Moon Gun Choi

Multistimuli Two-Color Luminescence Switching via Different Slip-Stacking of Highly Fluorescent Molecular Sheets

Color tuning and switching of the solid-state luminescence of organic materials are attractive subjects for both the fundamental research and practical applications such as optical recording. We report herein cyanostilbene-based highly luminescent molecular sheets which exhibit two-color fluorescence switching in response to pressure, temperature, and solvent vapor. The origin for the multistimuli luminescence switching is the two-directional shear-sliding capability of molecular sheets, which are formed via intermolecular multiple C-H···N and C-H···O hydrogen bonds. The resulting two distinctive crystal phases are promoted by different modes of local dipole coupling, which cause a substantial alternation of π-π overlap. These changes can be directly correlated with the subsequent intermolecular excitonic and excimeric coupling in both phases, as demonstrated by an in-depth theory-assisted spectroscopic and structural study. Finally, we have prepared a first device demonstrator for rewritable fluorescent optical recording media which showed multistimuli luminescence tuning with fast response. Our multistimuli responsive system is unique in terms of the slip-stacking of molecular sheets and thus provides a novel concept of rewritable fluorescent optical recording media.

Folding-Generated Molecular Tubes Containing One-Dimensional Water Chains.

A series of indolocarbazole-pyridine (IP) oligomers were prepared that fold into helical conformations, and their folding features in solution and in the solid state were revealed. Helical folding of these IP foldamers is induced by dipolar interactions through the ethynyl bond and π-stacking between two repeating units. Upon helical folding, (1)H NMR signals of aromatic protons were significantly shifted upfield by Δδ = 0.5-2.2 ppm. In addition, hypochromic shifts and fluorescence quenching were observed in the absorption and emission spectra. X-ray crystal structures clearly demonstrated that IP foldamers folded to helical structures with cylindrical internal cavities wherein 3 or 5 water molecules were occupied by hydrogen-bonding interactions in a 1-D array, reminiscent of transmembrane water channels, called aquaporins.

Synthetic K⁺/Cl⁻-selective symporter across a phospholipid membrane.

Synthetic molecules which selectively transport sodium or potassium chloride across a lipid membrane have been prepared. The salt carriers consist of two heteroditopic binding sites, an anion-binding cavity with three hydrogen bond donors and an azacrown ether for binding an alkali metal cation. The association constants between the carriers and chloride ion have been enhanced by 1 order of the magnitude in the presence of sodium or potassium ion in 10% (v/v) CD3OH/CD3CN, due to the formation of a contact ion-pair between the bound cation and chloride as demonstrated by the single-crystal X-ray structure of a sodium chloride complex. A series of transport experiments have demonstrated that the synthetic molecule functions as a mobile carrier of transporting salts via M(+)/Cl(-) symport. Among alkali metal chlorides, the carrier with an 18-azacrown-6 exhibits a strong selectivity toward potassium chloride, while the carrier with a 15-azacrown-5 displays a moderate selectivity for sodium chloride.

Self-organized spiral columns in laterally grafted rods

Attachment of a flexible coil on the mid-part of a rigid rod block generates T-shaped rod-coil block molecules that self-assemble into a stepped column. These layers, in turn, self-curve into a spiral column with tunable core structure in the solid state.

Liquid crystal phases generated by supramolecular self-assembly of biforked amphiphilic imidazoles

Functional groups with the capability of hydrogen bonding are widely used in the molecular design and preparation of liquid crystalline supramolecular systems, a rapidly growing area of materials showing a high sensitivity towards external stimuli. A series of novel imidazole-containing Schiffs bases replenishing the family of supramolecular liquid crystals has been synthesised and characterised by proton nuclear magnetic resonance, Fourier transform infrared, and ultraviolet–visible spectroscopy, and elemental analyses. Variation of lengths of the terminal alkyl substituents in the obtained amphiphilic imidazoles within 6, 8, 10, 12, 14 and 16 carbon atoms leads to significant changes in their thermal behaviour, micro-segregation and supramolecular self-assembly. Lower homologues were non-mesomorphic, while intermediate members of the homologous series exhibited monotropic bilayered smectic and columnar mesophases. A higher homologue with 16 carbon atoms has an increased trend towards crystallisation of the aliphatic chains and did not exhibit mesomorphism again. The liquid crystalline mesophases were identified and investigated by polarised optical microscopy, differential scanning calorimetry, X-ray diffraction and thermal emission microscopy methods. According to X-ray diffraction characteristics, the smectic mesophase has a bilayered structure where the hydrophilic imidazole groups form a continuous hydrogen bonded network. The interface curvature created by the second alkyl chain leads to the appearance of columnar nanostructures in homologues with 12 and 14 aliphatic carbon atoms.

Synthesis of cobalt cluster-based supramolecular triple-stranded helicates

A cobalt cluster-based triple-stranded helicate, Co8(PDA)6(PTA)3(DMF)3(H2O)3 () (PDA = 2,6-pyridinedicarboxylate, PTA = benzene-1,3-dicarboxylate, DMF = dimethylformamide) was successfully synthesized and fully characterized. Complex can be used as a supramolecular building block in constructing a higher-order helix-of-helix structure, [Co8(PDA)6(PTA)3(DMF)2(H2O)4-0.51(Co(OHn)2)] (n = 1 or 2) ().

cis-2-Aminocyclohex-4-enecarboxylic acid as a new building block of helical foldamers

cis-2-Aminocyclohex-4-enecarboxylic acid (cis-ACHE) is a conformationally constrained β-amino acid. We showed that the conformational preference of a cis-ACHE residue is similar to that of cis-2-aminocyclohexanecarboxylic acid. α/β-Peptides that consist of the alternation of (1S,2R)-cis-ACHE and L-alanine adopted 11/9-helical conformations in solution and in the crystal state.

Synthesis, structure, and ligand exchange of a copper(II)-based molecular helix with 2,6-pyridinedicarboxylates

An one-dimensional (1-D) metal–organic polymer, [Cu2(PDA)2(DMF)2]n (PDA = 2,6-pyridinedicarboxylate, DMF = dimethylformamide), was synthesized by reaction of copper(II) nitrate hemi(pentahydrate) and 2,6-pyridinedicarboxylic acid in DMF. The complex shows a molecular helix structure consisting of five-coordinate Cu(II) building blocks with distorted square pyramidal geometry. Tridentate chelating PDA, DMF, and an oxygen from the carboxylate of the adjacent Cu(II) building unit are coordinated to the copper(II) center. The weakly coordinated DMF groups in [Cu2(PDA)2(DMF)2]n easily exchange with a pyridine to generate a pyridine-coordinated non-helical 1-D metal–organic polymer with six-coordinate pseudooctahedral Cu(II) units.

Revisit to Synthesis of Allyl- and Propargyl-Phosphorylcholines: Crystal Structure of Allyl-Phosphorylcholine

GRAPHICAL ABSTRACT Abstract Allyl- and propargyl-phosphorylcholines were efficiently synthesized and fully characterized. The molecular structure of allyl-phosphorylcholine was studied by single-crystal X-ray diffraction for the first time.

Step-Growth Polymerization of Poly(phenylcarbosilane) and Its Structural and Physical Properties

The polymerization and molecular weight distribution of poly(phenylcarbosilane) (PPCS) depend on the temperature and pressure conditions under which it is synthesized using the Kumada rearrangement reaction. Its degree of polymerization is related not only to its physical properties such as solubility and boiling point but also to its chemical structure. In this study, soluble PPCS having a weight-average molecular weight (Mw) in the range of 2,500–3,000 was selectively isolated through distillation or fractional precipitation, and its chemical structure and physical properties were studied via FT-IR, NMR, GPC, and thermal analysis. The analytical data indicate that the Si–CH2–Si backbone and Si–Si bond are formed through step-growth polymerization and are clearly detected after purification. The differences in the thermal behavior of PPCS in air and in nitrogen were also investigated.