A high pressure study of phonons in oligophenyls using Raman scattering

We present a study of Raman phonons in three oligophenyls, namely, ter-, quarter- and hexaphenyl under hydrostatic pressure at low temperatures. The strongest modes observed are those due to the C-H bending mode at 1220 cm-1, the inter-ring C-C stretch Raman mode at 1280 cm-1, and the ring C-C stretch mode at 1600 cm-1. The intensity ratio of the first two modes is indicative of the degree of torsional motion between neighboring phenyl rings. Pressure brings the molecules closer to one another, thereby planarizing the molecule. In addition to the abrupt change in the intensity ratio caused by planarization, the frequencies of these modes increase discontinuously, and the linewidths change as well. The effect is clearly seen at pressures of 7.6, 7.8 and 13.6 kbar for the three materials, respectively.