Visible Photon Energy Storage by [2+2] Cycloaddition of Pd-Oxazolones

Abstract

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 in the UV 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 and adopt a clamshell geometry that that preorganizes the reactive alkenes in close spatial proximity. This unique coordination environment facilitates fast, near-quantitative, and reversible [2+2] photo-cycloaddition 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.