2-(2’-羟基苯基)苯并咪唑的光谱特性

Spectral Properties of 2-(2'-Hydroxyphenyl) Benzimidazole

2-(2’-羟基苯基)苯并咪唑(简称HBI)是一类具有激发态质子转移效应的有机分子。观测了HBI分子在甲苯、甲苯与乙醇混合及乙醇三种溶液的吸收光谱,发现其吸收光谱相类似。用317 nm光激发,观测其荧光光谱,在甲苯溶液中只观测到一个荧光带,其峰值位于470 nm;在甲苯与乙醇的混合溶液及乙醇溶液的荧光光谱中,都观测到两个荧光带,其峰值分别位于370和450 nm。根据激发态质子转移理论可知,470 nm的荧光带是由激发态质子转移生成的互变异构体,即HBI酮式构型分子的发射,峰值位于370 nm的荧光带是HBI稀醇式构型分子的发射。由于乙醇具有较大的极性,可与HBI分子相互作用形成分子间氢键,即生成溶剂化物。当溶剂化物被激发,在激发态发生激发态质子转移生成两性离子,两性离子发射荧光回到基态。因此,在极性溶剂中HBI发射峰值位于450 nm的荧光带应归于两性离子的发射。当用532 nm强光激发HBI溶液时,发现HBI分子在非极性溶剂中无双光子效应,而在极性溶剂中却存在双光子效应,表明HBI的溶剂化物具有双光子效应。

2-（2＇-hydroxyphenyl） benzimidazole （HBI） is one kind of organic molecule with excited-state proton transfer （ESPT） effect. The absorption spectra of HBI were observed in toluene, the mixture of toluene and ethanol, and ethanol, respectively. It was found that the absorption spectra in the three solvents are similar. The fluorescence of HBI was observed under the excitation of 317 nm light. Only one fluorescence band with a peak wavelength of 470 nm was observed in the toluene. There appeared two fluorescence bands in the mixed solvents and ethanol, of which the peak wavelength was 370 and 450 nm, respectively. Based on the ESPT theory, the fluorescence band with a peak at 370 nm is attributed to the emission from enol form of HBI molecule, while the band with a peak at 470 nm is attributed to the emission from tautomer form （i. e. keto form） via ESPT process. Because of the strong polarity of ethanol, the intermolecular H-bond can be formed between the HBI molecules and the ethanol and HBI molecules tend to exist in the solvated form. When HBI molecules in solvated form were excited, the zwitterionic form of HBI was formed via ESPT and returned to the ground state accompanied with fluorescence emission, so the fluorescence band with a peak at 450 nm is attributed to the zwitterionic emission of HBI. When the HBI in the three kinds of solvents was excited by the picosecond laser pulse at 532 nm, the twc-photon induced fluorescence was not observed in the nonpolar solvents but observed in the polar solvents, which indicate that two-photon effect occurred in solvated form.