Tuning the dihydroazulene - vinylheptafulvene couple for storage of solar energy

Dihydroazulene (DHA) is a molecular photoswitch that upon irradiation undergoes a ring-opening reaction to form a vinylheptafulvene (VHF) photoisomer. This VHF isomer will in time thermally return to the DHA isomer. As the isomerization is photo-induced only in one direction, the DHA/VHF couple has attracted interest as a molecular solar thermal energy storage device (MOST system). In this author review, we cover our systematic efforts to optimize the DHA/VHF couple for this purpose, with challenges being to achieve sufficiently high energy densities, to cover broad absorptions including the visible region, and to control the energy-releasing VHF-to-DHA back-reaction. By a combination of computations and experiments, we review the consequences of various structural modifications of the system (structure-property relationships), including the influence of donor-acceptor substitution at specific positions, benzannulations, and incorporation into macrocyclic structures. Synthetic protocols to reach the various modifications will also be covered.