Electronic Coupling--Decoupling-Dependent Single-Molecule Interfacial Electron Transfer Dynamics in Electrostatically Attached Porphyrin on TiO$_{\mathrm{2}}$ Nanoparticles*

Interfacial electron transfer (ET) is of crucial importance in a multitude of chemical and physical applications including solar energy conversion and photocatalytic reactions. Extensive ensemble-averaged studies have indicated complex dynamics involving various regulating parameters for interfacial ET. To characterize the role of electronic coupling in inhomogeneous and complex ET mechanism, we applied photon-stamping spectroscopy to study the interfacial ET dynamics of single cationic dyes electrostatically attached on the surface of TiO$_{\mathrm{2}}$ nanoparticles (NPs). By changing the surface charge on the TiO$_{\mathrm{2}}$ NP, positive or negative, we were able to change the coupling between dye and TiO$_{\mathrm{2}}$ NP. The interfacial ET activity of individual molecules altered depending on the electronic coupling strength between dye and TiO$_{\mathrm{2}}$ NP. Our data showed high ET activity of cationic dyes attached on negatively charged TiO$_{\mathrm{2}}$ NP surface compared to positively charged TiO$_{\mathrm{2}}$ NP surface. The observed difference in ET activity attributed to the change in purely electronic coupling factor via electrostatic interaction. Thus, our real-time single molecule experiment revealed the significant changes in ET activity with electronic coupling for electrostatically attached dyes on TiO$_{\mathrm{2\thinspace }}$surface.[\textit{J. Phys. Chem.} C \textbf{2016}, \textit{120}, 12313]