Living tissue uses stress-accumulated electrical charge to close wounds. Self-repairing synthetic materials, which are typically soft and amorphous, usually require external stimuli, prolonged physical contact, and long healing times. We overcome many of these limitations in piezoelectric bipyrazole organic crystals, which recombine following mechanical fracture without any external direction, autonomously self-healing in milliseconds with crystallographic precision. Kelvin probe force microscopy, birefringence experiments, and atomic-resolution structural studies reveal that these noncentrosymmetric crystals, with a combination of hydrogen bonds and dispersive interactions, develop large stress-induced opposite electrical charges on fracture surfaces, prompting an electrostatically driven precise recombination of the pieces via diffusionless self-healing.

Ductility, which is a common phenomenon in most metals and metal-based alloys, is hard to achieve in molecular crystals. Organic crystals have been recently shown to deform plastically, but only on one or two faces, and fracture when stressed in any other arbitrary direction. Here, we report an exceptional metal-like ductility in crystals of two globular molecules, BH 3 NMe 3 and BF 3 NMe 3 , with characteristic stretching, necking and thinning with deformations as large as ~ 500%. Surprisingly, the mechanically deformed samples not only retained good long range order, but also allowed structure determination by single crystal X-ray diffraction. Molecules in these high symmetry crystals interact predominantly via electrostatic forces (B-N +) and form columnar structures, thus forming multiple slip planes with weak dispersive forces among columns. While the former interactions hold molecules together, the latter facilitate exceptional malleability. On the other hand, the limited number of facile slip planes and strong dihydrogen bonding in BH 3 NHMe 2 negates ductility. We show the possibility to simultaneously achieve both exceptional ductility and crystallinity in solids of certain globular molecules, which may enable designing highly modular, easy-to-cast crystalline functional organics, for applications in barocalorimetry, ferroelectrics and soft-robotics. File list (2) download file view on ChemRxiv Manuscript_CMReddy.pdf (1.54 MiB) download file view on ChemRxiv Supporting Information_CMReddy.pdf (2.52 MiB) Metal-like ductility in organic plastic crystals: Role of molecular shape and dihydrogen bonding interactions in aminoboranes

Single crystals of optoelectronic materials that respond to external stimuli, such as mechanical, light, or heat, are immensely attractive for next generation smart materials. Here we report single crystals of a green fluorescent protein (GFP) chromophore analogue with irreversible mechanical bending and associated unusual enhancement of the fluorescence, which is attributed to the strained molecular packing in the perturbed region. Soft crystalline materials with such fluorescence intensity modulations occurring in response to mechanical stimuli under ambient pressure conditions will have potential implications for the design of technologically relevant tunable fluorescent materials.

Molecular crystals are not known to be as stiff as metals, composites and ceramics. Here we report an exceptional mechanical stiffness and high hardness in a known elastically bendable organic cocrystal [caffeine (CAF), 4-chloro-3-nitrobenzoic acid (CNB) and methanol (1:1:1)] which is comparable to certain low-density metals. Spatially resolved atomic level studies reveal that the mechanically interlocked weak hydrogen bond networks which are separated by dispersive interactions give rise to these mechanical properties. Upon bending, the crystals significantly conserve the overall energy by efficient redistribution of stress while perturbations in hydrogen bonds are compensated by strengthened π -stacking. Furthermore we report a remarkable stiffening and hardening in the elastically bent crystal. Hence, mechanically interlocked architectures provide an unexplored route to reach new mechanical limits and adaptability in organic crystals. This proof of concept inspires the design of light-weight, stiff crystalline organics with potential to rival certain inorganics, which currently seem inconceivable.