Inner shell excitation spectroscopy of biphenyl and substituted biphenyls: probing ring-ring delocalization

Quantitative optical oscillator strength spectra for C 1s excitation and ionization of gas-phase biphenyl, decafluorobiphenyl, and 2,2'-bis(bromomethyl)-1,1'-biphenyl have been derived from electron energy loss spectroscopy recorded under electric dipole dominated conditions. The C 1s X-ray absorption spectrum of hexaphenylbenzene has been recorded in the solid state. The C 1s spectral features are interpreted with the aid of ab initio calculations for core excitation of benzene, biphenyl, hexafluorobenzene, and decafluorobiphenyl. A weak feature at 287.7 eV in biphenyl is identified as a C 1s --> pi(deloc) transition, characteristic of ring-ring delocalization. Its intensity and position are shown to be related to the average torsion angle and thus the extent of pi-pi-interaction between adjacent aromatic rings. The effects of perfluoro substitution on core excitation spectra are also characterized and discussed.