Heekyoung Choi

Transfer and Dynamic Inversion of Coassembled Supramolecular Chirality through 2D-Sheet to Rolled-Up Tubular Structure

Transfer and inversion of supramolecular chirality from chiral calix[4]arene analogs (3D and 3L) with an alanine moiety to an achiral bipyridine derivative (1) with glycine moieties in a coassembled hydrogel are demonstrated. Molecular chirality of 3D and 3L could transfer supramolecular chirality to an achiral bipyridine derivative 1. Moreover, addition of 0.6 equiv of 3D or 3L to 1 induced supramolecular chirality inversion of 1. More interestingly, the 2D-sheet structure of the coassembled hydrogels formed with 0.2 equiv of 3D or 3L changed to a rolled-up tubular structure in the presence of 0.6 equiv of 3D or 3L. The chirality inversion and morphology change are mainly mediated by intermolecular hydrogen-bonding interactions between the achiral and chiral molecules, which might be induced by reorientations of the assembled molecules, confirmed by density functional theory calculations.

Roles of both amines and acid in supramolecular hydrogel formation of tetracarboxyl acid-appended calix[4]arene gelator

A tetracarboxylic acid-appended calix[4]arene derivative 1 which were insoluble in water could form a supramolecular hydrogel upon addition of both amines and HCl. The added amines such as primary, secondary and tertiary amines efficiently acted as base to enhance the solubility of gelator. The stability of supramolecular gels is strongly dependent to the number of amino groups involved in the amines and the number of equivalents of amines added, relative to the equivalents of carboxylic acid groups in the calix[4]arene. In addition, the sequence of addition of both the amines and acid is critically important to form the supramolecular hydrogel from a gelator which, without any other additives, is insoluble in pure water. Furthermore, chiral gelator 1 formed a right-handed chiral arrangement of the amide chromophore in the presence of achiral amines.

Metallogel of bis(tetrazole)-appended pyridine derivative with CoBr2 as a chemoprobe for volatile gases containing chloride atom

Abstract A bis(tetrazole)-appended ligand 1 formed the metallogel efficiently by mixing with Co2+ ion. Interestingly, the metallogel 1 with CoBr2 showed the orange yellow colour, which has octahedral structure. The rheological properties of metallogel obtained with CoBr2 were ca. 1.5-fold larger than that for the metallogel obtained with CoCl2. Upon addition of HCl, SOCl2, (COCl)2 and COCl2 containing chloride atoms in the metallogel 1 prepared with CoBr2 changed from orange yellow to blue-green colour. These results indicate that the octahedral structure of metallogel was converted into the tetrahedral structure. On the other hand, no significant colour changes were observed in the presence of an excess of other anions, namely HF, HBr, HI, HNO3 and H2SO4. These findings indicate that the metallogel 1 with CoBr2 is useful as a chemoprobe for gases containing chloride atom.

Origin of Both Right- and Left-Handed Helicities in a Supramolecular Gel with and without Ni2+ at the Supramolecular Level

We demonstrate the different origins of helical directions in polymeric gels derived from a hydrazone reaction in the absence and presence of Ni2+. The right-handed helicity of polymeric gels without Ni2+ originates from the enantiomeric d-form alanine moiety embedded in the building block. However, the right-handed helicity is inverted to a left-handed helicity upon the addition of Ni2+, indicating that added Ni2+ greatly affects the conformation of the polymeric gel by overcoming the influence of the enantiomer embedded in the building block on the helicity at the supramolecular level. More interestingly, the ratio of the right-toleft-handed helical fibers varies with the concentration of Ni2+, which converts from 100% right-handed helical fiber to 90% left-handed helical fiber. In the presence of Ni2+, both right- and left-handed helical fibers coexist at the supramolecular level. Some fibers also exhibit both right- and left-handed helicities in a single fiber.

Retracted Article: Spatially resolved mechanical properties of photo-responsive azobenzene-based supramolecular gels

The viscoelastic and stiffness properties of azobenzene-based supramolecular gels have been investigated by rheometry and AFM. Interestingly, the viscoelastic properties of cis-isomers were 8–10 fold enhanced in comparison to the trans-isomer, which was a unique phenomenon. More interestingly, Youngs modulus of gels increased linearly with time of UV irradiation. The stiffness of gels was finely controlled in the spatial micro-scale environment. The improved mechanical strength was attributed to electrostatic interaction with the production of radicals of the gelators.

Reversible cyanovinylcarbazole-based polymer gel via photo-cross-linking reaction

Abstract A polymeric gel was prepared from a bis(3-cyanovinylcarbazole)-based derivative and a thymine-bearing calix[4]arene derivative via photo-cross-linking between 3-cyanovinylcarbazole and thymine groups by irradiation with 365 nm UV light, a novel and facile method to obtain a soft material. Values for storage and loss moduli of the bis(3-cyanovinylcarbazole)-based derivative and the thymine-bearing calix[4]arene derivative gradually increased during photoirradiation. This gel was transformed back to the solution state by decomposition of the polymer network to the monomers via irradiation with 312 nm UV light.

Peculiar Triarylamine-Based Co-assembled Supramolecular Polymers That Exhibit Two Transition Temperatures in the Formation of a Coiled Helical Bundle

This paper describes the peculiar co-assembly supramolecular polymerization behavior of triphenylamine trisamide derivatives with d-alanine (T-ala) or glycine (T-gly) moieties. Concentration and temperature-dependent circular dichroism (CD) spectroscopy revealed that the heating curves of co-assemblies obtained at various molar ratios of T-ala to T-gly exhibited two distinct transition temperatures. The first transition was due to the transformation from coiled helical bundles to single helical fibers without handedness. The second was due to a change from typical elongation to nucleation. These phenomena were confirmed by solvent-dependent decoiling of coiled helical structures and concentration-dependent morphological analysis. The two transitioning temperatures were dependent on the concentration of T-ala in the co-assemblies, suggesting that T-ala concentration plays an important role in the formation of coiled helical bundles. Our study demonstrated the first observation of two distinct transition temperatures in supramolecular polymers.

A facile method to fabricate hydrogels from DMSO polymer gels via solvent exchange

A mixture of the bipyridine, phenyl and/or cyclohexanediamine-based building blocks 1, 2, and/or 3, having hydrazide, aldehyde or amine moieties, respectively, formed DMSO polymer gels by the hydrazone reaction under acidic conditions. These DMSO gels were transformed into hydrogels without a change in shape by solvent exchange. Interestingly, G′ and G′′ values of the hydrogels increased 9–12-fold due to the formation of a three-dimensional network structure by intermolecular hydrogen bonding interactions. In sweep measurements, the γ values of the hydrogels prepared without NaCl were enhanced from 18.5% to 46–55%, which was attributed to the three-dimensional network with additional intermolecular hydrogen-bonding interactions. Furthermore, a gel-to-quasi-liquid transition was completely reversible, where the G′ and G′′ values were recovered rapidly within 60 s. This reversibility was a thixotropic response.