Enhanced aggregation of derivatized tolan surfactants through donor - acceptor interactions at the air - water interface and in Langmuir - Blodgett films

Control of aggregation behavior of derivatized surfactants by moderately polar functional groups as well as the role of charge-transfer interactions governing photophysical phenomena was studied using surfactants incorporating the diphenylacetylene (tolan) chromophore (TFAs) modified with either a sulfone or an ether group (sTA or oTA, respectively). H aggregation is prevalent in these systems as monitored by the spectral shifts of absorption and emission. Packing of surfactants and aggregation were found to be strongly influenced by the presence of bivalent metal cations, Cd²⁺, in the subphase, as demonstrated by molecular area and chain orientation measurements. The nature of the aggregates formed from sTA surfactants is directly affected by complexation of the acid headgroup; in contrast, the associated oTA chromophore aggregation is unaffected by subphase modifications, despite the altered packing of surfactant chains. The photophysical behavior of aggregated TFAs in monolayer systems was followed at the air - water interface as well as with supported films on quartz, and conclusions can be made of the surfactant arrangements' stability. A unique mixed aggregate was formed when both the ether- and sulfone-derivatized TFAs were incorporated into a single phosphatidylcholine molecule (mixed PC) and deposited as an LB film. This monolayer exhibits additional enhancement in aggregation properties attributed to donor - acceptor interactions between the tolan chromophores that are distinct from a statistically mixed LB sample prepared from a mixture of TFAs. The current results are viewed in terms of general principles governing aggregation for a family of chromophore-derivatized surfactants in LB film.