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 (31 739 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 Cs 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.