Large uniaxial negative thermal expansion in pentacene due to steric hindrance

The uniaxial negative thermal expansion in pentacene crystals along a is a particularity in the series of the oligoacenes and is exceptionally large for a crystalline solid. Full x-ray structure analysis from 120 to 413 K reveals that the dominant thermal motion is a libration of the rigid molecules about their long axes, modifying the intermolecular angle which describes the herringbone packing within the layers. This herringbone angle increases with temperature (by 0.3 degrees-0.6 degrees per 100 K) and causes an anisotropic rearrangement of the molecules within the layers, i.e., an expansion in the b direction and a distinct contraction along a. Additionally, a larger herringbone angle improves the cofacial overlap between adjacent, parallel molecules, and thus enhances the attractive van der Waals forces.