Fine-Tuning the Excited-State Dynamics of Heteroleptic Ruthenium(II) Polypyridyl Complexes with Systematic Variation of Benzazole-Substituted 8-Hydroxyquinolines

A series of structurally related bistridentate heteroleptic Ru(II) polypyridyl complexes, [Ru^II(ttpy)(8-HQL^S/N/O)]⁺ (<b>Ru1</b>-<b>Ru3</b>), were synthesized, where <b>ttpy</b> = <i>p</i>-tolyl terpyridine and <b>8-HQL</b>^<b>S/N/O</b> are monoanionic N^N^O-donor tridentate ligands (<b>8-HQL</b>^<b>X</b>), derived from 8-hydroxyquinoline (8-HQ), namely, <b>8-HQL</b>^<b>S</b> = 2-(2'-benzothiazole)-8-hydroxyquinoline, <b>8-HQL</b>^<b>N</b> <b>=</b> 2-(2'-benzimidazole)-8-hydroxyquinoline, and <b>8-HQL</b>^<b>O</b> = 2-(2'-benzoxazole)-8-hydroxyquinoline. The electronic structures of these rigid ligands were systematically tuned by varying the noncoordinating heteroatoms (S, O, NH) in the five-membered heterocyclic ring, impacting the electronic properties, redox potentials, excited-state lifetime/dynamics, and deactivation pathways and photophysical behavior of the corresponding Ru(II) complexes. Notably, [Ru^II(ttpy)(8HQL^N)]⁺ (<b>Ru2</b>) exhibited an excited-state lifetime (τ > 1 ns in CH₃CN at RT) surpassing that of the homoleptic complex [Ru(ttpy)₂]²⁺ (τ ∼ 0.62 ns), despite its more distorted octahedral geometry. These heteroleptic complexes (<b>Ru1</b>-<b>Ru3</b>) showed extended excited-state lifetimes compared to their homoleptic counterpart <b>Ru4</b>. The complexes displayed absorption in the red region, which is favorable for phototherapeutic applications. Their relative singlet oxygen (¹O₂) quantum yields (Φ_Δ) in CH₃CN ranged from 0.03 to 0.10. Given their reasonable excited-state lifetimes and ¹O₂ generation ability, these Ru(II) complexes demonstrated potential as photocatalysts for organic substrates, as evidenced by their effectiveness in the photooxidation of PPh₃ to Ph₃P=O as a model reaction.