Molecular solar thermal energy storage in Dewar pyrimidone beyond 1.6 megajoules per kilogram.

Storing sunlight in a compact and rechargeable form remains a central challenge for solar energy utilization. Molecular solar thermal (MOST) energy storage systems, which harness photon energy and release it as heat on demand, provide a direct approach but have long failed to meet practical benchmarks. Inspired by the architecture of DNA, we report a pyrimidone-based MOST system that stores energy in the strained Dewar photoisomer upon excitation at 300 nanometers. Designed with sustainability in mind, the system operates solvent free and remains compatible with aqueous environments while overcoming one of the field's greatest hurdles—the controlled extraction and transfer of stored heat. When catalyzed by acid, the Dewar isomer releases enough heat to boil water (~0.5 milliliters). These advances help point the way toward decentralized solar heat storage and off-grid energy solutions. Editor's summary: Most fuels produce heat through combustion reactions that are hard to reverse. Photoswitches offer the opportunity to store light energy from the sun and then release heat in a sustainable cycle; however, they tend to release comparatively little heat. Nguyen et al. now report a pyrimidone compound that isomerizes under ultraviolet irradiation to form a highly strained bond between a nitrogen and the diametrically opposite carbon. Upon treatment with acid, that bond breaks to release more than a megajoule per kilogram of the compound, enough to rapidly boil water from a solution. —Jake S. Yeston INTRODUCTION: Heating accounts for nearly half of global energy consumption. However, nearly two-thirds of heating still relies on fossil fuels, such as natural gas, oil, and coal. As a result, heating is a major direct source of carbon emissions. Achieving a sustainable energy future, therefore, requires not only carbon-free electricity generation but also effective strategies for storing and delivering clean heat. Molecular photoswitches have recently emerged as media for renewable solar energy storage and release. This concept is known as molecular solar thermal (MOST) energy storage. In MOST systems, photon energy is stored in the strained chemical bonds of a metastable photoisomer. Upon activation by an external stimulus, the metastable photoisomer reverts to its thermodynamically stable form, releasing the stored energy as heat (ΔHstorage). In this work, we report a MOST system based on 2-pyrimidone and its Dewar isomer, engineered for solvent-free operation and water compatibility. The system delivers a record-high gravimetric energy density under both neat and dilute aqueous conditions, enabling heat release and transfer sufficient to raise water to its boiling point under ambient conditions. RATIONALE: Inspired by the aza-bicyclic framework of DNA lesions, we designed Dewar isomers that fuse highly strained 1,2-dihydroazete and diazetidine units to enhance the ring strain. We also incorporated a nitrogen atom at the reaction site, enabling labile C–N bond formation and cleavage. We hypothesized that combining high ring strain with a high-energy C–N bond would increase the energy of the metastable Dewar isomer, maximizing ΔHstorage and enabling controlled thermal release. RESULTS: Pyrimidone derivatives were designed and synthesized with different alkylation patterns. This approach promoted efficient valence isomerization while maintaining low molecular weight. Upon excitation at 300 nm, pyrimidones underwent valence isomerization to form the corresponding Dewar isomers. Differential scanning calorimetry showed that the Dewar isomers released exceptionally high ΔHstorage on thermal activation. The Dewar isomer reached a record-high gravimetric energy density of 1.65 MJ kg−1 (227.6 kJ mol−1), exceeding values reported for leading MOST photoswitches. To demonstrate practical heat release and transfer, Dewar pyrimidone was dissolved in water, and an acid catalyst was used to promote the back isomerization. Addition of hydrochloric acid (HCl) to Dewar pyrimidone (107 mg in 0.46 ml of water) increased the solution temperature to 100°C and induced boiling within 1 s, demonstrating rapid macroscopic heat transfer to an environmentally benign medium under ambient conditions. CONCLUSION: We developed a pyrimidone-based MOST system guided by two central design principles: (i) leveraging a highly strained, dearomatized aza-bicyclic scaffold to maximize stored energy and (ii) incorporating nitrogen at the reactive site to enable controllable thermal release. Together, these features yield a compact molecular architecture with an energy density of 1.65 MJ kg−1. As a proof of concept, we demonstrated that heat output sufficient to boil 0.46 ml (460 mg) of water could be generated using 107 mg of Dewar pyrimidone. This result highlights the potential of MOST technology as a practical route toward scalable, on-demand heat delivery for water heating, cooking, and surface defrosting. MOST energy storage in pyrimidone–Dewar pyrimidone.: Energy diagram showcasing the photon energy storage and heat release through reversible Dewar isomerization (left). ΔHstorage, energy storage density; ΔH‡, enthalpy of activation. Acid-triggered reversion of the Dewar isomer enables rapid heat release, and the protonated pyrimidone is neutralized for recharging (right). [ABSTRACT FROM AUTHOR]