Angew. Chem. Int. Ed. 51, 10319–10323 (2012)

Credit: © 2012 WILEY

Crystalline materials are notoriously brittle, and this is particularly true for organic crystals. Although metal–organic frameworks, which are held together by coordination bonds, can display structural flexibility — such as a 'breathing' behaviour that allows for their lattices' expansion and compression — organic crystals rely on weaker non-covalent interactions and are easily broken. Only a handful of flexible organic crystals have been described and in those cases, the irreversible bending arises from the concerted movements of molecular sheets.

Soumyajit Ghosh and C. Malla Reddy from the Indian Institute of Science Education and Research, Kolkata, have now prepared a co-crystal that is surprisingly, and reversibly, flexible, even at temperatures as low as −100 °C. Slow evaporation of an equimolar solution of caffeine (CAF) and 4-chloro-3-nitrobenzoic acid (CNB) in methanol produced needle-like crystals. Using tweezers and a metal pin, Ghosh and Reddy were able to bend a 5-mm-long and 0.1-mm-thick crystal into a loop — although thicker ones were less flexible. On removal of the pressure, the needles promptly returned to their original straight shape.

X-ray diffraction studies revealed that the co-crystals comprise CAF and CNB molecules in a 1:1 ratio, arranged into comb-like one-dimensional tapes. These tapes further stack together into two-dimensional sheets with a double-sided comb-like structure, which in turn pack into a zipper-like assembly. The lattice also features channels that are partially occupied by disordered methanol molecules. It is thought that the even distribution of weak interactions in all three dimensions, together with the mobile solvent channels, serve as release pathways for mechanical stress.

The material undergoes a reversible elastic bending process (in which the central part of the curved rod is stretched on one side and compressed on the other) in contrast to an irreversible plastic deformation. When dried, the crystals lost their elasticity; they became brittle and could not be resolvated. These findings show that small organic molecules can assemble into architectures with mechanical properties that have traditionally been observed with more complex materials.