Adsorption model of ethyl ester of fluorescein on colloidal TiO 2 and the mechanism of the interfacial electron transfer

Abstract

The adsorption of a sensitizing dye which has relatively high fluorescence quantum yield, ethyl ester of fluorescein (FLEt), on the surface of colloidal TiO 2 particles and the electron transfer process only from its singlet excited state to the colloidal TiO 2 particles were examined. By comparing the effects of colloidal TiO 2 on the absorption and fluorescence spectra of FLEt and its reference model compound, diethyl fluorescein (FLEt 2), it was demonstrated that FLEt can adsorb on the surface of colloidal TiO 2 particles through its phenol group. The apparent association constant ( K app) was about 1.47×10 2 mol −1 l from the effect of colloidal TiO 2 on the absorption spectra and 3.33×10 2 mol −1 l from fluorescence quenching. Exciting the ethyl ester of fluorescein with visible light can induce electron injection from its singlet excited state into the conduction band of TiO 2 ( k et=1.70×10 8 s −1). Electron spin resonance and laser spectroscopies demonstrated formation of the radical cation of FLEt. The lifetime of the radical cation FLEt + · determined from the decay profile was about 7.04 μs.