Enantioselective cyclization of racemic supramolecular polymers

Homochiral hydrogen-bonded cyclic assemblies are formed in dilute solutions of racemic supramolecular polymers based on the quadruple hydrogen bonding 2-ureido-4[1H]-pyrimidinone unit, as observed by 1H NMR and SEC experiments. Preorganization of the monomers and the combined binding strength of the eight hydrogen bonds result in a very high stability of the cyclic aggregates with pronounced selectivity between homochiral and heterochiral cyclic species, usually only observed in crystalline or liquid crystalline phases.     

Conformational control of hydrogen-bonded aromatic bis-ureas

The phenylurea moiety is a ubiquitous synthon in supramolecular chemistry because it contains strong complementary hydrogen bonding groups and is synthetically very accessible. Here we investigate the possibility to strengthen self-association by conformational preorganization of the phenylurea moiety. In fact, we show that it is possible to strongly enhance intermolecular interactions between hydrogen bonded aromatic bis-ureas by substitution at the ortho positions of the phenylurea groups. Ortho substituents enforce a noncoplanar conformation of the urea and phenyl moieties better suited for hydrogen bonding. Substitution by methyl groups is more efficient than with larger groups, probably because of reduced steric hindrance. These effects have been demonstrated in the case of two different supramolecular architectures, which points to the probable generality of the phenomenon. In addition, this study has led to the discovery of a new bis-urea able to form very stable self-assembled nanotubes in toluene up to high temperatures (boiling point) or low concentrations (10(-7) M) and in chloroform down to 3 × 10(-4) M.     

Supramolecular nanoribbons and nanoropes generated from hydrogen-bonded supramolecular polymers containing perylene bisimide chromophores

A perylene bisimide anchored with melamine hydrogen-bonding units has been prepared, and its supramolecular polymerization upon binding with N-dodecylcyanurate (CA) was examined. The resulting flexible supramolecular polymers self-organized via a pi-pi stacking interaction between perylene chromophores, affording ribbonlike aggregates in cyclic alkanes and ropelike aggregates in acyclic alkanes to form gels. [structure: see text]     

Gold nanoparticle assemblies through hydrogen-bonded supramolecular mediators

The synthesis of spherical gold nanoparticle assemblies with multicomponent double rosette molecular boxes as mediators is presented. These nine-component hydrogen-bonded supramolecular structures held together by 36 hydrogen bonds induce gold nanoparticle assembly. The morphologies of the nanoparticle assemblies can be tuned easily by changing the quantity of the building block chemisorbed on the nanoparticle surface.     

A hydrogen-bonded adduct containing seven-component supramolecular aggregates

The title compound is a salt, 3,6,9,16,19,22-hexaaza­tri­cyclo­[22.2.2.2^11,14]­triaconta-1(26),11(29),12,14(30),24,27-hexa­ene–3,5-di­nitro­benzoic acid–methanol (1/4/2), C₂₄H₄₂N₆⁴⁺·4C₇H₃N₂O₆⁻·2CH₄O, in which the cation lies across a centre of inversion and one of the two independent anions is positionally disordered over two sets of atom sites having equal occupancy. The components are linked by four types of N—H⃛O hydrogen bond [N⃛O 2.674 (2)–2.815 (2) Å; N—H⃛O 149–163°] and one type of O—H⃛O hydrogen bond in which the acceptor is disordered over two closely adjacent sites [O⃛O 2.67 (4) and 2.75 (4) Å; O—H⃛O 172 and 173°], forming centrosymmetric seven-component aggregates.     

Towards supramolecular polymers

The natural phenomenon of self-assembly has been used to make complex unnatural products. These new compounds have taken the form of catenanes and rotaxanes. We have identified several polymeric architectures containing the catenane and rotaxane motifs, and then described how the model structures, which we have identified and actually realized as chemical compounds, can be employed to assess the feasibility of introducing mechanical linkages into novel polymers by means of self-assembly processes.     

Designed supramolecular assembly of hydrogen-bonded anionic rosette layers

Self-assembly of five two-dimensional hydrogen-bonded honeycomb grids bearing the rosette motif has been conducted with the guanidinium cation and various anionic components as the building blocks, tetraalkylammonium ions being employed as the interlayer templates. The sinusoidal supramolecular guanidinium-carbonate (1:1) rosette layer reported previously has been induced to adopt a nearly planar configuration using 1H-imidazole-4,5-dicarboxylate as an auxiliary template and spacer. A novel three-component guanidinium-boric acid-carbonate (1:2:1) wavy layer has been constructed, which features two distinguishable rosette motifs. Deviating from conventional topological design, the generation of new rosette layers, albeit highly distorted, has also been accomplished with guanidinium ions and anionic building blocks (1,2-dithiosquarate and dianionic form of 1,1'-biphenyl-2,2',6,6'-tetracarboxylate) that do not conform to C3-symmetry.     

Photoresponsive self-assembly and self-organization of hydrogen-bonded supramolecular tapes

Self-assembling building blocks that are readily functionalizable and capable of achieving programmed hierarchical organization have enabled us to create various functional nanomaterials. We have previously demonstrated that N,N'-disubstituted 4,6-diaminopyrimidin-2(1 H)-one (DAP), a guanine-cytosine hybridized molecule, is a versatile building block for the creation of tapelike supramolecular polymer species in solution. In the current study, DAP was functionalized with azobenzene side chains. 1H NMR, UV/Vis, and dynamic light scattering studies confirmed the presence of nanometer-scale tapelike supramolecular polymers in alkane solvents at micromolar regimes. At higher concentrations (millimolar regimes), the supramolecular polymers hierarchically organized into lamellar superstructures to form organogels, as shown by X-ray diffraction and polarized optical microscopy. Remarkably, the azobenzene side chains are photoisomerizable even in the supramolecular polymers, owing to their loosely packed state supported by the rigid hydrogen-bonded scaffold, enabling us to establish photocontrollable supramolecular polymerization and higher order organization of the tapelike supramolecular polymers into lamellar superstructures.     

Ultrafast internal dynamics of flexible hydrogen-bonded supramolecular complexes

Supramolecular chemistry is intimately linked to the dynamical interplay between intermolecular forces and intramolecular flexibility. Here, we studied the ultrafast equilibrium dynamics of a supramolecular hydrogen-bonded receptor-substrate complex, 18-crown-6 monohydrate, using Fourier transform infrared (FTIR) and two-dimensional infrared (2DIR) spectroscopy in combination with numerical simulations based on molecular mechanics, density functional theory, and transition state theory. The theoretical calculations suggest that the flexibility of the macrocyclic crown ether receptor is related to an ultrafast crankshaft isomerization occurring on a time scale of several picoseconds and that the OH stretching vibrations of the substrate can serve as internal probes for the receptor's flexibility. The importance of population transfer among the vibrational modes of a given binding motif and of chemical exchange between spectroscopically distinguishable binding motifs for shaping the two-dimensional infrared spectrum and its temporal evolution is discussed.     

Characterization of supramolecular polymers

Supramolecular polymers are made of monomers that are held together by noncovalent interactions. This is the reason for the wide range of novel properties, such as reversibility and responses to stimuli, exhibited by supramolecular polymers. A range of supramolecular polymerization methods have been developed leading to a number of novel supramolecular materials. However, standard techniques for the characterization of supramolecular polymers have yet to be established. The dynamic nature of supramolecular polymers makes them difficult to be fully characterized using conventional polymer techniques. This tutorial review summarizes various methods for characterizing supramolecular polymers, including theoretical estimation, size exclusion chromatography, viscometry, light scattering, vapor pressure osmometry, mass spectrometry, NMR spectroscopy, scanning probe microscopy, electron microscopy, and atomic force microscopy-based single molecule force spectroscopy. Each of these methods has its own particular advantages and disadvantages. Most of the methods are used to characterize the supramolecular polymer chain itself. However, some of the methods can be used to study the self-assembled state formed by supramolecular polymers. The characterization of a supramolecular polymer cannot be realized with a single method; a convincing conclusion relies on the combination of several different techniques.     

Thermally induced structural transformation in a hydrogen-bonded supramolecular single-crystal

The thermally induced structural transformation of a hydrogen-bonded crystal formed from an amphoteric molecule of 6-[2-methoxy-4-(pyridylazo)phenoxy]hexanoic acid MeO was studied using differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction measurement (XRD). Crystal form of the hydrogen-bonded crystal was measured by single crystal four circle diffractometer (Mo-K_α radiation). As a result, the crystal of MeO was stabilized by many C–H⋅⋅⋅O hydrogen bonds, and the C–H⋅⋅⋅O hydrogen bonds were broken by thermal energy reversibly. After transformation the supramolecular architecture was composed of supramolecular polymer including free-rotation pentamethylene main chains.     

Propeller-like hydrogen-bonded banana-melamine complexes inducing helical supramolecular organizations

The results of the study presented here show a new example of the use of liquid crystals and the interactions involved in the mesomorphic state to build up complex molecular organizations. We have pursued a design strategy in which hydrogen bonding allows the combination of the pi-stacking tendency of melamine and the lateral interaction capability of V-shaped molecules, which has been extensively demonstrated through nematic, smectic, or columnar mesophases. This combination addresses the formation of columnar arrangements with inherent helical organization. In this work, nonmesomorphic as well as mesomorphic V-shaped acids, with a structure similar to banana liquid crystals, have been complexed to a 2,4,6-triamino-1,3,5-triazine derivative in a proportion 3 to 1, respectively. Hydrogen-bonded supramolecules whose formation and stability in solution have been proven by infrared and NMR techniques have been thus obtained. DOSY experiments have allowed us to assess in solution the presence of the complexes and their tetrameric composition. All the complexes display mesogenic ability, and their mesomorphic organization has been studied by X-ray diffraction and CD spectroscopy. Results allow us to propose a helical columnar model for the mesophase originated from a propeller-like conformation of the supramolecular complexes.     

Order-disorder transition in supramolecular polymers

In supramolecular polymers, directional interactions control the constituting units connectivity, but dispersion forces may conspire to make complex organizations. Here we report on the long-range order and order-disorder transition (ODT) of main-chain supramolecular polymers based on poly(propylene oxide) (PPO) spacers functionalized on both ends with thymine. Below the ODT temperature (T(ODT)), these compounds are semicrystalline with a lamellar structure, showing nanophase separation between crystallized thymine planes and amorphous PPO layers. Above T(ODT), they are amorphous and homogeneous even though their X-ray scattering spectrum reveals a peak. This peak is due to correlation hole effect resulting from contrast between end-functional groups and spacer. Macroscopically, the transition is accompanied by dramatic flow and mechanical properties changes.     

Mesophase behaviour of azobenzene-based angular supramolecular hydrogen-bonded liquid crystals

New types of angular 1:1 hydrogen-bonded supramolecular complexes via hydrogen-bond formation between 4-alkoxyphenylazo benzoic acids (In) and 4-(3?-pyridylazo)-4??-alkoxybenzoates (IIm) with various alkoxy chains (from 6 to 16 carbons) were prepared and investigated for their mesophase behaviour by differential scanning calorimetry (DSC) and polarised-light microscopy (PLM). All prepared homologues were found to be dimorphic, possessing smectic C and nematic mesophases. The formation of 1:1 hydrogen-bonded supramolecular liquid crystals (LCs) complexes was confirmed by FTIR and UV?visible (UV?vis) absorption spectroscopy. The study revealed that nematic transition enhancement (ΔT) decreases with the increase of the alkoxy chain length on the base complement, while it increases with the increase of the chain attached to the acid complement of the complex, that is the stability of the nematic phase is more dependent on the length of the acid component.     

Photoreversible supramolecular polymerisation and hierarchical organization of hydrogen-bonded supramolecular co-polymers composed of diarylethenes and oligothiophenes

Diarylethene 1 equipped with two monotopic melamine hydrogen-bonding sites and oligothiophene-functionalized ditopic cyanurate (OTCA) were mixed in a nonpolar solvent to form AA-BB-type supramolecular co-polymers (SCPs) bearing photoswitchable moieties in their main chains and extended π systems as side chains. UV/Vis, fluorescence, dynamic light scattering (DLS), TEM, and AFM studies revealed that the two functional co-monomers formed flexible quasi-one-dimensional SCPs in solution that hierarchically self-organized into helical nanofibers through H-aggregation of the oligothiophene side chains. Upon irradiating the SCPs with UV light, a transition occurred from the H-aggregated state to non-aggregated monomeric oligothiophene side chains, as shown by spectroscopic studies, which indicates the formation of small oligomeric species held together only by hydrogen-bonding interactions. TEM and AFM visualized unfolded fibrils corresponding to elongated single SCP chains formed upon removal of solvent. The helical nanofibers were regenerated upon irradiating the UV-irradiated solution with visible light. These results demonstrated that the supramolecular polymerisation followed by hierarchical organization can be effectively controlled by proper supramolecular designs using diarylethenes and π-conjugated oligomers.