Ligand tunability versus condition optimization: a guide to the structure–activity/stability nexus and mechanistic insights into photoinduced hydrogen evolution using hexacoordinated cobalt complexes

Abstract

Cobalt-containing molecular catalysts have become increasingly attractive for homogeneous photocatalytic H₂ production in aqueous/organic/organic–aqueous solvents containing photosensitizers, proton donors, buffers, and sacrificial electron donors. This feat has attracted immense attention from both academic and industrial scientists and is connected to their unique characteristics and impressive catalytic performance, alongside being cheap and earth-abundant. Mechanistic insights into H₂ generation photocatalysis have been brought to the limelight by the triad of experimental, spectroscopic, and computational investigations, leading to the seamless detection, the identification, and a detailed understanding of real-time charge transfer dynamics and electronic and geometric structural variations within homogeneous systems. In this review, we give an overview of the competition between ligand tunability and photocatalytic condition optimization and the consequences of this competition on the charge transfer dynamics between light-harvesting photosensitizers and catalytic species, variations in the geometric and electronic structures of catalytic species formed during photocatalysis, diverse reaction pathways, rate-limiting steps, and the stability, activity, and efficiencies of homogeneous H₂ generation photocatalysis. The use of cobaloxime and other materials such as TiO₂ as benchmarks for H₂-evolving photocatalysts is fast becoming a standard procedure and is discussed in this work. It is projected that the dynamics of these photophysical and photochemical events will be utilized as guidance for future catalyst designs that will be competitive and compatible with existing global energy conversion technologies.