Donor–acceptor type co-crystals of arylthio-substituted tetrathiafulvalenes and fullerenes

Summary A series of donor–acceptor type co-crystals of fullerene (as the acceptor) and arylthio-substituted tetrathiafulvalene derivatives (Ar-S-TTF, as the donor) were prepared and their structural features were thoroughly investigated. The formation of co-crystals relies on the flexibility of Ar-S-TTF and the size matches between Ar-S-TTF and fullerene. Regarding their compositions, the studied co-crystals can be divided into two types, where types I and II have donor:acceptor ratios of 1:1 and 1:2, respectively. Multiple intermolecular interactions are observed between the donor and acceptor, which act to stabilize the structures of the resulting co-crystals. In the type I co-crystals, the fullerene molecule is surrounded by four Ar-S-TTF molecules, that is, two Ar-S-TTF molecules form a sandwich structure with one fullerene molecule and the other two Ar-S-TTF molecules interact with the fullerene molecule along their lateral axes. In the type II co-crystals, one fullerene molecule has the donor–acceptor mode similar to that in type I, whereas the other fullerene molecule is substantially surrounded by the aryl groups on Ar-S-TTF molecules and the solvent molecules.

For the formation of the host-guest type supramolecular system, the shape and size complementarity between TTFs and fullerenes are key factors that give rise to the effective surface contact for the stabilization of the resulting supramolecular structures. Because pristine TTF cannot form good enough surface contact with fullerenes due to the shape and size mismatch [42], chemical modifications of TTF have been carried out. To this regard, introduction of substituents onto the peripheral sites [43][44][45][46][47][48][49][50][51][52][53][54] and expansion of the π-systems have been reported [55][56][57][58][59]. As reported, the π-extended TTFs (exTTF) can encapsulate fullerenes in solution, and form the inclusion complex with fullerenes as well [28][29][30][31][32][33][34][35][36][37]. Very recently, we have disclosed a facile approach toward the arylthio-substituted TTFs (hereafter denoted as Ar-S-TTF) [60][61][62], which bear four aryl groups on the peripheral positions of the TTF core through the sulfur bridges. The Ar-S-TTF molecules are size and shape matched for fullerenes (C 60 /C 70 ), and the peripheral aryls show large rotational freedom that could adjust their spatial alignment to adapt to the environmental variations [61].
Regarding the structural feature of Ar-S-TTF, we have performed the complexation of Ar-S-TTFs with C 60 /C 70 , and found that Ar-S-TTF could form D-A type inclusion complexes with C 60 /C 70 [63]. Crystallographic investigation reveals that the multidimensional interaction networks consisting of a central TTF core, peripheral aryls, and fullerenes are the key factors to stabilize the resulting supramolecular structures. Meanwhile, the solid state absorption study indicates that the inclusion complexes display a photoexcited electronic transition between Ar-S-TTF and C 60 /C 70 . To gain further insight into the structural features of Ar-S-TTF upon complexation with C 60 /C 70 , we have carried out the preparation a series of [(Ar-S-TTF)-(fullerene)] co-crystals and investigated their solid state structure, as reported herein. In this report we focus on the synthesis, composition (donor:acceptor ratio), and crystal structure of the resulting co-crystals.

Synthesis and compositions
On the basis of hundreds of experimental runs, we found that the Ar-S-TTFs possessing the first redox potential (E 1/2 1 ) smaller than 0.6 V could form the co-crystal-type complexes with fullerene molecules C 60 and/or C 70 , whereas those with E 1/2 1 > 0.6 V could not afford the desired complexes. The complexes obtained thus far are intrinsically neutral [63], which means the charge transfer does not take place between Ar-S-TTF and fullerenes in the ground state. In this regard, the E 1/2 1 values of Ar-S-TTFs would not affect the formation of co-crystals. On the other hand, the interaction between the aryls and fullerene molecules is very important to stabilize the structure of the co-crystals as reported in the crystal structures section. For Ar-S-TTFs exhibiting E 1/2 1 > 0.6 V, the aryl groups are more electron deficient than the phenyl [61,62]. The interaction between the electron-deficient aryls and fullerenes is weak; consequently, these TTFs (E 1/2 1 > 0.6 V) could not form the co-crystals with fullerenes.
Among the [(Ar-S-TTF)-(fullerene)] complexes obtained to this point, the complexes of Ar-S-TTFs 1-8 (Scheme 1) with C 60 /C 70 were cropped in the single crystalline form, and the others were obtained as powdery samples with difficult to determine compositions. The [(Ar-S-TTF)-(fullerene)] co-crystals were prepared by evaporating the mixed solution of Ar-S-TTF and the corresponding fullerenes at room temperature. As a typical example, compound 1 (12.8 mg, 2 mmol) and C 60 (7.2 mg, 1 mmol) were dissolved in carbon disulfide (CS 2 , 7 mL), and the resulting mixed solution was then placed in the dark hood and left standing without disruption. After 2 weeks, the black block-like single crystalline complex was cropped, and the composition of the complex was determined to be 1·(C 60 ) 2 ·(CS 2 ) 2 on the basis of the X-ray single crystal structure analysis. The synthetic conditions, compositions, yields, and morphologies for the 11 complexes are summarized in Table 1.
Concerning the compositions (donor:acceptor ratio), most of the co-crystals could be divided into two types (I and II), except for 2·(C 70 ) 4 ·(PhCl) 2 . The ratio of donor:acceptor (abbreviated as D:A) for the type I and type II co-crystals are 1:1 and 1:2, respectively. No solvent molecule was involved in most of the type I co-crystals, except for 4·C 60 ·CS 2 , which contains a small, linear solvent, CS 2 . On the other hand, all of the type II co-crystals contain solvent molecules in their matrix. Since one Ar-S-TTF is capable of encapsulating a fullerene molecule [63], the larger ratio of fullerenes would result in the formation of additional void space by fullerene molecules, which could potentially accommodate the solvent molecules. In a previous report, we have proposed that C 70 tends to form co-crystals with a larger acceptor ratio [63]. However, the present results suggest that this prediction would not hold because both C 60 and C 70 form the type I and type II co-crystals with Ar-S-TTFs as shown in Table 1. The D:A ratio for the co-crystals results from the cooperative effects of the geometry of Ar-S-TTF (particularly the peripheral aryls), the shape and size of the fullerene molecules, and the solvent molecules. Although we cannot presently provide a clear estimation of the D:A ratio of the Scheme 1: Chemical structures of Ar-S-TTFs 1-8. co-crystals, this work further demonstrates that Ar-S-TTFs are promising candidates to serve as receptors for fullerenes and have diverse supramolecular structures.

Crystal structures
The single crystalline structure of the complex is suitable for X-ray single crystal diffraction measurements. In most cases, the fullerene molecules and solvent molecules are disordered. The disorder of fullerenes and solvents cannot be suppressed even at low temperature, and can thus be characterized as having statistic rather than rotational disorder. The selected crystallographic data are summarized in Tables S1 and S2 in Supporting Information File 1. In the following sections, the structures of the type I and type II co-crystals will be discussed in sequence.

Type I co-crystals
The type I co-crystals include four C 60 complexes and two C 70 complexes, namely, 2·C 60 , 4·C 60 ·CS 2 , 7·C 60 , 8·C 60 , 1·C 70 , and 5·C 70 . It should be noted that we recently reported the structures of 2·C 60 and 4·C 60 ·CS 2 [63]. As a typical example of the newly obtained co-crystals, the structure of 5·C 70 is discussed here, and those of 7·C 60 , 8·C 60 , and 1·C 70 are provided in the Supporting Information File 1 (Figures S4-S15).
Complex 5·C 70 crystallizes in the triclinic space group P-1 with one molecule 5 and one C 70 crystallographically unique (Figure 1a). The central TTF core on molecule 5 has a chair conformation. The molecular geometry of 5 in the co-crystal, both the spatial alignment of pyridyl groups and the conformation of the TTF core, is very close to its neutral crystalline form obtained in CS 2 [61]. The C 70 molecule is surrounded by four molecules 5 (Figure 1b
Complex 1·(C 60 ) 2 ·(CS 2 ) 2 crystallizes in the triclinic space group P-1. The asymmetric unit contains half a molecule of 1, two halves of C 60    packing structure of C 60 in the bc-plane is shown in Figure 2c. Molecules A and B form two kinds of two-dimensional (2D) sheets in the ab-plane as shown in Figure 3, and two sheets alternate along the c-axis. In both 2D sheets, the center-tocenter distances between adjacent C 60 molecules are 10.4 Å along the aand b-axes.
Complex 1·(C 60 ) 2 ·PhCl crystallizes in the triclinic space group P-1. Unlike 1·(C 60 ) 2 ·(CS 2 ) 2 , the asymmetric unit of 1·(C 60 ) 2 ·PhCl contains one molecule of 1, four halves of C 60 (A, B, C, and D), and one PhCl molecule. The central TTF core of 1 is in a planar conformation. As shown in Figure S18 in Supporting Information File 1, C 60 molecules A and B have a donor-acceptor interaction mode similar to those in the type I co-crystals, and several C-C (3.17-3.39 Å) and C-S  On the other hand, two B molecules are located above and below the mean plane of 2 (Figure 4b), and there are C-S (3.44-3.50 Å) and C-C (3.13, 3.35 Å) contacts between the C 70 molecules and the central TTF core of 2. The C 70 molecules form the 3D network of the present co-crystals. As shown in Figure S30 Supporting Information File 1, one A molecule is surrounded by five B molecules, and one B molecule is surrounded by five A molecules. There are multiple C-C close contacts (3.20-3.38 Å) between the neighboring C 70 molecules along the different directions, which result in the 3D carrier transport pathway [38][39][40][41].

Structural comparison
As has previously been reported, the van der Waals length of the C 6 S 8 core in the Ar-S-TTF molecule is about 12.8 Å [61,63], which is larger than the van der Waals diameters of C 60 (10 Å) and C 70 (11 Å) [65,66]. In this regard, a single Ar-S-TTF molecule is able to encapsulate C 60 /C 70 , and the size difference between C 60 and C 70 would not be the sole factor determining the composition of the co-crystal. The D:A ratio for the co-crystal is attributed to the cooperation of the geometry of the Ar-S-TTF (particularly the geometry and rotational freedom of the peripheral aryls), the shape and the size of the fullerene molecules, and the solvent molecules. Furthermore, the dynamic effect on the crystal growth is also taken into account.
Additionally, the D:A ratio of the co-crystal plays a significant role on the donor-acceptor interaction mode and the packing motif of the fullerenes. In the type I co-crystals (D:A = 1:1), the donor-acceptor interactions mainly exist between the central TTF core of the Ar-S-TTF and the fullerene molecules. Therefore, Ar-S-TTF serves as the host and the fullerene molecule is the guest. The fullerene molecules form the 1D columnar stacks with a center-to-center distance of around 10.3 Å, which is comparable with that of superconducting C 60 complexes with alkali metals (e.g.,10.29 Å in RbCs 2 C 60 ) [67]. When the ratio of fullerene molecules increases (i.e., the type II co-crystals, D:A = 1:2), one fullerene molecule is substantially encapsulated by the central TTF core of Ar-S-TTF, and another fullerene molecule is surrounded by the aryls on the Ar-S-TTF and solvent molecules. In this case, the Ar-S-TTF molecule still acts as the host for at least one of the fullerene molecules. Moreover, the dimensionality of the packing motifs of fullerene molecules increases, resulting in a 2D network through multiple van der Waals forces. Upon further increase of the ratio of fullerene molecules in the co-crystal (e.g., 2·(C 70 ) 4 ·(PhCl) 2 ), the packing structure becomes dominated by the C 70 molecule, which forms the 3D network. The Ar-S-TTF and solvent molecules serve as the guests to occupy the void formed by the C 70 molecules.
The present results demonstrate that Ar-S-TTF molecules have three key features that enable formation of donor-acceptor type co-crystals with fullerenes: (1) size and shape complementarity, (2) flexibility, and (3) the ability to introduce an additional interaction with fullerene by peripheral aryls. While the interactions between the TTF framework and fullerenes have been observed in many TTF-fullerene supramolecular systems [38][39][40][41], the rotational freedom of the peripheral aryls on Ar-S-TTF causes the aryls to locate at the appropriate positions to form additional interactions with fullerenes. This therefore enhances the stability of the resulting co-crystals. By complexation with Ar-S-TTF molecules, the 1D columnar array, 2D sheets, and 3D networks of fullerene molecules have been successfully established, resulting in the carrier transport pathway, in principle. However, the ground state of the present co-crystals is intrinsically neutral, as demonstrated by the IR spectra ( Figures  S34-S40 in Supporting Information File 1). To improve the functionality of the co-crystals, one interesting strategy would be the generation of itinerant carriers in the co-crystals, i.e., charge transfer between the donor and acceptor in the ground state.

Conclusion
In summary, we have prepared eleven donor-acceptor type co-crystals of Ar-S-TTFs with fullerenes (C 60 /C 70 ), and performed thorough investigations on their solid state structures. These co-crystals mainly belong to two types according to the donor:acceptor ratios (D:A), types I and II having D:A of 1:1 and 1:2, respectively. The composition of the co-crystals is thought to be the cooperative consequence of the molecular geometry of Ar-S-TTF, the shape and size of the fullerene molecule, the solvent adduct, and the crystallization dynamics. The donor-acceptor interaction mode and the packing motif of the fullerenes largely depend on the composition of the co-crystal. The present results suggest that Ar-S-TTF molecules would be promising receptors for fullerenes as they are easily accessible. Meanwhile, Ar-S-TTFs possess the unique structural features to encapsulate fullerenes. That means the size matches with that of fullerenes and the flexibility is helpful to adapt to the shape of the fullerenes, which is supported by the additional stabilization force from the peripheral aryls moieties. Moreover, by varying the peripheral aryls and solvents, the 1D, 2D, and 3D packing motifs of fullerenes can be selectively achieved.

Supporting Information
Supporting Information File 1 Additional experimental data.

Supporting Information File 2
Crystallographic data in CIF format.