Tuning the dihydroazulene – vinylheptafulvene couple for storage of solar energy

Dihydroazulene (DHA) is a molecular photoswitch that undergoes a ring-opening reaction upon irradiation 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 discussed. The bibliography includes 60 references.