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Understanding the landscape of molecular photocatalysis is vital to enable efficient conversion of feedstock molecules to targeted products and inhibit off-cycle reactivity. In this study, the light-promoted reactivity of [RuCp*2]+ was explored via electronic structure, photophysical, and photostability studies and the reactivity of [RuCp*2]+ within a photocatalytic hydrogen evolution cycle was assessed. TD-DFT calculations support the assignment of a low-energy ligand-to-metal charge transfer transition (LMCT) centered at 500 nm, where an electron from a ligand-based orbital delocalized across both Cp* ligands is promoted to a dx2–y2-based β-LUMO orbital. Upon irradiating the LMCT absorption feature, ultrafast transient absorption spectroscopy measurements show that an initial excited state (τ1 = 1.3 ± 0.1 ps) is populated, which undergoes fast relaxation to a longer-lived state (τ2 = 12.0 ± 0.9 ps), either via internal conversion or vibrational relaxation. Despite the short-lived nature of these excited states, bulk photolysis of [RuCp*2]+ demonstrates that photochemical conversion to decomposition products is possible upon prolonged illumination. Collectively, these studies reveal that [RuCp*2]+ undergoes light-driven decomposition, highlighting the necessity to construct molecular photocatalytic systems resistant to off-cycle reactivity in both the ground and excited states. 

Ligand-to-metal charge transfer (LMCT) excited states are capable of undergoing a wide array of photochemical reactions, yet receive minimal attention compared to other charge transfer excited states. This work provides general criteria for designing transition metal complexes that exhibit low energy LMCT excited states and routes to drive photochemistry from these excited states. General design principles regarding metal identity, oxidation state, geometry, and ligand sets are summarized. Fundamental photoreactions from these states including visible light-induced homolysis, excited state electron transfer, and other photoinduced chemical transformations are discussed and key design principles for enabling these photochemical reactions are further highlighted. Guided by these fundamentals, this review outlines critical considerations for the future design and application of coordination complexes with LMCT excited states.