Comparative Ab-Initio Study of Substituted Norbornadiene-Quadricyclane Compounds for Solar Thermal Storage

Molecular photoswitches that are
capable of storing solar energy,
so-called molecular solar thermal storage systems, are interesting
candidates for future renewable energy applications. In this context,
substituted norbornadiene-quadricyclane systems have received renewed
interest due to recent advances in their synthesis. The optical, thermodynamic,
and kinetic properties of these systems can vary dramatically depending
on the chosen substituents. The molecular design of optimal compounds
therefore requires a detailed understanding of the effect of individual
substituents as well as their interplay. Here, we model absorption
spectra, potential energy storage, and thermal barriers for back-conversion
of several substituted systems using both single-reference (density
functional theory using PBE, B3LYP, CAM-B3LYP, M06, M06-2x, and M06-L
functionals as well as MP2 calculations) and multireference methods
(complete active space techniques). Already the diaryl substituted
compound displays a strong red-shift compared to the unsubstituted
system, which is shown to result from the extension of the conjugated π-system
upon substitution. Using specific donor/acceptor groups gives rise
to a further albeit relatively smaller red-shift. The calculated storage
energy is found to be rather insensitive to the specific substituents,
although solvent effects are likely to be important and require further
study. The barrier for thermal back-conversion exhibits strong multireference
character and as a result is noticeably correlated with the red-shift.
Two possible reaction paths for the thermal back-conversion of diaryl
substituted quadricyclane are identified and it is shown that among
the compounds considered the path via the acceptor side is systematically
favored. Finally, the present study establishes the basis for high-throughput
screening of norbornadiene-quadricyclane compounds as it provides
guidelines for the level of accuracy that can be expected for key
properties from several different techniques.