High-Resolution Spectroscopy of Jet-Cooled 1,1′-Diphenylethylene: Electronically Excited and Ionic States of a Prototypical Cross-Conjugated System

The photophysics of a prototypical cross-conjugated pi-system, 1,1'-diphenylethylene, have been studied using high-resolution resonance enhanced multiphoton ionization excitation spectroscopy and zero kinetic energy photoelectron spectroscopy, in combination with advanced ab initio calculations. We find that the excitation spectrum of S-1 displays extensive vibrational progressions that we identify to arise from large changes in the torsional angles of the phenyl rings upon electronic excitation. The extensive activity of the antisymmetric inter-ring torsional vibration provides conclusive evidence for a loss of symmetry upon excitation, leading to an inequivalence of the two phenyl rings. Nonresonant zero kinetic energy photoelectron spectroscopy from the ground state of the neutral molecule to the ground state of the radical cation, on the other hand, demonstrates that upon ionization symmetry is retained, and that the geometry changes are considerably smaller. Apart from elucidating how removal of an electron affects the structure of the molecule, these measurements provide an accurate value for the adiabatic ionization energy (65274 +/- 1 cm(-1) (8.093 eV)). Zero kinetic energy Photoelectron spectra obtained after excitation of vibronic levels in S-1 confirm these conclusions and provide us with an extensive atlas of ionic vibronic energy levels. For higher excitation energies the excitation spectrum of Si becomes quite congested and shows unexpected large intensities. Ab initio calculations strongly suggest that this is caused by a conical intersection between S-1 and S-2.