Dual Photoreactive Ternary Ruthenium(II) Terpyridyl Complexes: A Comparative Study on Visible-Light-Induced Single-Step Dissociation of Bidentate Ligands and Generation of Singlet Oxygen

The versatile and tunable ligand-exchange dynamics in ruthenium(II)-polypyridyl complexes imposed by the modulation of the steric and electronic effects of the coordinated ligands provide an unlimited scope for developing phototherapeutic agents. The photorelease of a bidentate ligand from the Ru-center is better suited for potent Ru(II)-based photocytotoxic agents with two available labile sites for cross-linking with biological targets augmented with possible phototriggered ¹O₂ generation. Herein, we introduced a phenyl-terpyridine (ptpy) ligand in the octahedral Ru(II) core of [Ru(ptpy)(L–L)Cl]⁺ to induce structural distortion for the possible photorelease of electronically distinct bidentate ligands (L–L). For a systematic study, we designed four Ru(II) polypyridyl complexes: [Ru(ptpy)(L–L)Cl](PF₆), ([<b>1</b>]–[<b>4</b>]), where L–L = 1,2-bis(phenylthio)ethane (SPH) [<b>1</b>], <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethylethylenediamine (TMEN) [<b>2</b>], <i>N</i>1,<i>N</i>2-diphenylethane-1,2-diimine (BPEDI) [<b>3</b>], and bis[2-(diphenylphosphino)phenyl]ether (DPE-Phos) [<b>4</b>]. The detailed photochemical studies suggest a single-step dissociation of L–L from the bis-thioether (SPH) complex [<b>1</b>] and diamine (TMEN) complex [<b>2</b>], while no photosubstitution was observed for [<b>3</b>] and [<b>4</b>]. Complex [<b>1</b>] and [<b>2</b>] demonstrated a dual role, involving both photosubstitution and ¹O₂ generation, while [<b>3</b>] and [<b>4</b>] solely exhibited poor to moderate ¹O₂ production. The interplay of excited states leading to these behaviors was rationalized from the lifetimes of the ³MLCT excited states by using transient absorption spectroscopy, suggesting intricate relaxation dynamics and ¹O₂ generation upon excitation. Therefore, the photolabile complexes [<b>1</b>] and [<b>2</b>] could potentially act as dual photoreactive agents via the phototriggered release of L–L (PACT) and/or ¹O₂-mediated PDT mechanisms, while [<b>4</b>] primarily can be utilized as a PDT agent.