A Polymorph Lost and Found: the High Temperature Crystal Structure of Pentacene

The high temperature structural phase transformation in bulk pentacene has been characterized by X-ray single crystal and powder diffraction. A well-defined transition temperature of 478K was observed in single crystals. In contrast, pentacene powders heated above the phase transformation temperature do not always fully convert, and upon cooling, coexistence of the two polymorphs in varying concentrations is observed down to room temperature. The phase transformation is martensitic in nature, whereby the close packed herringbone-type layers shift against each other. The present results demonstrate that the structure of pentacene first reported in 1961 is actually stable only at high temperatures, and thus metastable at room temperature. All single crystal structures of pentacene reported subsequently correspond to the low temperature form.