Pressure-Induced Conformational Change in Organic Semiconductors: Triggering a Reversible Phase Transition in Rubrene

A high-pressure polymorph of the organic semiconductor rubrene was obtained above 6.0 GPa by hydrostatic compression of the triclinic form. In the high-pressure phase, rubrene adopts an unexpected and previously unobserved conformation, which is ca. 70 kJ/mol less stable than the planar one observed in the ambient-pressure phase and is characterized by a unique "double twisting" of the tetracene core and "scissoring" of the lateral phenyl groups, which favor the formation of C-H center dot center dot center dot pi contacts. The evolution of the structure as a function of pressure is monitored and quantified by Hirshfeld surfaces analysis and calculations of lattice and intermolecular interaction energies. The isosymmetric single-crystal-to-single-crystal transition is fully reversible and is primarily driven by a reduction in molecular volume.