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-phenthroline 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 ~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 difference in photophysical and electrochemical properties of [1]2+ and [1-Me]3+ was rationalized by DFT calculations. Transient absorption spectroscopic analysis to unravel the relaxation pathway of complexes suggested 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. 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.
Keywords: Excited-state dynamics; Photophysics; Ruthenium(II) polypyridyls; Transient absorption spectroscopy; photochemistry.
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