Jet-cooled vibronic spectroscopy and asymmetric torsional potentials of phenylcyclopentene

The ultraviolet spectroscopy of the S₁← S₀ transition of 1-phenylcyclopentene (PCP) was studied by resonant-two-photon ionization (R2PI), laser-induced fluorescence (LIF) and single vibronic level fluorescence (SVLF). UV–UV hole-burning (UVHB) spectroscopy was used to determine that there is only one spectroscopically distinct conformer in the supersonic expansion. The excitation spectrum shows extensive vibronic structure extending to over 1000 cm⁻¹ above the electronic origin (34646 cm⁻¹). Much of the vibronic structure is similar to that of styrene and other singly substituted benzene derivatives, with Franck–Condon (FC) activity predominantly in substituent-sensitive benzene modes. Sizeable FC progressions were also found in the inter-ring torsion, reflecting a large displacement in the inter-ring angle upon electronic excitation. No evidence for FC activity in the ring-puckering coordinate is observed. The torsional potentials of the ground and excited states were determined from the experimental transition frequencies by fitting the calculated to the experimental torsional frequency spacings in an automated least-squares fitting procedure. The S₁ torsional potential is a symmetric single-well potential centered around a locally planar equilibrium geometry at a torsional angle of ϕ = 0°. The energy levels are reproduced by a cosine term potential function with torsional parameters V₂ = 3765 cm⁻¹ and V₄ = −183 cm⁻¹. The S₀ torsional potential possesses a twisted equilibrium geometry that is strongly asymmetric about ϕ = 0° due to the non-planarity of the cyclopentene ring. The best-fit potential parameters uses a sin/cos potential function (odd/even), with V^e₂ = 948 cm⁻¹, V^e₄ = −195 cm⁻¹, V^o₂ = −162 cm⁻¹ and V^o₄ = −268 cm⁻¹. The shape of the potentials are similar to those predicted by relaxed potential energy scans calculated at the DFT, CIS and TDDFT//CIS levels of theory. The change in the torsional angle ϕ upon electronic excitation was determined to be 15° from fits of the displacement δ of the S₀ torsional potential with respect to the S₁ potential. The simulated shift of the S₀ potential with respect to the S₁ potential of 15° is in very good agreement with that obtained from B3LYP calculations.