Franck S. Schoonbeek

Cyclic bis-urea compounds as gelators for organic solvents

The gelation properties of bis-urea compounds derived from opti- cally pure trans-1,2-diaminocyclohexane and 1,2-diaminobenzene, with pendant aliphatic, aromatic, or ester groups, as well as the structure of the resulting gels, have been studied by differential scan- ning calorimetry, infrared spectroscopy, small-angle X-ray diffraction, and elec- tron microscopy. These compounds have been found to be very potent gelators for organic solvents, such as aliphatic and aromatic hydrocarbons, esters, ke- tones, and alcohols, at concentrations well below 1 (w/v)%. Gelation by these compounds is completely thermorever- sible, with melting temperatures up to 1208C, and many of the gels display thixotropic properties. Even at low con- centrations these compounds self-as- semble into elongated and very thin fibers, which in turn form a three-dimen- sional network in the solvent. Infrared studies showed that aggregation is ac- companied by the formation of a hydro- gen-bonded network between urea moi- eties, and a single-crystal X-ray structure of one of the compounds showed that in crystals the molecules assemble into one-dimensional chains, which are sta- bilized by the formation of eight hydro- gen bonds between the urea groups and adjacent molecules. The molecular ar- rangement in gels is most likely very similar to that in the crystal, but the complete elucidation of the molecular arrangement in gels is complicated be- cause aggregation of these compounds is prone to polymorphism. It is concluded that the very efficient aggregation of these molecules and the elongated shape of the fibers most likely arise from the highly anisotropic hydrogen-bonding properties of these molecules, which is due to the presence of two coplanar oriented urea moieties in a single mol- ecule. Since the bis-urea compounds presented in this paper are very easy to synthesize and many structural varia- tions are possible without loss of the gelation ability, they are excellent build- ing blocks for the construction of func- tional gels.

Efficient Intermolecular Charge Transport in Self‐Assembled Fibers of Mono‐ and Bithiophene Bisurea Compounds

Hydrogen bonds between urea units allow self-organization of π systems in mono- and bithiophenes into fibers as shown schematically. In these fibers there is a surprisingly high mobility of charge carriers as determined by pulse-radiolysis time-resolved microwave conductivity measurements.

Geminal Bis-ureas as Gelators for Organic Solvents : Gelation Properties and Structural Studies in Solution and in the Gel State

Several geminal bis-urea compounds were synthesised by means of an acid-catalysed condensation of various benzaldehydes with different monoalkylureas. Many of these compounds form thermoreversible gels with a number of organic solvents at very low concentrations (<3mM) and which are stable to temperatures higher than 100 degrees C. Electron microscopy revealed a three-dimensional (3D) network of intertwined 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 evaluated by means of molecular mechanics calculations. 1H NMR, 2D NMR, 13C NMR and 13C-CP/MAS NMR techniques were used to obtain information about the aggregation and possible aggregate symmetry of geminal bis-ureas in solution, in the gel state, and in the solid state.

Low Molecular Weight Gelators for Organic Solvents

Everyone knows what a gel is, but from a scientific point of view the term gel encompasses chemically very diverse systems. Well known gel systems include, for instance, dilute solutions of polymers, proteins, and surfactants in water and organic solvents. These gel systems are important in medicine, biology, chemistry, and physics, and find many applications in the photographic, cosmetics, food, and petroleum industries [1]. However, as D. Jordan Lloyd already wrote in 1926: ‘The colloidal condition, the “gel”, is one which is easier to recognise than to define,…’. And although an exact definition of a gel is still a., problem, from a topological point of view gels can be defined as dilute mixtures of at least two components, in which both components form a separate continuous phase throughout the system [2]. This definition includes not only gels composed of a solid-like and a liquid phase, but also those composed of a solid and a gas phase (so called aerogels). For most gels a solid-like phase is the minor component which forms a three dimensional network structure within the fluid or gas phase. For solid-fluid gels it can be said that the network structure prevents the fluid from flowing, whereas the liquid phase prevents the network from collapsing [3]. The coexistence of a solid network structure together with a liquid phase distinguishes gels from pure solid, liquid crystalline, or fluid materials and gives gels their unique elastic properties.