Thomas J. Katz

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.

Preparations of polymers using metal-carbenes

Abstract Metal-carbenes initiate the polymerizations of cycloalkenes to polyalkenamers that are stereoselectively cis, cyclic trisubstituted olefins to polymers that are translationally invariant, and acetylenes to polyacetylenes. Some implications are described.

Simple preparation of a helical quinone

Abstract p -Divinylbenzene and p -benzoquinone combine to give a [5]-helicene-bis-quinone. This procedure uses no light and gives a helicene with an interesting functional group. It is notably simple. An enzymatic hydrolysis is used to obtain the quinone optically active.

Directive effect of bromine on stilbene photocyclizations. An improved synthesis of [7]helicene

Abstract Bromine atoms direct photocyolizations away both from positions they occupy and from those adjacent, providing previously unavailable selectivities in syntheses of [7]helicenes.

The 4-azepentalenyl anion

Abstract The simplest nitrogen heterocycle isoelectronic with the pentalenyl dianion is the 4-azapentalenyl anion (“azapentalenyl anion,” VI). The preparation and properties of this anion are described. It was prepared by reacting 3H-pyrrolizine (IX) in tetrahydrofuran (THF) with n-butyllithium, with potassium, and with sodium. That lithium, sodium, and potassium azapentalenide are ionic is indicated by the similarity of their PMR spectra in THF. The NMR and UV spectra of potassium azapentalenide are recorded in Figs 1 and 4. Lithium azapentalenide in THF reacts with benzophenone and with benzil to give fulvenes, X or XI. The anion is easily made and useful in preparing these derivatives, but the pKa of its conjugate acid, 3H-pyrrolizine, estimated to be 29 from the rate of exchange of its protons with deuterium oxide and triethylamine, is greater than anticipated on the basis of the Huckel MO theory.

Quasi-Phase-Matching in Chiral Materials

The second-order nonlinear optical coefficients associated with chirality differ in sign for the two mirror-image forms (enantiomers) of a chiral material. Structures comprised of alternating stacks of the enantiomers can therefore be used for quasi-phase-matched frequency conversion, as we demonstrate here by second-harmonic generation from Langmuir-Blodgett films of a helicenebisquinone. Such structures could lead to new types of frequency converters in which both the second-order nonlinear response and quasi-phase-matching arise from the chirality of a material rather than its polar order.

Optical activity effects in second harmonic generation from anisotropic chiral thin films

Circular-difference effects in second-harmonic generation have been used to study chiral, anisotropic thin films of a helicene derivative. For such samples, these effects arise both from the chirality of the film and from its anisotropy. We show theoretically and experimentally that there is a fundamental difference between a circular-difference effect originating from chirality and anisotropy. A method is described that distinguishes the two contributions.

Bromine auxiliaries in photosyntheses of [5]helicenes

Abstract Bromine substituents on benzene rings direct photocyclizations away from their ortho positions. This principle prevents [5]helicenes from photocyclizing and methoxyls from photoeliminating. It makes possible the photosynthesis of [5]helicene and of a tetramethoxy-derivative of [5]helicene.

Synthesis of phenols from metal-carbynes and diynes

Abstract Metal-carbynes RCM(CO)4Br with diacetylenes give specific phenols in good yield. The reactions take only a few minutes at or below room temperature.

A regression technique to analyze the second-order nonlinear optical response of thin films.

We present a new technique, based on regression analysis, to determine the second-order nonlinear optical susceptibility tensor of thin films. The technique does not require the absolute levels or phases of measured signals to be mutually calibrated. In addition it yields indicators that address the quality of theoretical models describing the sample. We use the technique to determine the susceptibility tensor of samples of a nonracemic chiral material which have very low symmetry (both chiral and anisotropic) and have many independent tensor components. The results show the importance of using detailed theoretical models that account for the linear optical properties of the sample.