Mechanistic Investigations of Photocatalytic Systems by Pump-Pump-Probe Spectroscopy

Understanding the mechanism through which a photochemical reaction proceeds grants access to thermodynamic and kinetic parameters that allow its optimization for large-scale applications. In the last few years, photocatalytic systems have been major players in scientific developments on research fields of primary importance, including solar fuel generation in artificial photosynthesis, and the use of excited organic radical ions as photocatalysts in thermodynamically challenging reactions of utmost synthetic relevance. Generally, time-resolved spectroscopic approaches are the main experimental tools to investigative the dynamics of photoactive systems, with pump-probe-based ones being the golden standard in photophysical and photochemical research. However, for photocatalytic systems in which multiple photons are required, which is generally the case for photosynthetic reactions and those promoted by excited organic radical ions, the pump-probe approach is no longer sufficient since it is based on a single photon-to-electron ratio. This is where a second actinic pump excitation comes into play in what is known as pump-pump-probe spectroscopy. In this review, we explore how this approach is used to unravel the mechanism of photocatalytic systems triggered by light using different probes of UV–vis absorption and resonance Raman scattering in varying time scales.