DING Ai-Xiang JIA Wen-Bin ZHANG Gao-Bin PAN Jian-Ting ZHANG Yu-Yang YANG Jia-Xiang

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Synthesis, Crystal Structure, and Photoluminescence Properties of One Novel D--D Type-Cyanostilbene Derivative①

DING Ai-Xiang JIA Wen-Bin ZHANG Gao-Bin PAN Jian-Ting ZHANG Yu-Yang YANG Jia-Xiang②

(230601)

-cyanostilbene, synthesis, crystal structure, UV-Vis and FL

1 INTRODUCTION

Recently, a lot of research work on-cyano- stilbene structure brings out excellent luminous cha- racteristics possessed by this kind of molecules[1-6]. As is well-known, most conventional organic lumino- phores emit strongly in dilute solutions but almost no emission in solid state and concentrated solutions, namely the notorious aggregation-caused quenching (ACQ). As a famous AIE or AIEE material,-cyano- stilbene derivatives have overcome the defect successfully and attracted wide interest. Therefore, they have been used in various fields such as organic light-emitting diodes (OLEDs)[7], liquid crystals[8], photochromism materials[9], organic gels[10, 11], bio- logical imagings[12], optoelectronic materials[13], fluorescent sensors and so on.

Intrigued by its ideal luminescent properties, a novel D--D type-cyanostilbene derivative, BPDPA, has been designed and synthesized, which is different from most D-A or D-A molecules[14, 15]. In this work, the structure of BPDPA was analyzed by X-ray single-crystal diffraction. Its molecular structure and 3-D framework were also studied. The results indicate that the molecular plane is somewhat twisted due to the additional steric interactions cau- sed by CN group, and the intermolecular hydrogen bonds, C–H···and···stacking interactions play significant roles in forming the 2-D and 3-D structures. In addition, the compound shows goodstability by thermogravimetric analysis.

2 EXPERIMENTAL

2.1 Materials and general method

All chemicals were commercially available and all solvents were purified by conventional methods before use. NMR spectra were recorded on a Bruker Advanced 400 MHz spectrometer. UV-Vis absorp- tion spectra were recorded on a TU-1901 of Beijing Purkinje General Instrument Co., Ltd, spectrometer using samples in solutions. Fluorescence measure- ments were carried out usingan Edinburgh FLS920 fluorescence spectrometer equipped witha 450W Xe lamp and a time-correlated single-photon counting(TCSPC) card. All the fluorescence spectra were collected. FT-IR spectra were obtained in KBr discs on a Nicolet 380 FT-IR spectrometer in the 4000～400 cm-1region. Thermogravimetric analysis was carried out on a Mettler-Toledo Star thermal analy- zer under N2protection at a heating rate of 20 ℃/min.

2.2 Synthesis

Scheme 1. Synthetic route for the D--D compound BPDPA

2.2.1 Synthesis of 2-(4-butoxyphenyl)acetonitrile (BPA)

2-(4-Hydroxyphenyl)acetonitrile (266 mg, 2 mmol) was added to DMF (8 mL) at 70 ℃, then K2CO3(553 mg, 4 mmol) was added quickly. After stirring for 30 min, 1-bromobutane (329 mg, 2.4 mmol) was slowly added. The mixture was stirred at 80 ℃ for 24 h, cooled to RT, and poured into water (50 mL). A yellow precipitate was thus formed. DCM (100 mL) was added to dissolve the preci- pitate. The organic layer was separated off and the aqueous phase was extracted with DCM three times (100 mL). The combined organic layers was washed with water three times (100 mL) and dried with Na2SO4. After filtering, the solvent was evaporated in vacuo. At last, a pale yellow powder was obtained with a yield of 73% (276 mg). FT-IR (KBr, cm?1): 2598 (s, CH), 2931 (s, CH), 2872 (s, CH), 2249 (w, C≡N), 1613 (m, CH), 1584 (w, CH), 1512 (s, CH), 1419 (w, CH), 1217 (m), 1174 (m), 817 (m, CH).1H NMR (CDCl3400 MHz ppm): 7.188 (d, 2H,= 8.0 Hz), 6.866 (d, 2H,= 8.4 Hz), 3.934 (t, 2H), 3.631 (s, 2H), 1.713～1.783 (m, 2H), 1.433～1.526 (m, 2H), 0.967 (t, 3H).13C NMR (CDCl3400 MHz ppm): 158.950, 129.046, 121.651, 118.285, 115.109, 67.833, 31.267, 22.745, 19.225, 13.821.

2.2.2 Synthesis of (E)-2-(4-butoxyphenyl)- 3-(4-(diethylamino)phenyl)acrylonitrile (BPDPA)

4-(Diethylamino)benzaldehyde (213 mg, 1.2 mmol) and BPA (189 mg, 1 mmol) were added to EtOH (26 mL) at 50 ℃. After the solid was dissolved, NaOH (48 mg, 1.2 mmol) dissolved in 5 mL EtOH was added quickly to the solution. The mixture was stirred for 4 h at 80 ℃, and an orange precipitate was formed and cooled to RT. The resulting precipitate was filtered off, and stirred with hot EtOH. The precipitate was hot filtered and dried in vacuo at 60 ℃ to get an orange solid with a yield of 87% (303 mg). FT-IR (KBr, cm?1): 2975 (s, CH), 2954.8 (s, CH), 2925 (s, CH), 2871 (s, CH), 2195 (s, C≡N), 1602 (s, CH), 1578 (s, CH), 1520 (s, CH), 1406 (m, CH), 1353 (s), 1288 (m), 1248 (m), 1222 (m), 1186 (s), 1156 (m), 1074 (m), 830 (m, CH).1H NMR (CDCl3400 MHz ppm): 7.801 (d, 2H,= 8.8 Hz), 7.531 (d, 2H,= 8.8 Hz), 7.264 (s, 1H), 6.914 (d, 2H,= 8.4 Hz), 6.673 (d, 2H,= 8.8 Hz), 3.980 (t, 2H), 3.383～3.436 (q, 2H), 1.740～1.810 (m, 4H), 1.453～1.546 (m, 2H), 1.198 (t, 6H), 0.981 (t, 3H).13C NMR (CDCl3400 MHz ppm): 195.075, 149.024, 140.663, 131.264, 128.008, 126.626, 121.077, 119.784, 114.813, 111.082, 103.654, 67.850, 44.500, 31.297, 19.260, 13.839, 12.631.

2.3 Structure determination

A pale yellow crystal of BPDPA with dimensions of 0.80mm × 0.70mm × 0.20mm was selected and mounted on a glass fiber. The single-crystal X-ray diffraction data were collected on a Bruker Smart 1000 CCD diffractometer equipped with a graphite- monochromatic Moradiation (= 0.71073 ?) situated in the incident beam by using an-2scan mode at 298(2) K. In the range of 1.49<<25.30o (–15≤≤13, –15≤≤16 and –18≤≤17), a total of 11990 reflections were collected, of which 7559 were independent (int= 0.0264). Unit cell dimen- sions were obtained with the least-squares refine- ments; the structure was solved by direct methods using SHELXS-97 and refined on2by full-matrix least-squares procedure with SHELXL-97 program. All non-H atoms were refined with anisotropic displacement parameters. All hydrogen atoms were located theoretically and refined with riding model position parameters and fixed isotropic thermal parameters. The final(reflections)= 0.0649 (3597),(reflections)= 0.2186 (7559).= 0.0649 and= 0.2158 (= 1/[2(F2) + (0.1000)2], where= (F2+ 2F2)/3), (Δ/)max= 0.000,= 1.067, (Δ)max= 0.219 and (Δ)min= –0.231 e/?3.

3 RESULTS AND DISCUSSION

3.1 Crystal structure analysis

Fig. 1. Crystalline molecular structure

The selected bond distances and bond angles for BPDPA are shown in Table 1. Table 2 gives the hydrogen-bonding geometry of BPDPA. The triple bond lengths of C(12)≡N(1) and C(35)≡N(3) are 1.143 and 1.141 ?, respectively, and the bond angles of N(1)–C(12)–C(11) and N(3)–C(35)–C(34) are 176.6 and 176.2°, respectively. These data are consistent with those of our reported derivates con- taining cyano group[4]. As shown in Fig. 2, there are two potential weak intermolecular interactions of C–H???O, one C(17)–H(17)???(with plane C(28)～C(33) and the distance is 3.815 ?) interaction and oneinteraction in the stacking diagram of BPDPA. The C(30)–H(30)···O(1) hydrogen-bonding interactions link the two molecules in the molecular structure and another hydrogen bond of C(21)– H(21B)···O(2) connects the adjacent molecules to form a supramolecular stacking structure. The C(17)–H(17)···and face-to-faceinteractions help rigidify the conformation. The ultima supramo- lecule formed due to the synergic effects of these weak interactions. Interestingly, the aggregate is neither traditional- nor-aggregates.

Table 1. Selected Bond Lengths (?) and Bond Angles (°)

Table 2. Hydrogen Bond Lengths (?) and Bond Angles (°)

Symmetry codes: (a) 1–x, 2–, 1–; (b) 1–, 1–, 2–

Fig. 2. 3-D stacking diagram of BPDPA viewed from the-axis, showing the weak interactions between the molecules (light green dotted line for C(30)–H(30)???O(1), sky blue dotted line for C(21)–H(21)B???O(2), pink dotted line for C(17)–H(17)???and plum dotted line forinteractions). The needless hydrogen atoms are omitted for clarity

3.2 Photophysical properties

The optical properties of D--D molecules in different solvents were studied. As shown in Fig. 3a, compound BPDPA exhibits one main peak in various solvents and generally red shifts with increa- sing the solvent polarity. In Fig. 3b, the phenomenon of FL keeps in consistence with its UV. The emis- sion has red-shifted gradually as the polarity of the solvents, from 433 to 481 nm, exhibits an obvious bathochromic effect. The maximum photolumine- scent position located in the large position could be ascribed to the intramolecular charge transfer (ICT), and the 48 nm red shift in the solvents from n-hexane to DMF could be caused by the twisted intramolecular charge transfer (TICT) in larger polar solvent.

3.3 Thermostability

The TGA curve is shown in Fig. 4. The analyzed result manifests the initial decomposition tempera- ture of BPDPA in a relative higher point, 288.8 ℃, indicating good thermal stability possessed by the material, which is necessary for perfect operation of an optical device. Therefore, BPDPA could have potential applications in the photoluminescent field with a high performance.

Fig. 3. UV-Vis (a) and FL (b) spectra, concentration: 1 M, FL was excited with each wavelength located at the maximum absorbance

Fig. 4. TGA for compound BPDPA

4 CONCLUSION

In conclusion, a novel D--D type cyanostilbene derivative has been designed and synthesized. The structure was characterized by FT-IR,1H NMR and13C NMR. The single crystal was cultivated with a method of slow evaporation. X-ray single-crystal analysis was carried out and the crystalline mole- cular structure and stacking mode were confirmed. The processing results showed an unusual aggregate formed through intermolecular weak interactions. FL study on the compound in different solvents indicated a fine photoluminescent property. Thermo- gravimetric analyses showed high initial decom- position temperature at 288.8 ℃. The good thermal stability and PL property of BPDPA made it a potential valuable material in photoelectric material field.

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18 February 2014;

22 May 2014 (CCDC 982234)

the Natural Science Foundation of Anhui Province (1208085MB21), and the National Natural Science Foundation of China (21101001)

. Yang Jia-Xiang, professor. E-mail: jxyang@ahu.edu.cn