title: Peculiar orientational disorder in 4-bromo-4'-nitrobiphenyl (BNBP) and 4-bromo-4'-cyanobiphenyl (BCNBP) leading to bipolar crystals.
abstract: 180° orientational disorder of molecular building blocks can lead to a peculiar spatial distribution of polar properties in molecular crystals. Here we present two examples [4-bromo-4'-nitrobiphenyl (BNBP) and 4-bromo-4'-cyanobiphenyl (BCNBP)] which develop into a bipolar final growth state. This means orientational disorder taking place at the crystal/nutrient interface produces domains of opposite average polarity for as-grown crystals. The spatial inhomogeneous distribution of polarity was investigated by scanning pyroelectric microscopy (SPEM), phase-sensitive second harmonic microscopy (PS-SHM) and selected volume X-ray diffraction (SVXD). As a result, the acceptor groups (NO2 or CN) are predominantly present at crystal surfaces. However, the stochastic process of polarity formation can be influenced by adding a symmetrical biphenyl to a growing system. For this case, Monte Carlo simulations predict an inverted net polarity compared with the growth of pure BNBP and BCNBP. SPEM results clearly demonstrate that 4,4'-dibromobiphenyl (DBBP) can invert the polarity for both crystals. Phenomena reported in this paper belong to the most striking processes seen for molecular crystals, demonstrated by a stochastic process giving rise to symmetry breaking. We encounter here further examples supporting the general thesis that monodomain polar molecular crystals for fundamental reasons cannot exist.
authors.affiliation: Department of Chemistry and Biochemistry, University of Berne , Freiestrasse 3, CH-3012 Berne, Switzerland.
authors.forename: Matthias
authors.initials: M
authors.lastname: Burgener
authors.affiliation: Department of Chemistry and Biochemistry, University of Berne , Freiestrasse 3, CH-3012 Berne, Switzerland.
authors.forename: Hanane
authors.initials: H
authors.lastname: Aboulfadl
authors.affiliation: Department of Chemistry and Biochemistry, University of Berne , Freiestrasse 3, CH-3012 Berne, Switzerland.
authors.forename: Gaël Charles
authors.initials: GC
authors.lastname: Labat
authors.affiliation: Department of Chemistry and Biochemistry, University of Berne , Freiestrasse 3, CH-3012 Berne, Switzerland.
authors.forename: Michel
authors.initials: M
authors.lastname: Bonin
authors.affiliation: Department of Chemistry and Biochemistry, University of Berne , Freiestrasse 3, CH-3012 Berne, Switzerland.
authors.forename: Martin
authors.initials: M
authors.lastname: Sommer
authors.affiliation: Department of Chemistry and Biochemistry, University of Berne , Freiestrasse 3, CH-3012 Berne, Switzerland.
authors.forename: Ravish
authors.initials: R
authors.lastname: Sankolli
authors.affiliation: Laboratory for Soft Matter and Biophysics, University of Leuven , Celestijnenlaan 200d, 3001 Leuven, Belgium.
authors.forename: Michael
authors.initials: M
authors.lastname: Wübbenhorst
authors.affiliation: Department of Chemistry and Biochemistry, University of Berne , Freiestrasse 3, CH-3012 Berne, Switzerland.
authors.forename: Jürg
authors.initials: J
authors.lastname: Hulliger
keywords: bipolar crystals; orientational disorder; structural defects