Test of the Orbital-Based LI3 Index as a Predictor of the Height of the 3MLCT →3MC Transition-State Barrier for [Ru(N∧N)3]2+ Polypyridine Complexes in CH3CN

Ruthenium(II) polypyridine compounds often have a relatively long-lived triplet metal-ligand charge transfer (³MLCT) state, making these complexes useful as chromophores for photoactivated electron transfer in photomolecular devices (PMDs). As different PMDs typically require different ligands and as the luminescence lifetime of the ³MLCT is sensitive to the structure of the ligand, it is important to understand this state and what types of photoprocesses can lead to its quenching. Recent work has increasingly emphasized that there are likely multiple competing pathways involved, which should be explored in order to fully comprehend the ³MLCT state. However, the lowest barrier that needs to be crossed to pass over to the nonluminescent triplet metal-centered (³MC) state has been repeatedly found to be a trans dissociation of the complex, at least in the simpler cases studied. This is the fourth in a series of articles investigating the possibility of an orbital-based luminescence index (LI3, because it was the most successful of three) for predicting luminescence lifetimes. In an earlier study of bidentate (N^∧N) ligands, we showed that the gas-phase ³MLCT → ³MC mechanism proceeded via an initial charge transfer to a single N^∧N ligand, which moves symmetrically away from the central ruthenium atom, followed by a bifurcation pathway to one of two ³MC enantiomers. The actual transition state barrier was quite small and independent, to within the limits of our calculations, of the choice of ligand studied. Here, we investigate the same reaction in acetonitrile, CH₃CN, solution and find that the mechanism differs from that in the gas phase in that the reaction passes directly via a trans mechanism. This has implications for the interpretation of LI3 via the Bell-Evans-Polanyi principle.