A New Role for Bboron in Advanced Materials
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PIOTR KASZYNSKI* AND DARIUSZ LIPIAK OrganicMaterialsResearch Group, Department of Chemistry, Vanderbilt University, Nashville, TN 37235 USA
ABSTRACT Application of highly polar dative bonds of heteroatoms to boron atoms for the preparation of a new class of liquid crystals is discussed. Semiempirical calculations show that compounds incorporating polar bonds of tetra- and higher-coordinated boron centers are characterized by large coaxial dipole moments and relatively large first hyperpolarizabilities P3.It is expected that these new liquid crystalline materials will be useful for electrooptical applications. INTRODUCTION Boron has been established as one of the key elements in inorganic materials such as borides, 1 boron nitride, 2 and inorganic borates. 3 More recently, boron was introduced into several types of organic-based materials. For instance, cyclic boronates have been used as structural elements in liquid crystalline molecules. 4 p-Carboranes have been added to the pool of rods for a molecular tinkertoy construction set. 5' 6 However, carboranes have been unsuccessfully used in the synthesis 7 of high-P non-linear optical (NLO) materials. ,8 Lewis acidity is inherently associated with the boron atom, but its advantages in advanced materials have not been explored. As a consequence of having only three valence electrons, boron forms highly polarized bonds to heteroatoms in tetra-, penta-, and hexacoordinated compounds and carries a formal negative charge (Figure 1). We have been studying 9-11 a new class of liquid crystals (LC) in which the polar boron-heteroatom bonds result in large dipole moments and large positive dielectric anisotropies. These materials may find practical applications in twisted nematic 13 LC displays,1 2 and some may show NLO properties. The formation of polar bonds to tricoordinated boron centers follows the Lewis acid/Lewis base reaction and is well exemplified in the literature. 14 Typical complexes formed with amines are characterized by relatively high enthalpies of dissociation (about 18 kcal/mol) and dipole moments,
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Boron Cluster Approach
Figure 1. Two methods of formation of highly polar bonds to boron: from triorganoboranes and from boron clusters.
341 Mat. Res. Soc. Symp. Proc. Vol. 374 @1995 Materials Research Society
usually between 3-5 D.15 Recently, we reported computational results for a class of liquid crystals based on 1-borabicyclo[2.2.2.]octane. 11 Semiempirical calculations show that compound 1 has an enthalpy of dissociation of at least 21.0 kcallmol and a dipole moment g.tof 6.0 D. Its isosteric and isoelectronic analog 216 displays a broad-range smectic phase (K 46 S 247 I), which promises good mesogenic properties for 1. Progress toward synthesis of 1 and related systems will be reported separately. 05 H,1 ~--
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The formation of polar bonds to penta-, and hexacoordinated boron atoms is a more complicated and more interesting case. Now, the highly coordinated boron c
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