Excited-state behavior of 7-diethylaminocoumarin dyes in AOT reversed micelles: Size effects

Photophysical properties of two 7-diethylaminocoumarin dyes belonging to the family of coumarinylbenzopyrano pyridines (BC I and BC II), and a third dye, 7-diethylamino-4-trifluoromethyl coumarin (C 35), are reported in Aerosol OT (AOT) and water-in-oil (n-heptane/AOT/water) microemulsions. In the case of BC I, emission arising from two different environments for the dye is observed. Results of the steady-state fluorescence anisotropy experiments also support the presence of two different environments. At low W-0 (=[water]/[AOT]) values (0 < W-0 < 10), the excited-state properties of the molecule in the interfacial region are found to be sensitive to the polarity of the surrounding microenvironment. The decrease in the fluorescence quantum yield at higher W-0 (greater than or equal to 10) values can be attributed to the intermolecular hydrogen bonding between the cyano (-CN) group of the dye and water molecules present in the micellar interface. These observations are explained on the basis of the comparable size of the probe dye molecule and the molecular dimensions of the interfacial region in the microemulsions. The large molecular size of BC II is also found to be the reason for the sensitivity of the dye to the microenvironment of n-heptane/AOT/water microemulsions. However, the effects are much smaller than those observed in BC I. A similar photophysical study involving C 35 is found to show no such effects, The smaller molecular size, charge neutrality, hydrophobic nature, and a preferential solvation in heptane are essentially the reasons for the results observed in C 35.