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Abstract:The synthesis, crystal structure, and fluorescence behavior of acetylene-bridged pentiptycene dimer (2), trimer (3), and tetramer (4) are reported. For comparison, a phenylene-pentiptycene-phenylene three-ring system (5) is also investigated. As a result of the unique intrachain pentiptycene-pentiptycene interactions in 3 and 4, their twisted conformers are populated in polar solvents and at low temperatures, and the phenomenon of nonequilibration of excited rotational conformers is observed. Twisting of the π-conjugated backbones leads to blue-shifted absorption and fluorescence spectra and increased fluorescence quantum yields and lifetimes. The fluorescence spectra of 2-4 undergo small red shifts but large intensity variations in the 0-1 vs 0-0 bands on going from solutions to thin solid films, which can be accounted for by the reabsorption effect. However, the reduction in fluorescence quantum yields for 2-4 in films vs solutions is mainly attributed to efficient interchain exciton migration to nonfluorescent energy traps. In contrast, the behavior of nonequilibration of excited rotamers is not observed for 5 in solutions. Compound 5 forms J-type aggregates through terminal phenylene π-stackings in the solid state, resulting in a new absorption band at 377 nm and large red shifts of the structured fluorescence spectra.

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Probing the Intrachain and Interchain Effects on theFluorescence Behavior of Pentiptycene-Derived

Oligo( p-phenyleneethynylene)s

Jye-Shane Yang,* Jyu-Lun Yan, Chung-Yu Hwang, Shih-Yi, Chiou,Kang-Ling Liau, Hui-Hsu Gavin Tsai, Gene-Hsiang Lee, and Shie-Ming Pe

ng

J. Am. Chem. Soc. 2006, 128, 14109-14119

Speaker： Po-yuan Chung

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Poly(phenyleneethynylene)s (PPEs)

• 高分子 (polymer)，顧名思義就是分子結構龐大、分子量高的物質，換言之，在分子主鏈上具有單鍵、雙鍵 (或參鍵 )交替之共軛結構，使電子可沿著分子鏈或跨分子鏈運動，因而具導電性的高分子量物質，我們稱之為共軛高分子。由於具有導電能力及電激發光的性質，可應用於有機電激發光元件。而 PPEs即為一種利用三鍵將單體 (monomer)連結起來的共軛高分子。

PPEs

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Intrachain Effect and Interchain Effect

• Intrachain effect： Undergo fast excited-state conformational relaxation to planarize the conjugated backbone before the fluorescence is emitted.

• Interchain effect ： The significant reduction in fluorescence quantum yields for 3 and 4 in films vs solutions is however attributed to efficient interchain exciton migration to nonfluorescent energy traps.

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Coplanar and Twisted Geometry

Rotational barrier ＜ 1 kcal/mol

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Swager’s Approach

Conformations and spatial arrangements of polymers 1-4 at the air-water interface and their reversible conversions between face-on, zipper and edge-on structures.

Kim, J.; Swager, T. M. Nature, 2001, 411, 1030-1034.

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Oligo(p-phenyleneethynylene)s (OPEs)

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Synthesis of 2

Lithium trimethylsilylacetylide

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Synthesis of 3

Quinone

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Synthesis of 4

Quinone

11

Synthesis of 5

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Rotational potential of 3’ and 5’ in the ground state were calculated with the AM1.

Transition energies were calculated with the ZINDO algorithm.

Rotational Potential of 3’ and 5’

Barrier = 2.6 kcal/mol Barrier = 0.06 kcal/mol

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Intrachain Effect

• Undergo fast excited-state conformational relaxation to planarize the conjugated backbone before the fluorescence is emitted.

• Be better investigated in dilute solutions or low-temperature solvent glasses.

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Jablonski Diagram

http://www.shsu.edu/~chemistry/chemiluminescence/JABLONSKI.html

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Franck-Condon Theory and Mirror Image Rule

曾炳墝教授 ,高等無機上課講義 ,2006;Chapter 1, with permission from Dr. Tzeng, B.-C

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The (a) Absorption and (b) Fluorescence Spectra of 3

3

314 nm

Polarity ： MeCN > THF > CHCl3 > hexane

λex = 302 nm

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Temperature Dependence of (a) Absorption and (b) Fluorescence Spectra of 3 in MTHF

λex = 302 nm

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Blue-shift20 nm

363 nm

5.6 ns 0.8 ns

Blue-shift4 nm

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Temperature Dependence of (a) Absorption and (b) Fluorescence Spectra of 4 in MTHF

λex = 303 nm

4

380 nm

Blue-shift41 nm

5.5 ns 0.8 ns

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Structure of Three-ring and Four-ring Zipper

296K λabs(nm) λfl(nm) Twisted form

λfl(nm)τ(ns) φfl

3 334(354) 366(387) 346(80K) 5.6(0.8) 0.63

4 341(367) 387(410) 346(80K) 5.5(0.8) 0.71

Red-shift

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Temperature Dependence of Fluorescence Spectra of 2 in MTHF

296K λabs(nm) λfl(nm) φfl

2 297(315) 327(342) 0.38

3 334(354) 366(387) 0.63

λex = 306 nm

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Σk： the sum of all nonactivated processes (fluorescence and intersystem crossing) A ： preexponentialEa： activation energy for the activated process

(1)

Compound 2Ea = 630 cm-1

DPAEa = 610 cm-1

Finney, N. S. J. Am. Chem. Soc. 2002, 124, 1178-1179.

Diphenylacetylene (DPA)

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Temperature Dependence of (a) Absorption and (b) Fluorescence Spectra of 5 in MTHF

λex = 320 nm

5

Compound 3

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Interchain effect

• The significant reduction in fluorescence quantum yields for 3 and 4 in films vs solutions is however attributed to efficient interchain exciton migration to nonfluorescent energy traps.

• Be better investigated in thin solid films.

• Be prepared by spin casting with 5×10-3 M chloroform solutions.

http://nmeg.group.shef.ac.uk/index.php?page=spincast

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Crystal Structure of 11

(a) the nonplanar conjugated backbones and layered packing motif.(b) head-to-tail tilted packing, included solvent molecules, and disor

dered octyl chains.

35°

8 Å

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Crystal Structure of 5

(a) the coplanar conjugated backbones and parallel interchain alignments.(b) interchain offset π-stacking of the terminal phenylene rings (the octyl groups were removed

for clarity).

5

3.46 Å

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Normalized Absorption and Fluorescence Spectra of 2, 3, 4, and 5

in CHCl3 in spin-cast films

377 nm

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Compd

296KMedia φfl

2CHCl3 0.40

film

3CHCl3 0.60

film 0.20

4CHCl3 0.69

film 0.20

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Normalized Fluorescence Spectra for Films of 3 Mixed (a) with PMMA and (b) with 2

Molar fractions of 3 = 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0

3 mixed with PMMA 3 mixed with 2

poly(methyl methacrylate)

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Comparison of the Fluorescence Data of 3-4 with those of 18

Compd Media φfl φfl 差距

3CHCl3 0.60

0.40film 0.20

4CHCl3 0.69

0.49film 0.20

18CHCl3 0.50

0.17film 0.33

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Conclusions

• The intrachain conformation and interchain exciton coupling effects on the fluorescence properties of PPEs have been studied.

• The unique intrachain pentiptycene-pentiptycene interactions in 3 and 4, as well as their twisted conformers and their photophysics can be characterized at low temperatures.