On two alizarin polymorphs

Abstract

For centuries alizarin has been used as natural pigment, yet, now it is also used in histochemistry and dermatology, and as a dye for semiconductor solar cells. Here, the crystal structure of the previously known form of alizarin is determined accurately and its new polymorph is discovered. The two polymorphs crystallize into the same monoclinic crystal system. The previously known form is centrosymmetric (P2₁/c space group) whereas the new one is not (space group Pc). Both crystals are twinned, disordered, and have the molecules packed into the crystal lattices in a comparable way. However the powder patterns of both forms are visibly different. In addition the analysis of the fingerprints of H⋯O intermolecular interactions based on Hirshfeld surfaces also indicates differences in the packing. Yet, these similarities between two forms of alizarin make their vibrational spectra hardly distinguishable.