Challenges of organic “cocrystals”

Abstract

ly pack in a DAD A···DADA fashion with sufficient D-A π-electron overlap, ii) segregated-stacking, wherein mole-cules pack in a DDDD···AAAA fashion with D-D and A-A π-overlap. As a novel research area, there are full of challenges for organic cocrystals. The first challenge is the basic mech-anism of co-crystallization, because not any two types of materials can recognize each other and self-assemble into a cocrystal. Hence, a question is what kind of material is suitable for co-crystallization, and how different molecules co-assemble, nucleate and crystallize. This is also related to the method applied for co-crystallization, including solu-tion [24], vapor-phase [6,27−29] and mechanochemical techniques [16,30]. In our experience, some fundamental conditions are also essential for solution co-crystallization, such as strong intermolecular interactions, planar molec-ular structure and similar solubility of donor (D) and ac-ceptor (A). In order to reveal the nucleation and growth mechanism of organic cocrystals and further control the self-assembling process of co-crystallization, we used perylene-tetracyanoquinodimethane (TCNQ) CT cocrys-tal as an example [31], and found that the morphology and phase of cocrystals mainly depend on the concentration of perylene (C perylene) rather than TCNQ (C TCNQ), and high C perylene is favourable for the formation of (perylene) 3 TCNQ (P3T1) microblocks, while low C perylene faciliates the forma-tion of (perylene) 1 TCNQ (P1T1) nanowires (Fig. 1). More importantly, we demonstrate that the formed D-A complex is regarded as a new material species, because the solubili-ty of the formed P3T1 in acetonitrile is obviously lowered than that of any single components. It actually allows us to define the cocrystal as a new molecular species. The struc-ture-function relationship of these cocrystals is further investigated. P1T1 cocrystals exhibit n-type behavior with electron mobility of 0.05 cm 2 V −1 s −1 , while P3T1 cocrystals display ambipolar behavior with high hole mobility of 0.03 cm 2 V −1 s −1 and low electron mobility of 2.1×10 −5 cm 2 V −1 s −1 . Moreover, white light responsivity of P1T1 is higher than Organic cocrystal (also " co-crystal "), formed with two or more different components via non-covalent intermolec-ular interactions, possesses novel, unpredicted and unique properties, which are not the simple sum of those molec-ular components, e.g., with effect of 1+1>2. In this regard, organic cocrystals provide a distinctive strategy for the