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

Type of publication Peer-reviewed
Publikationsform Original article (peer-reviewed)
Author Burgener Matthias, Aboulfadl Hanane, Labat Gaël Charles, Bonin Michel, Sommer Martin, Sankolli Ravish, Wübbenhorst Michael, Hulliger Jürg,
Project Polaritätsbildung in molekularen Kristallen, natürlichem Gewebe und biomimetischen Materialien
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Original article (peer-reviewed)

Journal IUCrJ
Volume (Issue) 3(3)
Page(s) 219 - 225
Title of proceedings IUCrJ
DOI 10.1107/s2052252516006709

Open Access

URL http://doi.org/10.1107/S2052252516006709
Type of Open Access Publisher (Gold Open Access)

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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