New pyrenyl fluorescent amphiphiles: synthesis and aggregation properties

Abstract

Four cationic amphiphilic fluorescent probes were synthesized and fully characterized. All amphiphiles feature a pyrrolidinium headgroup and a hydrocarbon tail tagged with a pyrene residue, and differ in the 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 were investigated by electrical conductivity, fluorescence, dynamic light scattering and transmission electron microscopy experiments. The obtained results show that changes in the molecular structure of the monomer, by affecting the aggregation behavior, have a strong influence on the conformation of the fluorophore inside the aggregate, and hence on its fluorescence features in aggregating conditions. In particular, the formation of pyrene dimers and/or excimers was observed only for the twin type surfactants, thus suggesting that a second alkyl chain is crucial for warranting a specific orientation of the pyrene moiety suitable for dimers or excimers formation. Moreover, the different linkage of the pyrene moiety to the alkyl chain in the twin type surfactants results in a remarkable difference in the organization of the aggregates they form, in fact only in the aggregates formed by one of the two surfactants the pyrene moiety is exposed at the lipid–water interface.