Rhodamine-6G photosensitized folic acid decomposition in aqueous solution, and its quantum yield in the presence of 10 mu M folic acid was 9.9 x 10(-6). A possible mechanism of this photodecomposition is direct oxidation through an electron transfer from folic acid to rhodamine-6G. The fluorescence lifetime of rhodamine-6G was slightly decreased by folic acid, suggesting electron transfer in the excited singlet state of rhodamine-6G. The quenching rate coefficient estimated from the Stern-Volmer plot of the fluorescence quenching supported that this electron transfer proceeds as a diffusion-controlled reaction. The quantum yields of the electron transfer and the following reaction could be determined. (C) 2015 Elsevier B.V. All rights reserved.