A Novel Pump-Pump-Probe Resonance Raman Approach Featuring Light-Induced Charge Accumulation on a Model Photosystem

Light-induced charge accumulation is at the heart of biomimetic systems aiming at solar fuel production in the realm of artificial photosynthesis. Understanding the mechanisms upon which these processes operate is a necessary condition to drive down the rational catalyst design road. We have built a nanosecond pump-pump-probe resonance Raman setup to witness the sequential charge accumulation process while probing vibrational features of different charge-separated states. By employing a reversible model system featuring methyl viologen (MV) as a dual electron acceptor, we have been able to watch the photosensitized production of its neutral form, MV⁰, resulting from two sequential electron transfer reactions. We have found that, upon double excitation, a fingerprint vibrational mode corresponding to the doubly reduced species appears at 992 cm^-1 and peaks at 30 μs after the second excitation. This has been further confirmed by simulated resonance Raman spectra which fully support our experimental findings in this unprecedented buildup of charge seen by a resonance Raman probe.