing

0

25

50

75

0.0

0.5

1.0

1.5

500 550 600 650 700 750 800

Ref

lect

ance

, %

Fluorescence, N

ormalized

Wavelength, nm

Figure 2.6. Fluid CLC film consisting of 2.0 wt% OF-r in CB-15:ZLI-2244-000 at a

35.6:64.4 mass ratio: (a) Reflection spectrum (green solid curve), OF-r fluorescence

spectrum from a ZLI-2244-000 film (dashed curve), and the lasing peak at 658 nm

(red solid curve) with a pump fluence of 30 mJ/cm2 at 10 Hz, and (b) Lasing output

energy as a function of pump energy; monodomain character of the glassy CLC film

verified with a polarizing optical micrograph included as the inset.

43

500 600 700 800

Flu

ores

cenc

e in

tens

ity, a

.u.

Wavelength, nm

I||

I⊥⊥⊥⊥

Sem=0.60

Figure 2.7. Polarized fluorescence spectrum of 22-µm-thick fluid nematic ZLI-2244-

000 film doped with 2.0 wt% OF-r to arrive at Sem=0.60.

44

0.0

1.0

2.0

3.0

4.0

5.0

6.0

1.0 1.5 2.0 2.5 3.0 3.5

Slo

pe e

ffici

ency

, %

Weight %

Figure 2.8. Slope efficiency as a function of wt% OF-r in glassy (●) and fluid (▲)

CLC lasers determined with the initial slopes of output-input relationships as

illustrated in Figures 2.3b and 2.6b. For glassy CLC films with OF-r at 1.5, 2.0, 2.5,

and 3.0 wt%, the F(MB)5-Ch:F(MB)5-N mass ratios are 24.0:76.0, 24.1:75.9,

24.2:75.8, and 24.3:75.7, respectively. For fluid CLC films with OF-r at 1.5, 2.0, 2.5,

and 3.0 wt%, the CB-15:ZLI-2244-000 mass ratios are 35.5:64.5, 35.6:64.4,

35.7:64.3, and 35.9:64.1, respectively. All the efficiency data were gathered from the

initial slopes of output-input relationships

45

0

20

40

60

80

100

120

140

0 50 100 150 200 250 300 350

Out

put e

nerg

y, n

J

Time, sec

0

20

40

60

80

100

120

140

0 50 100 150 200 250 300 350

Out

put e

nerg

y, n

J

Time, sec

72mJ/cm2

179 mJ/cm2

466 mJ/cm2(a)

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350

Out

put e

nerg

y, n

J

Time, sec

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350

Out

put e

nerg

y, n

J

Time, sec

186 mJ/cm2

553 mJ/cm274 mJ/cm2

(b)

0

20

40

60

80

100

120

140

0 50 100 150 200 250 300 350

Out

put e

nerg

y, n

J

Time, sec

0

20

40

60

80

100

120

140

0 50 100 150 200 250 300 350

Out

put e

nerg

y, n

J

Time, sec

72mJ/cm2

179 mJ/cm2

466 mJ/cm2(a)

0

20

40

60

80

100

120

140

0 50 100 150 200 250 300 350

Out

put e

nerg

y, n

J

Time, sec

0

20

40

60

80

100

120

140

0 50 100 150 200 250 300 350

Out

put e

nerg

y, n

J

Time, sec

72mJ/cm2

179 mJ/cm2

466 mJ/cm2(a)

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350

Out

put e

nerg

y, n

J

Time, sec

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350

Out

put e

nerg

y, n

J

Time, sec

186 mJ/cm2

553 mJ/cm274 mJ/cm2

(b)

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350

Out

put e

nerg

y, n

J

Time, sec

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350

Out

put e

nerg

y, n

J

Time, sec

186 mJ/cm2

553 mJ/cm274 mJ/cm2

(b)

Figure 2.9. Time dependence of lasing output from (a) a glassy CLC film and (b) a

fluid CLC film, both containing OF-r at 2.0 wt%, at specified pump fluences and 10

Hz each on a pristine spot on the films.

46

0

10

20

30

40

50

60

0 50 100 150 200 250 300 350

Pristine, 548 mJ/cm2

0.5 h, 563 mJ/cm2

20 h, 543 mJ/cm2

Out

put E

nerg

y, n

J

Time, sec

Figure 2.10. Lasing output from the same spot on a fluid CLC film containing OF-r

at 2.0 wt% pumped with 550±12 mJ/cm2 at 10 Hz rested for 0.5 and 20 h time

intervals between three runs.

47

Table 2.1. Dissymmetry Factor, ge, of Circularly Polarized Lasing from Glassy CLC

and Fluid CLC Films Pumped with 165 and 44 mJ/cm2, Respectively, with a Typical

Error of ±0.05

OF-r wt%

GCLC LCLC

lasing

wavelength (nm)

ge value lasing

wavelength (nm)

ge value

1.5

635 −1.65 658 −1.62 2.0

634 −1.64 658 −1.67

2.5

635 −1.69 660 −1.61 3.0

636 −1.62 657 −1.70

48

CHAPTER 3

SPATIALLY RESOLVED LASERS USING A GLASSY-

CHOLESTERIC LIQUID CRYSTAL FILM WITH LATERAL

PITCH GRADIENT

3.1. INTRODUCTION

As noted in Chapter 2, a cholesteric liquid crystal (CLC) is capable of self-

organization into a helical stack which constitutes a one-dimensional photonic band

gap, serving as an optical resonator for a light-emitting dopant [37,39]. Since the

definitive demonstration of CLC lasers more than two decades ago [37], both the

lasing threshold and slope efficiency have been steadily improved. For instance, a

lasing threshold of 0.13 mJ/cm2 [50] and a slope efficiency of 60% [71] have been

reported. In addition, temporally stable lasing has been achieved by solidification of

CLC films through photo-crosslinking [48] or glass formation [47,176]. Furthermore,

real-time tunability of lasing wavelength from a constant-pitch CLC film has been

demonstrated via photoinduced reactions [87-92], an applied electric field [81] or

mechanical stress [85,86], and through temperature modulation [73,80]. The potential

of CLC lasers has been further enriched by introducing a lateral pitch gradient to

realize spatially resolved, multiple wavelength lasing. The requisite lateral pitch

49

gradient has been achieved by creating either a temperature [94] or a chiral

concentration gradient in both fluid [59,96,97] and solidified [51,95,99] CLC films.

The performance of CLC lasers is known to be affected by a number of

factors, including the molar extinction coefficient, fluorescence quantum yield and

lifetime, singlet and triplet excited state absorption cross section, and orientational

order parameter within helical stacks of the light-emitting dopant (Sem), optical

birefringence and refractive index of the CLC host, in addition to film thickness

[39,62-66,177,178]. For dopants with emission dipoles aligned with the local quasi-

nematic director within the helical stack characteristic of CLC films, lasing output is

predicted to increase with increasing Sem [39]. In a couple of recent papers [171,176],

monodomain fluid and glassy CLC lasers were prepared using rodlike OF-r as

depicted in Figure 3.1. To evaluate its Sem, monodomain fluid ZLI-2244-000 and

glassy-nematic F(MB)5-N films were doped with OF-r for collecting linearly

polarized fluorescence spectra with unploarized photoexcitation at 525 nm to arrive at

I‖ and I⊥, representing the fluorescence intensity parallel and perpendicular to the

local director, respectively. The resultant Sem value evaluated by (I‖ – I⊥)/(I‖ + 2I⊥)

for OF-r in fluid and glassy-nematic films are 0.60 and 0.56, respectively. For a

comparison with OF-r , the Sem value of commonly used DCM in fluid nematic films

was characterized at 0.36.

Despite its superior Sem value, OF-r yielded a slope efficiency of 5.1% and a

lasing threshold of 7 mJ/cm2, comparable to those of DCM, 5.3% and 7 mJ/cm2. Both

50

dopants are at their respective optimum doping levels in fluid CLC films consisting of

ZLI-2244-000 lightly doped with CB15 [171]. It is evident that a higher Sem value

alone does not ensure a lower lasing threshold or a higher slope efficiency.

Furthermore, as the previous chapter shows, lasing from OF-r in a GCLC film was

temporally stable at pumping intensities up to 466 mJ/cm2, but lasing from the fluid

CLC film diminished with time at pumping intensities beyond 74 mJ/cm2 [176]. On

the other hand, OF-r -doped GCLC and fluid CLC lasers are characterized by

comparable slope efficiencies (2.7% and 5.2%) and lasing thresholds (6.8 and 7.0

mJ/cm2), both doped at their respective optimum doping levels. The temporal

instability from a fluid CLC film was attributed to susceptibility of its helical stack to

external perturbations, such as heating via optical pumping, light-induced pitch

dilation, and laser-induced flow [74,76-78]. To further exploit the temporal stability

afforded by a GCLC film, this chapter was motivated to demonstrate variable lasing

wavelengths using a spatially resolved GCLC film containing OF-r .

3.2. EXPERIMENTAL SECTION

The light-emitting dopant OF-r , glassy liquid crystals F(MB)5-N and

F(MB)5-Ch, and the procedures for preparing a constant-pitch GCLC laser described

in Chapter 2 were employed for the study in this chapter. To prepare a gradient-pitch

GCLC laser, films of F(MB)5-Ch:F(MB)5-N mixtures at 29.0:71.0 and 20.0:80.0

mass ratios, both doped with 2.5 wt% OF-r , were heated at 220 oC for 62 h to

51

expedite molecular diffusion across the interface, followed by shearing and thermal

annealing as described for the preparation of the constant-pitch GCLC film. The

gradient-pitch GCLC laser was characterized for selective reflection by a microscope

spectrometer (Leitz Orthoplan-Pol and Ocean Optics spectrometer USB-2000)

equipped with tungsten halogen lamp as white light source. The probe beam was

focused into a diameter of 30 µm. Fresnel reflections from the air-glass interfaces

were accounted for with a reference cell containing an index-matching fluid between

surface-treated substrates. The optical micrographs of GCLC films were generated

with a digital camera (MicroPix C-1024) mounted on a polarizing optical microscope

(Leitz Orthoplan-Pol). An 14-µm-thick OF-r -doped F(MB)5-N glassy-nematic film

was characterized for linearly polarized fluorescence spectroscopy to determine OF-

r ’s orientational order parameter of emission dipoles, Sem, using the approach noted in

Chapter 2. The lasing performance of each spatially resolved band was characterized

by the methodology described in the previous chapter using the experimental

apparatus shown in Figure 2.2.

3.3. RESULTS AND DISCUSSION

The requisite GCLC film with a lateral pitch gradient was prepared by

thermally activated molecular diffusion across the interface between two GCLC films

comprising F(MB)5-Ch:F(MB)5-N mixtures and thermal annealing described in

Experimental Section in previous Chapter. As a result of the laterally varying chiral

52

concentration, the emerging Grandjean-Cano lines [179] are located as white lines in

Figure 3.2. The spatial resolution of a GCLC film into Grandjean-Cano bands

originated from the strong surface anchoring by rubbed polyimide layers. As

illustrated in Figure 3.3a and b at three lateral positions, each Grandjean-Cano band is

characterized by a constant value of an apparent helical pitch length, pa, and a

constant number of half-pitches, m. As a result of confinement between two parallel

alignment layers, m and pa values across each Grandjean-Cano band must satisfy Eq.

(1) [179].

d = m pa/2 (1)

where d is the GCLC film thickness defined by a spacer.

It is further noted in Figure 3.2 that the width of the Grandjean-Cano band

increases toward both lateral edges of the GCLC film, an observation to be

interpreted as follows. At the center of each Grandjean-Cano band, helical pitch

adopts its natural value, po. Elsewhere in the same Grandjean-Cano band, helical

stacks adopt apparent pitch lengths, pa, imposed by the two substrates. The two

helical stacks across a Grandjean-Cano line are perturbed (dilated or contracted) from

their natural structure to the same extent, thereby storing identical elastic distortion

53

energy [179]. The elastic distortion energy of a helical stack with m half-pitches, Fm,

can be quantified by Eq. (2):

2

o2

2

oa2

2

2

122

2

1

−=

−=

pd

mK

ppKFm

ππππ

(2)

where K2 is the twist elastic constant of the CLC, 2π/pa the apparent helical twist (in

the perturbed helical stack), and 2π/po the natural helical twist. Across a Grandjean-

Cano line, Fm-1=Fm holds for the two perturbed helical stacks:

( ) 2

o2

2

o2

2

2

121

2

1

−=

−

−pd

mK

pd

mK

ππππ

(3)

from which po for the underlying unperturbed helical stack at the Grandjean-Cano

line is arrived at

2

12

o

−=

m

dp (4)

54

Let N be the number of pitches in the underlying unperturbed helical stack,

d=Npo. According to Eq. (4), a Grandjean-Cano line appears where N=(m/2−1/4), and

∆N= ½ from one Grandjean-Cano line to the next. With x defined as the lateral

position, the width of Grandjean-Cano band, ∆x, can be approximated as follows:

xNxdNdx ∆=∆∆≅ 2 (5)

In general, po is inversely proportional to the local chiral mole fraction, χ−−−−1, and hence

χ∝= opdN (6)

with a proportionality constant characteristic of a given GCLC material. Thus, ∆x is

inversely proportional to dχ /dx:

( )( ) ( ) 1121 −− ∝=∆ dxddxdNx χ (7)

55

With a sigmoid chiral concentration profile, χ(x), expected of the thermally

processed gradient-pitch GCLC film, a decreasing |dχ /dx| and hence an increasing ∆x

emerge toward both lateral edges of the GCLC film as observed in Figure 3.2.

Each Grandjean-Cano band of the GCLC film was optically pumped with a

532 nm laser for 35 ps at a 10 Hz repetition rate. The output-input relationship for an

arbitrary Grandjean-Cano band with lasing at 622 nm is shown in Figure 3.4 to arrive

at a lasing threshold of Γ=6.6 mJ/cm2 and a slope efficiency of η=1.5%. The spatially

resolved reflection and lasing spectra from every other Grandjean-Cano band of the

gradient-pitch GCLC film are presented in Figure 3.5a and b, respectively. Also

included in Figure 3.5(b) is the fluorescence spectrum, shown as the dashed curve,

with unpolarized photoexcitation at 525 nm of the same glassy-nematic F(MB)5-N

film as that used above to obtain OF-r ’s Sem=0.56 as displayed in Figure 3.6. The

glassy-nematic film was employed to represent quasi-nematic sublayers within the

helical stacks of a GCLC film. Across the Grandjean-Cano band identified in Figure

3.3a, lasing peaks overlap at 665 nm as shown in Figure 3.3b, consistent with the

presence of a single stop-band. Slope efficiencies and lasing thresholds for peaks

shown in Figure 3.5b are plotted in Figure 3.7a and b as a function of lasing

wavelength. The minimum threshold and maximum efficiency largely tracks the

fluorescence maximum shown in Figure 3.5b. Furthermore, the observed thresholds

are the lowest of all gradient-pitch CLC lasers reported to date [51,94,95], and the

maximum slope efficiency of 1.5% compares favorably with the maximum value of

0.5% reported for gradient-pitch fluid CLC lasers [94]. As indicated in Table 1, the

56

values of dissymmetry factor associated with lasing, ge as defined in Chapter 2, are

largely constant between −1.6 and −1.7 for all lasing wavelengths. Left in a desiccator

at room temperature for six months, GCLC films showed no evidence of

crystallization under polarizing optical microscope. During the six month monitoring

period, the Grandjean-Cano band with lasing at 636 nm exhibited overlapping

reflection and lasing spectra. For the same Grandjean-Cano band, stability in lasing

performance is further evidenced by the slope efficiency of 1.1% and lasing threshold

of 7.3 mJ/cm2, essentially identical to those characterized previously, 1.1% and 6.7

mJ/cm2.

Having demonstrated lasing from a Grandjean-Cano band of a gradient-pitch

GCLC film, let us proceed to appraise its merits against those from a constant-pitch

GCLC film of the same thickness, both lasing at 636 nm. A constant-pitch GCLC

film was prepared using F(MB)5-Ch:F(MB)5-N at a 24.0:76.0 mass ratio to match

the lasing peak at 636 nm from an arbitrary Grandjean-Cano band of a gradient-pitch

GCLC film. Lasing threshold and slope efficiency were found to be comparable: 6.7

mJ/cm2 and 1.1% for the gradient-pitch GCLC film, and 6.3 mJ/cm2 and 1.4% for the

constant-pitch GCLC film, respectively, as shown in Figure 3.8. While the

Grandjean-Cano bands furnish stop-bands across a wide spectral range from 515 to

719 nm, lasing is limited to 609 through 682 nm using OF-r in part because of the

required fluorescence intensity of the dopant. In principle, the range of lasing

wavelengths can be substantially extended with complementary dopants through

Förster energy transfer [97,99]. Thanks to the diverse properties that have been

57

demonstrated and the relative ease of fabrication into miniature devices, CLC lasers

may find numerous practical applications, including laser spectroscopy, medical

diagnostics and treatments, laser projection displays [58,59].

3.4. SUMMARY

The GCLC film with a lateral pitch gradient was prepared by thermally

activated molecular diffusion across the interface of two films with disparate chiral

contents. The formation of Grandjean-Cano bands was interpreted by taking into

account the strong surface anchoring at parallel polyimide alignment layers. Across

each Grandjean-Cano band, overlapping stop-bands emerged to produce essentially

the same lasing peak. The monotonically increasing widths of Grandjean-Cano bands

toward both lateral edges can be accountable by the sigmoid feature of chiral

concentration profile qualitatively, i.e. a decreasing |dχ /dx| toward both edges.

With OF-r as the single dopant, lasing peaks appeared at 609 to 682 nm from

discrete Grandjean-Cano bands. The minimum threshold and maximum efficiency

occurs at 622 nm, corresponding largely to OF-r ’s fluorescence maximum. The

observed lasing thresholds, 6.6−−−−7.6 mJ/cm2, are the lowest of all spatially resolved

CLC lasers reported to date. The slope efficiency, 0.2−−−−1.5%, reported herein

compares favorably with the maximum slope efficiency of 0.5% reported elsewhere

for gradient-pitch fluid CLC lasers.

58

An arbitrary Grandjean-Cano band in the gradient-pitch GCLC film performs

comparably to a constant-pitch GCLC film at the same lasing wavelength.

Furthermore, the GCLC film was demonstrated for temporal stability in film

morphology and lasing performance. Over the six-month monitoring period, the

GCLC film showed no evidence of crystallization, and the Grandjean-Cano band

under investigation exhibited overlapping selective reflection and lasing spectra with

the same lasing threshold and slope efficiency within experimental errors.

59

S S

N

S

N

OF-r G 104 oC N 304 oC I

F(MB)5-N G 92 oC N 171 oC I F(MB)5-Ch G 91 oC Ch 173oC I

Figure 3.1. Molecular structures of OF-r , F(MB)5-N, and F(MB)5-Ch accompanied

by their phase transition temperatures used for the preparation of constant-pitch and

gradient-pitch GCLC films. Symbols: G, glassy; N, nematic; and Ch, cholesteric.

60

400 µm

Bands capable of lasing

Figure 3.2. Polarizing optical micrograph of a 14-µm-thick GCLC film with

lateral pitch gradient selectively reflecting green to red, where Grandjean-Cano

bands are located by white lines to enhance visibility. The film was prepared by

thermally activated molecular diffusion at 220 oC for 62 h across the interface of

films comprising F(MB)5-Ch:F(MB)5-N mixtures at 29.0:71.0 and 20.0:80.0

mass ratios, both doped with 2.5 wt% OF-r . At the conclusion of thermal

annealing, the pitch gradient and the Grandjean-Cano bands were frozen in the

glassy state by quenching to room temperature. The widths of the Grandjean-

Cano bands from left to right are 1255, 584, 373, 323, 199, 180, 186, 161, 155,

161, 155, 143, 155, 143, 143, 149, 137, 137, 130, 130, 149, 143, 155, 230, 242,

298, 385, 528, 1168 µm.

61

0

25

50

75

500 600 700

Ref

lect

ance

, %

Wavelength, nm

Lasi

ng in

tens

ity, a

.u.

(a)

x xx

50 µµµµm

(b)(a)

Figure 3.3. (a) At three lateral positions identified as X’s in an arbitrary Grandjean-

Cano band of the gradient-pitch GCLC film, all three stop-bands are centered at 624

nm with overlapping lasing peaks at 665 nm, as shown in (b). The diameters of the

probe beam for selective reflection spectra measurements and the pump beam for

lasing experiments are 30 and 28 µm (full width at half maximum), respectively,

much less than the widths of all the Grandjean-Cano bands.

62

0

4

8

12

0.0 0.2 0.4 0.6 0.8

Out

put,

nJ

Input, µµµµJ

0

4

8

12

0.0 0.2 0.4 0.6 0.8

Out

put,

nJ

Input, µµµµJ

0

4

8

12

0.0 0.2 0.4 0.6 0.8

Out

put,

nJ

Input, µµµµJ

η =1.5 %Γ =6.6 mJ/cm2

Figure 3.4. Lasing output at 622 nm as a function of pump energy of a 14-µm-thick

gradient-pitch GCLC film doped with OF-r at 2.5 wt% to illustrate the determination

of lasing threshold and slope efficiency.

63

(a) (b)

0

20

40

60

400 500 600 700 800

Ref

lect

ance

, %

Wavelength, nm550 600 650 700 750

Em

issi

on in

tens

ity, a

.u.

Wavelength, nm

Figure 3.5. (a) Selective reflection spectra (i.e. stop-bands) of every other Grandjean-

Cano band in the gradient-pitch GCLC film shown in Figure 3.2; (b) lasing from

dopant OF-r using the gradient-pitch GCLC film shown in Figure 3.2, where optical

pumping was furnished with a 532 nm laser for 35 ps at a 10 Hz repetition rate; the

six lasing peaks from right to left are attributed to the 10th, 12th, 14th, 16th, 18th, and

20th Grandjean-Cano bands from the right shown in Figure 3.2. In addition, the

dashed curve in (b) represents the fluorescence spectrum of a 14-µm-thick glassy-

nematic F(MB)5-N film containing 2.5 wt% OF-r with unpolarized photoexcitation

at 525 nm.

64

500 600 700 800

Flu

ores

cenc

e in

tens

ity, a

.u.

Wavelength, nm

I||

I⊥⊥⊥⊥

Sem=0.56

Figure 3.6. Polarized fluorescence spectrum of 14-µm-thick glassy-nematic F(MB)5-

N film doped with 2.5 wt% OF-r .

65

(a) (b)

6.4

6.8

7.2

7.6

8.0

600 620 640 660 680 700

Lasi

ng th

resh

old,

mJ/

cm2

Lasing wavelength, nm

0.0

0.5

1.0

1.5

2.0

600 620 640 660 680 700

Slo

pe e

ffici

ency

, %

Lasing wavelength, nm

Figure 3.7. (a) Slope efficiency and (b) lasing threshold of OF-r at 2.5 wt% in 14-

µm-thick GCLC film as functions of lasing wavelength. Typical errors for both

efficiency and threshold are ±10%.

66

0

8

16

24

0.0 0.5 1.0 1.5 2.0

Out

put,

nJ

Input, µµµµJ

Figure 3.8. Lasing outputs at 636 nm as functions of pump energy for OF-r at 2.5

wt% from a Grandjean-Cano band of the 14-µm-thick gradient-pitch (circles) GCLC

film between F(MB)5-Ch and F(MB)5-N mixtures at 71.0:29.0 and 80.0:20.0 mass

ratio and the corresponding constant-pitch (triangles) GCLC film of F(MB)5-

Ch:F(MB)5-N at a 24.0:76.0 mass ratio. The lasing threshold and slope efficiency are

6.7 mJ/cm2, 1.1% and 6.3 mJ/cm2, 1.4% for the gradient-pitch and constant-pitch

GCLC films, respectively.

67

Table 3.1. Dissymmetry Factor Values, ge, of Spatially-Resolved Granjean-Bands

from the Gradient-Ptich Glassy CLC Film Pumped with 165 mJ/cm2 with a Typical

Error of ±0.05

Lasing Wavelength

ge value 682

−1.69 665

−1.65 650

−1.64 636

−1.67 622

−1.68 609

−1.67

68

CHAPTER 4

ROOM-TEMPERATURE PROCESSING OF ππππ-CONJUGATED

OLIGOMERS INTO UNIAXIALLY ORIENTED MONODOMAIN

FILMS ON COUMARIN-BASED PHOTOALIGNMENT LAYERS

4.1. INTRODUCTION

Polymers and monodisperse oligomers with an extended π-conjugation in

amorphous, crystalline, and liquid crystalline morphologies have found numerous

potential applications to electronics and photonics [100-106]. Uniaxial orientation of

conjugated backbones enables polarized light emission as energy-efficient backlights

for liquid crystal displays [108], anisotropic charge transport to suppress cross-talk in

logic circuit and pixel switching elements [107], and polarization-sensitive

photodiodes for sensing applications [109-111]. Monodomain glassy-nematic films

have been achieved with poly(fluorene)s [118,180], poly(fluorene-co-bithiophene)s

[112], and monodisperse oilgo(fluorene)s [113-117] and oligo(fluorene-co-

bithiophene)s [119] films through thermal annealing above their glass transition

temperatures on rubbed polyimide, PEDOT:PSS, and poly(p-phenylenevinylene)

alignment layers with subsequent cooling to room temperature.

To overcome the problems arising from rubbing, e.g. mechanical damage and

the generation of dust and electrostatic charges, photoalignment induced by polarized

UV-irradiation has emerged as an attractive alternative. Three distinct material

69

approaches have been demonstrated: axis-selective degradation of polyimides [122-

126], cis-trans isomerization of azobenzenes [127-129], and cycloaddition of

cinnamates [130-134] or coumarins [135-142]. Orientational order parameter values

emulating those on rubbed polymer alignment layers have been realized via thermal

annealing of films comprising π-conjugated polymers [143,145] and oligomers

[139,140] on photoalignment layers.

In a recent paper [119], we have reported the first demonstration of

chlorobenzene-vapor annealing at room temperature of oligo(fluorene-co-

bithiophene) films on rubbed polyimide alignment layers. The resultant monodomain

glassy-nematic films, referred to hereafter as uniaxially oriented monodomain films,

were as well ordered as those from thermal annealing at 110 to 120 oC, i.e. 10 oC

above glass transition temperatures. Nonetheless, the preparation of polyimide

alignment layers involved high-temperature baking. To pave the way for plastic

electronics, it is highly desirable to develop room-temperature processing of oriented

films, including the preparation of alignment layers.

This study was motivated to develop a new process in which both the

preparation of photoalignment layer and solvent-vapor annealing are conducted at

room temperature. A coumarin-containing polymer was chosen for its superior

thermal and photochemical stability without complication from photo-induced

isomerization [131,135]. Specific objectives are indentified as follows: (i) solvent-

vapor annealing of conjugated oligomers into uniaxially oriented monodomain films;

70

(ii) elucidation of how vapor absorption affects the vapor annealing process; (iii)

lyotropic nematic mesomorphism as the driving force for solvent-vapor annealing;

(iv) effectiveness of solvent-vapor annealing on photoalignment layers versus rubbed

polyimide alignment layers, and that of solvent-vapor annealing versus thermal

annealing on rubbed polyimide alignment layers; and (v) demonstration of uniaxially

oriented monodomain films on plastic substrates.

4.2. EXPERIMENTAL SECTION

4.2.a. Materials

The molecular structures of all materials used in this study are depicted in

Chart 4.1. The synthesis and properties of OF, OF2T and Polymer 1 have been

reported previously [113,119,139]; Polymer 1 was characterized to have a number

average molecular weight of 8,960 g/mol and a polydispersity index of 4.6. The

polymer analogue of OF2T, PF2T with a number-average molecular weight of

36,200 g/mol and a polydispersity index of 3.1, was used as received from American

Dye Source (Quebec, Canada); its thermotropic properties have been reported earlier

[119]. The differential scanning calorimetry (DSC) data accompanying the molecular

structures in Chart 4.1 were collected from 20 oC/min heating scans of samples that

had been preheated to isotropic liquids and then quenched at −100 oC /min to −30 oC

using a DSC-7 (Perkin-Elmer). The nature of the phase transition was characterized

71

by hot-stage polarizing optical microscopy (DMLM, Leica; FP90 central processor

and FP82 host stage, Mettler Toledo), which was also used to observe vapor

annealing of an OF film under saturated chlorobenzene in a sealed chamber at room

temperature.

4.2.b. Preparation and Characterization of Polymer 1 Photoalignment and

Rubbed Polyimide Alignment Layers

Photoalignment layers were prepared by spin-casting a 0.1 wt % solution of

Polymer 1 in chloroform at 4000 rpm on optically flat fused silica substrates (25.4

mm diameter × 3.0 mm thickness, EscoProducts) transparent to 200 nm, producing

photoalignment layers with a thickness of 10 nm determined by variable angle

spectroscopic ellipsometry (V-VASE, J. A. Woollam Corp.). After vacuum-drying at

room temperature for 3 h, the Polymer 1 films were irradiated at room temperature

under an argon atmosphere using a 500 W Hg-Xe lamp (model 66142, Oriel)

equipped with optical filters (model 87031, Oriel and model XUVB280, Asahi),

allowing transmission from 300 nm to 330 nm. The transmitted light was linearly

polarized using a polarizing beam splitter (HPB-308 nm, Lambda Research Optics,

Inc). The irradiation intensity was monitored by a UVX digital radiometer coupled

with a UVX-31 sensor (UVP, Inc.). The extent of coumarin dimerization, X, was

determined by the diminishing absorbance of coumarin monomers at 310 nm with

UV-Vis-NIR spectrophotometry (Lambda 900, Perkin-Elmer). The insolubility of

72

irradiated films was ascertained by UV-Vis absorption spectroscopy after submerging

in liquid chloroform for 2 min. Polyamic acid solution in 2-butoxyethanol (Sunever

grade 610 polyimide varnish, Nissan) was diluted with a mixed solvent, 2-

ethoxyethanol:2-butoxyethanol at a 4:1 volume ratio, for the preparation of 15-nm-

thick films by spin coating on fused silica substrates, followed by soft-baking at 80 oC

for 10 min, hard-curing at 250 oC for 1 h, and uniaxial rubbing with a velvet fabric for

use as an alignment layer.

4.2.c. Preparation and Characterization of Oriented ππππ-Conjugated Oligomer

Films

Conjugated oligomer films were prepared by spin coating from 1.0 wt %

chloroform solutions at 2000 rpm onto substrates containing either Polymer 1 or

polyimide alignment layers. The spin-cast films were dried under vacuum at room

temperature for 3 h before solvent-vapor annealing using the apparatus shown in

Figure 4.1. Defined as the solvent partial pressure over its vapor pressure (Ps) at room

temperature, the relative vapor pressure (Pr) in the equilibration chamber was

controlled by mixing a vapor-saturated nitrogen gas stream with a pure nitrogen

stream at varying ratios of volumetric flow rates regulated by flow meters (R-03227-

12 150 mm flow-meter, Cole-Parmer). After allowing for sufficient purge time

through the equilibration chamber, the oligomer films on Polymer 1 photoalignment

layers and rubbed polyimide alignment layers were loaded into the equilibration

73

chamber to start solvent-vapor annealing at room temperature for predetermined

durations and then vacuum-dried at room temperature for 3 h. Thermal annealing of

conjugated oligomer films on rubbed PI alignment layers was performed under an

argon atmosphere at 10 oC above the conjugated oligomers’ glass transition

temperatures for varying annealing times. The dried films were characterized for their

thickness using a white light optical profilometer (Zygo NewView 5000).

Absorption dichroism of the oriented conjugated oligomer films was

characterized using a Lambda 900 UV-Vis-NIR spectrophotometer equipped with

linear polarizers (HNP’B, Polaroid) for the calculation of orientational order

parameter, S=(R−1)/(R+2), in which the dichroic ratio R represents the absorbance

parallel divided by that perpendicular to the direction of linearly polarized UV-

irradiation of Polymer 1 photoalignment layer or the rubbing direction on polyimide

alignment layer. Micrographs of the oriented films were taken with a digital camera

(MicroPix C-1024) mounted on a polarizing optical microscope (Leitz Orthoplan-

Pol). The phase and topography images of the oligomer films were recorded on a

Nanoscope III atomic force microscope (AFM, Digital Instrument) in tapping mode

with a NT-MDT NSG01 cantilever at a scan rate of 1 Hz under ambient condition.

Films for electron diffraction were prepared under the same conditions except on

sodium chloride substrates (International Crystal Laboratories). The films were

floated off in a trough filled with de-ionized water for mounting onto copper grids.

Electron diffraction patterns were collected on a transmission electron microscope

(FEI Tecnai F20). For a demonstration of solvent-vapor annealing on a plastic

74

substrate, OF films were prepared and characterized on poly(allyl diglycol carbonate)

substrates (CR-39, 25.4 mm× 25.4 mm×1.5 mm thickness) with a UV absorption

cutoff at 380 nm (TASTRAK, Track Analysis Systems Ltd.) following the same

procedures as described above for fused silica substrates. The solvent and UV

resistance of the CR-39 substrates were demonstrated by UV-Vis absorption

spectroscopy after submerging in liquid chloroform for 2 min and UV irradiation to a

fluence of 1.0 J/cm2.

4.3. RESULTS AND DISCUSSION

Approximately 90-nm-thick OF films were prepared by spin coating on fused

silica substrates containing 10-nm-thick Polymer 1 photoalignment layers, which had

been irradiated with 1.0 J/cm2 linearly polarized light between 300 and 330 nm,

followed by vacuum drying at room temperature for 3 h. The UV-vis absorption

spectra of pristine Polymer 1 film and the film irradiated at room temperature are

included in Figure 4.2 for the calculation of X=0.31 using coumarin monomer’s

absorbance at 310 nm. In principle, a good solvent for the conjugated oligomer and

one that is also relatively volatile at room temperature would be effective for solvent-

vapor annealing. As a good solvent for OF and with Ps=170 mmHg at 22 oC,

chloroform was tested for vapor annealing of OF films with 25-min purge by flushing

two incoming gas streams through the equilibration chamber as shown in Figure 4.1.

75

The feasibility of solvent-vapor annealing was demonstrated with Pr=0.90 for

8 min exposure, resulting in S value of 0.74 as shown in Figure 4.3a. The S values are

plotted as a function of exposure time, t, in Figure 4.3b for films dried in vacuo after

vapor annealing at Pr=0.75 to 0.90. Doubling the purge time resulted in essentially

the same S profiles as shown in Figure 4.3b, thereby justifying 25 min as sufficient

for the equilibration chamber to reach the intended Pr values. All the curves through

data points associated with S values as functions of exposure time throughout this

chapter represent the least-squares fit to S=m1(1−exp(−m2t))+m3(1−exp(−m4t)),

yielding asymptotic S values at long exposure times. The OF film was found to be

dewetted at Pr=0.95, which limited solvent-vapor annealing to an asymptotic S value

of 0.74 at Pr=0.90. The S profiles presented in Figure 4.3b indicate that the time to

reach the asymptotic S value increases with a decreasing Pr value, suggesting a fast

absorption equilibrium of the OF film with ambient chloroform vapor relative to the

time for OF molecules to orient themselves with the underlying photoalignment

layer. This observation is consistent with time for the OF film on an impermeable

substrate to reach equilibrium with chloroform vapor, τ ∼ 4d2/D12, where d and D12

denote the OF film thickness and the diffusivity of chloroform (subscript 1) in the OF

film (subscript 2), respectively [181]. Given that d=90 nm encountered herein and

that D12 ≥ 10−10 cm2/s at room temperature [182], τ is estimated on the order of

seconds or less, which is orders of magnitude shorter than the solvent-vapor

annealing time.

76

The last two data points for Pr=0.90 and 0.85 and the last data point for

Pr=0.80 at the longest exposure times presented in Figure 4.3b correspond to

uniaxially oriented monodomain films, and the rest to polydomain films. The

morphological evolution of OF film annealed by chloroform vapor at Pr=0.90 is

illustrated by the polarizing optical micrographs shown in Figure 4.4. As displayed in

Figure 4.4a, a pristine OF film appears to be optically isotropic, which is

characteristic of glassy-amorphous morphology. A polydomain glassy-nematic film is

readily identifiable with threaded textures as shown in Figure 4.4b (1/2 min

exposure), whereas a uniaxially oriented monodomain film across the 2-cm-diameter

fused silica substrate is characterized by the absence of threaded textures, Figure 4.4c

(8 min exposure). The film morphology was further studied by atomic force

microscopy (AFM). For a polydomain glassy-nematic film as shown in Figure 4.4b,

the phase contrast observed in Figure 4.5a can be attributed to morphological

separation between LC domains. The surface root-mean-square roughness of 0.45 nm

observed in Figure 4.5b can be attributed in part to the domain boundaries and in part

to the film preparation process. The compositional and morphological uniformity of

monodomain film observed in Figure 4.5c are revealed by the phase images shown in

Figure 4.5c. A root-mean-square roughness of 0.35 nm resulting from the AFM

topography image, Figure 4.5d, can be attributed to the film preparation process. That

the pristine and vapor-annealed films are noncrystalline is verified by the electron

diffraction patterns shown in Figure 4.6.

77

In principle, room-temperature processing of oriented π-conjugated oligomer

films should be applicable to plastic substrates. Indeed, chloroform-vapor annealing

at Pr=0.90 of an OF film on a CR-39 substrate coated with a Polymer 1

photoalignment layer irradiated with 1.0 J/cm2 to X=0.30 produced a uniaxially

oriented monodomain film with S=0.75, the same value as that on a fused silica

substrate within experimental error.

In addition to OF, OF2T films were tested for annealing by chloroform vapor.

The results presented in Figure 4.7a and b for the polarized absorption spectra and S

values as a function of exposure time, respectively, are compared with those in Figure

4.3b, revealing a delayed arrival at asymptotic S values by OF2T compared to OF

films at the same Pr values. Under the condition of fast absorption equilibrium, OF

films must have been more absorptive of chloroform vapor than OF2T films at the

same Pr, which also appeared to be responsible for the OF film to undergo film

dewetting at a lower Pr value than OF2T, 0.95 versus 1.00. As a result, a higher

asymptotic S value of 0.82 was achieved at Pr=0.95 in OF2T film than 0.74 at

Pr=0.90 in OF film. Recall that thermal annealing of these two films on rubbed

polyimide alignment layers produced essentially the same S value, 0.83 for OF [113]

and 0.82 for OF2T [119] annealed at 160 and 115 oC, respectively, despite the

difference in their molecular structures. Thermal annealing of OF and OF2T films on

Polymer 1 photoalignment layers at 185 and 155 oC also produced S=0.82±0.01. It is

concluded (i) that solvent-vapor annealing on Polymer 1 photoalignment layer at

room temperature could achieve the same S value as conventional thermal annealing

78

on rubbed polyimide alignment layer, and (ii) that the highest S values achieved in

OF2T and OF films with chloroform-vapor annealing are determined largely by

maximum solvent contents without encountering dewetting.

Spin-cast films of both OF and OF2T were also tested for annealing by

chloroform vapor on rubbed polyimide alignment layers in addition to Polymer 1

photoalignment layers. The S values of OF film on rubbed PI are plotted as a function

of exposure time in Figure 4.8a for films dried in vacuo after vapor annealing at

Pr=0.90. The results from solvent-vapor annealing of OF, OF2T, and PF2T films on

Polymer 1 and rubbed polyimide alignment layers are summarized in Table 4.1.

Regardless of the underlying alignment layers, the same maximum S values-- namely,

0.82 and 0.74 for OF2T and OF, respectively-- resulted from annealing with

chloroform vapor at room temperature. The time to achieve asymptotic S values,

however, increased from 5−10 s on rubbed polyimide alignment layers to 6−8 min on

Polymer 1 photoalignment layers. The difference in time scale can be interpreted as

polyimide alignment layer offering π−π electronic interactions with OF2T and OF

molecules. Along the rubbing direction, grooves were generated and polyimide

backbones were reoriented on the alignment layer surface, both conducive to uniaxial

orientation of overlying nematic liquid crystal molecules thanks to the enhanced π-π

interactions [183,184]. These two factors are absent on the Polymer 1

photoalignment layer without rubbing, where liquid crystal orientation is dictated by

coumarin dimers through molecular interactions of dispersive and steric origins at the

early stage of photodimerization before cross-over [138], e.g. X=0.31 as implemented

79

in this study. In contrast to OF2T, its polymer analog, PF2T, could not be oriented

on either type of alignment layers after annealing under saturated chloroform vapor

up to 14 h, as revealed in Figure 4.8b.

Although polydomain glassy-nematic films were shown to result from

vacuum drying after chloroform vapor annealing (e.g. Figure 4.4b), it would be

informative to observe the evolution of mesomorphism during the annealing process.

As noted above, both OF and OF2T films lost their physical integrity to dewetting by

chloroform vapor at Pr≤1.00. In principle, a solvent less volatile than chloroform is

desired for in situ observation at Pr=1.00. Having a low vapor pressure,

chlorobenzene was selected for annealing an OF2T film under its saturated vapor,

Ps=10 mmHg at 22 oC. The low chlorobenzene content in the OF2T film is the

reason for the long 40 min annealing time to asymptotic S value, as shown in Figure

4.9a. The low chlorobenzene content during vapor annealing of OF2T films is also

responsible for the modest asymptotic S=0.56 in the thoroughly dried film, inferior to

0.82 achieved with chloroform. The polarizing optical micrograph is shown in Figure

4.9b for in situ observation of an OF2T film after exposure to saturated chorobenzene

vapor for 40 min, indicating lyotropic nematic mesomorphism as the driving force

behind solvent-vapor annealing of a solid film at room temperature well below its

glass transition temperature at 102 oC.

The chlorobenzene content in the OF2T film, however, can be substantially

increased by spin coating from a chlorobenzene solution under its own vapor at a

80

predetermined Pr value to slow down solvent evaporation. As an expedient approach,

quasi-solvent annealing was performed by spin coating in air from a chlorobenzene

solution onto a rubbed polyimide alignment layer before subjecting the relatively wet

film to saturated chlorobenzene vapor to obtain S=0.82 [119]. This value is identical

to that observed in an OF2T film spin-cast from chloroform on rubbed polyimide

alignment layer, dried and then exposed to chloroform vapor at Pr=0.95 (see Table

4.1). The same observations should apply to the Polymer 1 photoaligment layer,

although the annealing time is expected to be longer than that on the rubbed

alignment layer following the trend in time scale established in Table 4.1.

4.4. SUMMARY

This chapter reports the first demonstration of uniaxial orientation of π-

conjugated oligomers into macroscopic scale uniaxially oriented monodomain films

on coumarin-containing photoalignment layers through room-temperature processing.

Spin-cast films of π-conjugated oligomers, OF and OF2T, on Polymer 1

photoalignment layers were annealed at room temperature with chloroform vapor

diluted by nitrogen gas to varying extents to prevent film dewetting. In place of

Polymer 1 photoalignment layers prepared at room temperature, rubbed polyimide

alignment layers that had been cured at 250 oC were also used for chloroform-vapor

annealing of OF and OF2T films for a comparison of maximum attainable S values

in the resultant glassy-nematic films. In addition, saturated chlorobenzene vapor and a

81

CR-39 substrate were used to unravel the driving force behind solvent-vapor

annealing and to demonstrate its feasibility on plastic substrates, respectively. Key

observations are recapitulated as follows:

(i) As validated by polarizing optical microscopy, atomic force microscopy, and

electron diffraction, chloroform-vapor annealing yielded uniaxially oriented

monodomain OF and OF2T films on Polymer 1 photoalignment layers with

S values of 0.74 (at Pr=0.90) and 0.82 (at Pr=0.95), respectively. Compared

to S=0.82±0.01 for OF and OF2T films with thermal annealing on both

rubbed polyimide alignment layers and Polymer 1 photoalignment layers,

chloroform-vapor annealing is limited by film dewetting above a threshold

partial pressure of chloroform.

(ii) Chloroform-vapor annealing at room temperature of OF and OF2T films on

rubbed polyimide alignment layers was performed to further assess the

potential of Polymer 1 photoalignment layers. Maximum S values were found

to be independent of the underlying alignment layers. Because of rubbing-

induced grooves and reorientation of the polyimide backbone, both conducive

to π-π interactions of conjugated oligomers with the rubbed polyimide

alignment layer, the orientation of conjugated oligomers on rubbed polyimide

alignment layers proceeded much faster than on Polymer 1 photoalignment

layers. In contrast, PF2T was not amenable to solvent-vapor annealing on

either alignment layer even under saturated chloroform vapor.

82

(iii) With a much lower vapor pressure than chloroform at room temperature,

saturated chlorobenzene vapor facilitated in situ observation of lyotropic

nematic mesomorphism during the solvent-vapor annealing, equivalent to

thermotropic nematic mesomorphism behind thermal annealing above OF’s

and OF2T’s glass transition temperatures.

(iv) A uniaxially oriented monodomain OF film was demonstrated with S=0.75

through chloroform-vapor annealing at Pr=0.90 on CR-39, a plastic substrate,

coated with Polymer 1 photoalignment layer prepared at room temperature, as

opposed to polyimide alignment layers requiring high-temperature processing.

83

OF2T: G 102oC N 240oC I

OF: G 149oC N 366oC I

Polymer 1: G 68oC I PF2T: G+K 109oC, 129oC K 274oC N 320oC I

SymbolsG for glass transition, N for nematic mesomorphism, I for isotropic liquid, and K for crystallization or crystalline melting.

Chart 4.1. π-Conjugated oligomers and coumarin-based Polymer 1 for

photoalignment under present investigation

84

N2

N2

Flow-meters

Transfer Chamber

OligomerFilm

Exit Gas Stream

Equilibration Chamber

Vapor SaturatorElectric

Fan

Figure 4.1. A schematic diagram of the apparatus for solvent-vapor annealing under

controlled partial pressure.

85

250 300 350 400 4500.00

0.03

0.05

0.08

0.10

Abs

orba

nce

λλλλ , nm

Figure 4.2. UV-vis absorption spectra of 10-nm-thick pristine Polymer 1 film (blue

solid curve) and the film exposed to linearly polarized UV-irradiation to a fluence of

1.0 J/cm2 (red dotted curve) at room temperature.

86

0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60 80

S

Exposure Time, min

0.900.850.800.75

Pr

(a) (b)

300 400 5000.0

0.5

1.0

1.5

2.0

Abs

orba

nce

Wavelength , nm

A||

A⊥⊥⊥⊥

Figure 4.3. (a) Polarized absorption spectrum of 90-nm-thick spin-cast OF film on

10-nm-thick Polymer 1 photoalignment layer at X=0.31 annealed with chloroform

vapor at Pr=0.90 for 8 min. Symbols A|| and A⊥ represent absorbance parallel and

perpendicular to rubbing direction, respectively. (b) S of 90-nm-thick spin-cast OF

films on 10-nm-thick Polymer 1 photoalignment layers with X=0.31 as a function of

exposure time to chloroform vapor at specified Pr values preceded by 25-min purge.

The data points are accompanied by an error of ±0.03 overall.

87

30 µm

(c)

30 µm

(b)

30 µm

(a)

Figure 4.4. Polarizing optical micrographs of 90-nm-thick OF films on 10-nm-thick

Polymer 1 photoalignment layers at X=0.31 annealed with chloroform vapor at

Pr=0.90 followed by vacuum drying, both conducted at room temperature. (a)

pristine, (b) 1/2 min exposure to yield polydomain glassy-nematic film with S=0.23,

and (c) 8 min exposure to yield uniaxially oriented monodomain film with S=0.74.

88

(a)

1 µm

1 µm

1 µm

1 µm

(b)

(c) (d)

Figure 4.5. (a) AFM phase contrast, and (b) AFM topography with an rms roughness

of 0.45 nm of a polydomain glassy-nematic OF film on Polymer 1 after annealing

with chloroform vapor at Pr=0.90 for 1/2 min exposure. (c) AFM phase contrast, and

(d) AFM topography with an rms roughness of 0.35 nm of a monodomain glassy-

nematic OF film on Polymer 1 after annealing with chloroform vapor at Pr=0.90 for

8 min exposure.

89

(b)(a)

Figure 4.6. Electron diffraction patterns of (a) pristine OF film and (b) OF film

annealed with chloroform vapor at Pr=0.90 for 8 min exposure on Polymer 1

photoalignment layers.

90

(a) (b)

300 400 500 6000.0

0.5

1.0

1.5

2.0

Abs

orba

nce

Wavelength , nm

0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60

S

Exposure Time, min

0.950.900.850.80

Pr

0.0

0.2

0.4

0.6

0.8

1.0

0 20 40 60

S

Exposure Time, min

0.950.900.850.80

PrA||

A⊥⊥⊥⊥

Figure 4.7. (a) Polarized absorption spectra of 90-nm-thick spin-cast OF2T film on

10-nm-thick Polymer 1 photoalignment layer at X=0.31 annealed with chloroform

vapor at Pr=0.95 for 6 min exposure; symbols A|| and A⊥ are as defined in Figure

4.3(a). (b) S of 90-nm-thick OF2T films on 10-nm-thick Polymer 1 photoalignment

layers with X=0.31 as a function of exposure time to chloroform vapor at specified Pr

values followed by vacuum drying at room temperature for 3 h. At Pr=1.00 over ½

and 1 min exposure, S=0.28 and 0.00 (dewetting), respectively. The reported S values

are accompanied by an experimental error of ±0.02 overall. The last two data points

at Pr=0.95 correspond to uniaxially oriented monodomain films, and the rest to

polydomain glassy-nematic films.

91

 

(b)

300 400 500 6000.0

0.2

0.4

0.6

0.8

1.0

Ab

sorb

ance

Wavelength, nm

A||

A

(a)

0.0

0.2

0.4

0.6

0.8

1.0

0 40 80 120 160

S

Exposure Time, s

Pr=0.90

Figure 4.8. (a) S of 90-nm-thick OF films on a 15-nm-thick rubbed polyimide

alignment layer as a function of exposure time to chloroform vapor at Pr=0.90

followed by vacuum drying at room temperature for 3 h. The reported S values are

accompanied by an experimental error of ±0.02 overall. (b) Polarized absorption

spectra of 55-nm-thick spin-cast PF2T film on a 15-nm-thick rubbed polyimide

alignment layer and Polymer 1 photoalignment layer annealed with chloroform vapor

at Pr=1.00 for up to 14 h. Symbols A|| and A are as defined in Figure 4.3(a).

92

0.0

0.2

0.4

0.6

0 20 40 60 80

S

Exposure Time, min

1.000.95

Pr

(a)

30 µµµµm

(b)

Figure 4.9. 90-nm-thick OF2T films on 10-nm-thick Polymer 1 photoalignment

layers with X=0.31: (a) S of polydomain glassy-nematic films as a function of

exposure time to chlorobenzene vapor at specified Pr values followed by vacuum

drying at room temperature for 3 h; (b) In situ polarizing optical micrograph of

lyotropic nematic mesomorphism upon exposure to saturated chlorobenzene vapor for

40 min.

93

Table 4.1. S values of OF2T, OF, and PF2T films after annealing with chloroform

vapor at 22 oC on Polymer 1 and rubbed polyimide alignment layers

alignment layerd, e

asymptotic S, exposure time

OF2Ta

Polymer 1 0.82f, 6 min

rubbed PI 0.82f, ≤ 5s

OFb

Polymer 1 0.74g, 8 min

rubbed PI 0.74g, 10 s

PF2Tc

Polymer 1 0.00h, 14 h

rubbed PI 0.00h, 8 h

(a) 90-nm-thick OF2T films on Polymer 1 photoalignment and rubbed polyimide

alignment layers; (b) 90-nm-thick films OF on Polymer 1 photoalignment and

rubbed polyimide alignment layers; (c) 55-nm-thick PF2T films on Polymer 1

photoalignment and rubbed polyimide alignment layers; (d) X=0.31, 10-nm-thick

Polymer 1 photoalignment layers; (e) 15-nm-thick rubbed polyimide alignment

layers; (f) Pr=0.95, film dewetting at Pr=1.00; (g) Pr=0.90, film dewetting at

Pr=0.95; (h) Pr=1.00 without film dewetting.

94

CHAPTER 5

CONCLUSIONS AND FUTURE RESEARCH

5.1. CONCLUSIONS

Cholesteric liquid crystal lasers have emerged in recent years as an attractive

technology thanks to the ease of film preparation via molecular self-organization, their

compact size, real-time tunability and spatial resolution of lasing wavelength. To ensure

device robustness using solidified cholesteric liquid crystal films, in-situ

photopolymerization and glass transition have also been attempted albeit with erratic

optical quality. As part of this thesis research, monodomain glassy-cholesteric liquid

crystal films were prepared using mixtures of nematic and cholesteric pentafluorenes

through glass transition. Complete miscibility is assured by identical molecular structures

of the nematic and cholesteric components except pendant chirality. Doped with a

structurally similar red-emitting oligofluorene, glassy and fluid cholesteric liquid crystal

films are characterized by approximately the same levels of lasing threshold and

efficiency. The output energy of a cholesteric liquid crystal fluid laser diminished with

time upon repeated photopumping, but that of the pristine film was recovered after

having been left in the dark at room temperature for sufficient time. In contrast, glassy-

cholesteric liquid crystal film consistently exhibited superior temporal stability of lasing

output. The observed difference in temporal stability is attributable to external

perturbations on helical stacking in the fluid film but not in the solid film, such as heating

through optical pumping, light-induced pitch dilation, and laser-induced flow.

95

To take advantage of the temporal stability of solid films, spatially resolved

glassy-cholesteric liquid crystal lasers were prepared for an investigation of spatial

resolution. A lateral pitch gradient was introduced by thermally activated diffusion across

the interface of two films with disparate chiral contents, both containing the same light-

emitting dopant at the concentration. Frozen in the solid state by cooling through its glass

transition temperature at the end of thermal processing, the spatially resolved Grandjean-

Cano bands emerged because of the strong surface anchoring at rubbed polyimide

alignment layers and the lateral chiral concentration profile. Each Grandjean-Cano band

is characterized by a common stop-band, and a set of Grandjean-Cano bands produced

multiple lasing peaks across the spectral range determined by the light-emitter’s

fluorescence spectrum. The resultant lasing thresholds, 6.6–7.6 mJ/cm2, and slope

efficiencies, 0.2–1.5%, are superior to those reported to date for gradient-pitch CLC

lasers. An arbitrary Grandjean-Cano band in the gradient-pitch glassy-cholesteric liquid

crystal film exhibits lasing performance comparable to that of a constant-pitch

counterpart. The reproducible selective reflection spectra and lasing characteristics over a

period of six months further corroborates robustness of glassy-cholesteric liquid crystal

lasers.

Uniaxial alignment of π-conjugated polymers with high molecular weights and

monodisperse oligomers with modest molecular weights is highly relevant to organic

electronics and photonics, such as cholesteric liquid crystal lasers, organic polarized

light-emitting diodes, polarization-sensitive photodetectors, and anisotropic field-effect

transistors. Traditionally, π-conjugated backbones are oriented on rubbed polyimide

96

alignment layers, a process comprising high-temperature baking and thermal annealing

incompatible with plastic substrates for flexible electronics and roll-to-roll printing

process. Moreover, the rubbing process may cause mechanical damage in addition to

generating dust particles and electrostatic charges. A new approach to uniaxial

orientation of π-conjugated oligomers has been developed by solvent-vapor annealing of

spin-cast films on coumarin-containing photoalignment layers, a non-contact

methodology executed at room temperature.

Films of π-conjugated oligomers were solution cast on coumarin-containing

photoalignment layers for chloroform vapor annealing under controlled partial pressures.

As validated by polarizing optical microscopy, atomic force microscopy, electron

diffraction, and polarized absorption spectroscopy, the resulting monodomain glassy

films are uniaxially oriented with orientational order parameter, S=0.74 and 0.82 in an

heptafluorene and an oligo(fluorene-co-bithiophene) films, respectively. The maximum

S values, however, are limited by film dewetting short of annealing with saturated

chloroform vapor at room temperature. The time to arrive at maximum S values varied

from 6 to 8 min on photoalignment layers. Much shorter solvent-vapor annealing times, 5

to 10 s, were observed on rubbed polyimide alignment layers because of favorable π-π

electronic interactions between the conjugated oligomer film and the underlying

alignment layer. A polymer analog with a molecular weight twenty times that of an

oligo(fluorene-co-bithiophene) was not amenable to solvent-vapor annealing on either

photoalignment or rubbed polyimide alignment layers for up to 14 h. Annealing of an

oligo(fluorene-co-bithiophene) film under saturated chlorobenzene vapor at room

97

temperature permitted lyotropic nematic mesomorphism to be observed in situ, which is

equivalent to thermotropic nematic mesomorphism as the driving force behind thermal

annealing. This room-temperature alignment protocol was further demonstrated to be

applicable to plastic substrates. For example, chloroform-vapor annealing of a

heptafluorene film on a CR-39 plastic substrate coated with the photoalignment layer

produced a uniaxially oriented monodomain film with S=0.75, essentially the same as

that on a fused silica substrate.

5.2. FUTURE RESEARCH

Precise shaping of laser beam cross-section is essential to high-energy laser

systems. Conventional refractive and diffractive beam shapers are not suitable for

converting the Gaussian intensity profile typical of a laser beam to what is demanded by

the laser fusion project at the Laboratory for Laser Energetics. Although pixelated binary

beam shapers--such as metal plate, phase plate, and programmable spatial light modulator

devices are capable of delivering the desired intensity profile, these devices are limited by

the lack of ready scalability to large apertures or the low laser damage threshold. A new

device concept can be pursued with photopatterned nematic and twisted-nematic liquid

crystal pixels between coumarin-based photoalignment layers to overcome the limitations

encountered with existing technologies. Preliminary results acquired to date have shown

the promise of photoalignment layers for the fabrication of liquid crystal cells with

unprecedented laser damage thresholds at 1054 nm (1 ns pulse). This attribute will make

98

a significant impact on future research and development motivated to develop liquid

crystal beam shapers. Furthermore, this project will take advantage of the patterning

capability inherent to photoalignment methodology to achieve the desired intensity

profile and scalability to large apertures through solution processing and liquid crystal

cell assembly.

99

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