Molecular symmetry effects on the stability of highly ordered smectic phases

<p>In an effort to control the phase ranges of highly ordered smectic phases, we examined the impact of molecular symmetry on phase behaviour of a series of 12 symmetrical and unsymmetrical 4,4′-dialkanoyloxybiphenyl derivatives. Combined differential scanning calorimetry, polarised optical microscopy, and X-ray diffraction studies indicated that the compounds studied formed smectic F liquid crystals, and in some cases, G phases at lower temperatures. Although the clearing temperatures were largely unaffected by molecular symmetry, the transitions from the SmF liquid crystals to more ordered phases were consistently lowered upon reducing the molecular symmetry. As a result, unsymmetrical molecules had broader mesophases than their higher symmetry isomers, suggesting a strategy for tuning the phase behaviour of these highly ordered lamellar phases, which have been widely targeted for organic semiconductors.</p

Structural Design Parameters for Highly Birefringent Coordination Polymers

A series of coordination polymer materials incorporating the highly anisotropic 2-(2-pyridyl)-1,10-phenanthroline (phenpy) building block have been synthesized and structurally characterized. M­(phenpy)­[Au­(CN)₂]₂ (M = Cd, Mn) are isostructural and form a 1-D chain through bridging [Au­(CN)₂]⁻ units and extend into a 2-D sheet through aurophilic interactions. M­(phenpy)­(H₂O)­[Au­(CN)₂]₂·2H₂O (M = Cd, Mn, and Zn) are also isostructural but differ from the first set via the inclusion of a water molecule into the coordination sphere, resulting in a 1-D topology through aurophilic interactions. In­(phenpy)­(Cl)₂[Au­(CN)₂]·0.5H₂O forms a dimer through bridging chlorides and contains a free [Au­(CN)₂]⁻ unit. In the plane of the primary crystal growth direction, the birefringence values (Δ<i>n</i>) of 0.37(2) (Cd­(phenpy)­[Au­(CN)₂]₂), 0.50(3) (In­(phenpy)­(Cl)₂[Au­(CN)₂]·0.5H₂O), 0.56(3) and 0.59(6) (M­(phenpy)­(H₂O)­[Au­(CN)₂]₂·2H₂O M = Cd and Zn, respectively) were determined. β, a structural parameter defined by phenpy units rotated in the <i>A</i>–<i>C</i> plane relative to the light propagation (<i>C</i>) direction, was found to correlate to Δ<i>n</i> magnitudes. The addition of a carbon–carbon double bond to terpy has increased the molecular polarizability anisotropy of the building block, and all structures have reduced deviation from planarity in comparison to terpy and terpy derivative structures, leading to these higher Δ<i>n</i> values, which are among the highest reported for crystalline solids

Pi-Extended Ethynyl 21,23-Dithiaporphyrins: A Synthesis and Comparative Study of Electrochemical, Optical, and Self-Assembling Properties

21,23-Dithiaporphyrins were synthesized containing pi-extending ethynyl substituents at the meso positions. These porphyrins displayed highly bathochromic and broadened absorbance profiles spanning 400–900 nm with molar absorptivities ranging from 2500 to 300,000 M^–1 cm^–1. Electrochemically, these ethynyl dithiaporphyrins undergo a single oxidation at 0.44 or 0.57 V and reduction at −1.17 or −1.08 V versus a ferrocene/ferrocenium internal standard depending on the type of functionalization appended to the ethynyl group. DFT calculations predict that the delocalization of the frontier molecular orbitals should expand onto the meso positions of the ethynyl 21,23-dithiaporphyrins; shrinking the HOMO–LUMO energy gap by destabilizing the HOMO energy. Indeed, the DFT results agree with our optical and electrochemical assessments. Finally, differential scanning calorimetry combined with cross-polarized optical microscopy and powder X-ray diffraction was used to assess the ability of these porphyrins for long-range order. For the ethynylphenyl alkoxy 21,23-dithiaporphyin, birefringent, soft-crystalline-like domains were observed by polarized microscopy, which are marginally sustained by a low-level of crystallinity detected in the XRD, suggesting that long-range ordering is possible. Overall, ethynyl 21,23-dithiaporphyrins are able to harvest much lower energy light and possess lower oxidation and reduction potentials compared to their pyrrolic analogues, which are desirable properties for applications in organic electronics