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 ( NBD ) to quadricyclane ( QC ) is of great interest because of the generally high energy storage density (0.97 MJ kg -1 ) and long thermal reversion lifetime ( t 1/2,300K = 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 NBD and its dimethyl dicyano derivative ( DMDCNBD ) in the gas phase. Our multiconfigurational calculations and nonadiabatic molecular dynamics simulations have enumerated the possible pathways with 557 S 2 trajectories of NBD for 500 fs and 492 S 1 trajectories of DMDCNBD for 800 fs. The simulations predicted the S 2 and S 1 lifetimes of NBD (62 and 221 fs, respectively) and the S 1 lifetime of DMDCNBD (190 fs). The predicted quantum yields of QC and DCQC are 10 and 43%, respectively. Our simulations also show the mechanisms of forming other possible reaction products and their quantum yields.