The interfacial electron transfer processes between a fluorescent water-soluble perylene diimide dye (WS-PDI) and TiO2 nanoparticles were investigated using single-molecule fluorescence spectroscopy. Based on the single-molecule fluorescence spectral measurements, it was suggested that the local environment and/or the structural conformation of single WS-PDI molecules play important roles in the efficiency of the electron injection from WS-PDI in the singlet excited state to TiO2. The observed single-molecule fluorescence blinking behavior was interpreted in terms of the interfacial electron-transfer dynamics between the single WS-PDI molecules and TiO2 nanoparticles. (c) 2007 Elsevier B.V. All rights reserved.