New pyrenyl fluoroscent amphiphiles: synthesis and aggregation properties

Four cationic amphiphilic fluorescent probes were synthesized and fully characterized. All amphiphilesfeature a pyrrolidinium headgroup and a hydrocarbon tail tagged with a pyrene residue, and differ inthe connection between the headgroup and the tagged tail, and in the hydrophilic/hydrophobic balance,two being single tail surfactants and the others twin type surfactants. The aggregation features wereinvestigated by electrical conductivity, fluorescence, dynamic light scattering and transmission electronmicroscopy experiments. The obtained results show that changes in the molecular structure of themonomer, by affecting the aggregation behavior, have a strong influence on the conformation of thefluorophore inside the aggregate, and hence on its fluorescence features in aggregating conditions. Inparticular, the formation of pyrene dimers and/or excimers was observed only for the twin typesurfactants, thus suggesting that a second alkyl chain is crucial for warranting a specific orientation ofthe pyrene moiety suitable for dimers or excimers formation. Moreover, the different linkage of thepyrene moiety to the alkyl chain in the twin type surfactants results in a remarkable difference in theorganization of the aggregates they form, in fact only in the aggregates formed by one of the twosurfactants the pyrene moiety is exposed at the lipid-water interface.