Tuning intermolecular interactions:: A study of the structural and vibrational properties of p-hexaphenyl under pressure

Hydrostatic-pressure is used to modulate the intermolecular interactions in the conjugated oligophenyl, para-hexaphenyl. These interactions affect the structural properties and also cause changes in the molecular geometry that directly alter the electronic properties. We use Raman spectroscopy to investigate the nature of the structural changes. Our Raman studies in the temperature range of 12 K to 300 K, under pressures up to 70 kbar, indicate that the potential energy of two neighboring phenyl rings as a function of the torsional angle is "W"-shaped. The libration of the phenyl rings between the two minima of the "W"-shaped potential can be modulated by either promoting the molecule to a higher energy state (activation energy of 0.045 eV) by raising the temperature or by decreasing the intermolecular separation, which makes the potential more "U"-shaped. Both these situations make the molecule seem more planar. We, infer the shape of the potential from the relative intensity of the inter-ring C-C stretch Raman mode at 1280 cm(-1) to the C-H bending mode at 1220 cm(-1) (I-1280/I-1220) These results are interpreted within the framework of ab initio electronic and vibrational spectra calculations of a biphenyl molecule. We have also conducted X-ray studies to check the sample purity.