Molecular solar thermal energy storage in nitrone-oxaziridine pairs with widely tunable thermal half-lives

Oxaziridines have long been considered too unstable to isolate, as they are typically generated in situ and immediately consumed during synthesis. Here, we show that this instability can be controlled and harnessed, establishing nitrone-to-oxaziridine valence isomerization as a new platform for molecular solar thermal (MOST) energy storage. A series of para-substituted nitrones photoisomerize to isolable oxaziridines under UV-visible light (340 to 430 nm), with quantum yields reaching 49%. Substituent electronics tune the thermal half-lives of the oxaziridines by nearly four orders of magnitude, from 17 hours to 10 years, while the energy storage density stays nearly constant at 0.3 MJ kg^-1. This decoupling of storage density from thermal stability is uncommon among MOST systems, where extending the half-life usually comes at the cost of stored energy. The nitrones also isomerize efficiently under solvent-free conditions and under simulated sunlight and withstand repeated charge and discharge cycling with minimal loss, positioning nitrone-oxaziridine pairs as a structurally distinct and highly tunable addition to the MOST toolbox.