Mechanistic investigation of light-driven catalysis for solar fuel formation

Solar energy conversion into chemical fuels currently represents a viable solution to the global energy issue. In this regard, water splitting with formation of dihydrogen as an energy carrier has been usually considered as a target reaction. Due to the mechanistic complexity associated with both the oxidation (oxygen evolving reaction, OER) and reduction (hydrogen evolving reaction, HER) half-reactions, the design of active catalysts and their efficient coupling with photoactive components appears as a major target. Optical spectroscopies turn out to be key tools to monitor the photoreaction dynamics and extract detailed kinetic data which can be profitably employed towards performance optimization of both catalytic routines. This chapter will describe the application of steady-state and time-resolved absorption and emission spectroscopy to the investigation of the mechanistic aspects as- sociated with both the OER and HER performed using molecular components both as light- harvesting and catalytic units. Through the case studies examined, we will give an overview of how these spectroscopic tools allow proper identification of the photoreaction mech- anism, the rate and efficiency of each (photo)chemical step, the possible involvement of proton-coupled electron-transfer (PCET) processes and the occurrence of detrimental side- reactions, thus defining precise guidelines towards improvement of solar fuel formation.