Nanosecond time-resolved studies of long-lived photoinduced charge separation in the dyad fluorescein-anthraquinone-methyl ester adsorbed on TiO2 colloids

A dyad composed of fluorescein and 2-methyl-anthraquinone (FL-AQ) was synthesized and its photophysical properties were examined by absorption, fluorescence spectroscopy, and fluorescence lifetime. The charge-separated state formed by photoinduced intramolecular electron transfer was detected by nanosecond transient absorption spectroscopy for the first time. When FL is excited in solution, the photoinduced electron transfer from FL to AQ proceeds efficiently. The rate constant and the efficiency of intramolecular electron transfer are 3.95 x 10(9) s(-1) and 95%, respectively. Its charge-separated state lifetime is too short to detect by transient absorption spectroscopy. Adding nanometer colloidal TiO2 to an FL-AQ ethanol solution prolongs the lifetime of the charge-separated state, so its transient absorption signal is recorded significantly. 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 mus, respectively. (C) 2003 Elsevier Inc. All rights reserved.