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 MJᐧkg–1) and long thermal reversion lifetime (t1/2, 300K = 8346 years). However, the mechanistic details of the ultrafast excited-state [2+2]-cycloaddition is 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 in the gas phase. Our multiconfigurational calculations [SA6-CASSCF(4,7)/ANO-S-VDZP] and non-adiabatic molecular dynamics simulations have enumerated the possible pathways with 600 S2 initial conditions for 300 fs. The predicted S2 and S1 lifetimes are reported (62 and 221 fs). The QC: NBD formation ratio is 1:5; the predicted quantum yield of QC is 9%, which underscores the potential of NBD for MOST materials. Our simulations also show the mechanisms of forming other possible reaction products and their quantum yields.