A Norbornadiene-Based Molecular System for the Storage of Solar–Thermal Energy in an Aqueous Solution: Study of the Heat-Release Process Triggered by a Co(II)-Complex

It is urgent yet challenging to develop new environmentally friendly and cost-effective sources of energy. Molecular solar thermal (MOST) systems for energy capture and storage are a promising option. With this in mind, we have prepared a new water-soluble (pH > 6) norbornadiene derivative (HNBD1) whose MOST properties are reported here. HNBD1 shows a better matching to the solar spectrum compared to unmodified norbornadiene, with an onset absorbance of lambda(onset) = 364 nm. The corresponding quadricyclane photoisomer (HQC1) is quantitatively generated through the light irradiation of HNBD1. In an alkaline aqueous solution, the MOST system consists of the NBD1(-)/QC1(-) pair of deprotonated species. QC1(-) is very stable toward thermal back-conversion to NBD1(-); it is absolutely stable at 298 K for three months and shows a marked resistance to temperature increase (half-life t(1/2) = 587 h at 371 K). Yet, it rapidly (t(1/2) = 11 min) releases the stored energy in the presence of the Co(II) porphyrin catalyst Co-TPPC (Delta H-storage = 65(2) kJ center dot mol(-1)). Under the explored conditions, Co-TPPC maintains its catalytic activity for at least 200 turnovers. These results are very promising for the creation of MOST systems that work in water, a very interesting solvent for environmental sustainability, and offer a strong incentive to continue research towards this goal.