Long-Term Energy Storage Systems Based on the Dihydroazulene/Vinylheptafulvene Photo-/Thermoswitch

The dihydroazulene/vinylheptafulvene (DHA/VHF) couple presents a photo-/thermo-switch that has attracted interest for the development of molecular solar thermal energy storage systems. Here we present the synthesis and optical as well as switching properties of DHA derivatives incorporating alkyne and norbornadiene (NBD) substituents at position C2. The corresponding VHF isomers exhibited remarkably long lifetimes. Thus, the VHF-to-DHA back-reaction half-life for a derivative with a phenylethynyl substituent was 22 days in acetonitrile at room temperature, while the half-life of a VHF reference compound having a phenyl substituent is around 3.5 h under the same conditions. This finding is particularly attractive in the quest for long-term energy storage systems, considering limited enhancement of molecular weight and maintenance of good quantum yield photoisomerization. Moreover, from the thermal back-reaction of an NBD compound, used as a precursor for the DHA-NBD dyad, we estimated the radical-stabilizing influence of a C(Me)=C(CN)2 substituent.