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.

Photocontrollable Self‐Assembly

“…The authors who studied amphiphiles 11 and 12 suggested that the photoresponse of the azobenzene amphiphiles in the vesicles are strongly dependent on their aggregated state. [17,18] As counterparts of the self-assembly of azobenzene amphiphiles in aqueous conditions, self-assemblies of the azobenzenes 13…”

“…Thus, azobenzene-appended melamine 15 was prepared and its co-aggregation with 5,5'-diethylbarbituric acid (17) in CDCl 3 was investigated (Scheme 10). [22] For 15, steric crowding of the N-substituents increases with trans!cis isomerization in the aggregated state (Scheme 11).…”

“…This observation demonstrates that the steric crowding of the trans-azobenzene moiety in 15 is not enough to retain Scheme 7. Molecular structures of the azobenzene-containing amphiphilic molecules 8 by Kunitake, [14] 9 by Sakai, [15] 10 by Yianni, [16] 11 by Whitten, [17] and 12 by Engberts. [18] Scheme 8.…”

Synthesis and optical properties of new polyurethane cationomers with anchored stilbene chromophores

“…As shown in Figure 4, the PUC‐S1 film exhibits three characteristic absorption bands at 316, 328, and 338 nm, with the maximum peak occurring at 316 nm. Related to the maximum at 334 nm from the polymer solution, the redshift observed for the maximum at 338 nm in the UV spectrum of the polymer film could be tentatively associated with the predominance of stilbene aggregates, the size, structure, and properties of which were shown in other stilbene amphiphiles 28, 29. Subsequent irradiation of the polymer film indicated that the absorption spectra had curve profiles similar to those in solution, and the absorption bands decreased monotonously in favor of the formation of the cis species.…”

Interfacial Organization of Y‐Shaped Rod–Coil Molecules Packed into Cylindrical Nanoarchitectures

“…It can be seen that there is a blue shift (from 298 to 284 nm for Y21 and from 292 to 283 nm for Y34) in LB films deposited at high surface pressure compared with the films deposited at low surface pressure. Both theoretical [32] and experimental [33] reports in the literature suggest that small aggregates of from two to six p-conjugated molecules can alter the absorption and fluorescence spectra of the isolated chromophore, thus leading to blue-shifted absorption and red-shifted emission. In our case, the blue-shifted spectra for the film deposited at high surface pressure compared with the film deposited at low surface pressure are attributed to the formation of H aggregates, in which the long axes of the molecules are aligned in a general face-to-face arrangement.…”