Unraveling the Mechanisms of the Formations and Transformations of Metal–Ligand Charge Transfer States in [Ru(tpy)₂]^2+*: Consequences of Jahn–Teller Conical Intersections and the Pseudo–Jahn–Teller Effect

This work investigates Jahn-Teller conical intersections (CoIns) and the pseudo-Jahn-Teller effect on the formations and transformations of the low-lying singlet metal-ligand charge transfer (¹MLCT) excited states during the ultrafast evolution process of photoexcited [Ru(tpy)₂]^2+* (tpy = 2,2':6',2″-terpyridine). Longuet-Higgins' geometric phase analyses indicate that the potential energy surface (PES) crossing between charge transfer states ¹MLCT₁ and ¹MLCT₂ is a CoIn, originating from the change in diabatic Hamiltonian matrix elements around the CoIn. Moreover, an <i>E</i>⊗(<i>b</i>₁ + <i>b</i>₂) Jahn-Teller distortion can occur around the Franck-Condon and minimal energy CoIn (MECI) configurations, causing the molecule to distort spontaneously from the high-symmetry <i>D</i>_2<i>d</i> configuration to <i>C</i>_2<i>v</i> symmetry configurations that are close to it. Furthermore, the pseudo-Jahn-Teller effect can cause the molecule to distort further from <i>C</i>_2<i>v</i> to <i>C</i>₁ geometries since the former is a second-order saddle point on the whole dimensional PES but the latter is a true minimum. Eight minima in total are symmetrically distributed around the MECI. These minima are connected by the interligand electron transfer, the charge transfer, and the butterfly-like conformational inversion reactions, all of which have extremely small energy barriers.