Not All MC States Are the Same: The Role of MC States in the Photochemical NN Ligand Release from [Ru(bpy)(NN)] Complexes.

Ruthenium(II) complexes feature prominently in the development of agents for photoactivated chemotherapy; however, the excited-state mechanisms by which photochemical ligand release operates remain unclear. We report here a systematic experimental and computational study of a series of complexes [Ru(bpy)(NN)] (bpy = 2,2'-bipyridyl; NN = bpy (), 6-methyl-2,2'-bipyridyl (), 6,6'-dimethyl-2,2'-bipyridyl (), 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (), 1-benzyl-4-(6-methylpyrid-2-yl)-1,2,3-triazole (), 1,1'-dibenzyl-4,4'-bi-1,2,3-triazolyl ()), in which we probe the contribution to the promotion of photochemical NN ligand release of the introduction of sterically encumbering methyl substituents and the electronic effect of replacement of pyridine by 1,2,3-triazole donors in the NN ligand. Complexes to all release the ligand NN on irradiation in acetonitrile solution to yield [Ru(bpy)(NCMe)], with resultant photorelease quantum yields that at first seem counter-intuitive and span a broad range. The data show that incorporation of a single sterically encumbering methyl substituent on the NN ligand ( and ) leads to a significantly enhanced rate of triplet metal-to-ligand charge-transfer (MLCT) state deactivation but with little promotion of photoreactivity, whereas replacement of pyridine by triazole donors ( and ) leads to a similar rate of MLCT deactivation but with much greater photochemical reactivity. The data reported here, discussed in conjunction with previously reported data on related complexes, suggest that monomethylation in and sterically inhibits the formation of a MC state but promotes the population of MC states which rapidly deactivate MLCT states and are prone to mediating ground-state recovery. On the other hand, increased photochemical reactivity in and seems to stem from the accessibility of MC states. The data provide important insights into the excited-state mechanism of photochemical ligand release by Ru(II) tris-bidentate complexes.