Fluorescence based studies on the interaction and characterization of surface-active ionic liquids with polarity sensitive intramolecular charge transfer probe

Surface-active ionic liquids (SAILs) are of tremendous interest in recent times due to their improved application in the field of food, detergent, and health. The modulated fluorescence behavior of an intramolecular charge transfer (ICT) probe, trans-ethyl-p-(dimethylamino) cinnamate (EDAC), in presence of five different SAILs with varying head groups (viz. imidazolium, pyridinium, and morpholinium) and different alkyl chain length (decyl, dodecyl, and tetradecyl) was monitored by steady state and time-resolved fluorescence spectroscopy. Extreme sensitivity of EDAC fluorescence was used to characterize different physicochemical properties of the SAILs, including critical micelle concentration, static polarity, and empirical solvent polarity scale, ET(30) of the microenvironment. The estimated parameters are in good agreement with the literature reports and measured independently from other complimentary experiments discussed here. Time-resolved fluorescence experiments show a significant retardation in different nonradiative decay channels of EDAC, when compared to that in aqueous phase, indicating a preferential association of the probe in presence of SAILs. The results indicate that the physicochemical properties of SAILs can be tuned by controlling the nature of both the cations and as well as the chain length of the alkyl group. These properties also show significant modulation in solutions with varying SAIL concentration, particularly in the pre-and post-micellar region. The results, particularly the surface-active properties and the self-assembly behavior presented in this study, are expected to provide new knowledge towards the design and development of novel SAILs with specific industrial and biological applications.