The crystal structure of E-1,2-bis(3-methoxy-2-thienyl)ethene has been determined at five different temperatures, i.e. room temperature (293 K), 223, 173, 123 and 93 K. The solid-state work is complemented by the results of theoretical calculations of energies, geometries, difference electron densities and atomic charges of the free molecule. Analysis revealed static disorder caused by a higher energy conformer of E-1,2-bis(3-methoxy-2-thienyl)ethene, probably contaminating the crystal during its growth. The results support the contention that the electrical properties are mainly governed by the carbon backbone.

Blockhuys, F.; Vande Velde. C.M.L.; Maes, S.T.; Petern, C.; Geise, H.J.; Van Alsenoy, C.; Lenstra, A.T.H. Acta Cryst. B 2003, 59, 770-778.

Powder diffraction

The crystal structures of tetrabromothiophene and tetrabromoselenophene have been determined from X-ray powder diffraction data, using direct-space search techniques. In the case of tetrabromothiophene two crystalline phases occur, a stable orthorhombic and a metastable monoclinic phase. For the orthorhombic phase two different structural models were found that fit the experimental data equally well. The diversity in crystal structure models and packings of tetrabromothiophene is explained.

Helmholdt, R.B.; Sonneveld, E.J.; Vande Velde, C.M.L.; Blockhuys, F.; Lenstra, A.T.H.; Geise, H.J.; Peschar, R. Acta Cryst. B 2007, 63, 783-790.

Dynamic disorder and phase transitions

(E,E)-1-[2-(4-Nitrophenyl)ethenyl]-4-[2-(2,4-dimethoxyphenyl)ethenyl]benzene was characterized by X-ray diffraction and shown to be dynamically disordered at room temperature. The structure was re-determined over a range of temperatures to infer the thermodynamic parameters related to this disorder. A phase transition of third order according to the Ehrenfest classification scheme was discovered. To the best of our knowledge, this is the first experimentally observed phase transition of formal third order. It can be explained by the involvement of long-range lattice vibrations.

Vande Velde, C.M.L.; Collas, A.; De Borger, R.; Blockhuys, F. Chem. Eur. J. 2011, 17, 912-919.

Polar crystals

The synthesis of twelve asymmetric donor-acceptor distyrylbenzene derivatives with either one nitrile group or one, two or three nitro groups as electron acceptors, and one, two or three methoxy groups as electron donors is reported. Peak potentials obtained from cyclic voltammetry were combined with experimental UV/Vis data and molecular dipole moments obtained from quantum chemical calculations, yielding insight into the influence of the positions of the substituents on the electronic structure and charge distribution of this as yet unexplored class of organic semiconductors. The supramolecular structures of five of these compounds have been studied using single-crystal X-ray diffraction to monitor the influence of the positions of donor and acceptor groups on the organisation of the molecules in the solid state, and three crystal structures have been identified in which the molecular dipoles do not organize themselves in a centrosymmetric lattice. Analysis of the dipoles in the unit cell yields further insight into the possible non-linear optical properties of these three polar structures

Collas, A.; De Borger, R.; Amanova, T.; Vande Velde, C.M.L.; Baeke, J.K.; Dommisse, R.; Van Alsenoy, C.; Blockhuys, F. New J. Chem. 2011, 35, 649-662.