Multichromophoric photoswitches for solar energy storage: from azobenzene to norbornadiene, and MOST things in between

The ever-increasing global demands for energy supply and storage have led to numerous research efforts into finding and developing renewable energy technologies. Molecular solar thermal energy storage (MOST) systems utilise molecular photoswitches that can be isomerized to a metastable high-energy state upon solar irradiation. These high-energy isomers can then be thermally or catalytically converted back to their original state, releasing the stored energy as heat on-demand, offering a means of emission-free energy storage from a closed system, often from only organic materials. In this context, multichromophoric systems which incorporate two or more photochromic units may offer additional functionality over monosubstituted analogues, due to their potential to access multiple states as well as having more attractive physical properties. The extended conjugation offered by these systems can lead to a red shift in the absorption profile and hence a better overlap with the solar spectrum. Additionally, the multichromophoric design may lead to increased energy storage densities due to some of the molecular weight being 'shared' across several energy storage units. This review provides an overview and analysis of multichromophoric photoswitches incorporating the norbornadiene/quadricyclane (NBD/QC) couple, azobenzene (AZB), dihydroazulene (DHA) and diarylethene (DAE) systems, in the context of energy storage applications. Mixed systems, where two or more different chromophores are linked together in one molecule, are also discussed, as well as limitations such as the loss of photochromism due to inner filter effects or self-quenching, and how these challenges may be overcome in future designs of multichromophoric systems. In the context of energy storage, multichromophoric systems may offer additional functionality over monosubstituted analogues due to their potential to access multiple states as well as having more attractive physical properties.