Article pubs.acs.org/JPCC
Influence of Carbazolyl Groups on Properties of Piezofluorochromic Aggregation-Enhanced Emission Compounds Containing Distyrylanthracene Xiqi Zhang,† Zhenguo Chi,*,† Xie Zhou,‡ Siwei Liu,† Yi Zhang,† and Jiarui Xu† †
PFCM Lab, DSAPM Lab and KLGHEI of Environment and Energy Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering and ‡School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou 510275, China ABSTRACT: Distyrylanthracene derivatives (AnPCz and AnPCz3) with monocarbazolyl and tricarbazolyl groups were synthesized and characterized via nuclear magnetic resonance, mass spectrometry, elemental analysis, photoluminescence, ultraviolet absorption, wide-angle X-ray diffraction, and other techniques. Another compound, distyrylanthracene (AnP), without any carbazolyl group was synthesized for comparison. The results show that the piezofluorochromic properties of the three aggregation-enhanced emission (AEE) compounds were significantly influenced by the variety of carbazolyl groups in the molecular structures. Compounds AnPCz and AnPCz3 exhibited distinct piezofluorochromic properties and switched reversibly upon grinding− annealing or grinding−fuming. The piezofluorochromic activity can be greatly enhanced by the introduction of carbazole groups in the distyrylanthracene AEE structure. The general structure− property relationships established in this study will help researchers to identify and synthesize more piezofluorochromic materials.
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INTRODUCTION
pressurization in the solid state; however, the peak positions of both PL and EL were almost not changed with the increase in pressure. Therefore, a real fluorescence color switching compound that is responsive to pressure stimuli with AEE property was first reported by Park in 2010.24 Within the same period, a number of new fluorescence color switching compounds were synthesized in our laboratory and were named PAIE materials.25−32 The structure−property relationships of these compounds then are necessary to be established in help for the identification and synthesis of more novel piezofluorochromic materials. Recent studies in Tian’s group have demonstrated that 9,10-distyrylanthracene derivatives show high fluorescence efficiency in their solid states, and the restricted intramolecular rotations between the 9,10-anthylene core and the vinylene segment are the cause of the AIE phenomenon.33−36 In this study, a series of distyrylanthracene derivatives with different numbers of carbazolyl groups were synthesized, and their AEE and piezofluorochromic properties were characterized. It was found that the greater the number of carbazolyl groups, the more significant the piezofluorochromism would be.
Reversibly and dynamically tuning the fluorescence properties of solid-state materials for their potential applications in sensors, fluorescent probes, memory chips, and photoelectronic devices has attracted considerable attention.1−13 The newly developed piezofluorochromic materials, which show reversible luminescent color switching upon a mechanical stimulus, are particularly suited for this purpose. As previously reported, one promising approach to provide the materials with reversible luminescent color transition is to control the mode of molecular packing in the molecular assemblies by force.14 However, the aggregation-caused quenching (ACQ) effect results in a very weak fluorescence efficiency of organic piezofluorochromic materials in the solid state. In 2001, Tang et al.15 reported an important class of anti-ACQ materials, named aggregation-induced emission (AIE) materials that can emit more efficiently in the aggregated state. In 2002, Park et al.16 reported on aggregation-enhanced emission (AEE) materials similar to AIE materials. Since then, AIE (or AEE) materials have attracted considerable research attention for their potential application in various fields, such as optoelectronic devices and chemosensors.17−20 Several AIE compounds were reported by Tang’s group21,22 to have bright-dark switching properties between crystalline and amorphous states, which were termed crystallization-induced emission enhancement effect. In their amorphous states, the samples had longer emission wavelength, which was surmised that the amorphous powder might adopt a more planar conformation. To further investigate the AIE mechanism, pressure stimuli were used to hexaphenylsilole film and its device.23 It was found that photoluminescence (PL) and electroluminescence (EL) of hexaphenylsilole were enhanced by © 2012 American Chemical Society
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EXPERIMENTAL SECTION Materials and Measurements. Carbazole, 4-fluorobenzaldehyde, potassium tert-butoxide (t-BuOK), potassium iodide, potassium iodate, nitrobenzene, 9,10-bis(chloromethyl)anthracene, triethyl phosphate, and benzaldehyde purchased Received: June 29, 2012 Revised: September 26, 2012 Published: October 19, 2012 23629
dx.doi.org/10.1021/jp306452n | J. Phys. Chem. C 2012, 116, 23629−23638
The Journal of Physical Chemistry C
Article
Scheme 1. Synthetic Routes for AnP, AnPCz, and AnPCz3
Synthesis of AnP. 3 (0.24 g, 0.5 mmol) and benzaldehyde (0.13 g, 1.2 mmol) were dissolved in THF (20 mL), then t-BuOK (0.15 g) was added under Ar gas. The solution was stirred at room temperature overnight. After the solvent was removed under reduced pressure, the residue was recrystallized with THF/EtOH to give AnP (0.18 g, 94% yield). 1H NMR (300 MHz, CDCl3) δ: 6.92 (s, 1H), 6.98 (s, 1H), 7.38 (d, 2H, J = 7.5 Hz), 7.42−7.52 (m, 8H), 7.70 (d, 4H, J = 7.8 Hz), 7.91 (s, 1H), 7.96 (s, 1H), 8.40 (q, 4H, J = 3.3 Hz). MS (EI) calcd. for C30H22, 382; found, 382. Anal. Calcd for C30H22: C, 94.20; H, 5.80. Found: C, 94.22; H, 5.76. Synthesis of AnPCz. 3 (0.14 g, 0.29 mmol) and 1 (0.19 g, 0.7 mmol) were dissolved in THF (10 mL), then t-BuOK (0.10 g) was added under Ar gas. The solution was stirred at room temperature overnight. After the solvent was removed under reduced pressure, the residue was recrystallized with THF/EtOH to give AnPCz (0.16 g, 77% yield). 1H NMR (300 MHz, CDCl3) δ: 6.91 (s, 1H), 7.06 (s, 1H), 7.11 (s, 1H), 7.21 (s, 1H), 7.27− 7.38 (m, 6H), 7.40−7.63 (m, 12H), 7.69 (d, 4H, J = 8.1 Hz), 7.94 (d, 2H, J = 8.4 Hz), 8.05 (s, 1H), 8.10 (s, 1H), 8.18 (d, 4H, J = 7.5 Hz), 8.48 (q, 2H, J = 3.3 Hz). MS (EI) calcd. for C54H36N2, 712; found, 712. Anal. Calcd for C54H36N2: C, 90.98; H, 5.09; N, 3.93. Found: C, 91.02; H, 5.06; N, 3.91. Synthesis of AnPCz3. 3 (0.073 g, 0.15 mmol) and 2 (0.22 g, 0.37 mmol) were dissolved in THF (10 mL), then t-BuOK (0.10 g) was added under Ar gas. The solution was stirred at room temperature overnight. After the solvent was removed under reduced pressure, the residue was recrystallized with THF/EtOH to give AnPCz3 (0.16 g, 76% yield). 1H NMR (300 MHz, CDCl3) δ: 7.12 (s, 1H), 7.18 (s, 1H), 7.26−7.35 (m, 8H), 7.37− 7.48 (m, 14H), 7.52−7.91 (m, 18H), 8.03−8.22 (m, 12H), 8.25− 8.35 (m, 4H), 8.44−8.57 (m, 4H), 8.81 (q, 2H, J = 3.3 Hz). MS (FAB) calcd. for C102H64N6, 1372; found, 1373 [M+H]+. Anal. Calcd for C102H64N6: C, 89.12; H, 4.70; N, 6.12. Found: C, 89.03; H, 4.68; N, 6.15.
from Alfa Aesar were used as received. All other reagents and solvents were purchased as analytical grade from Guangzhou Dongzheng Company (China) and used without further purification. Intermediates 1 and 2 were synthesized according to the literature methods previously published by us.37 Intermediate 3 were synthesized according to procedures in the literature.38 Proton nuclear magnetic resonance (1H NMR) spectra were measured on a Mercury-Plus 300 spectrometer [CDCl3, tetramethylsilane (TMS) as the internal standard]. Mass spectra (MS) were measured on a Thermo DSQ EI-mass spectrometer or a VG ZAB-HS double-focusing mass spectrometer. Elemental analyses were performed with an Elementar Vario EL elemental analyzer. PL spectra were measured on a Shimadzu RF-5301pc spectrometer with a slit width of 1.5 nm for both excitation and emission. Ultraviolet−visible (UV−vis) absorption spectra were recorded on a Hitachi UV−vis spectrophotometer (U-3900). Wide-angle X-ray diffraction (WAXD) measurements were performed by using a Bruker X-ray diffractometer (D8 Advance, Germany) with an X-ray source of Cu Kα (λ = 0.15406 nm) at 40 kV and 40 mA at a scan rate of 4° (2θ) per 1 min. Time-resolved emission decay behaviors were measured on an Endinburgh Instruments spectrometer (FLSP920). The THF/water mixtures with different water fractions were prepared by slowly adding distilled water into the THF solution of samples under ultrasound at room temperature. Ground samples were prepared by grinding using a mortar and pestle. Annealing experiments were done on a hot-stage with automatic temperature control system for 5 min; the annealing temperature was 200 °C. The experiment of solvent vapor fuming treatment is filling the ground sample on a grooved glass slide, which was then placed in a large beaker saturated with CH2Cl2 vapor for 5 min at room temperature. The solid UV−vis absorption spectra experiments were done by filling the sample powder on a grooved quartz glass slide and determined by using a 60 mm integrating sphere, which was the standard fitting of the UV−vis spectrophotometer. 23630
dx.doi.org/10.1021/jp306452n | J. Phys. Chem. C 2012, 116, 23629−23638
The Journal of Physical Chemistry C
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Article
RESULTS AND DISCUSSION Structural units with monocarbazolyl or tricarbazolyl groups have been introduced to the distyrylanthracene cores to synthesize two new fluorescent compounds, which are abbreviated as AnPCz and AnPCz3, respectively. Another compound, AnP, without any carbazolyl group was synthesized for comparison. The target compounds were synthesized mainly by using Wittig reaction according to the routes shown in Scheme 1. The chemical structures of the compounds were characterized via nuclear magnetic resonance, mass spectrometry, and elemental analysis. We performed quantum mechanical computations with a Gaussian 03 software to obtain the lowest energy spatial conformation of the compounds.39 The highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) of these compounds were obtained according to the density functional method at the B3LYP/6-31G level after structural optimization (Figure 1). The HOMOs of the
solvents changed the existing compound solutions in pure THF into aggregated particles, thus changing their PL spectra. As shown in Figure 2, when the water fraction is
