Kwang-Ming Lee

Supramolecular liquid crystals of amide functionalized imidazolium salts

Liquid crystalline 1-acetamido-3-alkylimidazolium bromide, hexafluorophosphate and tetrafluoroborate salts with 3-alkyl chains of carbon number n = 10, 12, 14, 16 and 18 were prepared. The single crystal structure of the PF6− salt of n = 12 self-organizes to form a framework of a hydrogen bonded ribbon polymer. The mesomorphic behaviors of these compounds were studied by polarized optical microscopy, differential scanning calorimetry and powder X-ray diffraction. The presence of extended hydrogen bonding interactions in these acetamidoimidazolium salts allows them to have a wide temperature range of the mesophase. Anions have a profound effect on the phase transition temperature, suggesting that both ionic and hydrogen bonding interactions play dominant roles in the mesomorphic behavior of these liquid crystalline compounds.

Helical mono and dinuclear mercury(II) N-heterocyclic carbene complexes

Abstract A series of mono and dicationic carbene precursors have been synthesized. Reactions of these precursors with Hg(OAc)2 produce helical mononuclear and dinuclear mercury(II) carbene complexes. Five structures, including two ligand precursors and three mercury(II) carbene complexes have been analyzed by single crystal X-ray diffraction. Several intermediates have been identified during the course of reaction. Based on these observed intermediates, three different reaction pathways have been proposed.

Carboxylic acid functionalized imidazolium salts: sequential formation of ionic, zwitterionic, acid-zwitterionic and lithium salt-zwitterionic liquid crystals

Carboxylic acid functionalized imidazolium salts, their deprotonated carboxylate zwitterions and acid-zwitterion adducts have been studied, and are denoted as [N-Cn,N′-CO2H-Im][X] (Cn = CnH2n+1; X = Cl−, Br−, BF4− and PF6−), [N-Cn,N′-CO2-Im] and [(N-Cn,N′-CO2-Im)2H][anion] (anion = NO3− and Br−), respectively. Crystal structures of [N-C10,N′-CO2-Im]Br and [N-Cn,N′-CO2-Im]·2H2O studied by single-crystal X-ray diffraction indicate that carboxylic acid and carboxylate groups provide rich C–H⋯Br and/or C–H⋯O hydrogen bonding interactions in the construction of helical architecture. Except the PF6− salts, all the other types of compounds with long alkyl chains exhibit wide range SmA mesophase. Zwitterions and adducts represent two new classes of imidazolium based liquid crystals. The adducts [(N-Cn,N′-CO2-Im)2H]Br display wider mesophase range than the other series of compounds. Mixtures of lithium salts with zwitterion of [N-C10,N′-CO2-Im] form lithium containing liquid crystals at or near room temperature. A preliminary study of the mixtures of [N-C10,N′-CO2-Im]/[LiClO4] (3 : 1) and [N-C10,N′-CO2-Im]/[LiBF4] (3 : 1) placed between two ITO glasses shows that lithium ion conductivities increase upon increasing the temperature in the mesophase, yet decrease beyond the clearing temperature.

Rectangular architectures formed by acyclic diamido-metal-N-heterocyclic carbenes with skewed conformation

Acyclic acetamide functionalized mercury or silver carbene complexes, in which the coordination sphere adopts a skewed core conformation, self-assembled to form rectangular architectures.

Anion-controlled assemblies of C–H⋯O hydrogen bonded grid, stair or bilayer structures by L-shaped pyridinium salts

A series of 1-carbamoylmethyl-3-ethoxycarbonylmethylpyridinium salts with different anions (PF6− (1), BF4− (2), OTf− (3) and Br− (4)) have been synthesized and characterized by single-crystal X-ray diffraction. The L-shaped pyridinium cation moieties provide rich C–H donors and serve as building blocks to form grid structures with PF6− and BF4−, a zig-zag stair structure with OTf− and a bilayer structure with Br− anions.

Anion-controlled assemble of C–H⋯X hydrogen bonded helical tubes or catemers by crescent imidazolium salts

A series of 1-acetamido-3-(2-pyrimidyl)-imidazolium salts with different anions (Br− (1), NO3− (2), PF6− (3) and BF4− (4)) have been synthesized and characterized using single crystal X-ray diffraction. The crescent imidazolium moieties provide rich C–H donors and serve as building blocks to either convergently form helical tubes with Br− and NO3−, or divergently form 1-D catemers with PF6− and BF4− anions.

Ionic liquid crystal engineering of 3-carbamoyl-1-alkylpyridin-1-ium tetrachlorocuprate(II) and tetrachlorozincate(II) salts

Two series of nicotinamide based ionic liquid crystalline compounds, 3-carbamoyl-1-alkylpyridin-1-ium salts ([Nia–Cn]2X, X = CuCl42− and ZnCl42−; n = 12, 14 and 16), have been synthesized and characterized by single crystal and powder X-ray diffraction, DSC and POM. Both series form interdigitated bilayers in the solid state and liquid crystalline phase. The ionic layers of the two salts are stabilized mainly by four N–H⋯Cl and six C–H⋯Cl hydrogen bonds per anion and has sacrificed the formation of strong N–H⋯OC hydrogen bonding in the solid state. However, the N–H⋯Cl and N–H⋯OC hydrogen bonds dominate in the mesophase at high temperature to cause a significant increase of the thickness of the ionic layer.

N,N′-Dihexylbenzimidazolium salts. Anion-controlled bilayer structures via π–π dimer or C–H⋯π catemer motifs

A series of N,N′-dihexylbenzimidazolium salts with three anions, Br−, I− and PF6−, have been synthesized and characterized by single-crystal X-ray diffraction. These salts exhibit some anion-independent structural properties, which include the U-shaped conformation, the interdigitated bilayer packing and the thickness of the ionic layer. These salts also show anion-controlled inter-headgroup interactions; the benzimidazolium headgroups form a dimer motif via π–π interaction when the anions are Br− and I−, however, a 1D catemer motif is formed via C–H⋯π interactions when the anion is PF6−. The influence of the anion sizes on the arrangement of the benzimidazolium headgroups and the alkyl chains is discussed, with particular reference to the areas of the four adjacent headgroups and the degree of the alkyl chain tilting angles

Tetranuclear silver(I) clusters showing high ionic conductivity in a bicontinuous cubic mesophase.

The synthesis and characterization of tetranuclear silver triazole metallomesogens, [Ag4(L(4)-C(n))6][BF4]4 (L(4)-C(n) = 4-alkyl-1,2,4-triazoles where C(n) stands for C(n)H(2n+1) with n = 12, 14, 16, and 18), are reported. Upon heating, a phase transition sequence of Cr → SmC → Cub → SmA → isotropic liquid is observed for all of these compounds. Depending on the alkyl chain length, two types of cubic phases are found in this series of compounds. Those with shorter alkyl chains (n = 12 and 14) exhibit a micellar cubic phase, whereas long alkyl chains (n =16 and 18) show a bicontinuous cubic phase. Superior ionic conductivity at the bicontinuous cubic mesophase for [Ag4(L4-C16)6][BF4]4 is observed because of the presence of a three-dimensional ion-transporting channel. Doping a small amount of AgBF4 enhances the ionic conduction dramatically, presumably via promotion of the migration of Ag(I) ions in the channels.

Anion-controlled dimerized rectangular, herringbone and tape building blocks by L-shaped diamide-substituted pyridinium salts via N–H⋯O and C–H⋯O hydrogen bonding

A series of 1-(2-amino-2-oxoethyl)-4-carbamoylpyridin-1-ium salts with different anions (PF6− (1), OTf− (2), BPh4− (3) and Br− (4)) have been synthesized and characterized by single crystal X-ray diffraction. The results demonstrate that the unparallel diamide-substituted pyridinium salts form a dimerized rectangular building block and a 2-D rectangular structure via N–H⋯O and C–H⋯O hydrogen bonding (PF6− (1) and OTf− (2), respectively), a 2-D sheet structure (BPh4− (3)) via N–H⋯O hydrogen bonding catemers and a 2-D plate structure via N–H⋯O and C–H⋯O hydrogen bonding catemers (Br− (4)). The effects on the hydrogen bonding motifs, building blocks and structures by the four anion species with different hydrogen bonding acceptor abilities, sizes and shapes have been discussed.