Synthesis, polymorphic characterization and structural comparisons of the non-linear optically active and inactive forms of polymorphs of 3-(nitroanilino)cycloalk-2-en-1-ones

Abstract

There 3-(nitroanilino)cycloak-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)Å, β= 104.59 (5)°, Z= 4, D_c= 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)Å, Z= 4, D_c= 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 (A) 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.