Static and Dynamic Quenching of Ru(II) Polypyridyl Excited States by Iodide

The metal-to-ligand charge-transfer (MLCT) excited states of Ru(bpy)2(deeb)(PF6)2, where bpy is 2,2-bipyridine and deeb is 4,4‘-(CO2CH2CH3)2-2,2‘-bipyridine, in dichloromethane were found to be efficiently quenched by iodide at room temperature. The ionic strength dependence of the UV−visible absorption spectra gave evidence for ion pairing. Iodide was found to quench the excited states by static and dynamic mechanisms. Stern−Volmer and Benesi−Hildebrand analysis of the spectral data provided a self-consistent estimate of the iodide−Ru(bpy)2(deeb)2+ adduct in dichloromethane, K = 59 700 M-1. Transient absorption studies clearly demonstrated an electron-transfer quenching mechanism with transient formation of I2•- in high yield, φ = 0.25 for 355 or 532 nm excitation. For Ru(bpy)2(deeb)(PF6)2 in acetonitrile, similar behavior could be observed at higher iodide concentrations than that required in dichloromethane. The parent Ru(bpy)32+ compound also ion pairs with iodide in CH2Cl2, and light excitation gave a higher I2•- yield, φ = 0.50. X-ray crystallographic, IR, and Raman data gave evidence for interactions between iodide and the coordinated deeb ligand in the solid state.