Abstract
Three new enantiopure aryl-thioureas have been synthesized, N-(4-X-phenyl)-N-[1(S)-1-phenylethyl]thiourea, X= Cl, Br, and NO2 (compounds 1-3, respectively). Large single crystals of up to 0.5 cm3 were grown from methanol/ethanol solutions. Molecular structures were derived from X-ray diffraction studies and the crystal morphology was compared to calculations employing the Bravais-Friedel, Donnay-Harker model. Molecular packing was further studied with Hirshfeld surface calculations. Semi-empirical classical model calculations of refractive indices, optical rotation and the electro-optic effect were performed with OPTACT on the basis of experimentally determined refractive indices. Compound 3 (space group P 1 (No. 1)) was estimated to possess a large electro-optic coefficient r333 of approximately 30 pm/V, whereas 1 and 2 (space Group P 21 (No. 4) exhibit much smaller effects.
Highlights
Physical features of crystals are governed by their composition and symmetries (Neumann principle: a physical feature possesses at least the symmetry of the material of consideration)
In the absence of inversion symmetry, several non-linear optical properties can exist whereas the presence of Molecules 2010, 15 inversion symmetry prohibits optical rotation or the linear electro-optic effect for example [1]
Within the context of our goal to predict non-linear optical properties from the data provided in molecular crystal structures obtained via X-ray diffraction we were studying closely related chiral compounds for the impact of small differences in chemical composition on the structure-feature relationship
Summary
Physical features of crystals are governed by their composition and symmetries (Neumann principle: a physical feature possesses at least the symmetry of the material of consideration). Within the context of our goal to predict non-linear optical properties from the data provided in molecular crystal structures obtained via X-ray diffraction we were studying closely related chiral compounds for the impact of small differences in chemical composition on the structure-feature relationship. Crystals will grow in a way that minimizes as much as possible the sum of dipoles of all molecules inside a single unit cell. Geometrical strategies to minimize a total dipole include dimers of a right handed and a left handed molecule of the same chemical composition, related to each other through inversion symmetry. Inversion symmetry inside the unit cell is not possible in case of enantiopure molecules which causes difficulties for some molecules to grow into larger crystals.
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