Role of the Character of the Excited State of Singly Reduced Rh ₂ (II, II) Intermediates on Photocatalytic Activity

Singly reduced intermediates have recently been implicated as photoactive intermediates in a number of important reactions; however, their photophysical properties remain poorly understood. A series of dirhodium(II,II) complexes, cis-[Rh₂(p-R-Form)₂(bncn)₂]²⁺ (bncn = benzo[c]cinnoline; p-R-Form = N,N'-di-p-R-phenylformamidinate), where R = -OCH₃ (1), -CH₃ (2), -H (3), -F (4), - Cl (5), and -CF₃ (6), and their respective radical anions were prepared and their excited state properties were investigated. Complex 3 acted as a single-molecule photocatalyst for H₂ production with red light. Substitution on the formamidinate ligands in 1-6 affected the energy of the Rh₂(δ*)/Form(π,nb) highest occupied molecular orbital (HOMO), consistent with the metal/ligand-to-ligand charge transfer (¹ML-LCT) absorption maxima and the ³ML-LCT excited state lifetime, ranging from 1.6 ns in 1 to 54 ns in 6 in CH₃CN. The highest turnover number for photocatalytic H₂ evolution was observed for 3, and the lowest values were for 1 and 6. The radical anion, [Rh₂]⁻, formed during photocatalysis, was shown to absorb a photon and undergo a second reduction. The lifetimes of the doublet excited states of [3]⁻ and [6]⁻ were 0.49 and 0.24 ns, respectively. Calculations showed that the lowest energy excited state in [3]⁻ was ²ML-LCT, whereas that in [6]⁻ was a bncn⁻ → Rh₂(σ*) ligand-to-metal charge transfer (²LMCT) state. The ²LMCT state stabilized across the series from [1]⁻ to [6]⁻, pointing at its role in modulating the photophysical properties. This work highlights the importance of the reductive quenching of [Rh₂]⁻ and the generation of the doubly reduced species to effectively catalyze hydrogen evolution.