New features in the photophysics and photochemistry of 2-(2'-hydroxyphenyl)benzothiazoles introduced by amine substitution.
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'-diethylaminophenyl)benzothiazole (AHBT) was shown to undergo excited state intramolecular proton transfer (ESIPT) in the picosecond timescale (k greater than 3 x 10(10) 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 approximately 10(5). 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 approximately 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 photophysics 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.
Lenoble
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Department of Chemistry, University of Houston-University Park, TX 77204.
Becker
R S
RS
D000588:Amines; D052160:Benzothiazoles; D010777:Photochemistry; D010782:Photolysis; D013053:Spectrophotometry; D013844:Thiazoles / Q000737:chemistry* / Q000528:radiation effects
D000588:Amines; D052160:Benzothiazoles; D013844:Thiazoles; C068467:2-(4'-diethylaminophenyl)benzothiazole; C068466:2-(2'-hydroxy-4'-diethylaminophenyl)benzothiazole; C005465:benzothiazole