Steady state and time-resolved fluorescence spectroscopy of quinine sulfate dication in ionic and neutral micelles: Effect of micellar charge on photophysics

Steady state and time-resolved fluorescence studies have been carried out to investigate the effects of micellar surface charge on the photophysics of a well known fluorescent molecule quinine sulfate dication (QSD) in cationic, cetyltrimethylammonium bromide (CTAB), anionic, sodium dodecylsulphate (SDS) and neutral, triton X-100 (TX100) surfactants at concentrations above the critical micelle concentrations (c.m.c) in aqueous phase. Edge excitation red shift (EERS) in fluorescence maximum of QSD has been observed in all the surfactant solutions studied. The magnitude of observed EERS is less in anionic SDS surfactant solution compared to the EERS in CTAB and TX100 surfactants. The magnitude of EERS in CTAB and TX100 is almost the same as in bulk water solution. The EERS has been ascribed in terms of solvent relaxation process. The observed multi-exponential decay of fluorescence is due to the different locations of QSD in micellar systems. In SDS surfactant system, due to heterogeneous restricted motion of solvent molecules the solvent relaxation rate decreases which results in a decrease in net magnitude of EERS and fluorescence decay components fit in three exponentials. Following the two step and wobbling in a cone model for the analysis of the temporal fluorescence anisotropy decay of QSD in SDS micelles allows determination of restriction on the motion of fluorophore. Further, we have shown that the extraordinary capability to sense the surrounding environment makes QSD molecule very efficient for surface and interface studies and can also be used as a probe to investigate the mobility of solvent molecules around the excited molecules.