Photoinduced mixed-valence systems based on MLCT excited states in bimetallic complexes

Among the different products of charge separation in the photosynthetic reaction center, symmetry-breaking and non-symmetry-breaking charge separated species have been identified. However, photosynthetic reaction-center models have mainly focused on the former. Non-symmetry-breaking charge separated products involve an oxidized special pair and a reduced redox-active fragment that is external to the special pair. Photoinduced mixed-valence systems based on metal-to-ligand charge transfer excited states in bimetallic coordination complexes are excellent models thereof. Their systematic study allowed to derive structure-property correlations, and to understand the effects of the transiently reduced ligand (called charge-transfer counterpart) on the properties of the bimetallic mixed-valence core. In this review, bimetallic photoinduced mixed-valence systems are analyzed in detail, focusing on their differences with their corresponding bimetallic ground-state mixed-valence systems, which lack the charge-transfer counterpart. Electronic and redox effects are discussed in terms of their physical origin, globally analyzing recent examples from the literature. These effects modulate critical properties of bimetallic mixed-valence systems, like metal-metal electronic coupling and charge-density distribution between the redox-active metal centers, all of which have a profound impact on excited-state lifetimes and reactivity. Therefore, they are important in the design of solar-energy conversion schemes which exploit bimetallic complexes to mimic the photosynthetic reaction center and generate solar fuels.