Intraligand Charge Transfer in Metal-Organic Frameworks Facilitates Radical Anion-Mediated Hydrogen Evolution

Abstract

Metal-organic frameworks (MOFs) offer a structurally tunable platform for photocatalysis, yet their activity for hydrogen evolution is often constrained by inefficient charge separation and reliance on metal cocatalysts. Herein, a family of MOFs, BVR-19-M, constructed from divalent transition metal ions (M = Mg2+, Mn2+, Cu2+, Zn2+, Cd2+) and the aliphatic amino acid l-cystine is reported. Among them, BVR-19-Zn exhibits the highest photocatalytic activity, enabled by the zinc’s filled d-orbitals and the redox-active disulfide l-cystine ligand, promoting intraligand charge transfer (ILCT). Uniquely, the disulfide (S–S) bridges within BVR-19-Zn undergo transient homolytic cleavage upon light irradiation, generating a thiyl radical and thiolate anion pair that actively mediate charge separation and drive hydrogen evolution in the absence of a co-catalyst. Open-shell metal ions incorporated into BVR-19-M either introduce a mid-gap state that serves as a recombination center or promote metal-centric charge transfer, both of which diminish photocatalytic efficiency. The radical anion-mediated ILCT mechanism introduces a distinct molecular design strategy for MOFs, demonstrating how disulfide redox chemistry can be harnessed to unlock efficient, sustainable photocatalysis.