Brian D. Pate

Magnetic uniaxial alignment of the columnar superstructure of discotic metallomesogens over the centimetre length scale

The uniaxial alignments of the columnar superstructure of discotic metallomesogens, cobalt octa(n-dodecylthio)porphyrazine (CoS12), over the centimetre length scale have been achieved by spinning samples under a static magnetic field. The orientations of the columnar superstructures were investigated by small angle neutron scattering (SANS) and cryogenic transmission electron microscopy (cryo-TEM). Upon cooling from the isotropic phase to the columnar mesophase in the presence of a static magnetic field (0.4–1.1 T), CoS12 formed oriented columnar superstructures with the columnar directors being isotropically distributed in the plane normal to the external magnetic field. When the samples were continuously spun during cooling in a static applied magnetic field of 1.0 T, CoS12 was observed to form uniaxially aligned columnar superstructures with the columnar domain directors being parallel to the rotation axis which was normal to the external field. The optimal rotation speed for the alignment was found to be as low as 5–10 rpm, where the full width at half maximum of the domain director distribution is minimized. The uniaxial alignments were achieved over a macroscopic length scale (ca. 1 cm). Cryo-TEM measurements revealed that the persistence length of the uniaxially aligned columns was at least 1 µm.

9,10-anthracene dicarboxylate bridged complexes with M2 quadruply bonded dimetal units: [{M2(O2CtBu)3}2-(μ-9,10-An(CO2)2)], where M = Mo or W

From the reactions between [M2(O2CtBu)4] and 9,10-anthracenedicarboxylic acid in toluene, the dicarboxylate bridged complexes [[M2(O2CtBu)3]2(mu-9,10An(CO2)2)], have been obtained as microcrystalline yellow (M = Mo) and red (M = W) powders. The powders are soluble in THF forming intense red (M = Mo) and green (M = W) solutions. The electronic absorption spectra in 2-MeTHF have been recorded as a function of temperature (2-298 K) and show a small bathochromic shift on cooling. The electronic structures have been investigated by molecular orbital calculations employing density functional theory on the model compounds [(HCO2)3M2]2(mu-9,10-An(CO2)2) where the M4 unit is constrained to lie in a plane. These reveal a minimum energy, gas-phase structure wherein the plane of the anthracene is twisted by ca. 54 degrees with respect to its 9,10-carboxylate units for both Mo and W. The results of these calculations are correlated with the electronic absorption spectral data and the electrochemical measurements (CV and DPV) of the first and second oxidation waves. The EPR spectra of the radical cations formed by single-electron oxidation with [Cp2Fe](+)[PF6]- in a THF-CH2Cl2 solvent mixture show that the complexes are valence trapped at ambient temperature on the EPR timescale. These results are discussed in the light of recent studies of dicarboxylate-linked MM quadruple bonds.

On the electron delocalization in the radical cations formed by oxidation of MM quadruple bonds linked by oxalate and perfluoroterephthalate bridges

Electron paramagnetic resonance, electronic absorption, and resonance Raman spectroscopy reveal that in the oxalate-bridged compounds, [[(tBuCO2)3M2]2(mu-O2CCO2)]+[PF6]-, the unpaired electron is delocalized over four metal centers (M = Mo or W) as a result of M2 delta to bridge pi conjugation, but in the related cationic perfluoroterephthalate-bridged species, the tungsten complex is delocalized and the molybdenum analogue valence trapped.