We have applied single-molecule spectroscopy and ultrafast spectroscopy on single molecule studies of photosensitized interfacial electron transfer (ET) processes in Coumarin 343 (C343)-TiO2 nanoparticle (NP) and Cresyl Violet (CV+)-TiO2 NP systems, using time-correlated single photon counting coupled with scanning confocal fluorescence microscopy. Fluorescence intensity trajectories of individual dye molecules adsorbed on a semiconductor NP surface showed fluorescence fluctuations and blinking, with time constants distributed from milliseconds to seconds. The fluorescence fluctuation dynamics were found to be inhomogeneous from molecule to molecule and from time to time, showing significant static and dynamic disorders in the interfacial ET reaction dynamics. We attribute fluorescence fluctuations to the interfacial ET reaction rate fluctuations, associating redox reactivity intermittency with the fluctuations of molecule-TiO2 electronic and Franck-Condon coupling. The inhomogeneous nanoscale molecule-surface and molecule-molecule interactions are presumably the origins of the complexity in interfacial ET dynamics. These nanoscale inhomogenities at interfaces or on surfaces make it highly difficult for ensemble-averaged measurements to dissect the complex interfacial ET processes. The difficulty comes from both spatial and temporal inhomogeneities, which can be identified, measured, and analyzed best by studying one molecule at a time in a specific nanoscale local environment. Intermittent interfacial ET dynamics of individual molecules could be characteristic of a surface chemical reaction strongly involved with and regulated by molecule-surface interactions. The intermittent interfacial reaction dynamics that likely occur among single molecules in other interfacial and surface chemical processes can typically be observed by single-molecule studies, but not by conventional ensemble-averaged experiments.
| | | Back ... Home | | |