Remarkable Isomeric Effects on Optical and Optoelectronic Properties

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Remarkable Isomeric Effects on Optical and Optoelectronic Properties of N‑Phenylcarbazole-Capped 9,10-Divinylanthracenes Shanfeng Xue, Wei Liu, Xu Qiu, Yangyang Gao, and Wenjun Yang* Key Laboratory of Rubber-Plastics of Ministry of Education/Shandong Provincial Key Laboratory of Rubber-Plastics, School of Polymer Science and Engineering, Qingdao University of Science & Technology, 53 Zhengzhou Road, Qingdao 266042, P. R. China ABSTRACT: We synthesized three N-phenylcarbazole-capped 9,10-divinylanthrane isomers by changing the linking positions of N-phenylcarbazole, 9,10-bis(N-phenylcarbazol-2-/3-yl-vinyl2)anthracenes (CZ2/CZ3), and 9,10-bis(4-(carbazol-9-yl)styryl)anthracene (CZ9) to examine isomeric effects on the fluorescence and electroluminescence properties. The results showed that their fluorescence emission properties in both crystalline and amorphous states were affected by the linking position of N-phenylcarbazole. The three isomers were all strong crystallization-enhanced emission (CEE) dyes, whereas the strongest and the weakest fluorescence efficiencies were measured in crystalline and amorphous states of CZ9, respectively. When the isomers were used as bulk emitting layers by vapor deposition, the maximal luminous efficiency, luminance, and the turnon voltage of CZ9-based device were 0.10 cd A−1, 550 cd cm−2, and 7.8 V, respectively. In sharp contrast, CZ3 afforded the best EL performance, and the corresponding performances were 3.1 cd A−1, 13770 cd cm−2, and 3.2 V, respectively. The obvious isomeric effect indicated that subtle manipulation of peripheral groups was indeed a feasible and efficient way for tuning the optical and optoelectronic properties of CEE dyes.

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INTRODUCTION Anthracene was ever an important organic luminogen during the development of organic optoelectronics. For example, the first organic electroluminescence is observed from its single crystals.1 Because of its strong fluorescence and high chargetransporting properties, anthracene and its derivatives have been reused as building blocks for promising functional materials with unique optical and optoelectronic properties, including conjugated small molecules,2−8 oligomers,9−15 and polymers.16−22 In principle, aryl units could be linked to the 2,6-, 9,10-, and 2,6,9,10-positions of anthracene ring by single, double, and triple bond to form various conjugated molecules.23−29 Among them, strongly twisted 9,10-diarylanthracenes usually emit bright blue fluorescence in both solutions and solid states, but the nearly planar conformation 2,6-diaryl- and 2,6-bis(arylvinyl)anthracenes exhibit aggregation-quenched emission. Contrarily, 9,10-bis(arylvinyl)anthracenes not only have strongly twisted backbone but also display aggregation- and/or crystallization-enhanced emission (AEE and/or CEE) effect, whereas 2,6,9,10-tetra(arylvinyl)anthracenes exhibit not only AEE/CEE effect but also spatially separated frontier molecular orbital.30−32 Therefore, the optical, electronic, and optoelectronic properties of anthracenecentered conjugated molecules are highly dependent on both the nature of building blocks and the linking positions. Recently, 9,10-bis(arylvinyl)anthracene derivatives have been attracting considerable interest due to their AEE and stimulusresponse behaviors.33−39 Like other organic dyes with strongly twisted conjugation skeletons, 40−48 9,10-bis(arylvinyl)anthracenes exhibit high solid-state fluorescence, which is extremely desirable for organic light emitting and optical recording applications. Very recently, we and others have found © 2014 American Chemical Society

that a number of homologous 9,10-bis(alkylarylvinyl)anthracenes with different alkyl chain length exhibit strongly alkyl length-dependent solid-state luminescence and piezofluorochromic behavior, which could be regarded as a new means for tuning the aggregate structure and optical properties.38,49−51 Alternatively, we consider that changing the linking positions of peripheral aryl units of 9,10-bis(arylvinyl)anthracenes could afford the isomers with different chemical and electronic structure, which should influence the molecular aggregation behavior and tune the solid-state optical and optoelectronic properties to widen the scope of material candidates applicable in organic optical and optoelectronic fields. However, such investigations are rarely reported. In this paper, we report on changing the linking positions of Nphenylcarbazole on 9,10-divinylanthracene to synthesize three isomers (Scheme 1), 9,10-bis(N-phenylcarbazole-2-/3-yl-vinyl2)anthracenes (CZ2/CZ3), and 9,10-bis(4-(carbazol-9-yl)styryl)anthracene (CZ9). We highlight that the change of linking positions of N-phenylcarbazole could significantly affect not only the crystalline- and amorphous-state fluorescence properties but also the electroluminescent performances, demonstrating that the subtle manipulation of AIE/CEE molecular building blocks is indeed a feasible and efficient way for tuning the optical and optoelectronic properties. Received: June 4, 2014 Revised: July 21, 2014 Published: July 29, 2014 18668

dx.doi.org/10.1021/jp505527f | J. Phys. Chem. C 2014, 118, 18668−18675

The Journal of Physical Chemistry C

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Scheme 1. Synthetic Route and Structure of Isomers CZ2, CZ3, and CZ9

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EXPERIMENTAL SECTION Materials. Starting materials, charge-transporting materials, and catalysts were purchased from Aldrich Chemical Co. or Energy Chemical Co., China. Other solvents and reagents were all commercially available analytical-grade products and used as received without further purification, unless otherwise stated. Tetrahydrofuran (THF) was distilled over metallic sodium and dimethylformamide (DMF) over calcium hydride before use. 9,10-Bis(diethylphosphorylmethyl)anthracene is from previous work.11 Measurements. 1H NMR spectra were recorded on a Bruker AC500 (500 MHz) spectrometer at 298 K by utilizing deuterated chloroform (CDCl3) as solvents and tetramethylsilane (TMS) as internal standard. Elemental analysis was performed on a PerkinElmer 2400. UV−vis absorption and diffuse reflectance absorption spectra were recorded on a Hitachi U-4100 spectrophotometer. Fluorescence measurements were carried out with a Hitachi F-4600 spectrophotometer. Solution fluorescence quantum yield (Φ) was determined at room temperature by the dilution method using fluorescein in water (pH = 11) as the reference. Solidstate fluorescence efficiencies were measured on an integrating sphere (C-701, Labsphere) with a 325 nm Xe light as the excitation source, and the dense ground and then annealed pure solids were used as amorphous and crystalline states, respectively. Powder wide-angle X-ray diffraction experiments were performed on a powder X-ray diffractometer (INCA Energy, Oxford Instruments) operating at 3 kW. Cyclic voltammetry (CV) was performed with a BAS 100W Bioanalytical Systems, using a glass carbon disk (diameter = 3 mm) as the working electrode, a platinum wire as the auxiliary electrode with a porous ceramic wick, and Ag/Ag+ as the reference electrode standardized by the redox couple ferrocenium/ferrocene. All solutions were purged with a nitrogen stream for 10 min before measurement. The procedure was performed at room temperature, and a nitrogen atmosphere was maintained over the solution during measurements.52,53 Device Fabrication. Indium−tin oxide (ITO) coated glass with a sheet resistance of 15−20 Ω/cm2 was used as the cathode whose substrate was prepatterned by photolithography to give an effective device size of 6.25 mm2. Then, it was

cleaned in an ultrasonic bath with acetone, detergent, deionized water, and isopropanol as the clean agent. After being dried in an oven, it is treated with oxygen plasma for 4 min, and then 40 nm of PEDOT:PSS layer was spin-coated and baked overnight in a vacuum oven at 80 °C. On the top of above active layer, N,N′-di(1-naphthyl)-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine (NPB) as hole-transporting layer, isomer as bulk emitting layer (EML), and 1,3,5-tri(phenyl-2-benzimidazolyl)benzene (TPBi) as electron-transporting layer were vacuum deposited in a sequence. Finally, 0.5 nm of LiF film and 100 nm of Al film were vacuum deposited to form the anode at a base pressure of 3 × 10−4 Pa. The thickness of each deposition layer was monitored using a quartz crystal thickness/ratio monitor (STM-100/MF, Sycon). Electroluminescence (EL) spectra were measured by a PR650 fluorescence spectrophotometer. Luminance−voltage and current density−voltage characteristics were recorded simultaneously by combining the spectrometer with a Keithley model 2400 programmable voltage−current source. All measurements were carried out at room temperature under ambient conditions. Synthesis and Characterization. N-Phenyl-2-formylcarbazole (M1). A mixture of iodobenzene (2.51 g, 12.2 mmol), 2bromocarbazole (2.00 g, 8.13 mmol), CuI (0.15 g, 0.81 mmol), L-proline (0.09 g, 0.81 mmol), and K2CO3 (1.76 g, 16.26 mmol) in dimethyl sulfoxide (20 mL) was stirred for 3 days at 110 °C under nitrogen. After being cooled to room temperature, water was added and extracted with CH2Cl2 and dried over anhydrous MgSO4. Solvent was removed, and the residue was purified by column chromatography on silica gel using petroleum as the eluent. A white solid (1.31 g) was obtained, which was regarded as 2-bromo-9-phenylcarbazole without characterization and dissolved in 15 mL of anhydrous THF under nitrogen and added dropwise n-BuLi (0.1.9 mL, 4.7 mmol) at −78 °C. After stirring for 0.5 h, anhydrous DMF (0.76 mL, 9.92 mmol) was added, and the mixture was stirred for an additional 12 h at room temperature. Water and CH2Cl2 were added to the mixture, and the organic layer was separated, washed with saturated NaHCO3, and dried over MgSO4. The solvent was removed, and the residue was purified by a column chromatography (silica gel, ethyl acetate/hexane = 1/10) to give M1 as a white solid (0.62 g, 56% yield). 1H NMR (500 MHz, CDCl3): δ 10.03 (s, 1H), 8.18 (d, 2H), 7.87 (s, 1H), 7.76 18669

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anthracene ring, 9,10-bis(arylvinyl)anthracenes have the strongly twisted conformations, which could render molecular backbone capacity of intramolecular torsion motion. This relative motion facilitates the nonirradiative relaxation pathways and is freer in the molecular solution. As a sequence, weak fluorescence emissions are observed for three isomers in THF solution (Figure 1). We note that CZ9 solution shows the

(d, 1H), 7.58 (t, 2H), 7.47 (m, 4H), 7.37 (d, 1H), 7.30 (t, 1H) ppm. N-Phenyl-3-formylcarbazole (M2). The intermediate for M2, N-phenylcarbazole, was synthesized by the same procedure as described for 2-bromo-9-phenylcarbazole except that carbazole was used instead of 2-bromocarbazole. A mixture of phosphoryl chloride (0.42 mL, 4.5 mmol) and anhydrous DMF (1.04 mL, 13.5 mmol) was stirred at 0 °C for 1 h, and then 9phenylcarbazole (1.0 g, 4.1 mmol, in 20 mL sym-dichloroethane) was added. The reaction temperature was raised to 90 °C and stirred for 8 h. After cooling, the mixture was poured into ice water and extracted with dichloromethane. The solvent was evaporated, and the crude product was purified by a column chromatography on silica gel using ethyl acetate/ hexane (1/10) as the eluent. A white solid was obtained (1.02 g, 92% yield). 1H NMR (500 MHz, CDCl3): δ 10.09 (s, 1H), 8.65 (s, 1H), 8.19 (d, 1H), 7.93 (d, 1H), 7.62 (t, 2H), 7.53 (d, 3H), 7.42 (m, 4H) ppm. N-(4-Formylphenyl)carbazole (M3). This compound was synthesized by the same procedure as described for 2-bromo-Nphenylcarbazole except that 4-iodobenzaldehyde was used instead of 2-bromocarbazole. 1H NMR (500 MHz, CDCl3): δ 10.13 (s, 1H), 8.15, (t, 4H), 7.81 (d, 2H), 7.51 (d, 2H), 7.45 (t, 2H), 7.34 (t, 2H) ppm. 9,10-Bis(N-phenylcarbazol-2-yl-vinyl-2)anthracene (CZ2). 9,10-Bis(diethylphosphoryl-methyl)anthracene (0.10 g, 0.21 mmol) and M1 (0.12 g, 0.46 mmol) were dissolved in anhydrous THF (20 mL), and then potassium tert-butoxide (0.19 g, 1.67 mmol) was added under nitrogen. The suspension was stirred for 6 h at room temperature and added methanol. The resulting orange solid was collected by filtration, and the crude product was purified by a column chromatography on silica gel using petroleum ether/dichloromethane (5/1) as the eluent. An orange-yellowish solid was obtained (0.12 g, 80% yield). 1H NMR (500 MHz, CDCl3): δ 8.44 (d, 4H), 8.18 (m, 4H), 7.98 (d, 2H), 7.46 (d, 2H), 7.63 (t, 10H), 7.45 (m, 12H), 7.05 (d, 2H) ppm. HRMS (MALDI-TOF): m/z 713.8 [M+, calcd 712.29]. Anal. Calcd for C54H36N2: C, 90.98; H, 5.09; N, 3.93. Found: C, 91.07; H, 5.02; N, 3.90. 9,10-Bis[(N-phenylcarbazol-3-yl-vinyl-2]anthracene (CZ3). This compound was synthesized by the same procedure as described for CZ2 except that M2 was used instead of M1. Yellow solid was obtained (0.11 g, 73% yield). 1H NMR (500 MHz, CDCl3): δ 8.52 (s, 4H), 8.45 (s, 2H), 8.24 (d, 2H), 8.01 (d, 2H), 7.82 (d, 2H), 7.65 (d, 8H), 7.51 (d, 8H), 7.46 (s, 4H), 7.34 (d, 2H), 7.17 (d, 2H) ppm. HRMS (MALDI-TOF): m/z 713.8 [M+, calcd 712.29]. Anal. Calcd for C54H36N2: C, 90.98; H, 5.09; N, 3.93. Found: C, 90.71; H, 5.14; N, 3.97. 9,10-Bis(4-(carbazol-9-yl)styryl)anthracene (CZ9). CZ9 was synthesized by the same procedure as described for CZ2 except that M3 was used instead of M1, which affords yellow solid (0.13 g, 87% yield). 1H NMR (500 MHz, CDCl3): δ 8.48 (s, 2H), 8.18 (d, 4H), 8.10 (s, 1H), 8.05 (s, 1H), 7.94 (d, 2H), 7.70 (d, 4H), 7.51 (m, 12H), 7.33 (m, 6H), 7.21 (s, 1H), 7.11 (s, 1H), 7.06 (s, 1H), 6.91 (s, 1H) ppm. HRMS (MALDITOF): m/z 713.8 [M+, calcd 712.29]. Anal. Calcd for C54H36N2: C, 90.98; H, 5.09; N, 3.93. Found: C, 91.04; H, 5.05; N, 3.96.

Figure 1. Fluorescence images of isomers CZ2, CZ3, and CZ9 in THF solution and THF/water (1/9) dispersion at 1 × 10−5 M under illumination with a 365 nm UV lamp.

weakest fluorescence, implying that the symmetric introduction of a substituent relative to the intramolecular torsion motion axle could promote the nonirradiative decay (steric effect). When a large amount of water was added into the THF solution, the resulting aqueous dispersion shows stronger fluorescence emission, indicative of AEE effect. This is ascribed to aggregation-intensified the intermolecular interactions, which restricts the intramolecular vibration and rotational motions. Among them, the emission wavelength of CZ9 dispersion is the shortest, implying that this linking way of N-phenylcarbazole unit could afford weaker electron-donating ability to conjugation backbone (electronic effect) and/or render the molecules more twisted conformation in aggregate states (steric effect). Figure 2 shows the diffuse reflectance absorption and the emission spectra of three as-prepared isomer solids. The absorption bands of CZ2 and CZ3 are red-shifted compared to that of CZ9, which evidences that CZ9 has probably more twisted backbone conformation and/or weak intramolecular charge transfer (ICT) effect. It is observed that the peak emission wavelengths are increased in the order of CZ9, CZ3, and CZ2, whereas the fluorescence quantum yields are in the opposite order, i.e., ΦCZ9 (34.3%) > ΦCZ3 (26%) > ΦCZ2 (21%). This could be explained by their molecular structure features. The N-phenyl unit in N-phenylcarbazole should have a twisted conformation due to the intramolecular steric hindrance, which is not advantageous to ICT process and makes CZ9 show blue-shift emission. For CZ2 and CZ3, the linking positions are at planar carbazole ring, which will enhance ICT effect by extending the effective conjugation length (especially for CZ2) and promoting the electrondonating ability (especially for CZ3). The stronger ICT effect could decrease the bandgaps between ground and Franck− Condon states, red-shift the emission spectra, and facilitate nonirradiative decay while weakening the fluorescence efficiency of CZ2 and CZ3. Nevertheless, the three isomers could all exhibit AEE effect in both aqueous dispersions and solid states. Isomeric Effects on Solid-State Fluorescence and Piezofluorochromism. The powder wide-angle X-ray diffraction (PWXD) patterns of as-prepared isomer solids are shown in Figure 3, in which multiple sharp and intense

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RESULTS AND DISCUSSION Aggregation-Enhanced Emission Effect. Because of strong steric hindrance between 1,4,5,8-positions of anthracene ring and β-hydrogen of vinyl bonds linked at 9,10-positions of 18670

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Figure 2. Diffuse reflectance absorption (a) and emission (b) spectra of as-prepared isomer solids.

minimize its amount). As expected, the fluorescence colors of the pressed samples can be changed back to original colors by heat-annealing or solvent-fuming, and repressing still gives the fluorescence color as the first pressing (Figure 4). These observations demonstrate that CZ2, CZ3, and CZ9 are all PFC materials and exhibit reversible stimulus-response behavior. PWXD experiment on ground isomer solid indicates that the grinding has changed the aggregate structures (Figure 3). The diffractograms of the ground samples display broad and featureless reflections along with a series of overlapped peaks, reflecting notable amorphous features. The fluorescence efficiencies (Φ) of ground CZ3, CZ2, and CZ9 solids are 22%, 18%, and 9%, respectively. These values are lower than those of the crystalline states, especially for CZ9 (vide ante), implying that the organic luminogens with strong CEE and high crystalline Φ do not necessarily provide highly fluorescent amorphous state. Thus, the three isomers are all CEE dyes. It is also noteworthy that, to a given isomer, the crystalline fluorescence emission is blue-shifted compared to its amorphous one, which implies that the molecular conformation is more twisted in crystalline than amorphous states. Unfortunately, we are not able to obtain their single crystals suitable for crystal structure analysis. On the other hand, the real reason why the fluorescence efficiencies of CZ9 solid are the highest in crystalline state but the lowest in amorphous state is unclear at present. The normalized emission spectra of the three isomers mixed with KBr under various external stimuli are depicted in Figure 5. The emission peak wavelength of the pressed CZ2, CZ3, and CZ9 samples are 600, 593, and 565 nm, respectively. After annealing, they become 562, 552, and 528 nm, respectively, which are qualitatively consistent with those of as-prepared solids (Figure 2). These observations indicate that both pressed and annealed samples show blue-shifted emissions from CZ2 to CZ3 and then to CZ9, which makes their piezofluorochromic shifts (ΔλPFC = λpressed − λannealed) almost same (37−41 nm). Isomeric Effects on Electroluminescence Performance. The electroluminescent (EL) properties of organic luminogens are affected by many factors, such as the chemical structure of the molecules, the aggregation morphology of the emitting layer, and the device configurations. In order to examine isomeric effects on EL performance, devices with the same configuration of ITO/PEDOT:PSS (40 nm)/NPB (40 nm)/emitting layer (50 nm)/TPBi (50 nm)/LiF (0.5 nm)/Al (100 nm) have been fabricated, in which CZ2, CZ3, and CZ9 are used as the emitting layers, respectively. Figure 6a depicts

Figure 3. Powder X-ray diffraction patterns of as-prepared and ground CZ2, CZ3, and CZ9 solids at room temperature.

reflection peaks are observed, indicating the formation of ordered microcrystalline structure. Crystalline CZ2, CZ3, and CZ9 solids emit orange-yellow, yellow, and green fluorescence, respectively. However, when the isomer solids are ground with a spatula on glass plate, their fluorescence colors are obviously red-shifted (Figure 4), indicating a piezofluorochromic (PFC) phenomenon. Pressing experiments with IR pellet press (1 min at 1500 psi) afford same results (isomer is mixed with KBr to

Figure 4. Images of CZ2, CZ3, and CZ9 under a 365 nm UV lamp. (a) Pristine and ground powders. (b) Fluorescence colors upon brief pressed, annealing (2 min, at 150 °C, repressed and solvent-fuming (30 s on CH2Cl2)). 18671

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Figure 5. Normalized fluorescence emission spectra of CZ2, CZ3, and CZ9 solids upon grinding, annealing, repressed, and solvent-fuming. The experimental conditions are the same as for Figure 4.

Figure 6. (a) Current density−luminance−voltage (J−L−V) and (b) the luminous efficiency−power efficiency−current density (LE−PE−J) characteristics of the ITO/PEDOT:PSS (40 nm)/NPB (40 nm)/EML (50 nm)/TPBi (40 nm)/LiF (0.5 nm)/Al (100 nm).

To get insight into the significant isomeric effects, the electrochemical behaviors of the three isomers are investigated and the corresponding cyclic voltammograms are shown in Figure 7a. The reversible oxidative and reductive behaviors are observed and indicate that they could be utilized as the hole and electron transporters. The oxidative and reductive potentials are measured as Eox = 0.43, 0.61, and 0.67 eV and Ered = −1.81, −1.78, and −1.74 eV for CZ3, CZ2, and CZ9, respectively, implying that these isomers have smaller difference

the current density−luminance−voltage characteristics. The maximal luminance are 13370, 6280, and 550 cd cm−2, and the turn-on voltages are 3.2, 4.6, and 7.8 V for the devices with CZ3, CZ2, and CZ9 as emitting layer, respectively. Figure 6b shows that their luminous efficiencies are 0.76, 3.08, and 0.11 cd A−1 for CZ2-, CZ3-, and CZ9-based devices, respectively. These results distinctly reveal that the linking positions of Nphenylcarbazole could powerfully affect the EL performance of the isomeric luminogens. 18672

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Figure 7. (a) Cyclic voltammograms for CZ2, CZ3, and CZ9. (b) Energy diagram of the device. (c) EL spectra with the CZ2, CZ3, and CZ9 as emitting layer.

three isomers although their charge mobility is not determined because of our limited conditions. Based on their tremendous difference in EL performance, CZ3 with strong ICT effect, higher HOMO energy level, and more nonsymmetric backbone could exhibit better charge-transporting properties.

in n-doping than p-doping. Based on the onset oxidative and reductive potentials, their HOMO and LUMO energy levels are estimated to be −5.0, −5.18, and −5.24 eV (HOMO) and −2.84, −2.87, and −2.91 eV (LUMO) for CZ3, CZ2, and CZ9, respectively (Figure 7b). Thus, the corresponding bandgaps are calculated as 2.16, 2.31, and 2.33 eV, respectively, which are qualitatively consistent with the respective EL spectra (Figure 8). Since the HOMO energy level of hole-transporting layer

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CONCLUSIONS We have designed and synthesized three 9,10-bis(arylvinyl)anthracene isomers by changing the linking positions of peripheral N-phenylcarbazole units to demonstrate the isomeric effects on optical and optoelectronic properties. The changes of N-phenylcarbazole position could alter molecular backbone symmetry and intramolecular charge-transfer effect; thus, their solid-state fluorescence and electroluminescent properties exhibit remarkable isomeric effects. The three isomers are all aggregation-enhanced emission and piezofluorochromic dyes, and CZ9 exhibits the strongest and weakest fluorescence in crystalline and amorphous states, respectively, implying that highly fluorescent crystalline dyes do not necessarily provide highly fluorescent amorphous state. The most remarkable isomeric effect is their EL properties and in which the luminance, EL efficiency, and turn-on voltage of the device with CZ3 as emitting layer are greatly superior to those with CZ9 as emitting layer under the same device configuration. These findings suggest that subtle manipulation of end groups capped at 9,10-divinylanthracene could significantly alter and tune the optical and optoelectronic properties.

Figure 8. EL spectra with the CZ2, CZ3, and CZ9 as emitting layer.

NPB (−5.24 eV) and the LUMO energy level of electrontransporting layer TPBi (−2.70 eV) are respectively lower and higher than the corresponding those of isomers, it could be considered that there are no barriers for hole injection from NPB and electron injection from TPBi to emitting layer. Thus, the tremendous difference for three isomers in EL performance could be ascribed to their chemical structures and aggregate properties. In this context, the fluorescence efficiency and charge-transporting ability of emitting layer should play a crucial role. It is known that vapor deposition method commonly affords amorphous film, and the qualitative consistency between the EL spectra and the fluorescence spectra of ground isomer solids could evidence the amorphous features of evaporation films. Since the fluorescence quantum yields of three isomers in amorphous states have been measured to be ΦCZ3 > ΦCZ2 ≫ ΦCZ9 (vide supra), it is reasonable that the emitting layer with lower Φ gives lower EL performance. On the other hand, the linking positions of Nphenylcarbazole unit should affect charge-transporting ability of

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (W.Y.). Author Contributions

S.X. and W.L. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the National Science Foundation of China (51173092, 51303091), the Natural Science Foundation of Shandong Province of China (ZR2012EMQ003), and the Natural Science Foundation of Qingdao City of China (13-1-4-207-jch). We thank the Specialized Research Fund for the Doctoral Program of Higher Education (No. 20133371912 0005) and the State Key 18673

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The Journal of Physical Chemistry C

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Laboratories of Supra-molecular Structure and Materials of Jilin University (SKLSSM201403) and Luminescent Materials and Devices of South China University of Technology (2014skllmd-08).

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