Photochemically Induced Self-Healing in Cadmium(II) Coordination Complex

Self-healing in crystalline materials challenges the long-held perception of crystals as rigid and brittle. Here, we report a cadmium(II) coordination complex, Cd2(2-ohbz)4(4-nvp)4 (4-nvp = 4-(1-naphthyl)vinylpyridine; 2-ohbz = 2-hydroxybenzoate), that exhibits photochemical cracking, followed by light-activated repair under continuing ultraviolet (UV) irradiation. Upon 365 nm exposure, [2 + 2] cycloaddition of olefinic ligands generates anisotropic lattice strain that causes single crystals to fracture. Remarkably, ∼40 min of continued UV irradiation results in crystals regaining optical transparency and surface continuity. Atomic-force microscopy reveals time-dependent smoothing of surface fractures, and diffraction techniques support recovery of bulk crystallinity. UV-driven photochemical strain initiates fracture, while continued irradiation enables surface-level relaxation within the photoproduct phase that leads to fissure closure at the microscopic scale. To our knowledge, this system represents the first example of a crystal that self-heals following a photochemically induced pericyclic reaction.