Exploring Solvent Effects on the Proton Transfer Processes of Selected Benzoxazole Derivatives by Femtosecond Time-Resolved Fluorescence and Transient Absorption Spectroscopies.

Excited-state intramolecular proton transfer (ESIPT) is of great importance due to the large Stokes shift emission that can be observed in some ESIPT molecules. Although steady-state spectroscopies have been employed to study the properties of some ESIPT molecules, their excited-state dynamics have not been examined directly with time-resolved spectroscopy methods yet for a number of systems. Here, an in-depth investigation of the solvent effects on the excited-state dynamics of two prototypical ESIPT molecules, 2-(2'-hydroxyphenyl)-benzoxazole ( HBO ) and 2-(2'-hydroxynaphthalenyl)-benzoxazole ( NAP ), have been accomplished by using femtosecond time-resolved fluorescence and transient absorption spectroscopies. Solvent effects affect the excited-state dynamics of HBO more significantly than that of NAP . Particularly in the presence of water, the photodynamics pathways of HBO are changed, while only small changes can be found in NAP . An ultrafast ESIPT process that occurs within our instrumental response is observed for HBO , and this is followed by an isomerization process in ACN solution. However, in aqueous solution, the obtained syn -keto* after ESIPT can be solvated by water in about 3.0 ps, and the isomerization process is totally inhibited for HBO . The mechanism of NAP is different from HBO and is determined to be a two-step excited-state proton transfer process. Upon photoexcitation, NAP is deprotonated first in the excited state to generate the anion*, which can transfer to the syn -keto* form followed by an isomerization process.