Self‐Assembly of Bisurea Compounds in Organic Solvents and on Solid Substrates

Abstract

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 = ndodecyl, X = -(CH 2 ) 9 -), 3 (R = n-dodecyl, X = -(CH 2 ) 12 -), 4 (R = n-dodecyl, X = 4,4´-biphenyl), and 5 (R = benzyl, X = -(CH 2 ) 9 -) form thermoreversible gels with a wide range of organic solvents, at concentrations well below 10 mg 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 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.

Supramolecular Control of Two‐Dimensional Phase Behavior

“…The synthesis of compounds C12-12, C9-12, C6-12, and the bisthiophene (T2) (Scheme 1) has been reported. [51,58] The other linear bisurea compounds were synthesized by reaction of a,w-diamines with the appropriate isocyanates. Diaminooctane and diaminononane are commercially available.…”

Viscoelastic Behavior of a Supramolecular Polymeric System Consisting of Tri-3,7-dimethyloctyl cis-1,3,5-cyclohexanetricarboxamide and n-Decane

“…As shown above, neither the reptation nor living polymer model is suitable for the fast relaxation mode in the DO 3 If we assume that the relationship of ν<L> 3 = 33 is satisfied at a beak point found at c = 15 gL −1 in Fig.4, the value of β is evaluated to be ∼10 3 , which is close to the β value observed in rigid rodlike polymer solution. On the other hand, the total number density of DO 3 CH at c = 15 gL −1 is calculated to be n total = 1.4 × 10 22 L −1 ≈ n rod .…”

“…The supramolecular polymeric structure formed by DO 3 Because DO 3 B used in the previous study 24) bears achiral sidechains, the columnar supramolecular structure in the DO 3 B/C n systems is an equimolar mixture of left-handed and righthanded helical columns. A short columnar portion keeping one helicity contains perfect 3-fold hydrogen bonding and keeps relatively high rigidity, while a defect portion that contains one or two failures in 3-fold hydrogen bonding changes the helicity at high probability.…”

“…Because long columnar supramolecular structure has been also proposed for DO 3 Moreover, relaxation mechanisms will be discussed in conjunction with the proposed supramolecular polymeric structure formed by DO 3 CH. The concentration of DO 3 CH, c, in the prepared system was ranged from 0.6 to 60 gL −1 .…”

Metal ‘turn-off’, anion ‘turn-on’ gelation cascade in pyridinylmethyl ureas

“…21,22 We anticipate that changing from pyridyl ureas to simple pyridinylmethyl ureas (Scheme 1) should radically change the aggregation behaviour of the urea since the introduction of a methylene spacer between the urea and heterocycle means that the oxygen acceptor is no longer sterically hindered, hence markedly favouring urea···urea hydrogen bonding via the 2 1 (6) urea -tape motif. [23][24][25][26] In the presence of metal ions able to bind the pyridyl group and anions capable of both binding to the metal ion and hydrogen bonding to the urea NH groups, complex, stimuli-responsive behaviour is anticipated. We now report preliminary results into systems capable of gelsol-gel behaviour by a cascade of cation and anion interactions.…”