Polymorphic characterization and structural comparisons of the non-linear optically active and inactive forms of two polymorphs of 1,3-bis(m-nitrophenyl)urea

Abstract

1,3-Bis(m-nitrophenyl)urea (MNPU) has been found to exist in two polymorphic forms (α and β). The white colour of the β-(polar) polymorph transforms to the yellow colour of the α-(centrosymmetric) polymorph on heating. The transformation can be followed by second-harmonic generation measurement as well as more conventional methods of infrared spectroscopy and X-ray powder diffraction. The β-form was further characterized by X-ray single-crystal diffraction and the crystal structure obtained was compared with the known structure of the α-form to rationalize why two crystalline forms exhibit different colours and second-order non-linear optical properties. The β-form crystallizes in a non-centrosymmetric space group (C2) with a= 20.95(1)Å, b= 4.712(3)Å, c= 6.715(4)Å, β= 104.96(5)°, Z= 2, d_calc= 1.56 g cm^–3 and R= 0.050, whereas the α-form crystallizes in a centrosymmetric space group (P2₁/c). Both the α- and β-forms have a good agreement in bond distances and angles but they adopt quite different molecular conformations and hydrogen-bonding interactions. Based on the structural comparisons of the polymorphs of MNPU and other compounds, the colour change observed in the title compound could be due to the twisted conformation between the nitro and urea groups. Furthermore, the two-fold axis symmetry and the perpendicular conformation in the molecule of the β-form may be important to bias molecules to pack into a non-centrosymmetric crystal structure.