Simulating the Energy Capture Process in Push–Pull Norbornadiene-Quadricyclane Photoswitches

Molecular switches based on the norbornadiene-quadricyclane
(NBD-QC)
isomer pair are among the most promising candidates for applications
in molecular solar thermal energy storage (MOST). In these compounds,
solar energy is captured through a photoinduced [2 + 2] cycloaddition
reaction whose mechanism is only partially understood. This holds
true especially for NBD derivatives containing the type of push–pull
substitution pattern that was previously proven necessary to attain
reasonable photoisomerization quantum yields. In the present contribution,
we report a computational investigation of the photochemistry of NBD-QC
switches with precisely such a substitution pattern. Static calculations
provide information on the structures of the excited electronic states
involved in the photoinduced cycloaddition reaction, and the topographies
of the relevant ground- and excited-state potential energy surfaces.
Furthermore, nonadiabatic molecular dynamics (NAMD) simulations allow
an estimation of the reaction time scale and quantum yield. The simulation
results paint a detailed picture of the energy capture process: the
photoinduced cycloaddition reaction begins in the spectroscopically
bright excited state of the molecular switch. In the model compound
for which we performed NAMD simulations, ring closing takes place
on a time scale of roughly 150 fs, which makes it one of the fastest
known photoisomerization reactions.