Dynamics of Electron Transfers in Photosensitization Reactions of Zinc Porphyrin Derivatives

Photocatalytic systems for CO₂ reduction operate via complicated multi-electron transfer (ET) processes. A complete understanding of these ET dynamics can be challenging but is key to improving the efficiency of CO₂ conversion. Here, we report the ET dynamics of a series of zinc porphyrin derivatives (ZnPs) in the photosensitization reactions where sequential ET reactions of ZnPs occur with a sacrificial electron donor (SED) and then with TiO₂. We employed picosecond time-resolved fluorescence spectroscopy and femtosecond transient absorption (TA) measurement to investigate the fast ET dynamics concealed in the steady-state or slow time-resolved measurements. As a result, Stern-Volmer analysis of fluorescence lifetimes evidenced that the reaction of photoexcited ZnPs with SED involves static and dynamic quenching. The global fits to the TA spectra identified much faster ET dynamics on a few nanosecond-time scales in the reactions of one-electron reduced species (ZnPs) with TiO₂ compared to previously measured minute-scale quenching dynamics and even diffusion rates. We propose that these dynamics report the ET dynamics of ZnPs formed at adjacent TiO₂ without involving diffusion. This study highlights the importance of ultrafast time-resolved spectroscopy for elucidating the detailed ET dynamics in photosensitization reactions.