Journal of Materials Chemistry p. 123 - 129 (1996)
Update date:2022-08-11
Topics:
Huang, Kin-Shan
Britton, Doyle
Etter, Margaret C.
Byrn, Stephen R.
Three 3-(nitroanilino)cycloalk-2-en-1-ones have been synthesized and their non-linear optical (NLO) properties investigated. Two of these compounds have been found to exist in two polymorphic forms (α and β) that exhibit different second-order NLO properties. These polymorphic forms were prepared and characterized by second-harmonic generation measurements as well as the more conventional methods of X-ray powder diffraction and infrared spectroscopy. Of these polymorphs, the α- and β-forms of 3-(4-nitroanilino)cyclohex-2-en-1-one (4NACHD) have been further characterized by X-ray single-crystal diffraction and the crystal structures obtained have been compared with each other to rationalize why these two crystalline forms exhibit different second-order NLO properties. The α-form of 4NACHD crystallizes in a centrosymmetric space group (P21/c) with a = 6.863(7), b = 12.767(3), c = 13.410(4) A, β = 104.59 (5)°, Z = 4, Dc = 1.356 g cm-3 and R = 0.046, whereas the β-form crystallizes in a non-centrosymmetric space group (P212121) with a = 7.228(3), b = 12.064(3), c = 13.104(3) A, Z = 4, Dc = 1.350 g cm-3 and R = 0.076. Except for the slight difference in bond distances, both the α- and β-forms have the same orientation of the carbonyl group and hydrogen-bonding interactions. The carbonyl group is anti to the N-H group in both the two forms that result in the lambda (Λ) conformation. The whole molecule of 4NACHD is more twisted in the β-form than in the α-form. Based on structural comparisons of the polymorphs of 4NACHD and other compounds, the more twisted conformation in the β-form may bias molecules to pack into a non-centrosymmetric structure. Preliminary results suggest that 3-(nitroanilino)cycloalk-2-en-1-one compounds may have a higher chance of forming non-centrosymmetric crystal structures than normal achiral organic molecules.
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