Unravelling the Relaxation Pathway and Excited State Dynamics of Ruthenium(II) Polypyridyl Complexes Incorporating...

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.