Supramolecular Assemblies of Chromophores in LB Films and Related Media

Abstract

Formation of aggregates has been observed as a general phenomenon for a wide variety of organic molecules, especially aromatic compounds and dyes. Aggregation is most commonly encountered in crystals or in thin films. However, it has been increasingly observed in microheterogenous media or in other situations where high local concentrations occur or where specific orientation is favored. Two limiting types of aggregation have been defined based on the orientation of transition dipoles and their absorption spectral characteristics. These are the “J” aggregate, in which head-to-tail arrangements of transitiondipole moments are characterized by a sharp, intense, red-shifted transition compared to the isolated (solvated) monomer, and the “H” aggregate, where head-to-head (card-pack) orientations are characterized by a blue shift of the prominent transition compared with the monomer. Several treatments have been proposed to correlate the observed spectral shifts with the aggregate structure. For a number of compounds, the association of known x-ray-determined structures with spectral features has supported the theoretical predictions developed by Kasha and Hochstrasser or by Czikkely, Försterling, and Kuhn. The focus of the studies described here has been on aggregation occurring in Langmuir-Blodgett (LB) films and related media, such as bilayer vesicles, which are characterized by an assembly of molecules in an interfacial situation where a polar-nonpolar or water-hydrocarbon boundary should provide a strong organizing influence. In early cases where aggregates were encountered as prominent components of mixed LB films (even when relatively dilute mixtures were used), the phenomenon was usually dismissed as “microcrystallization,” which was considered an unavoidable nuisance and not really due to fundamental intermolecular interactions. More recent studies have shown that aggregation in these media is really a significant molecular phenomenon that shows dependence both on the specific molecules and the topology of the film-forming surfactant. Although some previous investigations have been carried out with different results for various substrates, we have embarked on a study to correlate aggregation behavior for a number of different chromophores incorporated into amphiphilic structures to obtain a general picture of the relative importance of different factors that can control aggregation phenomena.