Crystalline molecular materials are usually brittle and are prone to break upon external mechanical force. This fragility poses challenges for their application in next-generation technologies, including sensors, synthetic tissues, and advanced opto-electronics. The recent discovery of mechanical flexibility in single crystals of molecular materials has solved this problem and enable the design of smart flexible device technologies.[1] Mechanical flexibility of organic crystals can be tuned by altering the weak interactions in the crystal structure, for example through polymorphism. Here we report 4-bromo-6-[(6-chloropyridin-2-ylimino)methyl]phenol (BCMPMP) as a promising candidate for future waveguide technologies. It turns out that BCMPMP has two different polymorphs with distinct optical and mechanical properties. Form I shows brittle behavior under mechanical stress and exhibits very weak emission at 605 nm (λ_ex = 425 nm) together with a low photoluminescence quantum yield (Φ = 0.4 %). In contrast, Form II has a large plastic (irreversible bending) regime and a bright emission at 585 nm (λ_ex = 425 nm; Φ = 8.7 %). Making use of favorable mechanical flexibility and optical properties, form II was explored as a bendable optical waveguide. Light was successfully propagated through a straight-shaped and mechanically deformed BCMPMP crystal. Depending on the light source, active or passive waveguiding could be achieved. So BCMPMP can also be used as a flexible wavelength filter.

[1] Annadhasan, M., Agrawal, A. R., Bhunia, S., Pradeep, V. V., Zade, S. S., Reddy, C. M. & R. Chandrasekar (2020), Angew Chem Int Ed, 59, 13852-13858.

Mechanical flexibility in single crystals of covalently bound materials is a fascinating and poorly understood phenomenon.[1-2] We present here the first example of a plastically flexible one-dimensional (1D) coordination polymer (CP). The compound [Zn(µ-Cl)2(3,5-Cl2Py)2]n (1), is flexible over two crystallographic faces.[3] Through the combination of microscopy, diffraction, and spectroscopic studies we probe the structural response of the crystal lattice to mechanical bending. Our results suggest that mechanical bending occurs by displacement of the coordination polymer chains. Based on experimental and theoretical evidence, we propose a new model for mechanical flexibility in 1D coordination polymers.[4] To understand the role of weak interactions on mechanical flexibility of CP crystals, we explored a family of metal halide-based CPs isomorphous with 1, based on combination of two different metals (Zn and Cd) and two halogens (Cl and Br). We demonstrate how these simple modifications can tune the mechanical flexibilities across a significant range from plastic to delaminating, and ultimately to elastic. We rationalized these remarkable changes of mechanical properties by ab initio simulations.

[1] Saha, S., Mishra, M. K., Reddy, C. M. & Desiraju G. R. (2018). Acc. Chem. Res. 51, 2957–2967. [2] Đaković, M., Borovina, M., Pisačić, M., Aakeröy, C. B., Soldin, Ž., Kukovec, B. M. & Kodrin, I. (2018). Angew. Chem. Int. Ed. 57, 14801–14805. [3] Bhattacharya, B., Michalchuk, A. A. L., Silbernagl, D., Rautenberg, M., Schmid, T., Feiler, T., Reimann, K., Ghalgaoui, A., Sturm, H., Paulus, B. & Emmerling, F. (2020). Angew. Chem. Int. Ed. 59, 5557–5561.

[4] X. Liu, AAL Michalchuk, B Bhattacharya, F Emmerling, and CR Pulham (2021) Nat. Commun. 12, 3871.

Single crystals of optoelectronic materials that respond to external stimuli, such as mechanical, light or heat are immensely attractive for next generation smart materials.^[1,2] Here we report single crystals of a green fluorescent protein (GFP) chromophore analogue with irreversible mechanical bending and associated unusual enhancement of the fluorescence owing to the suppression of aggregation-induced quenching by aromatic stacked molecules in the perturbed structure.^[3] Such fluorescence intensity modulations, which were observed in high-pressure studies earlier,^[4] are now shown to occur as function of bending under ambient pressure, hence the study has potential implications for the design of technologically relevant tunable fluorescent materials.^[5]

Ductility, which is a common phenomenon in many metals, is difficult to achieve in molecular crystals. Organic crystals have been recently shown to bend plastically on one or two face specific directions, but they fracture when stressed in any other arbitrary directions.[1] Here, we present an exceptional metal-like ductility and malleability in the isomorphous crystals of two globular molecules, BH3NMe3 and BF3NMe3, with characteristic tensile stretching, compression, twisting and thinning (increase of width over 500%).[2] Surprisingly, the mechanically deformed samples, which transition to lower symmetry phases, not only retain good long range order, but also allow 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 ductility. On the other hand, the limited number of facile slip planes and strong dihydrogen bonding in BH3NHMe2 negates ductility. The structureproperty correlation established in these aminoboranes with exceptional ductility and ability to retain crystalline order may enable designing highly modular, easy-to-cast crystalline functional organics, for applications in solid-state electrolytes, adaptable
electronics[3] (soft ferro/piezo/pyro-electrics), barocalorimetry (solid coolants),[4] soft-robotics etc.[5]