Influence Of Substituents On The Fluorescence Properties Of 2-(4-substitutedphenyl)-4,5-bis (4-substituted Styryl) Imidazole

2,4,5-aryl-substituted imidazole derivative as an organic light-emitting material is a hot area of research at present. Double bond which connected with aryl in 4 and 5 position of imidazole ring can expand conjugation systerm of imidazole derivatives, and their organic light-emitting property are rarely reported.According to our previous reasearch, 18 kinds of unreported 2-aryl-4,5-bis(4-substituted styryl)imidazole compounds were synthesized and studied the influence of 2-position substituents on the fluorescence properties. The main contents are as follows:(1) 1,6-di(4-methylphenyl)-1,5-hexadiene-3,4-dione(MT-CH3) and 1,6-di(4-methoxyl phenyl)-1,5-hexadiene-3,4-dione(MT-OCH3) were synthesized with 2,3-butanedione,4-methylbenzaldehyde and 4-methoxylbenzaldehyde using piperidine as catalyzer. According to the study of the feeding mode and reaction time, it had been fuond that the yield was increased to 17.1% and 23.4% when 2,3-butanedione was added to the 4-substituted benzaldehyde and piperidine. The structures of MT-CH3 and MT-OCH3 were characterized by FT-IR,1H NMR and13 C NMR. UV analysis showed that the maximum absorption wavelength of MT-CH3 and MT-OCH3 was 341 nm and 365 nm, respectively,and the molar absorptivity was 3.17×104M-1cm-1 and 3.91×104 M-1cm-1,respectively.(2) 2-(4-substituted phenyl)-4,5-bis(4-substituted styryl)imidazoles were synthesized by MT-CH3, MT-OCH3,4-substituted benzaldehyde, and ammonium acetate. The structure and optical properties of these imidazole derivatives were characterized by FT-IR, 1H NMR and13 C NMR and UV. The maximum UV absorption wavelength was between 364 nm to 379 nm,and wasred-shifted 23 nm and 14 nm compared with MT-CH3 and MT-OCH3,respectively.The molar absorptivity was greater than 4.29×104 M-1cm-1.(3) Fluorescence properties of imidazole compounds were tested in the solvent of N,N-dimethlformamiade(DMF), ethanol and dichloromethane, which concentration were 10-3M,10-4 M and 10-5 M in each solvent, respectively. The result showed that luminescent emission wavelength between 450 nm and 570 nm, mainly in blue region, and some in green or near yellow region. Compared with the protic solvent ethanol, the emission wavelength of imidazole derivatives were red-shifted in aprotic solvent of DMF or dichloromethane;Compared with 2-phenyl imidazole compound, 2-(4-nitrophenyl) imidazole were "double" fluorescence emission which had a blue-shifted. However, the luminescent emission of2-(4-formylphenyl) or 2-(4-cyanophenyl) Imidazole compounds which has deactvating group were “single peak” and redshifted. Luminescent emission of other imidazole compoundswhich have actvating group or weak deactvating group were “double peak”, and the D-value between the double peak was 20-26 nm.(4) The influence of TBAH on 2-(4-substitutedphenyl)-4,5-bis(4-substituted styryl)imidazole was determined in DMF(10-5M). The fluorescence intensity of imidazoles which contained aldehyde group or cynao group were decreased and emission wavelength red shifted when added a little amount of TBAH. Imidazoles which cotained activiting groups or weak deactiviting groups were less than the sensitivity of imidazole contained deactiviting groups, When adding a little amount of TBAH, “double” fluorescence emission peak disappeared, but appeared “single” peak with weak fluorescence intensity and red shift. After gradually adding sulfuric acid(0.03 M), imidazole derivatives contaning both activting groups and blunt groups, which fluorescence performance recovery.(5) The influence of acid on 2-(4-substitutedphenyl)-4,5-bis(4-substituted styryl)imidazole were determined in DMF(10-5 M). The result showed that imidazole containing activiting groups were sensitive to acid, which a little amount could make“double” fluorescence emission peak decrease, and one of the peak decreased rapidly. When added more, “double” peak decreased, and appeared red shift “single” peak. A little amount of acid could make the fluorescence intensity of imidazole containing aldehyde group and cynao group decreased, but emission wavelength of imidazole which 2-position benzene with aldehyde group had a blue shift.(6) According to the fluorescent properties and the influence of TBAH and acid to imidazole derivatives, we predicted that 2-position benzene of imidazoles containning nitro group were mainly in the form of tight ion pairs and radical ion. 2-position benzene of imidazoles containning aldehyde group and cynao group were mainly in the form of tight ion pairs in DMF(10-5M), and 2-position benzene of imidazoles containning activiting groups or weak blunt groups were mainly in the form of dynamic balance of neutral molecules and tight ion pairs in DMF.