ARTICLE pubs.acs.org/JPCC
Synthesis and Properties of Aggregation-Induced Emission Compounds Containing Triphenylethene and Tetraphenylethene Moieties Bingjia Xu, Zhenguo Chi,* Haiyin Li, Xiqi Zhang, Xiaofang Li, Siwei Liu, Yi Zhang, and Jiarui Xu* PFCM Lab, DSAPM Lab and KLGHEI of Environment and Energy Chemistry, FCM Institute, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, People's Republic of China
bS Supporting Information ABSTRACT: Aggregation-induced emission compounds containing triphenylethene and tetraphenylethene moieties with high thermal stability and good device properties have been synthesized. Their maximum fluorescence emission wavelengths are 469493 nm in solid states. The glass transition temperatures range from 138 to 180 °C and the decomposition temperatures are 495557 °C. The unoptimized device fabricated with the triphenylethene compound combined with three tetraphenylethene groups as emitters turns on at ∼6 V and the maximum luminance is observed at ∼1908 cd/m2 and 15.5 V. The electroluminescence peak of the device is at 474 nm and the Commission Internationale de l’Eclairage (CIE) chromaticity coordinate values are (0.18, 0.31) at 10 V.
’ INTRODUCTION Most of organic luminescent materials exhibit very strong luminescence in dilute solutions. However, their light emissions dramatically decrease when the solution concentrations are increased and when they are aggregated in the solid states, due to both strong intermolecular ππ stacking interactions and nonradiative decay. This is commonly known as the aggregationcaused quenching effect (ACQ effect).14 The ACQ effect, which results in the unsatisfactory efficiency of organic luminescent materials in the solid states, has greatly limited the scope of the application of such materials as emitting layer materials of organic light-emitting diode devices (OLEDs). Thus, the ACQ effect causes many organic luminescent materials to be used in light-emitting devices only as dopants.59 Doping is difficult to control in the device fabrication because the optimum dopant concentration is usually low, and the effective doping range is extremely narrow. Consequently, dopant-based OLEDs are more difficult to produce massively than OLEDs based on nondoped host emitters. Organic light-emitting materials as nondoped emitters are becoming increasingly attractive because of their high brightness and efficiency.1014 Recently, several types of organic luminescent materials, including silole, 1-cyanotrans-1,2-bis-(4-methylbiphenyl)ethylene, triphenylethene, and tetraphenylethene derivatives, were found to possess unusual luminescent properties. In these materials, light emission is enhanced from an aggregated or solid state.1524 The interesting luminescent property has been named r 2011 American Chemical Society
aggregation-induced emission (AIE) or aggregation-induced emission enhancement (AIEE), both of which have been considered to be effective ways to overcome the notorious ACQ effect. Thus, AIE and AIEE materials are very suitable to be used as nondoped emitters. Triphenylethylene derivatives, a new kind of AIE molecules with excellent thermal properties and blue light emission capabilities, have been recently synthesized and studied in our laboratory.2531 The twisted configuration of triphenylethylene has been considered to be a key structural factor for the AIE property of these derivatives. Some triphenylethylene derivatives were also found to exhibit piezochromic fluorescent properties.3234 In this work, we report three compounds, VP0-(TPE)3, VP3-(TPE)3, and VP6-(TPE)3, in which three tetraphenylethene groups were linked to phenyl-substituted ethylene with different numbers of phenyl rings. The piezochromic fluorescent and AIE properties of VP3-(TPE)3 have been reported in our previous work,33 but its thermal and device properties have not been described. These compounds exhibited strong solid light emission properties, high thermal stabilities, and good device properties, and are potential nondoped light-emitting materials for luminescent devices. Received: June 13, 2011 Revised: July 28, 2011 Published: July 28, 2011 17574
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’ EXPERIMENTAL SECTION Materials and Measurements. Bis(4-bromophenyl)methanone, diethyl 4-bromobenzylphosphonate, potassium tert-butyloxide, n-butyllithium in hexane (2.2M), diphenylmethane, 4-(4-bromophenyl)benzophenone, tetrakis(triphenylphosphine) palladium(0), trimethylborate, p-toluenesulphonic acid, tetrabutyl ammonium bromide (TBAB), and tetrabutylammonium perchlorate (electrochemical grade) purchased from Alfa Aesar were used as received. All other reagents and solvents were purchased as analytical grade from Guangzhou Dongzheng Company (P.R. China) and used without further purification. Tetrahydrofuran (THF) was distilled from sodium/benzophenone. Ultrapure water was used in the experiments. VP3Br3,26 TPV-B,35 and TPE-B36 were synthesized according to the literature methods. Proton and carbon-13 nuclear magnetic resonance spectra (1H NMR and 13C NMR)) were measured on a Mercury-Plus 300 spectrometer (CDCl3 as solvent and tetramethylsilane, TMS, as the internal standard). Mass spectra (MS) were measured on a Thermo DSQ MS spectrometer. Elemental analyses (EA) were performed with an Elementar Vario EL elemental analyzer. FTIR spectra were obtained on a Nicolet NEXUS 670 spectrometer (KBr pellet). UVvisible absorption spectra (UV) were determined on a Hitachi U-3900 spectrophotometer. Fluorescence spectra (PL) were measured on a Shimadzu RF-5301PC spectrometer with a slit width of 1.5 nm for excitation and emission. Glass transition temperatures (Tg) and melting temperatures (Tm) were determined by differential scanning calorimetry (DSC) at heating and cooling rate of 10 °C/min under N2 atmosphere using a NETZSCH thermal analyzer (DSC 204F1). Thermogravimetric analyses (TGA) were carried out using a thermal analyzer (Shimadzu, TGA-50H) under N2 gas flow with a heating rate of 20 °C/min. The fluorescence quantum yields (ΦFL) of all of the compounds in different solvents or THF/ water mixtures were evaluated using 9,10-diphenylanthracene as ref 37 .Cyclic voltammetry (CV) measurement was carried out on a Shanghai Chenhua electrochemical workstation CHI660C in a three-electrode cell with a Pt disk counter electrode, a Ag/ AgCl reference electrode, and a glassy carbon working electrode. All CV measurements were performed under an inert argon atmosphere with supporting electrolyte of 0.1 M tetrabutylammonium perchlorate in dichloromethane at scan rate of 100 mV/s using ferrocene as standard. The energy gaps (ΔEg) for the compounds between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) were estimated from the absorption edges of UVvis absorption spectra. The glass substrate precoated with indiumtinoxide (ITO) was cleaned by an ultrasonic bath of acetone, ethanol, and deionized water, and then dried in an oven at 80 °C. Surface treatment was carried out by exposing ITO to UV-ozone plasma. The electroluminescence (EL) device was fabricated as follows: The hole-transporting layer, a 50-nm thick film of 4,40 -bis (1-naphthylphenylamino) biphenyl (NPB) was deposited on the ITO surface by high vacuum thermal evaporation. Compound VP3-(TPE)3 with 30 nm thick was thermally evaporated on the NPB layer, and a 20 nm thick 8-hydroxyquinoline aluminum (Alq3) layer was then deposited as the electron transporting layer. Finally, LiF (1 nm) and Al (100 nm) were deposited on top of the organic layers by thermal evaporation. The fabricated multilayer organic light emitting devices possessed the structure
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of ITO/NPB(50 nm)/VP3-(TPE)3(30 nm)/Alq3(20 nm)/LiF(1 nm)/Al(100 nm). Each layer was deposited under a base pressure lower than 5 106 mbar with BOC Edwards Auto 500 box chamber system. The thickness of layer was taken by Dektak 6 M profilometer (Veeco). EL spectrum at 10 V electric voltage was recorded by a spectrometer (Ocean Optics USB 2000) and the CIE coordinates were calculated from the EL spectrum of the device. Moreover, the current density-luminance-voltage characteristics were determined by a Keithley SMU 236 source measure unit and a commercial luminance meter equipped with a calibrated silicon photodiode (ST-86LA, from Photoelectric Instrument Factory of Beijing Normal University, P. R. China). Synthesis of TPE-PBr. A 2.2 M solution of n-butyllithium in hexane (23.8 mmol, 10.8 mL) was added to a solution of diphenylmethane (4.00 g, 23.8 mmol) in anhydrous tetrahydrofuran (50 mL) at 0 °C under an argon atmosphere. After stirring for 1 h at that temperature, the 4-(4-bromophenyl)benzophenone (6.41 g, 19.0 mmol) was added and the reaction mixture was stirred for 10 h allowing the temperature to rise gradually to room temperature. Then the reaction was quenched with an aqueous solution of ammonium chloride and the mixture was extracted with dichloromethane. The organic layer was evaporated after drying with anhydrous sodium sulfate and the resulting crude alcohol was dissolved in toluene (100 mL). The p-toluenesulphonic acid (1.00 g, 5.8 mmol) was added, and the mixture was refluxed overnight and cooled to room temperature. The mixture was evaporated and the crude product was purified by silica gel column chromatography using n-hexane as eluent to yield a light green powder (4.15 g, 45%). 1H NMR (300 MHz, CDCl3) δ(ppm): 7.49 (d, 2H), 7.38 (d, 2H), 7.28 (d, 2H), 7.126.98 (m, 17H); 13C NMR (75 MHz, CDCl3) δ(ppm): 143.81, 143.38, 141.52, 140.52, 139.77, 137.79, 132.06, 131.94, 131.49, 128.62, 127.90, 127.83, 126.72, 126.19, 121.55; FT-IR (KBr) υ (cm1): 3025, 1595, 1478, 813, 764, 699, 578; MS (EI), m/z: 488 ([M]+, calcd for C32H23Br, 488); Anal. Calcd for C32H23Br: C 78.85, H 4.76, Br 16.39; found: C 78.71, H 4.80. Synthesis of TPE-PB. A 2.2 M solution of n-butyllithium in hexane (12.0 mmol, 5.5 mL) was added to a solution of TPE-PBr (3.90 g, 8.0 mmol) in anhydrous tetrahydrofuran (50 mL) at 78 °C under an argon atmosphere. After stirring for 3 h, trimethylborate (2.2 mL) was added, and the mixture was warmed to room temperature. The reaction was terminated and acidified with hydrochloric acid (2 M) after stirring overnight. The mixture was extracted with dichloromethane and the organic layer was dried with anhydrous sodium sulfate. After filtration and solvent evaporation, the crude product was purified by silica gel column chromatography using n-hexane/acetone (v/v = 15/1) as eluent. Light green power of TPE-PB was obtained in 60% yield (2.18 g). 1H NMR (300 MHz, CDCl3) δ(ppm): 8.25(d, 2H), 7.707.54 (m, 3H), 7.467.32 (m, 3H), 7.126.98 (m, 17H); 13C NMR (75 MHz, CDCl3) δ(ppm): 144.83, 143.83, 143.60, 141.50, 140.61, 138.60, 136.28, 134.10, 132.05, 131.52, 127.90, 126.60; FT-IR (KBr) υ (cm1): 3025, 1600, 1492, 1348, 820, 752, 700. MS (EI), m/z: 408 ([M-B(OH)2+H]+, calcd for C32H24, 452); Anal. Calcd for C32H25BO2: C 84.97, H 5.57, B 2.39, O 7.07; found: C 84.89, H 5.61. General Procedure for the Synthesis of Compounds VP0(TPE)3, VP3-(TPE)3, and VP6-(TPE)3. VP3Br3 (0.29 g, 0.59 mmol) and the corresponding boric acid (2.7 mmol) were dissolved in toluene (30 mL), and then 2 M aqueous K2CO3 solution (1.5 mL) and TBAB (0.1 g) were added. The mixture was stirred for 40 min under an argon atmosphere at room 17575
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The Journal of Physical Chemistry C temperature. Then the Pd(PPh3)4 catalyst was added and the reaction mixture was stirred at 80 °C for 16 h. After cooling to room temperature, the product was concentrated and purified by silica gel column chromatography with dichloromethane/ n-hexane (v/v; VP0 -(TPE)3 : 1/6; VP 3-(TPE)3: 1/4; VP6 (TPE)3: 1/3) as eluent. VP0-(TPE)3. Light green powder. Yield 47%. 1H NMR (300 MHz, CDCl3) δ(ppm): 7.166.90 (d, 51H), 6.886.80 (t, 3H), 6.77 (d, 2H), 6.69 (d, 2H); 13C NMR (75 MHz, CDCl3) δ(ppm): 143.52, 143.44, 143.36, 143.19, 142.65, 142.57, 141.74, 141.53, 140.85, 140.76, 140.65, 140.53, 140.36, 138.00, 135.36, 131.09, 130.84, 130.63, 129.52, 128.59, 127.76, 127.45, 126.43, 126.22; FT-IR (KBr) υ (cm1): 3023, 1600, 1495, 1445, 824, 755, 700; MS (EI), m/z: 1019 ([M+H]+, calcd for C80H58, 1018); Anal. Calcd for C80H58: C 94.26, H 5.74; found: C 94.22, H 5.76. VP3-(TPE)3. Light green powder. Yield 54%. 1H NMR (300 MHz, CDCl3) δ(ppm): 7.53 (d, 2H), 7.48(d, 2H), 7.39 (d, 2H), 7.34 (d, 3H), 7.337.28 (t, 4H), 7.25 (d,2H), 7.166.96 (m, 55H); 13C NMR (75 MHz, CDCl3) δ(ppm): 143.46, 142.70, 142.59, 142.50, 141.97, 141.53, 140.88, 140.25, 139.47, 139.26, 139.02, 138.46, 137.98, 136.08, 131.53, 131.09, 130.60, 129.71, 127.76, 127.43, 126.75, 126.24, 126.05, 125.78, 125.59; FT-IR (KBr) υ (cm1): 3023, 1600, 1495, 1445, 817, 760, 700; MS (FAB), m/z: 1247 ([M+H]+, calcd for C98H70, 1246); Anal. Calcd for C98H70: C 94.34, H 5.66; found: C 94.26, H 5.62. VP6-(TPE)3. Light green powder. Yield 51%. 1H NMR (300 MHz, CDCl3) δ(ppm): 7.72 (d, 2H), 7.687.62(m, 8H), 7.60 (s, 5H), 7.45 (m, 6H), 7.38 (m, 6H), 7.19 (d, 2H), 7.167.01 (m, 52H); 13C NMR (75 MHz, CDCl3) δ(ppm): 143.43, 142.61, 142.13, 141.59, 140.90, 140.23, 139.54, 139.36, 139.20, 139.04, 138.95, 138.55, 137.96, 136.20, 131.56, 131.08, 130.82, 130.69, 129.83, 127.88, 127.43, 126.96, 126.80, 126.51, 126.22, 125.79; FT-IR (KBr) υ (cm1): 3023, 1600, 1490, 1445, 815, 757, 700. MS (FAB), m/z: 1475 ([M+H]+, calcd for C116H82, 1474); Anal. Calcd for C116H82: C 94.40, H 5.74; found: C 94.36, H 5.57.
’ RESULTS AND DISCUSSION For the comparison purposes, the three target compounds were designed according to the following principle: three tetraphenylethene groups were linked to phenyl-substituted ethylene with different numbers of phenyl rings [VP0-(TPE)3, VP3(TPE)3 and VP6-(TPE)3]. The target compounds were synthesized by the palladium-catalyzed Suzuki coupling reactions of aryl bromides with aryl boronic acids in moderate yields (47% 54%). The aryl bromide, VP3Br3, was synthesized through simple WittigHorner reaction of the ylide reagent diethyl 4-bromobenzylphosphonate with bis(4-bromophenyl)methanone. The synthesis of aryl boronic acid TPE-PB started with diphenylmethane and used three steps. The hydroxy intermediate, TPE-P(OH)Br, was not purified further and could be used directly in the next step. After the dehydration reaction of TPE-P(OH)Br in the presence of p-toluenesulphonic acid, the bromide intermediate TPE-PBr was obtained. Aryl boronic acid TPE-PB was then prepared from TPE-PBr by lithiation with n-butyllithium and boronation with trimethylborate. The aryl boronic acids TPV-B and TPE-B were synthesized according to methods reported in the literatures.35,36 The products were purified by column chromatography on silica gel using dichloromethane/n-hexane as the eluent. The chemical structures of these products were confirmed with proton and carbon-13 nuclear
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magnetic resonance spectra (1H NMR and 13C NMR), Fouriertransform infrared spectroscopy (FT-IR), mass spectrometry and elementary analysis (Scheme 1). The lifetimes of multilayer OLED devices strongly depend on the thermal and morphological stability of the materials in each layer. During OLED operation, both joule heating, due to the insulating nature of the organic materials, and inefficient energy transfers in the devices contribute to the thermal stress on the OLED. If an OLED is heated above the glass transition temperature (Tg) of one of the organic materials in the device, then irreversible failure can occur. Thus, good thermal stability is essential and necessary for the perfect operation of an OLED device. The thermal property and the morphological stability of the synthesized compounds were investigated by DSC and TGA, respectively, and the results are listed in Table 1. The DSC curves of the samples in the first and second heating runs are shown in Figures S1 and S2 of the Supporting Information, SI, respectively. In the first heating run, only the compound VP0-(TPE)3 exhibited a melting transition peak at ca. 320 °C. In the second heating run, only a glass transition could be observed for each compound. This indicates that the compounds exhibited an extremely low tendency toward crystallization from the melts. No melting peak could be detected in the second heating runs, which was ascribed to the starburst structures. The Tg values of VP0-(TPE)3, VP3-(TPE)3, and VP6-(TPE)3 were 138, 167, and 180 °C, respectively. The values were much higher than those of the two typical AIE-active compounds, 1-methyl-1,2,3,4,5-pentaphenylsilole (MPPS, 54 °C) and 1,1,2,3, 4,5-hexaphenylsilole (HPS, 65 °C), as reported.38 As expected, the values increased with the increase in number of phenyl ringsubstituted ethylene. The thermal decomposition temperatures (Td) of these compounds, corresponding to 5% weight loss under N2 atmosphere, were in the range of 495557 °C (Table 1 and Figure S3 of the SI). The values were significantly higher than those of the reported AIE-active compounds, MPPS (309 °C) and HPS (351 °C).6 The results indicate that the compounds had very high levels of thermal stability. The thermal stability of organic compounds is critical to the stability and lifetime of photoelectric devices during fabrication and use. Thus, the high level of thermal stability suggests that these compounds could be useful as photoelectric materials. Comparing the Td values of these compounds, it can be found that the Td values also increased with the increasing number of phenyl rings. The optical properties of the products were investigated by their UV and PL spectra in tetrahydrofuran (THF) solutions and in the solid states (Table 2). The UV and PL spectra of the compounds were provided in Supporting Information (Figure S4). The maximum absorption wavelengths (λabs max) and emission wavelengths (λem max) of VP0-(TPE)3, VP3-(TPE)3, and VP6-(TPE)3 em appeared at λabs max = 328, 344, and 346 nm; λmax = 491, 470, and em 468 nm in the THF solutions; and λmax = 469, 482, and 493 nm in the solid states, respectively. The λabs max of VP0-(TPE)3, VP3-(TPE)3 and VP6-(TPE)3 in the THF solutions exhibited monotonically red-shifts with the increasing number of phenyl rings. However, the λem max showed blue-shifts in the THF solutions (Figure 1), indicating the change of the maximum absorption wavelengths of the compounds was opposite to the changing of the maximum emission wavelengths in THF solution. In many cases, the changing regularities of UV absorption and PL emission spectra are often different in solution. For emission spectrum, the electron distribution of excited state and molecular planarity strongly affect its 17576
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Scheme 1. Synthetic Routes to the Compounds
Table 1. Thermal Properties of the Synthesized Compounds °
°
Table 2. Optical Properties of the Compounds
°
compound
Tg ( C)
Tm ( C)
Td ( C)
VP0-(TPE)3
138
320
495
VP3-(TPE)3
167
N/A
540
VP6-(TPE)3
180
N/A
557
emission spectrum. In the excited state, the twisted intramolecular charge transfer (TICT) state through intramolecular rotation often occurs. From the molecular structures, it can be seen that the capability of intramolecular rotation increase with the order from VP0-(TPE)3 to VP6-(TPE)3. Thus, it is possible that in excited state VP6-(TPE)3 possesses the most twisted molecular structure and the worst molecular planarity, resulting in the lowest conjugation and the shortest emission wavelength. It can also be seen from Figure 1 that the λem max of the compounds in the solid states exhibited red-shifts. Comparing the λem max values of the compounds in THF
compound
λabs max (nm)
λem max (nm)
ΦFL (%)
a
b
c
d
e
VP0-(TPE)3
328
491
469
0.16
1.0
VP3-(TPE)3
344
470
482
0.30
3.1
VP6-(TPE)3
346
468
493
0.43
3.6
a
In THF. b In THF. c Solid State; Fluorescence quantum yields. d THF. e Cyclohexane using 9,10-diphenyl-anthracene (DPA) as a standard.
solutions and in the solid states, VP0-(TPE)3 exhibited 22 nm blueshift, and however, VP3-(TPE)3 and VP6-(TPE)3 showed 12 and 25 nm red-shifts, respectively. In addition, the time-resolved emission decay behaviors of the compounds in solid state were studied and the decay profiles are illustrated in Figure 2. The fluorescence lifetimes (τ) of the compounds from VP0-(TPE)3 to 17577
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Table 3. Energy Levels of the Compounds
Figure 1. Changes in UV and PL wavelengths from VP0-(TPE)3 to VP6-(TPE)3.
Figure 2. Time-resolved emission decay curves of the compounds.
VP6-(TPE)3 were 0.61, 1.20 and 1.24 ns, respectively, indicating that the intramolecular or intermolecular interaction was decreased with the increasing number of phenyl rings. The quantum yield values (ΦFL) of the compounds in the THF and cyclohexane (CHX) solutions (10 μM) were determined by the standard method using 9,10-diphenylanthracene (DPA) as the standard. Table 2 shows that VP0-(TPE)3, VP3(TPE)3 and VP6-(TPE)3 had low ΦFL in both good (THF) and poor solvents (CHX), with the ΦFL values ranging from 0.16% to 0.43% and from 1.0% to 3.6%, respectively. This indicates that the compounds exhibited higher fluorescence efficiency in poor solvents than in good solvents. It is considered that the solute molecules form multimer aggregates in poor solvents through intermolecular associations, causing restricted vibro-rotational motions with reduced nonradiative relaxations and increased fluorescence quantum efficiency. Whether in a good solvent or in a poor solvent, the quantum yields of the compounds showed a slight increase with an increase in the number of phenyl rings. It is well-known that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), which are related to the redox potentials, are two important parameters for electroluminescent materials on account of their relationship with the injection and transport properties of charge carrier in OLEDs. To measure the HOMO levels of the synthesized compounds, cyclicvoltammetry (CV) analyses were carried out. The CV curves of the compounds are shown in Figure S5 (Supporting Information). The HOMO energy levels were obtained using the onset oxidation potentials. The energy
compound
HOMO (eV)
LUMO (eV)
ΔEg (eV)
VP0-(TPE)3
5.45
2.49
2.96
VP3-(TPE)3
5.56
2.55
3.01
VP6-(TPE)3
5.56
2.52
3.04
band gaps (ΔEg) of the compounds were estimated from the onset wavelengths of their UV absorptions. The LUMO energy levels were obtained from the HOMO energy and the energy band gap (ΔEg = LUMO HOMO). The HOMO, LUMO, and ΔEg values are listed in Table 3. The ΔEg values of the compounds were found to be in the range of 2.963.04 eV. In addition, VP0-(TPE)3 and VP6-(TPE)3 exhibited the narrowest band gap and widest energy band gap, respectively. By comparing the ΔEg values of the compounds, it can be derived that the band gaps of the compounds were hardly affected by the number of phenyl rings. The HOMO values ranged from 5.45 to 5.56 eV. VP0(TPE)3 had the lowest HOMO level, and VP6-(TPE)3 had the highest. VP3-(TPE)3 and VP6-(TPE)3 exhibited the same HOMO level (5.56 eV). The LUMO values ranged from 2.49 to 2.55 eV. Compound VP0-(TPE)3 possessed the lowest LUMO level, while VP3-(TPE)3 possessed the highest. In order to investigate the intriguing AIE phenomenon of the compounds, spectroscopic analyses of their diluted mixtures were performed in a mixture of water and tetrahydrofuran (THF) with different water fractions. Since the samples are completely insoluble in water, they will aggregate into nanoparticles when a substantial amount of water is added to their THF solutions under sonication. Meanwhile, the corresponding changes would be reflected in ultravioletvisible (UV) and photoluminescence (PL) spectra. For the THF solution of VP6-(TPE)3, an example of the compounds, the photoluminescence spectrum is nearly a flat line parallel to the abscissa with a fluorescence quantum yield (ΦFL) as low as 0.43% (Figures 3, 4, and S6 (of the SI) shows the other compounds) . The PL intensity and the quantum yield value remained unchanged until 40% water was added into the solution. However, dramatic enhancement in luminescence was observed for the ∼50:50 (v/v) water/THF mixture. In the solvent mixture with 95% water, the PL intensity and the quantum yield of VP6-(TPE)3 rose to 397 a.u. and 29.5%, which was 38-fold and 68-fold higher than those in the pure THF solution, respectively. Similar effects were observed for VP0-(TPE)3 and VP3-(TPE)3. Nevertheless, the amount of water added to show the dramatic enhancement was not the same for each compound due to the difference in solubility. The volume fractions of water in the water/THF mixtures where the PL intensity and ΦFL could be observed with noticeable change were 60%, 50% and 40% for VP0-(TPE)3, VP3-(TPE)3 and VP6-(TPE)3, respectively. Analogously, the extents of the increase of PL intensity and quantum yield were also not the same for each compound. For PL intensity, the enhancements from VP0-(TPE)3 to VP6-(TPE)3 were about 62, 54, and 38 times, respectively. Simultaneously, the ΦFL of VP0-(TPE)3, VP3-(TPE)3 and VP6-(TPE)3 rose from 0.16%, 0.43%, and 0.43% in THF solutions to 8.3%, 15.6% and 29.5% in the solvent mixture with 95% water which were about 51-, 52- and 68-fold higher than those in THF, respectively. These results indicate that all the synthesized compounds exhibit significant AIE effect. 17578
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Figure 3. PL spectra of VP6-(TPE)3 in water/THF mixtures. The inset depicts the changes in PL peak intensity.
Figure 5. Emission images of the compounds in pure THF (up) and 95% water/THF solution (v/v) (down) under UV light (365 nm) illumination at room temperature (10 μM).
Figure 4. Photoluminescent ΦFL of the compounds in water/THF mixtures with different volume fractions of water.
It can be seen that the PL intensities and quantum yields of all of the compounds decrease in the THF/water mixtures containing 70% and/or 80% of water. This phenomenon was often observed in some compounds with AIE properties, but the reasons remain unclear. There are two possible explanations for this phenomenon:2529,39 (1) after the aggregation, only the molecules on the surface of the nanoparticles emitted light and contributed to the fluorescent intensity upon excitation, leading to a decrease in fluorescent intensity. However, the restriction of intramolecular rotations of the aromatic rings around the carbon carbon single bonds in the aggregation state could enhance light emission. The net outcome of these antagonistic processes depends on which process plays a predominant role in affecting the fluorescent behavior of the aggregated molecules; (2) when water is added, the solute molecules can aggregate into two kinds of nanoparticle suspensions: crystal particles and amorphous particles. The former leads to an enhancement in the PL intensity, while the latter leads to a reduction in intensity. Thus, the measured overall PL intensity data depends on the combined actions of the two kinds of nanoparticles. Solutions of the compounds in their good solvent of THF were extremely weakly fluorescent, as can be seen from their emission images shown in Figure 5, which were taken under 365 nm UV illumination at room temperature. Nevertheless, the compounds in the solvent mixtures with 95% water exhibited very strong fluorescence, showing noticeable AIE effect.
Figure 6. UV absorption spectra of VP6-(TPE)3 in water/THF mixtures with different volume fractions of water.
The absorption spectra of VP6-(TPE)3 were investigated to observe the absorption behaviors when different fractions of water were added to the THF solution of the compound. As can be seen from Figure 6 (Figure S7 of the SI shows the other compounds), the spectral profile of the compound remained unchanged until ∼40% of water was added. Afterward, the absorbance of the entire spectrum increased swiftly showing an absorption tail extending well into the long wavelength region, which was caused by the light scattering or Mie effect of the nanoaggregate suspensions40 in the solvent mixtures. The result implied that the VP6-(TPE)3 molecules have aggregated into nanoparticles in the aqueous mixture containing ∼50% of water. Compared to the PL emissions, the abrupt broad change in the shape of the absorbance from 50% water fraction agreed well with the sudden jump in the PL intensity and the quantum yield shown in Figure. 2 and 3, thus confirming that the emission enhancements were induced by the aggregation of the molecules. The strong blue light emission and high thermal stability of the compounds prompted us to use them to construct electroluminescence devices. As an example, a multilayer electroluminescence 17579
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fluorescence emission wavelengths were 469493 nm in solid states. The unoptimized device fabricated with the triphenylethene compound combined with three tetraphenylethene groups as emitters turned on at ∼6 V, and the maximum luminance was ∼1908 cd/m2 at 15.5 V. The electroluminescence peak of the device was at 474 nm and the CIE chromaticity coordinate values were (0.18, 0.31) at 10 V.
’ ASSOCIATED CONTENT
bS
Supporting Information. Figures S1S7. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 7. Plots of current density and luminance versus voltage in a multilayer EL device of VP3-(TPE)3; inset shows the image taken at 10 V.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected];
[email protected].
Figure 8. Electroluminescence spectrum and CIE of the device.
device, ITO/NPB(50 nm)/ Sample(30 nm)/ Alq3(20 nm)/ LiF(1 nm)/ Al (100 nm), was fabricated using vapor deposition processes. 4,40 -Bis(1-naphthylphenylamino)biphenyl (NPB) was used as the hole-transporting layer, sample VP3-(TPE)3 as the emitting layer, and 8-hydroxyquinoline aluminum (Alq3) as the electron-transporting layer. LiF was used between the electron-transporting layer and cathode Al to enhance electron injection, and indiumtin oxide (ITO) coated glass was used as the substrate and anode. The currentvoltage-luminance characteristics of the unoptimized device without the packaging are presented in Figure 7. The turn-on voltage, defined as the voltage when the luminance is 1 cd/m2, was 6.0 V, and a luminance of ∼1908 cd/m2 was obtained at 15.5 V. The electroluminescence spectrum and CIE of the device are shown in Figure 8. The electroluminescence peak was at 474 nm, which exhibited 8 nm blue-shift relative to PL spectrum of solid powder sample (482 nm). On the basis of the emission spectrum, the CIE chromaticity coordinate values were calculated to be (0.18, 0.31) at an applied voltage of 10 V for the device.
’ CONCLUSIONS We have developed a new class of compounds containing triphenylethene and tetraphenylethene moieties. The compounds possessed aggregation-induced emission properties, high thermal stability, and good device properties. The Tg temperatures and Td temperatures of the compounds ranged from 138 to 180 °C and 495 to 557 °C, respectively. Their maximum
’ ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos.: 50773096 and 51073177), the Start-up Fund for Recruiting Professionals from “985 Project” of SYSU, the Science and Technology Planning Project of Guangdong Province, China (Grant Nos.: 2007A010500001-2, 2008B090500196), Construction Project for University-Industry cooperation platform for Flat Panel Display from The Commission of Economy and Informatization of Guangdong Province (Grant No.: 20081203), the Fundamental Research Funds for the Central Universities, Natural Science Foundation of Guangdong (S2011020001190), the Opening Fund of Laboratory Sun Yat-sen University (KF201026), and the Fund for Innovative Chemical Experiment and Research of School of Chemistry and Chemical Engineering. ’ REFERENCES (1) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: London, 1970. (2) Thomas, S. W., III; Joly, G. D.; Swager, T. M. Chem. Rev. 2007, 107, 1339–1386. (3) Grell, M.; Bradley, D. D. C.; Ungar, G.; Hill, J.; Whitehead, K. S. Macromolecules 1999, 32, 5810–5817. (4) Jakubiak, R.; Collison, C. J.; Wan, W. C.; Rothberg, L. J. Phys. Chem. A 1999, 103, 2394–2398. (5) Chen, C. H.; Shi, J.; Tang, C. W. Macromol. Symp. 1997, 125, 1–48. (6) Wu, W. C.; Yeh, H. C.; Chan, L. H.; Chen, C. T. Adv. Mater. 2002, 14, 1072–1075. (7) Sato, Y. Semicond. Semimet. 2000, 64, 209–254. (8) Rees, I. D.; Robinson, K. L.; Homes, A. B.; Towns, C. R.; O’Dell, R. MRS Bull. 2002, 27, 451–455. (9) Fuhrmann, T.; Salbeck, J. MRS Bull. 2003, 28, 354–359. (10) Yeh, H. C.; Yeh, S. J.; Chen, C. T. Chem. Commun. 2003, 2632–2633. (11) Wu, W. C.; Yeh, H. C.; Chan, L. H.; Chen, C. T. Adv. Mater. 2002, 14, 1072–1075. (12) Thomas, K. R. J.; Lin, J. T.; Velusamy, M.; Tao, Y. T.; Chuen, C. T. Adv. Funct. Mater. 2004, 14, 83–90. (13) Chiang, C. L.; Wu, M. F.; Dai, D. C.; Wen, Y. S.; Wang, J. K.; Chen, C. T. Adv. Funct. Mater. 2005, 15, 231–238. (14) Islam, A.; Cheng, C. C.; Chi, S. H.; Lee, S. J.; Hela, P. G.; Chen, I. C.; Cheng, C. H. J. Phys. Chem. B 2005, 109, 5509–5517. (15) Luo, J. D.; Xie, Z. L.; Lam, J. W. Y.; Cheng, L.; Chen, H. Y.; Qiu, C. F.; Kwok, H. S.; Zhan, X. W.; Liu, Y. Q.; Zhu, D. B.; Tang, B. Z. Chem. Commun. 2001, 1740–1741. 17580
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