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Polarized Blue Emission Based on a Side Chain Liquid Crystalline Polyacrylate Containing

Bis-tolane Side Groups

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2002 Jpn. J. Appl. Phys. 41 1374

(http://iopscience.iop.org/1347-4065/41/3R/1374)

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Polarized Blue Emission Based on a Side Chain Liquid Crystalline Polyacrylate

Containing Bis-tolane Side Groups

Shu-Wen CHANG, An-Kuo LI, Chih-Wei LIAO and Chain-Shu HSU*

Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan 30050, R.O.C.

(Received July 5, 2001; accepted for publication December 12, 2001)

A novel side chain liquid crystalline (LC) polyacrylate (1P) containing highly conjugated bis-tolane side groups wassynthesized and used for polymer light emitting diodes application. Polymer 1P shows a nematic phase at room temperature. Itexhibits a UV absorption peak at 330 nm and emits a blue light at 425 nm. Polyimide (PI), polyaniline (PA) and poly(3,4-diethylenedioxythiophene) (PEDOT) were used as alignment materials for polymer 1P. The photoluminescence dichroicratios of 1P on rubbed PI, PA and PEDOT films are 10.4, 8.3 and 7.3, respectively. The electroluminescence dichroic ratio of1P aligned on a PEDOT film is 6.1. [DOI: 10.1143/JJAP.41.1374]

KEYWORDS: polarized emission, photoluminescence, electroluminescence, bis-tolane liquid crystal, polyacrylate, side chain LCpolymer

1. Introduction

Progress in the field of polymer light emitting diodes(PLEDs) has been rapidly developed since 1990.1) Con-jugated polymers can be used for both single-layer andmulti-layer PLEDs by a spin coating process.2) Lightemitting polymers with anisotropic properties can providelinearly polarized emission, which is needed for liquidcrystal displays (LCDs) backlight applications. This willsolve both the brightness and power consumption problemsof LCDs. Polarized light emission has been reported forstretch aligned conjugated polymers,3) rubbing alignedconjugated polymers,4,5) Langmuir–Blodgett (LB) deposi-tion polymers6,7) and liquid crystalline self organization.8–11)

All four methods have been reviewed by Grell andBradley.12) The method of stretch alignment has severaldrawbacks. Apart from the intricate film transfer process,stretch alignment generally relies on the ability of polymermelts to elongate chain segments between entanglements. Itis extremely difficult to prepare a film thinner than 1�mbased on the stretch alignment method. The rubbing alignedconjugated polymers is a convenient method to obtain analigned polymer film. However, good alignment requireshigh rubbing strength. This will shorten the device lifetimedue to the scratches formed by strong rubbing. LBdeposition of conjugated polymers is a very suitable methodfor the fabrication of aligned polymer thin film. However,very few polymers with amphiphilic molecular structure canbe processed by the LB method. The LB method is alsodifficult to scale up.

Surface alignment method is widely used in the LCDindustry. Liquid crystal molecules are self-organized toachieve monodomain arrangement with the help of align-ment layer, typically a polyimide that has been rubbed by anylon cloth on a rotating dram. Conventional alignmentlayers such as rubbed polyimides are insulators, and thisproperty is very match for their traditional application inLCDs. However, this is unfavorable for PLED application.Grell et al.8) have reported on an alignment layer consistingof polyimide filled with a starburst-type amine, i.e., [4,40,400-tris (1-naphthyl)-N-phenylamino]triphenylamine. This align-ment layer shows hole transport properties and is used to

align poly[9,9-di(2-ethyl)hexylfluorine]. An electrolumines-cence (EL) device with a polarization ratio of 15 at anabsolute brightness of 45 cd/m2 was fabricated via thisapproach.

In our previous reports,13–16) we synthesized a series ofbis-tolane liquid crystals that show wide nematic tempera-ture ranges and low melting points. The liquid crystals showUV absorptions at about 330 nm and PL emission at bluelight region. They are ideal materials for polarized blueemission. In this study, we synthesized a side chain LCpolyacrylate containing bis-tolane side groups. Three align-ment materials, i.e., polyimide (PI), poly(3,4-diethylene-dioxythiophene) (PEDOT) and polyaniline (PA) film wereused to align the LC polyacrylate. The dichroic ratio inabsorption and emission spectra of the aligned LCpolyacrylate were measured. The alignment abilities ofthree alignment materials were compared.

2. Experimental

The monomer 6-{4-[2-{2-ethyl-4-[2-(4-propylphenyl)-1-ethynyl]phenyl}-ethynyl]phenoxy}hexylacrylate (1M) usedin this study was synthesized according to the processreported in the literature.17) To synthesize poly{6-{4-{2-{2-ethyl-4-[2-(4-propylphenyl)-1-ethynyl]phenyl}-1-ethynyl}-phenoxy}hexylacrylate} (1P), monomer 1M and initiator2,20-azobisisobutyronitrile (AIBN) (1wt%) were dissolvedin tetrahydrofuran (THF) (1.00 g/mL). The mixture wasstirred at 60�C for 24 h under N2 atmosphere. The productwas isolated by pouring the reaction mixture into methanoland purified by reprecipitation in methanol three times.Figure 1 presents the molecular structures of monomer 1Mand polymer 1P.

Three alignment materials were treated with differentmethods. Figure 1 presents also the molecular structures ofPI, PA and PEDOT. Polyamic acid solution (AL12G) waspurchased from Industrial Technology Research Institute(ITRI), Hsinchu, Taiwan. The PI film was obtained by spincoating of polyamic acid solution and baking at 100�C for1 h and subsequently at 230�C for 3 h. PA was synthesizedaccording to the literature procedure18) and dissolved in N-methyl-2-pyrolidinone for spin coating. The PA film wasobtained by spin coating of PA solution (5.00mg/mL) andbaked at 100�C for 1 h. PEDOT solution was purchased fromCovion Organic Semiconductors GmbH. The PEDOT film*Author for correspondence.

Jpn. J. Appl. Phys. Vol. 41 (2002) pp. 1374–1378

Part 1, No. 3A, March 2002

#2002 The Japan Society of Applied Physics

1374

was obtained by spin coating of commercial PEDOTsolution and baked at 100�C for 3 h. The alignment layerswere then rubbed unidirectionally with a rubbing machine.The rotating cylinder was covered with a rayon cloth androtated at 1000 rpm. The samples were passed once underthe cylinder at a translating speed of 10.0mm/s. Theimpression rubbing depth on the substrate was approxi-mately 1.0mm. Polymer 1P was spin coated onto thealignment layers and annealed at 50�C for 3 h to inducemonodomain alignment. For the measurements of UV andPL spectra, both alignment layer and monomer or polymerlayer were coated on quartz substrates.

In this study, we select PEDOT as an alignment layer forthe fabrication of PLED device. PLED device which wasprepared by spin coating of PEDOT solution on ITO coatedglass substrates, followed by a rubbing treatment and spincoating of the polymer 1P. Finally, the device was coated

with Al as cathode by thermal evaporation at 8� 10�6 Torrand encapsulated under Ar atmosphere.

3. Results and Discussion

The molecular weight and polydispersity of polymer 1Pare 38000 and 1.18, respectively. The phase transitiontemperatures of monomer 1M and polymer 1P weredetermined by differential scanning calorimetry (DSC) andpolarizing optical microscopy. The transition temperaturesand corresponding enthalpy changes for both monomer 1Mand polymer 1P are listed in Table I. Monomer 1M is aroom temperature nematic liquid crystal. It does notcrystallized even when the temperature was cooled to�50�C. Figure 2 presents the DSC thermograms of polymer1P. On the DSC heating scan, it shows a glass transition at4.8�C and a nematic to isotropic phase transition at 70.2�C.On the cooling scan, the isotropic to nematic phase transitionis revealed at 68.8�C. Polymer 1P also show nematic phaseat room temperature.

The UV-vis spectra of monomer 1M and polymer 1P weremeasured by Shimadzu UV-1601 spectrophotometer. Figure3 shows the UV absorption spectra of monomer 1M andpolymer 1P that were measured in THF and in solid state.Both monomer and polymer present absorption peaks atabout 330 nm, and both solution and film samples show verysimilar absorption spectra. The results demonstrate that theabsorption attributes to the �–�� transition of bis-tolane side

Fig. 1. The structures of monomer 1M, polymer 1P and three alignment

materials.

Table I. Phase transition temperatures of monomer 1M and polymer 1P.

Phase transitions,�C (Corresponding enthalpy changes, kcal/mol)

heating

cooling

1MK < �50�N92:3ð1:45ÞII89:5ð�1:51ÞN < �50�K

1PG4:8N70:2ð0:26ÞI

I68:8ð�0:22ÞN� 3:2G

K: crystalline, N: nematic, I: isotropic, G: glassy�The sample has been cooled to �50�C and shows no crystallizationtransition.

40 60 80

Second Heating

First Cooling

70.2 C

68.8 C-3.2 C

4.8 C

End

o

Exo

Temperature ( C)

100 120-20 0 20

Fig. 2. DSC thermograms of polymer 1P.

Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 3A S.-W. CHANG et al. 1375

groups. This is the reason why both monomer and polymerpresent similar absorption peaks. The absorption edge ofpolymer film is 370.6 nm, from which the �–�� band gapenergy is estimated to be 3.35 eV.

The fluorescence spectra of monomer and polymer weremeasured by Shimadzu RF-5301PC spectrofluorophot-ometer. The excitation wavelength was 335 nm. Figure 4

shows the fluorescence spectra of monomer 1M and polymer1P measured in THF and in solid state. The PL emissions ofmonomer 1M measured in THF and in solid state are at363.2 nm and 425.4 nm, respectively, and polymer 1Ppeaked at 364.8 nm and 425.2 nm, respectively. Notably,the PL emissions of both materials in solution presented ashoulder peak at a longer wavelength. This might be due tothe exciplex of the molecules, which formed in the solution.This figure also indicates that the fluorescence peaks of thefilms appeared at around 425 nm, which belongs to a blueemission. For 1M and 1P, the emission maxima of the filmsare red-shifted compared to the maxima in a solution. Thesephenomena indicate that the mesogenic side groups have the

tendency to aggregate in the solid-state and result in thedecrease of band gap. Figure 5 shows the EL spectrum ofpolymer 1P that was made as a single layer device [indium-tin oxide (ITO)/1P/Al]. The EL spectrum is very similar toits PL spectrum.

Figures 6(a)–6(c) show the polarized UV-vis absorptionand PL emission spectra of polymer 1P, which has beenaligned by three different alignment layers. Table II lists theexperimental results. The UV dichroism measurement wasperformed in the presence of a polarizer inserted between thesample and the light source parallel or perpendicular to therubbing direction. As shown in Fig. 6(a), a significantlylarger absorption ascribed to a �–�� transition can be seen atthe parallel polarized direction for 1P. Rubbing inducedchain alignment of 1P was also detected by a polarized PLmeasurement. The dichroic PL spectra of 1P are shown inFig. 6(a), indicating that the light emitted from the rubbedfilm was preferentially polarized parallel to the rubbingdirection. This result demonstrates that polymer 1P annealedat its nematic phase induces alignment of the bis-tolane sidegroups along the rubbing direction. The dichroic ratiodefined as the parallel to perpendicular PL intensity was 10.4for 1P, which was aligned by PI alignment layer. Table II

listed the dichroic ratio for polymer 1P aligned by threedifferent alignment materials. The alignment ability of threealignment materials for polymer 1P is PI > PA > PEDOT.PI shows the highest alignment ability. The reason could bedue to a more linear and rigid main chain structure of PI.

Finally, polymer 1P was fabricated to form a polarized ELdevice in which PEDOT served as an alignment layer.Figure 7 shows the polarized EL spectra of the aligned

0.0

0.3

0.5

0.8

1.0

Wavelength (nm)

(1M-THF solution) (1M-film) (1P-THF solution) (1P-film)

Abs

orpt

ion

(arb

. uni

ts)

200 250 300 350 400 450 500 550

Fig. 3. UV-vis absorption spectra of monomer 1M and polymer 1P in

THF and in films.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Wavelength (nm)

PL in

tens

ity (

arb.

uni

ts)

(1M-THF solution) (1M-film) (1P-THF solution) (1P-film)

300 350 400 450 500 550 600 650

Fig. 4. Photoluminescence spectra of monomer 1M and polymer 1P inTHF and in films.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

EL

Inte

nsity

(arb

. uni

ts)

Wavelength (nm)

300 350 400 450 500 550 600

Fig. 5. EL emission spectrum of an ITO/1P/Al device.

Table II. The dichroic ratios of polymer 1P aligned by three kinds ofalignment layer.

Dichroic ratio

PI PA PEDOT

UV (Ak=A?) 12.7 9.1 8.4

PL (Ek=E?) 10.4 8.3 7.3

k: rubbing direction parallel to the polarizer, ?: rubbing direction

perpendicular to the polarizer.

1376 Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 3A S.-W. CHANG et al.

device (ITO/aligned PEDOT/1P/Al) that was measuredparallel and perpendicular to the rubbing direction. Thepolarized EL emission maximum occurs at 432 nm, thedichroic ratio is 6.1 and the driving voltage is 10V. Themacroscopic order parameter was calculated from the

electroluminescence spectrum according the followingequation: S ¼ ðIk � I?)/(Ik þ 2I?), where Ik and I? are thevalue of the intensity of the electroluminescence parallel andperpendicular to the rubbing direction, respectively. Theorder parameter of polymer 1P that was coated on PEDOTalignment layer is 0.63, indicating a relatively high degree oforientational order. The current–voltage (J–V) and theluminance–voltage (L–V) curves are shown in Fig. 8. Thethreshold voltage is 3.0V. The power efficiency of thedevice is 1:0� 10�2 lm/W at 9.0 V.

4. Conclusion

A new side-chain LC polyacrylate containing bis-tolaneside group is reported herein. DSC analysis revealed thatboth monomer 1M and polymer 1P show a nematic phase atroom temperature and possesses a wide nematic temperaturerange. The low mesomorphic temperature made it possibleto align the polymer near room temperature. Measuring thedichroic ratios of polymer 1P aligned by different alignmentlayers confirmed that a more rigid and linear main chain hasbetter alignment ability. The PL dichroic ratios of polymer1P that has been aligned on rubbed PI, PA and PEDOT films

0.0

0.2

0.4

0.6

0.8

1.0

EL

Int

ensi

ty (

arb.

uni

ts)

Wavelength (nm)

350 400 450 500 550 600 650

Fig. 7. Polarized EL spectrum of an ITO/rubbed PEDOT/1P/Al device (k:rubbing direction parallel to the polarizer, ?: rubbing direction

perpendicular to the polarizer).

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

PL Intensity (arb. units)A

bsor

ptio

n (a

rb. u

nits

)

Wavelength (nm)

( ) ( ) ( ) ( )

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

PL Intensity (arb. units)A

bsor

ptio

n (a

rb. u

nits

)

Wavelength (nm)

( ) ( ) ( ) ( )

0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

0.8

1.0

PL intensity (arb. units)A

bsor

ptio

n (a

rb. u

nits

)

Wavelength (nm)

( ) ( ) ( ) ( )

(a)

(b)

(c)

250 300 350 400 450 500 550 600

300 350 400 450 500 550 600

300 350 400 450 500 550 600

Fig. 6. Polarized optical spectra of polymer 1P (a) on rubbed PI films (b)

on rubbed PA films (c) on rubbed PEDOT films (k: rubbing direction

parallel to the polarizer, ?: rubbing direction perpendicular to the

polarizer).

-2 0

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15C

urre

nt d

ensi

ty (

mA

/cm

2 )

Lum

inan

ce (

cd/m

2 )

Voltage (V)

(J-V) (L-V)

2 4 6 8 10 12 14 16

Fig. 8. The J–V (�) and L–V (�) curves of an ITO/PEDOT/1P/Al

device.

Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 1, No. 3A S.-W. CHANG et al. 1377

are 10.4, 8.3 and 7.3, respectively. The EL dichroic ratio ofpolymer 1P aligned on the rubbed PEDOT film is 6.1.Polymer 1P which emits a quite good polarized blue lightshows potential application for LCD backlights.

Acknowledgment

The authors are grateful to the National Science Councilof the Republic of China for financial support of this work.

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