Steady State and Time-Resolved Fluorescence Spectroscopy of Cinchonine Dication in Sodium Dodecylsulphate Micellar System.

This paper reports the influence of surface charge of the micelles on to the photophysical properties of a cinchonine dication (C²⁺) fluorophore in anionic, sodium dodecylsulphate (SDS), surfactant at premicellar, micellar and post-micellar concentrations in aqueous phase at room temperature. The magnitude of edge excitation red shift (EERS) in the fluorescence maximum of C²⁺ in bulk water solution is 1897 cm^- 1 whereas, in the case of SDS it is observed to be 1984 cm^- 1. The fluorescence decay curve of C²⁺ fits with multi exponential functions in the micellar system. The increase in lifetime of C²⁺ in SDS has been attributed to the increase in radiative rate due to the incorporation of C²⁺ at the micelle -water interface. The value of dynamic quenching constant determined is 16.9 M^- 1. The location of the probe molecule in micellar systems has been justified by a variety of spectral parameters such as dielectric constant, E_T (30), viscosity, EERS, average fluorescence decay time, radiative and non-radiative rate constants. All experimental results suggest that the C²⁺ molecule binds strongly with the SDS micelles and resides at micellar-water interface. The binding constant (K_b) calculated (3.85 × 10⁵ M^- 1) for C²⁺ in SDS revealed that the electrostatic forces mediate charge probe-micelle association.