AGGREGATION OF STILBENE DERIVATIZED FATTY-ACIDS AND PHOSPHOLIPIDS IN MONOLAYERS AND VESICLES

Synthetic fatty acid and phosphatidylcholine amphiphiles incorporating a trans-stilbene (TS) chromophore in the fatty acid chain have been f ound to exhibit sharp changes in absorption and fluorescence spectra u pon self-assembly in Langmuir-Blodgett films and aqueous dispersions. The spectral changes are readily associated with aggregates in which t here is a strong noncovalent interaction between the TS chromophores. In this paper, we report determination of the size, structure, and pro perties 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 tetram ers may be packed together to form an extended aggregate with only sma ll changes in absorption or fluorescence properties. While it was init ially expected that aggregation occurred as a consequence of amphiphil e self-assembly, studies of films of the stilbene fatty acids at the a ir-water interface show the predominance of aggregate prior to compres sion. Similarly in phospholipid dispersions aggregates persist above t emperature 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 observ ed for certain assemblies which can be attributed to formation of a ph otodimer. Fluorescence from aggregated TS chromophores is attributed t o extended aggregates and dimers (excimers), and the latter is likely responsible for the photodimerization. The supramolecular aggregates o bserved in this study appear quite general and closely related to thos e observed with a wide variety of amphiphiles incorporating aromatic c hromophores.