scanning tunneling microscopy
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
New low molecular weight gelators based on the structure R—NHCONH—X—NHCONH—R have been synthesized and tested for their ability to cause gelation of organic solvents. Compounds 2 (R = n-dodecyl, X = -(CH2)9-), 3 (R = n-dodecyl, X = -(CH2)12-), 4 (R = n-dodecyl, X = 4, 4′-biphenyl), and 5 (R = benzyl, X = -(CH2)9-) form thermoreversible gels with a wide range of organic solvents, at concentrations well below 10 mgmL-1. Depending on the nature of the R and X groups, the solvents that undergo gelation include hexadecane, p-xylene, 1-octanol, n-butyl actetate, cyclohexanone, and tetralin. The gels are stable up to temperatures well above 100°C, but are easily disrupted by mechanical agitation. Light microscopic investigations revealed that compounds 2-5 spontaneously aggregate to form thin flat fibers, which can be several hundreds of micrometers long and only 2-10 μm wide. Depending on the solvent, multiple twists in the fibers are observed. In the gels, these fibers form an extended three-dimensional network, which is stabilized by multiple mechanical contacts between the fibers. Electron microscopy and X-ray powder diffraction revealed that the fibers consist of stacks of sheets. The thickness of the sheets is 3.65 and 3.85 nm for 2 and 3, respectively. Scanning tunneling microscopic investigations of 2 absorbed on graphite showed that 2 forms long ribbons with a width of 5.0 nm. In the ribbons the molecules have a parallel arrangement, with the long molecular axis perpendicular to the long ribbon axis. The two urea groups within a given molecule are each part of mutually parallel extended chains of hydrogen bonds. Based on these observations a model is proposed for the arrangement of the molecules in the fibers. In this model the bisurea molecules aggregate through hydrogen-bond formation into long ribbons, which assemble into sheets. In these sheets the ribbons are tilted. Finally, the sheets stack to form long thin fibers. This model is supported by molecular dynamics simulations.
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