EPR Characteristics of a dye/colloidal TiO2 system under visible light irradiation

Abstract

An EPR study is reported of paramagnetic species and 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) spin-trapped adducts formed in a system composed of the ethyl ester of fluorescein (FLEt) and colloidal TiO2 particles under 532 nm pulsed laser irradiation in deaerated and aerated ethanolic media. Following excitation of FLEt, electron injection takes place from the singlet excited state 1FLEt* to the conduction band of TiO2 leading to formation of the radical cations FLEtNsbd+ in oxygen-free dispersions, and whose EPR signals have g = 2.0047. In the presence of oxygen, 532 nm irradiation of the ethanolic FLET/TiO2 dispersion first produced EPR signals with g = 2.0147, tentatively identified with the peroxy radical cation O2FLEtNsbd+, which on further irradiation decreased in intensity and were followed by the concomitant appearance of EPR signals (g = 2.0047) of increasing intensity of the FLEtNsbd+ radicals. Following the addition of the DMPO spin-trap, irradiation of the aerated dispersion gave EPR spectra consistent with formation of superoxide radical anions O2− through scavenging of the electrons on the colloidal TiO2 particles. The mechanistic details that led to changes in the EPR signals and to the generation of O2− are described.