DFT Insights on Ligand Photodissociation Pathways in Ruthenium–Terpyridine Complexes: 3 MLCT- or 3 MC-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 3MLCT–3MC energy-gap trends and revealed an atypical dissociative 3MC minimum dominated by tpy distortion rather than pronounced Ru–ACN elongation. From this distorted 3MC state, we located a transition structure for ACN loss leading to a pentacoordinate intermediate (3RuP), demonstrating that photosubstitution can proceed without a canonical elongated-bond 3MC geometry. For 1, the 3MC → 3RuP dissociation barrier exceeded the 3MLCT → 3MC internal-conversion barrier, whereas for 2, the two processes were competitive, rationalizing experimental photodissociation quantum yields. Relaxed scans suggest that both 3MC- and 3MLCT-mediated pathways for ACN loss are plausible and competitive across the series. The strongest support for 3MLCT-mediated dissociation is found for complex 1, where a transition state has been localized. Finally, sterically tuned tpy derivatives bearing ortho- or meta-methyl substituents (4, 5) biased 3MLCT–3MC conversion by stabilizing the distorted 3MC state while leaving the ACN-loss barrier largely unchanged, providing a design strategy to tune photosubstitution.