Nature of the Water Structure inside the Pools of Reverse Micelles Sensed by Laser-Induced Optoacoustic Spectroscopy

Abstract

Reaction volume changes, ΔVR, enthalpy content, ΔHMLCT, and decay lifetimes, τ2, of the metal-to-ligand charge transfer (MLCT) complex of (2,2‘-bipyridine)tetracyanoruthenate(II) complex, Ru(bpy)(CN)42-, inside the water pools of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) reverse micelles dispersed in n-alkanes, were determined using laser-induced optoacoustic spectroscopy (LIOAS). Emission quantum yields, Φem, of the complex were also determined by steady-state measurements. The enthalpy content of the MLCT state of Ru(bpy)(CN)42- was not dependent on the amount of water inside the reverse micelles (defined as the molar ratio R = [H2O]/[AOT]) and was, within experimental error, similar to that in water. However, the values of τ2 and Φem decrease while those of ΔVR increase as the values of R are incremented from 3 to 10. At the larger R values, τ2, Φem, and particularly ΔVR resemble the values in neat water. These results are interpreted to arise from the type of water involved in the hydrogen bond interactions between the cyanide ligands of the complex and the water molecules (second-sphere donor−acceptor interactions). In this framework, for reverse micelles solutions at R ≥ 10, the hydrogen bonds are made with the “free” water in the inner pool. For R < 10 the properties of Ru(bpy)(CN)42- are indicative of a more rigid microenvironment in the micellar core. Thus, the photophysical properties (and particularly ΔVR) of Ru(bpy)(CN)42- are very sensitive to its microenvironment and serve as a probe of the type of water involved in the hydrogen bonding. A combination of the LIOAS data from temperature-dependent measurements in neat water with those for the same complex in reverse micelles dispersed in various n-alkanes at high R values improved in 1 order of magnitude the accuracy of the ΔHMLCT and ΔVR values for the MLCT state.