A computational investigation on the photo-isomerization of 2,4,6-octatriene and its UV-visible spectrum

The dynamical processes following photoexcitation of all-trans 2,4,6-octatriene to the two lowest singlet excited states are investigated theoretically, from two different points of view. The S1-S0 photoisomerization is characterized with a focus on structural aspects (stationary points and reactive modes), while for the UV-visible spectrum, arising from excitation to the S2 state, nonadiabatic quantum dynamics calculations with four planar degrees of freedom are performed. The underlying electronic structure data are obtained from CASSCF and MS-CASPT2 ab initio computations, the quantum dynamical calculations rely on the Multiconfiguration Time-Dependent Hartree (MCTDH) method. The observed envelope of the UV-visible spectrum is well reproduced and the substantial broadness of the spectral features related to a nonradiative S2-S1 transition proceeding within few tens of femtoseconds. The vertical excitation energy from the Ag to the Bu state shows a red-shift (∼0.2 eV with the cc-pVTZ basis set) with reference to the all-trans hexatriene system. This affects the S2-S1 nonradiative decay, while the methylation of the terminal carbon atoms seems to have a minor influence on the photo-isomerization path.