Electron affinities and lowest triplet and singlet state properties of para-oligophenylenes (n = 3–5): theory and experiment

Abstract

We apply photodetachment–photoelectron spectroscopy to measure the electron affinities and the energetics of the lowest excited electronic states of the neutral molecules para-terphenyl (p3P), para-quaterphenyl (p4P) and para-quinquephenyl (p5P), including especially the triplet states below S₁. The interpretation of the experimental data is based on the comparison to calculated 0–0 energies and Dyson norms, using density functional theory and multireference configuration interaction methods, as well as Franck–Condon patterns. The comparison between calculated and experimental vibrational fine-structures reveals a twisted benzoid-like molecular structure of the S₀ ground state and nearly planar quinoid-like nuclear arrangements in the S₁ and T₁ excited states as well as in the D₀ anion ground state. For all para-oligophenylenes (ppPs) in this series, at least two triplet states have been identified in the energy regime below the S₁ state. The large optical S₀–S₁ cross sections of the ppPs are rationalised by the nodal structure of the molecular orbitals involved in the transition. The measured electron affinities range from 380 meV (p3P) over 620 meV (p4P) to 805 meV (p5P). A saturation of the electron binding energy with the increasing number of phenyl units is thus not yet in sight.