Between ethylene and polyenes - The non-adiabatic dynamics of cis-dienes

Using Ab Initio Multiple Spawning (AIMS) with a Multi-State Multi-Reference Perturbation theory (MS-MR-CASPT2) treatment of the electronic structure, we have simulated the non-adiabatic excited state dynamics of cyclopentadiene (CPD) and 1,2,3,4-tetramethyl-cyclopentadiene (Me₄-CPD) following excitation to S₁. It is observed that torsion around the carbon-carbon double bonds is essential in reaching a conical intersection seam connecting S₁ and S₀. We identify two timescales; the induction time from excitation to the onset of population transfer back to S₀ (CPD: ∼25 fs, Me₄-CPD: ∼71 fs) and the half-life of the subsequent population transfer (CPD: ∼28 fs, Me ₄-CPD: ∼48 fs). The longer timescales for Me₄-CPD are a kinematic consequence of the inertia of the substituents impeding the essential out-of-plane motion that leads to the conical intersection seam. A bifurcation is observed on S₁ leading to population transfer being attributable, in a 5:2 ratio for CPD and 7:2 ratio for Me₄-CPD, to two closely related conical intersections. Calculated time-resolved photoelectron spectra are in excellent agreement with experimental spectra validating the simulation results.