DFT Insights on Ligand Photodissociation Pathways in Ruthenium–Terpyridine Complexes: 3MLCT- or 3MC-Triggered?

We computationally elucidated the mechanism of acetonitrile (ACN) photodissociation in Ru-terpyridine PACT complexes from Turro's series, [Ru(tpy)(L)(ACN)] n+ , where tpy = 2,2':6',2''-terpyridine and L = 4,4'-dimethyl-2,2'-dipyridylbipyridine (1), acetylacetonate (2) or 1,3-diphenylpropane-1,3-dione (3).DFT reproduced the experimental 3 MLCT-3 MC energy-gap trends and revealed an atypical dissociative 3 MC minimum dominated by tpy distortion rather than pronounced Ru-ACN elongation.From this distorted 3 MC state, we located a transition structure for ACN loss leading to a pentacoordinate intermediate ( 3 RuP), demonstrating that photosubstitution can proceed without a canonical elongated-bond 3 MC geometry.For 1, the 3 MC →dissociation barrier exceeded the 3 MLCT→ 3 MC internal-conversion barrier, whereas for 2, the two processes were competitive, rationalizing experimental photodissociation quantum yields.Transition-state searches and relaxed scans supported direct ACN loss on the 3 MLCT surface, confirming the pathway postulated by Turro and co-workers in previous works.Finally, sterically tuned tpy derivatives bearing orthoor meta-methyl substituents (4, 5) biased 3 MLCT-3 MC conversion by stabilizing the distorted 3 MC state while leaving the ACN-loss barrier largely unchanged, providing a design strategy to tune photosubstitution.Overall, the results indicate that 3 MC-driven (tpy-distortion) and 3 MLCT-mediated mechanisms can operate in parallel, with their relative contributions depending on the specific complex of the series.