Visible photon energy storage by [2+2] cycloaddition of Pd-oxazolones

We report a new class of ortho-palladated oxazolone complexes that capture and release solar energy through visible-light-triggered [2 + 2] photocycloaddition reactions. The free oxazolone ligands weakly absorb near 400 nm and undergo slow and incomplete intermolecular dimerization, yielding multiple stereoisomers of cycloadducts. Upon ortho-palladation, the resulting Pd(ii) complexes exhibit strong charge-transfer absorption bands in the visible range (430-570 nm) and adopt a clamshell geometry that preorganizes the reactive alkenes in close spatial proximity. This unique coordination environment facilitates fast, near-quantitative, and reversible [2 + 2] photocycloaddition in both solution and amorphous solid state under the irradiation of green light or standard sunlight. The cycloadducts formed are stable, isolable solids with long thermal half-lives (up to 504 days at 298 K), enabling long-term energy storage. The combination of visible-light responsiveness, robust reversibility, and solid-state reactivity demonstrates the potential of transition metal complexes as a promising molecular platform for molecular solar thermal energy storage applications.