Multiconfigurational Calculations and Photodynamics Describe Norbornadiene Photochemistry.

Storing solar energy is a vital component of using renewable energy sources to meet the growing demands of the global energy economy. Molecular solar thermal (MOST) energy storage is a promising means to store solar energy with on-demand energy release. The light-induced isomerization reaction of norbornadiene () to quadricyclane () is of great interest because of the generally high energy storage density (0.97 MJ kg) and long thermal reversion lifetime ( = 8346 years). However, the mechanistic details of the ultrafast excited-state [2 + 2]-cycloaddition are largely unknown due to the limitations of experimental techniques in resolving accurate excited-state molecular structures. We now present a full computational study on the excited-state deactivation mechanism of and its dimethyl dicyano derivative () in the gas phase. Our multiconfigurational calculations and nonadiabatic molecular dynamics simulations have enumerated the possible pathways with 557 S trajectories of for 500 fs and 492 S trajectories of for 800 fs. The simulations predicted the S and S lifetimes of (62 and 221 fs, respectively) and the S lifetime of (190 fs). The predicted quantum yields of and are 10 and 43%, respectively. Our simulations also show the mechanisms of forming other possible reaction products and their quantum yields.