Enantiotropic Polymorphism in Di-Indenoperylene*

The enantiotropic polymorphic phase transformation of di-indenoperylene (DIP), an organic semiconductor material, with transition temperature of 403 K, has been structurally characterized using single crystal X-ray diffraction. Both the low temperature $\alpha $- and the high temperature $\beta $-phase have a herringbone-type structure, with the $\alpha $-phase being triclinic with doubled unit cell volume compared to the monoclinic $\beta $-phase. In the latter, the molecules have a more upright orientation in the herringbone plane. The epitactic transformation from the $\beta $- to the $\alpha $-phase involves strong shearing displacements as well as bending and torsional deformations of the DIP molecules. The $\beta $-phase of DIP is equivalent to the thin film phase.