ARTICLE pubs.acs.org/JPCC
Bent-Core-Liquid-Crystalline-Based Smart Material with Switchable Photoluminescence in Two Distinct Modulating Modes Mingjun Teng,† Dan Wang,† Xinru Jia,*,† Xing Fan,† Guichao Kuang,† Xiaofang Chen,*,† Dechun Zou,† and Yen Wei‡ †
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of the Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China ‡ Department of Chemistry, Tsinghua University, Beijing 100084, China
bS Supporting Information ABSTRACT: We reported herein a smart material (BCLC-FM) with tunable emission by combining a fluorescent molecule (FM) with a bent-core liquid crystal (BCLC). The photoluminescence of BCLC-FM could be modulated in two distinct switching ways by changing the transmittance under different electric fields. The two distinct operating modes are interchangeable and thermoerasable, which may provide a novel designing strategy to realize the multiswitching modes of photoluminescence of organic compounds.
1. INTRODUCTION Dynamically modulating the photoluminescence of organic materials under the external stimuli such as temperature,1 chemicals,2 forces,2e,f,3 and light4 has been an attractive prospect and investigated extensively because of their wide applications in a variety of fields. However, the voltage-responsive photoluminescent materials are rarely reported for the requirements of sophisticated equipments and lack of feasible design principle, although the electric stimulus is facile and easy handled.5 Recent development of liquid-crystalline (LC) physical gels provides a new opportunity to tune the luminescent property of organic materials by exploiting different optical multistability of LC6 and light scattering of LC multidomains.7,8 For example, Zhao and Kato independently reported that the emission intensity of luminescent LC physical gels could be switched by electric fields.8a,d Specifically, Zhao demonstrated that the photoluminescent intensity of quantum dots could be modulated by voltages based on either self-assembly of gelators or cross-linked polymers.8b Their studies show novel ways to control electrically the photoluminescence of solid materials, whereas developing smart materials with switchable photoluminescence by different and interchangeable modes still remains challenge. Bent-core liquid crystals (BCLCs), showing distinct polar mesophases, unique supramolecular chirality, multistability, and fast electric responsive properties,9 have gained much attention for several years. The extensively studied B2 phase of BCLCs has four basic structures10,11 of chiral (SmCsPF and r 2011 American Chemical Society
SmCaPA) and racemic (SmCsPA and SmCaPF) states based on the tilting and polar direction in the neighboring layers. (The subscripts “s” and “a” represent synclinic and anticlinic arrangement of the adjacent layers, whereas the “F” and “A” represent ferroelectricity and antiferroelectricity, respectively.) The four basic structures are switchable under different external electric fields,10 which could be identified not only by the LC texture but also by the transmittance alternation. In general, SmCsPF and SmCsPA show a highly scattering state, whereas SmCaPA and SmCaPF display high transmittance in some BCLCs.12 Inspired by the characteristic of B2 phase of BCLCs, we aim to develop an organic material with adjustable photoluminescence through a simple and interchangeable way under electric field. Our designing strategy here is to combine a BCLC 1 with a fluorescent molecule FM 2 (BCLC-FM, Scheme 1), by which we expect to modulate and switch the photoluminescent intensity of FM 2 in two different ways via controlling the orientational state of BCLC 1. The molecular structure of FM 2 should be carefully designed to keep the phase separation and not to depress the B2 phase of BCLC 1 (vide infra). As a proof of our concept, the photoluminescent intensity of FM 2 was successfully modulated in two distinct and interchangeable ways by electric-field-controlled chirality orientation of BCLC 1. Received: June 3, 2011 Revised: October 6, 2011 Published: October 24, 2011 22540
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2. EXPERIMENTAL SECTION General. 1H NMR was recorded on 400 MHz (Bruker
ARX400), and 13C NMR spectra were recorded on Bruker 100 MHz spectrometer at room temperature with CDCl3 as the solvent and tetramethylsilane (TMS) as the internal standard. Scheme 1. Chemical Structures of BCLC 1 and FM 2
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High-resolution mass spectra (HRMS) were acquired on a Bruker Apex IV FTMS mass spectrometer. UV vis spectra were acquired on a Varian CARY 1E or Lambda 35 Perkin-Elmer UV vis spectrophotometer. Differential scanning calorimetry (DSC) measurements were carried out on TA Instruments DSC Q100 at the atmosphere of N2 with flowing rate of 50 mL/min. Polarized optical microscopy (POM) images were recorded using a Leica DLMP microscope. Confocal laser scanning microscopy (CLSM) measurement was performed in A1R-Si instrument. Wide-angle X-ray diffraction (WAXD) examination was conducted with a Bruker GADDS D8 discover X-ray diffractomer. Switching current was measured by applying triangular voltage wave using Instec automatic LC tester (ALCT) equipment. The synthesis route is shown in the Scheme 2. The compound 1 was synthesized according to the similar methods in the literature and our previous studies13a d with minor modification. 5 (1.31 g, 0.0022 mol), 3 (0.63 g, 0.0022 mol), DCC (0.45 g, 0.0022 mol), and DMAP (0.027 g, 0.22 mmol) were dissolved in dichloromethane. The solution was stirred at room temperature for 3 h. The precipitate was filtered and evaporated under the vacuum. The obtained solid was purified by silica gel column chromatography with dichloromethane as the eluent to yield 1.82 g (96%) of 1. 1H NMR (400 MHz, CDCl3 TMS, T = 298 K): 8.32 6.97 (m, 20H, Ar H), 4.07 4.03 (m, 4H, OCH2 ),
Scheme 2a
a (a) K2CO3, acetone, reflux; (b) KOH, ethanol, reflux; (c) DCC, DMAP, CH2Cl2 rt; (d) H2, 5% Pd C, dioxane, rt; (e) DEAD, PPh3, THF, reflux; (f) p-sulfonbenzene acid, chloroform, reflux; (g) Mg, THF, reflux; (h) Trimethyl borate, THF, 78 °C; and (i) Pd(PPh3)4, NaHCO3, H2O, glyme, reflux.
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Scheme 3
2.22 2.17 (m, 2H, CHtC CH2 ), 1.95 (t, 1H, CHtC ), 1.86 1.79 (m, 4H, OCH2 CH2), 1.56 1.27 (m, 30H, CH2 ), 0.89 (t, 3H, CH3). 13C NMR (100 MHz, CDCl3 TMS, T = 298 K): 164.94, 164.50, 164.34, 163.86, 163.60, 155.45, 151.35, 150.88, 142.19, 137.78, 132.44, 132.33, 131.85, 129.86, 128.26, 126.89, 122.20, 122.14, 121.50, 120.99, 120.56, 120.44, 114.45, 114.34, 84.75, 77.23, 68.42, 68.33, 68.10, 31.93, 29.67, 29.65, 29.60, 29.57, 29.39, 29.36, 29.30, 29.10, 29.03, 28.72, 28.48. 26.00, 22.70, 18.41, 14.13 HR-ESI MS C56H64O8: m/z calcd, 864.4601; found, [M + Na]: 887.4493. Gly-Asp G2-NH3+CF3COO (200 mg, 0.20 mmol),13e K2CO3 (0.030 g), and triethylamine (TEA) (0.5 mL) were mixed in dichloromethane (30 mL) with stirring. Dansyl chloride (62 mg 0.23 mmol) was added after 10 min. The reaction mixture was stirred for 1 day and concentrated under vacuum. Pure 2 (green solid) was obtained by the purification of silica gel column chromatography using CHCl3/CH3OH (10:1) as the eluent (Scheme 3, yield: 70%). 1H NMR (400 MHz, CDCl3 TMS, T = 298 K): 8.53 (d, 1H, Ar H), 8.30(d, 1H, Ar H), 8.18 (d, 1H, Ar H), 8.08 (d, 1H, Ar H), 7.75 7.12 (m, 27H, Ar H, NH ), 6.77 (t, 1H, Ar H), 5.07 4.97 (m, 8H, CH2 Ar), 4.96 4.76 (m, 3H, CH ), 4.04 3.79 (m, 4H, CH2 ), 3.58 3.53 (m, 2H, CH2 ), 3.02 2.92 (m, 4H, CH2 ), 2.86 (s, 6H, CH3), 2.81 2.60 (m, 2H, CH2 ). 13C NMR (100 MHz, CDCl3 TMS, T = 298 K): 171.44, 171.17, 170.59, 170.55, 170.50, 169.50, 169.40, 169.30, 151.81, 135.35, 135.29, 135.11, 135.03, 133.79, 130.58, 129.80, 129.42, 128.42, 128.24, 128.19, 128.14, 128.06, 128.03, 123.10, 118.79, 115.24, 67.41, 67.36, 66.76, 66.70, 50.61, 48.85, 48.81, 45.75, 45.27, 43.06, 37.35, 36.10. HR-ESI MS C58H61N7O15S m/z calcd, 1127.3946; found, [M + H]: 1128.4015, [M + Na]: 1150.3849. Preparation of BCLC-FM. The sample was prepared on a small scale. In general, the weighed BCLC 1 and FM 2 were dissolved in chloroform to obtain a homogeneous solution and then dried in vacuum overnight to remove the solvent. For example, 10 mg of BCLC 1 and 0.5 mg of FM 2 were mixed together to prepare the sample containing 5 wt % FM 2, which were adequate to fabricate a LC cell device. Characterization of Transition Temperature. The phase transition temperature of the samples was measured by DSC measurement accompanied by POM measurement. The heating and cooling rate was 10 °C/min.
Measurement of Electro-Optical Property. ITO (indium tin oxide) glass sandwich cells (2 cm 2 cm 8 μm) coated with parallel rubbing polyimide layers were used in the electro-optical and fluorescence measurements. The sample was heated to its isotropic state and introduced to the ITO-coated cell and then cooled to the required temperature. The change of optical transmittance was measured by using a He Ne laser at the wavelength of 633 nm as the incident light and a high-speed photodiode combined with a digital oscilloscope. For the electroswitching of emission intensity and optical transmittance, a highvoltage generator (Keithley 238) was used to apply a square-wave electric field (200 ms duration) to the sample. The device from BCLC-FM can properly work in the temperature ranging from 50 to 70 °C. The optimal temperature for voltage-switching is 65 °C, which is evidenced by a series of experiments. All measurements were performed at 65 °C. The visual image of transmittance change was taken on the hot-stage, and the LC cell was located on transparent glass plate (thickness of 6 mm), which contacted with hot stage. Fluorescence Measurements. Fluorescence emission measurements were carried out using a Hitachi F-4500 fluorescence spectrophotometer. The cell was located in the position in which its normal makes an angle of 45° both to the incident light and emission path to the photodetector. A heater was used to control the temperature of the LC cell. The electric field switching fluorescence spectra were measured at the temperature of 65 °C. The variation of emission intensity was realized by applying voltages generated by Keithley 238 high-voltage generator.
3. RESULTS AND DISCUSSIONS BCLC 1 was designed and synthesized according to the similar procedures reported in the literatures and our studies.13a d POM, DSC, and X-ray diffraction (XRD) were implemented to investigate the liquid crystal property of BCLC 1. The melting point of BCLC 1 appears at 91.5 °C, and two phase transitons including isotropic-to-LC and LC-to-crystal transtion are 81.5 and 48.5 °C, respectively (Figure S1 of the Supporting Information). Strong birefringence was observed upon cooling the sample from the isotropic state. The typical B2 phase texture of fingerprint and circular domains was identified under POM between 81.5 and 48.5 °C, as shown in Figure 1. XRD patterns at 22542
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Figure 1. POM picture of 1 taken at 65 °C.
the mesophase exhibited two clear diffraction peaks in the 2θ = 2.54 and 5.13°, respectively. According to Bragg diffraction equation, the values of d spacing are 3.4 and 1.7 nm, which can be assigned to the (100) and (200) reflection of the smectic layers (Figure 2a). The d spacing of (100) reflection corresponds to the interlayer distance (3.4 nm), which was shorter than the extended molecular length (L, as shown in Figure S2 of the Supporting Information), indicating the tilted arrangement of the molecules. The tilted angle of the molecules to the layer is 40° calculated by the equation of sin 1(d/L). The B2 phase and the antiferroelectric (AF) behaviors of BCLC 1 were identified by triangular voltage wave experiments. Two current peaks were detected per half period of triangular wave, revealing a tristable switching and AF switching behavior (Figure 2b). The spontaneous polarization (Ps) was calculated with the value of 500 nC/cm2. The ground state of BCLC 1 was assigned to the racemic synclinic AF state (SmCsPA) because of the opaque LC cell without the voltage.14 The doping dye of FM 2 was designed with polypeptide as building blocks because amino acids derivatives were suitable alternatives for the fabrication of light scattering electro-optic material because of its easily controlled microphase-separated morphology by varying concentration as reported in the literature.7a FM 2 was synthesized by a convergent approach and further modified with a dansyl group at the focal point so as to endow the molecule with green luminescence (a broad emission band around 520 nm) (Figure S3 of the Supporting Information). The readily modulating morphology may provide the possibility to optimize the device performance as demonstrated below. In DSC profile, FM 2 only showed a glass transition temperature around 43 °C (Figure S4 of the Supporting Information). The BCLC-FM was prepared by combining a small amount of FM 2 (2 6 wt %) with BCLC 1; the mixture sample was then introduced to the ITO cell in its isotropic state. (See the Experimental Section.) As shown in Figure S1 of the Supporting Information, the phase-transition temperatures of BCLC 1 showed no significantly change after doping with FM 2 (
