New pyrenyl fluoroscent amphiphiles: synthesis and aggregation properties

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.