Organic co-crystal engineering involves assembling two or more types of chemical species in stoichiometric ratios into highly ordered superstructures.  The assemblies are formed by exploiting intermolecular noncovalent interactions,  and this strategy has been used in recent years to fabricate multifunctional materials that exhibit unpredicted and versatile chemico-physical properties.  Unlike traditional chemical synthesis involving covalent bond breakage and formation, organic co-crystal engineering possesses the advantages for creating functional materials, including: ⅰ) the preparation techniques are facile and low-cost, and do not include the execution of complicated synthetic procedures and harsh experimental conditions; ⅱ) the structures, sizes, morphologies, and stoichiometric ratios associated with the co-crystals can be tuned by selecting suitable conformers or changing solvents; ⅲ) the co-crystals not only retain the inherent properties of the individual component but also exhibit multifunctional properties via the synthetic and synergistic effects exerted by the constituent units. To date, organic co-crystals have been extensively applied in organic semiconductor, ferroelectricity, fuorescence and organic room-temperature phosphorescence,  stimuli-responsive materials,  pharmaceutics,  etc.

      Organic co-crystals, built by planar small molecules, are most commonly packed following the segregated- and mixed-stacking modes. Macrocycles with intrinsic cavities and polygonal structures are principal tools of supramolecular chemistry. In theory, macrocycle-based organic co-crystals should exhibit various molecular arrangements.  These co-crystals should be characterized by tunable stoichiometric ratios and interesting properties, the origin of which can be traced back to the polygonal topological structures and the presence of interior cavities.  It is challenging to construct MCCs with identical co-components but varying donor (D)-acceptor (A) stoichiometries and study the superstructure–property relationships characterizing the molecules.

      Herein, we report three sets of macrocycle co-crystals with identical co-constitutions. The macrocycle co-crystals differ in the stoichiometric ratios (2:1, 1:1, and 2:3) of the constituents and molecular packing modes. The co-crystals are constructed using triangular pyrene-macrocycle  and 1,2,4,5-tetracyanobenzene exploiting exo-wall charge-transfer  interactions. Interestingly, the three co-crystals exhibit distinct, tunable emission properties. The corresponding emission peaks appear at 575, 602, and 635 nm, covering yellow via orange to red. The X-ray diffraction analyses and the density functional theory  calculations reveal the superstructure-property relationships that is attributed to the formation of different ratios of charge-transfer transition states between the donor and acceptor motifs, resulting in red-shifted luminescence.

Fig. 1. Chemical structures.  The components (Pe[3], Pe, and TCNB) of CT co-crystals.

In this study,  a triangular macrocycle Pe[3] bearing three pyrene units in the skeletal structure was selected as the co-crystal component as the pyrene group functions as an electron-rich donor and luminophore (Fig. 1).  1,2,4,5-tetracyanobenzene (TCNB) unit is chosen as the acceptor. We attempted to develop macrocycle-based co-crystals by carefully regulating the crystallization conditions and using pyrene and TCNB units as the building blocks, as these were suitable conformers that could be efficiently used for the development of co-crystals exploiting CT interactions.  It was found that solvent modulation played a critical role in controlling the D-A stoichiometric ratio and molecular arrangement in the crystal superstructures (Fig. 2). Co-crystallization of electron-rich Pe[3] with electron-deficient TCNB at a ratio of 1:3 in tetrahydrofuran (THF), dioxane, and CHCl₃ resulted in the formation of three sets of CT co-crystals, represented as MCC-1, MCC-2, and MCC-3. The Pe[3]:TCNB molar ratios in the crystal structures were 2:1, 1:1, and 2:3, respectively . 

      The structures of the  three sets of MCCs are completely different. The remarkable difference among the structures of the three co-crystals can be attributed to the CT participation ratio of macrocyclic skeleton (1/3 for MCC-1, 2/3 for MCC-2 and 3/3 for MCC-3) (Fig. 2). In MCC-1, one TCNB molecule binds with two Pe[3] molecules to form a sandwich-typed CT complex exploiting the face-to-face π···π interactions.   In the case of MCC-2, the two edges of Pe[3] interact with two types of crystallographically distinct TCNB molecules (TCNB-1 and TCNB-2) in a face-to-face fashion by CT interactions. In the crystal structure of MCC-3, the three edges of the Pe[3] unit participated in generating CT interactions with two types of crystallographically distinct TCNB molecules (TCNB-3 and TCNB-4).  For comparison,   we grew co-crystals of the monomer (Pe, Fig. 1) with TCNB in the above mentioned solvents. Only the  1:1 ratio of  Pe-TCNB co-crystals was obtained, and this was similar to the case of the traditional small-molecule CT co-crystals. The results indicate that the polygonal skeleton of the macrocycles dictates the formation of structurally diverse MCCs.

Fig. 2 Solid-state superstructures of MCC-1, MCC-2, MCC-3 and Pe-TCNB. (a–c) Schematic representation of the charge-transfer interactions and stoichiometric ratios between Pe[3] and TCNB in MCCs. (d) Crystal structure of Pe-TCNB. (e–h) Stacking modes of three MCCs and Pe-TCNB co-crystals in a 2D plane. Hydrogen atoms and solvents are omitted for clarity. Different colors represent symmetry equivalence. Crystallographically distinct TCNB molecules were marked as TCNB-1, TCNB-2, TCNB-3, and TCNB-4. The arrows represent π…π distances (Å) and the dashed lines represent dihedral angles.

      We then explored the luminescence properties of the MCCs. The fluorescence microscopy images revealed that the three as-prepared co-crystals exhibited tunable luminescence properties. MCC-1 exhibited yellow luminescence, MCC-2 exhibited orange luminescence, and MCC-3 exhibited red luminescence (Fig. 3a). Individual Pe[3] crystals exhibited blue luminescence, and the results indicated that co-crystallization significantly affected the optical properties of the molecules.

Fig. 3 Luminescence properties. (a) Fluorescence microscopy images recorded for Pe[3], MCC-1, MCC-2, and MCC-3. Scale bar: 100 μm. The luminescence property could be tuned, and the MCCs exhibited yellow, orange, and red luminescence, respectively. (b) Solid-state fluorescence spectral profiles recorded for Pe[3] crystals and three MCCs. The peaks significantly red-shifted relative to the peak corresponding to the Pe[3] crystals. (c) Fluorescence decay curves corresponding to Pe[3] crystals and three MCCs.

      To investigate the effect of solvents on the structure and stoichiometry of MCCs, we made many attempts and obtained 6 sets of MCCs and 2 individual macrocycle crystals  from solvents with different morphologies and colors (Fig. 4a-h). Their crystal structures show that the formed D-A ratios of MCCs are 2:1 (in ClCH₂CH₂Cl and 1,3-dioxolane), 1:1 (in CH₂Cl₂, benzene and 2,3-dihydrofuran), 2:3 (in o-xylene) and 1:0 (in DMSO and DMF) . The varied stoichiometries of MCCs are depended on the solubility of TCNB (Fig. 4i). The lower the solubility, the stronger the solvophobic forces,  and the CT participation ratio of TCNB is higher. Low solubility (<10 mM) of TCNB forms 2:3 MCC. Moderate solubility (41–85 mM) of TCNB tends to generate 1:1 MCC. High solubility (277–436 mM) of TCNB prefer to obtain 2:1 MCC. While TCNB solubility is more than 1000 mM, only individual macrocycles of Pe[3] were crystallized. The solubility (2.3 – 10 mM) of Pe[3] did not change too much in these solvents. Therefore, the effect of solvents on the diverse structures and tunable stoichiometric ratios of MCCs is decided by the solubility of TCNB and solvophobic forces.

Fig. 5 Optical microscopy images and solubilities. The photographs of MCCs (a–f) and individual Pe[3] crystals (g, h) in different solvents, showing various morphologies. Scale bar: 40 μm. (i) Solubility of Pe[3] and TCNB in 11 solvents at 20 ^oC (mean ± SD, n = 3).

      In summary, we designed and constructed three sets of MCCs with diverse D-A stoichiometric ratios. The self-assembled superstructures were constructed using an electron-rich Pe[3] macrocycle and an electron-deficient TCNB unit. Solvent modulation plays a critical role in controlling the stoichiometry and molecular arrangement of the crystal superstructures. The three MCCs exhibited tunable luminescence properties from yellow to orange, to red. The crystal structure analyses and DFT calculations revealed the structure–property relationships. The results indicated that the CT interaction between Pe[3] and TCNB is the dominant factor of the tunable photophysical properties of these co-crystals. The results reported herein offer a deep insight into the structure–CT interaction-based luminescence relationship of the molecules and present a platform for the facile synthesis of solid-state multicolor CT luminescent materials.