The electronic structure of pyracene: a spectroscopic and computational study

Abstract

We report a synthetic, spectroscopic and computational study of the polycyclic aromatic molecule pyracene, which contains aliphatic five-membered rings annealed to a naphthalene chromophore. An improved route to synthesize the compound is described. Gas-phase IR and solid-state Raman spectra agree with a ground-state D_2h structure. The electronically excited S₁ A¹B_3u state has been studied by resonance-enhanced multiphoton ionisation. An adiabatic excitation energy T₀ = 30 798 cm⁻¹ (3.818 eV) was determined. SCS-ADC(2) calculations found a D_2h minimum energy structure of the S₁ state and yielded an excitation energy of +3.98 eV, including correction for zero point vibrational energy. The spectrum shows a rich low-frequency vibrational structure that can be assigned to the overtones of out-of-plane deformation modes of the five-membered rings by comparison with computations. The appearance of these modes as well as the frequency reduction in the excited state indicate that the potential in the S₁ state is very flat. At higher excess energies most bands can be assigned to fundamentals, overtones and combination bands of either totally symmetric a_g modes or of b_2g modes that appear due to vibronic coupling. Lifetimes between 43 ns and 76 ns were measured for a number of vibronic bands. For the S₂ state an equilibrium geometry with a non-planar carbon framework was computed. In addition a signal from the pyracene dimer was present. The spectrum shows several broad and structureless transitions. The origin band has a maximum at around 329 nm (30 400 cm⁻¹). Again lifetimes between 60 ns and 70 ns were found. The dimer ion signal rises within less than 10 ps. Computations show that a crossed geometry with the long axis of one unit aligned with the short axis of the second constitutes the most stable structure. The broadening of the bands is most likely caused by excimer formation.