Electronic, optical, and structural properties of oligophenylene molecular crystals under high pressure: An ab initio investigation

The goal of this ab initio study is to investigate the role of intermolecular interactions in molecular semiconductors. We discuss the pressure-dependent electronic, optical, and structural properties of biphenyl and para-terphenyl molecular crystals. These materials can be viewed as prototypes for conjugated molecular crystals. We optimize all internal structural parameters, including bond lengths and the molecular orientation, as a function of pressure, where the lattice parameters are taken from experiment. We compare the computed bulk modulus to available experimental data. The electronic band structures are calculated for the optimized structures and are debated as a function of pressure. We find evidence that one has to dismiss the picture of pure van der Waals crystals, which has been discussed in the literature for oligo-para-phenylenes as well as for other fundamental types of molecular crystals, such as oligothiophenes or polyacenes. Furthermore, we observe a redshift and a broadening of the optical transitions under pressure. Analysis shows that these effects are of purely intermolecular origin while changes in the internal molecular geometry are of less importance in the high-pressure regime up to 5 GPa.