Abstract
Flash-excitation of degassed solutions of 2-(2′-hydroxyphenyl)benzothiazole (HBT) in an inert solvent leads to the metastable triplet state of HBT and to a cis → trans isomerization of the HBT keto tautomer. Both processes proceed from the first excited singlet state of the HBT keto tautomer, 1K*. The trans-keto tautomer is not formed below 150 K, whereas the triplet yield increases at lower temperatures, like the fluorescence yield. Besides fluorescence, intersystem crossing and cis → trans isomerization, an additional deactivation channel of 1K* is proposed in order to explain the different temperature dependencies of the quantum yields of fluorescence and cis → trans isomerization. It is suggested that in the singlet ground state the keto-trans isomer decays by a second-order reaction to the enol form, 1E, by mutual hydrogen exchange: 2 1K tr → 2 1E tr → 2 1E cis.
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