Geminal Bis-ureas as Gelators for Organic Solvents: Gelation Properties and Structural Studies in Solution and in the Gel State Franck S. Schoonbeek, [a] Jan H. vanEsch,* [a] Ron Hulst, [b] Richard M. Kellogg, [a] and Ben L. Feringa* [a] Abstract: Several geminal bis-urea compounds were synthesised by means of an acid-catalysed condensation of various benzaldehydes with different monoalkylureas. Many of these com- pounds form thermoreversible gels with a number of organic solvents at very low concentrations (< 3mm) and which are stable to temperatures higher than 100 8C. Electron microscopy revealed a three-dimensional (3D) network of in- tertwined fibres, which are several tens of micrometers long and have a width ranging from approximately 30 to 300 nm. The possible aggregate forms and aggregate symmetries were evalu- ated by means of molecular mechanics calculations. 1 H NMR, 2D NMR, 13 C NMR and 13 C-CP/MAS NMR tech- niques were used to obtain information about the aggregation and possible ag- gregate symmetry of geminal bis-ureas in solution, in the gel state, and in the solid state. Keywords: aggregation ´ gels ´ hydrogen bonds ´ molecular model- ing ´ self-assembly Introduction Low molecular weight organic gelling agents are currently the subject of much attention, not only because of the numerous applications of gels, but also because gelation phenomena by low molecular weight organic gelling agents are still poorly understood. [1, 2, 3] These organogelators have in common that in organic solvents they self-assemble into long fibre-like structures through highly specific noncovalent interactions, such as hydrogen bonding and p ± p stacking. These fibres then entangle to form a three-dimensional (3D) network which retains the solvent within the pores through capillary forces. The molecular structures of known (very potent) organogelators are, however, very diverse, and often structur- ally closely related compounds do not exhibit any gelation properties. Most of the research has thus focussed on the exploration of the molecular diversity of organogelators, but many of these studies are hampered by the lack of knowledge on the self-assembly properties of the compounds under study. A different approach towards a better understanding of organogels and the elucidation of the molecular prerequisites for gelation is by the design of novel organogelators. Recently, Hanabusa et al. and our group independently demonstrated that bis-urea compounds are exceptionally well-suited for the design of low molecular weight gelators owing to the rigidity, strength and high directionality of the multiple intermolecular hydrogen bonds that can be formed. [4, 5] A key feature in the design of novel gelators appeared to be unidirectional gelator± gelator interactions, as is most clearly the case in trans-1,2-cyclohexyl- and 1,2-phenyl-bis(ureido) gelators. [6] The proximity of the urea groups in these gelating scaffolds, leads to adoption of a coplanar orientation, which strongly favours one-dimensional aggregation through hydrogen-bond formation (Figure 1). However, molecular modelling studies and experimental results indicate that one-dimensional ag- gregation by these compounds is prone to polymorphism, although the 1,2-bis(urea)cyclohexane and 1,2-bis(urea)ben- zene gelating scaffolds have little conformational freedom and the actual or dominant aggregate structure in the gels is not known in detail. So the problem that is faced is not trivial: is it possible to establish a clear relationship between crystal structure, gel structure, aggregation in solution and monomer structure? Reports on crystal structures of gelator molecules have been scarce up to now, and we were able to demonstrate in one case that the arrangement of gelator molecules in the crystal does not correspond fully with the molecular arrange- ment in a gel. [6b, 7, 8] In the present work we turned our attention to the aggregation behaviour and gelation capability of novel geminal bis-urea compounds. For geminal bis-ureas it is expected that the two urea fragments do not have much [a] Dr. J. H. vanEsch, Prof.Dr. B. L. Feringa, F. S. Schoonbeek, Prof. Dr. R. M. Kellogg Department of Organic and Molecular Inorganic Chemistry Stratingh Institute, University of Groningen, Nijenborgh 4 9747 AG Groningen (The Netherlands) Fax: ( 31)50-363-4296 E-mail: esch@chem.rug.nl feringa@chem.rug.nl [b] Dr. R. Hulst Department of Chemical Analysis, University of Twente P.O. Box 217, 7500 AE Enschede (The Netherlands) FULL PAPER Chem. Eur. J. 2000, 6, No. 14  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 0947-6539/00/0614-2633 $ 17.50+.50/0 2633