Nature of ultrafast dynamics in the lowest-lying singlet excited state of [Ru(bpy)₃]²

This work presents mechanisms to rationalize the nature of ultrafast photochemical and photophysical processes on the first singlet metal-ligand charge transfer state (¹MLCT₁) of the [Ru(bpy)₃]²⁺ complex. The ¹MLCT₁ state is the lowest-lying singlet excited state and the most important intermediate in the early evolution of photoexcited [Ru(bpy)₃]²⁺*. The results obtained from simple but interpretable theoretical models show that the ¹MLCT₁ state can be very quickly formed via both direct photo-excitation and internal conversions and then can efficiently relax to its equilibrium geometry in ca. 5 fs. The interligand electron transfer (ILET) on the potential energy surface of the ¹MLCT₁ state is also extremely fast, with a rate constant of ca. 1.38 × 10¹³ s^-1. The ultrafast ILET implies that the excited electron can dynamically delocalize over the three bpy ligands, despite the fact that the excited electron may be localized on either one of the three ligands at the equilibrium geometries of the three symmetric equivalent minima. Since rapid ILET essentially suggests delocalization, the long-standing controversy in inorganic photophysics-whether the excited electron is localized or delocalized-may therefore be calmed down to some extent.