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 Δ T of 39.6 K and fully recyclable use. These results highlight a promising avenue for the practical application of MOST technology.