SELF-ASSEMBLY OF BISUREA COMPOUNDS IN ORGANIC-SOLVENTS AND ON SOLID SUBSTRATES

New low molecular weight gelators based on the structure R-NHCONH-X-NH CONH-R have been synthesized and tested for their ability to cause gel ation 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'-biphe nyl), and 5 (R = benzyl, X = -(CH2)(9)-) form thermoreversible gels wi th a wide range of organic solvents, at concentrations well below 10 m g mL(-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 temp eratures well above 100 degrees C, but are easily disrupted by mechani cal agitation. Light microscopic investigations revealed that compound s 2-5 spontaneously aggregate to form thin flat fibers, which can be s everal hundreds of micrometers long and only 2-10 mu m wide. Depending on the solvent, multiple twists in the fibers are observed. In the ge ls, these fibers form an extended three-dimensional network, which is stabilized by multiple mechanical contacts between the fibers. Electro n microscopy and X-ray powder diffraction revealed that the fibers con sist of stacks of sheets. The thickness of the sheets is 3.65 and 3.85 nm for 2 and 3, respectively. Scanning tunneling microscopic investig ations of 2 absorbed on graphite showed that 2 forms ion ribbons with a width of 5.0 nm. In the ribbons the molecules have a parallel arrang ement, with the long molecular axis perpendicular to the long ribbon a xis. The two urea groups within a given molecule are each part of mutu ally parallel extended chains of hydrogen bonds, Based on these observ ations a model is proposed for the arrangement of the molecules in the fibers, In this model the bisurea molecules aggregate through hydroge n-bond formation into long ribbons, which assemble into sheets. In the se sheets the ribbons are tilted. Finally, the sheets stack to form lo ne thin fibers. This model is supported by molecular dynamics simulati ons.