Crystal Engineering DOI: 10.1002/ange.201204604 Elastic and Bendable Caffeine Cocrystals: Implications for the Design of Flexible Organic Materials** Soumyajit Ghosh and C. Malla Reddy* Dedicated to Professor Gautam R. Desiraju on the occasion of his 60th birthday Molecular crystals are among the most ancient and highly investigated materials in chemistry. However, mechanical properties of these materials have remained relatively unex- plored despite their unique applications in optoelectronics, mechanical actuators, artificial muscles, pharmaceuticals, and explosives. [1] Conserving the orientational order of molecules and bonds is important for efficient charge transport and for the lifetime of organic light-emitting diodes, transistors, and solar cells. [2] Hence, the realization of high-performance materials with excellent self-healing capabilities or efficient stress dissipating behaviors is attractive. For this reason, the remarkable properties displayed by natural fibres such as spider silk, [3] muscle protein titin, [1e, 4] cytoskeleton micro- tubules, [5] etc. have recently sparked tremendous interest in establishing a reliable structure–property correlation to guide the design of their mimics for various applications. A good starting point for achieving such a goal is to study much simpler and easy-to-characterize organic crystals, which self- assemble through the same noncovalent interactions. It remains a challenge to simultaneously achieve both flexibility and crystallinity in organic materials because crystallinity positively correlates with brittleness. For exam- ple, compared to highly ordered molecular crystals, liquid crystals show greater flexibility, but are less crystalline. Desiraju and co-workers showed irreversible mechanical bending in organic crystals as mediated by the movement of molecular sheets through weak interactions between them. [6, 7] The plastic deformation disrupts the long-range order per- manently. It was also shown that reversible molecular move- ments in organic crystals (e.g., in photomechanical bending), can perform work in devices. [8] Herein we report a remarkably flexible, elastically bendable cocrystal solvate 1, formed from caffeine (CAF), 4-chloro-3-nitrobenzoic acid (CNB), and methanol in a 1:1: < 1 ratio (Figure 1). The cocrystal solvate 1 retains a high internal order through an efficient stress dissipation mechanism, and hence is important in the context of crystal engineering [9] and for the design of flexible organic materials. The single crystals of 1 could be obtained from a 1:1 molar solution of CAF and CNB in methanol by using a slow evaporation method (Figure 1). [10] 1 H NMR and thermogra- vimetric (TG) analyses have confirmed the presence of CAF, CNB, and methanol molecules in a 1:1: < 1 ratio within the lattice (see Figures S1 and S2 in the Supporting Information). The typically long needle crystals of 1 grow along the a axis (Figure 1 and Figure S4). When a straight crystal, having about a 0.1 mm thickness and 5 mm length, was pushed with a metal pin while being held with a pair of forceps (tweezers) from the opposite end, it transformed into a bent shape without breaking (Figure 2a–d and Figure S5). Further, it could be made into a loop by joining the two ends with a smooth curve (see Videos S1–S3 in the Supporting Infor- mation). Upon withdrawal of the force, the crystal quickly Figure 1. Single-crystal preparation of the cocrystal solvate 1 from a methanol solution of caffeine and 4-chloro-3-nitrobenzoic acid. Figure 2. Elastic bending in the cocrystal solvate 1. a–f) First bending cycle resulting from applied mechanical stress, using a pair of forceps and a metal pin. g) The subsequent recovery upon withdrawal of the force. h) Second bending cycle. i) The breaking of the crystal when the elastic limit is exceeded. Notice the remarkable shape recovery in the two broken halves in (i). [*] S. Ghosh, Prof. Dr. C. M. Reddy Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata (IISER-K) 741252, Nadia, West Bengal (India) E-mail: cmallareddy@gmail.com [**] S.G. thanks the CSIR (New Delhi) for SRF. C.M.R. is grateful for financial support from the DST (SR/FT/CS-074/2009). We thank Dr. C. C. Sun (Univ. of Minnesota) for his comments, Dr. L. Straver (Bruker) for suggestions on the X-ray structure, and S. B. Sinha (IISER-K) for assistance with videos. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201204604. A ngewandte Chemi e 10465 Angew. Chem. 2012, 124, 10465 –10469  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim