Autonomous self-healing organic crystals for nonlinear optics

Multifunctional adaptive dynamic crystalline materials with ability to self-repair, self-actuate, or with mechanical adaptability, hold great potential for uses in smart technologies such as in flexible devices, soft-robotics, energy, optical, electrical, bio-medical, actuators, and high precision sensors. Despite this, autonomous self-healing – the ability of a material to recover and restore its pristine properties without the need of any external intervention – is poorly understood in synthetic materials, and even more so in crystalline and hard materials, which is highly desirable to meet future needs of high-end technologies. It is not always viable to repair components, such as in a space equipment or micro-devices deployed in remote operations. Hence discovery of new ways of incorporating self-healing property in synthetic materials has become critical for overcoming current challenges. Nature offers many self-healing strategies (e.g., skin, bone) that inspire synthetic approaches.

Over two decades, physical and chemical methodologies for self-healing have leveraged diffusion, shape-memory effects, dynamic covalent bond reshuffling, supramolecular dynamics, or their combinations, but commonly require external stimuli (heat, pressure, light, chemicals) and long contact periods, limiting practical use. Most prior self-healing studies target restoration of mechanical properties in gels, polymer films, and composites. For single crystalline materials with applications in piezoelectricity, ferroelectricity, and SHG, preserving non-centrosymmetric order is crucial. Realizing self-healing in molecular crystals with crystallographic precision remains a significant challenge with very few examples.

Here the authors target an organic, non-centrosymmetric single crystal that can autonomously self-heal rapidly, maintain crystallographic order, and preserve nonlinear optical performance (SHG), while quantifying mechanical limits and actuation dynamics underlying the healing.

The paper situates the work within self-healing materials research spanning polymers, gels, composites, and dynamic crystals. Prior strategies include diffusion-based healing, shape-memory effects, dynamic covalent exchange, and supramolecular interactions, typically requiring external stimuli and long healing times. In crystalline materials, especially non-centrosymmetric systems essential for piezoelectric, ferroelectric, and SHG applications, maintaining long-range order is critical but rarely demonstrated with self-healing at macroscopic scales. Previous dynamic crystals show various motions (bending, twisting, thermosalient effects) but generally require external triggers. Autonomous, rapid, crystallographically precise self-healing in organic single crystals is identified as a gap.

• Synthesis and crystallization: Dimethyl-4,4'-(methylenebis(azanediyl))dibenzoate synthesized by condensing methyl 4-aminobenzoate with formaldehyde in acetonitrile with catalytic formic acid. Product isolated (~60% yield), characterized by 1H NMR and HRMS, and crystallized by slow evaporation from methanol (3–5 days) to afford needle-shaped single crystals.
• Structural characterization: Single-crystal X-ray diffraction (Rigaku, CuKα, 293 K) solved with SHELXT/SHELXL; powder XRD for phase purity; SEM imaging after gold sputtering; calculated morphology and packing analysis using Mercury.
• Thermal analysis: TGA (N2, 2 °C/min, 25–275 °C) and DSC (N2, 5 °C/min, 25–275 °C) to assess stability (crystals stable up to ~200 °C per text and Supplementary data).
• Mechanical testing and healing experiments: Three-point bending and uniaxial compression under stereomicroscopy with high-speed imaging (1250–1600 fps) to induce fractures and observe autonomous recombination. Quantification of load using a force sensor with ~190 µm spherical tip; crystals (thickness ~70–100 µm) fixed on a substrate to enable repeated loading cycles. Loads for gentle vs excessive pushes recorded directly and plotted vs crystal thickness.
• Healing time measurement: Time between load withdrawal and disappearance of crack-line determined from high-speed video frames across multiple crystals.
• Nanoindentation: Performed on (001) and (100)/(010) faces using Hysitron TI Premier with Berkovich tip (~150 nm radius) at max load 1 mN. Hardness and modulus extracted via Oliver–Pharr method; SPM imaging used to assess indent impressions and pile-up.
• Kelvin probe force microscopy (KPFM): Non-contact KPFM (Cypher ES, Ti/Ir-coated tip) to measure nanoscale surface potential on fractured surfaces (~20 min post-fracture) to support charge generation underpinning actuation/healing.
• SHG setup and measurements: Femtosecond Ti:sapphire laser (810 nm, 80 MHz, 100 fs, 100 mW) focused to ~8 µm spot; collection through dichroics into spectrograph/CCD. Verified SHG at 405 nm, power dependence (log–log slope ~2), FWHM ratio vs pump spectrum; polarization-dependent SHG by rotating half-waveplate; spatial SHG line-mapping along c-axis (1 µm steps) on pristine, imperfectly healed, and healed crystals.
• Computational analyses: Energy frameworks via CrystalExplorer17 (B3LYP-D2/6-31G(d,p)) to quantify intermolecular interaction energies; DFT/TDDFT (Gaussian 16; B3LYP and CAM-B3LYP/6-31G(d)) for ground/excited and transition dipole moments and their orientations relative to crystal axes.
• Actuation performance analysis: From high-speed videos, extracted linear/angular displacements and times; computed response times, average linear/angular accelerations, forces (F = ma), moments of inertia (rectangular prism approximation), torques, and work capacities for angular and linear actuation modes.
• SCXRD of healed crystals: Compared reciprocal space reconstructions and diffraction profiles of pristine vs healed vs imperfectly healed to assess domain alignment and single-crystallinity retention.

• Autonomous self-healing: Needle-shaped single crystals of 1 autonomously recombine after gentle mechanical fracture via attractive forces between oppositely charged fracture surfaces, achieving macroscopic, often crack-free healing within milliseconds. Fragments can attract and rejoin even when tens of micrometers apart.
• Load threshold for healing: Quantified uniaxial compression shows perfect self-healing occurs for loads in the mN range; for crystals ~70–100 µm thick, loads ≲70 mN generally yield repeatable, perfect healing (up to 10 cycles), whereas ≳70 mN often cause imperfect healing or crushing and permanent damage.
• Healing timescale: Ultrafast healing of 10–30 ms (among the fastest autonomous repairs reported), with actuation response times 0.6–8 ms captured by high-speed imaging.
• Actuation performance: Two modes observed: • Angular actuation (pivoted crack): Average angular acceleration 13×10^3–71×10^3 rad s^-2; torque 0.15×10^-12–96×10^-12 kg m^2 s^-2; work capacity 0.2–15 N m^3. • Linear actuation (separated shards): Average linear acceleration 1–62 m s^-2; force 3×10^-3–3×10^-2 mN; work capacity 0.04–2 N m^2. These place performance metrics comparable to certain MEMS and surpass some electroactive polymers.
• Crystal structure and interactions: Crystallizes in non-centrosymmetric polar space group I41/acd. V-shaped molecules (arm angle 105.48°) form columns along c-axis with aligned –CH2– groups, giving a net dipole along c. Packing involves strong N–H···O (2.03 Å, 163.82°) and supportive C–H···O (2.49 Å, 140.15°) hydrogen bonds, and weak C–H···π interactions (2.92 Å, 166.20°). Inter/intra-column interaction energies are comparable (−37 and −43 kJ/mol), consistent with similar mechanical response across faces.
• Nanomechanics: Elastic modulus E ≈ 7.50 ± 0.15 GPa and hardness H ≈ 0.60 ± 0.02 GPa on (100)/(010); E ≈ 7.1 ± 0.3 GPa and H ≈ 0.50 ± 0.04 GPa on (001). Pile-up indicates incompressible character.
• SHG confirmation and retention after healing: SHG at 405 nm upon 810 nm excitation; power-law slope 2.01 ± 0.01; FWHM(SHG)/FWHM(pump) = 0.36 ± 0.01 (theory 0.354). Polarization-dependent SHG maximal when E is ⟂ to needle (c-axis). Fitted ratios of second-order coefficients reported as γ33/γ31 = 3 ± 0.02 and γ32/γ31 = 7 ± 0.11. Line-mapping shows uniform SHG intensity across pristine and neatly healed regions, with a sharp drop only at visible crack-lines in imperfectly healed samples, indicating restoration of nonlinear optical performance in healed crystals.
• Crystallographic precision: Reconstructed reciprocal space images and diffraction profiles of perfectly healed crystals are indistinguishable from pristine, indicating effective single-crystallinity restoration; imperfectly healed samples show split spots due to domain misalignment.
• Stability: Crystals stable at ambient conditions for >1 year and thermally stable up to ~200 °C (Supplementary data).

The study demonstrates that an organic, non-centrosymmetric single crystal can autonomously and rapidly self-heal macroscopic fractures while preserving crystallographic order and nonlinear optical functionality. The key mechanism involves complementary surface charges generated upon brittle fracture, producing strong attractive forces that drive ultrafast actuation (angular or linear) to rejoin fragments. Quantitative load thresholds delineate operational regimes enabling repeatable healing cycles, while high-speed imaging and kinematic modeling establish response times and performance metrics that are competitive with established actuator classes (MEMS, electroactive polymers). Structural analysis links the polar packing (dipoles along c-axis) and robust hydrogen-bonded network to the mechanical integrity and piezoelectric-like charging upon fracture, enabling actuation-driven healing. SHG studies validate that neatly healed regions retain optical performance comparable to pristine crystals, underscoring practical viability for nonlinear optical applications. Together, these results address the challenge of achieving self-healing with crystallographic precision in hard, single-crystalline organic materials and highlight a generalizable framework for actuator-enabled autonomous repair in functional crystals.

This work introduces dimethyl-4,4'-(methylenebis(azanediyl))dibenzoate single crystals that autonomously self-heal within milliseconds via charge-driven actuation, restore single-crystallinity, and retain high-efficiency SHG. The authors quantify mechanical load limits for repeatable healing, establish ultrafast response times and strong actuation metrics, and correlate structure, mechanics, and nonlinear optics. These self-healing crystals offer durability advantages for piezoelectric/ferroelectric/SHG devices and suggest pathways toward robust, low-maintenance components in optics, sensors, and soft robotics. Future directions include: elucidating the structural origins and universality of charge generation upon fracture in non-centrosymmetric crystals; optimizing crystal morphology and interfaces to increase perfect-healing propensity and alignment; integrating such crystals into device architectures; and expanding materials discovery for tailored actuation and optical properties.

• Perfect healing requires gentle loads (<~70 mN for 70–100 µm thick crystals) and precise self-alignment; excessive loads lead to imperfect healing or permanent damage.
• Healing propensity decreases with misalignment or uneven fracture surfaces; autonomous recombination over tens of micrometers is possible but with lower success for perfect registry.
• Quantitative pressure at contact was not reported due to uncertainties in contact area with the force sensor; raw load values were used.
• Nanoindentation loads were limited to ≤1 mN due to tip contamination by debris, potentially constraining full mechanical characterization.
• The exact structural origins of fracture-induced surface charge generation in non-centrosymmetric crystals are not fully understood.
• Demonstrations are at micro- to millimeter scales; translation to larger-scale components and diverse environments needs further validation.