Adsorption model of ethyl ester of fluorescein on colloidal TiO2 and the mechanism of the interfacial electron transfer

The adsorption of a sensitizing dye which has relatively high fluorescence quantum yield, ethyl ester of fluorescein (FLEt), on the surface of colloidal TiO2 particles and the electron transfer process only from its singlet excited state to the colloidal TiO2 particles were examined. By comparing the effects of colloidal TiO2 on the absorption and fluorescence spectra of FLEt and its reference model compound, diethyl fluorescein (FLEt,), it was demonstrated that FLEt can adsorb on the surface of colloidal TiO2 particles through its phenol group. The apparent association constant (K-app) was about 1.47 x 10(2) mol(-1) l from the effect of colloidal TiO2 on the absorption spectra and 3.33 x 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 TiO2 (k(et) = 1.70 x 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 mu s. (C) 1998 Elsevier Science B.V. All rights reserved.