INVESTIGATION OF TRIPLET-STATES OF 2-(2'-HYDROXYPHENYL)BENZOTHIAZOLE AND 2-(2'-HYDROXYPHENYL)BENZOXAZOLE BY TRANSIENT ABSORPTION-SPECTROSCOPY AND AB-INITIO CALCULATIONS

Properties of the 3(pi,pi*) states of 2-(2'-hydroxyphenyl)benzothiazole (HBT) and 2-(2'-hydroxyphenyl)-benzoxazole (HBO) were investigated by means of transient absorption spectroscopy and ab initio calculations. When the volume fraction of ethanol in a mixed solvent of toluene and ethanol is small, the intramolecularly proton-transferred cis-keto form of HBT predominates over the intermolecularly hydrogen-bonded trans-enol form in the triplet state. However, when the volume fraction of ethanol becomes large, the relative composition is reversed. The triplet species of HBO at 10 mus after photoexcitation contain a cis-enol form in both nonpolar and alcoholic solvents. For HBO, the cis-keto form produced immediately after photoexcitation is partly converted to the intramolecularly hydrogen-bonded cis-enol form in the triplet state within 10 mus. Interpretation of the solvent effect is made on the basis of the ab initio calculations for the ground state. The difference in solvent effect between HBT and HBO can be explained in terms of the hydrogen bonding involving the sulfur or oxygen atom at the 1-position. The results of the ab initio calculations for the triplet state show that the stable triplet species of HBT and HBO can be regarded as a kind of cis-keto form. This result can be explained by considering the pi electron nodal pattern of the wave function in the lowest 3(pi,pi*) state.