Alkyl chains, assembly

Aqueous dispersion of a triple-chain glutamic acid diester monoamide shows beautiful helical fibers under the light microscope, which slowly close to form tubules. Above 50°C the fibers melt and rearrange to vesicles. Twisted ribbons made of similar molecules with azobenzene in the hydrophobic core polymerize upon UV irradiation. This leads to the disappearance of an induced CD spectrum of surface adsorbed methyl orange. Alanine-derived amphiphiles with azobenzene units in the chain also form helical fibers in water. They absorb at 350 run when the number of the methylene groups in the outer chain is even and at 320 nm when odd. The chromophore in the fiber is presumably less stacked in even-numbered alkyl chain assemblies (Fig. 9.5.i). 

Figure 6. Positions of the methylene symmetric (open symbols) and antisymmetric (filled symbols) stretching modes in die PM-IRRA spectrum of the series of phosphates. The horizontal dotted line corresponds to the typical values of these modes observed in crystalline ordered alkyl chain assemblies.

Because these carboxylate salts also create films on aqueous surfaces, they act as soaps. The polar groups stick into the water while the alkyl chains assemble into a hydrophobic... 

A second class of monolayers based on van der Waal s interactions within the monolayer and chemisorption (in contrast with physisorption in the case of LB films) on a soHd substrate are self-assembled monolayers (SAMs). SAMs are well-ordered layers, one molecule thick, that form spontaneously by the reaction of molecules, typically substituted-alkyl chains, with the surface of soHd materials (193—195). A wide variety of SAM-based supramolecular stmctures have been generated and used as functional components of materials systems in a wide range of technological appHcations ranging from nanoHthography (196,197) to chemical sensing (198—201). 

Tetrakis(cumylphenoxy)phthalocyanine (5) a PC derivative, having liquid—crystalline-like substituents (38—43) was studied because the cross-section area of the substituents is much larger than that of a normal alkyl chain, and therefore, the requirement of minimized free volume in the assembly may be easier to accomplish. 

Using this strategy, constmction of multilayer films of - O. fim thickness by self-assembly of methyl 23-ttichlorosilyltticosanoate (MTST) on siUcon substrates has been demonstrated (Fig. 9) (165). The linear relationship between the film thickness and the layer number showed a slope of 3.5 nm /layer. Filipsometry data, absorbance intensities, and dichroic ratios for the multilayers all suggest that the samples were composed of distinct monolayers. However, ir data indicated that there maybe more tilting or disordering of the alkyl chains in the seven-layer sample than for the monolayer samples. 

The fdr studies reveal that the alkyl chains in SAMs of thiolates on Au(lll) usually are tilted 26-28° from the surface normal, and display 52-55° rotation about the molecular axis. This tilt is a result of the chains reestabUshing VDW contact in an assembly with - 0.5 nm S—S distance, larger than the distance of - 0.46 nm, usually quoted for perpendicular alkyl chains in a close-packed layer. On the other hand, thiolate monolayers on Ag(lll) are more densely packed owing to the shorter S—S distance. There were a number of different reports on chain tilt in SAMs on Ag(lll), probably owing to different amounts of oxide, formed on the clean metallic surface (229,230,296,297). In carefully prepared SAMs of alkanethiolates on a clean Ag(lll) surface, the alkyl chains are practically perpendicular to the surface. 

Monolayers of alkyl chains on siUcon are a significant addition to the family of SAMs. An abiUty to directly connect organic materials to siUcon allows a direct coupling between organic materials and semiconductors. The fine control of supedattice stmctures provided by the self-assembly technique offers a route for building organic thin films with, for example, electrooptic properties on siUcon. 

Schemes 14 and 15 outline the polyol chain assembly. Alkylation of cyanohydrin 93 with iodide 94 provided the chlorocyanohydrin 95, which was converted to the required iodide (96). A second alkylation of 93, this time with 96, provided bisacetonide 97 in 70-80% yield. Conversion of 97 to iodide 87 completed the synthesis of the C6-C15 fragment.

The use of ordered supramolecular assemblies, such as micelles, monolayers, vesicles, inverted micelles, and lyotropic liquid crystalline systems, allows for the controlled nucleation of inorganic materials on molecular templates with well-defined structure and surface chemistry. Poly(propyleneimine) dendrimers modified with long aliphatic chains are a new class of amphiphiles which display a variety of aggregation states due to their conformational flexibility [38]. In the presence of octadecylamine, poly(propyleneimine) dendrimers modified with long alkyl chains self-assemble to form remarkably rigid and well-defined aggregates. When the aggregate dispersion was injected into a supersaturated... 

The AFM tip plays also the dominant role in another technique where CH3 end groups of a self-assembled monolayer of alkyl chains are oxidized by a conductive tip... 

Kittredge KW, Fox MA, Whitesell JK (2001) Effect of alkyl chain length on the fluorescence of 9-alkylfluorenyl thiols as self-assembled monolayers on gold. J Phys Chem B 105 10594-10599... 

Figure 29. For the latter case, it is considered that the semiconducting re-electron systems are separated by insulating hydrocarbon spacers, resulting in alternate thin layers of organic semiconductor and insulator in these monolayer assemblies. The direct current - voltage (I - V) characteristics were mea- sured for the multilayers H2Pc(R)8 and Cu-Pc(R)8 in directions perpendicular and parallel to the film plane. In both cases, the linear I - V relationships of Ohm s law were observed at low electric field and obtained DC conductivities are summarized in Table 3. The normal conductivity (ajJ were ca. 10 13 S cm-1, while the lateral ones p//) were 3.4 x 10-7 and 9.9 x 10 7 S cm 1 for films of the metal-free and copper Pc derivatives, respectively. The former (ojJ tended to decrease slightly with increase of Figure 29. Schematical illustration of the substituent alkyl chain length,...

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