Unexpected Role of Ru(II) Orbital and Spin Contribution on Photoinduced Ligand Exchange: New Mechanism To Access the Photodynamic Therapy Window

Abstract

A series of 12 Ru(II) complexes of the type [Ru(tpy)(L)(CH3CN)]n+, 1–12, containing the tridentate tpy ligand (tpy = 2,2′:6′,2″-terpyridine) and various bidentate ancillary ligands, L, were synthesized and evaluated for their ability to photodissociate CH3CN, a model for nitrile-containing drugs. Although the bidentate ligands chosen display a similar degree of steric bulk around the metal center in the ground state, the photosubstitution efficiencies of 1–12 vary by approximately an order of magnitude. The complexes containing the most electron-donating bidentate ligands, 8–11, exhibit the larger quantum yield values for ligand exchange in water. Complexes 8–11 also possess the smallest energy gap between the ground state and the lowest energy triplet metal-to-ligand charge transfer (3MLCT) excited state. In addition, density functional theory calculations indicate that a large degree of ligand character is present in their highest occupied molecular orbitals (HOMOs) and in their 3MLCT excited states. These results show that ligand mixing affects the π-backbonding ability of the metal center in the excited state, increasing the quantum yields of nitrile ligand dissociation. Linear relationships were observed between the quantum yield of ligand exchange and both the Mulliken spin density on the metal center in the excited state and the percent of Ru(d) character in the ground state HOMO. These findings, driven by the presence of π-donating ancillary ligands that enhance excited state reactivity, can be used to design new complexes with higher quantum yields for the release of nitrile-containing drugs and biological probes without affecting their stability in the dark.