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

Abstract

<p></p><p>Excited-state intramolecular proton transfer(ESIPT) is of great importance due to the large Stokes shift emissionthat can be observed in some ESIPT molecules. Although steady-statespectroscopies have been employed to study the properties of someESIPT molecules, their excited-state dynamics have not been examineddirectly with time-resolved spectroscopy methods yet for a numberof systems. Here, an in-depth investigation of the solvent effectson 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-resolvedfluorescence and transient absorption spectroscopies. Solvent effectsaffect the excited-state dynamics of HBO more significantlythan that of NAP. Particularly in the presence of water,the photodynamics pathways of HBO are changed, whileonly small changes can be found in NAP. An ultrafastESIPT process that occurs within our instrumental response is observedfor HBO, and this is followed by an isomerization processin ACN solution. However, in aqueous solution, the obtained syn-keto* after ESIPT can be solvated by water in about3.0 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 protontransfer process. Upon photoexcitation, NAP is deprotonatedfirst in the excited state to generate the anion*, which can transferto the syn-keto* form followed by an isomerizationprocess.<br></p>