3,3′-Diethyloxacarbocyanine iodide: A new microviscosity probe for micelles and microemulsions

Fluorescence studies of a carbocyanine dye 3,3′-diethyloxacarbocyanine iodide (DOCI) in a series of monohydroxy alcohols reveal a monotonic increasing trend for both the fluorescence quantum yield and fluorescence lifetime with solvent viscosity. The results were interpreted in terms of retardation of the photoisomerization rate of DOCI with increasing solvent viscosity, and a definite inverse relationship between the two parameters was established. This relationship was utilized to estimate the microviscosity experienced by DOCI in supramolecular assemblies like micelles and microemulsions of several well-known surfactants and amphiphilic block copolymer. In most of the assemblies, the local environment around DOCI was characterized by rather high microviscosity, comparable to those of high alcohols between butanol and decanol. Our observations suggest that the rod-like DOCI molecule is largely embedded among the hydrophobic tails of the surfactant molecules, which exert a viscous drag strong enough to hinder its photoisomerization. The fluorescence properties of DOCI were also utilized to determine the critical micellization concentration of several surfactants in water. Interestingly, the microviscosity reported by DOCI in water-free AOT reverse micelles is significantly high, and can be explained by considering enhanced electrostatic attraction between the DOC+ cation and the anionic AOT head-groups.