Norbornadiene/Quadricyclane System in the Spotlight: The Role of Rydberg States and Dynamic Electronic Correlation in a Solar‐Thermal Building Block

Abstract In this contribution, we studied the photophysics and photochemistry of an archetypal molecular switch in terms of solar energy storage and release. In detail, we characterized the valence and Rydberg states of norbornadiene (NBD) and quadricyclane (QC) by means of CASPT2//CASSCF theory level, finding a good agreement of the calculated vertical excitation energies with the experimental counterparts. The NBD↔QC thermal and photochemical valence isomerization [2 π +2 π ] reactions have been addressed and investigated. Low energy crossing points between excited states were identified for both the NBD↔QC photochemical reactions, through the calculation of minimum energy paths which revealed that the photochemistry is ruled by the deformation of two coupled reaction coordinates. Such coordinates were also used to build potential energy surfaces through relaxed energy scans with the goal of building a simplified, still accurate, model system able to catch the NBD↔QC photorelaxation. Also, the S 1 /S 0 respective conical intersection were characterized and related to the reaction quantum yield. Interestingly, we found that reverse photoisomerization from QC to the NBD progenitor could in principle occur by direct excitation of σ ‐3 s and σ ‐3 p Rydberg excited states in the UV spectral range. In both NBD and QC photoreactivity, the doubly excited valence state has shown to play a crucial role for reaching crossing points leading to nonadiabatic population transfers.