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.
Highlights
Among many known types of structural defects in crystals, 180 orientational disorder of dipolar molecules can lead to peculiar phenomena when investigating as-grown crystals by new physical methods revealing the spatial distribution of polar properties (Hulliger et al, 2002; Hulliger, Wust, Brahimi et al 2013)
Taking the benefit of these techniques, a structural analysis of molecular crystals exhibiting significant and spatially variable orientational disorder may follow a different strategy than usual in crystallography: In view of the polarization distribution shown in Figs. 5–9, we may ask: For which parts of the crystal is a structural refinement appropriate? Surprisingly, such types of inhomogeneous crystal objects can express a regular morphology
The analysis presented attempts to understand polarity evolution in 4bromo-40-nitrobiphenyl (BNBP) and 4-bromo-40-cyanobiphenyl (BCNBP) crystals in the frame of a general theory describing stochastic polarity formation due to 180 orientational disorder of dipolar molecules (Hulliger et al, 2002; Hulliger, Wust, Brahimi et al 2013)
Summary
Among many known types of structural defects in crystals, 180 orientational disorder of dipolar molecules can lead to peculiar phenomena when investigating as-grown crystals by new physical methods revealing the spatial distribution of polar properties (Hulliger et al, 2002; Hulliger, Wust, Brahimi et al 2013). The analysis presented attempts to understand polarity evolution in 4bromo-40-nitrobiphenyl (BNBP) and 4-bromo-40-cyanobiphenyl (BCNBP) crystals in the frame of a general theory describing stochastic polarity formation due to 180 orientational disorder of dipolar molecules (Hulliger et al, 2002; Hulliger, Wust, Brahimi et al 2013)
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