Aggregation of Stilbene Derivatized Fatty Acids and Phospholipids in Monolayers and Vesicles1

Abstract

Synthetic fatty acid and phosphatidylcholine amphiphiles incorporating a trans-stilbene (TS) chromophore in the fatty acid chain have been found to exhibit sharp changes in absorption and fluorescence spectra upon self-assembly in Langmuir−Blodgett films and aqueous dispersions. The spectral changes are readily associated with aggregates in which there is a strong noncovalent interaction between the TS chromophores. In this paper, we report determination of the size, structure, and properties of these “supramolecular” aggregates using both experiments and simulations. Important findings are that the key “unit aggregate” having distinctive spectroscopic properties is a cyclic “pinwheel” tetramer characterized by strong edge−face interactions. These tetramers may be packed together to form an extended aggregate with only small changes in absorption or fluorescence properties. While it was initially expected that aggregation occurred as a consequence of amphiphile self-assembly, studies of films of the stilbene fatty acids at the air−water interface show the predominance of aggregate prior to compression. Similarly in phospholipid dispersions aggregates persist above temperature at which chain melting occurs. Aggregation produces strong effects on the photophysics and photochemistry of the TS chromophore. No photoisomerization occurs; however, a slow photobleaching is observed for certain assemblies which can be attributed to formation of a photodimer. Fluorescence from aggregated TS chromophores is attributed to extended aggregates and dimers (excimers), and the latter is likely responsible for the photodimerization. The supramolecular aggregates observed in this study appear quite general and closely related to those observed with a wide variety of amphiphiles incorporating aromatic chromophores.