T

NKa

b

a

ARRAA

KCSEOB

1

eltirfabsbadelsl

(

S

0d

Synthetic Metals 161 (2011) 466–473

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

uning of HOMO levels of carbazole derivatives: New molecules for blue OLED

eeraj Agarwala,∗,1, Pabitra K. Nayaka,2, Farman Ali a, Meghan P. Patankarb,.L. Narasimhanb,∗, N. Periasamya,∗

Department of Chemical Sciences, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, IndiaDepartment of Condensed Matter Physics and Material Science, Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India

r t i c l e i n f o

rticle history:eceived 20 April 2010eceived in revised form 3 December 2010ccepted 1 January 2011vailable online 31 January 2011

eywords:

a b s t r a c t

The Suzuki–Miyaura coupling between 3,6-dibromo-N-hexylcarbazole and aryl-boronic acids cat-alyzed by Pd(PPh3)4 afforded blue emitting molecules, 3,6-diaryl-N-hexylcarbazole: aryl = phenyl (1),p-methoxyphenyl (2), p-cyanophenyl (3), p-acetophenyl (4), pyrenyl (5), biphenyl (6), difluorophenyl (7)and p-nitrophenyl (8), in good yield. The HOMO level of the carbazole derivative varied between −5.67and −6.02 eV due to the electron-withdrawing or donating substituent in the aryl group. This is confirmedby the linear correlation of the Hammett parameter of the substituent in the phenyl group with oxidation

arbazoleuzuki–Miyaura couplinglectroluminescenceLEDlue EL

potential or energy levels. Density functional theory calculation (B3LYP/6-31G*) of carbazole derivativesconfirmed the experimentally observed trend. Blue fluorescence quantum yield of these molecules ishigh (0.19–0.96), except for the nitrophenyl derivative (<0.1). Single layer organic light emitting devices(OLEDs) using spin-coated films of 2–5 showed blue electroluminescence (EL) with a turn-on voltage at∼6–7 V. The performance of blue EL of molecules 3 and 4 improved dramatically (turn-on at ∼3 V, bright-ness at ∼1000 cd/m2, efficiency at ∼1 cd/A) in a multilayer OLED designed using standard molecules as

rt lay

electron and hole transpo

. Introduction

Design and synthesis of organic molecules for use in organiclectronics materials has become an active area of research [1–6]. Aarge effort to develop organic light-emitting device (OLED) basedechnology for commercially viable displays and lighting devicess under way. Essentially, blue, green, and red color OLEDs areequired to develop a full color display. It has been noted that per-ormance of blue emitting materials in blue OLEDs is not as efficients that of green or red and the main problems are color purity, sta-ility and lifetime of blue OLEDs. This provided the motivation toearch for new stable blue emitting materials for OLEDs. Carbazole-ased compounds are known for their intense blue luminescencend some derivatives are already used in OLEDs [7,8]. For efficientevice operation it is also necessary to have HOMO and LUMO lev-

ls of emitter suitably aligned with the HOMO of hole transportayer on anode and LUMO of electron transport layer on cathodeide, respectively. Carbazole derivatives have to satisfy the energyevel conditions to be potentially useful in blue OLEDs.

∗ Corresponding author. Tel.: +91 22 22782383; fax: +91 22 22804610.E-mail addresses: nagarwal12@gmail.com (N. Agarwal), kln@tifr.res.in

K.L. Narasimhan), peri@tifr.res.in (N. Periasamy).1 Present address: Centre for Basic Sciences, University of Mumbai Kalina Campus,

antacruz (E), Mumbai-400098, India.2 Present address: Weizmann Institute of Science, Rehovot, Israel.

379-6779/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2011.01.001

ers for efficient and balanced carrier injection.© 2011 Elsevier B.V. All rights reserved.

Blue photoluminescence of carbazole is not affected by alkylsubstitution in N position [9,10]. Attaching N-hexyl group incarbazole increases its solubility in organic solvents, which isuseful to make thin films by spin coating. N-hexylcarbazole istherefore chosen as the core structure to make new carbazolederivatives. Herein, we report the synthesis of eight 3,6-diaryl-N-hexylcarbazole derivatives (1–8), and their properties such asfluorescence quantum yield, Tg value, energy levels, etc. Singlelayer OLEDs were fabricated using these molecules to demonstrateblue electroluminescence in four molecules. It is also demonstratedthat the performance of blue EL improved dramatically when thecarbazole derivative is used as blue emitter, sandwiched betweenstandard hole and electron transport layers in a multilayer device.

2. Experimental

2.1. Materials

All the solvents, N-bromosuccinimide (NBS), K2CO3, wereobtained from SD Fine Chemicals (India). Carbazole waspurchased from Sigma–Aldrich and was used as received. 2,3,5,6-

tetrafluoro-7,7′,8,8′-tetracyano-p-quinodimethane (F4TCNQ)was purchased from Lumetech (Taiwan). N-N′-diphenyl-N,N′-(bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD)and 4,7-diphenyl-1,10-phenanthroline (BCP) were purchasedfrom Sigma–Aldrich (USA) and were used as received 3,6-

N. Agarwal et al. / Synthetic Metals 161 (2011) 466–473 467

NH

NH

BrBr

I

Br

F

F

n-BuLi

Mg

B(OH)2

F

F

B(OH)2

N

BrBr NBS, DMF

2 h, RT

1-Bromohexane

NaOH, DMSO

900 C, 12 h

+

+

(i) THF

(ii) i-propylborate

(i) Diethylether

(ii) i-propylborate

9

10

80 %

81 %

s of bu

daoEswgnpm(p1

w1

rCL(2CFP

2

wt0

2

cswauw

Scheme 1. Synthesi

ibromocarbazole was prepared as shown in Scheme 1. Differentryl boronic acids were either purchased from Sigma–Aldrichr were synthesized. Pd(PPh3)4 was freshly prepared. poly(3,4-thylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) wasourced from Baytron. Reactions that require anhydrous conditionsere carried out under inert atmosphere of nitrogen in oven-dried

lassware. Diethylether was dried over Na wire and distilled underitrogen. Progress of reaction was monitored using Silica Gel TLClates by UV detection (254 and 365 nm). Silica gel (100–200esh) was used for column chromatography. Melting points

uncorrected) and Tg were measured using a capillary meltingoint apparatus and/or thermal analysis using Netzsch STA 409PC.H and 13C NMR spectra were recorded using Bruker spectrometerith working frequency 500 MHz for 1H NMR and 125 MHz for

3C NMR. For 1H NMR and 13C NMR, the chemical shifts wereeferenced to CHCl3 present as impurity in the deuterated solvent,DCl3. Mass spectra were measured using a Thermo FinniganCQ Deca Electrospray quadrupole ion trap mass spectrometerThermo Electron Co., UK) or on MALDI-TOF (Micromass Tof SpecE). Photoluminescence was measured using SPEX Fluorolog 1681.yclic voltammetry was done using CH Instruments 600C. Atomicorce Microscopy was measured using Molecular Imaging Picolus.

.2. Roughness measurements by atomic force microscopy

The uniformity of the spin coated films of the molecules on ITOas checked by atomic force microscopy (AFM). For molecules 2–5,

he surface roughness (rms value) was estimated to be in the range.3–1.4 nm (see supporting information).

.3. OLED fabrication

OLEDs were fabricated on pre-patterned, pre-cleaned ITOoated glass substrates. The ITO substrates were patterned using

tandard lithography and then cleaned by mechanical scrubbingith detergent, followed by ultrasonic cleaning in hot water

nd finally blow drying using dry N2 gas. For a few devicessing PEDOT:PSS, ITO-coated substrates were spun at 5000 rpmith poly (3,4-ethylenedioxythiophene) (PEDOT):poly(styrene sul-

11 %53

ilding blocks 9–11.

fonate) (PSS) and baked at 150 ◦C for 30 min. The thickness ofPEDOT:PSS was nominal at ∼800 A. Compounds 2–5 were dis-solved in chloroform (25 mg/ml) and spin-coated at 3000 rpmonto ITO or PEDOT:PSS coated ITO. Ca and Al layers were thendeposited by thermal evaporation in a high-vacuum (∼10−6 Torr)chamber through a shadow mask. The thickness of various lay-ers was monitored by quartz crystal thickness monitor. For thesingle active layer device, the typical thickness of each layerwas as follows: 1000 A of 2–5, 1000 A of Ca and 800 A of Al.All the devices were capped with LiF (1000 A). Multilayer OLEDs(area: 2 mm2), ITO/F4TCNQ(20 A)/TPD(400 A)/CBP(100 A)/3 or 4(400 A)/BCP(60 A)/LiF(15 A)/Al, were fabricated by deposition of allorganic materials under high vacuum. The electroluminescenceintensity in cd/m2 was measured using a calibrated large area Sili-con detector placed in close proximity to the sample.

3. Results and discussion

3.1. Theoretical calculations

Theoretical calculations for N-methylcarbazole and 3,6-diphenyl-N-methyl carbazole were carried out by densityfunctional theory (DFT) using Gaussian 03 [11]. The hexylgroup in the synthesized molecules was replaced by methyl groupin the calculations. B3LYP method was used with 6-31G(d) basisset for geometry optimization [12]. These calculations provideddistribution of the HOMO densities at different positions in themolecular framework. Fig. 1 shows the contribution of pz atomicorbital for HOMO of N-methyl carbazole and its 3,6-diphenylderivative. Quantitative data indicate that 3- and 6-positionsof carbazole have largest contribution to HOMO and hence theeffect of electron donating/withdrawing groups (EDG/EWG) inthese positions would effect maximal changes in the HOMO level.Similarly, the density distribution in the model compound 3,6-diphenyl-N-methylcarbazole shows that ortho and para position

of the phenyl group contribute maximally to the HOMO (Fig. 1b).Hence, substitution at ortho and para position of the phenyl groupwith EDG/EWG should tune the HOMO energy levels. Therefore,3,6-diphenylcarbazole derivatives having EDG/EWG substituentsat para position of the phenyl group were synthesized and studied.

468 N. Agarwal et al. / Synthetic Metals 161 (2011) 466–473

F above( le.

3

assbs31daiwtu1ippDaih

ig. 1. Representation of the pz contribution of the atomic orbitals for HOMO (B3LYP/6-31G(d)) for (a) N-methylcarbazole and (b) 3,6-diphenyl-N-methylcarbazo

.2. Synthesis of starting materials

The synthetic route and structures of synthesized moleculesre shown in Schemes 1 and 2. Starting materials, 9–11 wereynthesized as shown in Scheme 1. 3,6-Dibromocarbazole wasynthesized by bromination of carbazole using 2.2 equiv. of N-romosuccinimide in DMF at room temperature [13]. Pale yellowolid was washed with ethanol and recrystallized to obtain pure,6-dibromocarbazole. Reaction of 3,6-dibromo carbazole with-bromohexane in DMSO under basic condition gave the 3,6-ibromo-N-hexylcarbazole, 9 in 80% yield. 2,4-Difluoro boroniccid, 10 and biphenyl boronic acid, 11 were synthesized as shownn Scheme 1. 1-Bromo-2,4-difluorobenzene was allowed to react

ith freshly cleaned Mg turnings in dry diethylether followed byhe slow addition of tri-i-propylborate. Purification of crude prod-ct by recystallization afforded 10 as pure white solid. Compound1 was prepared by a slightly different procedure. Lithiation of

odobiphenyl with n-BuLi at −78 ◦C followed by addition of tri-i-ropylborate and stirring for 10 h gave crude product which on

urification by recrystallization yielded 11 as white solid. 3,6-ibromo-N-hexylcarbazole, 9 and aryl boronic acids were useds building blocks for the synthesis of molecules 1–8 as shownn Scheme 2. Suzuki–Miyaura coupling [14] of 3,6-dibromo-N-exylcarbazole with ∼2.2 equiv. of aryl boronic acid catalyzed by

N

BrBr

B

Ar

OHHO

OCH3 COCHCN

+

Ar =

1 32 4

Pd(PPh

DME, H

Scheme 2. Synthesis of carb

) and molecular orbital plots (below) calculated using density functional theory

Pd(PPh3)4 and K2CO3 was carried out in a mixture of water and 1,2-dimethoxyethane (1:1, v/v). Reaction mixture was refluxed at 80 ◦Cfor 12–16 h under inert atmosphere. Crude product was obtainedwhich on further purification by silica gel column chromatographyusing hexanes and ethyl acetate as eluent produced the carbazolederivatives 1–8 in 60–85% yields (see supporting information forfull details).

The carbazole derivatives 1–8 were characterized by 1HNMR, 13C NMR, IR and elemental analysis. In 1H NMR of 3,6-dibromocarbazole a singlet of two protons (4- and 5-positions) at8.16 ppm was observed which is shifted to ∼8.44 ppm in 1–8 show-ing that aryl substitution occurred at positions 3 and 6, respectively.As representative NMR spectra, aromatic regions of 9 and 1 areshown in Fig. 2 (see supporting information for all spectra). Synthe-sis and characterization of molecules 3 and 4 have been reportedpreviously [15].

3.3. Photophysical properties

The absorption and emission spectra of 1–8 in air-saturatedacetonitrile were recorded at room temperature. Molecules 1–8showed similar pattern of absorption spectra (see supportinginformation), characteristic of ��* transition of carbazole, with apeak at ∼300 nm and a marginal blue or red shift as compared to

3

N

ArAr

F

F NO2

60-85%

3)4, K2CO3

2O, 800 C

5 76 8

azole derivatives 1–8.

N. Agarwal et al. / Synthetic Metals 161 (2011) 466–473 469

ppm(t1)7.508.008.50

Fig. 2. 1H NMR (aromatic region) of 9 (above) and 1 (below).

700600500400

0.0

0.2

0.4

0.6

0.8

1.0

Inte

nsit

y (

a.u

.)

1

2

3

4

5

6

7

8

Fo

1ttrtis

7006506005505004504003500.0

0.2

0.4

0.6

0.8

1.0

Inte

nsit

y (

a.u

.)

1

2

3

4

5

6

7

8

TP

Wavelength (nm)

ig. 3. Emission spectra of 1–8 in solid thin films on quartz (Please see web versionf the figure in color).

. The peak at 279 nm in the absorption spectrum of 5 is due tohe pyrene moiety [16]. Steady state fluorescence spectra of 1–8 as

hin films and as solution in acetonitrile are shown in Figs. 3 and 4,espectively. 1–7 emit in the blue region (�em = 385–479 nm in solu-ion) while the molecule 8 emits in orange region (�em at 600 nmn solution). The fluorescence quantum yields of 1–8 were mea-ured in dilute solution (OD ∼ 0.2) in acetonitrile using anthracene

able 1hotophysical and Tg data of 1–8.

Molecule �max absorption (nm) �maxa emission (nm) �max

b

1 294, 349e, 364e 395 315, 32 295, 352e, 367e 407 4083 307, 345 421 430c,4 310, 350 479 4595 279, 345 443 4586 302, 350f 401 4237 286, 342e, 355e 366, 385 366, 38 382 600 552

a In acetonitrile.b Thin solid film on quartz.c Evaporated film.d Spin coated film, nd = not determined.e Molar extinction coefficient <10,000 M−1 cm−1.f Shoulder.

Wavelength (nm)

Fig. 4. Emission spectra of 1–8 in acetonitrile (Please see web version of the figurein color).

as standard (ϕ = 0.27 in ethanol [17]). The p-cyanophenyl deriva-tive, 3 showed the highest quantum yield of 0.96. Solid thin filmdata are important for OLEDs and hence we measured the solidstate emission properties of 1–8. Thin films were made by spincoating on quartz substrate using the solutions of 1–8 in chloroform(25 mg/ml). The emission spectra of 1–8 in thin films (Fig. 3) wereslightly broader than the solution spectra and showed a peak shift ofup to 20 nm except 8, which showed a large blue shift. Fluorescencedecays were measured by TCSPC technique [18]. The fluorescencedecays were multiexponential (see supporting information) andaverage lifetimes were calculated. The optical band gap energyfor the materials was taken to be the intersection of normalizedabsorption/excitation and emission spectra of thin films. All thephotophysical results are summarized in Table 1.

3.4. Electrochemical properties

Cyclic voltammetry was used to study the redox behaviour of1–8 and to determine their HOMO and LUMO energy levels. Car-bazole derivatives, except 5, showed reversible oxidation peak inacetonitrile in cyclic voltammetry (conc. [1–8] ∼1 mM, tetrabutylammonium hexafluorophosphate 0.1 M as supporting electrolyte)

(see supporting information). Peak potential was determinedversus ferrocene as internal standard. 1–8 were not reducible inthe electrochemical range accessible in acetonitrile. Thus, only theHOMO level could be determined from the cyclic voltammetricpeak potential vs. ferrocene [19,20] using EHOMO of ferrocene as

emission (nm) �a (ns) �b (ns) ˚a Tg [◦C]

94 7.30 2.32 0.24 nd7.32 5.80 0.19 61

455d 2.56 1.00 0.96 1532.73 0.38 0.21 1372.93 0.82 0.56 1223.34 0.67 0.39 76

86 3.04 0.25 0.19 nd1.63 0.36 <0.10 110

470 N. Agarwal et al. / Synthetic Metals 161 (2011) 466–473

Table 2Electrochemical data for 1–8.

Compound aEpox (V) bEHOMO (eV) cELUMO (eV) Optical Band gap (eV)

1 0.72 −5.82 −2.48 3.342 0.57 −5.67 −2.50 3.173 0.87 −5.97 −2.93 3.044 0.80 −5.90 −3.00 2.905 0.64 −5.74 −2.80 2.946 0.70 −5.80 −2.65 3.157 0.87 −5.97 −2.55 3.428 0.92 −6.02 −3.55 2.47

−tE−ufo

aHroot

3

oiiIbt∼wwfi

tIi

0

0.05

0.1

3020100

Voltage (V)

Cu

rren

t (m

A)

0

50

100

840

Current Density (mA/m2)

EL

In

ten

sit

y (

a.u

.)

0.01

0.1

1

20100

Voltage (V)

EL

In

ten

sit

y (a

.u.) c

a

b

a Determined from cyclic voltammetry experiments vs. Fc/Fc+ in acetonitrile.b EHOMO = −(Epox (vs. ferrocene) + 5.1) eV.c Obtained from EHOMO and optical band gap.

5.1 eV [21–23]. The LUMO levels of 1–8 were determined by addi-ion of the optical band gap energy to the HOMO level. EHOMO andLUMO of 1–8 fall in the range of −5.67 to −6.02 and −2.48 to3.55 eV, respectively. Therefore, electron injection in EL devicessing these molecules can be made using Ca as cathode whose workunction is −2.9 eV. The band gap energies, HOMO and LUMO levelsf 1–8 are summarized in Table 2.

Theoretical calculations indicated that aryl substituents at 3-nd 6-positions in carbazole provide an effective tool to tune theOMO level of these molecules. This is confirmed by the linear cor-

elation of EHOMO of six molecules with the Hammett constants (�p)f the substituents, which is a measure of its electron withdrawingr donating character. Fig. 5 shows the plot of EHOMO with �p forhe substituents.

.5. Properties of OLEDs using 2–5 as active layers

The molecules 1–8 were designed to have high solubility inrganic solvents to enable fabrication of OLED devices by spin coat-ng. Thus, it was decided to fabricate single layer, spin coated OLED,n which the carbazole derivative was sandwiched between ITO orTO/PEDOT:PSS as the anode and Ca cathode which was protectedy a layer of thick Al. Single layer OLEDs were fabricated by solu-ion processing using molecules 2–5 as the active layer of thickness1000 A. Molecules 2–5 showed blue EL (see below). Molecule 8as not suitable because of low quantum yield. Other moleculesere not successful presumably due to poor quality of spin coatedlms.

Fig. 6a shows the current–voltage characteristics of one ofhese molecules in the following configuration: ITO/4/Ca/Al andTO/PEDOT:PSS/4/Ca/Al. We see from Fig. 6a and c that spin coat-ng PEDOT:PSS on the ITO anode significantly reduces the turn-on

-0.4 -0.2 0.0 0.2 0.4 0.6 0.8

-6.1

-6.0

-5.9

-5.8

-5.7

-5.6

Hammett constant

EH

OM

O (

eV

)

Fig. 5. Correlation of EHOMO with Hammett constant.

Fig. 6. (a) Current as a function of applied voltage, (b) EL intensity as a func-tion of current density and (c) EL intensity as a function of voltage for the device,ITO/PEDOT:PSS/4/Ca/Al (�) and ITO/4/Ca/Al (�).

voltage for the device. Fig. 6b shows the effect of PEDOT:PSS onthe EL light output as a function of current density. The signifi-cantly improved slope for the EL-current density in the presenceof PEDOT:PSS is due to the improved hole injection efficiency.PEDOT:PSS has been widely used in polymer devices and onlyvery recently for small molecule based devices as an efficient holeinjection layer [24]. Use of PEDOT:PSS obviates the need for aseparate hole injection layer simplifying the device architecture,especially in single layer devices for testing new molecules. Allother molecules (2, 3 and 5) reported here also showed low holeinjection barrier when contacted with a PEDOT:PSS layer. EL spectraof the devices matched well with the PL spectra of 2–5 in solid state,as shown in Fig. 7. Table 3 summarizes the electroluminescenceresults.

While the turn-on voltage for current and EL for the single layer

devices was reduced considerably with PEDOT:PSS, the EL intensity(typically 5–10 cd/m2) and current efficiency of EL did not improvewith the reduction in turn-on voltage. The single layer devices weremade with spin-coated molecules. The performance of thermallyevaporated single layer devices was also studied. It was observed

Table 3Electroluminescence data for 2–5.

Molecule EL peak (nm) Turn-on voltage (V) b� (cd/A) CIE coord (X, Y)

2 420 8.5 0.062 0.22, 0.213 460 a5.0 0.091 0.17, 0.174 450 a6.5 0.059 0.20, 0.245 460 9.0 0.112 0.18, 0.21

a PEDOT:PSS was used.b Efficiency of EL.

N. Agarwal et al. / Synthetic Metals 161 (2011) 466–473 471

7006005004000

1

2In

ten

sit

y

Wavelength (nm)

7006005004000

1

3

Inte

nsit

y

Wavelength (nm)

7006005004000

1

4

Inte

nsit

y

Wavelength (nm)

7006005004000

1

5

Inte

nsit

y

Wavelength (nm)

F . (Fort

tmEtrdnetdm

ig. 7. Solid state PL (black line) and EL spectra (red line with filled squares) of 2–5o the web version of the article.)

hat the current turn-on voltage was smaller in devices with ther-ally evaporated films (see Supplementary Information) but the

L intensity still remained very poor. The poor performance ofhe single layer devices may be understood as follows. The cur-ent injection into the single layer device is unbalanced due to theominance of one carrier, namely, the hole in this case. Thus, it isecessary to design a device structure using additional organic lay-

rs such that the current in the device is balanced in which casehe probability of electron–hole recombination is maximum. Weescribe below such a multilayer OLED for which the EL perfor-ance parameters were excellent.

-5.40

-2.20

TPD

ITO

-6.0

-2.9

CBP

-5.24

-8.34

F 4TCNQ

Fig. 8. Energy level diagram of OLEDs:ITO/

interpretation of the references to color in this figure legend, the reader is referred

3.6. Multilayer blue OLEDs using 3 and 4 as blue emitters

3.6.1. Multilayer OLED of the following structure was fabricatedusing 3 and 4

ITO/F4TCNQ(20 A)/TPD(400 A)/CBP(100 A)/3 or 4 (400 A)/BCP(60 A)/LiF(15 A)/Al. The device was fabricated by vapour depositionof all the molecules in high vacuum. F4TCNQ, TPD, CBP and BCP are

standard organic layers used in multi layer OLEDs. The functionof each layer in the multilayer OLED may be understood from theenergy levels of the molecules which are shown in Fig. 8. F4TCNQfacilitates hole injection into TPD, which is the hole transporter.

-5.97 (3) -5.90 (4)

-6.50

-2.93 (3) -3.00 (4)

LiF/Al

3 or 4

BCP

-3.0

F4TCNQ/TPD/CBP/(3 or 4)/BCP/LiF/Al.

472 N. Agarwal et al. / Synthetic Metals 161 (2011) 466–473

Table 4Electroluminescence data for multilayer OLEDs for molecules 3 and 4.

Molecule EL peak (nm) Turn-on Voltage (V) � (cd/A) Brightness (cd/m2) CIE coord (X, Y)

3 (with CBP) 430 3.0 1.1 628 (@10 mA) 0.17, 0.083 (without CBP) 440 3.0 1.1 628 (@10 mA) 0.18, 0.134 (with CBP) 450 2.8 0.77 882 (@5 mA) 0.20, 0.224 (without CBP) 460 2.8 0.77 1088 (@5 mA) 0.19, 0.23

0

1

2

3

4

5

1050-5

Voltage (V)

Cu

rren

t (m

A)

0

500

1000

1050-5

Voltage (V)

Inte

nsit

y (

cd

/m2)

4

4

3

3

FO

BCf

ttwsPclw

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

864

200

400

600

800

1000

cd

/AVoltage (V)

cd

/m2

ig. 9. Current and EL intensity as a function of applied voltage for the multilayerLED: ITO/F4TCNQ/TPD/CBP/(3 or 4)/BCP/LiF/Al.

CP serves as an electron injection and hole blocking layer. A thinBP layer between TPD and 3 or 4 helps in suppressing exciplex

ormation at the TPD/3(4) interface.The current and EL intensity as a function of applied voltage for

he multilayer OLED using 3 and 4 are shown in Fig. 9. The EL spec-ra of the blue OLED are shown in Fig. 10. The EL spectra matchedith the PL spectra of molecules 3 and 4. It was noted that the EL

pectrum of 3 showed a peak at 430 nm which matched with the

L of evaporated film of 3 and not that of the spin coated film. Thisonfirms that electron–hole recombination occurs in the carbazoleayer resulting in blue electroluminescence. The turn-on voltage

as also low at 3.0 V for 3 and 2.8 V for 4. The performance of the

0.0

0.5

1.0

700500300

Wavelength (nm)

EL

in

ten

sit

y (

a.u

.)

4

3

Fig. 10. EL spectra of multilayer OLEDs using 3 and 4 as blue emitters.

Fig. 11. EL intensity (cd/m2) and current efficiency (cd/A) at different applied volt-ages for the device ITO/F4TCNQ/TPD/CBP/4/BCP/LIF/Al.

OLEDs with and without the thin CBP layer was similar. However,the EL spectra of the devices without CBP showed a red shift of10 nm and higher intensity in the long wavelength tail presumablydue to exciplex formation at the interface of TPD/3(4). Thus, thepresence of CBP prevents exciplex formation and improves bluecolor purity.

The brightness of the multilayer OLED device increased withcurrent. Brightness as high as 900–1000 cd/m2 was achieved at acurrent level of 5 mA. However, maximum current efficiency of1.1 cd/A (3) and 0.77 cd/A (4) was achieved only at a lower cur-rent level. Fig. 11 shows the current efficiency and brightness as afunction of applied voltage for the blue OLED using molecule 4. Itmay be noted that the peak efficiency is maintained for a bright-ness level of 100 cd/m2, which is a significant brightness level forpractical applications. EL performance data for the blue OLEDs aresummarized in Table 4.

4. Conclusion

A series of highly soluble, blue and blue green lumi-nescent N-hexylcarbazole derivatives were synthesized usingSuzuki–Miyaura coupling. The HOMO levels of these carbazolederivatives with different substituents varied considerably. Thephotophysical and electrochemical properties were studied. Wehave demonstrated blue EL in spin coated, single layer devices usinga few of these molecules as active layer. It is also demonstrated thatthe turn-on voltage, brightness and efficiency are vastly improvedin multilayer devices in which the carbazole derivative as usedas blue emitter sandwiched between standard hole and electrontransport layers.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.synthmet.2011.01.001.

tic Me

R

[

[

[[[

[

[

[[[

[

[

N. Agarwal et al. / Synthe

eferences

[1] J. Shinar, in: J. Shinar (Ed.), Organic Light-Emitting Devices: A Survey, Springer-Verlag, New York, 2003, pp. 1–34.

[2] Z. Li, H. Meng (Eds.), Organic Light-emitting Materials and Devices, CRC Taylorand Francis, New York, 2007.

[3] R.D. Miller, E.A. Chandross, Chem. Rev. 110 (2010) 1–2.[4] N. Koch, ChemPhysChem 8 (2007) 1438–1455.[5] T.W. Kelley, P.F. Baude, C. Gerlach, D.E. Ender, D. Muyres, M.A. Haase, D.E. Vogel,

S.D. Theiss, Chem. Mater. 16 (2004) 4413–4422.[6] F. Garnier, Acc. Chem. Res. 32 (1999) 209–215.[7] K.R.J. Thomas, J.T. Lin, Y.T. Tao, C. Ko, J. Am. Chem. Soc. 123 (2001) 9404–9411.[8] J. Ding, J. Gao, Y. Cheng, Z. Xie, L. Wang, D. Ma, X. Jing, F. Wang, Adv. Funct.

Mater. 16 (2006) 575–581.[9] J. Zheng, C. Zhan, J. Qin, R. Zhan, Chem. Lett. 31 (2002) 1222–1223.10] Z.-B. Zhang, M. Fujiki, H.-Z. Tang, M. Motonaga, K. Torimitsu, Macromolecules

35 (2002) 1988–1990.11] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheese-

man, J.A. Montgomery, Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S.Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A.

Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P.Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Strat-mann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K.Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich,A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari,

[

[

[

tals 161 (2011) 466–473 473

J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Ste-fanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith,M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. John-son, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian Inc., Wallingford,CT, 2004.

12] C.J. Cramer, Essentials of Computational Chemistry, Wiley, 2002.13] Y. Li, J. Ding, M. Day, Y. Tao, J.P. Lu, M. Diorio, Chem. Mater. 16 (2004) 2165–2173.14] N. Miyaura, in: A. De Meijere, F. Diederich (Eds.), Metal-catalyzed Cross-

Coupling Reactions, Wiley-VCH, Weinheim, 2004.15] P.K. Nayak, N. Agarwal, N. Periasamy, M.P. Patankar, K.L. Narasimhan, Synth.

Met. 160 (2010) 722–727.16] I.B. Berlman, Hand Book of Fluorescence Spectra of Aromatic Molecules, Aca-

demic Press, New York, 1966.17] W.R. Dawson, M.W. Windsow, J. Phys. Chem. 72 (1968) 3251–3260.18] Ira, A.S.R. Koti, G. Krishnamoorthy, N. Periasamy, J. Fluoresc. 13 (2003) 95–103.19] A. Mishra, P.K. Nayak, D. Ray, M.P. Patankar, K.L. Narasimhan, N. Periasamy,

Tetarhedron Lett. 47 (2006) 4715–4719.20] B.W.D. Andrade, S. Datta, S.R. Forrest, P. Djurovich, E. Polikarpov, M.E. Thomp-

son, Org. Electron. 6 (2005) 11–20.21] I. Noviandri, K.N. Brown, D.S. Fleming, P.T. Gulyas, P.A. Lay, A.F. Masters, L.

Phillips, J. Phys. Chem. 103 (1999) 6713–6722.22] P. Zacharias, M.C. Gather, M. Rojahn, O. Nuyken, K. Meerholz, Angew. Chem.

Int. Ed. 46 (2007) 4388–4392.23] T. Johansson, W. Mammo, M. Svensson, M.R. Andersson, O. Inganäs, J. Mater.

Chem. 13 (2003) 1316–1323.24] S.C. Tse, S.W. Tsang, S.K. So, J. Appl. Phys. 100 (2006) 6370.