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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.

There are three parameters in Eq. (2) which determine the behavior of the model and these can, within limits, be determined without reference to surface order. We took J 2 Mi as -0.2, quite close to the preferred value of -0.25 suggested in Ref. 7. To illustrate best the contrast between results from the bulk and from the surface, we adjusted J ± so that the calculated points for the bulk order parameter agree with experiment 4 up to the experimental transformation temperature. This gave J 1 /k = 23TK, a value about 7% higher than would have been obtained by the procedure of Ref. 7. Finally we let AE = 2.5^, corresponding to a constant average surface composition.In Fig. 4 we show the average of the order parameters for the two topmost Cu-Au layers, for the two boundary conditions of the surface model, together with the measured surface order and the calculated bulk order as a function of temperature. It appears from the figure that the difference between the observed behavior of the longrange order parameter at the surface and that in the bulk can be explained by a simple model involving only nearest-and next-nearest-neighbor interactions which are the same for both surface and bulk. Furthermore, it is evident from the analysis that consideration of a region of accommodation between surface and bulk is a necessary Considerable interest exists in examining the properties of both organic and inorganic structures which are highly anisotropic and can be visualized as containing one-dimensional chainsmetallic "spines"-surrounded by a matrix whose properties are highly important in determining the transport properties of that system. The or-feature of any model meant to describe the phenomenon of long-range order at a crystal surface.The temperature dependence of the electrical conductivity and Seebeck coefficient for the polymer chain system (SN) X has been measured along the chain axis from 4.2 to 300°K. The data show the system to be metallic over the entire temperature range studied, with a small conductivity maximum at ~33°K.

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We analyzed experimentally and by calculations the crystal packing of eight ammonium and phosphonium halide salts, each with four equivalent n-alkyl chains containing 10-18 carbon atoms. All of the salts crystallize as stacked monolayers with an "ionic plane" consisting of an array of anions and positively charged N or P atoms in the middle of each layer. The four alkyl chains of each molecule adopt either a biradial or a tetraradial shape. In the biradial salts, the chains are paired and each pair is projected on opposite sides of the ionic plane. In the tetraradial case, the chains are unpaired and take the shape of a crude tetrahedron. There are gauche bends along at least some of the chains of each salt. MM2 force-field computational analyses, coupled with entropy calculations based on conformational statistics, predict correctly that the tetraradial f biradial shape transformation will occur when the alkyl chains reach ca. 12 carbon atoms in length. Detailed analyses of the several factors responsible for the crystal packing in these salts and comparisons with salts containing either four shorter chains or one or two long chains are presented.

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