Fig. 1. The four S\begin{document}$ _0 $\end{document}-state configurations of MVCI (where A and S stand for \begin{document}$ anti $\end{document} and \begin{document}$ syn $\end{document}, respectively, the first A/S indicate the \begin{document}$ anti $\end{document}/\begin{document}$ syn $\end{document} configuration with respect to the orientations of the vinyl group and terminal O atom beside C3-O1 bond; the second one corresponds to that with respect to C3-C4 bond). Geometries optimized at the MS3-CASPT2/def2-TZVP level, along with the important bond lengths (in Å) are shown on the left, and definitions of key (dihedral) angels are shown on the right panel.

Fig. 1. The active orbitals selected in CAS(14_e, 10_o) for SCI and CAS(12_e, 9_o) for MVCI in CASSCF and MS-CASPT2 calculations.

Fig. 2. Important geometries with key parameters (bond lengths are given in Angstrom) and relative energies (in kcal/mol) calculated at the PBE0/def2-SVP (in black), SA3-CASSCF/def2-SVP (in red), MS3-CASPT2/def2-SVP (in blue) and MS3-CASPT2/def2-TZVP (in green) levels.

Fig. 2. The CASSCF/def2-SVP computed S\begin{document}$ _1 $\end{document}-state energy profiles along the C3-O2 rotary coordinate (i.e., dihedral angle \begin{document}$ \phi $\end{document}) (a) between SA and AA and (b) between SS and AS, FCs are shown in red triangles, fully optimized S\begin{document}$ _1 $\end{document}-SA and S\begin{document}$ _1 $\end{document}-SS are shown in red circles; (c) S\begin{document}$ _1 $\end{document}-SA, S\begin{document}$ _1 $\end{document}-SS and the CIs optimized on the same level, with important geometrical parameters. The CIs are named as follows: subscript S and A stand for the configuration around the unchanged C3-C4 bond, and +/- indicates the dihedral angle \begin{document}$ \phi $\end{document} larger or smaller than 90\begin{document}$ ^\circ $\end{document}, respectively.

Fig. 3. Distributions of the hopping points near the S\begin{document}$ _0 $\end{document}/S\begin{document}$ _1 $\end{document}-CIs for the trajectories starting from the S\begin{document}$ _1 $\end{document} state of (a) AA, (b) AS, (c) SA and (d) SS configurations, following the dihedral angle \begin{document}$ \phi $\end{document} and pyramidalization angle \begin{document}$ \tau $\end{document}. The CASSCF/def2-SVP optimized CIs are shown in blue and red "+" symbols, respectively.

Fig. 3. The CASSCF/def2-SVP computed S₁-state energy profiles along the C3-C4 rotary coordinate. (a) The isomerization between S₁-AS and S₁-AA, (b) The isomerization between S₁-SS and S₁-SA. In all calculations, the dihedral angle ϕ was fixed.

Fig. 4. (a) (b) The CASSCF/def2-SVP level 2-D PESs for S₁-state photoisomerization (relative energies are shown in kcal/mol). Evolutions of the corresponding (dihedral) angles are shown in (c) and (d), and bond lengths (or distances) are shown in (e) and (f), along the C3-O rotation (dihedral angle ϕ) coordinate.

Fig. 4. The major photoisomerization channels of S\begin{document}$ _1 $\end{document}-state MVCI started from (a) AA (in black) and AS (in purple), (b) SA and SS configurations. Yields for each process relative to the total successful trajectories are shown in light-red and purple background. The relative energies (in kcal/mol) of minima are calculated at the MS3-CASPT2/def2-TZVP level, whereas those of the S\begin{document}$ _0 $\end{document}-TSs and CIs are calculated at the MS-CASPT2/def2-TZVP//PBE0/def2-SVP and MS-CASPT2/def2-TZVP//CASSCF/def2-SVP level, respectively.

Fig. 5. The state population of the S₁-state trajectories and excited-state lifetime fitted by a kinetic model.

Fig. 5. Geometrical evolution of trajectories in four configurations of (a) AA, (b) AS, (c) SA and (d) SS, initiated from the S\begin{document}$ _2 $\end{document} state. Trajectories in red and black are that of O-O dissociation and unreactive one during the simulation time, respectively.

Fig. 6. Geometry distribution of the first hopping points. The black and green balls show the S₁ → S₀ and S₁ → S₂ hops, respectively. For ease of observing, their projections on R-α and α-ϕ surfaces are also shown in grey (S₁ → S₀) and light-green (S₁ → S₂). These hops can be classified into three types: Those of which projection distributed in red boxes centred at R(O1-O2) = 1.5 Å, α = 95˚ and ϕ = 90~100˚ correspond to hops near the perpendicular S₁/S₀-CI; Those with projection distributed in the blue boxes kept the initial structures and are hops in FC region; and that in purple boxes are characterized with obviously longer O-O distances (R(O1-O2)>1.7 Å).

Fig. 7. Evolution of ϕ and θ dihedral, α angle, O1-O2 and C3-O2 distances of trajectories for four configurations initiated from the S₁ state for configurations (a) AA, (b) AS, (c) SA and (d) SS. The trajectories leading to O-O dissociation (R(O1-O2) > 3.0 Å) are shown in red, and those for ring-closure (with bond angle α < 60˚ and C-O bond R(C3-O2) < 1.5 Å) are in green, anti/syn(CO) isomerization (ϕ = 0˚ or 180˚) are in blue, and the rest trajectories (unreactive) are in black. Product yields for three isomerization are also given.

Fig. 8. Evolution of dihedral angle ϕ for trajectories in four configurations initiated from the S₂ state. Trajectories shown in red correspond to those lead to dissociation, and the ones in black are for unreactive.

Fig. 9. Quantum populations for an ensemble of all the successful trajectories start from S₂ state.

Fig. 10. Geometry and time distribution of all hopping points. The red, blue and black balls show the S₂ → S₁, S₁→S₀ and S₂ → S₁ hops, respectively. For ease of observing, their projections on R(O1-O2)-time and time-ϕ surfaces are also shown in light-colored points.

Table 1. Relative Boltzmann populations of the four configurations of MVCI. The temperature is 273.15 K.

Table 2. Excitation energies (in kcal/mol) calculated at the SA3-CASSCF/def2-SVP, MS3-CASPT2/def2-SVP and MS3-CASPT2/def2-TZVP levels, and excitation energy windows set for each state in the dynamic simulations.

Table 3. Relative energies (in kcal/mol) of important geometries calculated at the PBE0/def2-SVP, SA3-CASSCF/ def2-SVP, MS3-CASPT2/def2-SVP as well as MS3-CASPT2/def2-TZVP levels. The energies of S₀-TSs and CIs are obtained basing on the PBE0/def2-SVP and CASSCF/def2-SVP optimized geometries, respectively.