Multiconfigurational Static and Dynamic Calculations Explain Tricyclo[4,2,0,02,5]octa-3,7-diene Photochemistry

Sunlight is a renewable energy source that can be stored in chemical bonds using photochemical reactions. The synthesis of exotic and strained molecules is especially attractive with photochemical techniques because of the associated efficient and mild reaction conditions. We have used complete active space self-consistent field (CASSCF) calculations with an (8,7) active space and the ANO-S-VDZP basis set to understand the photophysics and subsequent photochemistry of a possible cubane precursor tricyclo[4,2,0,0^2,5]octa-3,7-diene (1). The energies were corrected with a second-order perturbative correction CASPT2(8,7)/ANO-S-VDZP. The S₀→S₁ vertical excitation energy of 1 is 6.25 eV, whose nature is π→π^* excitation. The minimum energy path from the S₁ Franck-Condon point leads directly to a 4π-disrotatory electrocyclic ring-opening reaction to afford bicyclo[4,2,0]octa-2,4,7-triene. The 2D potential energy surface scan located a rhomboidal S₁/S₀ minimum energy crossing point that connects 1 and cubane, suggesting that a cycloaddition is theoretically possible. We used the fewest switches surface hopping to study this reaction: 85% of 1,722 trajectories relaxed to 8 products; the major products are bicyclo[4,2,0]octa-2,4,7-triene (30%) and cycloocta-1,3,5,7-tetraene (32%). Only 0.4% of trajectories undergo a [2+2] cycloaddition to form cubane.