Raman modes in oligophenyls under hydrostatic pressure

We present a low temperature study of the Raman modes in three oligo(para-phenylene) materials (p-terphenyl, 3P; p-quaterphenyl, 4P; and p-hexaphenyl, 6P) under hydrostatic pressure up to 80 kbar. The 1220 cm(-1) (C-H in plane bend), 1280 cm(-1) (C-C interring stretch), 1600 cm(-1) (aromatic C-C on-ring stretch) and 1610 cm(-1) (combination mode of 610 cm(-1) and 990 cm(-1)) modes provide indicators of planarization in their frequency and intensity behaviors. In this comparative study of 3P, 4P, and 6P, the most remarkable feature is the clearly defined change in the slope of the 1220 cm(-1) in-plane bending mode upon planarization. In contrast, modes in the 1600 cm(-1) region do not have as clear a two-slope behavior; however, a more sensitive measure of planarization is the frequency difference between the 1610 cm(-1) and the 1600 cm(-1) modes, which exhibits a distinct drop before planarization, and a gradual increase thereafter. The resonance conditions that are responsible for the 1610 cm(-1) mode make the two-slope behavior less pronounced as compared to the 1280-1220 pair of modes. The intensity of the Raman modes from all three oligophenyls also indicate planarization: the intensity of the 1280 cm(-1) mode decreases sharply relative to the 1220 cm(-1) mode until the molecule planarizes, and increases slowly thereafter.