J. van Esch

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.

Di-urea compounds as gelators for organic solvents

Abstract Simple diurea compounds form thermoreversible gels with several organic solvents. These gels are stable up to temperatures of 100°C, and can be stored for months. Electron microscopy reveals that in these solvents the gelation agents assemble into very thin rectangular sheets which are several tens of micrometers long.

A new synthetic route to symmetrical photochromic diarylperfluorocyclopentenes

Abstract Symmetrical and photochromic diarylperfluorocyclopentene has been prepared by reaction of 3-lithio-5-chloro-2-methylthiophene with the ethyl ester of hexafluoroglutaric acid, followed by ring closure via a McMurry coupling. Compound 7 is a versatile intermediate for the development of photochromic materials.

Molecular switches get wired: Synthesis of diarylethenes containing one or two sulphurs

ABSTRACT Two series of novel diarylethenes, the first containing one and the second two acetyl protected thiol groups, were synthesised in two and three steps from an easily available known intermediate. Newly synthesised compounds present ideal examples of model substrates for photochromic studies on metal surfaces.

Supramolecular chemistry at the liquid/solid interface probed by scanning tunnelling microscopy

The liquid/solid interface provides an ideal environment to investigate self-assembly phenomena, and scanning tunnelling microscopy (STM) is one of the preferred methodologies to probe the structure and the properties of physisorbed monolayers on the nanoscale. Physisorbed monolayers are of relevance in areas such as lubrication, patterning of surfaces on the nanoscale, and thin film based organic electronic devices, to name a few. It is important to gain insight in the factors which control the ordering of molecules at the liquid/solid interface in view of the targeted properties. STM provides detailed insight into the importance of molecule-substrate (epitaxy) and molecule-molecule interactions to direct the ordering of both achiral and chiral molecules on the atomically flat surface. The electronic properties of the self-assembled physisorbed molecules can be probed by taking advantage of the operation principle of STM, revealing spatially resolved intramolecular differences within these physisorbed molecules.

Location and aggregation behaviour of tetra-aryl-porphyrins in dioctadecyldimethylammonium chloride vesicles

5,10,15,20-Tetrakis(4-hexadecyloxyphenyl)porphyrin forms unusual edge to edge type of aggregates in bilayers of dioctadecyldimethylammonium chloride vesicles.

Reaction dynamics and applications in patterning of bisthienylcyclopentene-based photochromic switches

The structural and optical properties of the gel-forming photochromic switch 1,2-bis(2’-methyl-5’-{{((R) phenylethyl)amino}carbonyl}thienyl-3’-yl)cyclopentene are studied by means of the linear absorption and holographic grating techniques. The use of diffractive optics enables recording of holographic gratings with high long-term phase stability. The diffraction efficiency of the recorded holographic gratings approach values of 30% for low writing beam energies when diffusion is the rate determining factor. At higher writing pulse energies the competition between the diffusion and photodecomposition processes causes lower diffraction efficiencies. At irradiation doses above 10 mJ the spatial profile of the recorded gratings is strongly influenced by saturation effects. Because of the well-determined grating profile the holographic grating technique is potentially applicable for the quantitative characterisation of the diffusion process in photochromic gels.

Observation of instability of faceted crystals in lipid monolayers

Abstract The morphological instabilities of two-dimensional hexagonal crystals in lipid monolayers are studied. Fluorescence microscopy indicates that beyond a critical size the faceted crystal develops into dendrites with unusual tip growth behavior, which is not consistent with the current theory of dendritic growth. The morphological transitions in relation to the driving force for two-dimensional crystal growth are also studied.

Surfactants containing imidazole ligands. Spontaneous formation of vesicles by addition of metal ions

Metal ions increase the headgroup size of the ligand-surfactant dihexadecyl[5-(1-imidazolyl)-3-oxapentyl]-methylammonium chloride and trigger the conversion of open to closed bilayer structures.

Illumination-related pattern formations in lipid monolayers

We report on the phenomenon that in the two phase coexistence region (LC-LE) of a lipid monolayer film the fractal-like solid domains may grow simply by continuous illumination of a fluorescence microscope. The mechanism of this 2D domain growth is discussed. This phenomenon gives insight into the two-dimensional ramified crystallization in monolayers.