Surfactant-induced fluorescence enhancement of a quinoline-coumarin derivative in aqueous solutions and dropcast films

Abstract

Fluorescent aggregates, with a very small quantum yield and a large Stokes shift, are formed in aqueous solutions of 7-(diethylamino)-3-(2,3-dihydrofuro[3,2-c]quinolin-4-yl)–2H-chromen-2-one (DDQC). They comprise not only the neutral but also the cationic form of the molecule. Surfactants disrupt these aggregates, yielding strongly emissive micellized monomers, and comparable degrees of enhancement with cetyltrimethylammonium bromide (CTAB) and triton X-100 (TX-100). A lesser degree of enhancement with sodium dodecyl sulfate (SDS) is rationalized by the preponderance of the less emissive cationic form of DDQC, due to the smaller local pH in the vicinity of the negatively charged Stern layer. Interestingly, time-resolved fluorescence experiments indicate the presence of the cation in TX-100 micelles as well, even though the headgroup is neutral in this case. This surprising observation is rationalized by the presence of a significant amount of water in the palisade layer of TX-100, which leads to the retention of the protonation-deprotonation equilibrium to some extent. Upon dropcasting films from these solutions, rodlike fibres are obtained with CTAB and SDS, while spherical structures are observed with TX-100. Fluorescence lifetime imaging microscopy (FLIM) at different ranges of emission provides an insight into the species that contribute to the enhanced fluorescence of DDQC in these nanostructures.