Highly oriented poly(paraphenylene vinylene): Polarized optical spectroscopy under pressure

We investigate the role of intermolecular interactions in thick highly oriented poly(p-phenylene-vinylene) under applied hydrostatic pressure by polarized reflectance and Raman spectroscopy. Evidence of intramolecular and intermolecular effects is observed. The analysis of the C=C stretching modes and of the optical transition at about 3.7 eV previously assigned to conjugation chain ends indicates that the conjugation length of this highly ordered polymer is weakly affected by the applied pressure. On the other hand, the absorption onset and the spectral shape of the vibronic progression are deeply influenced by pressure, which generates a bathochromic shift joined to a broadening of all the optical transitions. By analyzing the data with a suitable optical model for an anisotropic system, a significant difference between the optical components polarized parallel and perpendicular to the stretching direction is observed. For the parallel component, a low energy gain of the oscillator strength, joined to a correspondent reduction in the high energy spectral range, is observed. For the perpendicular component, an increase in the oscillator strength is instead observed for all wavelengths. These effects are mainly assigned to intermolecular interactions even though a contribution from electronphonon coupling cannot be excluded to explain the observed broadening.