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      Peculiar orientational disorder in 4-bromo-4′-nitrobiphenyl (BNBP) and 4-bromo-4′-cyanobiphenyl (BCNBP) leading to bipolar crystals

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          Abstract

          Two dipolar biphenyls show significant 180° orientational disorder resulting in bipolar as-grown crystals. An added symmetrical biphenyl interferes with polarity formation and inverts the initial bipolar state.

          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 (NO 2 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.

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          Author and article information

          Journal
          IUCrJ
          IUCrJ
          IUCrJ
          IUCrJ
          International Union of Crystallography
          2052-2525
          01 May 2016
          28 April 2016
          28 April 2016
          : 3
          : Pt 3 ( publisher-idID: m160300 )
          : 219-225
          Affiliations
          [a ]Department of Chemistry and Biochemistry, University of Berne , Freiestrasse 3, CH-3012 Berne, Switzerland
          [b ]Laboratory for Soft Matter and Biophysics, University of Leuven , Celestijnenlaan 200d, 3001 Leuven, Belgium
          Author notes
          Article
          bi5055 IUCRAJ S2052252516006709
          10.1107/S2052252516006709
          4856144
          27158508
          cb2ba4cd-601b-4a8b-b24e-28637b1bdb08
          © Matthias Burgener et al. 2016

          This is an open-access article distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.

          Categories
          Research Papers

          orientational disorder,bipolar crystals,structural defects

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