Non-centrosymmetric molecular crystals possess unique structural order absent in other synthetic materials, leading to applications in piezoelectric transducers, energy storage, and nonlinear optical materials. However, their brittleness limits their utility due to fatigue and catastrophic failure. Nature provides examples of self-healing materials (bone, enamel, etc.) that maintain efficiency over a lifetime. Incorporating self-healing into crystalline materials would greatly expand their applications. While methods exist for inducing self-healing in materials (diffusion, shape memory effect, covalent bond reshuffling, etc.), they often require long contact times and external stimuli (heat, pressure, light, chemical agents). Most research focuses on restoring mechanical properties in soft materials, neglecting the importance of preserving internal order in crystalline materials for applications requiring non-centrosymmetric structures, such as piezoelectric, ferroelectric, and SHG materials. This work aims to address this gap by introducing a self-healing organic crystal that maintains its nonlinear optical properties after self-repair.

The literature extensively documents self-healing materials, primarily focusing on restoring mechanical properties in gels, polymer films, and composites. Examples of self-healing in single crystalline materials are scarce. Strategies for maintaining internal order during healing are crucial for preserving the performance of crystals in piezoelectric, ferroelectric, and SHG applications. Existing self-healing methods often require external stimuli, limiting their practical usability. The challenge lies in achieving self-healing in organic materials with crystallographic precision for broader applications.

Single crystals of dimethyl-4,4'-(methylenebis(azanediyl))dibenzoate (1) were synthesized via slow evaporation from a methanol solution. Three-point bending tests and uniaxial stress experiments (using a force sensor) were performed to quantify the mechanical forces required for self-healing. High-speed cameras (1200–1600 fps) recorded the fracture and healing processes. Crystal structure was determined via single-crystal X-ray diffraction (SCXRD). Nanoindentation characterized the nanomechanical response. Kelvin probe force microscopy (KPFM) measured surface potential. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) determined thermal stability. Second harmonic generation (SHG) measurements evaluated self-healing efficiency, including polarization-dependent studies. Density functional theory (DFT) calculations were used to understand the molecular dipole moments and their alignment in the crystal.

Single crystals of compound 1 demonstrated ultrafast autonomous self-healing (10–30 ms) after mechanical fracture, even with cracks up to tens of micrometers. The crystals fractured in a brittle manner under excessive force, leading to imperfect healing if the shards misaligned. A gentle force induced perfect healing. Statistical analysis revealed a clear limit to the load for complete self-healing (typically <70 mN). Repeatability of healing cycles was demonstrated (up to 10 cycles). The self-healing mechanism involves the generation of opposite charges on the broken surfaces, attracting the fragments and enabling recombination. SCXRD confirmed the retention of long-range order in perfectly healed crystals. Nanoindentation revealed comparable mechanical properties on different crystal faces. The material showed exceptional fast-healing and ultrafast actuation motion. Analysis of this motion provided performance metrics comparable to MEMS and electroactive polymers. SHG studies demonstrated that the healed regions retained SHG efficiency comparable to the unaffected regions, even in cases of perfect healing, confirming the self-healing capability on a crystallographic level. Polarization-dependent SHG studies confirmed the anisotropy and the presence of strong charge transfer interactions, consistent with DFT calculations.

The findings demonstrate the successful integration of self-healing capabilities into a non-centrosymmetric organic crystal while maintaining its SHG activity. The ultrafast autonomous self-healing and actuation mechanisms are remarkable. The material's stability under moderate-to-harsh environmental conditions and the high SHG efficiency after self-healing highlight its potential for applications where maintaining structural order and functionality are critical. The quantitative analysis of mechanical loads and performance metrics provides valuable data for future material design and optimization.

This study introduces a novel dibenzoate single crystal exhibiting exceptional autonomous self-healing and ultrafast actuation. The ability to self-heal efficiently while maintaining high SHG activity presents significant potential for applications in various fields, including piezoelectric actuators, SHG frequency modulation devices, and soft robotics. Future research should explore the versatility of this self-healing mechanism in other organic crystals and investigate potential applications in advanced technologies.

While the crystals exhibit excellent self-healing capabilities under gentle forces, excessive forces lead to imperfect healing due to shard misalignment. The study mainly focuses on macroscopic-scale self-healing; further investigation at the nanoscale may provide additional insights. The synthesis yield is moderately low. The exploration of self-healing efficiency in harsher conditions is needed.