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.