Chain-length-dependent intermolecular packing in polyphenylenes: a high pressure study

We report on pressure-induced structural changes in crystalline oligo(para-phenylenes) containing two to six phenyl rings. The results are discussed with particular emphasis put on the implications these changes in intermolecular distances and molecular arrangement have on important bulk properties of this class of materials, such as optical response and charge transport. We performed energy dispersive x-ray diffraction in a systematic study on polycrystalline powders of biphenyl, para-terphenyl, p-quaterphenyl, p-quinquephenyl and p-sexiphenyl under hydrostatic pressure up to 60 kbar. Revisiting the crystal structures at ambient conditions reveals details in the packing principle. A linear relationship between the density at ambient conditions and the number of phenyl rings is found. High pressure data not only yields pressure-dependent lattice parameters and hints towards pressure-induced changes in the molecular arrangement but also allows for an analysis of the equations of state of these substances as a function of oligomer length. We report the previously unknown bulk modulus of p-quaterphenyl, p-quinquephenyl and p-sexiphenyl (B-0 = 83, 93 and 100 kbar, respectively) and its pressure derivative (B'(0) = 6.4, 7.5 and 5.6). A linear dependence of the bulk modulus on the inverse number of phenyl rings in the molecules and on their ambient conditions density is found.