Not All (3)MC States Are the Same: The Role of (3)MC(cis) States in the Photochemical N(∧)N Ligand Release from [Ru(bpy)(2)(N(∧)N)](2+) Complexes

[Image: see text] 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)(2)(N(∧)N)](2+) (bpy = 2,2′-bipyridyl; N(∧)N = bpy (1), 6-methyl-2,2′-bipyridyl (2), 6,6′-dimethyl-2,2′-bipyridyl (3), 1-benzyl-4-(pyrid-2-yl)-1,2,3-triazole (4), 1-benzyl-4-(6-methylpyrid-2-yl)-1,2,3-triazole (5), 1,1′-dibenzyl-4,4′-bi-1,2,3-triazolyl (6)), in which we probe the contribution to the promotion of photochemical N(∧)N 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 N(∧)N ligand. Complexes 2 to 6 all release the ligand N(∧)N on irradiation in acetonitrile solution to yield cis-[Ru(bpy)(2)(NCMe)(2)](2+), 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 N(∧)N ligand (2 and 5) leads to a significantly enhanced rate of triplet metal-to-ligand charge-transfer ((3)MLCT) state deactivation but with little promotion of photoreactivity, whereas replacement of pyridine by triazole donors (4 and 6) leads to a similar rate of (3)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 2 and 5 sterically inhibits the formation of a (3)MC(cis) state but promotes the population of (3)MC(trans) states which rapidly deactivate (3)MLCT states and are prone to mediating ground-state recovery. On the other hand, increased photochemical reactivity in 4 and 6 seems to stem from the accessibility of (3)MC(cis) states. The data provide important insights into the excited-state mechanism of photochemical ligand release by Ru(II) tris-bidentate complexes.