Supramolecular Aggregates of Azobenzene Phospholipids and Related Compounds in Bilayer Assemblies and Other Microheterogeneous Media:  Structure, Properties, and Photoreactivity¹

Abstract

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.

Metal–Molecule–Metal Junctions in Langmuir–Blodgett Films Using a New Linker: Trimethylsilane

“…[39] In addition, the band has a marked blue-shift of approximately 33 nm with respect to the absorption band in solution ( Figure 2, dotted line), attributable to the formation of H-aggregates (side by side alignment of transition dipole moments of dyes). [40] There is a small red shift of 4 nm of the maximum wavelength of the normalised reflection spectra upon compression, which could be attributed to a less polar environment when the surface pressure increases. In keeping with this suggestion, a bathochromic shift of 2 nm in this characteristic absorption band of TMS-OPE-NH 2 is observed in CCl 4 solution (nonpolar) compared to the CHCl 3 solution (polar).…”

Solvent‐Polarity‐Tunable Dimeric Association of a Fullerene (C60)–N,N‐Dimethylaminoazobenzene Dyad: Modulated Electronic Coupling of the Azo Chromophore with a Substituted 3D Fullerene

“…The positions of the oxygen atoms to which the aryl/alkoxy substitution was attached essentially controlled the extent of aggregation. Supramolecular aggregates of azobenzene phospholipids and related compounds in bilayer assemblies have been studied for structure, properties, and photoreactivity by Song et al [46] Strong evidence of H-aggregate formation in the pure and mixed dispersions has been indicated. On the basis of simulations and studies with similar stilbene phospholipids and the induced circular dichroism signals from the aggregate, a chiral "pinwheel" unit aggregate structure, similar to that for several aromatics, was proposed.…”

Optical switching studies of an azobenzene rigidly linked to a hexa-peri-hexabenzocoronene derivative in solution and at a solid–liquid interface

“…We attribute this to aggregate formation, which prevents the switching, probably due to steric hindrance [17].…”