High-pressure studies on molecular crystals—relations between structure and high-pressure behavior

This paper summarizes attempts to understand structure-property relationships for a large class of aromatic diphenyl-1,3,4-oxadiazole molecules. Starting from the investigation of the crystal structure several common packing motifs as well as characteristic differences are derived. Many different molecules show a rather planar conformation in the solid state. A stronger intermolecular twist is only observed for compounds with substituents occupying the ortho-positions of the phenyl rings. Most crystal structures are characterized by the formation of stacks leading to intense pi-pi acceptor-donor interactions between oxadiazole and phenyl rings. High-pressure investigations result in a soft compression behavior typical for organic molecular crystals. The bulk behavior may be described by the Murnaghan equation of state with similar coefficients (bulk modulus and its pressure derivative) for nearly all investigated compounds but also for related substances. The compression shows a strong anisotropy resulting from the specific features and packing motifs of the crystal structure. This is clearly indicated by a corresponding strain analysis. Additionally to the crystal structure the Raman spectrum was also investigated under increasing pressure. The different pressure behavior of external and internal modes reflects the difference between intra- and intermolecular interactions.