Twin derivatives of fluorophenyl, difluorophenyl or trifluorophenyl substituted carbazoles as electroactive amorphous materials

Synthetic Metals 203 (2015) 122–126

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Twin derivatives of fluorophenyl, difluorophenyl or trifluorophenyl substituted carbazoles as electroactive amorphous materials G. Krucaite a , D. Tavgeniene a , D. Volyniuk a , J.V. Grazulevicius a , L. Liu b , Z. Xie b , B. Zhang b, * , S. Grigalevicius a, * a b

Department of Polymer Chemistry and Technology, Kaunas University of Technology, Radvilenu plentas 19, LT50254, Kaunas, Lithuania State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 26 September 2014 Received in revised form 29 January 2015 Accepted 17 February 2015 Available online xxx

Twin derivatives containing two 4-fluorophenyl, 3,5-difluorophenyl or 2,4,6-trifluorophenyl substituted carbazole rings were synthesized by the multi-step synthetic routes. Thermal properties and hole injecting/transport properties of the materials were examined by thermogravimetric analysis, differential scanning calorimetry, electron photoemission and time of flight techniques, respectively. Hole-transporting properties of thin amorphous layers of the derivatives were also tested in the structures of organic light emitting diodes (OLEDs) with Alq3 as the green emitter. The bilayer device based on a twin derivative composed of 3-(3,5-difluorophenyl) carbazol-9-yl blocks exhibited the best overall performance with a maximal photometric efficiency of about 2.1 cd/A and maximum brightness of 560 cd/m2. An analogous multilayer OLED with an additional PEDOT:PSS hole injecting layer demonstrated turn on voltage of 6.0 V, maximum brightness of about 6200 cd/m2 and maximal photometric efficiency of 3.4 cd/A. ã 2015 Elsevier B.V. All rights reserved.

Keywords: Twin derivative Carbazole Amorphous material Ionization potential Hole drift mobility Light emitting diode

1. Introduction Efficient organic light emitting diodes (OLEDs) are obtained only by building multilayer structures. The main approach that has been employed to improve efficiency of the devices is the formation of an additional hole transporting layer in the multilayer devices [1–6]. Carbazole-based derivatives are among the most studied hole transporting materials for various devices of organic electronics due to their good hole-transporting properties and high thermal stability [7,8]. Some carbazole containing materials have even been commercialized for a number of devices and processes such as electrophotographic photoreceptors of photocopying machines and laser printers [9]. We have earlier synthesized series of twin derivatives containing aryl or arylamino substituted carbazole rings and tested them as materials for hole transporting layers (HTL) [10,11]. It was observed that the compounds demonstrate good amorphous film forming properties and better charge transporting properties than that of derivatives containing unsubstituted carbazole rings.

* Corresponding authors. Tel.: +370 67946213. E-mail addresses: [email protected] (B. Zhang), [email protected] (S. Grigalevicius). http://dx.doi.org/10.1016/j.synthmet.2015.02.027 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.

In this work, we have designed and synthesized new twin derivatives containing fluorophenyl, difluorophenyl or trifluorophenyl substituted carbazole rings, which were expected to show different electron donating properties, and could be suitable as HTL materials for multilayer OLEDs. 2. Experimental 2.1. Instrumentation 1

H NMR spectra were recorded using a Varian Unity Inova (300 MHz) apparatus. Mass spectra of the low-molar-mass derivatives were obtained on a Waters ZQ 2000 spectrometer. Differential scanning calorimetry (DSC) measurements were carried out using a Bruker Reflex II thermosystem. Thermogravimetric analysis (TGA) was performed on a TGAQ50 aparatus. The TGA and DSC curves were recorded in a nitrogen atmosphere at a heating rate of 10
C/min. The ionization potentials of the layers of the compounds synthesized were measured by the electron photoemission method. The measurement method was, in principle, similar to that demonstrated by Miyamoto et al. [12]. The samples for the ionization potential measurements were prepared as it was described earlier [13].

G. Krucaite et al. / Synthetic Metals 203 (2015) 122–126

The charge carrier mobility (m) measurements were carried by the time of flight method (TOF) [14]. Samples of the form ITO/(4, 5 or 6)/Al were fabricated for the measurements. Commercially available indium tin oxide (ITO) coated glass with a sheet resistance of 70–100 V/sq was used as a substrate. The aluminum (Al) layer was used as a top cathode. The layers were vacuum deposited onto pre-cleaned ITO-coated glass substrates under the vacuum of 1–5  106 mBar using vacuum evaporation equipment. The charge carriers were generated at the layer surface by illumination with pulses of Nd:YAG laser (pulse duration was 25 ps, wavelength 355 nm). The transit time was determined from the kink point in the transient photocurrent curves. The transit time tt with the applied bias (V) indicates the passage of holes through the entire thickness of the cell (d) and enables determination of the hole mobility as m= d2/U  tt. The experimental setup consists of a delay generator Stanford Research DG 535 and a digital storage oscilloscope Tektronix TDS754C. The electroluminescent devices were fabricated on glass substrates containing a bottom indium tin oxide (ITO) anode (125 nm). Before use for the device fabrication, the ITO-coated substrates were carefully cleaned and treated with UV/ozone right before deposition of the organic layers. The hole-transporting layers (HTL) of 40 nm thickness were prepared by spin-coating from chloroform solutions (5 mg/ml) of the derivatives 4–6. Poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) layers (40 nm) were also deposited by spin-coating and heated at 120
C for 30 min. The layers were used in a control device and for optimized multilayer OLEDs. Tris(quinolin-8-olato) aluminium (Alq3) was used as green light emitter as well as electron transporting layer. Evaporation of Alq3 (80 nm) and of LiF (1 nm)/Al (100 nm) cathode was done at a pressure of 4  104 Pa in vacuum evaporation equipment. The final structure of the devices was ITO(125 nm)/HTL(40 nm)/Alq3(80 nm)/LiF(1 nm)/Al(100 nm). An optimized device ITO (125 nm)/PEDOT:PSS (40 nm)/material 5 (15 nm)/Alq3(80 nm)/LiF(1 nm)/Al(100 nm) containing an additional hole injecting PEDOT:PSS layer was also formed and tested. The luminance of the fabricated OLEDs was measured using a Minolta CS-100 luminance-meter. A Keithley 2400 electrometer was used to measure the current–voltage characteristics of the

123

devices. All the measurements were performed at ambient conditions in air. 2.2. Materials 9H-carbazole (1), 4-fluorophenyl boronic acid, 3,5-difluorophenyl boronic acid, 2,4,6-trifluorophenyl boronic acid, 1,6-dibromohexane, tetra-N-butylammonium hydrogen sulfate (TBAHS), bis(triphenylphosphine) palladium(II) dichloride (Pd (PPh3)2Cl2), Alq3 and potassium hydroxide were purchased from Aldrich and used as received. 3-Iodo-9H-carbazole (2) was obtained by a procedure of Tucker [15]. 1,6-Bis(3-iodocarbazol-9-yl) hexane (3) was prepared as described earlier [16]. 1,6-Bis[3-(4-fluorophenyl) carbazol-9-yl]hexane (4). 1.9 g (2.8 mmol) of 1,6-bis(3-iodocarbazol-9-yl) hexane (3), 1 g (7.2 mmol) of 4-fluorophenyl boronic acid, 0.08 g (0.11 mmol) of PdCl2(PPh3)2 and 0.8 g (14.2 mmol) of powdered potassium hydroxide were stirred in 15 ml of THF containing degassed water (1.5 ml) at 80
C under nitrogen for 24 h. After TLC control the reaction mixture was cooled and quenched by the addition of ice water. The product was extracted by ethyl acetate. The combined extract was dried over anhydrous Na2SO4. The crude product was purified by silica gel column chromatography using the mixture of ethyl acetate and hexane (vol. ratio 1:10) as an eluent. Yield: 0.74 g of white crystals. M.p.: 152
C (DSC). MS (APCI+, 20 V): 605.2 ([M + H], 100%). 1H NMR (300 MHz, CDCl3, d, ppm): 8.22 (d, 2H, J = 1.5 Hz, Ar), 8.11 (d, 2H, J = 7.8 Hz, Ar), 7.65-7.53 (m, 6H, Ar), 7.47-7.39 (m, 2H, Ar), 7.32 (d, 2H, J = 8.4 Hz, Ar), 7.26-7.07 (m, 8H, Ar), 4.25 (t, 4H, J = 7.35 Hz, NCH2(CH2)4CH2N), 1.90-1.79 (m, 4H, NCH2CH2(CH2)2CH2CH2N), 1.43-1.33 (m, 4H, NCH2CH2(CH2)2CH2CH2N). 1,6-Bis[3-(3,5-difluorophenyl) carbazol-9-yl]hexane (5). 1 g (1.5 mmol) of 1,6-bis(3-iodocarbazol-9-yl) hexane (3), 0.7 g (7.4 mmol) of 4-fluorophenyl boronic acid, 0.04 g (0.056 mmol) of PdCl2(PPh3)2 and 0.4 g (7.4 mmol) of powdered potassium hydroxide were stirred in 15 ml of THF containing degassed water (1.5 ml) at 80
C under nitrogen for 24 h. After TLC control the reaction mixture was cooled and quenched by the addition of ice

I H N

H N

KI, KIO3 1

2

Br(CH2 )6Br N

N

KOH I

3 I Pd(PPh3 )2Cl2

4

KOH

F

*

OH Ar B OH

Ar

F

Ar :

5

*

F 6

F

F

*

N

N 4-6

Ar

F Scheme 1. Synthesis scheme of the materials 4–6.

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The synthesis of twin derivatives (4–6) containing 4-fluorophenyl, 3,5-difluorophenyl or 2,4,6-trifluorophenyl substituted carbazole rings was carried out by a multi-step synthetic route shown in Scheme 1. The key material, i.e., 3-iodo-9H-carbazole (2) was synthesized from commercially available 9H-carbazole by Tucker iodination reaction by described in literature procedure [15]. Twin derivative 3 containing two 3-iodocarbazol-9-yl fragments was prepared by the reaction of 1,6-dibromohexane with an excess of the 3-iododerivative 2 under basic conditions as we described earlier [16]. The objective electro-active derivatives (4– 6) were prepared by Suzuki reaction of the 1,6-bis(3-iodocarbazol9-yl) hexane (3) with an excess of 4-fluorophenyl boronic acid, 3,5difluorophenyl boronic acid or 2,4,6-trifluorophenyl boronic acid, correspondingly. The synthesized compounds were identified by mass spectrometry and 1H NMR spectroscopy. The data were found to be in good agreement with the proposed structures. The materials are soluble in common organic solvents, i.e., tetrahydrofuran (THF), chloroform, ethyl acetate, chlorobenzene. Transparent thin films of compounds 4–6 can be prepared from their solutions by spin coating method. The behavior under heating of the synthesized materials 4–6 was studied by DSC and TGA under a nitrogen atmosphere. The results are presented in Table 1. It was observed during the TGA analysis that thermal resistance of these derivatives depend strongly on their chemical composition. For example, material 4 Table 1 Thermal characteristics of the objective materials 4–6. Material

Tm (
C)

Tg (
C)

Td (
C)

4 5 6

152 86 178

54 48 51

395 346 278

st

1 heating

o

T g= 51 C nd

2 heating

Cooling

0

40

80

120

160

200

o

Temperature ( C) Fig. 1. DSC curves of the material 6. Heating rate: 10
C/min.

containing the lowest amount of fluorine has the highest temperature of 5% mass loss (Td). Its value of Td (395
C) is visibly higher than that of derivative 6 (Td = 278
C) containing the biggest amount of fluorine in its structure. The thermal transitions under heating of the derivatives 4–6 was studied by DSC. Melting temperatures (Tm) and glass transition temperatures (Tg) of the compounds are presented in Table 1. It was observed that all the objective derivatives were obtained as crystalline materials after synthesis; however they could form also stable amorphous materials upon cooling of the melt. The DSC thermograms of the material 6 are shown in Fig. 1 as an example. When the crystalline sample 6 was heated during the DSC test, the endothermic peak due to melting was observed at 178
C. When the melt sample was cooled down and heated again, the glasstransition was observed at 51
C and on further heating no peaks due to crystallization and melting appeared. The crystalline samples of compounds 4 and 5 demonstrated similar behavior as material 6. They melted on first heating at 152
C (4) and at 86
C (4), respectively, and formed amorphous state during cooling. The amorphous samples demonstrated only glass transitions upon second heating at 54
C (4) and at 48 (5)
C, respectively. The investigations confirmed that the crystalline derivatives 4–6 can be converted into amorphous materials and used for the preparation of thin amorphous layers on substrates. It can be also observed that values of Tg of the materials do not depend considerably on chemical composition of the derivatives and are in the narrow range from 48
C to 54
C. Ionization potentials (Ip) of layers of the materials synthesized were measured by the electron photoemission method. The

6 Ip=5.92 eV 4 Ip=5.70 eV 5 Ip=5.75 eV

Photocurrent (a. u.)

3. Results and discussion

o

T m = 178 C

< Exo - Endo >

water. The product was extracted by ethyl acetate. The combined extract was dried over anhydrous Na2SO4. The crude product was purified by silica gel column chromatography using the mixture of ethyl acetate and hexane (vol. ratio 1:8) as an eluent. Yield: 0.36 g of white crystals. M.p.: 86
C (DSC). MS (APCI+, 20 V): 641.2 ([M + H], 100%). 1H NMR (300 MHz, CDCl3, d, ppm): 8.25 (d, 2H, J = 1.5 Hz, Ar), 8.12 (d, 2H, J = 7.8 Hz, Ar), 7.58(dd, 2H, J1 = 1.8 Hz, J2 = 8.4 Hz, Ar), 7.49-7.42 (m, 2H, Ar), 7.33 (m, 2H, J = 9 Hz Ar), 7.28-7.15 (m, 8H, Ar), 6.80-6.72 (m, 2H, Ar), 4.26 (t, 4H, J = 7.2 Hz, NCH2(CH2)4CH2N), 1.90-1.80 (m, 4H, NCH2CH2(CH2)2CH2CH2N), 1.42-1.34 (m, 4H, NCH2CH2(CH2)2CH2CH2N). 1,6-Bis[3-(2,4,6-trifluorophenyl) carbazol-9-yl]hexane (6). 1.4 g (2.1 mmol) of 1,6-bis(3-iodocarbazol-9-yl) hexane (3), 1 g (5.3 mmol) of 4-fluorophenyl boronic acid, 0.1 g (0.14 mmol) of PdCl2(PPh3)2 and 1.1 g (14.1 mmol) of powdered potassium hydroxide were stirred in 15 ml of THF containing degassed water (1.5 ml) at 80
C under nitrogen for 24 h. After TLC control the reaction mixture was cooled and quenched by the addition of ice water. The product was extracted by ethyl acetate. The combined extract was dried over anhydrous Na2SO4. The crude product was purified by silica gel column chromatography using the mixture of ethyl acetate and hexane (vol. ratio 1:20) as an eluent. Yield: 0.44 g of brown crystals. M.p.: 178
C (DSC). MS (APCI+, 20 V): 677.2 ([M + H], 100%). 1H NMR (300 MHz, CDCl3, d, ppm): 8.32(dd, 2H, J1 = 0.6 Hz, J2 = 1.8 Hz, Ar), 8.02 (m, 2H, J = 7.5 Hz, Ar), 7.65 (dd, 2H, J1 = 1.2 Hz, J2 = 8.4 Hz, Ar), 7.51-7.17 (m, 10H, Ar), 7.07 (d, 2H, J = 8.7 Hz, Ar), 4.19 (t, 4H, J = 6.9 Hz, NCH2(CH2)4CH2N), 1.84-1.74 (m, 4H, NCH2CH2(CH2)2CH2CH2N), 1.35-1.29 (m, 4H, NCH2CH2(CH2)2CH2CH2N).

5,6

5,8

6,0

6,2

6,4

Photon energy (eV) Fig. 2. Electron photoemission spectra of the layers prepared using materials 4–6.

10

-3

10

-4

10

-5

10

-6

10

-7

125

ionization potentials of the materials as well as with characteristics of OLED devices using these materials. To investigate the hole-transporting properties of the materials 4–6, OLED devices were fabricated with a device configuration of ITO/HTL/Alq3/LiF/Al. In this device configuration 4–6 were used as HTL materials, Alq3 was used both as a light emitting and electron transporting material, ITO and LiF/Al were used as an anode and cathode, respectively. A reference device of the structure ITO/PEDOT:PSS/Alq3/LiF:Al was also fabricated for comparison. All the devices emitted bright green luminescence (lmax = 512–

4 5 6

120

0

200

400 1/2

E

600 1/2

800

1000

4 5 6 PEDOT:PSS

100

2

-2

Current Density (mA/cm )

10

2

-1 -1

Mobility (cm V s )

G. Krucaite et al. / Synthetic Metals 203 (2015) 122–126

-1/2

(V cm )

Fig. 3. Electric field dependencies of mh in charge transport layers of the materials 4–6.

EL Intensity (a. u.)

60 40 20 0 0

3

6

9

12

15

18

21

24

15

18

21

24

Voltage (V) 600

4 5 6 PEDOT:PSS

2

Luminance (cd/m )

500 400 300 200 100 0 0

3

6

9

12

Voltage (V) 2,5

Luminous Efficiency (cd/A)

photoemission spectra of thin amorphous layers of the compounds 4–6 as well as values of Ip for the layers are presented in Fig. 2. It could be seen that Ip’s of the newly synthesized compounds depend on the nature of substituents attached to carbazole core, i.e., number of fluorine atoms at phenyl ring. 4-Fluorophenyl substituted derivative 4 demonstrated the lowest Ip of 5.7 eV. The layers of 2,4,6-trifluorophenyl substituted derivative 6 showed the highest Ip of 5.92 eV, which is similar with that of derivatives having electronically isolated carbazole rings (Ip  5.9 eV) [17,18]. These results demonstrate that hole transporting properties of thin layers of 4–6 should also depend on chemical composition of the materials. Amorphous films of 4 or 5 should demonstrate better hole injecting and transporting properties in optoelectronic devices than that of material 6. Time of flight measurements (TOF) were used to characterize the magnitudes of charge drift mobility in thin layers of the synthesized materials 4–6. It was observed from the measurements that positive charges (holes) are transported in the thin films. Electric field dependencies of the hole drift mobility (mh) for the layers are shown in Fig. 3. The mh of the materials 4–6 range from 4  106  103 to 1.1 103 cm2/(V s) at high electric field at 25
C. It is obvious that charge transporting properties depend on chemical structures of the electroactive materials. The highest mh, exceeding 103 cm2/(V s) at high electric field, was observed for the layers of the derivative 5 containing 3-(3,5-difluorophenyl) carbazolyl fragment. Charge mobility observed for the layers of the material 6 containing 3-(2,4,6-trifluorophenyl) carbazolyl fragments are the lowest and reach 4  106 cm2/Vs at high electric fields. It is evident that these results correlate with

80

4 5 6 PEDOT:PSS

2,0

1,5

1,0

0,5

0,0

0

400

500

600

700

Wavelength (nm) Fig. 4. EL spectrum of the device: ITO/4/Alq3/LiF/Al.

800

20

40

60

80

100

2

Current Density (mA/cm ) Fig. 5. OLED characteristics of the devices with the configuration: ITO/4, 5, 6 or PEDOT:PSS/ Alq3/LiF/Al.

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520 nm) in agreement with the photoluminescent spectrum of Alq3. The electroluminescence (EL) spectrum of the device containing HTL of material 4 is shown in Fig. 4 as an example. The results clearly show that, in all the devices, charge recombination occurred in the Alq3 layer. No emission at the longer wavelength from the exciplex species formed at the interface of HTL or electron transporting layer were observed from the OLED devices, which generally occurs in devices containing planar molecules as the HTL. Fig. 5 shows current density- voltage and luminance – voltage characteristics as well as the luminous efficiencies for the OLEDs containing HTL of 4–6. It was observed that HTL of these derivatives demonstrate very different hole transporting properties in the devices due to their different electron donating characteristics. The OLED devices in general exhibit turn-on voltages of 7.5–10.5 V, a photometric efficiency of 0.13–2.27 cd/A, and a maximum brightness of about 30–560 cd/m2 (at 18–21 V). Among these OLEDs, the device containing hole transporting material 5 exhibits the best overall performance, i.e., maximum brightness of 560 cd/m2 and maximal photometric efficiency of about 2.1 cd/A. The characteristics observed for this device are notably superior to those of reference device containing widely used PEDOT:PSS as HTL. The charge injection/transporting properties of material 4 could also be compared with those of similar derivate 1,6-bis(3-phenylcarbazol-9-yl) hexane, which was described by us earlier [11]. It could be observed that one fluorine atom at phenyl ring has not an evident influence on charge injection/transporting characteristics of the similar derivatives. They demonstrate very close Ip values and very similar properties in the analogous OLED devices. Electron photoemission spectroscopy demonstrated that layers of the materials 4–6 still have rather high values of Ip and large barrier for hole injection from ITO anode into the layers of these material. Due to the reason derivative 5, which demonstrated the most promising OLED characteristics, was further studied in the device ITO/PEDOT:PSS/5/Alq3/(LiF/Al) containing an additional hole injecting layer of PEDOT:PSS. The slightly optimized device demonstrated turn on voltage of 6.0 V, maximum brightness of about 6200 cd/m2 and maximal photometric efficiency of 3.4 cd/A. The device characteristics observed for this devices are superior to those of related multilayer systems containing widely used bis (naphthalen-1-yl)-N,N0 -bis(phenyl) benzidine (NPB) as HTL and the Alq3 emitter as reported earlier [19]. It should be pointed out that these characteristics were obtained in test devices under ordinary laboratory conditions. The device performance may be further improved by an optimization of the layer thicknesses and processing conditions [20]. In conclusion, twin derivatives containing two 4-fluorophenyl, 3,5-difluorophenyl or 2,4,6-trifluorophenyl substituted carbazole rings were synthesized and characterized as hole-transporting

layer materials for organic light emitting devices. The derivatives form homogeneous amorphous layers with glass transition temperatures of 48–54
C. The ionization potential values of amorphous layers of the derivatives and the hole drift mobility studies show that these compounds are potential materials for the preparation of hole transporting layers for multilayer optoelectronic devices. The compounds were tested as hole-transporting materials in organic light emitting diodes using Alq3 as the green emitter and electron transporting layer. The device based on a twin derivative containing 3-(3,5-difluorophenyl) carbazolyl moieties exhibited the best overall performance with a maximal photometric efficiency of 2.1 cd/A and maximum brightness of about 560 cd/ m2. The analogous multilayer device containing an additional PEDOT:PSS hole injecting layer demonstrated turn on voltage of 6.0 V, maximum brightness of about 6200 cd/m2 and maximal photometric efficiency of 3.4 cd/A. Acknowledgement This research was funded by a grant No. MIP-024/2013 from the Research Council of Lithuania. References [1] K. Mullen, U. Scherf (Eds.), Organic Light Emitting Devices – Synthesis, Properties and Applications, Wiley-VCH, Weinheim, 2005. [2] M.S. AlSalhi, J. Alam, L.A. Dass, M. Raja, Int. J. Mol. Sci. 12 (2011) 2036. [3] J. Kalinowski, Opt. Mater. 30 (2008) 792. [4] L.K. Alexander, F.P. Igor, J.S. Peter, Chem. Soc. Rev. 39 (2010) 2695. [5] U. Mitschke, P. Bauerle, J. Mater. Chem. 10 (2000) 913. [6] M. Baoxiu, H. Wang, Z.Q. Gao, X.P. Wang, R.F. Chen, W. Huang, Prog. Chem. 23 (2011) 136. [7] J.-F. Morin, D. Ades, A. Siove, M. Leclerc, Macromol. Rapid Commun. 26 (2005) 761. [8] V. Vaitkeviciene, A. Kruzinauskiene, S. Grigalevicius, J.V. Grazulevicius, R. Rutkaite, V. Jankauskas, Synth. Met. 158 (2008) 383. [9] J.V. Grazulevicius, Polym. Adv. Technol. 17 (2006) 694. [10] S. Grigalevicius, G. Blazys, J. Ostrauskaite, J.V. Grazulevicius, V. Gaidelis, V. Jankauskas, J. Photochem. Photobiol. A: Chem. 154 (2003) 161. [11] R. Griniene, G. Krucaite, J.V. Grazulevicius, L. Liu, Z.Y. Xie, B. Zhang, S. Grigalevicius, Opt. Mater. 35 (2013) 553. [12] E. Miyamoto, Y. Yamaguchi, M. Yokoyama, Electrophotography 28 (1989) 364. [13] A. Balionyte, E. Lideikis, S. Grigalevicius, J. Ostrauskaite, E. Burbulis, V. Jankauskas, E. Montrimas, J.V. Grazulevicius, J. Photochem. Photobiol. A: Chem. 162 (2004) 187. [14] C.A. Amorim, M.R. Cavallari, G. Santos, F.J. Fonseca, A.M. Andrade, S. Mergulhao, J. Non-Cryst. Solids 358 (2012) 484. [15] S.H. Tucker, J. Chem. Soc. 1 (1926) 548. [16] S. Grigalevicius, L. Ma, G. Qian, Z. Xie, M. Forster, U. Scherf, Macromol. Chem. Phys. 208 (2007) 349. [17] P. Strohriegl, J.V. Grazulevicius, J. Pielichowski, K. Pielichowski, Prog. Polym. Sci. 28 (2003) 1297. [18] G. Blazys, S. Grigalevicius, J.V. Grazulevicius, V. Gaidelis, V. Jankauskas, V. Kampars, J. Photochem. Photobiol. A: Chem. 174 (2005) 1. [19] R. Griniene, J.V. Grazulevicius, K.Y. Tseng, S.H. Peng, J.H. Jou, S. Grigalevicius, Synth. Met. 161 (2011) 2466. [20] J.H. Jou, M.F. Hsu, W.B. Wang, C.P. Liu, Z.C. Wong, J.J. Shyue, C.C. Chiang, Org. Electron. 9 (2008) 291.