Self-assembly of styrylthiophene amphiphiles in aqueous dispersions and interfacial films: Aggregate structure, assembly properties, photochemistry, and photophysics

Fatty acids and phosphatidyl choline derivatives incorporating the photoreactive trans-styrylthiophene chromophore have been prepared and studied in Langmuir-Blodgett films and aqueous dispersions, respectively. Both absorption and fluorescence spectra in the assemblies show prominent shifts indicative of aggregation similar to that observed with similar trans-stilbene and ti-ans-azobenzene derivatives previously investigated. Studies of aqueous dispersions of the pure styrylthiophene phospholipids indicate the formation of structures much larger than the typical small unilamellar bilayer vesicles formed from saturated phospholipids of comparable chain length. Studies of the disaggregation process for two of the phospholipids give aggregation numbers of approximately 2 and 6, corresponding to 4 and 12 styrylthiophene units per aggregate. The stability of these aggregates is very similar to those of corresponding stilbene aggregates, and on the basis of spectral similarities it seems reasonable to propose a "pinwheel" fraction of a glide or herringbone structure as the "unit aggregate" and predominate species present in the aqueous dispersions. However, the chief photoreaction observed for the aqueous dispersions of the phospholipids is dimerization to form the syn head-to-head dimer, consistent with topologically controlled reaction from a translation structure in which nearest-neighbor chromophores are aligned parallel. Simulations suggest that while a translation layer structure may be of lowest energy, glide layer structures that show good agreement between measured and predicted properties are also energetically accessible. A possible explanation for the observed reactivity and photophysics is that within the aqueous dispersions there may be equilibration between "glide" and "translation" structures. When mixtures of the styrylthiophene phospholipids and saturated phospholipids are codispersed, bilayer vesicles are formed that are capable of entrapping reagents such as carboxyfluorescein. irradiation of the aggregated trans-styrylthiophene in these vesicles leads to release of the entrapped carboxyfluorescein concurrent with photodimerization of the styrylthiophene. The pattern of reagent release is similar to that observed upon photoisomerization of the corresponding azobenzene aggregates and suggests a "catastrophic" destruction of the vesicles.