Synthesis, Structure and Physical Properties of (trans-TTF-py 2 ) 1.5 (PF 6 ) · EtOH: A Molecular Conductor with Weak CH ··· N Hydrogen Bondings

: The studies of crystal structures with hydrogen bonds have been actively pursued because of their moderate stabilization energy for constructing unique structures. In this study, we synthesized a molecular conductor based on 2,6-bis(4-pyridyl)-1,4,5,8-tetrathiafulvalene (trans-TTF-py 2 ). Two pyridyl groups were introduced into the TTF skeleton toward the structural exploration in TTF-based molecular conductors involved by hydrogen bonds. In the obtained molecular conductor, (trans-TTF-py 2 ) 1.5 (PF 6 ) · EtOH, short contacts between the pyridyl group and the hydrogen atom of the TTF skeleton were observed, indicating that hydrogen bonding interactions were introduced in the crystal structure. Spectroscopic measurements and conductivity measurement revealed semiconducting behavior derived from π -stacked trans-TTF-py 2 radical in the crystal structure. Finally, these results are discussed with the quantiﬁed hydrogen bonding stabilization energy, and the band calculation of the crystal obtained from density functional theory calculation.


Introduction
Since the first metallic molecular conductor, TTF-TCNQ was reported [1], many researchers have been eagerly exploring TTF-based charge-transfer complexes [2,3], and exotic physical properties have been reported in well-designed molecular crystals in recent years [4,5]. In the progress of molecular conductors, modulating crystal structures has been one of the biggest challenges because the physical properties of molecular conductors heavily depend on their structures. Up to now, there have been several attempts to manipulate crystal structures by introducing supramolecular interactions into the TTF skeleton, such as intermolecular hydrogen bond [6][7][8][9][10][11][12] and halogen bond [6,[13][14][15][16]. Among them, utilizing the hydrogen bond has been gathering attention because of its ability to form various unique structures owing to the moderate bond-dissociation energy (~40 kcal/mol) [17]. Although developments of TTF-based molecular crystals with hydroxy, amide, and cyano groups have been reported so far [6][7][8][9][10][11][12], TTF-based molecular conductors with pyridyl groups have been scarcely reported [18,19]. Furthermore, the pyridyl group in the TTF skeleton has often been used for metal coordination [20,21] but not for hydrogen bonding. The hydrogen bonding via pyridyl groups has many advantages in the arrangement of crystal structures because of its well-directed interaction derived from the rigid structure of the pyridyl group. In addition, the pyridyl group is easy to be introduced into π-conjugated molecular backbones by applying a hetero coupling reaction [22]. To investigate a novel crystal structure of TTF-based molecular conductors, we chose 2,6-bis(4-pyridyl)-1,4,5,8-tetrathiafulvalene (trans-TTF-py 2 ) as a starting material. This molecule has two pyridyl groups on the TTF skeleton (Scheme 1), and a unique structure based on a hydrogen bonding via the pyridyl groups can be expected. Although TTF-py 2 has been reported as a neutral molecular crystal [23] and as a ligand in coordination polymers [24][25][26][27][28], there is no report of trans-TTF-py 2 -based conductive crystal. Even in trans-bis-substituted TTF molecules, the crystal structure of the molecular conductor was reported in only one paper to the best of our knowledge [29]. In our paper, a molecular conductor, (trans-TTF-py 2 ) 1.5 (PF 6 )·EtOH (TTF-py 2 _PF 6 ), was prepared through electrochemical crystallization. Single crystal X-ray diffraction (SXRD) revealed a one-dimensional π-stacking structure of oxidized trans-TTF-py 2 , where the nitrogen atoms of the pyridyl groups have short contacts with hydrogen atoms of the adjacent TTF-py 2 molecules, suggesting the existence of significant hydrogen bondings. Spectroscopic analyses and the measurement of electrical conductivity of TTF-py 2 _PF 6 were carried out. Its band structure and the stabilization energy of hydrogen bondings calculated by density functional theory (DFT) are also discussed. Scheme 1. Synthesis of TTF-py 2 .

Methods
All solvents and reagents used in the syntheses were obtained from commercial sources without further purification. IR spectrum was recorded as KBr pellets on a FT/IR-4200 spectrometer of JASCO, Tokyo, Japan at room temperature (RT). UV-Vis-NIR absorption spectrum was measured as KBr pellets on a V-670 spectrophotometer of JASCO, Tokyo, Japan at RT. Both spectra were connected to represent the solid-state absorption spectrum in a wide range (Figure 2b). The ESR spectra were acquired by using a JES-FA100 of JEOL, Tokyo, Japan. Microscopic Raman Spectrum was measured by using a LabRAM HR-800 of HORIBA, Kyoto, Japan at RT. 1

Single X-ray Diffraction
The diffraction data for TTF-py 2 _PF 6 were collected on a XtaLAB AFC10 diffractometer with a HyPix-6000HE hybrid pixel array detector, graphite monochromated Mo Kα radiation (λ = 0.7107 Å) and a cryogenic equipment GN-2D/S of Rigaku, Tokyo, Japan. The crystal structure was solved using direct methods (SHELXT) followed by Fourier synthesis. Structure refinement was performed using full matrix least-squares procedures with SHELXL [30,31] on F [2], where F is the crystal structure factor, in the Olex2 software. [32] 2.1.2. Computational Methods DFT calculations were performed using the gaussian 16 package [33] for estimation of hydrogen bonding with the counterpoise method [34]. The B3LYP functional [35][36][37] and the cc-pVTZ basis sets [38] were used for the calculation of hydrogen bonding, because the B3LYP functional has been validated in previous studies for the calculation of hydrogen bonds in several models such as clathrate, radical, and cation-anion systems [39][40][41][42]. The atomic coordination of trans-TTF-py 2 ligands was extracted from the cif files of TTF-py 2 _PF 6 reported herein and used for the calculation without any optimizations. The molecular orbital energies were presented against the vacuum level standard.
Amsterdam Modeling Suite (AMS) packages were applied for the calculations of charge transfer integrals [43,44] and band structure [45] with tight-binding approximation. Charge transfer integrals between adjacent trans-TTF-py 2 molecules and band structure of TTF-py 2 _PF 6 were investigated by the B3LYP/TZP method, and third-order density-functional-based tight binding (DFTB3) model, respectively, without structural optimization.

Crystal Structure
The trans-TTF-py 2 -based molecular conductor, TTF-py 2 _PF 6 , was obtained through electrochemical crystallization in a CHCl 3 /EtOH (7:3, v/v) solution dissolving TTF-py 2 and TBAPF 6 [47]. The similar conditions with TBAClO 4 , TBABF 4 , or TBAX (X = Cl and Br) did not give any crystals. Single-crystal X-ray structure analysis shows two hexafluorophosphates for every three trans-TTF-py 2 molecules, indicating that the average charge of trans-TTF-py 2 is +0.66 (Figure 1). The cis isomer was not incorporated into the crystal. Two crystallographically independent trans-TTF-py 2 (A and B) molecules exist in the crystal structure (Figure 1a), repeating the A-B-B order to form a one-dimensional columnar structure by π-π stacking. The closest intermolecular S···S distances between adjacent trans-TTF-py 2 molecules along the π-stack axis are summarized in Table 1. This table indicates that the S···S distances between A-B are shorter than the sum of van der Waals radius of the sulfur atoms (3.60 Å) [48], whereas those between B-B are longer than that value. In fact, the side view of the π-stacking structure ( Figure 1c) clearly shows that the shift of π-stacking between B and B is larger than that between A and B. Focusing on the environment of π-stack columnar structures (Figure 1d,e), one π-stack column is surrounded by four other columns. Figure 1d,e highlight the interactions of A and B with neighboring molecules, respectively. It is noteworthy that there are short contacts between the nitrogen atoms of the pyridyl groups and the hydrogen atoms of TTF cores, emphasized with orange broken lines in Figure 1d and e. A is surrounded by four B molecules and has short contacts with two of them. B, on the other hand, is surrounded by two each of A and B, and it interacts with all but one A. The intermolecular N···H distances are 2.365 Å (between A and B) and 2.626 Å (between B and B), which are shorter than the sum of the van der Waals radii of hydrogen and nitrogen atoms [48], suggesting the construction of C-H···N hydrogen bondings. To obtain a deeper insight into the hydrogen bondings connecting trans-TTF-py 2 molecules, a DFT calculation was applied for estimating stabilization energies of the hydrogen bondings between A and B and B and B depicted in Figure 1d,e. The calculation indicated that the stabilization energies are -0.74 kcal/mol for the hydrogen bonding between A and B and -1.72 kcal/mol for that between B and B. Hence, the sum of the stabilization energies of the hydrogen bondings around A is -1.48 kcal/mol (= 2 × (-0.74) kcal/mol) and that around B is -4.18 kcal/mol (= 2 × (-1.72) -0.74 kcal/mol). Calculated effective transfer integrals (V eff ) between neighboring trans-TTF-py 2 molecules along the columnar structure are V eff (A-B) = 390.8 meV for A-B and V eff (B-B) = 222.9 meV for B-B (Figure 1c), reflecting the difference in the stacking feature. On the other hand, V eff of trans-TTF-py 2 molecules between adjacent columns are less than 10 meV in all combinations. This much difference of V eff obviously demonstrates the character of one-dimensional electron conductor in TTF-py 2 _PF 6 .

Electronic States and Conductivity of TTF-py 2 _PF 6
To understand the electronic state of TTF-py 2 _PF 6 , microscopic Raman spectrum, solid-state absorption spectrum, and ESR spectrum of TTF-py 2 _PF 6 were recorded and shown in Figure 2. Microscopic Raman spectrum of TTF-py 2 _PF 6 shows a strong peak around 1450 cm -1 and weak peaks around 1420 and 1520 cm -1 . It is well known that there is a correlation between the valence of the TTF molecule and Raman frequency [49], and the valence corresponding to 1450 cm -1 is found to be about +0.6 based on previous studies [49]. This value is in approximate agreement with the valence obtained from the formula, +0.66. The absorption spectrum consists of major absorption bands above 1.7 eV, a broad band around 0.4 eV, and a minor band around 1.5 eV (Figure 2b). Each band was assigned by following previous reports [50][51][52]. Major bands above 1.7 eV correspond to the mixture of the π-π* transition for the radical monomer trans-TTF-py 2 •+ and radical dimer (trans-TTF-py 2 •+ ) 2 . A broad band around 0.4 eV and a minor band around 1.5 eV are characteristics of intermolecular charge transfer from trans-TTF-py 2 •+ to trans-TTF-py 2 0 (forming trans-TTF-py 2 0 and trans-TTF-py 2 •+ ) and from trans-TTF-py 2 •+ to trans-TTF-py 2 •+ (forming trans-TTF-py 2 0 and trans-TTF-py 2 2+ ), respectively [53].
The existence of trans-TTF-py 2 •+ was also supported by the results of the ESR spectrum (Figure 2c), which shows a singlet peak with g = 2.003 derived from a TTF radical. A single crystal direct-current conductivity (= σ) of TTF-py 2 _PF 6 along the π-stack direction is shown in Figure 2d. The electrical conductivity of TTF-py 2 _PF 6 is 12 S cm -1 at RT, and the decreases with cooling temperature, suggesting semiconducting behavior. The slope of the Arrhenius plot is constant with 1000T -1 above 4.5 K -1 . From the fitting of the Arrhenius plot above 4.5 K -1 with an Arrhenius dependence of σ = σ 0 exp[−(E g /2kT)] where σ 0 is a constant, E g is the bandgap and k is Boltzmann constant [54], it shows that the E g of the carriers is estimated to be 162 meV.

Discussion
The structural study shows short contacts by the hydrogen bondings, and the DFT calculation with the counterpoise correction shows the significant stabilization energy. It is noteworthy that although the distance of the hydrogen bonding between A and B (2.365 Å) is shorter than that between B and B (2.626 Å), the stabilization energy of the hydrogen bonding between A and B (= -0.74 kcal/mol) is less than that between B and B (= -1.72 kcal/mol). This tendency did not change even when the exchange correlation functional was changed from B3LYP to B3PW91 or PBEPBE in the DFT calculations. The nature of the hydrogen bond is complicated by the fact that the hydrogen bond has multiple parameters such as angles, distances and chemical species (elements of hydrogen donor and acceptor atoms). [17,55]. In the present case, the N···H distance seems to be less important to determine the energy of the hydrogen bond. As shown in Figure 1f,g, the angles between the axis of the pyridyl groups and the hydrogen atoms of the TTF core (∠C(position 4)···N···H) are 143.7 • for A and B and 162.5 • for B and B. Because the latter angle is closer to 180 • , where the lone pair of nitrogen atom is headed to the hydrogen atom, the hydrogen bonding between B and B is stronger than that between A and B. Wood et al. examined the distance-and the angle-dependence of hydrogen bondings with pyridine in terms of computational chemistry [55]. In the paper, the structural dependence of the stabilization energy varies with chemical species, and the dependence of the energy on the interatomic distance is found to be relatively weak for the weak hydrogen bondings such as those with benzene. Even in this system, the hydrogen bondings are also weak C-H···N bonds, thus the dependence of the energy on the N···H distance is small, and the change of the angle is more likely to be involved in the stabilization energy.
The temperature-dependent electrical conductivity of TTF-py 2 _PF 6 shows semiconducting behavior along the a axis. Effective charge transfer integrals of adjacent TTF-py 2 clarify the nature of one-dimensional electron conductivity along the a axis, which is also denoted from the band structure. Given the difference of the effective charge transfer integral between A and B (V eff = 390.8 meV), and B and B (V eff = 220.9 meV), the semiconducting behavior of TTF-py 2 _PF 6 is probably due to the localization of carriers in the columnar structure. In fact, band calculation by using tight-binding approximation without structural optimization ( Figure 3) shows an inherent bandgap, E g = 140 meV, and hence a significant localization of carrier can be assumed from the crystal structure. The Calculated bandgap is almost consistent with the bandgap acquired from temperature-dependent conductivity (E g = 162 meV). The carriers in the dominant part of the electron conduction were located where the path of the band structure is along a* axis, the direction of π-π stacking of TTF-py 2 . Hence, not only the effective charge transfer integrals but also the band structure show the one-dimensional conducting character of TTF-py 2 _PF 6 .

Conclusions
In this paper, we discussed the crystal structure and the physical properties of the molecular conductor TTF-py 2 _PF 6 . It is the first molecular conductor containing trans-TTF-py 2 molecules and is the second one with trans-bis-substituted TTF molecules, to the best of our knowledge. Although TTF-py 2 _PF 6 has one dimensional electron transport properties, which is typical in molecular conductors, the hydrogen bondings between the pyridyl groups and the hydrogen atoms of the TTF skeleton were successfully introduced in the crystal structure. We believe that this result provides a potential for further structural explorations and physical properties of TTF-based molecular conductors with a substitution group with hydrogen bondings such as pyridyl and other moieties. Additionally, TTF-py 2 is a promising ligand for constructing both conductive π-stacked arrays and coordination networks in a crystal, such as porous molecular conductors [56,57] and conductive π-stacked metal-organic frameworks [58][59][60][61].
CCDC-2040487 contains the supplementary crystallographic data for this paper. Crystal structure information is available online at the Cambridge Crystallographic Data Centre (CCDC) database via www.ccdc.cam.ac.uk/data_request/cif.