Xiangbing Zeng

Supramolecular dendritic liquid quasicrystals

A large number of synthetic and natural compounds self-organize into bulk phases exhibiting periodicities on the 10-8–10-6 metre scale as a consequence of their molecular shape, degree of amphiphilic character and, often, the presence of additional non-covalent interactions. Such phases are found in lyotropic systems (for example, lipid–water, soap–water), in a range of block copolymers and in thermotropic (solvent-free) liquid crystals. The resulting periodicity can be one-dimensional (lamellar phases), two-dimensional (columnar phases) or three dimensional (‘micellar’ or ‘bicontinuous’ phases). All such two- and three-dimensional structures identified to date obey the rules of crystallography and their symmetry can be described, respectively, by one of the 17 plane groups or 230 space groups. The ‘micellar’ phases have crystallographic counterparts in transition-metal alloys, where just one metal atom is equivalent to a 103 - 104-atom micelle. However, some metal alloys are known to defy the rules of crystallography and form so-called quasicrystals, which have rotational symmetry other than the allowed two-, three-, four- or six-fold symmetry. Here we show that such quasiperiodic structures can also exist in the scaled-up micellar phases, representing a new mode of organization in soft matter.

Frank–Kasper, quasicrystalline and related phases in liquid crystals

The review covers 3-dimensional and some 2-dimensional self-assembly patterns of supramolecular liquid crystals possessing either quasi-periodic or closely related truly periodic order. Compounds showing such structures are amphiphilic, and most often wedge-shaped, with dendrons being the most common examples. The topology is described in terms of 3D and 2D tiling of a variety of polyhedra or polygons, respectively. Analogy is made with structures of metallic alloys and soap froth. The recently discovered dodecagonal liquid quasicrystal is compared with the different tetrahedrally close packed Frank-Kasper phases in thermotropic and lyotropic liquid crystals. Parallels are also drawn with honeycomb columnar phases with related plane tilings, including that of distorted pentagons. The potential for the creation of nearly isotropic photonic bandgap materials is mentioned.

Self-Assembly of Dendronized Perylene Bisimides into Complex Helical Columns

The synthesis of perylene 3,4:9,10-tetracarboxylic acid bisimides (PBIs) dendronized with first-generation dendrons containing 0 to 4 methylenic units (m) between the imide group and the dendron, (3,4,5)12G1-m-PBI, is reported. Structural analysis of their self-organized arrays by DSC, X-ray diffraction, molecular modeling, and solid-state (1)H NMR was carried out on oriented samples with heating and cooling rates of 20 to 0.2 °C/min. At high temperature, (3,4,5)12G1-m-PBI self-assemble into 2D-hexagonal columnar phases with intracolumnar order. At low temperature, they form orthorhombic (m = 0, 2, 3, 4) and monoclinic (m = 1) columnar arrays with 3D periodicity. The orthorhombic phase has symmetry close to hexagonal. For m = 0, 2, 3, 4 ,they consist of tetramers as basic units. The tetramers contain a pair of two molecules arranged side by side and another pair in the next stratum of the column, turned upside-down and rotated around the column axis at different angles for different m. In contrast, for m = 1, there is only one molecule in each stratum, with a four-strata 2(1) helical repeat. All molecules face up in one column, and down in the second column, of the monoclinic cell. This allows close and extended π-stacking, unlike in the disruptive up-down alteration from the case of m = 0, 2, 3, 4. Most of the 3D structures were observed only by cooling at rates of 1 °C/min or less. This complex helical self-assembly is representative for other classes of dendronized PBIs investigated for organic electronics and solar cells.

Complex Multicolor Tilings and Critical Phenomena in Tetraphilic Liquid Crystals

X-shaped molecules undergo reversible thermal transitions between phase-separated and mixed states. T-shaped molecules with a rod-like aromatic core and a flexible side chain form liquid crystal honeycombs with aromatic cell walls and a cell interior filled with the side chains. Here, we show how the addition of a second chain, incompatible with the first (X-shaped molecules), can form honeycombs with highly complex tiling patterns, with cells of up to five different compositions (“colors”) and polygonal shapes. The complexity is caused by the inability of the side chains to separate cleanly because of geometric frustration. Furthermore, a thermoreversible transition was observed between a multicolor (phase-separated) and a single-color (mixed) honeycomb phase. This is analogous to the Curie transition in simple and frustrated ferro- and antiferromagnets; here spin flips are replaced by 180° reorientations of the molecules.

Hollow Six-Stranded Helical Columns of a Helicene†

Much studied have been the helical columnar liquid-crystalline phases of many materials, including such lyotropic systems as the polymers DNA, poly(g-benzyl-l-glutamate), and xanthan and such thermotropic materials, some with chiral side chains, as triphenylenes, banana-shaped molecules, mesomorphic twin molecules, foldamers, wedgeshaped molecules (including dendrons, either free or attached to polymer backbones), crown ethers, polyphenylacetylenes, and star-shaped molecules. But the columnar liquid-crystalline phases of helicenes have been studied very little, even though they are particularly interesting because the cores of their constituent molecules are chiral. Herein we analyze by means of X-ray scattering, in conjunction with electron-density reconstruction, molecular dynamics, and scattering pattern simulation, the supermolecular structures of the materials formed when molecules of one such helicene (1 in Figure 1a), both racemic and enantiopure, assemble in well-aligned fibers. The enantiopure material, synthesized previously, was shown to exhibit strong circularly polarized fluorescence and strong second harmonic generation. Like a related structure, it was thought to be comprised of molecules that stack on one another as in discotic phases. Instead, it is shown herein that the molecules assemble into hexagonal phases comprised of hollow-centered 132 helical columns, illustrated in Figure 1 and Figure 3. Both racemic and enantiopure 1 are ductile (plastic) solids that melt into isotropic liquids at 215 8C (40 kJmol ) and 242 8C (47 kJmol ), respectively. No other transitions are observed by polarized light microscopy, differential scanning calorimetry (DSC), and X-ray diffraction. Thermal gravimetric measurements confirm that both compounds under nitrogen are chemically stable up to 285 8C, but DSC heating and cooling experiments show that enantiomerically pure 1 slowly racemizes upon melting into the isotropic phase. Fibers were drawn from 10 wt% solutions in heptane at 80 8C and investigated by wide-angle (WAXS) and smallangle (SAXS) diffraction of synchrotron-generated X-rays. Figure 2a, c shows the WAXS and SAXS patterns of the racemic fiber. The pattern of enantiopure ( )-1 is indistinguishable from that of rac-1, except for the position of the small-angle reflection marked by the arrow in Figure 2c,d. Additional still wider angle diffraction patters show no sharp reflections in the 2p/q region of 0.3–0.5 nm, thus confirming the liquid-crystallinity of 1. The experimental fiber X-ray patterns show sharp Bragg reflections on layer lines 0 and 2 and diffuse streaks on the others. The equatorial reflections, with q values in the ratio 1:3:4:7:9:12:13 (see Table S1 in the Supporting Information) typify hexagonal order. The indexing in Figure 2c is in terms of h and k for a 2D hexagonal lattice. The cell parameter ahex= 4.001 nm. The diffuse streaks on the higher layer lines form a triple cross pattern, with the centers of the first, second, and third meridional crosses on the 0, 13, and 26 layer lines. Near the 0th and 26th layers only even lines are observed, namely, the 0th, 2nd, 4th, and 24th (weak), 26th, 28th, and 30th (weak). In contrast, near the 13th layer Figure 1. a) Compound ( )-1. b) The definition of coordinates. x,y,z are principal axes of the molecular mass tensor. Z, R, f are cylindrical coordinates with the column axis as reference (note that the OC12H25 groups are replaced by OH groups in the derivation of the tensor). Adjustable parameters: R0 is the distance of the molecular center of mass from the column axis; w0 is the molecular tilt, that is, rotation around y ; 10 is the “propeller blade” rotation around R. c) Arrangement of the heterohelicenes in the six-molecule asymmetric unit: molecule i+1 is related to molecule i by rotation around Z by f0 and translation along Z by Z0. The best-fit parameters are listed.

Simple Cubic Packing of Gold Nanoparticles through Rational Design of Their Dendrimeric Corona

The first simple-cubic liquid crystal was obtained by coating monodisperse Au nanoparticles (NPs) with a thick corona of amino-substituted organic dendrons. This unusual structure was determined by grazing-incidence diffraction and electron density reconstruction and explained by analyzing the radial density profile of the corona. Another novel structure is proposed for the phase preceding the cubic one: a hexagonal superlattice composed of alternating dense and sparse strings of Au NPs.

Transformation from Kinetically into Thermodynamically Controlled Self-Organization of Complex Helical Columns with 3D Periodicity Assembled from Dendronized Perylene Bisimides

The dendronized perylene 3,4:9,10-tetracarboxylic acid bisimide (PBI), (3,4,5)12G1-1-PBI, was reported by our laboratory to self-assemble into complex helical columns containing dimers of dendronized PBI with one molecule in each stratum, with different intra- and interdimer rotation angles but identical intra- and interdimer distance of 3.5 Å, exhibiting a four-strata 2(1) helical repeat. A thermodynamically controlled 2D columnar hexagonal phase with short-range intracolumnar order represents the thermodynamic product at high temperature, while a kinetically controlled monoclinic columnar array with 3D periodicity is the thermodynamic product at low temperature. With heating and cooling rates higher than 10 °C/min to 1 °C/min, at low temperature the 2D columnar periodic array is the kinetic product for this dendronized PBI. Here the synthesis and structural analysis of a library of (3,4,5)nG1-m-PBI with n = 12 to 6 and m = 1 are reported. A combination of differential scanning calorimetry, X-ray diffraction on powder and orientated fibers, including pattern simulation and electron density map reconstruction, and solid-state NMR, all as a function of temperature and heating and cooling rate, was employed for their structural analysis. It was discovered that at low temperature the as-prepared n = 12 to 10 exhibit a 3D layered array that transforms irreversibly into columnar periodicities during heating and cooling. Also the kinetically controlled 3D columnar phase of n = 12 becomes thermodynamically controlled for n = 10, 9, 8, 7, and 6. This unprecedented transformation is expected to facilitate the design of functions from dendronized PBI and other self-assembling building blocks.

Self-Repairing Complex Helical Columns Generated via Kinetically Controlled Self-Assembly of Dendronized Perylene Bisimides

The dendronized perylene 3,4:9,10-tetracarboxylic acid bisimide (PBI), (3,4,5)12G1-3-PBI, was recently reported to self-assemble in complex helical columns containing tetramers of PBI as basic repeat unit. These tetramers contain a pair of two molecules arranged side-by-side and another pair in the next stratum of the column turned upside-down and rotated around the column axis. Intra- and intertetramer rotation angles and stacking distances are different. At high temperature, (3,4,5)12G1-3-PBI self-assembles via a thermodynamically controlled process in a 2D hexagonal columnar phase while at low temperature in a 3D orthorhombic columnar array via a kinetically controlled process. Here, we report the synthesis and structural analysis, by a combination of differential scanning calorimetry, X-ray and electron diffraction, and solid-state NMR performed at different temperatures, on the supramolecular structures generated by a library of (3,4,5)nG1-3-PBI with n = 14-4. For n = 11-8, the kinetically controlled self-assembly from low temperature changes in a thermodynamically controlled process, while the orthorhombic columnar array for n = 9 and 8 transforms from the thermodynamic product into the kinetic product. The new thermodynamic product at low temperature for n = 9, 8 is a self-repaired helical column with an intra- and intertetramer distance of 3.5 Å forming a 3D monoclinic periodic array via a kinetically controlled self-assembly process. The complex dynamic process leading to this reorganization was elucidated by solid-state NMR and X-ray diffraction. This discovery is important for the field of self-assembly and for the molecular design of supramolecular electronics and solar cell.

Dynamic Mirror-Symmetry Breaking in Bicontinuous Cubic Phases**

Chiral segregation of enantiomers or chiral conformers of achiral molecules during self-assembly in well-ordered crystalline superstructures has fascinated chemists since Pasteur. Here we report spontaneous mirror-symmetry breaking in cubic phases formed by achiral multichain-terminated diphenyl-2,2′-bithiophenes. It was found that stochastic symmetry breaking is a general phenomenon observed in bicontinuous cubic liquid crystal phases of achiral rod-like compounds. In all compounds studied the

Self-Assembly of Hybrid Dendrons into Doubly Segregated Supramolecular Polyhedral Columns and Vesicles

{{\it Im}\bar 3m}