Photoinduced Charge Separation across Colloidal TiO2 and Fluorescein Derivatives

Abstract

The photoinduced interaction of the dyad of fluorescein's anthraquinone methyl ester (FL-AQ) and its model compound of fluorescein's butyl ester (FL4) with colloidal TiO2 was examined by absorption, fluorescence spectroscopy, fluorescence lifetime, photoinduced ESR, and transient laser flash photolysis. The dyad and the model compound are adsorbed on colloidal TiO2 through their phenolic groups. Their apparent association constants with colloidal TiO2 (Kapp) obtained from absorption spectra are 3060 and 3590 M-1, and the degrees of association (α) are 0.84 and 0.86, respectively. In the photoinduced ESR study, the cation radical of fluorescein (FL·+) was always observed for the FL4/TiO2 colloid system no matter if fluorescein was excited at 532 nm or TiO2 excited at 355 nm, while, for the FL-AQ/TiO2 colloidal system, the cation radical of fluorescein was detected only when TiO2 was excited at 355 nm. The charge-separated state of the FL-AQ/TiO2 colloidal system formed by photoinduced intramolecular electron transfer was detected by nanosecond transient absorption spectroscopy immediately. The lifetimes of FL·+ at 480 nm and AQ·- at 560 nm in the FL-AQ/TiO2 colloidal system are 11.1 and 8.93 μs, respectively. The related phenomena are discussed in this paper.