Applications of Photoswitches in the Storage of Solar Energy

Abstract Photoswitches are organic or organometallic chromophores that undergo a reversible chemical transformation upon absorption of light. Among the most commonly studied photoswitches are stilbenes and azobenzenes, capable of efficient interconversion between cis and trans isomers. When one isomer is significantly less thermodynamically stable than the other, photoisomerization of the stable to the metastable isomer converts a fraction of the absorbed photon energy into excess free energy (chemical potential). If the metastable isomer is sufficiently inert at room temperature, its photoconversion provides a means of storing solar energy, which is recovered by triggering heat‐releasing thermal conversion of the metastable to the stable isomer. In other words, such a photoswitch acts as a battery that captures solar energy, stores it as chemical potential and releases it on demand as heat. This process is known as molecular solar thermal energy storage or a molecular solar thermal battery. Unlike the more established conventional solar thermal storage, which uses sunlight to heat, melt or vaporize material, molecular solar thermal energy storage does not require thermal insulation to prevent discharge but relies on the kinetic activation barrier separating the two isomers. Unlike solar‐to‐chemical energy conversion by photosplitting of H 2 O or photoreduction of CO 2 , which comprise open‐system cycles, photoswitches are thermodynamically closed storage media. Successful deployment of molecular solar thermal energy storage requires new photoswitches that combine a seemingly contradictory set of molecular parameters: a large difference in the free energies of the two isomers separated by a large kinetic barrier; a high quantum yield of photogeneration of the metastable isomer that itself is either photochemically inactive or transparent to sunlight; highly selective isomerizations that allow many charge/discharge cycles without accumulation of side‐products even at high discharge temperatures. While the optimal photoswitch for molecular solar thermal energy storage remains to be invented, a large body of empirical observations acquired in the past decade provides several potentially valuable starting points for such a search.