Crystal structures of two charge–transfer complexes of benzo[1,2-c:3,4-c′:5,6-c′′]trithiophene (D 3h-BTT)

Benzo[1,2-c:3,4-c′:5,6-c"]trithiophene (D 3h-BTT) is an easily prepared electron donor that readily forms charge–transfer complexes with organic acceptors. We report here two crystal structures of its charge–transfer complexes with 7,7,8,8-tetracyanoquinodimethane (TCNQ) and buckminsterfullerene (C60). The D 3h-BTT·TCNQ complex crystallizes with mixed layers of donors and acceptors, with an estimated degree of charge transfer at 0.09 e. In the D 3h-BTT·C60·toluene complex, the central ring of BTT is ‘squeezed’ by the C60 molecules from both faces. However, the degree of charge transfer is low.


Chemical context
Conjugated sulfur-containing aromatic molecules remain the most popular choices for the preparation of organic electronic materials. They are typically electron-rich. Their planar shapes and the large 3p orbitals on sulfur atoms allow extensive intermolecular orbital overlap in the solid state. Both of these features make them very good candidates as donors in binary charge-transfer (CT) complexes (Holiday et al., 2014). Although CT complexes have long been known, their potential as electronic materials has not been noted until relatively recently (Goetz et al., 2014). Previous reports indicate that binary charge-transfer (CT) complexes show high conductivity and other promising optoelectronic properties such as ambipolar transport and photoconductivity (Goetz et al., 2014). We recently prepared and studied the structures and properties of CT complexes using C 3h -symmetric benzotrithiophene (C 3h -BTT) as the donor (Qin et al., 2017) with a variety of organic acceptors. The D 3h isomer of benzotrithiophene (D 3h -BTT) is the most highly symmetric isomer of the BTTs, and all three of its sulfur atoms point away radially from the central ring. It is also one of the most easily prepared BTT isomers (Hart et al., 1978). The outwardly directed sulfur atoms might maximize intermolecular S-S contact, a feature that has proven important in promoting high electrical conductivity in some organic conductors (Saito et al., 2011). Although Hart et al. reported that D 3h -BTT formed CT complexes with several acceptor molecules including TCNQ, no structural information for any of the CT complexes was provided. In this communication, we report the X-ray crystal structures of the CT complexes D 3h -BTTÁTCNQ and D 3h -BTTÁC 60 Átoluene, the latter of which exhibits the second closest pair of bilateral arene-C 60 contacts. ISSN 2056-9890 2. Structural Commentary D 3h -BTTÁTCNQ D 3h -BTT forms a 1:1 binary charge-transfer complex with TCNQ, in the space group P2 1 /n. The asymmetric unit (Fig. 1) consists of four independent pairs of donor and acceptor molecules arranged in two columns along the a-axis of the unit cell. Within the columns, D 3h -BTT and TCNQ are stacked pairwise. The faces of these planar molecules are roughly parallel. The closest donor-acceptor distance is at 3.396 (3) Å (C43Á Á ÁC87). The closest contact between the two columns is 3.209 (3) Å (N1Á Á ÁS8).
Two methods were used to estimate the extent of charge transfer for the TCNQ complexes. The first is based on bonddistance ratios in the acceptor molecules. The degree of charge transfer is given by = ( x -0 )/( 1 -0 ), where is the ratio of bond distances c/(b + d) for the indicated bonds in the TCNQ derivative in Fig. 2. (Kistenmacher et al., 1982;Sugano et al., 1988).
The degree of charge transfer based on the bond ratio is 0.09 e. This is very close to the degree of charge transfer of C 3h -BTTÁTCNQÁtoluene (0.10 e). A second method utilizes infrared spectroscopy. It has been shown that for TCNQ there is an excellent linear correlation of the degree of ionicity with the nitrile stretching frequency ( CN ), and that the frequencies are relatively insensitive to the crystal environment (Chappell et al., 1981). In D 3h -BTTÁTCNQ, a frequency of 2218 cm À1 was observed, a slight decrease of CN from that in neutral TCNQ. This frequency is identical to that of the C 3h -BTTÁTCNQÁtoluene complex and correlates to a charge transfer of 0.20 e. Based on both methods, the degree of charge transfer in the two TCNQ complexes is nearly identical. This is not surprising, inasmuch as the HOMO-LUMO gaps for D 3h -BTT and C 3h -BTT only differ by 0.2-0.3 eV (Guo et al., 2011).
D 3h -BTTÁC 60 ÁToluene D 3h -BTT and C 60 form a 1:1 complex with the inclusion of a toluene molecule, which was used as the solvent for CT formation. The shortest donor-acceptor contact is between one of the carbons of the central 'benzene' ring of D 3h -BTT and C 60 at 3.014 (6) Å (C50Á Á ÁC61). This distance is 0.39 Å shorter than the sum of the van der Waals radii for the two carbon atoms. On the other side of the BTT molecule, a second C 60 makes a contact of only 3.051 (6) Å to C66, the carbon adjacent to C61 (C23Á Á ÁC66) (Fig. 3) Bonds in TCNQ used to estimate the degree of CT, .

Figure 1
The asymmetric unit of D 3h -BTTÁTCNQ. Carbon atom labels have been omitted for clarity.

Figure 3
BTT-D 3h 'squeezed' between two C 60 molecules in D 3h -BTTÁC 60 Átoluene. The minor disordered fullerene moiety is omitted for clarity. clearly disordered. Initial refinement with P2 1 as the space group led to elongated ellipsoids for the carbon atoms of C 60 . Refinement was noticeably improved and the ellipsoids became more reasonable in appearance using a model in which the C 60 unit was disordered about a pseudo mirror plane.
We attempted to estimate the charge transfer by comparing the C-C and C C bond-length variations between those of the of D 3h -BTT donor itself and those in the CT complex. However, the bond-ratio results were not informative. We compared the IR spectra of the CT complex with those of the donor and acceptor. The IR spectrum of the CT complex is not a simple sum of the spectra of the donor and acceptor, suggesting that there is charge transfer, but no quantitative estimate can be drawn from the data. In addition, a preliminary measure of the magnetic susceptibility showed diamagnetism, which suggests a very low degree of charge transfer to be present.

Supramolecular features
In D 3h -BTTÁC 60 Átoluene, the C 60 molecules form straight columns along the a-axis direction. These columns are sandwiched by corrugated sheets of D 3h -BTT. Adjacent C 60 columns form a zigzag pattern along the b-axis direction. The toluene molecules reside as an array down the a axis in a pocket formed between the donor and acceptor. The toluene molecule sits in an edge-to-face relationship with the -system of the donor but it showed no particular close contact with either the donor or the acceptor (Figs. 4 and 5).

Database survey
An extensive search of the Cambridge Structural Database (Version 5.40, update of May 2019; Groom et al., 2016) for close C 60 -arene contacts found only one example in which two C 60 molecules make contacts shorter than 3.05 Å to carbon atoms on both sides of an organic -system. In that case (CSD refcode VOPNEV; Sun et al. 2014), two C 60 molecules touch a substituted ethylene that lies on a center of inversion. The two (symmetry-related) contact distances are 3.013 Å . The present case, with contacts of 3.014 (6) and 3.051 (6) Å on the two sides of the central aromatic ring of the donor, yields the next smallest sum of contact distances 6.065 (12) Å ] after the pair in VOPNEV (6.026 Å ).

Synthesis and crystallization
D 3h -BTT was prepared by a literature procedure (Hart et al., 1978). D 3h -BTTÁTCNQ: A solution of D 3h -BTT (10 mg, 4.1 mmol) in acetonitrile (3 mL) and a solution of TCNQ (8.3 mg, 4.1 mmol) in acetonitrile (3 mL) were mixed. This solution was then left to evaporate slowly in the dark at room temperature. After 30% of solvent had evaporated, dark needles formed in the test tube. We note that the color of TCNQ solution is green, but it turns dark immediately upon mixing with a solution of D 3h -BTT, which is almost colorless. D 3h -BTTÁC 60 ÁToluene: D 3h -BTT (5 mg, 2.0 mmol) was dissolved in toluene (1.5 mL). C 60 (14 mg, 4.0 mmol) was dissolved in toluene (4 mL) to give a dark-purple solution. C 60 is sparingly soluble in toluene, and this solution was warmed and filtered before mixing with the solution of D 3h -BTT. The mixture was warmed briefly, and then it was left to evaporate in the dark at room temperature. Dark, square plates of the CT complex formed upon complete solvent evaporation.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. In the final stages of the refinement of D 3h -BTTÁC 60 Átoluene, it became evident that there was disorder in the C 60 portion. The refinement was continued using two orientations of the C 60 portion, starting with idealized units and continuing with allowing them to 'relax' somewhat subject to the restraints ISOR 0.01 and SIMU 0.01 for all their carbon atoms. In addition, the geometries of the two orientations were restrained to be similar using a SAME instruction for the second component. The final refined occupancies for the two components are 0.766 (3) and 0.234 (3). Additionally, the structure was refined as twocomponent inversion twin. Hydrogen atoms in both structures were included as riding contributions with isotropic displacement parameters tied to those of the attached atoms with C-H distances of 0.93, 0.95 or 0.98 Å , and U iso (H) equal to 1.2 or 1.5 times U eq (C) of the carrier atom. Selected H atoms in D 3h -BTTÁC 60 Átoluene were freely refined (H67-H72). SAINT (Bruker, 2016). Program(s) used to solve structure: SHELXT (Sheldrick, 2015a) for (I); SHELXT (Sheldrick, 2015a) for (II). Program(s) used to refine structure: SHELXL2018/1 (Sheldrick, 2015b) for (I); SHELXL2018/1 (Sheldrick, 2015b) for (II). For both structures, molecular graphics: DIAMOND (Brandenburg & Putz, 2012); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).  (2) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.