Redetermination of the crystal structure of α-copper phthalocyanine grown on KCl

The crystal structure of a polymorph of copper phthalocyanine (CuPc) grown on a KCl substrate is redetermined by transmission electron diffraction. It has a triclinic unit cell containing one molecule; the crystal does not have a herringbone-type molecular arrangement, which is a common packing mode of planar phthalocyanines. The molecular packing is determined by the diffraction intensity with the aid of the calculation of molecular packing energy. One of the striking features of this polymorph is its stacking mode within a molecular column: the molecular stacking direction projected on a molecular plane is different by an angle of about 45degrees from that of the alpha-modifications of platinum phthalocyanine (PtPc) and metal-free phthalocyanine (H2Pc). A powder X-ray diffraction profile calculated for the polymorph agrees well with that of so-called alpha-CuPc and Rietveld analysis for alpha-CuPc indicates that the CuPc crystals grown on KCl are actually alpha-CuPc; hence, alpha-CuPc is not isostructural with either alpha-PtPc or alpha-H2Pc. On the basis of the present results and the reported crystal structures of the planar phthalocyanines that form molecular columns, the polymorphs of the phthalocyanines can be classified into four types distinguished by the molecular stacking mode within the column: alpha(x)-, alpha(+)-, beta(x)- and beta(+)- types.