Electronic Coupling Decoupling-Dependent Single-Molecule Interfacial Electron Transfer Dynamics in Electrostatically Attached Porphyrin on TiO2 Nanoparticles

Factors controlling the interfacial electron-transfer (ET) dynamics in molecule semiconductor systems have been intensively investigated in recent years. Here, we study dynamics of interfacial ET on Zn(II) meso-tetra (N-methyl-4-pyridyl) porphine tetrachloride (ZnTMPyP)-TiO2 nanoparticle (NP) system using single-molecule photon, stamping spectroscopy while electrostatically controlling the Coupling between ZnTMPyP and TiO2 NP by changing the surface charge of the TiO2 NP. The single-molecule fluorescence trajectories show strong fluctuation and blinking between, bright and dark states providing clear indication for the binding affinity between ZnTMPyP and the TiO2 NP via electrostatic interaction. By changing the surface charge on the TiO2 NP, positive or negative, we are able to change the coupling between ZnTMPyP and the TiO2 NP, which is revealed from the dominant dark states distribution in fluorescence trajectories and shorter fluorescence lifetime of ZnTMPyP attached on negatively charged, TiO2 NP surface compared to positively charged TiO2 NP surface. The observed difference in fluorescence trajectories and lifetime of ZnTMPyP can be qualitatively accounted for by considering the change in purely electronic coupling factor caused by the positively or negatively charged TiO2 NP surface in electrostatically bound dye-sensitized TiO2 systems. Strong binding interaction between ZnTMPyP and negatively charged TiO2 NP is further observed by higher fluorescence anisotropy compared to ZnTMPyP on positively charged TiO2 NP.