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One of the main concerns in the photochemistry and photophysics of ruthenium complexes is the de-excitation of the triplet metal centered ligand-field state MC. To understand the mechanism by which theMC states in some reversible photochemical reactions could avoid the fate of fast decay and ligand dissociations, the photoinduced chiral inversion at the metal center of the complexes [Ru(diimine)(l-ser)] (diimine = 1,10-phenanthroline or 2,2'-bipyridine, l-ser = l-serine) has been analyzed at the first principle level of theory. The calculated equilibrium constants and ECD curves for the photoinduced equilibrium mixtures are in agreement with the observed ones. The results showed that the reversible photochemical process Δ(δS) ⇌ Λ(δS) on the potential surface of the lowest triplet excited state proceeds in three steps: CT ↔ MC, MC ↔ MC, MC ↔ CT, where the first and the third steps involve mainly the elongation and compression of the octahedral core of the reactant Δ(δS) and product Λ(δS), respectively. The chiral inversion Δ ↔ Λ takes place in the second step through a much distorted square-pyramid-like transition state, and actually proceeds on the triplet ground state MC due to the crossover of the triplet T and singlet S states. Inspecting the transient structures at the crossing points, we found that they become less distorted and their lowest or imaginary-frequency displacement vectors in triplet state still dominate the reaction path, which makes the reaction reversible without ligand release. Thus, the triplet ground-state-bridged photoinduced mechanism offers a new angle of view to understand the related reversible photochemical reactions.

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