Observation of Excited State Proton Transfer between the Titania Surface and Dye Molecule by Time-Resolved Fluorescence Spectroscopy

A temporal change in acidity on the titania surface during UV irradiation was indirectly monitored by evaluating the deposited fluorescein dye using surface plasmon resonance spectroscopy such that gold nanoparticles were embedded under the titania thin film in order to increase the sensitivity. The fluorescence signal mainly originated from the highly fluorescent dianion species of fluorescein in the fluorescence time profiles after the femtosecond laser excitation. The signal monotonously decayed with a single exponential function on silica, whereas the signal rose and decayed with a double exponential function on the titania. The present femtosecond time-resolved fluorescence study revealed that a process involving the transformation from the monoanion to dianion of fluorescein “in the excited states”, i.e., the excited state proton transfer from the dye molecule to the titania surface, induced the subsequent fluorescence process, which had not been detected by the previous transient absorption or nanosecond fluorescence measurements.