Large Negative Thermal Expansion in Pentacene due to Steric Hindrance

TL;DR

This study reveals that pentacene crystals exhibit large negative thermal expansion along one axis due to molecular libration and herringbone angle changes, affecting intermolecular interactions and packing.

Contribution

It provides detailed structural analysis showing how libration and herringbone angle variations cause negative thermal expansion in pentacene.

Findings

Negative thermal expansion along axis a observed

Herringbone angle increases with temperature

Molecular libration influences intermolecular forces

Abstract

The uniaxial negative thermal expansion in pentacene crystals along $a$ is a particularity in the series of the oligoacenes, and exeptionally large for a crystalline solid. Full x-ray structure analysis from 120 K 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 -- 0.6$^{\circ}$ 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.