Evidence for static quenching of MLCT excited states by iodide

The metal-to-ligand charge-transfer (MLCT) excited states of Ru(deeb)(bpy)(2)(PF6)(2) [where bpy is 2,2-bipyridine and deeb is 4,4'-(CO2CH2CH3)2-2,2'-bipyridine] in acetonitrile or dichloromethane were found to be quenched by iodide at room temperature. The ionic strength dependence of the optical spectra gave evidence for ion pairing. Iodide is found to quench the photoluminescence (PL) intensity and influence the spectral distribution of the emitted light. A static component to the time-resolved PL quenching provided further evidence for ground-state adduct. Stern-Volmer analysis of the static component provided an estimate of the iodide-Ru(deeb)(bpy)(2)(2+) adduct equilibrium constant in dichloromethane, K-sv = 40 000 M-1. Transient absorption studies clearly demonstrate that an electron-transfer quenching mechanism is operative and that I-2(-center dot) can be photoproduced in high yield, phi = 0.25. For Ru(bpy)(3)(PF6)(2) in acetonitrile, similar behaviour could be observed at iodide concentrations > 100 times that required for dichloromethane.