Strain Engineered Bridged Bicyclic Diene Photoswitches in the Race of Next-Generation Molecular Solar Thermal Energy Storage

Norbornadiene/Quadricyclane (NBD/QC) is a prototypical bridged bicyclic diene (BBD)-based photoswitch that has been well-studied for molecular solar thermal energy storage (MOST). Inspired by the recent synthetically accessed BBDs, herein several photoswitches are rationally designed with modulated ring strain energies (RSE) in photoisomers to incorporate high energy storage density (ESD) and storage time in a single couple. The storage energy ($$ ) calculated at DLPNO-CCSD(T)/Def2TZVP level is correlated with difference in RSE of two isomers (ΔRSE) whereas thermal back reaction (TBR) barrier calculated at (8,8)-CASPT2/6-311++G** shows correlation with RSE in metastable photoproduct. On the basis of these structure-property-RSE relationships, we recognized that two photoisomers need not to be highly strained. Instead, the RSE in the photoproduct and diene should be minimized while maintaining a large enthalpy difference between them to increase ESD and extend energy storage times in a single photoswitch. TBR barrier is governed by RSE in photoproduct and increasing strain in photoproduct may improve the $$ but at the cost of the TBR barrier. Herein, the structural skeletons are explored that holds promise to remarkably improve thermochemical properties relative to the unsubstituted BBD-based photoswitches reported so far. The BBD molecules with short saturated bridge length but elongated unsaturated bridges could bestow desirable thermochemical parameters and can be regarded as excellent candidates for MOST application. The work lays a theoretical foundation that guides to improve thermochemical properties via strain engineering of BBD-based photoswitches and opens a new avenue for designing principles and future experimental investigations of MOST systems.