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 O 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, ([]-[]), where L-L = 1,2-bis(phenylthio)ethane (SPH) [], ,,','-tetramethylethylenediamine (TMEN) [], 1,2-diphenylethane-1,2-diimine (BPEDI) [], and bis[2-(diphenylphosphino)phenyl]ether (DPE-Phos) []. The detailed photochemical studies suggest a single-step dissociation of L-L from the bis-thioether (SPH) complex [] and diamine (TMEN) complex [], while no photosubstitution was observed for [] and []. Complex [] and [] demonstrated a dual role, involving both photosubstitution and O generation, while [] and [] solely exhibited poor to moderate O production. The interplay of excited states leading to these behaviors was rationalized from the lifetimes of the MLCT excited states by using transient absorption spectroscopy, suggesting intricate relaxation dynamics and O generation upon excitation. Therefore, the photolabile complexes [] and [] could potentially act as dual photoreactive agents via the phototriggered release of L-L (PACT) and/or O-mediated PDT mechanisms, while [] primarily can be utilized as a PDT agent.