Molecular Conductors Based on Dimethylcyclohexene-Fused Tetrathiafulvalene

Abstract

Chiral electroactive materials have attracted attention for the effects of electrical magnetochiral anisotropy (eMChA) and chirality-induced spin selectivity (CISS). The combination of tetrathiafulvalene (TTF) with chiral moieties is one way to access chiral electroactive materials. In this paper, we have focused on the fused 2,3-dimethylcyclohexene (DMCh) ring as a substituent with chiral carbon atoms and without heteroatoms, which has not been used in the field of molecular conductors, and we synthesized a new TTF derivative (rac-DMCh-EDT-TTF). We have developed novel molecular conductors (rac-DMCh-EDT-TTF)₂X (X⁻ = PF₆⁻, AsF₆⁻ and ClO₄⁻), which have bilayer conducting sheets composed of the two crystallographically independent molecules. All salts exhibited semiconducting behavior from room temperature down to low temperatures, and a resistivity anomaly was observed at 180–250 K. X-ray structure analysis at 100 K and 263 K and molecular orbital calculations using the results of X-ray structure analysis indicated the emergence of a charge disproportionation between Layers 1 and 2 at the low-temperature phase.

1. Introduction

Chiral electroactive materials exhibit unique properties, where the chirality of the material and the anisotropy of the electrical conductivity are correlated [1]. In particular, attention has been focused on the effects of electrical magnetochiral anisotropy (eMChA) [2,3,4] and chirality-induced spin selectivity (CISS) [5,6,7]. The eMChA effect is the dependence of the resistance of a chiral conductor on the chirality of the molecule itself and the relative orientation of the current and the applied magnetic field. On the other hand, the CISS effect is the selective transfer of preferential electron spin orientation through chiral materials. The similarities and differences between these two effects have been recently the subject of discussion [8]. There is also growing interest in chiral superconductors, with reports of eMChA [9] and CISS [10] in superconductors. Organic molecules are advantageous for the development of chiral materials, because asymmetric synthesis and optical resolution have been well studied. Tetrathiafulvalene (TTF) has a promising π-electron framework as a conductive component of molecular conductors, because TTF and its derivatives and analogs have yielded a large number of molecular conductors with metallic conductivity [11]. Studies on the synthesis and physical properties of chiral molecular conductors based on derivatives of TTF and bis (dithiolene), an analog of TTF, with chiral substituents have been reported [12]. The dimethylethylenedithio (DM-EDT) group [13], which contains two chiral centers, has been used in many of the above chiral conducting materials [14,15,16,17]. An eMChA [18] and CISS [19] have been observed in TTF derivatives only with molecules containing the DM-EDT group. Since eMChA and CISS have been observed under magnetic fields, substances that combine chiral TTF with achiral magnetic anions or achiral TTF with chiral magnetic anions are also interesting research targets, and there have been several reports on them [20,21,22]. Regarding the CISS effect, it has been demonstrated that the chirality-induced optical activity and spin polarization properties are closely related [23,24]. Since the chiral properties such as circular polarization vary depending on the atomic groups attached to the asymmetric carbon, we focused on the 2,3-dimethylbutylene group, in other words, the fused 2,3-dimethylcyclohexene (DMCh) ring, in which the sulfur atoms in the DM-EDT group are replaced by methylene groups. To establish a synthetic method for this new substituent-containing TTF derivative, we first synthesized rac-DMCh-EDT-TTF. We also prepared several radical cation salts based on rac-DMCh-EDT-TTF with octahedral and tetrahedral anions and found that the molecular conductors obtained are composed of bilayer conducting sheets separated by insulating anion layers not seen in the DM-EDT conductors. We also report electrical conducting behavior attributed to the energy difference between the bilayer molecular arrangement layers.

2. Materials and Methods

2.1. General

Unless otherwise specified, reagents and solvents were purchased commercially and used without additional purification. Acetonitrile was dried over Molecular Sieves 3A. Silica gel (Kanto Chemical Co., Inc., Tokyo, Japan, 60N, 63–210 µm and Fuji Silysia Chemical Ltd., Kasugai, Aichi, Japan, PSQ100B, 100 µm) was employed for column chromatography. ¹H NMR spectra were measured on a Bruker Biospin AVANCE 400 spectrometer and were analyzed using Delta 6.2.0 software (JEOL). Tetramethylsilane was used as an internal standard in ¹H NMR measurements (δ = 0.00 ppm). Mass spectra were recorded on a Bruker compact QTOF. The uncorrected melting points were measured using a Yanaco MP-500D. IR spectra were recorded on a JASCO FT/IR-460 plus spectrometer using a KBr disk. Cyclic voltammetry (CV) measurements were performed using a BAS ALS/chi 617B electrochemical analyzer and a conventional three-electrode cell, which consisted of a Pt disk, a Pt wire counter, and Ag/AgNO₃ (0.01 M) reference electrodes. The measurements were performed in a benzonitrile solution containing 0.1 M tetra-n-butylammonium hexafluorophosphate as a supporting electrolyte at 25 °C. All the redox potentials were converted to a ferrocene/ferrocenium (Fc/Fc⁺) couple.

2.2. Synthesis

2.2.1. Compound rac-2

A solution of 1 [25] (5.00 g, 15.6 mmoL) and tetraethylammonium iodide (12.1 g, 46.9 mmoL) and diethyl fumarate (7.60 mL, 46.8 mmoL) in acetonitrile (200 mL) was refluxed for 3 h under an argon atmosphere. After removing the solvent in vacuo, the residue was extracted with CH₂Cl₂. The organic layer was washed with saturated Na₂S₂O₃ and then with water. After drying over anhydrous MgSO₄, the solvent was evaporated in vacuo to afford yellow oil. The crude product was purified by column chromatography on silica gel (CH₂Cl₂). The solvent was evaporated in vacuo, and the residue was washed with hexane to obtain the pure product rac-2 (4.22 g, 12.7 mmoL) at 81% yield. Yellow solid; mp 114–115 °C; ¹H NMR (CDCl₃, 400 MHz) δ 4.20 (q, J = 7.2 Hz, 4H), 3.25–3.18 (m, 2H), 2.90–2.72 (m, 4H), 1.28 (t, J = 7.1 Hz, 6H); ¹³C NMR (CDCl₃, 101 MHz) δ 14.2, 26.7, 40.8, 61.6, 134, 172, 211. IR (KBr) ν 2982, 2904, 1724, 1477, 1438, 1375, 1303, 1260, 1227, 1192, 1125, 1112, 1063, 1034, 867, 557, 515 cm⁻¹; HRMS (APCI): m/z Calcd. for C₁₃H₁₇O₄S₃⁺: 333.0283 (M+H⁺); Found 333.0288 (error −1.4 ppm).

2.2.2. Compound rac-3

To a solution of rac-2 (2.50 g, 7.53 mmoL) and lithium bromide (1.32 g, 15.2 mmoL) in THF-CH₃OH (5:1, 60 mL), sodium borohydride (1.14 g, 30.2 mmoL) was slowly added at 0 °C under an argon atmosphere. The reaction mixture was then warmed to room temperature and further stirred for 1 h. The reaction mixture was then heated to reflux overnight, and the reaction was quenched with H₂O at 0 °C. The reaction mixture was extracted with AcOEt, and the organic phase was washed with brine. After drying over anhydrous MgSO₄, the solvent was evaporated in vacuo to obtain the yellow solid. The yellow solid was washed three times with hexane to obtain crude product rac-3 (1.8 g, 7.1 mmoL) at 94% crude yield. The compound rac-3 was used for the subsequent reaction without further purification because it gradually decomposed during purification. ¹H NMR (CDCl₃, 400 MHz) δ 3.87–3.70 (m, 4H), 2.59–2.42 (m, 4H), 2.27 (br, 2H), 2.05–1.99 (m, 2H).

2.2.3. Compound rac-4

To a mixture of the crude rac-3 (1.8 g, 7.1 mmoL) with imidazole (2.90 g, 42.6 mmoL) and triphenylphosphine (5.60 g, 21.4 mmoL) in CH₂Cl₂ (130 mL), iodine (5.45 g, 21.5 mmoL) was added at 0 °C under an argon atmosphere. The reaction mixture was then warmed to room temperature and stirred overnight in the dark. The reaction mixture was washed with saturated Na₂S₂O₃ and then with water. After drying over anhydrous MgSO₄, the solvent was evaporated in vacuo to obtain the yellow solid. The crude compound was purified by column chromatography on silica gel (CS₂:CH₂Cl₂ = 10:1), followed by reprecipitation with hexane to obtain the pure product rac-4 (2.87 g, 6.13 mmoL) at 87% yield. Yellow powder: mp 163–164 °C; ¹H NMR (CDCl₃, 400 MHz) δ 4.44–3.30 (m, 4H), 2.60–2.58 (m, 4H), 1.63–1.62 (m, 2H); ¹³C NMR (C₆D₆-CS₂, 101 MHz) δ 11.8, 31.3, 38.2, 134, 209. IR (KBr) ν 2999, 2906, 2891, 2832, 1590, 1513, 1432, 1417, 1269, 1180, 1053, 1027, 920, 888, 860, 806, 764, 610, 540, 515 cm⁻¹; HRMS (APCI): m/z Calcd. for C₉H₁₁I₂S₃⁺: 468.8107 (M+H⁺); Found 468.8123 (error –3.4 ppm).

2.2.4. Compound rac-5

To a solution of rac-4 (2.68 g, 5.72 mmoL) in DMF (116 mL) under argon, a solution of sodium borohydride was added dropwise (698 mg, 18.5 mmoL) in DMF (20 mL). The reaction mixture was stirred for 30 min at room temperature, and the reaction was quenched with 1M HCl aq. at 0 °C. The mixture was extracted with toluene 3 times, and the organic layer was washed with water and then with brine 3 times, dried over anhydrous Na₂SO₄, and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel (CS₂) to obtain rac-5 (1.06 g, 4.89 mmoL) at 86% yield. Yellow solid; mp 60.5–61.4 °C; ¹H NMR (CDCl₃, 400 MHz) δ 2.61–2.56 (m, 4H), 2.20–2.14 (m, 2H), 1.65 (m, 4H), 1.07–1.05 (d, J = 6.0 Hz, 6H); ¹³C NMR (CDCl₃, 101 MHz) δ 18.9, 33.2, 34.7, 137, 213. IR (KBr) ν 2969, 2951, 2882, 2823, 1594, 1452, 1426, 1377, 1348, 1290, 1059, 1024, 883, 514 cm⁻¹; HRMS (APCI): m/z Calcd. for C₉H₁₃S₃⁺: 217.0174 (M+H⁺); Found 217.0164 (error 4.7 ppm).

2.2.5. Compound rac-6

A solution of rac-5 (1.08 g, 4.83 mmoL) in (CH₃)₂SO₄ (20 mL) was heated to 80 °C for 1 h under an argon atmosphere. The reaction mixture was precipitated with Et₂O to obtain a pink solid and washed three times with Et₂O. The pink solid was dissolved in CH₃CN (10 mL), and sodium borohydride (340 mg, 8.55 mmoL) was slowly added at 0 °C. The reaction mixture was stirred for 30 min at room temperature. The reaction mixture was extracted three times with CH₂Cl₂, and the organic layer was washed twice with water and then with brine, dried over anhydrous Na₂SO₄, and concentrated under reduced pressure to obtain the crude product rac-6 (962 mg, 4.00 mmoL) as orange oil at 83% yield. The compound rac-6 was used for the subsequent reaction without further purification because it gradually decomposed during purification. Orange oil: ¹H NMR (CDCl₃, 400 MHz) δ 5.83 (s, 1H), 2.44–2.20 (m, 5H), 2.02–1.79 (m, 2H), 1.53–1.48 (m, 2H), 0.98 (d, J = 4.8 Hz, 6H).

2.2.6. Compound rac-7

To a solution of crude rac-6 (962 mg, 4.00 mmoL) in Ac₂O (3 mL), HPF₆ aq. (55% solution, 2.16 g, 8.14 mmoL) was added dropwise at 0 °C. The reaction mixture was stirred for 1 h at room temperature, and Et₂O was added to the reaction mixture to obtain the pink solid. The resulting precipitate was collected by filtration, and the solid was washed three times with Et₂O to obtain crude rac-7 (1.12 g, 3.39 mmoL) at 85% yield. The compound rac-7 was used for the subsequent reaction without further purification because it gradually decomposed during purification. Pink solid: ¹H NMR (CD₃CN, 400 MHz) δ 10.81 (s, 1H), 3.38–3.32 (m, 2H), 2.80–2.74 (m, 2H), 1.79–1.73 (m, 2H), 1.11 (d, J = 6.4 Hz, 6H).

2.2.7. rac-DMCh-EDT-TTF

To a solution of crude rac-7 (89 mg, 0.27 mmoL) and 8 [26] (158 mg, 0.29 mmoL) in CH₃CN (4.4 mL), Et₃N (0.36 mL, 2.7 mmoL) was added dropwise at 0 °C under an argon atmosphere. After the reaction mixture was stirred for 5 h at room temperature, methanol was added to the reaction mixture until no precipitation was observed. The resulting precipitate was collected and subjected to column chromatography on silica gel (CS₂). The target compound rac-DMCh-EDT-TTF was isolated as an orange powder (77 mg, 0.20 mmoL) at 76% yield. Orange powder: mp 236–238 °C; ¹H NMR (C₆D₆-CS₂, 400 MHz) δ 3.02 (s, 4H), 2.20–2.25 (m, 2H), 1.81–1.87 (m, 2H), 1.44 (m, 2H), 0.95 (d, J = 5.6 Hz, 6H); This compound is too insoluble to record a ¹³C NMR spectrum. IR (KBr) ν 2955, 2920, 2896, 2864, 2825, 1630, 1535, 1449, 1431, 1407, 1369, 1346, 1287, 1151, 971, 903, 831, 772, 679 cm⁻¹; HRMS (APCI): m/z Calcd. for C₁₄H₁₈S₆^•+: 375.9571 (M^•+); Found 375.9567 (error 1.0 ppm).

2.3. Preparation of Radical Cation Salts

Single crystals of the rac-DMCh-EDT-TTF salts with PF₆⁻, AsF₆⁻, and ClO₄⁻ anions were prepared by galvanostatic electrochemical oxidation in chlorobenzene (17 mL) containing 6% (v/v) ethanol (1 mL) under an argon atmosphere. A total of 2 mg of rac-DMCh-EDT-TTF was used. The corresponding tetra-n-butylammonium salts (28 mg for the PF₆⁻ salt, 31 mg for the AsF₆⁻ salt, and 20 mg for the ClO₄⁻ salt) were used as supporting electrolytes. A 2.0 mm φ diameter wire platinum electrode and H-shaped cells were employed. The current was applied in steps varying from 0.2 to 0.3 µA at 5 °C for 7 days.

2.4. X-Ray Crystallographic Analysis

The diffraction data of single crystals for rac-DMCh-EDT-TTF and the rac-DMCh-EDT-TTF salts with PF₆⁻, AsF₆⁻, and ClO₄⁻ anions were collected on a Rigaku Saturn724 diffractometer equipped with multilayer-mirror-monochromated Mo-Kα radiation (λ = 0.71075 Å) at the Advanced Research Support Center (ADRES), Ehime University. The diffraction data of single crystals for rac-DMCh-EDT-TTF salts with ClO₄⁻ anions were collected on a Rigaku HyPix-6000 diffractometer equipped and multilayer-mirror-monochromated Mo-Kα radiation at the Institute for Molecular Science. The single crystal was cooled to 263 K and 100 K using a continuous nitrogen-gas-flow cryostat. The data were corrected for Lorentz and polarization effects using CrysAlisPro (ver. 1.171.42.49) [27]. The structures were solved by the dual-space method (SHELXT) [28] and refined by full-matrix least squares on F² (SHELXL) [29]. The disordered DMCh moiety and the EDT groups were divided into major and minor orientations, and we applied geometric constraints to the bond lengths and angles using the SADI command in SHELXL to refine the structure. In addition, the RIGU command in SHELXL was used to apply reasonable constraints on anisotropic thermal vibration parameters, and some of the disordered carbon atoms with minor orientation were analyzed isotropically. For the ClO₄⁻ anion, the structure was refined without using these restraint commands. See Table S5 of the Supplementary Material for each occupancy. All calculations were performed using the Orex2 crystallographic software package [30].

2.5. Band Calculations

The calculation of intermolecular overlap integrals (S) was performed using the HOMOs of rac-DMCh-EDT-TTF based on the extended Hückel method with parameters listed in Table S1. The transfer integral (t) was in proportion to S, t = ES (E = −10 eV). The electronic band dispersions, DOSs, and Fermi surfaces were calculated based on the tight-binding approximation using the estimated t [31]. The atomic orbitals for the sulfur (3s, 3p, and 3d) built up from Slater-type orbitals were used for the extended Hückel calculation [32].

2.6. Electrical Conductivity

Electrical resistivities were measured by the four-probe method. Gold wires (10 µm diameter) were attached to a single crystal using a carbon paste. DC resistivities of (rac-DMCh-EDT-TTF)₂X (X⁻ = PF₆⁻, AsF₆⁻, and ClO₄⁻) were measured with a KEITHLEY 2001 digital multimeter and YOKOGAWA 7651 programmable direct current source. During measurement, the sample was cooled using an Iwatani CryoMini model CRT-HE05-RE cooling system and a Lakeshore S331 digital program temperature controller in the temperature range of 10–320 K. For the resistivity measurement under hydrostatic pressure, we employed a clump-type cell made of Cu-Be/NiCrAl alloy and Daphne 7373 oil as a pressure medium.

3. Results and Discussion

3.1. Synthesis and Electrochemistry

The synthetic route of rac-DMCh-EDT-TTF is shown in Scheme 1. The reaction of compound 1 with tetraethylammonium iodide in the presence of diethyl fumarate in refluxing acetonitrile afforded the Diels–Alder cycloadduct rac-2 at 81% yield. After the hydride reduction of rac-2 with NaBH₄ in a mixed solvent of THF and methanol at 60 °C, yielding crude rac-3 with a crude yield of 94%, the crude rac-3 was subjected to the Appel reaction using iodine, imidazole, and triphenylphosphine in dichloromethane at room temperature to obtain the diiodide (rac-4) at 82% yield based on rac-2. Rac-4 was then hydride-reduced again using NaBH₄ in DMF at room temperature, producing the DMCh-fused 1,3-dithiole-2-thione (rac-5) at 86% yield. Rac-5 was methylated in dimethyl sulfate at 80 °C, followed by a hydride reduction with NaBH₄ in acetonitrile at 0 °C to afford rac-6 as a crude product. The treatment of rac-6 with aqueous HPF₆ in anhydrous acetic acid at 0 °C gave the dithiolylium salt rac-7 at 85% yield. The target compound rac-DMCh-EDT-TTF was obtained at 76% yield by a reaction with the phosphonium salt 8 in the presence of an excess of triethylamine in acetonitrile. In cyclic voltammetry, rac-DMCh-EDT-TTF exhibited two pairs of one-electron redox waves at −0.06 V and +0.38 V (vs. Fc/Fc⁺, in PhCN at 25 °C) (Figure S1).

3.2. Preparation and Electrical Conductivities of (rac-DMCh-EDT-TTF)₂X (X⁻ = PF₆⁻, AsF₆⁻, and ClO₄⁻)

The radical cation salts of rac-DMCh-EDT-TTF were prepared using an electrochemical oxidation method, and black plate-like crystals of (rac-DMCh-EDT-TTF)₂X (X⁻ = PF₆⁻, AsF₆⁻, and ClO₄⁻) were obtained. The electrical conductivities of the obtained salts were measured on a single crystal using the four-probe technique, and their electrical properties are summarized in

Table 1. The electrical properties of rac-DMCh-EDT-TTF salts.

	σr.t.a/S cm−1	Ea/eV
		300–275 K	265–240 K	210–180 K
(rac-DMCh-EDT-TTF)2PF6 b	4.4 × 10−2	2.1 × 10−1	4.3 × 10−1	1.6 × 10−1
(rac-DMCh-EDT-TTF)2AsF6 b	2.5 × 10−1	1.8 × 10−1	5.4 × 10−1	2.0 × 10−1
(rac-DMCh-EDT-TTF)2ClO4 b	4.1 × 100	4.8 × 10−2	—	1.7 × 10−1

^a σ_r._t.: Conductivity at 300 K. ^b The stoichiometry was determined by single-crystal X-ray structure analysis.

. The temperature dependence of the resistivity is shown in Figure 1. All salts exhibited semiconducting behavior from room temperature down to low temperature, with inflection points observed at 250 K for (rac-DMCh-EDT-TTF)₂X (X⁻ = PF₆⁻ and AsF₆⁻) and at 180 K for (rac-DMCh-EDT-TTF)₂ClO₄. Additionally, the activation energy changed around these inflection points. For (rac-DMCh-EDT-TTF)₂ClO₄, electrical conductivity measurements under pressure were also conducted (Figure 2). As the pressure increased, the resistivity decreased, maintaining semiconducting behavior up to 1.8 GPa.

3.3. X-Ray Structure Analyses of (rac-DMCh-EDT-TTF)₂X (X⁻ = PF₆⁻, AsF₆⁻, and ClO₄⁻)

Single-crystal X-ray structure analysis of (rac-DMCh-EDT-TTF)₂X (X⁻ = PF₆⁻, AsF₆⁻, and ClO₄⁻). The crystallographic data are summarized in

Table 2. Crystallographic data of (rac-DMCh-EDT-TTF)₂X (X⁻ = PF₆⁻, AsF₆⁻, and ClO₄⁻).

	(rac-DMCh-EDT-TTF)2PF6		(rac-DMCh-EDT-TTF)2AsF6	(rac-DMCh-EDT-TTF)2ClO4
Formula	C28H32PF6S12		C28H32AsF6S12	C28H32ClO4S12
T/K	100	263	100	100	263
crystal system	triclinic	triclinic	triclinic	triclinic	triclinic
space group	P$\overline{1}$ (#2)	P$\overline{1}$ (#2)	P$\overline{1}$ (#2)	P$\overline{1}$ (#2)	P$\overline{1}$ (#2)
a/Å	6.6189 (2)	6.6582 (3)	6.6442 (2)	6.6158 (2)	6.6663 (3)
b/Å	8.1402 (2)	8.2694 (4)	8.21630 (10)	8.0542 (3)	8.2020 (4)
c/Å	32.8895 (8)	32.8995 (9)	32.9681 (6)	32.5826 (7)	32.6647 (13)
α/°	94.785 (2)	94.801 (3)	95.051 (2)	93.731 (3)	93.919 (4)
β/°	90.361 (2)	90.106 (3)	90.064 (2)	92.451 (2)	92.093 (2)
γ/°	99.344 (2)	99.151 (4)	98.937 (2)	99.953 (3)	100.116 (4)
V/Å3	1742.10 (8)	1781.88 (13)	1770.80 (7)	1703.83 (9)	1751.99 (14)
Z	2	2	2	2	2
Dcalc/g cm−3	1.712	1.674	1.767	1.662	1.616
μ/mm−1	0.855	0.836	1.723	0.884	0.860
independent reflections	8026	8170	8143	7831	8044
observed reflections [I > 2σ(I)]	6213	4627	7328	6436	5769
variable parameters	500	438	485	476	447
Rint	0.0543	0.1166	0.0348	0.0916	0.1018
GOF	1.031	0.962	1.028	1.033	1.056
R1 [I > 2σ(I)]	0.0409	0.0584	0.0303	0.0419	0.0556
wR2 [I > 2σ(I)]	0.0792	0.1049	0.0719	0.0992	0.1378
R1 (all data)	0.0620	0.1209	0.0353	0.0537	0.0800
wR2 (all data)	0.0860	0.1266	0.0741	0.1055	0.1506
CCDC No.	2404093	2404094	2404095	2404096	2404097

. These three salts were isostructural with each other, regardless of the shape and size of the counter anions, and crystallized in the triclinic space group P$\overline{1}$ (#2). The unit lattice volume of the three salts differs slightly with the anion size, but there is no clear change in the lattice constant; a comparison of measurements at 100 K and 263 K shows no clear change in lattice parameters except for thermal contraction for all salts. The crystal structure of (rac-DMCh-EDT-TTF)₂PF₆ at 100 K is shown in Figure 3. Two donor molecules (molecule 1 and molecule 2) are crystallographically independent. The methine carbons (C13, C14, C17, and C18) of the DMCh moiety were disordered in both molecules 1 and 2, and they formed a racemic solid solution. Disorders of the methine carbon were found in all donor molecules in the three salts at 100 K. These donor molecules construct a bilayer structure of Layer 1 formed by molecule 1 and Layer 2 formed by molecule 2. The donor arrangement in Layer 1 is a so-called β-type arrangement [33], in which the molecules are stacked in a ring-over-bond overlap mode (Figure 3c). The donor arrangement in Layer 2 has a so-called β’-type arrangement [33], in which ring-over-bond and ring-over-atom overlap modes are stacked alternately (Figure 3d). The PF₆⁻ anions are situated in a cavity between Layer 1 and Layer 2, and there are interatomic contacts shorter than the van der Waals radius [34] between the carbon atoms of molecule 2 and the fluorine atoms of PF₆⁻. At 100 K, there are two C•••F contacts: C21-F5 (3.100(3) Å) and C28-F4 (3.157(5) Å), whereas at 263 K, only one contact is observed: C21-F5 (3.163(3) Å). This difference is attributed to the increase in the lattice volume due to thermal expansion. No similar contacts are observed for molecule 1.

3.4. Band Calculations of (rac-DMCh-EDT-TTF)₂X (X⁻ = PF₆⁻, AsF₆⁻, and ClO₄⁻)

Based on the results of a single-crystal X-ray structural analysis, the HOMO diagram calculated by the extended Hückel method is shown in Figure 4. For both molecule 1 and molecule 2, the HOMO is distributed over the TTF skeleton and the sulfur of the EDT group but not on the DMCh moiety. Molecule 1 has a larger bending angle of the TTF skeleton (Figure S2, Table S4) compared to molecule 2 (molecule 1: 13.0°, molecule 2: 4.5°), and it is considered that molecule 1 has less charge than molecule 2. The calculated HOMO energy levels and the molecular charges estimated from the bond lengths [35,36] (Table S3) are summarized in

Table 3. Calculated HOMO level (extended Hückel method) and estimated charge by bond lengths.

	Molecule 1		Molecule 2		Molecule 1–Molecule 2
	HOMO Level/eV	Charge	HOMO Level/eV	Charge	HOMO Level/eV	Charge
PF6 (100 K)	−9.345	+0.33	−9.270	+0.67	−0.075	−0.34
PF6 (263 K)	−9.313	+0.41	−9.256	+0.59	−0.057	−0.18
AsF6 (100K)	−9.348	+0.33	−9.274	+0.67	−0.07	−0.34
ClO4 (100 K)	−9.354	+0.32	−9.262	+0.68	−0.092	−0.36
ClO4 (263 K)	−9.336	+0.33	−9.258	+0.67	−0.078	−0.34

. The results of the bending angles of the TTF framework, the calculated HOMO energy levels, and the molecular charge estimated from bond lengths are consistent. Additionally, only molecule 2 has contacts within the van der Waals radius [34] between the carbon and fluorine atoms. In other words, molecule 1 has a lower estimated molecular charge from the bond lengths compared to molecule 2. Comparing the results at 100 K and 263 K, the difference in bending angles Δ_angle (molecule 1 bending angle–molecule 2 bending angle) is 1.2° smaller at 263 K than at 100 K (Table S4). The difference between the calculated HOMO energy level and the molecular charges estimated from the bond length is smaller at 263 K than at 100 K (Table 3). This is consistent with fewer contacts within the van der Waals radius between the carbon and fluorine atoms at 263 K compared to 100 K. These results demonstrate that the degree of charge disproportionation between Layer 1 and Layer 2 is enhanced at 100 rather than 263 K.

The overlap integral between the HOMOs of rac-DMCh-EDT-TTF molecules was calculated using the extended Hückel method based on the results of the single-crystal X-ray structural analysis [31,32]. The molecular arrangement is shown in Figure 3c,d, and the overlap integral values and geometric parameters are shown in

Table 4. Overlap integrals ^a S (× 10⁻³) and geometrical parameters ^b ϕ (°), x (Å), y (Å), and z (Å).

	(rac-DMCh-EDT-TTF)2PF6
	Layer 1						Layer 2
100 K	S	ϕ/°	x/Å	y/Å	z/Å	100 K	S	ϕ/°	x/Å	y/Å	z/Å
b1	19.0	88.8	1.49	0.08	3.66	b3	30.8	89.8	1.45	0.01	3.38
b2	4.23	89.4	4.54	0.04	3.89	a2	4.20	7.97	1.70	6.34	0.89
a1	−3.09	14.8	1.29	6.28	1.65	p2	−2.62	48.5	4.36	3.12	3.55
p1	0.80	19.4	5.83	6.32	2.23	p3	5.32	21.5	3.15	6.32	2.49
q1	6.31	17.9	0.20	6.20	2.01	q2	−1.11	54.2	6.06	3.19	4.43
ΔS/<S> c	1.27					ΔS/<S> c	1.69
263 K	S	ϕ/°	x/Å	y/Å	z/Å	263 K	S	ϕ/°	x/Å	y/Å	z/Å
b1	18.3	89.4	1.49	0.04	3.70	b3	29.1	89.7	1.45	0.02	3.43
b2	3.56	89.1	4.62	0.06	3.95	a2	4.51	7.62	1.70	6.38	0.85
a1	−3.30	15.2	1.33	6.29	1.71	p2	−3.12	49.1	4.45	3.13	3.62
p1	0.69	19.4	5.96	6.36	2.24	p3	4.60	22.0	3.15	6.37	2.58
q1	5.83	17.6	0.15	6.26	1.99	q2	−0.94	54.0	6.15	3.25	4.47
ΔS/<S> c	1.35					ΔS/<S> c	1.61
	(rac-DMCh-EDT-TTF)2AsF6
	Layer 1						Layer 2
100 K	S	ϕ/°	x/Å	y/Å	z/Å	100 K	S	ϕ/°	x/Å	y/Å	z/Å
b1	19.6	89.1	1.44	0.05	3.66	b3	30.1	90.0	1.44	0.00	3.39
b2	3.86	89.8	4.60	0.02	3.93	a2	3.94	8.16	1.73	6.35	0.91
a1	−2.94	14.7	1.35	6.29	1.65	p2	−2.83	48.7	4.43	3.15	3.58
p1	0.78	19.8	5.95	6.31	2.28	p3	5.11	21.4	3.17	6.35	2.48
q1	6.12	17.9	0.09	6.24	2.01	q2	−1.00	54.5	6.16	3.20	4.49
ΔS/<S> c	1.34					ΔS/<S> c	1.66
	(rac-DMCh-EDT-TTF)2ClO4
	Layer 1						Layer 2
100 K	S	ϕ/°	x/Å	y/Å	z/Å	100 K	S	ϕ/°	x/Å	y/Å	z/Å
b1	17.3	88.1	1.68	0.12	3.67	b3	31.2	87.7	1.49	0.13	3.36
b2	4.23	88.9	4.62	0.08	3.83	a2	3.64	8.32	1.61	6.35	0.93
a1	−3.03	15.0	1.23	6.28	1.67	p2	−0.04	46.2	4.38	3.31	3.46
p1	0.47	18.8	5.85	6.36	2.16	p3	6.53	21.3	3.10	6.21	2.43
q1	6.14	18.0	0.45	6.16	2.00	q2	−1.38	55.3	5.99	3.04	4.38
ΔS/<S> c	1.21					ΔS/<S> c	1.95
263 K	S	ϕ/°	x/Å	y/Å	z/Å	263 K	S	ϕ/°	x/Å	y/Å	z/Å
b1	15.8	88.3	1.68	0.11	3.72	b3	28.4	88.0	1.50	0.12	3.43
b2	3.66	88.6	4.65	0.10	3.92	a2	3.47	8.38	1.63	6.39	0.94
a1	−3.17	15.4	1.26	6.31	1.74	p2	0.24	46.0	4.42	3.39	3.51
p1	0.62	18.8	5.91	6.41	2.18	p3	5.84	21.6	3.13	6.28	2.48
q1	5.78	17.8	0.42	6.20	1.99	q2	−1.42	55.9	6.05	3.01	4.45
ΔS/<S> c	1.25					ΔS/<S> c	1.97

^a Definition of intermolecular interactions a1, a2, b1, b2, b3, p1, p2, p3, q1, and q2 are depicted in Figure 3c,d. ^b See reference [33]. ^c ΔS/<S> = ||b1 (b3)| − |q1 (q2)||/((|b1 (b3)| + | q1 (q2)|)/2).

. For both Layer 1 and Layer 2, the intradimer overlap integral values (b1 and b3) are the largest, about 3 to 5 times larger than the interdimer overlap integral values (q1 and q2). The values of ∆S/<S>, which indicate the degree of dimerization, are 1.22–1.34 for Layer 1 and 1.66–1.99 for Layer 2, indicating strong electronic dimerization. These dimers are arranged two-dimensionally in the direction of the a and b axes. The overlap integral values between dimers are isotropic for both Layer 1 and Layer 2, forming a two-dimensional conductive layer in terms of electronic structure. Additionally, no significant changes in the overlap integral values were observed between 100 K and 263 K.

The density of states, band dispersion, and Fermi surface of (rac-DMCh-EDT-TTF)₂PF₆ calculated using the tight-binding approximation are shown in Figure 5. The calculated bandwidth (W) and energy gap (E_g) are shown in

Table 5. Bandwidth W (eV) and energy gap (eV).

	PF6 (100 K)/eV	PF6 (263 K)/eV	AsF6 (100 K)/eV	ClO4 (100 K)/eV	ClO4 (263 K)/eV
WU	0.11	0.45	0.11	0.04	0.05
WM	0.33	–	0.32	–	–
WL	0.32	0.34	0.31	0.63	0.59
Eg (WU–WL)	–	0.02	–	0.11	0.07
Eg1 (WU–WM)	0.03	–	0.03	–	–
Eg2 (WM–WL)	0.01	–	0.10	–	–

. The band dispersion or Fermi surface is represented by the red lines for Layer 1 and the blue lines for Layer 2. Since the donor-to-anion ratio is 2:1, the overall band filling is 3/4. At 263 K, there is a gap (E_g) between the upper two and lower two band branches. Therefore, the Fermi level crosses the upper band. Both of the calculated Fermi surfaces of Layer 1 (red) and Layer 2 (blue) were two-dimensional. The upper band had a narrow bandwidth (PF₆ = 0.45 eV, Table 5) and was effectively half-filled, suggesting it behaves as a Mott insulator. This is consistent with the conductivity measurements, which show semiconducting behavior. In contrast, at 100 K, the upper two of the four band dispersions do not intersect, forming a band gap (PF₆ = 0.06 eV, Table 5). Consequently, it is considered to be a band insulator around 100 K. The energy level difference between the HOMOs of molecule 1 and molecule 2 is 0.057 eV at 263 K, while at 100 K, it is 0.075 eV, larger than at 263 K. This difference in the HOMO energy levels leads to the energy level difference in the band dispersion of Layer 1 and Layer 2, causing the band gap at 100 K. From the results of the band calculations, the inflection point observed in the conductivity measurements was concluded to be a semiconductor–semiconductor transition from a Mott insulator to a band insulator. At 263 K, (rac-DMCh-EDT-TTF)₂ClO₄ remains a band insulator, with a band gap (E_g) of 0.07 eV, which is smaller than the 0.11 eV at 100 K (Figure S3, Table 5). Similar to (rac-DMCh-EDT-TTF)₂PF₆, at 263 K, the energy difference between the HOMO levels of molecule 1 and molecule 2 is 0.078 eV, while this difference increases to 0.092 eV at 100 K. This difference in HOMO energy levels generates the energy level difference in the band dispersion between Layer 1 and Layer 2, which is the cause of the larger band gap at 100 K compared to 263 K. Based on the results of the band calculations, the inflection point observed in the electrical conductivity measurements was concluded to be a semiconductor-to-semiconductor transition from one band-insulator state to another.

4. Conclusions

We successfully synthesized a new TTF derivative rac-DMCh-EDT-TTF as a new component molecule for molecular conductors, where two chiral carbon atoms are contained in the 3,4-dimethylcyclohexene (DMCh) moiety. Single crystalline cation radical salts of rac-DMCh-EDT-TTF with octahedral PF₆⁻ and AsF₆⁻ and tetrahedral ClO₄⁻ anions have been prepared by electrocrystallization. They showed semiconducting behavior from room temperature (300 K) with a resistivity anomaly at 250 K for PF₆⁻ and AsF₆⁻ anions and at 180 K for the ClO₄⁻ anion. Their single-crystal structure analyses revealed that they are isostructural with the stoichiometry to be (rac-DMCh-EDT-TTF)₂X (X = PF₆⁻, AsF₆⁻ and ClO₄⁻), implying a +0.5 mean formula charge per one donor molecule. However, these crystals involve two crystallographically independent rac-DMCh-EDT-TTF molecules (molecules 1 and 2), with different molecular charges estimated by bond lengths. Molecules 1 and 2 construct bilayer conducting sheets Layer 1 and Layer 2, respectively; that is, it was found that a charge disproportionation emerged between Layers 1 and 2. Interestingly, there is a distinct difference in the degree of charge disproportionation at 263 and 100 K for PF₆⁻ and ClO₄⁻ salts. It is associated with the conducting behavior showing a resistivity anomaly at 250 K for the PF₆⁻ salt and at 180 K for the ClO₄⁻ salt. To investigate the charge degree of freedom in rac-DMCh-EDT-TTF-based molecular conductors, we have currently prepared cation radical salt with another counter anion. Moreover, we are working on the synthesis of enantiopure TTF derivatives ((S,S)- and (R,R)-DMCh-EDT-TTF) and the preparation of conductive crystals.