Luminescence Properties and the Primary Process of Photochromism of 2-(2-Hydroxyphenyl)benzothiazole

Abstract

The luminescence behavior of 2-(2-hydroxyphenyl)benzothiazole at 77 K was studied in detail, special attention being paid to dual fluorescence at ≈500 and ≈370 nm and also to phosphorescence at ≈500 nm. From the dependence of these emission bands upon the hydrogen-bonding ability of solvents and the excitation spectra, it is concluded that the short- and long-wavelength fluorescence bands are due to the intermolecularly hydrogen-bonded enol imine and the keto amine produced from the intramolecularly hydrogen-bonded enol, respectively, and that the phosphorescence is due to the intermolecularly hydrogen-bonded enol imine. The estimated quantum yield of the enol fluorescence gives further support to the formation scheme of the photochromic colored species previously presented. The rate constants of the intramolecular hydrogen transfer at 77 and 293 K are evaluated to be larger than 4×1010 s−1 and 3×1012 s−1, respectively.