Hydrogen Production from Water: Photocatalysis

Photocatalytic hydrogen production from water provides an attractive but challenging approach to meeting our global energy needs. Design and development of systems that utilize visible light to drive the energetically favorable, multielectron reduction of water to produce hydrogen fuel have been of long-term interest. Addressing the challenges of efficient harvesting of solar energy to produce transportable fuels requires ongoing, sustained research in this field. Current technologies utilize photobiological methods, photoelectrochemical methods, and molecular photocatalysis. This article focuses on the recent progress of proton or water reduction using supramolecular photocatalysts. These photochemical molecular devices often couple charge-transfer light absorbers (LAs) to reactive metals in complex molecular architectures. Linking multiple components into large molecular assemblies allows for applications requiring complex functions. Early successes in supramolecular photocatalysts for solar hydrogen production are summarized. The redox, excited state properties, and photochemical properties of supramolecular systems applicable in photocatalytic hydrogen production are presented. Fundamental studies of the perturbations of subunit properties upon incorporation into supramolecular arrays are paramount to their successful application in this forum. Optical excitation typically affords a metal-to-ligand charge-transfer state that is quenched by electron transfer, providing reduced complexes with reactive metals able to deliver the electrons to substrates. Sacrificial electron donors provide coupling to oxidative chemistry, allowing independent study of photocatalysts and hydrogen production schemes. Supramolecular chemistry allows modulation of redox, photophysical, and photochemical properties by component modification. Coupling multiple LAs to an electron acceptor, which, when reduced by multiple electrons, provides photoinitiated electron collectors (ECs). Photoinitiated ECs, which collect multiple electrons on a reactive rhodium center with the supramolecular architecture remaining, reduce water to hydrogen fuel. Mixed-valence systems that promote HX reduction to produce hydrogen have also been described. Coupling of charge-transfer LAs to cobalt, iron, palladium, or platinum is a topic of recent interest in the design of water reduction photocatalysts. The general design considerations for the development of supramolecular assemblies that function as photocatalysts for proton reduction are discussed, providing valuable insight into engineering efficient solar hydrogen production catalysts for water reduction. Mechanistic studies remain important to understanding this complex multielectron photochemistry, which will aid in future molecular design. Much remains to be mastered in this complicated, multielectron photochemistry but recently there is increased awareness and interest in this field. Keywords: supramolecular complexes; ruthenium polyazine light absorbers; bridging ligands; reactive metals; electrochemistry; photocatalysis; multielectron chemistry; solar energy conversion; solar hydrogen production; photoinitiated electron collection; metal-to-ligand charge transfer; metal-to-metal charge transfer