Two polymorphs of fluoren-9-ylidene malononitrile

Abstract

Fluoren-9-ylidene malononitrile (FM), a compound with two different crystal structures, has been explored to better understand its polymorphism. Two polymorphs were obtained from different solvents used in crystallization. The form 1, obtained from chloroform crystallizes in triclinic P1¯ space group. On the other hand, the form 2 obtained from acetonitrile crystallizes in orthorhombic Sohncke space group P212121. Two symmetry independent molecules (Z′ = 2) were observed in the asymmetric unit of form 1 representing it to be a rare type of crystals for which Z′ > 1. Distinctly different crystal packing was observed. While in form 1, the molecules stack infinitely in head-to-tail manner, in form 2, the molecules stack in head-to-head manner. The form 1 showed stronger intermolecular interaction compared to form 2, as evidenced by Hirshfeld surface analysis. In addition to PXRD, Raman, FT-IR spectroscopy measurements, analyses of shape index, curvedness, and 2-D finger print plots are discussed, which showed distinct features of these two polymorphs.