High pressure studies on the packing forces and intermolecular interaction in polyphenyls
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High pressure studies on the packing forces and intermolecular interaction in polyphenyls G. Heimel1, P. Puschnig2, M. Oehzelt1, K. Hummer2, B. Koppelhuber-Bitschnau3, F. Porsch4, C. Ambrosch-Draxl2, and R. Resel1 1 Institute of Solid State Physics, Graz University of Technology, Petersgasse 16, A-8010 Graz, Austria 2 Institute for Theoretical Physics, University of Graz, Universitätsplatz 5, A-8010 Graz, Austria 3 Institute of Physical and Theoretical Chemistry, Graz University of Technology, Technikerstrasse 4, A-8010 Graz, Austria 4 Institute of Mineralogy and Petrology, University of Bonn, Poppelsdorfer Schloss, D-53115 Bonn, Germany ABSTRACT In this work, we report on pressure induced structural changes in crystalline oligo(paraphenylenes) containing two to six phenyl rings. 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. Energy dispersive X-ray diffraction has been performed in a systematic study on polycrystalline powders of biphenyl, paraterphenyl, p-quaterphenyl, p-quinquephenyl and p-sexiphenyl under hydrostatic pressure up to 60 kbar. Our investigations not only yield pressure dependent lattice parameters and hints towards pressure induced changes in the molecular arrangement, but also allow 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 (B0 = 83 kbar, 93 kbar, and 100 kbar respectively) and its pressure derivative (B0’ = 6.4, 7.5, and 5.6). A linear dependence of the bulk modulus on the inverse number of phenyl rings in the molecules is found. INTRODUCTION In the present study, we focus on the oligomers of poly(para-phenylene) (PPP), the oligo(para-phenylenes) (OPPs) containing two to six benzene repeat units: biphenyl (2P), paraterphenyl (P3P), p-quaterphenyl (P4P), p-quinquephenyl (P5P), and p-sexiphenyl (P6P). These molecules are representatives of the group of rigid rod like, polycyclic, π-conjugated compounds. While 2P can be seen as the simplest model system for studying this series of materials, P3P, P4P, and P6P are technologically applicable. E.g., P3P and P4P are used as UV laser dyes [1], in scintillation counters [2], and as wavelength shifters [3], whereas the longer P6P is used as active layer in thin film organic LEDs (OLEDs) showing polarized blue emission [4]. Also the polymer PPP [5] and its planarized derivatives [6] have been extensively studied in optoelectronic applications. A deeper insight into the nature of the intermolecular interactions and the packing forces acting between molecules can be gained by modulating the intermolecular distances. Applying pressure to the sample is a “clean” way to tune the degree of intermolecular interaction.
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