TEMPERATURE-DEPENDENT TAUTOMER FLUORESCENCE-SPECTRA OF 3',4'-BENZO-2'-HYDROXYCHALCONE - DIRECT EVIDENCE FOR PHOTOENOLIZATION FOLLOWED BY Z-]E ISOMERIZATION IN THE SINGLET MANIFOLD

Direct evidence for the S-1 --> S-1' photoenolization of 3',4'-benzo-2 '-hydroxychalcone was obtained by the detection of the S-1' --> S-0' t automer fluorescence spectra (lambda(max) = 600-610 nm) at 77-298 K. T he enol-tautomer is barely fluorescent at 298 K, and the fluorescence intensity increases with decreasing temperature. The drastic increase in the S-1' --> S-0' fluorescence intensity with the 180 --> 77 K temp erature drop demonstrates the solvent-derived viscosity barrier for th e Z --> E isomerization of S-1' to generate S-0'' (E-enol-tautomer). S uch primary photochemical processes are supported by the detection of the respective transient absorption spectra of E- and Z-enol-tautomers (S-0'' and S-0') with lifetimes of 190 and 13 mu s at 298 K. (C) 1998 Elsevier Science B.V. All rights reserved.