Hierarchical self-assembly of a bow-shaped molecule bearing self-complementary hydrogen bonding sites into extended supramolecular assemblies

The bow-shaped molecule 1 bearing a self-complementary DAAD-ADDA (D=donor A=acceptor) hydrogen-bonding array generates, in hydrocarbon solvents, highly ordered supramolecular sheet aggregates that subsequently give rise to gels by formation of an entangled network. The process of hierarchical self-assembly of compound 1 was investigated by the concentration and temperature dependence of UV-visible and (1)H NMR spectra, fluorescence spectra, and electron microscopy data. The temperature dependence of the UV-visible spectra indicates a highly cooperative process for the self-assembly of compound 1 in decaline. The electron micrograph of the decaline solution of compound 1 (1.0 mM) revealed supramolecular sheet aggregates forming an entangled network. The selected area electronic diffraction patterns of the supramolecular sheet aggregates were typical for single crystals, indicative of a highly ordered assembly. The results exemplify the generation, by hierarchical self-assembly, of highly organized supramolecular materials presenting novel collective properties at each level of organization.     

N,N'-disubstituted ureas: influence of substituents on the formation of supramolecular polymers

Symmetrical N,N'-disubstituted ureas have been synthesized and characterized. Among them, the branched dialkylureas prepared are highly soluble in organic media. Moreover, the solutions obtained are very viscous in heptane, if the branched alkyl groups are not too bulky (i.e. a methyl group on the alpha carbon, or an ethyl group on the beta carbon). Due to the strong, bifurcated hydrogen bonds between the urea moieties, linear supramolecular polymers are formed. The degree of association of these supramolecular polymers has been determined by FTIR spectroscopy.     

Self-healing supramolecular gels formed by crown ether based host-guest interactions

Automatic repair: a polymer with pendent dibenzo[24]crown-8 units (purple in picture) was cross-linked by two bisammonium salts (green) to form two supramolecular gels based on host-guest interactions. These two gels are stimuli-responsive materials that respond to changes of the pH value and are also self-healing materials, as can be seen by eye and as evidenced by rheological data.     

High-tech applications of self-assembling supramolecular nanostructured gel-phase materials: from regenerative medicine to electronic devices

It is likely that nanofabrication will underpin many technologies in the 21st century. Synthetic chemistry is a powerful approach to generate molecular structures that are capable of assembling into functional nanoscale architectures. There has been intense interest in self-assembling low-molecular-weight gelators, which has led to a general understanding of gelation based on the self-assembly of molecular-scale building blocks in terms of non-covalent interactions and packing parameters. The gelator molecules generate hierarchical, supramolecular structures that are macroscopically expressed in gel formation. Molecular modification can therefore control nanoscale assembly, a process that ultimately endows specific material function. The combination of supramolecular chemistry, materials science, and biomedicine allows application-based materials to be developed. Regenerative medicine and tissue engineering using molecular gels as nanostructured scaffolds for the regrowth of nerve cells has been demonstrated in vivo, and the prospect of using self-assembled fibers as one-dimensional conductors in gel materials has captured much interest in the field of nanoelectronics.     

Photo gel-sol/sol-gel transition and its patterning of a supramolecular hydrogel as stimuli-responsive biomaterials

In a focused library of glycolipid-based hydrogelators bearing fumaric amide as a trans-cis photoswitching module, several new photoresponsive supramolecular hydrogelators were discovered, the gel-sol/sol-gel transition of which was pseudo-reversibly induced by light. Studying the optimal hydrogel by NMR spectroscopy and various microscopy techniques showed that the trans-cis photoisomerization of the double bond of the fumaric amide unit effectively caused assembly or disassembly of the self-assembled supramolecular fibers to yield the macroscopic hydrogel or the corresponding sol, respectively. The entanglement of the supramolecular fibers produced nanomeshes, the void space of which was roughly evaluated to be 250 nm based on confocal laser scanning microscopy observations of the size-dependent Brownian motion of nanobeads embedded in the supramolecular hydrogel. It was clearly shown that such nanomeshes become a physical obstacle that captures submicro- to micrometer-sized substrates such as beads or bacteria. By exploiting the photoresponsive property of the supramolecular nanomeshes, we succeeded in off/on switching of bacterial movement and rotary motion of bead-tethered F(1)-ATPase, a biomolecular motor protein, in the supramolecular hydrogel. Furthermore, by using the photolithographic technique, gel-sol photopatterning was successfully conducted to produce sol spots within the gel matrix. The fabricated gel-sol pattern not only allowed regulation of bacterial motility in a limited area, but also off/on switching of F1-ATPase rotary motion at the single-molecule level. These results demonstrated that the photoresponsive supramolecular hydrogel and the resulting nanomeshes may provide unique biomaterials for the spatiotemporal manipulation of various biomolecules and live bacteria.     

Recognition-induced polymersomes: structure and mechanism of formation

Random polystyrene copolymers grafted with complementary recognition elements were combined in chloroform producing vesicular aggregates, that is, recognition-induced polymersomes (RIPs). Reflection interference contrast microscopy (RICM) in solution, coupled with optical microscopy (OM) and atomic force microscopy (AFM) on solid substrates, were used to determine the wall thickness of the RIPs. Rather than a conventional mono- or bilayer structure (approximately 10 or approximately 20 nm, respectively) the RIP membrane was 43+/-7 nm thick. Structural arrangement of the polymer chains on the RIP wall were characterized by using angle-resolved X-ray photoelectron spectroscopy (AR-XPS). The interior portion of the vesicle membrane was found to be more polar, containing more recognition units, than the exterior part. This gradient suggests that a rapid self-sorting of polymers takes place during the formation of RIPs, providing the likely mechanism for vesicle self-assembly.     

Chiral bis(amino alcohol)oxalamide gelators-gelation properties and supramolecular organization: racemate versus pure enantiomer gelation

Four new chiral bis(amino alcohol)oxalamides (1-4: amino alcohol=leucinol, valinol, phenylglycinol, and phenylalaninol, respectively) have been prepared as low-molecular-weight organic gelators. Their gelation properties towards various organic solvents and mixtures were determined and these were then compared to related bis(amino acid) oxalamide gelators. Spectroscopic (FTIR, (1)H NMR) and X-ray diffraction studies revealed that the primary organization motif of (S,S)-1 and racemate 1 (rac-1) in lipophilic solvents involved the formation of inverse bilayers. The X-ray crystal structure of (S,S)-1 also shows this type of bilayer organization. The crystal structure of rac-2 reveals meso bilayers of hydrogen-bonded aggregates. Within the bilayers formed, the gelator molecules are connected by cooperative hydrogen bonding between oxalamide units and OH groups, while the interbilayer interactions are realized through lipophilic interactions between the iBu groups of leucinol. Oxalamide meso-1 lacks any gelation ability and crystallizes in monolayers. In dichloromethane rac-1 forms an unstable gel; this is prone to crystallization as a result of the formation of symmetrical meso bilayers. In contrast, in aromatic solvents rac-1 forms stable gels; this indicates that enantiomeric bilayers are formed. Oxalamide rac-1 is capable of gelling a volume of toluene three times larger than (S,S)-1. A tranmission electron microscopy investigation of rac-1 and (S,S)-1 toluene gels reveals the presence of thinner fibers in the former gel, and, hence, a more compact network that is capable of immobilizing a larger volume of the solvent. The self-assembly of these types of gelator molecules into bilayers and subsequent formation of fibrous aggregates can be explained by considering the strength and direction of aggregate forces (supramolecular vectors) in three-dimensional space.     

G-quartets 40 years later: from 5'-GMP to molecular biology and supramolecular chemistry

Molecular self-assembly is central to many processes in both biology and supramolecular chemistry. The G-quartet, a hydrogen-bonded macrocycle formed by cation-templated assembly of guanosine, was first identified in 1962 as the basis for the aggregation of 5'-guanosine monophosphate. We now know that many nucleosides, oligonucleotides, and synthetic derivatives form a rich array of functional G-quartets. The G-quartet surfaces in areas ranging from structural biology and medicinal chemistry to supramolecular chemistry and nanotechnology. This Review integrates and summarizes knowledge gained from these different areas, with emphasis on G-quartet structure, function, and molecular recognition.     

Boronic Acids in Molecular Self‐Assembly

The reversibility of boronic acid and diol interaction makes it an ideal candidate for the design of self‐assembled molecular structures. Reversibility is required to ensure that the thermodynamically most stable structure is formed. Reversibility also ensures that any errors produced during the assembly process are not permanent.     

When Self‐Assembly Fails: Stepwise Metal‐Directed Synthesis of [2]Catenanes

On the attempted synthesis of a series of homo‐ and heterotrimetallic [2]catenanes by the self‐assembly of a 2‐(pyridin‐4‐ylmethyl)‐2,7‐diazapyrenium ligand, (ethylenediamine)palladium(II) or platinum(II) nitrate, and a dioxoaryl bis(N‐monoalkyl‐4,4′‐bipyridinium) salt as building blocks, both the one‐pot direct self‐assembly of the components and the so called “magic ring” approach fail to produce the expected trinuclear [2]catenanes under thermodynamically driven conditions. However, one of the target supramolecules is obtained by following a stepwise protocol, consisting of the threading of a dinuclear Pt^II metallacycle and the dioxoaryl bis(N‐monoalkyl‐4,4′‐bipyridinium) axle, followed by kinetically controlled Pt^II‐directed cyclization of the corresponding pseudorotaxane.     

Hydrogel Formation upon Photoinduced Covalent Capture of Macrocycle Stacks from Dynamic Combinatorial Libraries

Stacks of macrocycles, assembled using reversible disulfide-bond formation, are covalently captured by photoinitiated exchange of disulfide bonds, inducing the formation of hydrogels. This strategy allows access to structures beyond the thermodynamic minima traditionally targeted by dynamic combinatorial chemistry.