[Spectral properties of 2-(2'-hydroxyphenyl) benzimidazole]

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 two-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.