Single-Molecule Interfacial Electron Transfer Dynamics of Porphyrin on TiO₂ Nanoparticles: Dissecting the Complex Electronic Coupling Dependent Dynamics

The photosensitized interfacial electron transfer (ET) dynamics of the zinc(II)–5,10,15,20-tetra(3-carboxyphenyl)porphyrin ( m -ZnTCPP)–TiO 2 nanoparticle (NP) system has been studied using single-molecule photon-stamping spectroscopy. The single-molecule fluorescence intensity trajectories of m -ZnTCPP on TiO 2 NP surface show fluctuations and blinking between bright and dark states, which are attributed to the variations in the reactivity of interfacial ET, i.e., intermittent interfacial electron transfer dynamics. Comparing the results with that from our earlier studied p -ZnTCPP–TiO 2 nanoparticle system, we show the effect of anchoring group binding geometry (meta or para), hence electronic coupling of sensitizer ( m -/ p -ZnTCPP) and TiO 2 substrate, on interfacial ET dynamics. Compared to p -ZnTCPP on TiO 2 NP surface, with m -ZnTCPP, dark states are observed to dominate in single-molecule fluorescence intensity trajectories. This observation coupled with the large difference in lifetime derived from bright and dark states of m -ZnTCPP demonstrates higher charge injection efficiency of m -ZnTCPP than p -ZnTCPP. The nonexponential autocorrelation function decay and the power-law distribution of the dark-time probability density provide a detailed characterization of the inhomogeneous interfacial ET dynamics. The distribution of autocorrelation function decay times (τ) and power-law exponents ( m dark ) for m -ZnTCPP are found to be different from those for p -ZnTCPP, which indicates the sensitivity of τ and m dark on the molecular structure, molecular environment, and molecule–substrate electronic coupling of the interfacial electron transfer dynamics. Overall, our results strongly suggest that the fluctuation and even intermittency of excited-state chemical reactivity are intrinsic and general properties of molecular systems that involve strong molecule–substrate interactions.