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)22+ (bncn = benzoccinnoline; 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.