Order-disorder transition in a quasi-two-dimensional colloidal system

A 2D colloidal system governed by repulsive dipolar forces tends to form a more ordered system when the interaction strength between the particles increases. Here we report an order-disorder transition of the colloidal system followed by chain formation upon increasing the dipolar interactions and show that the critical field scales with the density of colloids. Our system can do this by changing its dimensionality and therefore exhibits novel behavior that could help us understand colloidal ordering phenomena.     

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.     

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.     

CO2 in supramolecular chemistry: preparation of switchable supramolecular polymers

CO2 gas was used to construct novel types of supramolecular polymers. Self-assembling nanostructures 11 and 13 were prepared, which employ both hydrogen bonding and dynamic, thermally reversible carbamate bonds. As precursors, calixarene ureas 1 and 2 were synthesized, which strongly aggregate/dimerize (K(D)>/=10(6) M(-1) per capsule) in apolar solution with the formation of self-assembling capsules 7 and linear polymeric chains 8, respectively, and also possess "CO2-philic" primary amino groups on the periphery. CO2 effectively reacts with molecules 7 and 8 in apolar solvents and cross-links them with the formation of multiple carbamate salt bridges. Oligomeric aggregate 11 and three-dimensional polymeric network 13 were prepared and characterized by 1H and 13C NMR spectroscopy. The morphology of supramolecular gel 13 was studied by scanning electron microscopy. Addition of a competitive solvent destroyed the hydrogen bonding in assembling structures 11 and 13, but did not influence the carbamate linkers; carbamate salts 12 and 14, respectively, were obtained. On the other hand, thermal release of CO2 from 11 and 13 was easily accomplished (1 h, 100 degrees C) while retaining the hydrogen-bonding capsules. Thus, three-dimensional polymeric network 13 was transformed back to linear polymeric chain 8 without breaking up. Encapsulation and storage of solvent molecules by 11 and 13 was demonstrated. This opens the way for switchable materials, which reversibly trap, store, and then release guest molecules. A two-parameter switch and control over hydrogen bonding and CO2-amine adducts was established.     

“Order-disorder” transition model in aqueous solutions of aliphatic alcohols

A model of “order-disorder” type structural transitions in dilute aqueous solutions of lower aliphatic alcohols is proposed. Analytical expressions for the thermodynamic potential of the solution, its isobaric heat capacity increment, and the dependence of the temperature of a solution transition to a structurally ordered state on the alcohol concentration are obtained. A linear correlation between the concentration dependence of the heat capacity and the compressibility of the solution is proved to exist.     

Physical organic chemistry of supramolecular polymers

Unlike the case of traditional covalent polymers, the entanglements that determine properties of supramolecular polymers are defined by very specific, intermolecular interactions. Recent work using modular molecular platforms to probe the mechanisms underlying mechanical response of supramolecular polymers is reviewed. The contributions of supramolecular kinetics, thermodynamics, and conformational flexibility to supramolecular polymer properties in solutions of discrete polymers, in networks, and at interfaces, are described. Molecule-to-material relationships are established through methods reminiscent of classic physical organic chemistry.     

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.     

New supramolecular transition metal catalysts

The covalent connection of a catalytically active transition metal center with a water-soluble receptor (host molecule) generates a new type of supramolecular catalyst in which the features of molecular recognition, phase transfer catalysis and transition metal catalysis are combined in a single system. The first examples of this principle make use of commercially available β-cyclodextrin (β-CD) as the receptor and rhodium complexes of diphosphanes as the catalytically active center, these being covalently connected to one another via a spacer. In competitive hydrogenation of certain olefins, unusual degrees of substrate selectivity based on molecular recognition are observed, not possible by conventional transition metal catalysts. The two-phase (water/organic) hydrogenation of nitro-aromatics also is a smooth process catalyzed by these supramolecular complexes. They also constitute an unusually active catalyst system for the selective hydroformylation of higher olefins such as 1-octene in a two-phase system. Dendrimers having diphosphane moieties on the surface provide ligands for transition metals, the corresponding metal complexes (e.g., Pd) functioning as efficient catalysts which can be recycled due to their nanoscopic properties.     

Isothermal titration calorimetry of supramolecular polymers

A method to characterize the self-association of supramolecular polymers by isothermal titration calorimetry (ITC) has been designed. Association constants in the range 10(4)-10(6) dm(3) mol(-1) have been successfully determined from the heat exchange occurring when a supramolecular polymer solution is injected into a calorimetric cell containing pure solvent. Very good agreement with literature values has been obtained. Compared to other techniques (such as NMR or Fourier transform infrared spectroscopy), the use of ITC presents several advantages: (i) the enthalpy of association is obtained together with the association constant from the same experiment, (ii) the measurements can be performed in almost any solvent, and (iii) systems with higher association constants can be characterized.     

Hydrogen bonded supramolecular polymers in moderately polar solvents

Hydrogen bonded assemblies are usually decomposed by polar organic solvents. However, we have succeeded in preparing a strongly associated supramolecular polymer which forms viscous solutions in competitive solvents such as tetrahydrofuran.     

New directions in supramolecular transition metal catalysis

Supramolecular chemistry has grown into a major scientific field over the last thirty years and has fueled numerous developments at the interfaces with biology and physics, clearly demonstrating its potential at a multidisciplinary level. Simultaneously, organometallic chemistry and transition metal catalysis have matured in an incredible manner, broadening the pallet of tools available for chemical conversions. The interface between supramolecular chemistry and transition metal catalysis has received surprisingly little attention. It provides, however, novel and elegant strategies that could lead to new tools in the search for effective catalysts, as well as the possibility of novel conversions induced by metal centres that are in unusual environments. This perspective describes new approaches to transition metal catalyst development that evolve from a combination of supramolecular strategies and rational ligand design, which may offer transition metal catalysts for future applications.     

Thermodynamics of formation of host-guest supramolecular polymers

The interactions between three beta-cyclodextrin hosts (having 1-3 binding sites) and two adamantyl guests (having 1-2 binding sites) have been studied by ITC, ROESY, static and dynamic light scattering (SLS and DLS), and AFM and TEM techniques. The enthalpy and free energy values (determined from ITC experiments) evidence that the single interaction between one binding site of the guest and one binding site of the host is independent of the number of binding sites of the interacting species. The average values are deltaH degrees = -26.6 +/- 2.3 kJ mol(-1) and deltaG degrees = -30.4 +/- 3.2 kJ mol(-1), indicating that the process is mainly enthalpy driven. In all cases, the experimental molar ratio (from ITC experiments) agrees with the expected one from the number of binding sites of both the host and guest. The formation of polymer-like entities was demonstrated by SLS, DLS, AFM, and TEM measurements. The structure of polymers is linear when both the host and the guest are ditopic entities and dendritic (or Cayley tree type) when the host and the guest have three and two binding sites, respectively.     

Advanced supramolecular polymers constructed by orthogonal self-assembly

Large aggregates, constructed by linking together monomer building blocks via non-covalent interactions with polymer properties, are regarded as supramolecular polymers. Many kinds of non-covalent interactions, such as metal-ligand coordination, hydrogen bonding, π-π stacking, ionic interaction, and host-guest interaction etc., can be involved in the binding interactions of monomer building blocks, as well as in the modification of the side chain for the construction of variable supramolecular polymers. In this tutorial review, we summarized the reported supramolecular polymers fully- or partially-created from the combination of multiple non-covalent binding interactions, mainly of two kinds, in the orthogonal way.     

Chiral supramolecular polymers formed by host-guest interactions

alpha-Cyclodextrin with a p-t-butoxyaminocinnamoylamino group in the 3-position (3-p-(t)()BocCiNH-alpha-CD) has been found to form a supramolecular polymer in an aqueous solution. The degree of polymerization of the supramolecular polymer is higher than 15 at 20 mM, as proved by VPO (vapor pressure osmometry) measurements and turbo ion spray TOF MS measurements. The existence of substitution/substitution interactions between adjacent monomers of the supramolecular polymer have been confirmed by the observation of positive and negative Cotton bands in circular dichroism spectra. The mechanism for the induction of the chirality was confirmed using model compounds. The substituents were found to exist as a left-handed anti configuration in supramolecular polymers. The supramolecular polymer was found to take a helical structure. The structure of the supramolecular polymer was observed by STM measurements.     

Thermosolutal self-organization of supramolecular polymers into nanocraters

The ability of two complementary molecular modules bearing H-bonding uracilic and 2,6-(diacetylamino)pyridyl moieties to self-assemble and self-organize into submicrometer morphologies has been investigated by means of spectroscopic, thermogravimetric, and microscopic methods. Using uracilic (3)N-BOC-protected modules, it has been possible to thermally trigger the self-assembly/self-organization process of the two molecular modules, inducing the formation of objects on a mica surface that exhibit crater-like morphology and a very homogeneous size distribution. Confirmation of the presence of the hydrogen-bonding-driven self-assembly/self-organization process in solution was obtained by variable-temperature (VT) steady-state UV-vis absorption and emission measurements. The variation of the geometric and spatial features of the morphologies was monitored at different T by means of atomic force microscopy (AFM) and was interpreted by a nonequilibrium diffusion model for two chemical species in solution. The formation of nanostructures turned out to be affected by the solid substrate (molecular interactions at a solid-liquid interface), by the matter-momentum transport in solution (solute diffusivity D(0) and solvent kinematic viscosity ν), and the thermally dependent cleavage reaction of the BOC functions (T-dependent differential weight loss, θ = θ(Τ)) in a T interval extrapolated to ～60 K. A scaling function, f = f?(νD(0), ν/D(0), θ), relying on the onset condition of a concentration-driven thermosolutal instability has been established to simulate the T-dependent behavior of the structural dimension (i.e., height and radius) of the self-organized nanostructures as ?h? ≈ f (T) and ?r? ≈ 1/f (T).     

Constructing supramolecular polymers from phototrigger containing monomer

A new type of strategy for photo-induced supramolecular polymerization based on hostguest interaction has been explored. A kind of monomer containing dibenzo-24-crown-8 (DB24C8) macrocycle and dibenzylammonium (DBA) site was designed and synthesized. The coumarin, as a photocleavable protector, was introduced to the end of the monomer whose polymerization can be irradiated by UV light via the hostguest interactions between DB24C8 moieties and DBA units.     

Order-disorder antiferroelectric phase transition in a hybrid inorganic-organic framework with the perovskite architecture

[(CH3)2NH2]Zn(HCOO)3, 1, adopts a structure that is analogous to that of a traditional perovskite, ABX3, with A = [(CH3)2NH2], B = Zn, and X = HCOO. The hydrogen atoms of the dimethyl ammonium cation, which hydrogen bond to oxygen atoms of the formate framework, are disordered at room temperature. X-ray powder diffraction, dielectric constant, and specific heat data show that 1 undergoes an order-disorder phase transition on cooling below 156 K. We present evidence that this is a classical paraelectric to antiferroelectric phase transition that is driven by ordering of the hydrogen atoms. This sort of electrical ordering associated with order-disorder phase transition is unprecedented in hybrid frameworks and opens up an exciting new direction in rational synthetic strategies to create extended hybrid networks for applications in ferroic-related fields.     

Conformational control in the cyclization of hydrogen-bonded supramolecular polymers

Bifunctional 2-ureido-4[1H]-pyrimidinone (UPy) derivatives can form small cyclic oligomers as well as long supramolecular polymers in chloroform solutions using the quadruple hydrogen-bonding motif. Ring-chain equilibria of a set of supramolecular monomers containing methyl-substituted alkyl linkers between the hydrogen-bonding UPy moieties were investigated by (1)H NMR spectroscopy and viscometry. The data were characterized in terms of critical concentration (CC, denoting the onset of polymerization) and equilibrium cyclic dimer concentration (EDC, representing preorganization of the monomer toward selective formation of cyclic dimer). Methyl substituents in the monomer were found to promote conformations favorable for cyclic dimerization, leading to an increase in both the EDC and the CC with respect to unsubstituted monomer. Furthermore, we observed an odd-even effect in the CC and EDC with increasing length of the linker between the hydrogen-bonding units. The combined results allow tuning of the critical concentration over a broad range and offer detailed information on the correlation between monomer structure, conformation, and polymerizability which may provide new insights for the study and design of other ring-chain equilibria or helix-random coil transitions.