| Summary: | 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 λ<sub>onset</sub> = 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<sup>−</sup>/QC1<sup>−</sup> pair of deprotonated species. QC1<sup>−</sup> is very stable toward thermal back-conversion to NBD1<sup>−</sup>; it is absolutely stable at 298 K for three months and shows a marked resistance to temperature increase (half-life <i>t</i><sub>½</sub> = 587 h at 371 K). Yet, it rapidly (<i>t</i><sub>½</sub> = 11 min) releases the stored energy in the presence of the Co(II) porphyrin catalyst Co-TPPC (Δ<i>H</i><sub>storage</sub> = 65(2) kJ∙mol<sup>−1</sup>). 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.
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