Ultrafast photo-induced ligand solvolysis of cis-[Ru(bipyridine) sub(2)(nicoti namide) sub(2)] super(2+): experimental and theoretical insight into its photoactivation mechanism

• Published in: Physical chemistry chemical physics : PCCP Vol. 16; no. 36; pp. 19141 - 19155
• Main Authors: Greenough, Simon E, Roberts, Gareth M, Smith, Nichola A, Horbury, Michael D, McKinlay, Russell G, Zurek, Justyna M, Paterson, Martin J, Sadler, Peter J, Stavros, Vasilios G
• Format: Journal Article
• Language: English
• Published: 01.08.2014
• Subjects: Absorption spectroscopy; Density functional theory; Effectiveness; Irradiation; Ligands; Photodynamic therapy; Solvents; Solvolysis
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c4cp02359e

Mechanistic insight into the photo-induced solvent substitution reaction of cis-[Ru(bipyridine) sub(2)(nicoti namide) sub(2)] super(2+) (1) is presented. Complex 1 is a photoactive species, designed to display high cytotoxicity following irradiation, for potential use in photodynamic therapy (photochemotherapy). In Ru(ii) complexes of this type, efficient population of a dissociative triplet metal-centred ( super(3)MC) state is key to generating high quantum yields of a penta-coordinate intermediate (PCI) species, which in turn may form the target species: a mono-aqua photoproduct [Ru(bipyridine) sub(2)(nicotinamide)(H sub(2 )O)] super(2+) (2). Following irradiation of 1, a thorough kinetic picture is derived from ultrafast UV/Vis transient absorption spectroscopy measurements, using a 'target analysis' approach, and provides both timescales and quantum yields for the key processes involved. We show that photoactivation of 1 to 2 occurs with a quantum yield greater than or equal to 0.36, all within a timeframe of similar to 400 ps. Characterization of the excited states involved, particularly the nature of the PCI and how it undergoes a geometry relaxation to accommodate the water ligand, which is a keystone in the efficiency of the photoactivation of 1, is accomplished through state-of-the-art computation including complete active space self-consistent field methods and time-dependent density functional theory. Importantly, the conclusions here provide a detailed understanding of the initial stages involved in this photoactivation and the foundation required for designing more efficacious photochemotherapy drugs of this type.