β-(BDA-TTP)₂I₃ at ambient pressure exhibited semiconducting behavior with E_a (activation energy) = 54 meV, and the temperature dependence of its susceptibility obeyed the 1D (one-dimensional) antiferromagnetic Heisenberg model (Figure 4). The susceptibility behavior implies a possibility of a pressure-induced superconductivity in β-(BDA-TTP)₂I₃, because the phase diagram of BEDT-TTF superconductors suggests the superconducting state lies adjacent to the antiferromagnetic insulating state, as mentioned above. Thus, hydrostatic-pressure experiments using β-(BDA-TTP)₂I₃ under up to 14 kbar were undertaken.

The semiconducting behavior of β-(BDA-TTP)₂I₃ observed at ambient pressure remained almost unaltered under pressures of up to 9 kbar (Figure 5a). To our surprise, a marked change in conductivity was found above 10 kbar (Figure 5b). At 10.0 kbar, the resistivity showed an almost temperature-independent value from room temperature, a change to semiconducting behavior below about 190 K and a drop with an onset at 4.5 K. In addition, the onset temperature of a resistivity drop reached 8.2 K by increasing the pressure by only 0.3 kbar (Figure 5c); however, with further increases in the pressure, it decreased slowly and no resistivity drop was observed at 14.0 kbar. To gain an insight as to whether the resistivity drop is attributable to a superconducting transition, we investigated a Meissner effect of this salt. As shown in Figure 5d, β-(BDA-TTP)₂I₃ under a magnetic field of 0.1 T at 10 kbar exhibited a sudden stepwise decrease in magnetization below 10.5 K. Furthermore, MR (magnetoresistance) measurements under pressures of 10.3 to 11.5 kbar at 1.6 K indicated that the resistance is restored by increasing the magnetic field (Figure 5e). Another sample at 9.7 kbar also showed a two-step resistivity drop with an onset at 9.5 K, which completely disappeared by applying a magnetic field of 15 T (Figure 5f). These results added to the superconductivity of β-(BDA-TTP)₂I₃. However, we failed to detect zero resistance in the salt probably due to a small volume fraction in the superconducting state observed.

In order to elucidate the derivation of the structure-property relationship in β-(BDA-TTP)₂I₃, X-ray structural studies at ambient pressure and at a hydrostatic pressure of 7.5 kbar were carried out. The salt consists of alternate layers of BDA-TTP donor molecules arranged in the β-packing mode and layers of I₃⁻ anions (Figure 6a). At ambient pressure, the BDA-TTP molecules in the donor layer are dimerized with interplanar distances of 3.65 and 3.49 Å to form a stack along the a-axis. Intermolecular chalcogen···chalcogen contacts shorter than the sum of the van der Waals radii (or less than the van der Waals distance between chalcogen···chalcogen atoms) are known to be important in studying the electronic structures of organic molecular conductors. In the case of β-(BDA-TTP)₂I₃, several S···S contacts shorter than the van der Waals distance (3.70 Å) occur between donor stacks rather than within a donor stack (Figure 6b). The conformation of BDA-TTP donor molecules, including the orientations of two trimethylene end groups with respect to the molecular plane, belongs to the same category as found in β-(BDA-TTP)₂X (X = SbF₆, AsF₆ and PF₆, conformation A in Figure 3b) and in the neutral structure of BDA-TTP (Figure 2c). At 7.5 kbar, the conformation of BDA-TTP is slightly different from that at ambient pressure (Figure 6c): The dihedral angles around the intramolecular sulfur-to-sulfur axes in two outer dithiane rings are 51.3° and 29.2°, respectively, whereas the corresponding dihedral angles at ambient pressure are 50.9° and 32.0°, respectively. To gain further information on the electronic structure of β-(BDA-TTP)₂I₃, we calculated the overlap integrals between the HOMO’s (highest occupied molecular orbitals) of adjacent donor molecules by the extended Hückel method using the reported parameters [22] at ambient pressure and at 7.5 kbar (see the legend to Figure 6b). A comparison of the intrastack overlap integral ratio of p1 to p2 (2.73) at ambient pressure with the corresponding ratio (2.13) at 7.5 kbar indicates that the dimerization of donor molecules becomes small by applying pressure. However, the Fermi surface of β-(BDA-TTP)₂I₃ at 7.5 kbar, which was obtained by the tight-binding calculation method, is still open along the stacking a-axis (Figure 6d), similar to that at ambient pressure (Figure 6e). These open Fermi surfaces would be responsible for the semiconducting behavior of the salt observed under hydrostatic pressures ranging from ambient pressure to 9 kbar.

On the basis of their band structures, organic superconductors fall into two main systems: The half-filled band system and the quarter-filled one [23]. In both systems, the T_c’s (superconducting transition temperatures) are sensitive to pressure and the electronic states are also changed by pressure. In the case of many organic superconductors with the half-filled band system, with increasing pressure, the Mott insulating ground state is suppressed and then the superconducting state appears [24]. On the other hand, in the case of organic superconductors with the quarter-filled band system, the pressure enables the charge-ordered insulating state to change into the superconducting state [25,26]. Therefore, the application of pressure plays an important role in inducing organic superconductivity. In addition to the hydrostatic-pressure application with an isotropic pressure effect, the uniaxial-strain method with modification of the intermolecular distance along a desired direction is a powerful tool for the research of organic superconductors [27,28]. In this subsection, we address the following question: What changes take place in the T_c and P_c (critical superconducting pressure) of the pressure-induced superconductor β-(BDA-TTP)₂I₃ by making fine adjustments to the orientation of applying uniaxial strain. Before that, the electronic states and band structures of β-(BDA-TTP)₂I₃ at ambient pressure and at a hydrostatic pressure of 7.5 kbar are described.

The infrared and Raman spectroscopic study revealed that the BDA-TTP donor molecule in β-(BDA-TTP)₂I₃ is in the +0.5 oxidation state [29]. The result coupled with the susceptibility behavior explained by the 1D antiferromagnetic Heisenberg model (Figure 4) indicates that the ground state of β-(BDA-TTP)₂I₃ at ambient pressure is a Mott insulator with the half-filled band system, although there is an overlap between the upper and lower bands in the energy dispersion curve obtained by the tight-binding calculation using the extended Hückel method as shown in Figure 7a, where W, W_U and W_O stand for the total bandwidth of the upper and lower bands, the bandwidth of the upper band and the overlap between the upper and lower bands, respectively. The values of W and W_U are essential to the control of the effective electronic correlation, because the effective electronic correlation is given by U/W and V/W, where V is the intersite Coulomb repulsion, in the quarter-filled band system, and is expressed as U/W_U and V/W_U in the half-filled band system. Based on X-ray data of β-(BDA-TTP)₂I₃ at 7.5 kbar, it was found that the overlap W_O increases (Figure 7b) with a reduction of about 4% in the unit cell volume [917.2(11) ų] relative to that at ambient pressure [953.9(2) ų]. Therefore, the ground state of the salt is likely to change from the half-filled band into the quarter-filled one under applied pressure, because a large value of W_O is seen in the quarter-filled band structure [26].

We investigated the temperature dependence of the resistivity of β-(BDA-TTP)₂I₃ by applying uniaxial strains along the crystallographic a-, b- and c-axes and then along different directions in the conducting ac plane to find favorable pressure orientations for inducing superconductivity. The orientation of uniaxial strain in the ac plane is defined by the angle φ from the c-axis fixed as a reference axis (Figure 8). Figure 9a–c shows the resistivity of the salt as a function of temperature under uniaxial strains parallel to the a-, b- and c-axes, respectively. Under the a-axis strains up to 12 kbar, increase of resistivity began to be suppressed in a low temperature region at 7, 8, 9 and 10 kbar, and the salt behaved like a metal from 50 to 10 K under 12 kbar. On the other hand, under the b-axis strains, the insulating behavior of the salt remained unchanged at all pressures up to 12 kbar. Under the c-axis strains up to 10 kbar, a drop in resistivity occurred with an onset at 8.5 K under 8.5 kbar. The onset temperature of the resistivity drop increased up to 10.5 K at 9.5 kbar, but decreased slightly to 9.5 K at 10 kbar. We observed a recovery of the resistance in the resistivity measurement of another single crystal by application of a uniaxal strain of 10 kbar with φ = ~7° under applied magnetic fields (Figure 9d); therefore, the drop in resistivity observed by applying the c-axis strain can be attributed to a superconducting transition. It was ultimately found that the maximum value of resistive T_c (abbreviated as T_cmax) under the c-axis strain rises by 1 K compared to that under hydrostatic pressure (T_cmax = 9.5 K).

Figure 10a summarizes the pressure dependence of T_c in β-(BDA-TTP)₂I₃ by varying the orientation of uniaxial strain in the ac plane. Superconductivity was found in the orientation angle range of −47° ≤ φ ≤ 80°; however, outside this range, no superconductivity was observed. It should be noted that at the uniaxial strain with φ = 45°, the T_c’s varied from 4.3 to 10.3 K in a wide pressure range from 7 to 13.5 kbar and the P_c (7 kbar) is lower than that under hydrostatic pressure (P_c = 9.7 kbar). Figure 10b,c depicts the plots of the values of T_cmax and P_c, respectively, at different pressure orientations with φ = −47°, −32°, −1°, 16°, 45°, 58° and 80°. Higher values of T_cmax from 8.5 to 10.5 K are recorded in the orientation angle range of −32° ≤ φ ≤ 58°, whereas the values of T_cmax decrease outside this range. The plots of P_c display a convex-shaped pressure-orientation dependence within the same orientation angle range (−32° ≤ φ ≤ 58°), outside which the values of P_c increase.

Taking account of the approximately 4% reduction of the unit cell volume by application of a hydrostatic pressure of 7.5 kbar, we calculated the tight-binding band structures under uniaxial strains parallel to the ac plane by the extended Hückel method based on the assumption that the distance between the BDA-TTP donor molecules along each uniaxial strain decreases by 5% with no change in the HOMO level of BDA-TTP at ambient pressure. Figure 11 shows the curves of W, W_U and W_O, which were obtained from the band calculations by varying the pressure orientation angle at an interval of 5° in the ac plane, together with the values of W, W_U and W_O at ambient pressure. The value of W_O exhibits a gradual increase as φ approaches 0° (the direction of the c-axis), and a gradual decrease as φ come close to 106° (the direction of the a-axis) and −74° (the direction of the –a-axis). A maximum value of W_O is observed at φ = 16° (the direction perpendicular to the a-axis). The variation of W_O suggests that the ground state of β-(BDA-TTP)₂I₃ is changed from the half-filled band into the quarter-filled one by applying the c-axis strain, whereas the half-filled band is dominant in the ground state of β-(BDA-TTP)₂I₃ under the a-axis strain. It is noteworthy that compared to the value of W_O at ambient pressure, larger values of W_O are observed in the orientation angle range in which higher T_cmax’s are recorded (Figure 10b); therefore, the quarter-filled band system is likely to be favorable for the achievement of superconductivity in β-(BDA-TTP)₂I₃.

Application of pressure is generally thought to lead to an enhancement of the bandwidth and, consequently, to decrease the electron correlation [30]. In addition, a key parameter to control the ground state in the quarter-filled band system is proposed to be the effective electronic correlation U/W and V/W [23]. The ground state is in the metallic phase when the values of U/W and V/W are small, while the ground state is in the insulating phase when those are large. Superconductivity appears when U/W and V/W are in between. In the conducting ac plane of β-(BDA-TTP)₂I₃, the total bandwidth W under the a-axis strain exhibits a value close to the maximum (Figure 11), whereas the value of W at the uniaxial strain with φ = 16° is minimal. With decreasing W, the values of U/W and V/W increase, so that the achievement of superconductivity requires an increase of pressure. It is thus predicted that the value of P_c shows a maximum around φ = 16° and decreases as the orientation angle moves away from φ = 16°. This prediction is in good agreement with the pressure-orientation dependence of P_c in the orientation angle range of −32° ≤ φ ≤ 58° (Figure 10c) where the ground state of β-(BDA-TTP)₂I₃ seems to be the quarter-filled band. At the pressure orientation angles of −47° and 80°, the ground states are close to the half-filled band, the values of P_c significantly increase (Figure 10c) and the values of T_cmax decrease (Figure 10b).

The effective electronic correlation U/W_U and V/W_U in the half-filled band system acts as a key parameter to control the ground state [23], akin to the case of the quarter-filled band system. In the ac plane of β-(BDA-TTP)₂I₃, the values of the upper bandwidth W_U at different uniaxial strains are slightly larger than that at ambient pressure (Figure 11) and almost independent of φ. This pressure-orientation dependence of W_U differs from that of W. Thus it seems in the half-filled band system that a higher pressure is necessary for increase of W_U, and hence for decrease of U/W_U and V/W_U to induce superconductivity. As mentioned above, the ground state of β-(BDA-TTP)₂I₃ is regarded as the half-filled band for the pressure orientation along the a-axis, and we were actually unable to find superconductivity by applying the a-axis strains up to 12 kbar, though a metallic resistive behaviour as well as suppression of the resistivity increase were observed (Figure 9a). There is therefore a possibility that further increases in pressure along the a-axis achieve superconductivity in β-(BDA-TTP)₂I₃ with the half-filled band.

Our approach to the study of organic superconductivity described here, that is, the application of uniaxial strains with a variety of orientation angles demonstrates that the pressure orientation enables the ground state to change into another one, where the control of T_c is feasible by adjusting the intensity of uniaxial strain, considering the contribution of the effective electronic correlation. This work is thus the first example that realizes two competing insulating states depending on the pressure orientation in the same material, which is sure to shed light on the research of organic superconductors associated with those ground states. On the other hand, it remains to be proved that further application of the a-axis strain to β-(BDA-TTP)₂I₃ leads to superconductivity. Work currently in progress is addressing this issue.

We planned the synthesis of DHOT-TTP beginning with the oxathiolane-attached oxone 1a (Figure 13). However, as previously reported by us [41], the preparation of 1a from its dioxolane analogue 1b results in a mixture of 1a,b and the dithiolane-attached oxone 1c. We thus examined the exclusive formation of 1a. Treatment of 1b with conc. H₂SO₄ in refluxing MeOH gave dimethyl acetal 2 in 72% yield, which upon heating with 2-mercaptoethanol in benzene containing a catalytic amount of p-TsOH led successfully to 1a in 74% yield. Conversion of 1a into the corresponding tin dithiolate 3 by sequential treatment with MeMgBr and Cl₂SnBu₂ in THF followed by reaction with 2-ethoxycarbonyl-1,3-dithiolane 4 in the presence of Me₃Al in CH₂Cl₂ gave the coupling product 5 in 39% overall yield. Subsequent oxidation of 5 with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) in refluxing toluene allowed the construction of DHOT-TTP in 88% yield.

According to structural information on DHOT-TTP in the neutral state, the DHOT-TTP molecules crystallize in an orientationally disordered manner with respect to the oxygen atom (O4 or O4′ in the atomic numbering scheme of Figure 14a) and the sulfur atom (S4 or S4′). In the molecular structure, the dihedral angles around the intramolecular sulfur-to-sulfur axes in two inner dithiole rings, hereafter defined as φ, are equivalently 152.2° (Figure 14b), which is smaller than the corresponding dihedral angle (φ= 173°, Figure 2b) in the BDH-TTP molecule. It is therefore clear that DHOT-TTP has a nonplanar structure compared to BDH-TTP.

CV (cyclic voltammetry) measurements of DHOT-TTP [E₁ = 0.52, E₂ = 0.80, E₃ = 1.16 V (vs. SCE)] and BDH-TTP [E₁ = 0.56, E₂ = 0.85, E₃ = 1.24, E₄ = 1.44 V (irreversible)] under identical conditions (0.1 M n-Bu₄NClO₄ in PhCN, Pt electrode, scan rate 50 mVs⁻¹) revealed that the E₁ value of DHOT-TTP is smaller than that of BDH-TTP and the ∆E (E₂ − E₁) value of DHOT-TTP (0.28 V) is almost equal to that of BDH-TTP (0.29 V). These results indicate that the replacement of one sulfur atom with oxygen slightly enhances the electron-donating ability, but scarcely affects the on-site Coulomb repulsion, similar to the electrochemical tendency found among BEDT-TTF and its oxygen-substituted analogues [38,42].

In contrast to the metallic I₃ and BF₄ salts of BDH-TTP [15], DHOT-TTP gave the semiconducting I₃ and BF₄ salts with E_a = 17 and 45 meV, respectively [39]. In addition, the Au(CN)₂ salt of DHOT-TTP exhibited semiconducting behavior with E_a = 36 meV and σ_rt (room-temperature conductivity) = 2.4 S cm⁻¹ [43]. The Au(CN)₂^– anion with BDH-TTP also gave a semiconducting salt with E_a = 7 meV and σ_rt = 11 S cm⁻¹ [43]; therefore, the Au(CN)₂ salt of BDH-TTP is an exception to the tendency that BDH-TTP forms metallic salts with linear, tetrahedral and octahedral anions [14]. On the other hand, as shown in Figure 15a, the resistivities of the AuI₂, PF₆ and AsF₆ salts of DHOT-TTP monotonically decreased with decreasing temperatures to 1.4 K: Similarities can be seen in the BDH-TTP salts with the same anions [15]. Variable-temperature resistivity measurement of the FeCl₄ salt of DHOT-TTP showed some jumps in the cooling process and a large hump in the heating process (Figure 15b). The resistivity jumps are likely to be attributed to micro cracks caused by the stresses of the resistance-measuring probes. Furthermore, unlike κ-(BDH-TTP)₂FeCl₄ remaining metallic down to 1.5 K [31], a small upturn in resistivity was observed around 5 K. The magnetic susceptibility of the FeCl₄ salt of DHOT-TTP was fitted to a Curie–Weiss law from 2 to 300 K, giving a C (Curie constant) of 4.33 emu K mol⁻¹ and a θ (Weiss constant) of −0.50 K. The fitted C is close to the value of 4.38 emu K mol⁻¹ expected for a high-spin Fe³⁺ ion (S = 5/2, g = 2.0), thereby indicating that the Fe atom in the anion dominates the magnetization observed from 2 to 300 K. The small negative θ implies a very weak antiferromagnetic interaction between the Fe centers. Such a magnetic interaction, though very weak, does not occur in the paramagnetic κ-(BDH-TTP)₂FeCl₄ salt with θ = 0.041 K [31].

While we were unable to obtain a single crystal of the semiconducting I₃ salt of DHOT-TTP suitable for an X-ray structural study, the stoichiometry and crystal structure of the metallic AuI₂ salt was determined by X-ray analysis [39]. The salt has a donor-to-anion ratio of 2:0.87 and consists of α-type donor layers alternating with anion layers (Figure 16a). In the donor layer, there are two crystallographically independent DHOT-TTP molecules, in one of which, unlike the neutral DHOT-TTP molecule, orientational disorder with respect to the oxygen atom is negligibly small within the accuracy of X-ray analysis. However, another independent DHOT-TTP molecule has orientational disorder between the oxygen and sulfur atoms in the oxathiolane ring: Occupancy factors for the oxygen sites are 0.62 and 0.38. The structures of both independent DHOT-TTP molecules take on planarity: The respective φ's in the disorder-free DHOT-TTP molecule (molecule A) are 169.8° and 175.2° (Figure 17a), whereas those in the disordered DHOT-TTP molecule (molecule B) are 177.1° and 174.5° (Figure 17b). DHOT-TTP donor molecules are stacked with some dimerization along the c-axis to form two types of columns (columns A and B in Figure 16b). The interplanar distances in column A are 3.66 and 3.61 Å, whereas those in column B are 3.57 and 3.66 Å. The molecular plane of DHOT-TTP on column B is tilted by 50.1° from that on column A. Instead of no intermolecular S···S contact shorter than the van der Waals distance (3.70 Å) within each column, there are several short S···S contacts between columns A and B. Because of the orientational disorder between the oxygen and sulfur atoms in molecule B, the intermolecular overlap integrals were calculated by fixing the oxygen atom at the site with a larger occupancy factor (0.63). The absolute values of the overlap integrals between columns A and B (corresponding to a1, a2, q1 and q2) are all larger than those within both columns (corresponding to c1, c2, c3 and c4), reflecting the short S···S contact pattern. The calculation result suggests a 2D interaction in the donor layer, which would be responsible for the metallic conductivity to 1.4 K.

Since molecule A is free from disorder, its HOMO could be investigated by the extended Hückel method (Figure 18a). It should be noted that the coefficient of HOMO on the oxygen atom is considerably smaller than those on the sulfur atoms. Differences in the C–O and C–S bond lengths in the oxathiolane ring can be also discerned (Figure 18b). The lengths of the two C–O bonds [C18–O2, 1.45(2) Å; C20–O2, 1.30(2) Å] are obviously shorter than those of the two C–S bonds [C18–S14, 1.74(1) Å; C19–S14, 1.82(2) Å]; nonetheless, molecule A lacks such nonplanarity as is seen in the molecular structure of DHOT-TTP in the neutral state (Figure 14b). This is probably because the central conjugated system of DHOT-TTP spreads out towards the outer chalcogen atoms with increasing charge, so that the distortion of molecular structure in the neutral state is relieved.

Unlike the donor packing motif in α-(DHOT-TTP)₂(AuI₂)_0.87, the BDH-TTP donor molecules in the metallic (BDH-TTP)₂AuI₂ salt form a uniform stack with a constant interplanar spacing of 3.63 Å along the a-axis (Figure 19a) [43,44]. On the other hand, the S···S contact pattern is similar to that observed in α-(DHOT-TTP)₂(AuI₂)_0.87: Short S···S contacts are found between donor stacks rather than within a donor stack (Figure 19b). In addition, the absolute values of the interstack overlap integrals b and q are larger than that of the intrastack overlap integral a, which would lead to a 2D interaction responsible for the metallic state to 2.0 K. The planarity of BDH-TTP in (BDH-TTP)₂AuI₂ is somewhat enhanced (φ = 177.0, Figure 20a) compared to that of BDH-TTP in the neutral state (Figure 2b), and the distribution of its HOMO (Figure 20b) is quite similar to that of BDH-TTP in κ-(BDH-TTP)₂PF₆ [15]. Comparing the coefficients of HOMO of BDH-TTP in (BDH-TTP)₂AuI₂ with those of the disorder-free DHOT-TTP molecule (molecule A, Figure 18a) in α-(DHOT-TTP)₂(AuI₂)_0.87, it is thought that the replacement of only one sulfur atom with oxygen in BDH-TTP corresponds just to the removal of the sulfur atom from BDH-TTP, and that the formation of a somewhat weak 2D interaction is possible. Actually, the largest absolute value of the interstack overlap integral in α-(DHOT-TTP)₂(AuI₂)_0.87 (q2 = 9.07 × 10⁻³, Figure 16b) is less than two thirds of that in (BDH-TTP)₂AuI₂ (b = 14.5 × 10⁻³, Figure 19b).

An additional comparative study of the structural aspects of DHOT-TTP and BDH-TTP salts was carried out with the semiconducting Au(CN)₂ salts of DHOT-TTP and BDH-TTP [43]. By X-ray analyses, both the crystal structures were found to be isostructural with each other and to consist of alternate layers of donor molecules arranged in the α-packing mode and layers of Au(CN)₂⁻ anions [45,46]. Figure 21a shows the crystal structure of α-(DHOT-TTP)₂Au(CN)₂, in which the donor packing motif and the short S···S contact pattern (Figure 21b) resemble those in α-(DHOT-TTP)₂(AuI₂)_0.87 (Figure 16b). The asymmetric unit contains two crystallographically independent donor molecules. These structural characteristics also hold for the α-(BDH-TTP)₂Au(CN)₂ salt. Figure 22a,b shows the ORTEP drawings of two independent DHOT-TTP donor molecules in α-(DHOT-TTP)₂Au((CN)₂: One (molecule C) has almost no orientational disorder for the oxygen atom, whereas the other (molecule D) has an orientational disorder among the oxygen atom and the three outer sulfur atoms in the oxathiolane and dithiolane rings: Occupancy factors for the oxygen sites are 0.15, 0.18, 0.23 and 0.44. The respective two dihedral angles φ's in molecules C and D are comparable to those in molecules A and B in α-(DHOT-TTP)₂(AuI₂)_0.87 (Figure 17a,b) and also to those in two independent BDH-TTP donor molecules (molecules E and F) in α-(BDH-TTP)₂Au(CN)₂ (Figure 23a,b). Two ethylene carbon atoms on one side in molecule E have thermal disorder (Figure 23a). The distribution of HOMO in the disorder-free molecule C (Figure 22c) is almost the same as that in the disorder-free molecule A in α-(DHOT-TTP)₂(AuI₂)_0.87 (Figure 18a). In the donor layers of both α-(DHOT-TTP)₂Au(CN)₂ and α-(BDH-TTP)₂Au(CN)₂, the dihedral angles between the molecular planes of π-donors on neighboring stacks are 50.2° and 44.9°, respectively, and the absolute values of the intrastack overlap integrals c1, c2, c3 and c4, which were calculated by fixing the oxygen atom at the site with the largest occupancy factor (0.44) in the disordered molecule D in the case of α-(DHOT-TTP)₂Au(CN)₂, are smaller than those of the interstack overlap integrals a1, a2, q1 and q2 (Figure 21b). In particular, a very small value and relatively small values are estimated for the intrastack overlap integral c1 (0.08 × 10⁻³) in α-(DHOT-TTP)₂Au(CN)₂ and for the intrastack overlap integrals c2 and c4 (0.37 and 0.75 × 10⁻³) in α-(BDH-TTP)₂Au(CN)₂, respectively, which would be responsible for the semiconducting behaviors of both salts. Comparing the absolute values of the interstack overlap integrals in both salts, no remarkable differences in the a1 and q1 values [6.47 and 9.20 × 10−3 in α-(DHOT-TTP)2Au(CN)2; 5.67 and 9.34 × 10−3 in α-(BDH-TTP)2Au(CN)2] are found, whereas the a2 and q2 values in α-(DHOT-TTP)₂Au(CN)₂ (5.44 and 6.65 × 10⁻³) are smaller than those in α-(BDH-TTP)₂Au(CN)₂ (8.38 and 10.7 × 10⁻³). Therefore, it appears that the side-by-side interaction between donor molecules is somewhat weakened by introducing the oxygen atom instead of the sulfur atom.

As mentioned above, α-type donor packing motifs occurred in the AuI₂ and Au(CN)₂ salts of DHDO-TTP. On the other hand, DHOT-TTP with the PF₆⁻, AsF₆⁻ and FeCl₄⁻ anions formed CT salts with κ-type donor packing motifs [40], the structures of which are described in this subsection.

X-ray structural studies of the PF₆ and AsF₆ salts of DHOT-TTP established that both salts crystallize isostructurally and, additionally, are isostructural to κ-(BDH-TTP)₂PF₆ [15]. Figure 24a shows the donor packing motif of κ-(DHOT-TTP)₂PF₆, in which the DHOT-TTP molecule has orientational disorder among four outer chalcogen atoms in the oxathiolane and dithiolane rings: Occupancy factors for the oxygen sites are 0.28, 0.27, 0.23 and 0.22. There are several short S···S contacts (less than the van der Waals distance of 3.70 Å) between donor pairs, whereas no short S···S contact is observed within a donor pair. The S···S contact pattern is somewhat different from that observed in κ-(BDH-TTP)₂PF₆, in which each BDH-TTP donor molecule is linked by short intrapair and interpair S···S contacts. Intermolecular overlap integrals were calculated by fixing the oxygen atom at the site with the largest occupancy factor (0.28). According to the extended Hückel calculation, the coefficient of HOMO on the fixed oxygen atom in DHOT-TTP is very small, close to those on the oxygen atoms in the disorder-free DHOT-TTP molecules, molecule A (Figure 18a) in α-(DHOT-TTP)₂(AuI₂)_0.87 and molecule C (Figure 22c) inα-(DHOT-TTP)₂Au(CN)₂. Despite no short intrapair S···S contact, large values of the intermolecular overlap integrals are found within a donor pair (p1 = 20.5 × 10⁻³) as well as between donor pairs (p2 = 22.3 × 10⁻³). Compared to the corresponding values of intrapair and interpair overlap integrals in κ-(BDH-TTP)₂PF₆ (20.7 and 19.6 × 10⁻³ [15]), the p1 value is almost equal and the p2 value is slightly large: The same is true of κ-(DHOT-TTP)₂AsF₆ (p1 = 20.1 × 10⁻³, p2 = 22.3 × 10⁻³). Therefore, similar to κ-(BDH-TTP)₂PF₆, κ-(DHOT-TTP)₂X (X = PF₆ and AsF₆) are regarded as 2D metals, and hence would remain metallic all the way down to 1.4 K.

Analogous to an isostructural series of κ-(DHOT-TTP)₂X (X = PF₆ and AsF₆) and κ-(BDH-TTP)₂PF₆, the structure of the FeCl₄ of DHOT-TTP was found by X-ray analysis to be isostructural to κ-(BDH-TTP)₂FeCl₄ [31]. Figure 24b shows the donor arrangement of κ-(DHOT-TTP)₂FeCl₄. The oxygen atom of the DHOT-TTP molecule in the salt is orientationally disordered with the three outer sulfur atoms in the oxathiolane and dithiolane rings: Occupancy factors for the oxygen sites are 0.37, 0.36, 0.19 and 0.08. As is the case in κ-(BDH-TTP)₂FeCl₄, short S···S contacts are observed between donor pairs, but not within a donor pair. Nevertheless, calculation of the intermolecular overlap integrals by fixing the oxygen atom at the site with the largest occupancy factor (0.37) revealed that the value of the intrapair overlap integral p1 (17.8 × 10⁻³) is almost equal to the largest value (p2 = 17.9 × 10⁻³) among the interpair overlap integrals. Although the p1 and p2 values are, respectively, slightly smaller and slightly larger than the corresponding values calculated on the donor layer of κ-(BDH-TTP)₂FeCl₄ (19.3 and 15.7 × 10⁻³ [31]), these values seem to be large enough to lead to 2D interaction similar to that in κ-(BDH-TTP)₂FeCl₄ with metallic conductivity to 1.5 K. Accordingly, it is at present difficult to rationalize the resistivity upturn observed around 5 K (Figure 15b). The arrangement of FeCl₄⁻ anions is also similar to that in the paramagnetic κ-(BDH-TTP)₂FeCl₄ salt with θ = 0.041 K, featuring the formation of pairs. The Fe···Fe distance within a pair [5.563(2) Å] is close to that in κ-(BDH-TTP)₂FeCl₄ [5.559(2) Å], whereas the shortest Fe···Fe distance between pairs [8.060(6) Å] is slightly shorter than that in κ-(BDH-TTP)₂FeCl₄ [8.169(2) Å], but yet would be too long to induce a direct interaction between the Fe moments. In addition, a superexchange mechanism via the Cl atoms would be deniable because the shortest Cl···Cl distance between the anions [3.578(4) Å] is close to that in κ-(BDH-TTP)₂FeCl₄ [3.594(2) Å]. However, the shortest S···Cl distance [3.602(5) Å] between the donor molecule and the FeCl₄⁻ anion in κ-(DHOT-TTP)₂FeCl₄ is somewhat shorter than that in κ-(BDH-TTP)₂FeCl₄ [3.66(1) Å]. Eventually, it is likely that an interplay of the DHOT-TTP donor molecules and the FeCl₄⁻ anions contributes to a very weak antiferromagnetic interaction observed in κ-(DHOT-TTP)₂FeCl₄ with θ = −0.50 K.

The results above demonstrate that the DHOT-TTP donor molecules in κ-type salts have a cohesive force comparable to BDH-TTP. Our studies of the α- and κ-salts of DHDO-TTP suggest that the replacement of one sulfur atom in BDH-TTP with oxygen holds the following two conflicting effects: (i) A decrease of overlap between donor molecules, which arises from a very small coefficient of HOMO on the oxygen atom, and (ii) an increase of chemical pressure caused by the oxygen atom smaller in size than the sulfur atom, which is reflected by a decrease of the unite cell volume as shown in Table 1, where the unit cell volumes of two isostructural series of α- and κ-salts of DHDO-TTP and BDH-TTP are summarized. It seems that both effects are in competition with each other in the κ-salts with orthogonal arrangement of donor dimers, whereas the former effect is preferentially exerted in columnar donor arrangements such as α-type packing mode in which the side-by-side interaction between donor molecules plays a crucial role in the formation of 2D interactions. Chemical modification combining the chalcogen substitution described here with the extension of σ-framework may lead to a fine-tuning of intermolecular interaction to develop new strongly correlated organic electron systems. We continue further studies along this line, one of which has been communicated in reference [47].

The synthetic route to DMDH-TTP (Scheme 1) is essentially the same as that to BDH-TTP [15] except to use commercially available 2,3-butanedithiol 6 as a mixture of meso-, (R,R)- and (S,S)-forms instead of 1,2-ethanedithiol. Thus, the BF₃·OEt₂-mediated reaction of the dioxane-fused oxone 7 with 6 in CH₂Cl₂ gave oxone 8 with a dimethyl-substituted dithiolane ring in 94% yield as a 3.4:1 mixture of meso- and trans-dimethyl isomers. The ratio was determined by ¹H NMR integration for the methyl methine proton, and comparison with the ¹H NMR spectrum of (S,S)-8, prepared via the BF₃·OEt₂-meadiated reaction of 7 with (S,S)-6 [52], ascertained that the minor product corresponds to the trans-isomer. Then, oxone 8 was converted to the corresponding tin dithiolate 9 by sequential treatment with MeMgBr and Cl₂SnBu₂ in THF followed by the Me₃Al-promoted reaction with ester 4 in CH₂Cl₂ to afford the coupling product 10 in 40% overall yield. Finally, DDQ oxidation of 10 in refluxing toluene furnished, after purification by silica gel chromatography followed by recrystallization, DMDH-TTP in 63%–70% yields in repeated experiments. The ¹H NMR spectrum showed two sets of signals to the methyl and methine protons, and the intensity rations for the two methine protons ranged from 3.7:1 to 4.7:1 in repeated experiments. Taking into account the meso-to-trans ration of 8, it is most likely that the major isomer is meso-DMDH-TTP. The electrochemical behavior of the resulting DMDH-TTP was investigated by CV under the same conditions as used for the CV measurement of BDH-TTP (see Subsection 3.1). DMDH-TTP shows three reversible oxidation waves at 0.54, 0.81 and 1.36 V (vs. SCE). Both the values of E₁ (0.54 V) and ∆E (E₂ − E₁) (0.27 V) are somewhat smaller than those of BDH-TTP [E₁ = 0.56 V, ∆E (E₂ − E₁) = 0.29 V], indicating that the attachment of two methyl substituents causes a slight enhancement in the donor ability and a slight decrease in the on-site Coulomb repulsion due to an electron-donating effect of the methyl substituent.

Controlled-current electrocrystallization [53,54] of a diastereomeric mixture of DMDH-TTP in 1,1,2-trichloroethane with n-Bu₄NX (X = AsF₆, PF₆ and BF₄) gave single crystals, together with other crystals of unsuitable quality or size for an X-ray structure determination [51,55]. Notably, the fact that all the single crystals contain only meso-DMDH-TTP as donor components with the composition (meso-DMDH-TTP)₂X (X = AsF₆, PF₆ and BF₄) was ascertained by X-ray analyses (vide infra), which is different from the report that electrocrystallization of a diastereomixture of the corresponding diethyl-substituted derivative of BEDT-TTP [2,5-bis(4,5-ethylenedithio-1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene] with n-Bu₄NHgI₃ provides a single crystal of (Et₂BEDT-TTP)₂HgI₃ in which a donor stacking column consists of (R,R)-, (S,S)- and meso (or cis)-isomers [56]. Figure 26 shows the temperature dependence of the resistivities of the AsF₆ (σ_rt = 31 S cm⁻¹), PF₆ (σ_rt = 32 S cm⁻¹) and BF₄ (σ_rt = 54 S cm⁻¹) salts of meso-DMDH-TTP. Despite the presence of two methyl substituents in meso-DMDH-TTP, these salts maintained metallic conductivity down to a liquid helium temperature of 4.2 K, similar to the corresponding salts of BDH-TTP [15].

X-ray structural studies of the AsF₆ and PF₆ salts proved that their structures are isostructural with each other [55]. As shown in Figure 27a, the crystal structure of the AsF₆ salt contains two distinct types of donor layers, that is, a donor layer of the β-type (Figure 27b) and a donor layer of the β"-type (Figure 27c), which are interleaved by an anion layer. The asymmetric unit includes two crystallographically independent meso-DMDH-TTP molecules, both of which are, respectively, stacked in a head-to-tail manner to form the different donor layers. In the β-type donor layer, parallel stacks of somewhat dimerized donor molecules with separations of 3.61 and 3.78 Å occur along the b-axis, and intermolecular S···S contacts close to or shorter than the van der Waals distance (3.70 Å) exist within a donor stack as well as between donor stacks. According to the overlap integrals calculated by the extended Hückel method, the values of the intrastack overlap integrals p1 and p2 and of the interstack overlap integral q1 are large, whereas the absolute value of the interstack overlap integral a along the crystallographic a-axis is smallest. This anisotropic interaction in the β-layer results in a 1D Fermi surface being open along the interstacking a-direction (Figure 27d). Meanwhile, the β″-type layer has donor stacks aligned along the [101] direction, and the donor stack contains meso-DMDH-TTP donor molecules alternating at almost the same separation of about 3.74 Å. Short S···S contacts from 3.549(4) to 3.708(3) Å are observed between donor stacks rather than within a donor stack. Also the values of the interstack overlap integrals q1 and q2 are larger than those of the intrastack overlap integrals p1 and p2, leading to an open Fermi surface along the intrastaking [101] direction (Figure 27e). Although both the β- and β″-donor layers have 1D Fermi surfaces, the respective opening directions of the two 1D Fermi surfaces differ, so that the salt would exhibit metallic behavior down to a low temperature. These structural characteristics hold good in the case of the PF₆ salt. Such a dual-layered donor packing motif as found in β,β″-(meso-DMDH-TTP)₂X (X = AsF₆ and PF₆) has also occurred in the BDH-TTP salt, α'-(BDH-TTP)₆[Hg(SCN)₃][Hg(SCN)₄] [34], which features two different α'-types of donor layers responsible for two subsequent phase transitions, that is, a metal-semiconductor transition followed by a semiconductor-semiconductor transition. Furthermore, other examples of dual-layered conductors based on TTF donors have been reported [57].

On the other hand, the λ-type donor packing motif was found in the BF₄ salt of meso-DMDH-TTP (Figure 28a) [55]. The BF₄⁻ anion has a large thermal motion around the boron atom and exhibits positional disorder of the fluorine atoms. The asymmetric unit contains two crystallographically discrete meso-DMDH-TTP molecules, which are stacked along the [01] direction in a head-to-tail manner with a four-folded period being 3.87, 3.75 and 3.61 Å apart. Intermolecular S···S contacts shorter than the van der Waals distance (3.70 Å) are observed among three consecutive donormolecules in the four-fold period as well as between donor stacks (Figure 28b). Compared to the absolute values of the interstack overlap integrals a1, a2, q1, q2 and q3, larger absolute values are estimated for all the intrastack overlap integrals p1, p2 and p3. The tight-binding band calculation led to the energy dispersion curve and the Fermi surface shown in Figure 28c,d, respectively. Two highest bands are partially filled, and the Fermi surface associated with these bands contains a pair of wave-like lines and a closed rhombus-like loop centered at Γ. Thus, this salt has both 1D and 2D Fermi surfaces, and the 2D Fermi surface would cause the metallic conductivity down to 4.2 K.

The donor stacking modes in β,β″-(meso-DMDH-TTP)₂X (X = AsF₆ and PF₆) and λ-(meso-DMDH-TTP)₂BF₄ are characterized as follows; (i) one methyl substituent in meso-DMDH-TTP is situated in an axial-like position almost perpendicular to the molecular plane including the π-electron system to relieve steric repulsion between the adjoining two methyl substituents and (ii) meso-DMDH-TTP donor molecules are stacked in a head-to-tail manner with formation of pairs in which two axial-like methyl substituents face each other; for instance, as seen in two meso-DMDH-TTP donor molecules with an interplanar spacing of 3.78 Å (d2) in the β-layer of β,β″-(meso-DMDH-TTP)₂AsF₆ (Figure 27a). It can be seen that donor stacking modes having these structural features cause the metallic state with the itinerant electron system. Therefore, a subtle chemical modification for altering these structural features would be required to control the electron correlation. To this end, we focused on a different stereochemistry between two methyl substituents, and accomplished the synthesis of a chiral DMDH-TTP, (S,S)-DMDH-TTP, and a racemic mixture of DMDH-TTP, (±)-DMDH-TTP, in both of which two methyl substituents are in a trans configuration. Preparation, conductivity measurements and structural studies of CT materials derived from (S,S)-DMDH-TTP and (°)-DMDH-TTP are currently in progress in our laboratory.