Cracking and Self-Healing in a Molecular Crystal Driven by Topochemical Polymerization

Topochemical reactions in molecular crystals are often accompanied by severe lattice strain, frequently leading to crystal fracture and loss of crystallinity. Here we report an unusual case in which heat-driven topochemical azide-alkyne cycloaddition (TAAC) polymerization in a non-centrosymmetric molecular crystal induces macroscopic cracking that is followed by autonomous self-healing. Single-crystal x-ray diffraction reveals a regiospecific single-crystal-to-single-crystal transformation to a helical polymer, despite large-amplitude molecular motion and anisotropic lattice deformation (a +19.9%, b -2.9%, c -13.3%). Face-indexing and morphology analysis correlate the crystallographic axes with macroscopic crystal dimensions, rationalizing the observed breadth expansion, length contraction, and cleavage along a weakly bound crystallographic plane. Mechanistically, crack formation originates from reaction-induced bimorphic lattice strain, while self-healing is driven by electrostatic attraction between oppositely charged fracture surfaces in the polar crystal, followed by continued intradomain polymerization that eliminates the bimorphic interface. Autonomous healing occurs upon completion of the topochemical polymerization in the crystalline material.