Synthesis, photophysical and electroluminescent properties of phenanthroimidazole derivatives with terminal carbazole or pyrene

Synthetic Metals 193 (2014) 94–101

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Synthesis, photophysical and electroluminescent properties of phenanthroimidazole derivatives with terminal carbazole or pyrene Jiang Peng, Kaiqi Ye, Gonghe Zhang, Yong Zhan, Junhui Jia, Pengchong Xue, Ran Lu ∗ State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, PR China

a r t i c l e

i n f o

Article history: Received 20 January 2014 Received in revised form 31 March 2014 Accepted 10 April 2014 Available online 6 May 2014 Keywords: Phenanthroimidazole Carbazole Pyrene Conjugated molecules Photoluminescence Electroluminescence

a b s t r a c t New phenanthroimidazole derivatives PSC, PDC, PCC and PPP bearing terminal carbazole or pyrene unit have been synthesized. They exhibited excellent solubility in common organic solvents. It was found that the obtained phenanthroimidazole derivatives could emit strong fluorescence in cyclohexane, and the fluorescence quantum yields of the phenanthroimidazole derivatives were in the range of 0.49–0.83 using quinine sulfate (0.1 N in H2 SO4 ) as a standard. We fabricated the organic light-emitting diode with the configuration of ITO/NPB (45 nm)/phenanthroimidazole derivatives (15 nm)/TPBI (40 nm)/LiF/Al, and a saturated blue emission with CIEx,y of (0.16, 0.17) was achieved from the device based on PSC. In addition, the devices based on other phenanthroimidazole derivatives could emit bluish green or green light. The turn-on voltage, current efficiency and maximum luminance of the devices based on PSC, PDC, PCC and PPP were 3.4 V, 3.4 V, 3.8 V, 3.4 V and 0.92 cd/A, 1.83 cd/A, 1.28 cd/A, 0.32 cd/A, as well as 1190 cd/m2 , 1930 cd/m2 , 1270 cd/m2 , 1149 cd/m2 , respectively, meaning the phenanthroimidazole derivatives might become candidates as emitting materials employed in OLEDs. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Organic conjugated molecules have attracted much attention due to their unique optoelectronic properties because they have potential applications in many fields [1], such as organic thin film transistors (OTFTs) [2], organic light-emitting diodes (OLEDs) [3], and photovoltaic cells [4]. Till now, lots of conjugated systems, including porphyrins, thiophene, fluorene, and so on, have been prepared and employed in optoelectronic devices. Among the conjugated units, phenanthroimidazole is a typical near-ultraviolet emitter [5] as well as electron injection or hole blocking material [6]. Therefore, some deep-blue emitters based on phenanthroimidazole derivatives exhibiting high performance in OLEDs were synthesized [7]. For example, Wang and co-workers have prepared 4,4 -bis(1-phenylphenanthro[9,10d]imidazol-2-yl)biphenyl, which was used as electron transport layer and fluorescence host material in OLED, and the maximum luminance and turn-on voltage for the corresponding device reached 40 000 cd/m2 and 2.8 V [5]. Lee et al. have synthesized triphenylamino terminal phenanthroimidazole (TPA-TPI), and found the device based on TPA-TPI showed maximum current, power, and external quantum efficiencies were of 2.63 cd/A,

∗ Corresponding author. Tel.: +86 431 88499179; fax: +86 431 88923907. E-mail address: [email protected] (R. Lu). http://dx.doi.org/10.1016/j.synthmet.2014.04.004 0379-6779/© 2014 Elsevier B.V. All rights reserved.

2.53 lm/W and 3.08%, respectively [7a]. On the other hand, as a candidate in OLEDs, pyrene has attracted significant attention because of its pure blue fluorescence and long fluorescence lifetime. Meanwhile, carbazole has high hole-transporting ability arising from the lone pair electrons in nitrogen atom and its derivatives are also usually used as promising blue light-emitting materials [8]. Although blue-emitting materials based on phenanthroimidazole derivatives have been synthesized [5,7], the requirement in practical application could not be fulfilled. In addition, to the best of our knowledge, the reports on the relationship between the performance of phenanthroimidazole derivatives in devices and their molecular structures are rare. Herein, we designed four phenanthroimidazole derivatives with terminal carbazole units linked by different bridges (such as, phenylene for PSC, phenylvinyl for PDC and phenylvinyl carbazolylvinyl for PCC) or pyrene unit linked by phenylvinyl (PPP) (Scheme 1) in order to obtain new blue-emitting materials. It was found that the obtained phenanthroimidazole derivatives were high emissive in cyclohexane. Meanwhile, we fabricated the organic light-emitting diode with the configuration of ITO/NPB (45 nm)/phenanthroimidazole derivatives (15 nm)/TPBI (40 nm)/LiF/Al, in which PSC, PDC, PCC and PPP were used as emitting materials. A saturated blue emission with CIEx,y of (0.16, 0.17) was achieved from the device based on PSC. In addition, the devices based on other phenanthroimidazole derivatives could emit bluish green or green light. The turn-on voltage, current efficiency and maximum luminance of

J. Peng et al. / Synthetic Metals 193 (2014) 94–101

N

HO B OH

Br

N

95

+

N

Pd(PPh3)4, 85OC, 48h

N C8H17

2

1

PSC N

C8H17 N

N

o

N C8H17

3 (O Pd N N

Br

C8H17 N

N

K2CO3, H2O, Toluene, N2

,1

,K c) 2 A

C

10

h 24 C,

PDC

O3

2

C8H17 N N

C8H17 N

N

4

N C8H17

Pd(OAc)2, K2CO3, 110oC, 24h PCC

,K c) 2 A (O Pd

1

N C8H17

,1 3 CO

2

5 o

10

N

2 C,

N

4h

PPP Scheme 1. The synthetic routes for the phenanthroimidazole derivatives.

the devices based on PSC, PDC, PCC and PPP were 3.4 V, 3.4 V, 3.8 V, 3.4 V and 0.92 cd/A, 1.83 cd/A, 1.28 cd/A, 0.32 cd/A, as well as 1190 cd/m2 , 1930 cd/m2 , 1270 cd/m2 , 1149 cd/m2 , respectively. It suggested that the phenanthroimidazole derivatives might become candidates as emitting materials employed in OLEDs. 2. Results and discussion In order to improve the solubility of the phenanthroimidazole derivatives, tert-butylphenyl was introduced into the target molecules. The synthetic routes for the phenanthroimidazole derivatives with terminal carbazole or pyrene unit (PSC, PDC, PCC and PPP) are summarized in Scheme 1. 1-(4-tert-butylphenyl)-2-(4-bromophenyl)-1H-phenanthro[9,10d]imidazole (1) [7a], 9-octyl-9H-carbazol-3-yl-3-boronicacid (2) [9], 9-octyl-3-vinyl-9H-carbazole (3) [10], 9-octyl-3-((E)-2-(9octyl-9H-carbazol-6-yl)vinyl)-6-vinyl-9H-carbazole (4) [10] and 1-vinylpyrene (5) [11] were synthesized according to the procedures reported in the literatures. Compound PSC was prepared from compounds 1 and 2 via Suzuki cross-coupling reaction catalyzed by Pd(PPh3 )4 in a yield of 46%. Compounds 3–5 were easily transformed into PDC, PCC, PPP through Heck reaction

with compound 2 in yields of 70%, 65% and 65%, respectively. The final products were purified by column chromatography and characterized by 1 H NMR, 13 C NMR, FT-IR, and MALDI/TOF mass spectrometry. It was found that the vinyl groups adopted the trans-conformation in the synthesized compounds on account of the absence of the signal at ∼6.56 ppm in 1 H NMR spectra, which was assigned to the proton in the cis-form. In addition, in the FT-IR spectra the appearance of the vibration absorption at ca. 960 cm−1 and the absence of the absorption at ca. 830 cm−1 due to cis-double bond further indicated CH CH were in trans-forms. 2.1. UV–vis absorption and fluorescence emission spectra The UV–vis absorption spectra of PSC, PDC, PCC and PPP in dilute solution (cyclohexane, 4 × 10−6 M) and in the films were given in Fig. 1. The corresponding photophysical data were listed in Table 1. As shown in Fig. 1a, all the compounds exhibited similar absorption bands at ca. 250 nm, which can be assigned to the ␲–␲* translation of the K band of benzene ring [12]. As to compounds PSC, PDC, PCC with terminal carbazole, the bands at ca. 300 nm were ascribed to the ␲–␲* translation of the carbazole moieties. The maximal absorption bands of PSC appeared at 338 nm,

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Fig. 1. Normalized UV–vis absorption spectra of PSC, PDC, PCC and PPP in cyclohexane (a, 4 × 10−6 M) and in the films (b).

which red-shifted to 371 nm for PDC due to the increased conjugation. As to PCC, two strong absorption bands appeared at 338 nm and 370 nm, meaning that the longer spacer of phenylvinylcarbazolylvinyl between terminal carbazole and phenanthroimidazole could increase the conjugation length of the molecules. The maximal absorption band of PPP emerged at 394 nm. Additionally, the absorption bands of the four compounds red-shifted obviously in the films compared with those in solutions because of the intermolecular interactions (Fig. 1b), for example, the absorption at 394 nm for PPP in cyclohexane red-shifted to 423 nm in the film. From Fig. 2a we could find that PSC, PDC, PCC and PPP gave two strong fluorescence emission peaks in cyclohexane. The emission bands of PSC with terminal carbazole linked to phenanthroimidazole via phenylene appeared at 390 nm and 412 nm, and red-shifted to 418 nm and 443 nm for PDC, in which phenylvinyl was used as the spacer, due to the larger conjugation length in PDC. Similarly, with further increasing the conjugation length, the emission bands of PCC appeared in lower energy region (426 nm and 450 nm). On account of introduction of pyrene unit into phenanthroimidazole, the emission at 450 nm and 477 nm was observed. On the other hand, only one broad emission band could be detected for each compound in the film, and red-shifted obviously compared with that in solution due to ␲–␲ interaction in the solid state (Fig. 2b). Taking PPP as an example, the emission band red-shifted to 510 nm in the film compared with those (450 nm and 477 nm) in cyclohexane. We also showed the solvent-dependent UV–vis absorption and fluorescence emission spectra of PSC, PDC, PCC and PPP in Figs. S4–S7. It was found that the maximal absorption band of PSC gave no obvious shift in the solvents with different polarities (Fig. S4), indicating that the structural and electronic characteristics of the ground and Franck–Condon (FC) excited states was similar in the solvents with different polarities [13]. Compounds PDC, PCC and PPP exhibited similar absorption spectral behaviors. However, the fluorescence emission band of PCC red-shifted obviously with increasing the solvent polarities, and it red-shifted from 451 nm in cyclohexane to 491 nm in DMF (Fig. S6 and Table S3). It illustrated

the occurrence of intramolecular charge transfer (ICT) upon photoexcitation [14], and a large solvent relaxation as well as strong stabilization of the excited state in the polar solvents [15]. However, as to compounds PSC, PDC and PPP, no obvious shift of the emission with increasing the polarities of the solvents was detected (Figs. S4–S5 and S7, Tables S1–S2 and S4), meaning no ICT took place. The fluorescence quantum yields of PSC, PDC, PCC and PPP in cyclohexene were 0.72, 0.54, 0.49 and 0.83, respectively, using quinine sulfate (0.1 N in H2 SO4 ) as a standard, suggesting the perylene functionalized phenanthroimidazole and the phenanthroimidazole with terminal carbazole linked by phenylene group were high emissive. The reason for the low fluorescence quantum yields of PDC and PCC compared with those of PDC and PCC might be that the longer spacer in later ones would lead to increased non-radiative transition. 2.2. Electrochemical properties The electrochemical behaviors of PSC, PDC, PCC and PPP were examined by cyclic voltammetry (CV) using a standard threeelectrode cell and an electrochemistry workstation (CHI 604) under N2 atmosphere. Platinum button was used as the working electrode, salt bridge as the reference electrode, a platinum wire as the counter electrode, and ferrocene was used as a standard. The cyclic voltammetry (CV) diagrams of PCC, PSC, PDC and PPP in CH2 Cl2 in the presence of Bu4 NBF4 as the supporting electrolyte were shown in Fig. 3 and Figs. S1–S3, and the corresponding electrochemical data listed in Table 2. Compounds PSC, PDC, PCC and PPP displayed an irreversible redox process with onset oxidation peaks at +1.27 V, +1.07 V, +1.01 V and +1.39 V, respectively. The lower oxidation potentials could be originated from the stronger electron-donating ability. On the basis of the first oxidation potential, the highest occupied molecular orbital (HOMO) energy levels were estimated as −5.63 eV for PSC, −5.31 eV for PDC, −5.51 eV for PCC and −5.25 eV for PPP based on the equation of EHOMO = −(Eonset + 4.24) eV. The lowest unoccupied molecular orbital (LUMO) energy levels of these compounds could be estimated from their HOMO energy levels and

Table 1 Photophysical data of PSC, PDC, PCC and PPP. Compounds

a

PSC PDC PCC PPP a b c d e

Fluorescence emissionb

Absorption

a

Solution

Film

Solution

Film

243, 290, 338 247, 371 251, 338, 370 240, 394

257, 305, 371 251, 387 418 265, 423

390, 412 418, 443 426, 450 450, 477

438 475 485 510

˚F c

Tm (◦ C)d

Td (◦ C)e

0.72 0.54 0.49 0.83

76 88 237 254

471 459 456 469

Measured in cyclohexane. Excited at 365 nm. The fluorescence quantum yields were determined against quinine sulfate in 0.1 M H2 SO4 (˚F = 0.546) as a standard excited at 365 nm. Obtained from DSC measurement. Obtained from TGA measurement (temperature at 5% weight loss under nitrogen, 10 ◦ C/min ramp rate).

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Fig. 2. Normalized fluorescence emission spectra of PSC, PDC, PCC and PPP in cyclohexane (a, 4.0 × 10−6 M) and in the films (b) (ex = 365 nm).

2.4. Thermal properties The thermal properties of PSC, PDC, PCC and PPP were investigated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) under a nitrogen atmosphere at a heating and cooling rate of 10 ◦ C/min. As shown in Fig. S8 and Table 1, PCC and PPP gave high melting points (Tm ) at 237 ◦ C and 254 ◦ C, while the Tm of PSC and PDC were of 76 ◦ C and 88 ◦ C. Their decomposition temperatures, which correspond to a 5% weight loss upon heating during TGA processes, were 471 ◦ C for PSC, 459 ◦ C for PDC, 456 ◦ C for PCC and 469 ◦ C for PPP. These results indicated that the four compounds exhibited good thermal stability, favoring for the lifetime of devices. Fig. 3. Cyclic voltammetry diagrams of compound PCC in anhydrous CH2 Cl2 with 0.1 M Bu4 NBF4 as electrolyte at a scan rate of 100 mV/s.

2.5. Electroluminescent properties energy band gaps (Eg ), which were calculated from the absorption edge of these compounds in solutions (ELUMO = EHOMO + Eg ). The LUMO energy levels were −2.65 eV, −2.44 eV, −2.46 eV and −2.48 eV for PSC, PDC, PCC and PPP, respectively. The HOMO levels of PDC and PPP were close to that of PEDOT:PSS (−5.2 eV), indicating a good hole injection contact. Therefore, these compounds might be used as hole-transporting materials in OLEDs.

2.3. Molecular orbital calculations To gain a deeper insight into the electronic structures of these compounds, we carried out the density functional theory (DFT) calculation at the B3LYP/6-31G(d) level. To minimize calculation cost, the octyl group was substituted by methyl group in the calculation, and corresponding energy levels were listed in Table 2. As shown in Fig. 4, the electron densities of HOMO of these compounds were distributed on the whole ␲-conjugated backbone, which were affected by the electron donating substituent. The ␲-electrons in LUMO for the four compounds were mostly populated on the linker between the acceptor and the donor as well as on the imidazole and pyrene ring.

To evaluate the electroluminescent properties of phenanthroimidazole derivatives the devices with configuration of ITO/NPB (45 nm)/phenanthroimidazole derivatives (15 nm)/TPBI (40 nm)/LiF/Al, in which PSC, PDC, PCC and PPP were used as emitting layer, ITO as the anode, NPB as the hole-transporting layer, TPBI as the electron-transporting layer, and LiF/Al as the cathode, were fabricated. Fig. 5 showed the EL spectra of the devices, it was clear that blue-light emission with a peak at 436 nm, which was similar to the PL emission of PSC in the film (438 nm), was detected from the device based on PSC, and Commission Internationale de L’Eclairage (CIE) coordinates of the device was (0.16, 0.17), suggesting a saturated blue emission (Table 3). Additionally, the devices based on PDC, PCC and PPP emitted bluish green light located at 484 nm, green light emission at 492 nm, and green light at 520 nm, respectively, and their Commission Internationale de L’Eclairage (CIE) coordinates were (0.21, 0.36), (0.23, 0.38) and (0.33, 0.53), respectively (Table 3). The luminance–voltage (L–V) and voltage–current density (V–J) characteristics for the EL devices based on phenanthroimidazole derivatives were shown in Fig. 6a and b. The turn-on voltage of the devices based on PSC, PDC, PCC and PPP were 3.4 V, 3.4 V, 3.8 V and 3.4 V, respectively, and the maximum luminance was 1190 cd/m2 at 10.5 V for PSC, 1930 cd/m2

Table 2 Electrochemical and theoretical calculated data for PSC, PDC, PCC and PPP. Compounds

ox Eonset (V)a

HOMO (eV)b

LUMO (eV)b

Eg (eV)c

HOMO (eV)d

LUMO (eV)d

PSC PDC PCC PPP

1.39 1.07 1.27 1.01

−5.63 −5.31 −5.51 −5.25

−2.48 −2.44 −2.65 −2.46

3.51 2.87 2.86 2.79

−4.99 −4.81 −4.71 −4.97

−1.07 −1.42 −1.41 −1.81

ox a Eonset (V), onset oxidation potential; potentials versus salt bridge, working electrode platinum button, 0.1 M Bu4 NBF4 –CH2 Cl2 , scan rate 100 mV/s, Fc/Fc+ was used as external reference. ox b Calculated using the empirical equation: HOMO = −(4.24 + Eonset ) and LUMO = HOMO + Eg . c Estimated from the onset of the absorption spectra (Eg = 1240/onset ). d Obtained from quantum chemical calculation using TDDFT/B3LYP/6-31G.

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Fig. 4. The frontier orbital plots of the HOMO and LUMO of PSC, PDC, PCC and PPP.

at 11 V for PDC, 1270 cd/m2 at 12.5 V for PCC and 1149 cd/m2 at 11 V for PPP. The current efficiency and power efficiency versus current density of the devices were shown in Fig. 6c and d. It was found that the maximum current efficiency and the maximum power efficiency of the device based on PSC were 0.92 cd/A and 0.85 lm/W, respectively, suggesting that PSC could be used as blue-emitting material in light-emitting diodes. Additionally, the devices based on PDC and PCC showed the maximum current efficiencies of 1.83 cd/A and 1.28 cd/A, respectively, while the maximum power efficiencies were 1.38 lm/W and 1.05 lm/W, respectively. As a result, we deduced that the performance of the

device based on PDC was better than PCC because the conjugated degree of PDC was moderate compared with PCC. These results indicated that the performance of the device decreased as the conjugated degree of the emitter was extended. The possible reason might be that the luminescence quenching would be increased in the ␲-aggregates of the compound with more carbazole–vinylene units (PCC). Additionally, the device based on PPP gave the maximum current efficiency and maximum power efficiency of 0.32 cd/A and 0.24 lm/W, respectively. The obtained results suggested that the synthesized phenanthroimidazole derivatives could be used as emitting material in light-emitting diodes although

Table 3 EL performance of the devices with the configuration of ITO/NPB (45 nm)/phenanthroimidazole derivatives (15 nm)/TPBI (40 nm)/LiF/Al. Emitters

EL a (nm)

Von b (V)

Lmax c (cd/m2 )

c d (cd/A)

p e (lm/W)

CIEf

PSC PDC PCC PPP

436 484 492 520

3.4 3.4 3.8 3.4

1190 1930 1270 1149

0.92 1.83 1.28 0.32

0.85 1.38 1.05 0.24

0.16, 0.17 0.21, 0.36 0.23, 0.38 0.33, 0.53

a b c d e f

Peak electroluminescence. Turn-on voltage. Maximum brightness. Maximum current efficiency. Maximum power efficiency. The Commission International de L’Eclairage (CIE) coordinates at 10 V.

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derivatives (15 nm)/TPBI (40 nm)/LiF/Al, in which PSC, PDC, PCC and PPP as the emitting materials, were fabricated. The device based on PSC gave a saturated blue emission with CIEx,y of (0.16, 0.17), and the turn-on voltage, maximum luminance and power, current efficiency of the device were of 3.40 V, 1190 cd/m2 , 0.85 lm/W and 0.92 cd/A, respectively. The device based on PDC and PCC showed similar performance besides the color of the emitting light. Therefore, the phenanthroimidazole derivatives might become candidates as emitting materials employed in OLEDs, and other related fields.

4. Experimental Fig. 5. Normalized electroluminescence (EL) spectra of the devices with configuration ITO/NPB (45 nm)/phenanthroimidazole derivatives (15 nm)/TPBI (40 nm)/LiF/Al.

the performance of the obtained devices were not as high as the reported ones [5,7]. 3. Conclusions In summary, we synthesized new phenanthroimidazole derivatives bearing terminal carbazole linked by different spacers, including 1,4-phenylene, phenylvinyl and phenylvinylcarbazolylvinyl (PSC, PDC, PCC), and bearing terminal pyrene (PPP), which exhibited excellent solubility in common organic solvents. It was found that the obtained phenanthroimidazole derivatives were high emissive in cyclohexane, for example, the fluorescence quantum yields were 0.72 and 0.83 for PSC and PCC, respectively, using quinine sulfate (0.1 N in H2 SO4 ) as a standard. Meanwhile, they exhibited high thermal stability with initial decomposition temperature over 450 ◦ C, which might be helpful for improvement of the device stability and lifetime. The organic light-emitting diodes with the configuration of ITO/NPB (45 nm)/phenanthroimidazole

4.1. Materials and measurements 1 H NMR spectra were recorded on a Mercury plus 400 MHz using CDCl3 as solvent in all cases. 13 C NMR spectra were recorded on a Mercury plus 100 MHz using CDCl3 as solvent. UV–vis absorption spectra were determined on a Shimadzu UV-1601PC spectrophotometer. Fluorescent emission spectra were carried out on a Shimadzu RF-5301 luminescence spectrometer. FT-IR spectra were measured using a Germany bruker vertex 80v FT-IR spectrometer by incorporating samples in KBr disks. Mass spectra were performed on Agilent 1100 MS series and AXIMA CFR MALDI/TOF (Matrix assisted laser desorption ionization/Time-offlight) MS (COMPACT). Cyclic voltammetry (CV) measurements were performed using CHI 604B electrochemical working station and were carried out in DCM containing 0.1 M tetrabutylammonium tetrafluoroboron (Bu4 NBF4 ) as a supporting electrolyte at room temperature. Platinum button was used as a working electrode and a platinum wire as a counter electrode, and the potentials were recorded versus Hg/HgCl2 (saturated) as a reference electrode. The scan rate was maintained at 100 mV/s. DMF was distilled from phosphorous pentoxide, and other chemicals were used as received without further purification.

Fig. 6. (a) L–V characteristics and (b) J–V characteristics (c) current efficiency–current density, (d) power efficiency–current density of the devices with configurations of ITO/NPB (45 nm)/phenanthroimidazole derivatives (15 nm)/TPBI (40 nm)/LiF/Al.

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4.2. Synthetic procedures and charactrerizations 1-(4-tert-butylphenyl)-2-(4-bromophenyl)-1H-phenanthro (1) [7a], 9-octyl-9H-carbazol-3-yl-3[9,10-d]imidazole boronicacid (2) [9], 9-octyl-3-vinyl-9H-carbazole (3) [10], 9-octyl-3-((E)-2-(9-octyl-9H-carbazol-6-yl)vinyl)-6-vinyl-9Hcarbazole (4) [10] and 1-vinylpyrene (5) [11] were synthesized according to the procedures reported in the literatures.

4.2.1. 1-(4-tert-butylphenyl)-2-(4-(9-octyl-9H-carbazol-6yl)phenyl)-1H-phenanthro[9,10-d]imidazole (PSC) A mixture of compound 1 (0.81 g, 1.60 mmol), compound 2 (1.03 g, 3.17 mmol), Pd(PPh3 )4 (0.29 g, 0.25 mmol), degassed toluene (30 mL) and degassed K2 CO3 aqueous solution (25 mL, 2 mol/L) was stirred at 85 ◦ C for 48 h under N2 atmosphere. After cooled to room temperature, the mixture was extracted with DCM. The combined organic extracts were dried with anhydrous Na2 SO4 , followed by removal of the solvent. The residue was purified by column chromatography (silica gel) using petroleum ether/CH2 Cl2 (v/v = 1/1) as eluent to give a white solid. Yield: 62%. Mp 112.0–114.0 ◦ C. 1 H NMR (400 MHz, CDCl3 ) ı 8.94 (d, J = 7.4 Hz, 1H), 8.77 (d, J = 8.4 Hz, 1H), 8.71 (d, J = 8.3 Hz, 1H), 8.31 (s, 1H), 8.13 (d, J = 7.7 Hz, 1H), 7.68 (ddd, J = 31.6, 15.8, 7.9 Hz, 10H), 7.46 (dp, J = 23.3, 8.2 Hz, 7H), 7.21 (d, J = 8.6 Hz, 1H), 4.30 (t, J = 7.1 Hz, 2H), 1.87 (p, J = 7.2 Hz, 2H), 1.46 (s, 9H), 1.42–1.20 (m, 10H), 0.91–0.83 (m, 3H) (Fig. S9). 13 C NMR (100 MHz, CDCl3 ) ı 153.26, 150.92, 142.24, 140.90, 140.15, 137.42, 136.14, 131.22, 129.69, 129.22, 128.60, 128.34, 128.26, 127.31, 127.23, 127.06, 126.78, 126.21, 125.85, 125.52, 124.97, 124.73, 124.05, 123.37, 123.20, 123.09, 122.93, 122.84, 120.92, 120.42, 118.96, 118.74, 108.95, 108.87, 77.35, 77.22, 77.03, 76.71, 60.42, 43.19, 35.04, 31.80, 31.44, 29.38, 29.17, 28.99, 27.31, 22.60, 14.08, 0.00 (Fig. S10). FT-IR (KBr, cm−1 ): 3053, 2928, 2855, 1600, 1508, 1464, 1451, 746. MS (MALDI-TOF): m/z 705.3 [M+H+ ] (Calcd for C51 H49 N3 : 704.0, Fig. S11).

4.2.2. 1-(4-tert-butylphenyl)-2-(4-((E)-2-(9-octyl-9H-carbazol6-yl)vinyl)phenyl)-1H-phenanthro[9,10-d]imidazole (PDC) Compound 1 (5.00, 9.90 mmol), 9-octyl-3-vinyl-9H-carbazole 3 (7.30 g, 12.00 mmol), Pd(OAc)2 (10.00 mg, 0.04 mmol), anhydrous K2 CO3 (1.67 g, 24.00 mmol), and Bu4 NBr (3.90 g, 12.00 mmol) in dry DMF (50 mL) were added to a 100 mL two-necked round bottom flask. The mixture was stirred at 110 ◦ C for 24 h under N2 . After cooling to room temperature, the mixture was poured into water (500 mL), and extracted with DCM. The combined organic phases were washed with brine, and dried with anhydrous Na2 SO4 . After the solvent was removed, the residue was purified by silica gel column chromatography using petroleum ether/CH2 Cl2 (v/v = 2/3) as eluent to give yellow solid. Yield: 70%. Mp 142.0 ◦ C. 1 H NMR (400 MHz, CDCl3 ) ı 9.06–8.89 (m, 1H), 8.77 (d, J = 8.4 Hz, 1H), 8.71 (d, J = 8.3 Hz, 1H), 8.25–8.18 (m, 1H), 8.11 (d, J = 7.7 Hz, 1H), 7.76 (t, J = 7.4 Hz, 1H), 7.70–7.59 (m, 6H), 7.54–7.44 (m, 6H), 7.38 (dd, J = 11.1, 8.6 Hz, 2H), 7.32 (s, 1H), 7.30–7.26 (m, 1H), 7.25–7.18 (m, 2H), 7.10 (d, J = 16.2 Hz, 1H), 4.28 (t, J = 7.2 Hz, 2H), 1.87 (p, J = 7.4 Hz, 2H), 1.47 (s, 9H), 1.42–1.16 (m, 10H), 0.86 (t, J = 6.8 Hz, 3H) (Fig. S12). 13 C NMR (100 MHz, CDCl3 ) ı 153.28, 150.80, 140.85, 140.36, 138.32, 137.32, 136.07, 130.59, 129.54, 129.22, 128.83, 128.55, 128.26, 127.23, 127.05, 126.21, 125.91, 125.83, 125.54, 125.18, 124.76, 124.47, 124.04, 123.19, 123.14, 123.09, 122.84, 120.91, 120.39, 119.03, 118.82, 108.90, 77.34, 77.23, 77.02, 76.70, 43.18, 35.04, 31.78, 31.43, 29.35, 29.16, 28.98, 27.28, 22.60, 14.06, 0.00 (Fig. S13). FT-IR (KBr, cm−1 ): 3050, 2928, 2858, 1599, 1510, 1470, 960, 750. MS (MALDI-TOF): m/z 731.3 [M+H+ ] (Calcd for C53 H51 N3 : 730.0, Fig. S14).

4.2.3. 1-(4-tert-butylphenyl)-2-(4-((1E)-2-(9-octyl-3-((E)-2-(9octyl-9H-carbazol-3-yl)vinyl)-9H-carbazol-6-yl)vinyl)phenyl)1H-phenanthro[9,10-d]imidazole (PCC) The synthetic method for PCC was similar to that of PDC. It was synthesized from compounds 1 and 4 under Heck reaction condition in a yield of 65%. The crude product was purified by column chromatography (silica gel) using petroleum ether/DCM (v/v = 1/1) as eluent to give a bright yellow solid. Mp >200.0 ◦ C. 1 H NMR (400 MHz, CDCl3 ) ı 8.98–8.91 (m, 1H), 8.76 (d, J = 8.4 Hz, 1H), 8.70 (d, J = 8.3 Hz, 1H), 8.27 (d, J = 6.6 Hz, 3H), 8.15 (d, J = 7.7 Hz, 1H), 7.80–7.69 (m, 4H), 7.64 (ddd, J = 15.8, 8.4, 2.7 Hz, 6H), 7.54–7.44 (m, 6H), 7.43–7.31 (m, 7H), 7.30–7.26 (m, 1H), 7.21 (t, J = 8.6 Hz, 1H), 7.12 (d, J = 16.2 Hz, 1H), 4.28 (q, J = 9.2, 8.2 Hz, 4H), 1.87 (q, J = 6.9 Hz, 4H), 1.47 (s, 9H), 1.44–1.14 (m, 20H), 0.87 (t, J = 6.5 Hz, 6H) (Fig. S15). 13 C NMR (100 MHz, CDCl ) ı 153.25, 150.80, 140.84, 140.71, 140.30, 3 139.98, 138.29, 137.35, 136.08, 130.53, 129.53, 129.21, 129.11, 128.56, 128.37, 128.31, 128.26, 127.25, 127.21, 127.03, 126.86, 126.19, 125.92, 125.68, 125.52, 125.22, 124.73, 124.61, 124.52, 124.23, 124.03, 123.31, 123.24, 123.21, 123.15, 123.08, 122.93, 122.85, 120.91, 120.40, 118.86, 118.81, 118.26, 118.18, 109.04, 108.87, 108.82, 77.34, 77.22, 77.02, 76.70, 43.25, 43.15, 35.03, 31.79, 31.43, 29.37, 29.18, 29.01, 27.30, 22.61, 14.08, 0.00 (Fig. S16). FT-IR (KBr, cm−1 ): 3486, 3022, 2925, 2856, 1600, 1491, 1348, 960, 754. MS (MALDI-TOF): m/z 1034.5 [M+H+ ] (Calcd for C75 H76 N4 : 1033.4, Fig. S17). 4.2.4. 1-(4-tert-butylphenyl)-2-(4-((E)-2-(pyren-3yl)vinyl)phenyl)-1H-phenanthro[9,10-d]imidazole (PPP) The synthetic method for compound PPP was similar to that of PDC. It was synthesized from compounds 1 and 5 under Heck reaction condition with a yield of 65%. The crude product was purified by column chromatography (silica gel) using petroleum ether/CH2 Cl2 (v/v = 1/1) as eluent to give a yellow solid. Mp >200.0 ◦ C. 1 H NMR (400 MHz, CDCl3 ) ı 8.98 (d, J = 7.1 Hz, 1H), 8.78 (d, J = 8.3 Hz, 1H), 8.72 (d, J = 8.3 Hz, 1H), 8.49 (d, J = 9.3 Hz, 1H), 8.31 (d, J = 8.1 Hz, 1H), 8.25 (s, 1H), 8.23–8.17 (m, 3H), 8.15 (d, J = 9.5 Hz, 1H), 8.06 (s, 2H), 8.01 (t, J = 7.6 Hz, 1H), 7.78 (t, J = 7.3 Hz, 1H), 7.73–7.59 (m, 7H), 7.56–7.46 (m, 3H), 7.34 (s, 1H), 7.29 (d, J = 8.5 Hz, 1H), 7.20 (d, J = 7.8 Hz, 1H), 1.49 (s, 9H) (Fig. S18). 13 C NMR (100 MHz, CDCl3 ) ı 153.35, 150.55, 137.95, 137.39, 136.05, 131.58, 131.50, 130.97, 130.91, 129.63, 129.27, 128.54, 128.44, 128.29, 127.64, 127.42, 127.36, 127.25, 127.19, 127.09, 126.44, 126.42, 126.23, 126.00, 125.59, 125.34, 125.12, 125.08, 125.04, 124.92, 124.81, 124.05, 123.58, 123.11, 122.90, 122.82, 120.92, 77.33, 77.21, 77.01, 76.70, 63.39, 35.06, 31.44, 0.00 (Fig. S19). FT-IR (KBr, cm−1 ): 3050, 2960, 2873, 1508, 1469, 1452, 1426, 960, 752. MS (MALDI-TOF): m/z 654.2 [M+H+ ] (Calcd for C49 H36 N2 : 652.8, Fig. S20). 4.3. Theoretical calculation methods The geometrical structures of PSC, PDC, PCC and PPP were optimized by employing the density functional theory at the B3LYP/6–31 level with the Gaussian 03W program package. Molecular orbitals were visualized using Gaussview. 4.4. Fabrication of the OLEDs and EL measurements The devices were grown on a glass, which was pre-coated with indium tin oxide (ITO) having a sheet resistance equal to 20  cm−2 . The ITO glass was routinely cleaned by ultrasonic treatment in detergent solutions, followed by rinsed with acetone, boiled in isopropanol, rinsed in methanol, and then in de-ionized water. The glass was dried in vacuum oven between each cleaning step above. To reduce the possibility of electrical shorts on the ITO anode and increase the value of its work function, the ITO substrate was treated using a Plasma Cleaner (PDC-32G-2, 100W) with

J. Peng et al. / Synthetic Metals 193 (2014) 94–101

the oxygen ambient. Prior to the deposition, all the organic materials were purified by sublimation method. The organic layer was sequentially deposited onto the substrate without breaking vacuum at a pressure of about 10−4 Pa. A very thin layer of TPBI could enhance electron injection from aluminum cathode. A shadows mask with 2 mm × 3 mm openings was used to define the cathodes. The EL spectrum, brightness and the current–brightness–voltage characteristics of the devices were measured with a rapid scan system using a spectrophotometer (PR-650, Photo Research) and a computer-controlled, programmable, direct-current (DC) source (Keithley 2400). Luminance-voltage and current–voltage characteristics were measured at room temperature under an ambient atmosphere. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (51073068 and 21374041), and Open Project of State Key Laboratory of Supramolecular Structure and Materials (SKLSSM201407), the Open Project of State Laboratory of Theoretical and Computational Chemistry (K201302). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.synthmet. 2014.04.004.

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