Synthesis, Fluorescence Properties and Theoretical Calculations of Novel Stilbene Derivatives Based on 1,3,4-Oxadiazole Bearing Anthracene Core
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- Li, X., Lu, H., He, D. et al. J Fluoresc (2013) 23: 1039. doi:10.1007/s10895-013-1231-9
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Two novel stilbene derivatives bearing anthracene core based on 1,3,4-oxadiazole were efficiently synthesized and characterized by 1H-NMR, mass spectrometry and elemental analysis. The optical properties of the title compounds were investigated by UV–vis absorption and fluorescence emission spectra in different solvents. Chemical calculations were performed by density functional theory (DFT) at the (B3LYP)/6-31G* level. The results show the two compounds exhibit strong green fluorescence emission ranged from 489–493 nm, and the fluorescence quantum yield ranged from 0.78–0.92. Their HOMO and LUMO levels are (−5.44 eV, −2.25 eV) and (−5.45 eV, −2.28 eV), respectively. The influence of the solvent on the fluorescence intensities was also discussed.
Over the past few years, π-conjugated organic materials have attracted much attention due to the increasing development of potentially active components for a wide range of electronic and optoelectronic devices . As an important class of five-membered hetero cyclic compounds, 1,3,4-oxadiazoles are well-known not only for their considerable biological and medical activities, which include anti-inflammatory, antifungal, anti-HIV, anticancer, and antimicrobial activities [2–5], but also for their sufficient fluorescence in the visible light range, large Stokes shifts, high quantum yields [6–8], and so on. Another feature of the 1,3,4-oxadiazole derivatives is that the photophysical and spectroscopic properties can be readily modified by the introduction of substituents in the structure of the oxadiazole, giving more flexibility to fit well in various applications . In addition, oxadiazole unit can improve electron injection and transport properties of the molecules. Thus, many 1,3,4-oxadiazole derivatives with good hole-transporting capabilities and high fluorescence emission, have actually been used as electron-injection materials . Nowadays, compounds with blue luminescence are highly desirable because of their wide applicability in organic materials to tune the emission colours in the entire region of visible spectrum . The fluorescent characteristics mainly depend on molecular structure, such as conjugate system, coplanarity and rigidity. The longer the conjugate system is, the stronger the fluorescent intensity and greater emission wavelength will be . Anthracene derivatives have rigid structure, wide energy gaps and high fluorescent quantum efficiency [13, 14]. Moreover, anthracene derivatives have been extensively utilized as fluorescence sensors for metal ions and biological molecules . Great deals of compounds which contain anthracene core electroluminescent materials have been developed till now.
Melting points were determined using RY-1 melting point apparatus and were uncorrected. 1H NMR spectra were recorded in CDCl3 on a Bruker AVANCE-400 MHz NMR spectrometer using TMS as internal standard. Mass spectra were obtained with a HPLC/MS LCQDECA spectrometer(APCI). Elemental analyses were performed on a Vario EL Ш CHN elemental analyzer. UV–vis absorption spectra were recorded on a Hitachi UV-3010 spectrophotometer. Fluorescence spectra were obtained on a Hitachi F-4500 spectrophotometer at room temperature.
Synthesis of 2-(anthracen-9-yl)-5-(p-tolyl)-1,3,4-oxadiazole (1)
Synthesis of 2-(anthracen-9-yl)-5-(4-(bromomethyl)phenyl)-1,3,4-oxadiazole(2)
Synthesis of 4-(5-(anthracen-9-yl)-1,3,4-oxadiazol-2-yl)benzylphosphonate(3)
Intermediates of compounds 1, 2 and 3 were prepared by the reported methods .
Typical Procedure for the Synthesis of Compounds(4)
To a stirred solution of aromatic aldehydes (1.7 mmol) and the intermediate 3 (0.9 g, 1.7 mmol) in anhydrous N, N-dimethylformamide (15 mL) under nitrogen atmosphere was added dropwise a solution of t-BuOK (2 g, 3 %) in ethanol. The reaction proceeded at room temperature overnight. Then the resulting mixture was filtered and washed with ethanol. The residue was recrystallized from ethanol/DMSO.
2-(anthracen-9-yl)-5-(4-(2,4-dichlorostyryl)phenyl)-1,3,4-oxadiazole (4a), yellow crystals, yield 92.5 %; m.p. 256–257 °C; 1H NMR (400 MHz, CDCl3) δ 8.71 (s, 1H, anthracen-H), 8.21 (d, J = 8.4 Hz, 2H, C6H4 2,6-H), 8.14–8.00 (m, 4H, anthracen-H), 7.71 (d, J = 8.4 Hz, 2H, C6H4 3,5-H), 7.65 (d, J = 8.5 Hz, 1H, C6H3 6-H), 7.61–7.52 (m, 4H, anthracen-H), 7.57 (d, J = 16.2 Hz, 1H, CH=CH), 7.44 (d, J = 2.1 Hz, 1H, C6H3 3-H), 7.28 (d, J = 8.5 Hz, 1H, C6H3 5-H), 7.12 (d, J = 16.3 Hz, 1H, CH=CH); APCI MS: m/z, 493(M+, 100), 495(M+2, 80), 496(M+3, 20); Anal. Calcd. for C30H18Cl2N2O (493.2):C, 73.03; H, 3.68; N, 5.68; Found: C, 73.41; H, 3.65; N, 5.72.
2-(anthracen-9-yl)-5-(4-(3,4-dichlorostyryl)phenyl)-1,3,4-oxadiazole (4b), yellow crystals, yield 91.2 %; m.p. 253–254 °C; 1H NMR (400 MHz, CDCl3) δ 8.71 (s, 1H, anthracen-H), 8.20 (d, J = 8.2 Hz, 2H, C6H4 2,6-H), 8.15–8.02 (m, 4H, anthracen-H), 7.68 (d, J = 8.3 Hz, 2H, C6H4 3,5-H), 7.63 (s, 1H, C6H3 2-H), 7.61–7.51 (m, 4H, anthracen-H), 7.45 (d, J = 8.3 Hz, 1H, C6H3, 5-H), 7.37 (d, J = 8.4 Hz, 1H, C6H3 6-H), 7.14 (s, 2H, CH=CH); APCI MS: m/z, 493(M+, 100), 495(M+2, 70), 496(M+3, 25); Anal. Calcd. for C30H18Cl2N2O(493.2):C, 73.03; H, 3.68; N, 5.68; Found: C, 72.99; H, 3.75; N, 5.76.
Results and Discussion
1H NMR Spectra
In the 1H NMR spectra of two compounds, 4a showed two fine doublets corresponding to the olefinic protons (CH=CH) from stilbene at δ: 7.57 ppm and 7.12 ppm (J = 16.3 Hz). At δ: 7.65, 7.28, and 7.44 ppm, two doublets and a singlet could be assigned to 6-H, 5-H and 3-H from aromatic protons (C6H3-protons), respectively. At δ: 8.71, 8.14–8.00, and 7.61–7.52 ppm, two multiplets and a singlet could be assigned to the protons of anthracen. In addition, another two downfield doublets were also found at δ: 8.21 and 7.71 ppm which could be assigned to the contribution of aromatic protons (C6H4-protons). In contrast, the compound 4b in its 1H NMR spectrum showed the similar resonance peaks accountable to the protons of anthracen and aromatic protons (C6H4-protons), respectively, only a singlet was found corresponding to the olefinic protons (CH=CH) from stilbene at δ: 7.14 ppm . From the 1H NMR spectra of two compounds, it was found that their olefinic protons (CH=CH) from stilbene had different coupling, which indicated the different substituents on benzene had an effect on olefinic proton-proton coupling.
The fluorescence characteristics of the title compounds in CHCl3, THF, and DMF solution at room temperature (concentration: 1 × 10−5 mol/L)
Two novel stilbene derivatives bearing anthracene core based on 1,3,4-oxadiazole were synthesized, characterized by spectral studies, theoretical calculations and their fluorescence spectra were recorded in order to study the changes in the photophysical properties with different solvents. The results showed the compounds exhibited strong green fluorescence emission with high fluorescence quantum yield (0.78–0.92). The influence of solvent on the fluorescence intensities of the compounds indicated that the solvents played an important role in determining the fluorescence intensity of the compounds. The DFT calculations establish that they possess good charge-transport characteristics. Thus, they may serve as potential applications in organic electroluminescent materials.
The authors thank the Fundamental Research Funds for the Central Universities (No.2012ZM0035) and Guangdong Provincial Natural Science Foundation of China (No.04300531) for the financial assistance.