A Water Soluble and Solid‐State Molecular Solar Thermal (MOST) System as Renewable Solar Fuel

Solar energy storage is key to overcome the intermittent character of sunlight. We present a sustainable solution based on norbornadiene-quadricyclane pairs for molecular solar thermal (MOST) energy storage working in highly concentrated neutral water solutions and solid state. Photochemical preparation of high-energy, metastable isomers in previously unattainable 1.64 M solutions was achieved. The best compound can be stored for 1.9 years and is stable in solid state, allowing for the preparation of up to 3 M water solutions. Catalyzed back-conversion ensures efficient heat release and starting material recovery with remarkable fatigue resistance in water solution and solid state, exhibiting macroscopic heat release in both states, providing a ΔT of 39.6 K and fully recyclable use. These results highlight a promising avenue for the practical application of MOST technology.