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

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, elect:ron injection takes place from the singlet excited state (FLEt)-F-1* to the conduction band of TiO2 leading to formation of the radical cations FLEt.+ 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 O2FLEt.+, 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 FLEt.+ radicals. Following the addition of the DMPO spin-trap, irradiation of the aerated dispersion gave EPR spectra consistent with formation of superoxide radical anions O-2(.-) 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 O-2(.-) are described.