Solvent-induced crystalline-state emission and multichromism of a bent π-surface system composed of dibenzocyclooctatetraene units.

All bent out of shape: The solvent of crystallization effectively enhances the emission of flexible, bent, π-conjugated molecules in the crystalline state owing to control of the packing of the molecules in the structure. Multichromism such as thermochromism and vapochromism also arises from the solvent-controlled packing. This crystalline-state emission is attributable to the flexibility of cyclooctatetraene units of the bent π-conjugated molecules in the solid state (see figure).

Dynamic molecular tweezers composed of dibenzocyclooctatetraene units: synthesis, properties, and thermochromism in host-guest complexes.

Novel dynamic molecular tweezers (DMTs) 3 a, 3 b, 4 a, 4 b, and 5 b, composed of two tub-shaped dibenzocyclooctatetraene (DBCOT) units, were designed and synthesized. The cyclooctatetraene (COT) rings of these DMTs readily invert in solution, and the molecular structure shows rigid syn and anti forms in an equilibrium mixture in solution. The syn and anti conformers can be observed by NMR. The isomerization barriers of 3 a, 3 b, 4 a, 4 b, and 5 b are in the range of 16.5-21.3 kcal mol(-1), depending on steric repulsion between substituents of the COT rings and protons of the central benzene ring. These DMTs form complexes with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and 1,2,4,5-tetracyano-benzene (TCNB) in solution and in the solid state. The binding abilities of these DMTs increase with electron-donating substituents on COT, which increase the electron densities of the cavity of the syn form, as supported by theoretical calculations. In addition, elongation of the terminal alkoxy chains of the DMTs was found to cause the enhancement of van der Waals contact with guest molecules. Therefore, 5 b, which has CH(2)OMe groups on the COT rings and longer ethoxy groups on the terminal benzene rings, showed the highest electron density of the cavity and hence the highest binding ability with the electron-deficient guest molecules. Interestingly, solutions of 3 b, 4 b, and 5 b show thermochromism in the presence of DDQ. A solution of 3 b or 4 b with DDQ in CHCl(3) is green due to charge-transfer interaction at room temperature and the color changes from green to yellow upon heating to 60 degrees C and from green to blue upon cooling to -40 degrees C, whereas the high complexation ability of 5 b with DDQ only shows a change in the shade of blue.

Giant macrocycles composed of thiophene, acetylene, and ethylene building blocks.

Fully conjugated giant macrocyclic oligothiophenes with 60pi, 90pi,120pi, 150pi, and 180pi frames (1, 2, 3, 4 and 5) have been designed, and their butyl-substituted derivatives (1a, 2a, 3a, 4a, and 5a) have been synthesized using modified Sonogashira and McMurry coupling reactions as key steps. The 60-180pi systems 1-5 are circular with 1.8-6 nm inner cavities and 3.3-7.5 nm outside molecular diameters. Compound 1a containing ten 3,4-dibutyl-2,5-thienylene, eight ethynylene, and two vinylene units has been converted into macrocyclic oligo(3,4-dibutyl-2,5-thienylene-ethynylene) 6a using bromination/dehydrobromination procedure. Giant macrocycles 1a-6a exhibit a red shift of their absorption spectra and a fairly strong fluorescence with a large Stokes shift as compared to a linear conjugated counterpart having five thiophene rings. Compounds 1a-6a exhibit multistep reversible redox behaviors with fairly low first oxidation potentials, reflecting their cyclic conjugation. Furthermore, chemical oxidation of 1a-6a with FeCl3 shows drastic changes of spectroscopic properties due to intramolecular and intermolecular pi-pi interactions. Doping of 1a-3a with iodine forms semiconductor due to its pi-donor properties and pi-pi stacking ability. X-ray analysis of 1a confirmed a round, planar structure with nanoscale inner cavity, and revealed host ability for alkanes and unique packing structure. Interestingly, 2a and 3a self-aggregate in the solid state to form "molecular wires," which are about 200 nm thick and more than 1 mm long. The internal structures of fibrous aggregates have been investigated by optical microscope, scanning electron microscopy, atomic force microscopy, and X-ray diffraction analyses.

Efficient construction of biaryls and macrocyclic cyclophanes via electron-transfer oxidation of Lipshutz cuprates.

An efficient method for the homocoupling of aryl halides by electron-transfer oxidation of Lipshutz cuprates (Ar2Cu(CN)Li2) with organic electron acceptors is disclosed. Thus, various types of Lipshutz cuprates are prepared by successive treatment of aryl or heteroaryl bromides with tert-butyllithium and CuCN. The electron-transfer oxidation of Lipshutz cuprates with p-benzoquinones proceeds smoothly to afford the corresponding homocoupling products in moderate to good yields. Furthermore, it can be applied to the construction of either thiophene- or benzene-fused 10-membered ring cyclophanes. For the synthesis of 10-membered cyclophanes, the linear C-Cu-C structure of Lipshutz cuprates should be maintained in the dimetallacyclic intermediates, producing the large ring cyclophanes efficiently. The X-ray analysis of the cyclophanes reveals that the difference in the bridging atoms results in the different conformations of the macrocyclic rings. Thus, the silicon-bridged cyclophane 5a adopts a D2-symmetric structure with a twisted rhombic arrangement of four thiophene rings, whereas the methylene- and oxygen-bridged cyclophanes 5b and 5c possess C2h- and C2-symmetric structures with chair- and boatlike conformations, respectively. The 1H NMR spectrum of C2-symmetric 5c is temperature-dependent, and the activation energy (DeltaG) for the conformational change is 10.1 kcal/mol.

d-Electron-induced negative magnetoresistance of a pi-d interaction system based on a brominated-TTF donor.

A new pi-d interaction system (EDT-TTFBr2)2FeBr4 (EDT-TTFBr2 = 4,5-dibromo-4',5'-ethylenedithiotetrathiafulvalene) and its nonmagnetic anion analogue (EDT-TTFBr2)2GaBr4 based on a brominated TTF-type organic donor are investigated. The salts featured by quasi-1D pi-electronic systems are metallic with metal-insulator transitions taking place at about 20 and 70 K for the FeBr4- and GaBr4- salts, respectively, where the low-temperature insulating state is associated with charge ordering or a Mott insulator followed by an antiferromagnetic transition at lower temperatures. The FeBr4- salt is featured with an antiferromagnetic transition of the anion d spins at a Neel temperature (TN) = 11 K, which is significantly high despite its long anion-anion Br-Br contact, suggesting the importance of the pi-d interaction in the magnetism. The surprisingly strong pi-d interaction, ca. -22.3 K estimated from the magnetization curve, evidences the usefulness of the chemical modification of the donor molecule with bromine substitution to achieve strong intermolecular interaction. The antiferromagnetic state of the anion d spins affects the transport of the conducting pi electrons through the strong pi-d interaction, as evidenced by the presence of a resistivity anomaly of the FeBr4- salt at TN. Below TN, the FeBr4- salt shows negative magnetoresistance that reaches -23% at the highest magnetic field investigated (B=15 T), whereas only a small positive magnetoresistance is observed in the pi-electron-only GaBr4- salt. The mechanism of the negative magnetoresistance is explained by the stabilization of the insulating state of the pi electrons by the periodic magnetic potential of the anion d spins in the FeBr4- salt, which is modified by applying the external magnetic field.

Novel pi-expanded radialene macrocycles with inner cavity.

[structure: see text] Polyenyne macrocycles with pi-extended [9]- and [12]radialene frameworks have been synthesized. These radialenes exhibit restricted rotation of the aromatic rings, and the D3- and D4-symmetric structures in solutions have been determined by dynamic NMR. The macrocyclic radialenes bear small to medium inner cavities, and the small cavity of the pi-extended [9]radialene can incorporate a silver cation.

Mono- and bis(tetrathiafulvaleno)hexadehydro[12]annulenes.

Hexadehydro[12]annulenes annelated with one or two TTF units have been synthesized to investigate their pi-amphoteric properties based on the TTF and [12]annulene moieties; these compounds show multi-redox potentials, solvatochromism and the formation of large sandwich complexes.

Twin-rotor system created on a [4]radialene frame.

[reaction: see text] Three extended [4]radialenes with two tricyclic rings connected with exocyclic butatriene units have been synthesized. The compounds, possessing thioxanthene and dihydroanthracene moieties as the terminal substituents, show a fast rotation around the butatriene bonds at ambient temperatures (DeltaG() = 13.7 and 14.9 kcal/mol, respectively). In contrast, the fluorene-substituted analogue shows a much higher rotational barrier (DeltaG() = 17.8 kcal/mol) of the butatriene bonds due to the reduced steric repulsion between the two fluorene moieties at the ground state.