Nanosecond time resolved emission spectroscopy of aminocoumarins in AOT reversed micelles

Abstract

Time resolved emission spectroscopy of a water insoluble aminocoumarin derivative, BC I, in n-heptane–AOT–water reversed micelles is investigated using a nanosecond fluorescence spectrometer. The extent of the time-dependent Stokes shift is found to be smaller for W₀=4 than for W₀=10 and also for W₀=40 (W₀, molar ratio of water and AOT). A monoexponential decay of the dynamic Stokes shift correlation function with a time constant of 4.5 ns is observed at W₀=4. At W₀=10 the time-correlation function shows a bi-exponential decay with a fast component of ∽600 ps and another of 3.3 ns comparable to the fluorescence lifetime of the dye. The time correlation function at W₀=40 shows an extremely slow component of the solvation time which is at least an order of magnitude larger than the fluorescence lifetime of the probe dye. In addition a faster component with a solvation time of 2.15 ns is also observed. The solvation time observed is likely due to the diffusion of the probe dye within the micellar interface or due to the reorganization of the water molecules bound to the polar head groups of AOT. Steady-state absorption and fluorescence, and time resolved fluorescence study of another aminocoumarin, C 480, give evidence of equilibrium between the distribution of the dye molecules between the interface and water pool. The time resolved shift of the emission spectra is more likely due to the diffusion of the probe between the interface and water pool rather than reorientation of the water molecules (solvent relaxation) in the core of the reversed micelles as it has been reported earlier.