A number of phosphatidyl choline derivatives containing trans-azobenzene units in the fatty ester backbone have been synthesized and studied in aqueous dispersions both pure and in the presence of saturated and unsaturated phospholipids. The structures of the assemblies formed have been investigated by microcalorimetry, dynamic light scattering, cryo-transmission electron microscopy, and reagent entrapment. While many of the mixed phospholipid dispersions give evidence for the formation of small unilamellar vesicles, the aqueous dispersions of pure azobenzene phospholipids (APL's) give evidence for several different structures, including relatively large plates in at least one case. The azobenzenes show strong evidence of “H” aggregate formation both in the pure and mixed dispersions. The aggregation number has been estimated for several of the APL's and found to be typically 3 or a multiple thereof. On the basis of simulations and studies with similar stilbene phospholipids as well as on the strong induced circular dichroism signals observed for the aggregate, we infer a chiral “pinwheel” unit aggregate structure similar to that found for several aromatics. The azobenzenes in the aqueous dispersions have been found to photoisomerize to give cis-rich photostationary states; the cis-azobenzenes show no evidence for aggregation and no induced circular dichroism. The cis-azobenzenes can be isomerized back to the trans either by irradiation or by thermal paths. Mixed aqueous dispersions of trans-APL's with saturated or unsaturated phospholipids can be prepared which entrap the fluorescent dye carboxyfluorescein (CF) under conditions where the CF fluorescence is very low due to self-quenching. By varying the APL/host phospholipid ratio the azobenzene can be aggregate, monomer, or dimer. In cases where the azobenzene is monomer or dimer, irradiation produces complete isomerization but little “leakage” of CF from the vesicle interior. In contrast, where the azobenzene is predominantly aggregate, irradiation results in both photoisomerization and reagent release. That photoisomerization in the latter case can result in “catastrophic” destruction of the vesicle can also be shown by cryo-transmission electron microscopy.

This feature article presents an overview of a study of several different aromatic-functionalized amphiphiless fatty acid and phospholipid derivatives. These amphiphiles form organized assemblies when the fatty acids are spread as monolayers at the air-water interface or when the phospholipids are dispersed in aqueous solutions. For a wide range of aromatic chromophoresstrans-stilbene derivatives and a series of "vinylogues" (1,4-diphenyl-1,3-butadiene and 1,6-diphenyl-1,3,5-hexatriene), diphenylacetylenes, and azobenzenes such as phenyl, biphenyl, and terphenyl derivatives and modified stilbenes (styryl thiophenes and styryl naphthalenes)sassembly formation is accompanied by formation of aggregates of the aromatic groups. Results of experimental studies and simulations indicate that in many cases the aromatics form a small, stable "unit aggregate" characterized by strong "noncovalent" edge-to-face interactions among adjacent aromatics. Although the unit aggregates exhibit characteristic spectral shifts and strong induced circular dichroism indicating a chiral "pinwheel" aggregate structure, they may be packed together in pure films or dispersions to form an extended glide or herringbone structure. Although the "pinwheel" unit aggregate and the extended glide structure is favored for the majority of aromatics studied, for certain aromatics (styrenes, styrylthiophenes, and R-styrylnaphthalenes) a translation layer, characterized by face-to-face noncovalent interactions, is preferred. The glide or herringbone aggregates are readily distinguished from the translation aggregates by different spectral signatures and different photochemical and photophysical behavior. Factors controlling the type of aggregate and hence extended structure formed from different aromatic functionalized aromatics include shape and steric factors and strength of the competing noncovalent edge-face and face-face interactions.

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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.