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

IUCrJ

Sankolli, R.¹  Burgener, M.¹  Bonin, M.¹  Hulliger, J.¹  Sommer, M.¹  Aboulfadl, H.¹  Labat, G.C.¹  Wübbenhorst, M.² 

[1] Department of Chemistry and Biochemistry, University of Berne, Freiestrasse 3, CH-3012 Berne, Switzerland;Laboratory for Soft Matter and Biophysics, University of Leuven, Celestijnenlaan 200d, 3001 Leuven, Belgium

关键词: ORIENTATIONAL DISORDER;    BIPOLAR CRYSTALS;    STRUCTURAL DEFECTS;   

DOI  :  10.1107/S2052252516006709

学科分类：数学（综合）

来源: International Union of Crystallography

【 摘 要 】

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.

【 授权许可】

CC BY