Polymorphism, polar morphology and absolute structure determination of 4-iodo-4′-nitrobiphenyl (INBP)

Abstract

4-Iodo-4′-nitrobiphenyl (INBP), a typical nonlinear optical molecule, features three polymorphs: Fdd2 as known before and two new forms, P2/c and P2. Monopolar crystals of the Fdd2 structure express a polar morphology and allowed an absolute structure determination. Iodo substituents cover the (001) and related faces. A negative pyroelectric coefficient was observed. The P2/c form was obtained from sublimation. Heating INBP (Fdd2) in the range of 150 to 180 °C ended in a second polar form (P2). DSC and dielectric measurements allowed us to assume a diffuse type of phase change from Fdd2 into P2. Lattice energy calculations for Fdd2 and P2/c yielded very similar energies. The twinning state observed for crystals not expressing polar symmetry is explained as a 50 : 50 % intergrowth of Fdd2 and P2/c. Periodic fragments allowed the close packing mode of all three polymorphs to be understood. A Hirshfeld surface analysis makes evident that apart from a strong linear synthon (I⋯NO₂) other interactions in all three forms are very similar.