A Sustainable Molecular Solar Thermal (MOST) System as Renewable Solar Fuel in Water and Solid-State

Solar energy storage is key to overcoming the intermittent character of sunlight. Herein, we present new norbornadiene-quadricyclane pairs for molecular solar thermal energy storage working in highly concentrated neutral water solutions and solid state. This provides a sustainable approach to the storage of solar energy through the photochemical preparation of high-energy, metastable isomers in previously unattainable 1.64 M solutions. New compounds can be stored for 1.9 years and are 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. Affordable and scalable synthesis enables gram-scale use and macroscopic heat release in two different regimes: prompt temperature boost or extended constant heating. The best candidate can also be utilized in solid state, providing a ΔT of 39.6ºC and fully recyclable use. These results highlight a promising avenue for the practical application of MOST technology.