Jet-cooled vibronic spectroscopy of potential intermediates along the pathway to PAH: phenylcyclopenta-1,3-diene

The vibronic excitation spectrum of phenylcyclopenta-1,3-diene (PCP3D) has been recorded in a supersonic expansion using resonant-two-photon ionization (R2PI) and laser-induced fluorescence (LIF) techniques. The spectrum is dominated by the S₀–S₁ origin transition (31739 cm⁻¹), with several low-frequency vibronic bands in the first 400 cm⁻¹, followed by a sharp cut-off in intensity due to turn-on of a non-radiative process. Single vibronic level fluorescence (SVLF) spectra were recorded for the S₁ origin and several vibronic bands of PCP3D. The excitation and emission spectra show that the molecule is planar with C_s symmetry in both the ground and excited states. Torsional potentials were simulated from the observed torsional structure in the excitation and emission spectra. The S₀ potential (V₂ = 1237 cm⁻¹, V₄ = −256 cm⁻¹) is associated with a flat-bottomed potential supporting large inter-ring angular changes with little cost in energy (±36° at 200 cm⁻¹), with a barrier of 1237 cm⁻¹ at the perpendicular geometry. The S₁ potential is much stiffer about the planar geometry, with a calculated barrier five times larger than in S₀ (V₂ = 6732 cm⁻¹, V₄ = −477 cm⁻¹). Based on the torsional assignments, weak bands in the same frequency region assigned earlier to the structural isomer phenylcyclopenta-1,4-diene [J. J. Newby, J. A. Stearns, C. P. Liu and T. S. Zwier, J. Phys. Chem. A, 2007, 111, 10914–10927] have been re-assigned as hot bands arising from v″ = 1 in the inter-ring torsion, ν₅₇.