Unravelling the Relaxation Pathway and Excited State Dynamics of Ruthenium(II) Polypyridyl Complexes Incorporating Phosphorus‐Based Monodentate Ligands

Herein, the photophysical, photochemical properties and photogenerated excited state dynamics of two new Ru(II) complexes, viz. [Ru(p-ttp)(bpy)(PTA)]²⁺ [1]²⁺, [Ru(p-ttp)(phen)(PTA)]²⁺ [2]²⁺ having a phosphorus-based ligand PTA [p-ttp=p-tolyl terpyridine; bpy=2,2'-bipyridyl; phen=1,10-phenanthroline and PTA=1,3,5-triaza-7-phosphaadamantane] are reported. Upon excitation with 470 nm LED, [1]²⁺ and [2]²⁺ neither undergo ligand release nor exhibit room temperature luminescence/¹O₂ generation. However, when excited around their respective MLCT bands at 77 K, luminescence bands at ~600 nm were observed. N-methylation of the coordinated PTA in complex [1]²⁺ resulted in [Ru(p-ttp)(bpy)(MePTA]³⁺ {[1-Me]³⁺} (MePTA=N-methyl-1,3,5-triaza-7-phosphaadamantane). The differences in the photophysical and electrochemical properties of [1]²⁺ and [1-Me]³⁺ were rationalized by DFT calculations. The transient absorption spectroscopic analysis was used to unravel the relaxation pathway of the complexes, suggesting the involvement of the ³MC state in the relaxation process. The effect of electronic modulation resulted from the N-methylation of PTA on the excited state dynamics of the complexes was observed. The MTT assays in various cancer cells revealed their non-cytotoxic nature in the dark or upon irradiation with 470 nm blue LED. The non-toxicity of the complexes, despite being internalized in cancer cells as established by ICP-MS, is attributed to their dark and photostability and the inability to generate ¹O₂.