Quenching of fluorescence from Ru(dipy) 2+3 and Ru(dipy)2(CN)2 in solutions of sodium dodecyl sulphate

Abstract

The kinetics of the fluorescence quenching of Ru(dipy)²⁺₃ and Ru(dipy)₃(CN)₂ by Cu²⁺ and MV²⁺ are reported. At SDS concentrations above the c.m.c. the kinetics of Ru(dipy)²⁺₃ quenching by both quenchers are simple first order with constants which increase linearly with quencher concentration and which decrease with increasing SDS concentration. The results quantitatively agree with those predicted by the model in which the emitter is completely micellised and the quenching is slow enough to allow equilibration of the quencher between solution and micelles. With Ru(dipy)₂(CN)₂ above the c.m.c., the quenching by MV²⁺ has fast and slow components, the latter being that of the natural emission decay. The results are shown to be consistent with almost complete micellisation of the quencher and a high micellar quenching rate constant. With Cu²⁺ the micellar quenching is too fast to follow so that ‘quasi-static quenching’ is seen, i.e. there is a decrease in the initial emission intensity followed by the slower unquenched decay. However, analysis shows that specific interaction of Cu²⁺ with the emitter on the micelle must be assumed to account for the abnormally high efficiency of Cu²⁺.

Enhanced quenching of Ru(dipy)²⁺₃ emission by Cu²⁺ and MV²⁺ is also observed below the c.m.c. This is explained in terms of the formation of clusters of the emitter and SDS with which the quenchers associate. With Ru(dipy)₂(CN)₂ only Cu²⁺ gives enhanced quenching below the c.m.c. It is suggested that Cu²⁺ complexes with SDS to form a micelle-like species with which the emitter can associate.