Pressure studies on the intermolecular interactions in biphenyl

We investigate the influence of intermolecular interactions on the optical and structural properties of oligophenyls in solid films of polycrystalline nature. To this end, we have performed X-ray powder diffraction (XRD) experiments in a pressure range up to 2 kbar yielding the lattice constants of biphenyl (2P) as a function of pressure. The optimized geometry of the 2P molecule — including the inter-ring torsion angle — is then determined using three-dimensional (3D) band structure calculations within density functional theory (DFT). We find a red shift upon increasing pressure in the computed optical absorption spectra which is due to conformational changes of the 2P molecule. A planarization accompanied by a reduction of the inter-ring bond length increases the conjugation, thereby reducing the band gap. Pressure dependent Raman measurements on 2P, terphenyl, quaterphenyl and sexiphenyl support the calculated conformational changes. Moreover, experimental optical absorption spectra on 2P films under hydrostatic pressure were measured in a pressure range up to 3 kbar. The observed red-shift in the absorption quantitatively agrees with the computed values.