Photoexcited hydroxyarenes as probes for microenvironments

Part I. Oxygen and nitrogen acids and bases can form strong hydrogen bonds to protic solvents, which because of their amphoteric nature can allow fast proton transfer between the solvated reactants by a Grofthuss chain-type mechanism. This mechanism rationalizes proton transfer through a relay process in which the proton "hops" from molecule to molecule. In order to test this mechanism, 2-(2'-hydroxyphenyl)ethynylbenzothiazole (HEBT) a derivative of 2-(2'-hydroxyphenyl)benzothiazole (HBT), in which an ethynyl group provides fixed distances between a proton donor and a proton acceptor, were synthesized and investigated.; Intermolecular proton transfers to solvent water quenches the emission of 2-(2'-hydroxyphenyl)ethynylbenzothiazole. Hydrogen bonding solvents also quench the fluorescence emission and fluorescence lifetimes could not be determined in these solvents. The results show a lack of dual emission in organic solvent-water mixtures. The lack of anion emission in organic solvent-water mixtures and the very weak emission of the anion in 90% glycerol suggests that the excited state proton transfer to water is the mechanism of quenching. Therefore, important kinetic information regarding proton transfer in water is not possible on the picosecond timescale. Longer decay profiles for 2-(2'-hydroxyphenyl)ethynylbenzothiazole were observed in nonpolar solvents and solvent systems of high viscosity. The fluorescence decay is dependent on the dielectric constant of the solvent and slightly on solvent viscosity. These results show that 2-(2' -hydroxyphenyl)ethynylbenzothiazole is not an ideal excited state proton transfer probe for microheterogeneous systems such as biological systems, polymers and micelles.; Part II. Camptothecin and its analogs (in particular, 10-hydroxycamptothecin) exhibit unusual fluorescence properties. Time-resolved experiments in methanol and water suggested a mechanism consistent with excited state proton transfer rather than just a simple solvent effect. The fluorescence behavior of 10-hydroxycamptothecin is not the result of a simple solvent effect but shows a mechanism of excited state proton transfer which is the result of the solvent water acting as a proton acceptor. There is no prototropic equilibrium in the excited state due to the lack of proton recombination.