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) expe riments 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 m olecule - including the interring torsion angle - is then determined using three-dimensional (3D) band structure calculations within density functiona l theory (DFT). We And a red shift upon increasing pressure in the computed optical absorption spectra which is due to conformational changes of the 2 P molecule. A planarization accompanied by a reduction of the inter-ring bo nd length increases the conjugation, thereby reducing the band gap. Pressur e dependent Raman measurements on 2P, terphenyl, quaterphenyl and sexipheny l support the calculated conformational changes. Moreover, experimental opt ical absorption spectra on 2P films under hydrostatic pressure were measure d in a pressure range up to 3 kbar. The observed red-shift in the absorptio n quantitatively agrees with the computed values. (C) 2001 Elsevier Science B.V. All rights reserved.