NEW FEATURES IN THE PHOTOPHYSICS AND PHOTOCHEMISTRY OF 2-(2'-HYDROXYPHENYL)BENZOTHIAZOLES INTRODUCED BY AMINE SUBSTITUTION

Abstract— –The photophysics and photochemistry of the 4′‐diethylamino derivative of both 2‐phenyl‐benzothiazole and 2‐(2′‐hydroxyphenyl)benzothiazole have been studied by nanosecond and microsecond laser flash photolysis and picosecond emission spectroscopy. For the non‐hydroxy substituted molecule, the singlet excited state was shown to relax primarily via fluorescence emission, and a very weak triplet transient was observed after laser flash excitation. The 2‐(2′‐hydroxy‐4′‐diethylamino‐phenyl)benzothiazole (AHBT) was shown to undergo excited state intramolecular proton transfer (ESIPT) in the picosecond timescale (k > 3 times 1010 s‐1) to form a colored zwitter‐ion/keto form in solution at room temperature while the ground state back proton transfer was slower by a factor of?105. However, in marked contrast with other derivatives of 2‐(2′‐hydroxyphenyl)benzothiazole and related molecules, the ESIPT was not the only deactivation process of the lowest singlet excited state of the enol form. Under steady‐state excitation at room temperature (and low temperature), the fluorescence emission of the enol form was observed. The T‐T absorption of the enol form was also observed and furthermore, the ESIPT was shown to have an activation energy which was estimated to be ? 4 kJ. None of the foregoing, fluorescence and T‐T absorption of the enol nor activation energy for proton transfer have been observed for the parent or derivatives of 2‐(2′ hydroxyphenyl)benzothiazoles. The striking new features for the ESIPT photochemistry and photo‐physics for the 4′‐diethylamino derivative of 2‐(2′‐hydroxyphenyl)benzothiazole are discussed and MO calculations are used to aid in the interpretation of some of the experimental results. Copyright © 1990, Wiley Blackwell. All rights reserved