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 pho toreactive trans-styrylthiophene chromophore have been prepared and st udied in Langmuir-Blodgett films and aqueous dispersions, respectively . Both absorption and fluorescence spectra in the assemblies show prom inent shifts indicative of aggregation similar to that observed with s imilar trans-stilbene and ti-ans-azobenzene derivatives previously inv estigated. Studies of aqueous dispersions of the pure styrylthiophene phospholipids indicate the formation of structures much larger than th e typical small unilamellar bilayer vesicles formed from saturated pho spholipids of comparable chain length. Studies of the disaggregation p rocess for two of the phospholipids give aggregation numbers of approx imately 2 and 6, corresponding to 4 and 12 styrylthiophene units per a ggregate. The stability of these aggregates is very similar to those o f corresponding stilbene aggregates, and on the basis of spectral simi larities it seems reasonable to propose a ''pinwheel'' fraction of a g lide or herringbone structure as the ''unit aggregate'' and predominat e species present in the aqueous dispersions. However, the chief photo reaction observed for the aqueous dispersions of the phospholipids is dimerization to form the syn head-to-head dimer, consistent with topol ogically controlled reaction from a translation structure in which nea rest-neighbor chromophores are aligned parallel. Simulations suggest t hat while a translation layer structure may be of lowest energy, glide layer structures that show good agreement between measured and predic ted properties are also energetically accessible. A possible explanati on for the observed reactivity and photophysics is that within the aqu eous dispersions there may be equilibration between ''glide'' and ''tr anslation'' structures. When mixtures of the styrylthiophene phospholi pids and saturated phospholipids are codispersed, bilayer vesicles are formed that are capable of entrapping reagents such as carboxyfluores cein. irradiation of the aggregated trans-styrylthiophene in these ves icles 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 cor responding azobenzene aggregates and suggests a ''catastrophic'' destr uction of the vesicles.