Molecular Solar Thermal Energy Storage Systems with Long Discharge Times Based on the Dihydroazulene/Vinylheptafulvene Couple

Molecular solar thermal energy storage (MOST) systems based on photochromic molecules that undergo photoisomerization to high‐energy isomers are attractive for storage of solar energy in a closed‐energy cycle. One challenge is to control the discharge time of the high‐energy isomer. Here we show that incorporation of a strong acceptor substituent in the seven‐membered ring of the dihydroazulene/vinylheptafulvene (DHA/VHF) couple increases the half‐life of the energy‐releasing VHF‐to‐DHA back‐reaction from hours to more than a day in a polar solvent. For some derivatives, the absorption maximum of the photo‐active DHA is also significantly redshifted, thereby better matching the solar spectrum. Synthetic protocols and kinetics studies are presented together with a computational study of the energy densities of the systems and excitation spectra. The computations show that the increased lifetime of the high‐energy isomer is counter‐balanced by a lower energy storage capacity in vacuo than for the parent system, but a slightly higher energy density than for the parent system in a polar solvent.