Effect of anionic molecular assembly environments on fluorescence quenching by electron transfer

Abstract

The quenching of the excited state of [tris(2,2'-bipyridine) ruthenium(II)][Ru(bpy) 2+ 3] by cationic quenchers [methylviologen, diquat, copper(II) and thallium(I)] in molecular assemblies such as polyelectrolytes, polysoaps and micelles have been investigated by steady-state and time-resolved luminescence techniques. The emission quenching of photoexcited positively charged sensitizers was greatly enhanced in anionic molecular assemblies in comparison to neat water. The quenching in poly(styrenesulfonate)—(PSS) solution is found to be dynamic in nature. In the absence of a quencher, the emission decay ∗Ru(bpy) 2+ 3 in polyelectrolyte solution is single exponential. Upon addition of cationic quencher the emission decay is clearly nonexponential. The detailed studies showed that emission decay of ∗Ru(bpy) 2+ 3 over the whole range of cationic quencher concentration, is well described by a general model of dispersed kinetics, in which there is a Gaussian distribution of the logarithm of the rate constants about some mean. The distribution can be characterized by a mean rate constant, k̄, and parameter γ—the width of the Gaussian distribution. Laser flash photolysis was used to study the yield of electron transfer as well as the rate of the back reaction. Addition of the polyelectrolyte markedly decreases the efficiency of charge separation in the organized system as compared to the aqueous solution. Steady-state spectroscopic and pulsed laser techniques have been used to investigate electron transfer from excited Ru(bpy) 2+ 3 to methylviologen in the PSS film. The kinetics of luminescence quenching are consistent with electron-tunneling mechanism where the rate constant k( r) has an exponential dependence on the reactant separation distance, r, with the matrix vibration, and is shown to be quite temperature dependent. No quenching of ∗Ru(bpy) 2+ 3 by methylviologen was observed in glassy solutions or polymer films at 77 K.