Excited state proton transfer in 2-substituted benzothiazoles

Abstract

The photophysics of 2-(2'-hydroxyphenyl)benzothiazole (HBT) and related compounds has been studied as a function of solvent, pH or H 0 and temperature. Measurements of absorption and fluorescence spectra and fluorescence decay profiles under these various conditions have been combined with theoretical calculations to arrive at an understanding of the
excited state behaviour of HBT and derivatives.
An investigation has been made into the protonation and deprotonation of 2-phenylbenzothiazole, 2-(2'-methoxyphenyl)benzothiazole), HBT and derivatives of HBT with bromine, chlorine, hydroxy, methoxy, methyl and nitro substituents in the 2-phenyl ring. Absorption and fluorescence spectra have been combined with Forster cycle calculations to yield pKa, pKb, pKa* and pKb* values for the various compounds. The results have been compared with those for the corresponding phenols and many similarities noted. The overlap of the pK a* and pKb* values for HBT and derivatives has been noted. This has explained the occurrence of excited state intramolecular proton transfer (ESIPT) in these compounds.
A study of the photophysics of HBT in non-polar and alcoholic mixtures for temperatures in the range 96-298K has been made using the Berlin synchrotron source, BESSY. In all solvents a rise in both fluorescence quantum yield and lifetime is observed as the temperature is decreased. It is proposed that a viscosity dependent non-radiative process leading to a nonemissive, twisted excited state accounts for these observations. Application of quantum chemical calculations to the system appears to confirm this interpretation.
The fluorescence kinetics of HBT have been studied as a function of alkaline pH. Combination of the lifetime data with quantum yields and pK a* values have allowed calculation of all the rate constants in the neutral c-> anion equilibrium for HBT. Similar measurements have been undertaken for substituted HBTs and for HBT at high acidities, but the data obtained has been less easy to interpret.