Abstract
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-[Rh(2)(p-R-Form)(2)(bncn)(2)](2+) (bncn = benzo[c]cinnoline; p-R-Form = N,N'-di-p-R-phenylformamidinate), where R = -OCH(3) (1), -CH(3) (2), -H (3), -F (4), -Cl (5), and -CF(3) (6), and their respective radical anions were prepared and their excited state properties were investigated. Complex 3 acted as a single-molecule photocatalyst for H(2) production with red light. Substitution on the formamidinate ligands in 1-6 affected the energy of the Rh(2)(δ*)/Form(π,nb) highest occupied molecular orbital (HOMO), consistent with the metal/ligand-to-ligand charge transfer ((1)ML-LCT) absorption maxima and the (3)ML-LCT excited state lifetime, ranging from 1.6 ns in 1 to 54 ns in 6 in CH(3)CN. The highest turnover number for photocatalytic H(2) evolution was observed for 3, and the lowest values were for 1 and 6. The radical anion, [Rh(2)](-), 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 (2)ML-LCT, whereas that in [6](-) was a bncn(-) → Rh(2)(σ*) ligand-to-metal charge transfer ((2)LMCT) state. The (2)LMCT 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 [Rh(2)](-) and the generation of the doubly reduced species to effectively catalyze hydrogen evolution.