Exploiting molecular self-assembly: from urea-based organocatalysts to multifunctional supramolecular gels.

We describe the self-assembly properties of chiral N,N'-disubstituted urea-based organocatalyst 1 that leads to the formation of hierarchical supramolecular gels in organic solvents at low concentrations. The major driving forces for the gelation are hydrogen bonding and π-π interactions according to FTIR and (1)H NMR spectroscopy, as well as quantum-mechanical studies. The gelation scope could be interpreted based on Kamlet-Taft solvatochromic parameters. TEM, SEM, and AFM imaging revealed that a variety of morphologies including helical, laths, porous, and lamellar nanostructures could be obtained by varying the solvent. Experimental gelation tests and computational structural analysis of various structurally related compounds proved the existence of a unique set of molecular interactions and an optimal hydrophilic/hydrophobic balance in 1 that drive the formation of stable gels. Responses to thermal, mechanical, optical, and chemical stimuli, as well as multifunctionality were demonstrated in some model gel materials. Specifically, 1 could be used for the phase-selective gelation of organic solvent/water mixtures. The gel prepared in glycerol was found to be thixotropic and provided a sensitive colorimetric method for the detection of Ag(I) ions at millimolar concentrations in aqueous solution. Moreover, the gel matrix obtained in toluene served as a nanoreactor for the Friedel-Crafts alkylation of 1H-indole with trans-ß-nitrostyrene.

Crossover experiments applied to network formation reactions: improved strategies for counting elastically inactive molecular defects in PEG gels and hyperbranched polymers.

Molecular defects critically impact the properties of materials. Here we introduce a paradigm called "isotopic labeling disassembly spectrometry" (ILDaS) that facilitates unprecedented precise experimental correlations between elastically inactive network defects (dangling chains and primary loops) and network formation kinetics and precursor structure. ILDaS is inspired by classical crossover experiments, which are often used to interrogate whether a reaction mechanism proceeds via an inter- or intramolecular pathway. We show that if networks are designed from labeled bifunctional monomers that transfer their labels to multifunctional junctions upon network formation, then the extent of junction labeling correlates directly with the number of dangling chains and cyclic imperfections within the network. We demonstrate two complementary ILDaS approaches that enable defect measurements with short analysis times, low cost, and synthetic versatility applicable to a broad range of network materials including polydisperse polymer precursors. The results will spur new experimental and theoretical investigations into the interplay between polymer network structure and properties.

C-C Bond formation catalyzed by natural gelatin and collagen proteins.

The activity of gelatin and collagen proteins towards C-C bond formation via Henry (nitroaldol) reaction between aldehydes and nitroalkanes is demonstrated for the first time. Among other variables, protein source, physical state and chemical modification influence product yield and kinetics, affording the nitroaldol products in both aqueous and organic media under mild conditions. Significantly, the scale-up of the process between 4-nitrobenzaldehyde and nitromethane is successfully achieved at 1 g scale and in good yield. A comparative kinetic study with other biocatalysts shows an increase of the first-order rate constant in the order chitosan < gelatin < bovine serum albumin (BSA) < collagen. The results of this study indicate that simple edible gelatin can promote C-C bond forming reactions under physiological conditions, which may have important implications from a metabolic perspective.

Proton-conducting supramolecular metallogels from the lowest molecular weight assembler ligand: a quote for simplicity.

Oxalic acid has been proven to be the lowest molecular weight organic ligand able to form robust supramolecular metallogel networks in the presence of metal salts. In particular, two novel multifunctional metallogels were readily prepared at room temperature by simple mixing of stock solutions of Cu(II) acetate monohydrate or Cu(II) perchlorate hexahydrate and oxalic acid dihydrate. Formation of different polymorphs and unprecedented proton conduction under anhydrous conditions were also demonstrated with some of these materials.