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

High Resolution Image Download MS PowerPoint Slide 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 → 3 RuP dissociation barrier exceeded the 3 MLCT → 3 MC internal-conversion barrier, whereas for 2, the two processes were competitive, rationalizing experimental photodissociation quantum yields. Relaxed scans suggest that both 3 MC- and 3 MLCT-mediated pathways for ACN loss are plausible and competitive across the series. The strongest support for 3 MLCT-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 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.