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 (³MLCT) state deactivation but with little promotion of photoreactivity, whereas replacement of pyridine by triazole donors (4 and 6) leads to a similar rate of ³MLCT 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 ³MC_cis state but promotes the population of ³MC_trans states which rapidly deactivate ³MLCT 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 ³MC_cis states. The data provide important insights into the excited-state mechanism of photochemical ligand release by Ru(II) tris-bidentate complexes.

中文翻译：

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并非所有 3MC 状态都相同：3MCcis 状态在 [Ru(bpy)2(N∧N)]2+ 配合物光化学 N∧N 配体释放中的作用

钌 (II) 配合物在光活化化疗药物的开发中占有重要地位；然而，光化学配体释放的激发态机制仍不清楚。我们在此报告了一系列配合物 [Ru(bpy) ₂ (N ^∧ N)] ²⁺ (bpy = 2,2'-联吡啶；N ^∧ N = bpy ( 1 ), 6-甲基-2,2'-联吡啶 ( 2 ), 6,6'-二甲基-2,2'-联吡啶 ( 3 ), 1-苄基-4-(pyrid-2-yl)-1,2,3-三唑 ( 4 ), 1-benzyl-4-(6-methylpyrid-2-yl)-1,2,3-triazole ( 5 ), 1,1'-dibenzyl-4,4'-bi-1,2,3-三唑基 ( 6)), 其中我们探讨了引入空间阻碍甲基取代基对促进光化学 N ^{∧ N 配体释放的贡献以及 N ∧} N配体中 1,2,3-三唑供体取代吡啶的电子效应. 配合物2至6在乙腈溶液中辐照时全部释放配体 N ^∧ N 以产生顺式- [Ru(bpy) ₂ (NCMe) ₂ ] ²⁺，由此产生的光致释放量子产率乍一看似乎违反直觉并且跨越广泛范围。数据表明，在 N ^∧ N 配体 (2和5 ) 导致三线态金属到配体电荷转移 ( ³ MLCT) 状态失活率显着提高，但几乎没有促进光反应性，而用三唑供体取代吡啶（4和6）导致相似的速率³ MLCT 失活但具有更大的光化学反应性。此处报告的数据与先前报告的相关复合物数据一起讨论，表明2和5中的单甲基化空间抑制³ MC_顺式状态的形成，但促进³ MC_{反式的数量}迅速停用^{3 个}MLCT 状态并且易于调解基态恢复的状态。另一方面，4和6中光化学反应性的增加似乎源于³ MC_顺式状态的可及性。这些数据为 Ru(II) 三双齿配合物释放光化学配体的激发态机制提供了重要见解。