Ordering in two dimensions: Optical textures in monolayers

Summary Optical studies of Langmuir monolayers of simple amphiphiles such as fatty acids and esters by polarized fluorescence microscopy and Brewster-angle microscopy reveal a large variety of textures including stripes, stars, and spirals that closely resemble textures observed in liquid crystals. The textures represent large-scale (>10 μm) spontaneous organization of the molecular tilt azimuth. Phase transitions within the monolayer can be directly observed by marked changes in texture, revealing a complex phase diagram. Many of the textures can be explained by a Landau-deGennes theory of tilted hexatic phases that takes into account the broken-symmetry characteristic of amphiphiles at an interface. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Tetraethynylethenes: Fully cross-conjugated π-electron chromophores and molecular scaffolds for all-carbon networks and carbon-rich nanomaterials

The preparation of tetraethynylethene (3,4-diethynylhex-3-ene-1,5-diyne) 1 as well as of a great diversity of differentially mono-, di-, and triprotected derivatives by newly developed synthetic routes is described. These fully cross-conjugated molecules are versatile building blocks and precursors to two-dimensional all-C networks and novel C-rich nanoarchitecture with unusual structural and electronic properties, such as perethynylated expanded radialenes, or molecular wires and polymers with the novel polytriacetylene backbone. A key step in all of these routes was the Corey-Fuchs dibromoolefination of aldehydes and ketones. Dibromoolefination of silyl-protected penta-1,4-diyn-3-ones yielded the corresponding dibromomethylidene derivatives which, by twofold Pd-catalyzed alkyne coupling, were transformed into tetraethynylethene derivatives. In routes to tetraethynylethenes with free cis-or trans-enediyne moieties, dibromoolefination of aldehyde groups produced geminal dibromoethenes which, upon elimination/metallation with LDA followed by quenching with H⁺ or other electrophiles, yielded free or substituted ethynyl groups in high yields. Tetra- and triprotected tetraethynylethenes are rather stable compounds that could be isolated in pure form, whereas derivatives with two or more free ?C? H termini were only stable in dilute solution and polymerized rapidly in pure form. A trans-bis(triisopropylsilyl)-protected derivative represented an exception and could be isolated as stable crystals. X-Ray analysis revealed that the two bulky (i-Pr)₃Si groups isolate the reactive chromophores from one another in the crystal and prevent intermolecular reactions. The structures of several tetraethynylethenes were revealed in high-quality X-ray crystal structures.     

Assembly of triple-stranded beta-sheet peptides at interfaces

A 30-residue peptide, BS30, which incorporates two proline residues to induce reverse turns, was designed to form a triple-stranded beta-sheet monolayer at the air-water interface. To discern the structural role of proline, a second peptide, BS30G, identical to BS30 but with glycine residues replacing proline, was prepared and examined in parallel fashion. Surface pressure-molecular area isotherms indicated a limiting area per molecule (ca. 460 A(2)) for BS30 that corresponds well to that estimated from the known dimensions of crystalline beta-sheet monolayers (492 A(2)). Comparable measurements on BS30G yielded a smaller molecular area (380 A(2)). Grazing incidence X-ray diffraction measurements performed on the BS30 monolayer at nominal area per molecule of 500 A(2), exhibited two Bragg peaks corresponding to 4.79 and 34.9 A spacings, consistent with formation of triple-stranded beta-sheet structures that assemble into two-dimensional crystallites at the air-water interface. Visualized by Brewster angle microscopy, BS30 monolayers displayed uniform, solidlike domains, whereas BS30G appeared to be disordered.     

Carboracycles: Macrocyclic Compounds Composed of Carborane Icosahedra Linked by Organic Bridging Groups

A family of macrocyclic compounds are described, together with their precursors. These cycles are composed of icosahedral carboranes linked via their carbon vertices through 1,3-trimethylene, alpha,alpha'-1,3-xylylene, or alpha,alpha'-2,6-lutidylene groups. The compounds cyclo-(1,3-trimethylene-1',2'-closo-1',2'-C(2)B(10)H(10))(4) (6a), cyclo-(1,3-trimethylene-1',2'-closo-9',12'-dimethyl-1',2'-C(2)B(10)H(8))(4) (6b), cyclo-(1,3-trimethylene-1',2'-closo-1',2'-C(2)B(10)H(10))(3) (9), cyclo-(alpha,alpha'-1,3-xylylene-1',2'-closo-1',2'-C(2)B(10)H(10))(2) (11a), cyclo-(alpha,alpha'-1,3-xylylene-1',7'-closo-1',7'-C(2)B(10)H(10))(2) (11b), cyclo-(alpha,alpha'-1,3-xylylene-1',7'-closo-9',10'-dimethyl-1,7-C(2)B(10)H(8))(2) (11c), cyclo-(alpha,alpha'-1,3-xylylene-1',2'-closo-1',2'-C(2)B(10)H(10))(4) (12), cyclo-(alpha,alpha'-1,3-xylylene-1',7'-closo-1',7'-C(2)B(10)H(10))(3) (13), cyclo-(alpha,alpha'-2,6-lutidylene-1',7'-closo-1',7'-C(2)B(10)H(10))(2) (19), and cyclo-(alpha,alpha'-2,6-lutidylene N-oxide-1',7'-closo-1',7'-C(2)B(10)H(10))(2) (20) have been synthesized. The structures of 6a, 6b, 9, 11a, 11b, 11c, 12, and 19 have been determined by X-ray crystallography. Crystal data: for 6a, triclinic, space group P&onemacr;, a = 11.131(2) ?, b = 12.642(2) ?, c = 12.996(2) ?, alpha = 84.383(6) degrees, beta = 65.884(6) degrees, gamma = 97.292(5) degrees, Z = 1, R = 0.079; for 6b, monoclinic, space group P2(1)/a, a = 13.500(2) ?, b = 31.141(3) ?, c = 13.831(2) ?, beta = 99.90(1) degrees, Z = 2, R = 0.097; for 11a, monoclinic, space group C2/c, a = 14.5682(8) ?, b = 14.5046(8) ?, c = 16.1998(8) ?, beta = 95.631(2) degrees, Z = 4, R = 0.081; for 11b, monoclinic, space group P2(1)/n, a = 11.650(2) ?, b = 10.606(2) ?, c = 11.730(2) ?, beta = 104.951(6) degrees, Z = 2, R = 0.069; for 11c, orthorhombic, space group Pbca, a = 12.532(2) ?, b = 14.271(2) ?, c = 18.143(3) ?, Z = 4, R = 0.076; for 19, orthorhombic, space group Pcab (No. 61, standard setting Pbca), a = 11.0428(6) ?, b = 11.3785(6) ?, c = 22.533(1) ?, Z = 4, R = 0.074.     

Octahedral coordination of halide ions (I-, Br-, Cl-) sandwich bonded with tridentate mercuracarborand-3 receptors

The "anti-crown" B-hexamethyl 9-mercuracarborand-3 (1) was shown to complex halide ions (I-, Br-, Cl-) in an eta(3)-sandwich fashion. Symmetry-allowed interactions of the filled halide ion p-orbitals and the corresponding empty mercury p-orbitals result in three equivalent p(Hg)-p(halide)-p(Hg) three-center two-electron bonds and a sandwich structure. The molecular structures of [Li.(H(2)O)(4)][1(2).I].2CH(3)CN, MePPh(3)[1(2).Br].((CH(3))(2)CO)(2).(H(2)O)(2), and PPN[1(2).Cl] were determined by single-crystal X-ray diffraction studies. Compound [Li.(H(2)O)(4)][1(2).I].2CH(3)CN crystallized in the triclinic space group P-1, a = 13.312(8) A, b = 13.983(9) A, c = 13.996(9) A, alpha = 61.16(2) degrees, beta = 82.34(2) degrees, gamma = 86.58(2) degrees, V = 4365(2) A(3), Z = 1, R = 0.063, and R(w) = 0.171. Compound MePPh(3)[1(2).Br].((CH(3))(2)CO)(2).(H(2)O)(2) crystallized in the monoclinic space group C2/c, a = 24.671(8) A, b = 17.576(6) A, c = 26.079(8) A, beta = 106.424(6) degrees, V = 10847(6) A(3), Z = 8, R = 0.0607, and R(w) = 0.1506. Compound PPN[1(2).Cl] crystallized in the monoclinic space group C2/m, a = 37.27(2) A, b = 29.25(1) A, c = 10.990(4) A, beta = 100.659(7) degrees, V = 11774(8) A(3), Z = 4, R = 0.0911, and R(w) = 0.2369.     

The effect of gravity-induced gradients on coexistence curve measurements in binary mixtures

An analysis is made of the effect of gravity-induced gradients on the measurement of coexistence curves for binary mixtures near the critical solution temperature. Results are obtained for a classical model and for a scaled equation of state. It is shown that gradients cause experimental artifacts that affect the determination of the critical temperature and the value of the critical exponent β.