Tetrathiapentalene-based organic conductors

Abstract

The synthesis, structure and properties of tetrathiapentalene-based (TTP) organic conductors are reviewed. Among various TTP-type donors, bis-fused tetrathiafulvalene, 2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene (BDT-TTP) and its derivatives afford many metallic radical cation salts stable down to low temperatures, regardless of the size and shape of the counter anions. Most BDT-TTP conductors have a β-type donor arrangement with almost uniform stacks. Introduction of appropriate substituents results in molecular packing that differs from the β-type. A vinylogous TTP, 2-(1,3-dithiol-2-ylidene)-5-(2-ethanediylidene-1,3-dithiole)-1,3,4,6-tetrathiapentalene (DTEDT) has yielded an organic superconductor (DTEDT)₃Au(CN)₂ as well as metallic radical cation salts, regardless of the counter anions. (Thio)pyran analogs of TTP, namely (T)PDT-TTP and its derivatives produce molecular conductors with novel molecular arrangements. A TTP analog with reduced π-electron system 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene (BDA-TTP) has afforded several organic superconductors. Highly conducting molecular metals with unusual oxidation states (+1, +5/3 and neutral) have been developed on the basis of 2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene (BDT-TTP) derivatives and analogous metal derivatives M(dt)₂ (M = Ni, Au).

• molecular conductor
• tetrathiapentalene
• cyclic voltammetry
• x-ray structure analysis
• band calculation

Introduction

Since the publication of the report on the metallic conductivity in a tetrathiafulvalene-tetracyanoquinodimethane (TTF·TCNQ) complex [1] and the subsequent discovery of the first organic superconductor (TMTSF)₂PF₆ (TMTSF=tetramethyltetraselenafulvalene) [2], TTF and its derivatives have played a leading role in the development of new molecular metals and superconductors [3–5]. To explore molecular organic superconductors, it is important to stabilize the metallic state down to low temperatures ( 4.2 K) by enhancing the dimensionality of electrical conductivity. In this way, the Peierls transition, which is observed in conventional one-dimensional (1D) conductors represented by TTF·TCNQ, can be suppressed. Since the discovery of 2D bis(ethelynedithio)-TTF (BEDT-TTF) metals and superconductors, substitution of ‘capped’ alkyl-dichalcogeno groups on TTF has been regarded as the most promising strategy for preparation of 2D metals [6, 7]. However, ‘capped’ alkyl-dichalcogeno groups on π-electron frameworks other than TTF must be introduced. This limits the search for novel conducting components of metals stable down to low temperatures (scheme 1).

Scheme 1 Molecular structures of TTF and TCNQ derivatives in this paper.

In this context, a new π-electron framework, having the ability to produce multidimensional metals without any substituent, is of significant interest. Thus, the introduction of substituent alkyl, benzo and/or methylthio groups, as well as unsubstitution, may also be effective in preparing 2D (or at least quasi-1D) metals for such π-electron systems, whereas the corresponding TTF derivatives usually yield completely 1D conductors. We have noticed a bis-fused TTF, 2,5-bis(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentalene (abbreviated as BDT-TTP or simply TTP) as a promising candidate for the construction of 2D metals. This molecule can also be designed by insertion of the 1,3,4,6-tetrathiapentalene moiety between two 1,3-dithiole rings of TTF. Because BDT-TTP has the so-called ladder-like array of sulfur atoms, as in BEDT-TTF, its cation radical salts are expected to form 2D conducting sheets thanks to the lateral intermolecular interactions based on sulfur–sulfur contacts incorporated in the BDT-TTP framework (scheme 2). This article reviews the synthesis, structure and electrochemical properties of TTP donors, and the structure and properties of molecular conductors based on these TTP donors.

Scheme 2 Lateral interactions in BEDT-TTF and BDT-TTP conductors.

Synthesis, structure and properties of TTP molecules

Synthesis

The first synthesis of BDT-TTP derivatives (1) (scheme 3) was reported in 1980 [8]. Schumaker and co-workers synthesized BDT-TTP derivatives with strongly electron-withdrawing substituents such as cyano, methoxycarbonyl or trifluoromethyl groups by means of a phosphite-mediated cross-coupling reaction between 1,3,4,6-tetrathiapentalene-2,5-dione (2) and corresponding 1,3-dithiole-2-thiones (3) (scheme 3). However, this methodology has not been applied to the synthesis of the other BDT-TTP derivatives, probably because of difficulties in optimizing the reaction conditions for cross-coupling using 2. Furthermore, the details of the structure and properties of the products have not been reported, particularly the crystal structure, redox behavior and formation of conducting salts. The author and co-workers developed a versatile route for the synthesis of BDT-TTP derivatives early in 1990 as shown in scheme 4[9, 10]. First, an unsymmetrical TTF moiety is constructed with protection groups for dithiolate. Then, the fused 1,3-dithiole ring is formed by deprotection of TTF dithiolate and successive reaction with triphosgene. Finally, the trialkylphosphite-mediated cross-coupling reaction between the resultant 1,3-dithiol-2-one fused with TTF (5) and 1,3-dithiole-2-thione derivatives afford the desired BDT-TTP derivatives. Unfortunately, the cross-coupling reaction in the final step does not proceed at all when unsubstituted or alkyl-substituted 3 is used. Thus, the unsubstituted derivative has been synthesized by demethoxycarbonylation of the bis(methoxycarbonyl) derivative using lithium bromide monohydrate in hexamethylphosphoric triamide (HMPA). In contrast, the tetramethyl derivative has not been synthesized thus far in spite of the expectation of as an analog of TMTSF.

Scheme 3 Molecular structures of 1–3.

Scheme 4 Synthesis of BDT-TTP derivatives.

In a similar manner, selenium analogs ST-TTP and BDS-TTP (6) (scheme 5) have been prepared in the cross-coupling reaction by the combination of 1,3-dithiole-2-thiones fused with TTF (or STF) (7) and 1,3-diselenol-2-ones (8) [11]. More than 80 types of BDT-TTP derivatives and selenium analogs have been synthesized by this method to date [9–41]. Yamada et al developed another synthetic strategy for 5 (scheme 6) [42]. One of the TTF frameworks is constructed via the Me₃Al-promoted reaction (Me=methyl) of organotin compound 9 prepared from 2 with ester 1 0 in this procedure. However, P(OEt)₃-mediated cross coupling is still required because of the extremely low solubility of 5 and 11 in common solvents.

Scheme 5 Synthesis of selenium analogs of BDT-TTP.

Scheme 6 Synthesis of 5 and11 via Me₃ Al-promoted reaction.

Structure and electrochemical properties of BDT-TTP derivatives

Figure 1 shows the molecular structure of neutral BDT-TTP determined by x-ray structure analysis [10]. The BDT-TTP molecule adopts a chair conformation, in which the molecule is folded at the four sulfur atoms in the central tetrathiapentalene moiety. The dihedral angle is 27.17° (27.33°) between the two planes composed of S1-S2-C1-C2-S3-S4 (S5-S6-C7-C8-S7-S8) and S3-S4-C5-C6-S5-S6.

Figure 1 Top (a) and side (b) views of the molecular structure of BDT-TTP. (Reprinted with permission from [10].)

Table 1 summarizes the redox potentials of BDT-TTP derivatives measured by cyclic voltammetry. The cyclic voltammograms of most BDT-TTP derivatives exhibit four pairs of redox waves because they have four redox-active 1,3-dithiole rings. The first redox potential of BDT-TTP, E₁=+0.44 V versus saturated calomel electrode (SCE) in benzonitrile, is higher than that of TTF (+0.35 V) by about 0.1 V in spite of the extension of π-conjugation. Instead, the E₁ value of BDT-TTP is comparable to those of 4, 5-bis(methylthio)-TTF (12, 0.44 V) and 2-isopropylidene-1, 3-dithiolo[4,5-d]-TTF (13, 0.45 V) [43] measured under identical conditions. In contrast, the tetrakis(methylthio) derivative (TTM-TTP) has an E₁ value (+0.53 V) similar to the corresponding TTF TTM-TTF (+0.51 V). These results indicate that the positive charge formed by the first oxidation mainly distributes on one TTF moiety, while the other acts as a sulfur-based substituent. The ΔE (=E₂−E₁) values of BDT-TTP (0.18 V) and TTM-TTP (0.19 V) are smaller by 0.24 and 0.08 V compared with those of TTF (0.42 V) and TTM-TTF (0.27 V), respectively. Because such a small ΔE corresponds to a reduction of on-site Coulomb repulsion in the dicationic state of TTP derivatives, TTP conductors may be expected to show high conductivity.

Table 1 Redox potentials^a of BDT-TTP derivatives and the related compounds (scheme 7).

Donor	E1	E2	E3	E4	ΔE(=E2−E1)
BDT-TTP	0.44	0.62	1.05	1.13^a	0.18
TTM-TTP	0.53	0.72	0.99	1.11	0.19
12	0.44	0.77			0.33
13	0.45	0.76	1.14^b		0.31
TTM-TTF	0.51	0.78			0.27

^a versus SCE in benzonitrile containing 0.1 M Bu₄NClO₄.

^b Irreversible step (an anodic peak potential).

Scheme 7 Molecular structures of the donors listed in table 1.

Structures and conducting properties of TTP conductors

Numerous molecular conductors based on TTP donors have been synthesized (see scheme 8). Table 2 summarizes the molecular packing patterns and conducting properties of TTP-type conductors whose crystal structures have been determined. In this section, representative materials based on BDT-TTP derivatives are reported.

Scheme 8 Molecular structures of BDT-TTP derivatives listed in table 2.

Table 2 Molecular packing patterns and electrical properties of conducting salts based on BDT-TTP derivatives (scheme 8); σ_rt refers to room-temperature conductivity.

Material	Donor packing	σrt (S cm^−1)	Conducting behavior^a	Ref
(BDT-TTP)Ax	Uniform β-type	160–400	M′	[44]
(A=ClO4, ReO4, BF4)
(BDT-TTP)2SbF6	β-type	48	M	[45]
(BDT-TTP)6A(TCE)2^b	β-type	83–285	M	[46, 47]
(A=Mo6Cl14, Re6S6Cl8)
(BDT-TTP)7A0.5[Re6S6Cl8]0.5(CH2Cl2)2	β-type	180–800	M	[47]
(A=Mo6Cl14, Re6S6Cl8)
(BDT-TTP)8[Re6S7Cl7](TCE)4^b	β-type	200–1400	M	[47]
(BDT-TTP)2[Re6S5Cl9]		2	S	[47]
(BDT-TTP)6[Ce(NO3)6](C2H5OH)x	β-type	200	M	[48, 49]
(x ≈ 3)
(BDT-TTP)5[M(NO3)5]	β-type	10–500	M	[48, 49]
(M=Nd, Sm, Eu, Gd, Tb, Dy, Ho,
Er, Tm, Yb, Lu)
(BDT-TTP)2I	β-type	206	M	[50]
(BDT-TTP)3I		0.4	S	[50]
(BDT-TTP)3[M^IICl4](EtOH)x		≈5	TMI=30 K	[51]
(M=Co, Mn, Zn,x ≈ 1.0)
(ST-TTP)2AsF6	β-type	200	M′	[11]
(ST-TTP)6Re6S6Cl8 (TCE)2^b	β-type	600	M	[47]
(BDS-TTP)2AsF6	β-type	70	M	[12]
(CH-TTP)(I3)0.31	κ-type	38	M	[52]
(CHEO-TTP)(ReO4)0.38	κ-type	11	M	[25]
(CHET-TS-TTP)(SbF6)0.36	κ-type	–	–	[26]
(DM-TS-TTP)2PF6	β-type	55	M	[23]
(EO-TTP)2AsF6	β-type	600	M	[53]
(CPEO-TTP)(SbF6)0.40	κ-type	60	M	[22]
(EOET-TTP)3AsF6	κ-type	600	M	[14]
(BTM-TTP)2A	β-type	73–120	M	[34, 54]
(A=SbF6, TaF6)
(BTM-TTP)2SbF6	θ-type	0.8	S	[54]
(BTM-TTP)(TCNQ)(TCE)	Uniform 1D stack	200	TMI=100K	[55]
(TMEO-TTP)2Au(CN)2	β″-type	200	M′	[56]
(TMEO-TTP)3SbF6	β′-type	9	S	[57]
(TMEO-TTP)AuBr2(THF)	Dimerized 1D stack	0.12	S	[58]
(TMEO-ST-TTP)Au(CN)2	Dimerized 1D stack	3.8×10^−3	S	[59]
(TMEO-ST-TTP)2A	β-type	5–75	M	[59, 60]
(A=PF6, AsF6, TaF6)
(TMEO-ST-TTP)2ClO4(DCE)^b	β-type	0.07	S	[24]
(TMEO-ST-TTP)(TCNQ)(PhCl)	D-A stack	5.3×10^−5	S	[24]
(SMEO-TTP)Au(CN)2	Dimerized 1D stack	5.2×10^−4	S	[27]
(SMEO-ST-TTP)2PF6	β-type	4.6	M20 K	[27]
(BEDT-TTP)2I3	β-type	900	M	[61]
(EP-TTP)2Au(CN)2	β-type	400	M	[62]
(TMET-TTP)Ax	θ-type	9–45	TMI=200 K	[63]
(A=AuI2, PF6, ClO4, ReO4, IO4,x=
0.13–0.34)
(TMES-TTP)4I3	θ-type	13	S	[19]
(TMET-TS-TTP)2TCNQ	θ-type	1–10	TMI=120 K	[20]
(SMET-ST-TTP)(AsF6)0.32	θ-type	6	S	[35]
(SMES-BDS-TTP)(AsF6)0.35	θ-type	11	S	[35]
(TMET-DSDTP)A0.30	θ-type	13–45	S	[36]
(A=PF6, AsF6)
(TMET-DTDSP)Ax	θ-type	10–16	S	[36]
(A=PF6, AsF6,x=0.30–0.33)
(C2TET-TTP)2ClO4	Uniform β-type	12–55	M	[28]
(C2TET-TS-TTP)2A	Uniform β-type	15–55	M	[29]
(A = PF6, ClO4, BF4)
(C2TMT-TTP)2PF6	Uniform β-type	500	M	[37]
(C1TMT-TTP)2PF6	Uniform β-type	500	M	[37]
(C4TET-TTP)I3	Dimerized non-	2.5×10^−2	S	[28]
	columnar structure)
(TTM-TTP)2I3	Trimerized stack	0.03	S	[64]
(TTM-TTP)I3	Uniform β-type	700	TMI=160 K	[64]
(TTM-TTP)(I3)5/3	Uniform 1D stack	200	TMI=20 K	[54]
(TTM-TTP)AuX2(X=Br, I)	Dimerized 1D stack	10–40	S	[66]
(TTM-TTP)A(PhCl)0.5	Dimerized 1D stack	0.01–0.07	S	[67, 68]
(A=FeCl4, FeBr4, GaCl4, GaBr4)
(TTM-TTP)FeBr1.8Cl2.2	Uniform 1D stack	1000	TMI =160 K	[67]
(TTM-TTP)(PF6)0.267(THF)0.6	Pentamerized stack	3×10^−3	S	[69]
(TSM-TTP)(I3)5/3	Uniform 1D stack	200	TMI=20 K	[21]
(TSM-TTP)3(I3)2	Trimerized stack	3×10^−2	S	[21]
(TTM-DSDTP)I3	Trimerized stack	2.3	S	[38]
(TTM-DSDTP)GaCl4	Uniform 1D stack	280	TMI=60K	[38]
(DTM-DSM-TS-TTP)(I3)5/3	Uniform 1D stack	520	TMI=25K	[38]
(TTM-BDS-TTP)(I3)5/3	Uniform 1D stack	630	M	[38]
(CPTM-TTP)4A	λ-type	7–110	M’ (A=PF6, AsF6)	[17]
(A=PF6, AsF6, SbF6)			TMI=50 K
			(A=SbF6)
(ChETTM-TTP)2A	β-type	20–150	TMI=100–150K	[18, 70]
(A=AuBr2, Au(CN)2, GaCl4)
(ChETTM-TTP)ReO4	β-type	8×10^−3	S	[70]
(Et2BEDT-TTP)2HgI3	Pseudo λ-type	2	S	[30]
(TTP-PROXYL)MCl4 (M=Ga, Fe)		1×10^−3	S	[71]
(CP-TTP-PROXYL)2AsF6		1	S	[40]

^a M: metallic down to low temperature (4.2 K), M′: increase of resistivity at low temperature, but no metal–insulator transition, S: semiconductor, T_MI: metal-to-insulator transition temperature.

^b TCE = 1,1,2-trichloroethane, DCE=1, 2-dichloroethane, cn=1-chloronaphthalene.

(BDT-TTP)A_x and (ST-TTP)₂A

Several radical cation salts with various anions have been synthesized on the basis of BDT-TTP and its selenium analog ST-TTP. Most of these salts exhibit high room-temperature electrical conductivity σ_rt=1–1000 S cm⁻¹ and show metal-like conducting behavior down to low temperatures (0.6–4.2 K) regardless of the size and shape of the counter anions [11, 12, 44–51]. Figure 2 shows the crystal structure of (BDT-TTP)₂SbF₆. BDT-TTP molecules form 2D conducting sheets, each of which is divided by an insulating layer composed of the anions, as expected. The packing pattern of the donors is classified as the β-type in BEDT-TTF conductors (figure 3). The donors form a face-to-face stack with interplanar distances of 3.46 and 3.47 Å. The overlap mode of the donor molecules in the stack is the so-called ring-over-bond type. The slip distance is 1.6 Å along the long molecular axis in the stack. A similar molecular arrangement has also been found in most molecular conductors based on BDT-TTP and its selenium analogs ST-TTP and BDS-TTP with various anions (ClO₄⁻, BF₄⁻, ReO₄⁻, PF₆⁻, AsF₆⁻, SbF₆⁻, Re₆S₆Cl₈²⁻, Mo₆Cl₁₄²⁻, etc.). Therefore, BDT-TTP is regarded as having the strong self-aggregating property of β-type molecular packing with little dimerization. This contrasts to the behavior of the array of donors in BEDT-TTF conductors, which strongly depends on both the shape and the size of the counter anions.

Figure 2 Crystal structure of (BDT-TTP)₂SbF₆ where dotted lines represent intermolecular S···S contacts. (Reprinted with permission from [45].)

Figure 3 Donor sheet structure of (BDT-TTP)₂SbF₆. The intermolecular overlap integrals are a1=25.1, a2=25.3, c=0.8, p1=7.9 and p2=8.6× 10⁻³. (Reprinted with permission from [45].)

The overlap integrals of BDT-TTP salts were calculated by a molecular orbital method based on the extended Hückel approximation. The calculated intrastack overlap integrals between the highest occupied molecular orbitals (HOMOs) of (BDT-TTP)₂SbF₆ are almost identical (a1=25.1, a2=25.3× 10⁻³), which is consistent with the interplanar distances and the slip distances along the long molecular axis. Thus, the donors form an almost uniform stack in this salt, while β-BEDT-TTF salts have a strongly dimerized stack. On the other hand, the calculated lateral overlaps (S_l) are relatively large compared with the stacking ones (S_s). The ratio of the largest intrastack overlap to the largest lateral one (S_s/S_l) is 2.9, which corresponds to an intermediate value between a typical 2D metal, β-(BEDT-TTF)₂I₃ (S_s/S_l=1.9) [72] and a typical quasi-1D metal, (TMTTF)₂Br (S_s/S_l=6.6) [73]. Thanks to such relatively large interstack interactions, the calculated Fermi surface is closed in the conducting layer (figure 4(a)). On the other hand, the Fermi surface, as deduced from the experimental reflection spectrum, is open in the k_c direction, as shown in figure 4(b) [74]. When the theoretical calculation is performed by excluding 3d orbitals of sulfur atoms, the shape of the calculated Fermi surface approaches that of the Fermi surface obtained from the reflection spectrum. However, the overlap integrals are underestimated by about 30%.

Figure 4 Fermi surface of (BDT-TTP)₂SbF₆ calculated from the transfer integrals obtained (a) theoretically and (b) from optical experiments.

Three exceptional molecular packing patterns that differ from uniform β-type packing have been identified for BDT-TTP conductors. (BDT-TTP)₂Cu(NCS)₂ adopts θ-type arrangement of the donors, and exhibits semiconducting temperature dependence of resistivity. Yakushi and co-workers reported that this salt shows a phase transition accompanied with a change in the activation energy at 250 K, below which the donors are in a charge-ordering state as is often observed in θ-phase conductors [75]. Kobayashi and co-workers reported that (BDT-TTP)₃I has a unique 3D donor packing with six crystallographically independent donor molecules, as shown in figure 5 [50]. This salt shows semiconducting behavior with a small activation energy of 0.014 eV due to charge disproportionation. Kobayashi and co-workers also developed BDT-TTP conductors with divalent anions [M^IICl₄]²⁻ (M=Co, Mn, Zn) [51]. Figure 6 shows the crystal structure of (BDT-TTP)₃[M^IICl₄](EtOH)_x (x ≈ 1.0). Three independent halves of BDT-TTP molecules (A, B, C) and half of a [M^IICl₄]²⁻ anion are crystallographically independent. Comparison of the bond lengths indicates that the formal charge of molecule C is nearly 0 and those of molecules A and B are +1. Donors A and B form a column along the c-axis with a ring-over-bond overlap mode. In contrast, donor C forms a unique 1D ‘BDT-TTP tape’ along the c-axis and the molecular plane of donor C is approximately perpendicular to the planes of donors A and B. These salts exhibit a relatively high conductivity of σ_rt ≈ 5 S cm⁻¹ and weak metal like behavior down to about 150 K. At low temperature, the resistivity gradually increases with decreasing temperature, however, the conductivity is high even at 30 K. Magnetic susceptibility measurement reveals temperature-independent metal-like behavior in the [Zn^IICl₄]²⁻ salt.

Figure 5 Crystal structure of (BDT-TTP)₃I. (Reprinted with permission from [50].)

Figure 6 Donor-sheet structure of (BDT-TTP)₃[M^IICl₄](EtOH)_x (M=Co, Mn, Zn, x≈1.0). (Reprinted with permission from [51].)

κ-Type salts based on TTP donors

Molecular packing that differs from the β-type can be realized if appropriate substituents are introduced into the BDT-TTP framework. Construction of a κ-type structure is of significant interest because many κ-type radical cation salts based on TTF derivatives show superconducting transitions [4, 5, 7]. For TTF systems, the introduction of capped alkyl-dichalcogeno groups is indispensable for constructing κ-type conductors. The roles of the capped alkyl-dichalcogeno groups are as follows: (i) steric hindrance to avoid formation of stacking structure revealing in the formation of dimers and (ii) realization of 2D electronic structure through sulfur–sulfur lateral interaction. In fact, all TTF donors affording κ-type salts have one or two ethylenedithio, methylendithio or ethylenedioxy groups. In contrast, a ‘capped’ alkyl-dichalcogeno group is not needed for the BDT-TTP system, because it has enough sulfur atoms in the π-electron framework to realize lateral interactions. Thanks to the ability to form a 2D conduction layer, the introduction of a non-planar substituent such as a fused cyclohexene ring may be possible to construct a κ-type structure. In fact, 2-(4,5-cyclohexeno-1,3-dithiol-2-ylidene)-5-(1,3-dithiol-2-ylidene)-1,3,4,6-tetrathiapentanlene (CH-TTP) has afforded a κ-type salt with an I₃⁻ anion (figure 7) [52]. This salt exhibits high conductivity of σ_rt=38 S cm⁻¹ and metal-like temperature dependence down to 1.4 K. Superconductivity has not been observed in spite of the κ-type molecular packing, probably because the upper band of this salt is not half-filled due to a nearly 3 : 1 donor : acceptor composition. Whereas all the κ-type superconductors discovered thus far have a 2:1 composition and a half-filled upper band as a result. The other BDT-TTP derivatives with a fused cyclohexene ring, CHEO-TTP [25] and CHET-TS-TTP [26], have also yielded κ-type conductors with ReO₄⁻ or SbF₆⁻ anion, both of which retain metallic conductivity down to 4.2 K.

Figure 7 Donor-sheet structure of (CH-TTP)(I₃)_0.31. The intermolecular overlap integrals are c=−4.32, p=22.2, r=7.61 and b=12.0× 10⁻³. (Reprinted with permission from [52].)

In this regard, BDT-TTP derivatives fused with other six-membered rings are of interest to elucidate a correlation between molecular structure and the molecular array in the TTP conductors. BDT-TTP fused with dehydro-1,4-dioxane, in other words, ethylenedioxy-substituted BDT-TTP (EO-TTP), seems to afford the κ-type salts. However, x-ray structure analysis of (EO-TTP)₂AsF₆ revealed a β-type packing pattern with little dimerized stacking as observed in (BDT-TTP)₂A [53]. The torsion angle around the ethylene bridge in (EO-TTP)₂AsF₆ is 37°, which is about two-thirds of that in (CH-TTP)(I₃)_0.31 (57°). Considering a smaller dihedral angle, as well as fewer hydrogen atoms in the dehydrodioxane ring (four hydrogen atoms) than in the cyclohexene rings (eight hydrogen atoms), steric hindrance between the donors in (EO-TTP)₂AsF₆ should be smaller than that in (CH-TTP)(I₃)_0.31. Smaller steric repulsion may favor the formation of the stacking β-type structure. A tight-binding band calculation indicates that this salt has a closed Fermi surface similar to (BDT-TTP)₂SbF₆, although the 1D character is still strong owing to large overlaps in the stacking direction. EO-TTP salts with tetrahedral and octahedral anions including (EO-TTP)₂AsF₆ exhibit very high conductivity of σ_rt=380–1300 S cm⁻¹ with a simple metallic temperature dependence down to 1.5–4.2 K. On the other hand, CPEO-TTP [22] and EOET-TTP [14], which have the additional substituent of an annelated cyclopentene ring or ethylenedithio group, have yielded κ-type salts with octahedral anions.

BTM-TTP conductors

The methylthio group seems to be more sterically hindered than the annelated cyclohexane group, being expected to afford κ-type or other type of packing of the donors. Bis(methylthio)-substituted TTP (BTM-TTP) has afforded two types of radical cation salts (needles and plates) with octahedral anions. x-ray structure analysis revealed that needle-like crystals of (BTM-TTP)₂A (A=SbF₆, TaF₆) have the so-called β-type array of donors [34, 54]. BTM-TTP molecules form a nearly uniform column in spite of the presence of bulky methylthio groups. Namely, both the interplanar distances and the slip distances are almost the same (1.6 Å) along the donor long axis. Calculation of the overlap integrals indicates that the donors are slightly dimerized in the stack. On the other hand, the interstack overlap integrals are 14–30% larger than the intrastack ones. A tight-binding calculation suggests that the calculated Fermi surface is quasi-1D.

The plate-like crystal of (BTM-TTP)₂SbF₆ adopts the so-called θ-type array (figure 8(a)) [54]. The donors are uniformly stacked with an interplanar distance of 3.44 Å. The donors slip along both the short and long axes of the donor (figure 8(b)), in contrast to θ-BEDT-TTF salts, in which only slipping along the short axis is observed. The dihedral angle of the donors between the neighboring columns is large (130.7°), indicating smaller overlap integrals than those of the θ-salts with smaller dihedral angles. The calculated bandwidth (0.57 eV) is about half of that in β-(BTM-TTP)₂SbF₆. The calculated Fermi surface is closed, as is characteristic of a 2D metal.

Figure 8 (a) Conducting sheet structure of θ-(BTM-TTP)₂SbF₆ viewed along the donor long axis. The intermolecular overlaps between HOMOs are a=7.7, p=6.5 and q=3.6× 10⁻³. (b) The overlap mode of the donors in the stack. (Reprinted with permission from [54].)

The β-type salts exhibit relatively high conductivity of σ_rt=30–120 S cm⁻¹ and metallic temperature dependence down to 5 K. In contrast, θ-(BTM-TTP)₂SbF₆ exhibits relatively low conductivity of σ_rt=0.8 S cm⁻¹ and semiconducting temperature dependence with an activation energy of 0.13 eV, probably because of narrow bandwidth and resultant large electron correlation in the quarter-filled band system.

TMEO-TTP and TMEO-ST-TTP salts

The BDT-TTP derivative with ethylenedioxy and two methylthio groups TMEO-TTP affords many metallic radical cation salts with octahedral and linear anions down to low temperatures (0.6–4.2 K), whereas the salts with tetrahedral anions are semiconductors [14]. Among the metallic salts, the crystal structure of only (TMEO-TTP)₂Au(CN)₂ has been determined [56]. This salt has the so-called β″-type array of donors similar to (BEDT-TTF)₂ClO₄(TCE) [6] (see figure 9). It shows a resistivity maximum around 180 K (figure 10), while the other metallic TMEO-TTP salts exhibit simple metallic temperature dependence. The thermoelectric power and electron spin resonance (ESR) spectrum of this salt also exhibit anomalies around this temperature, indicating a metal-to-metal transition (see also figure 10).

Figure 9 Donor-sheet structure of (TMEO-TTP)₂Au(CN)₂. The intermolecular overlap integrals are c=4.2, a1=18.3, a2=7.3, p1=−11.5 and p2=−9.7× 10⁻³. (Reprinted with permission from [56].)

Figure 10 Electrical properties of (TMEO-TTP)₂Au(CN)₂. (Reprinted with permission from [56].)

On the other hand, a selenium analog TMEO-ST-TTP forms a 1 : 1 salt with an Au(CN)₂⁻ anion [59]. The Au(CN)₂⁻ anion is located in a cavity surrounded by four donor molecules in the ac-plane (see figure 11). Although the TMEO-ST-TTP molecules seem at first glance to form a 2D network in the ac-plane, there is no significant lateral interaction along either a- or c-axis. The donors are strongly dimerized in the column, because one of the overlaps makes a large slip along both the long and short axes of the donor, as shown in figure 12. Thus, a tight-binding band calculation indicates that this is a 1D band insulator. In contrast to the Au(CN)₂⁻ anion, the octahedral anions (PF₆⁻, AsF₆⁻ and TaF₆⁻) have afforded conducting materials that retain metallic conductivity down to 4.2 K [59, 60]. X-ray structure analysis revealed that these materials have β-type donor packing with the composition of (TMEO-ST-TTP)₂A. In these salts, the overlap mode of the donors in the stack is the ring-over-bond type (figure 13). The slip distances along the long molecular axis are about half of the 1,3-dithiole ring for p1 (1.6–1.7 Å), and one-and-a-half for p2 (4.9–5.0 Å). As a result, the donors are electronically dimerized along the stacking direction, although the interplanar distances are almost the same (3.50 and 3.52 Å for the PF₆⁻ salt). The ratio of the calculated overlap integrals (p1/p2) is 2.3 for the PF₆⁻ salt. A tight-binding band calculation suggests that the upper and lower bands are completely separated, which corresponds to strong dimerization along the stack, and results in the formation of an effectively half-filled band. The interstack overlaps are 10–20% of the larger intrastack overlap. Thus, the calculated Fermi surface is closed along the stacking direction, but open along the interstack direction as is observed in a quasi-1D system. However, the interstack interactions are sufficiently strong to suppress the metal-to-insulator transition at low temperature.

Figure 11 Crystal structure of (TMEO-ST-TTP)Au(CN)₂. (Reprinted with permission from [59].)

Figure 12 Overlap modes of donor molecules in (TMEO-ST-TTP)Au(CN)₂. (Reprinted with permission from [59].)

Figure 13 Overlap modes of donor molecules in (TMEO-ST-TTP)₂AsF₆. (Reprinted with permission from [60].)

(BEDT-TTP)₂I₃ and (EP-TTP)₂Au(CN)₂

The title materials have the donor packing close to β-(BEDT-TTF)₂I₃ [61, 62]. However, the degree of dimerization in the stack is much smaller than for BEDT-TTF salts, because the longer π-electron framework of BDT-TTP is less sensitive to overlap integral than TTF systems. A tight-binding band calculation of (BEDT-TTP)₂I₃ suggests the existence of a 2D Fermi surface as shown in figure 14, which is also supported by angle-dependent magnetoresistance [76]. On the other hand, the calculated Fermi surface of (EP-TTP)₂Au(CN)₂ is quasi-1D, because the lateral interaction is smaller than that of (BEDT-TTP)₂I₃ owing to steric hindrance by the projected propylenedithio group along the lateral direction. Both the salts exhibit high conductivity of σ_rt=600–900 S cm⁻¹ and retain metallic conductivity down to 0.5–0.6 K. The temperature dependence of resistivity approximately obeys the T² law. Such simple metallic behavior indicates that the metallic states of these salts are significantly stabilized. As a result, they show no superconductivity in spite of being isostructural with the ambient-pressure superconductor β-(BEDT-TTF)₂I₃.

Figure 14 The Fermi surface of (BEDT-TTP)₂I₃.

Conducting materials based on TMET-TTP and its analogs

A BDT-TTP derivative with both methylthio and ethylenedithio groups TMET-TTP and its selenium analogs have afforded θ-type salts regardless of the counter anions [19, 20, 35, 37, 63]. It is noteworthy that even TCNQ has yielded the θ-type conductors [20]. In these materials, the donors make pseudo-stacks, in which they slip along both the short and long axes of the donor as observed for θ-(BTM-TTP)₂SbF₆ [54]. The dihedral angles between molecular planes of adjacent stacks are as large as 118–128°. Most of the θ-type salts based on TMET-TTP-type donors have uniform stacks, whereas the donors form dimerized stacks in (TMES-TTP)₄I₃. The calculated Fermi surface is 2D and is similar to that of θ-type BEDT-TTF salts. These θ-type salts show a metal-to-insulator transition at low temperatures or exhibit semiconducting behavior at and below room temperature. Destabilization of the metallic state in TMET-TTP conductors is probably due to the narrow bandwidth and resultant strong electron correlation derived from large dihedral angles between molecular planes, as pointed out in θ-BEDT-TTF salts by H Mori et al [77].

The BDT-TTP derivatives with long alkyl chains have been investigated by T Mori and co-workers to change the ratio of donor to anion in TMET-TTP salts, most of which have the composition (TMET-TTP)₄A. Among them, the crystal structures of (C₂TET-TTP)₂ClO₄, (C₂TET-TS-TTP)₂A (A=PF₆, ClO₄, BF₄) and (C₄TET-TTP)I₃ have been determined [28, 29]. The radical cation salts based on C₂TET-TTP and C₂TET-TS-TTP are isostructural. They have β-type donor packing contrary to the initial expectation of the θ-type array, although the ratio of the donor to anion (2 : 1) has indeed changed from 4 : 1. Surprisingly, the donors form a uniform stack in spite of the presence of sterically hindered ethylthio groups, whereas most donors with large substituents are dimerized to avoid steric hindrance as is observed in CH-TTP (κ-type) and TMEO-ST-TTP (dimerized β-type) salts, with the exception of TMET-TTP (θ-type) and BTM-TTP (both β- and θ-types). In the C₂TET-TTP salts (figure 15), large steric hindrance in the stack derived from the ethyl chain is reduced by a larger slip along the donor long axis (4.7 Å) than that of (BDT-TTP)₂ClO₄ (1.6 Å), which also has a uniform β-type donor array. Whereas the donors form a pseudo stack so that steric hindrance is avoidable by a slip along the donor short axis in the θ-type conductors. All the C₂TET-(TS)-TTP salts with tetrahedral and octahedral anions exhibit metallic conductivity down to 1.5 K owing to the uniform stacks of a relatively large bandwidth. On the other hand, two donor molecules and an anion are alternatively stacked in (C₄TET-TTP)I₃, in which the donors are stacked in a head-to-tail manner (figure 16). The dimer is surrounded by four I₃⁻ anions in the ac-plane, and the interdimer overlaps p1 and p2 are only −0.6 and 0.03×10⁻³, respectively. Therefore, the bandwidth of this material is expected to be very small. It shows low conductivity of 0.025 S cm⁻¹ at room temperature, and has a large activation energy of 0.23 V, which corresponds with a small bandwidth and a 1 : 1 composition.

Figure 15 Donor sheet structure of (C₂TET-TTP)₂ClO₄. The intermolecular overlap integrals are b=−20.2, p1=7.0, p2=2.1, p3=−0.9 and p4=9.6× 10⁻³. (Reprinted with permission from [28].)

Figure 16 Donor sheet structure of (C₄TET-TTP)I₃. The intermolecular overlap integrals are c=−16.6, p1=−0.6 and p2=0.03× 10⁻³. (Reprinted with permission from [28].)

(TTM-TTP)A_x

Tetrakis(thiomethyl) derivative TTM-TTP has afforded three types of I₃⁻ salts: (TTM-TTP)₂I₃, (TTM-TTP)I₃ and (TTM-TTP)(I₃)_5/3 [64, 65]. Among them, (TTM-TTP)₂I₃ is a low-conductive (σ_rt=0.03 S cm⁻¹) semiconductor with a trimerized donor stack [64]. The crystal includes a neutral donor molecule in the anion layer. (TTM-TTP)I₃ exhibits very high conductivity of σ_rt=700 S cm⁻¹, in spite of the fully oxidized +1 state of the donor. It shows metal-like temperature dependence down to T_MI=160 K (figure 17) [64]. TTM-TTP molecules form uniform columns with a relatively short interplanar distance of 3.45 Å (figure 18). The arrangement of the donors may be classified as β-type. However, effective lateral interaction is inhibited, because the methylthio groups protrude from the long molecular axis. A tight-binding band calculation suggests that (TTM-TTP)I₃ has a large bandwidth of W=1 eV. The nature of the insulating state of (TTM-TTP)I₃ has been investigated, but has not get been elucidated [76–81]. On the other hand, (TTM-TTP)(I₃)_5/3 also has uniform donor stacks similar to (TTM-TTP)I₃, and it exhibits metallic conductivity down to T_MI=20 K (see also figure 17). The donor molecule in this material has an anomalously large positive charge of +5/3. The metallic conductive behavior in spite of large positive charges of +1 and +5/3 in TTM-TTP conductors is due to the small on-site Coulomb energy as well as the large bandwidth derived from uniform stacks [65]. The AuI₂⁻ and MX₄⁻ (M=Fe, Ga,X=Cl, Br) anions also yield 1:1 salts; however, anions that are shorter than I₃⁻ make dimerized columns [66–68]. As a result, they exhibit the semiconducting behavior characteristic of band insulators. In contrast, (TTM-TTP)FeBr_1.8Cl_2.2 exhibits metallic behavior similar to (TTM-TTP)I₃, because it occasionally forms uniform donor columns [68].

Figure 17 Temperature dependence of resistivity of (TTM-TTP)I₃ and (TTM-TTP)(I₃)_5/3.

Figure 18 Stacking structure of (TTM-TTP)I₃. (Reprinted with permission from [64].)

(CPTM-TTP)₄A(A = PF₆, AsF₆, SbF₆)

The title materials are isostructural and form molecular packing similar to β-type salts [17]. However, they have a tetramerized column in which two crystallographically independent donors A and B are stacked as AABB as shown in figure 19. Therefore, the donor sheet structure of these salts is closer to that of λ–(BETS)₂GaCl₄, an ambient-pressure superconductor with T_c=8 K [82], rather than to that of β-type BEDT-TTF salts. The donor molecules are stacked in a head-to-tail manner in A–A (p1) and B–B (p3), and in a head-to-head manner in A–B (p2). The interplanar distances are 3.58, 3.61 and 3.40 Å for p1, p2 and p3, respectively (for the AsF₆⁻ salt). All the overlap modes are the ring-over-bond type: however, the slip distance along the long molecular axis (D) of p3 (4.73 Å) is much larger than those of p1 (1.55 Å) and p2 (1.68 Å). Such a large D value of p3 does not affect the overlap integral very much in this case, because BDT-TTP is about twice as long as TTF. Thus, the overlap of p3 (18.1× 10⁻³ for the AsF₆⁻ salt) is only about 20% smaller than those of the other intrastack interactions. On the other hand, there are relatively large interstack interactions (q2 and q4). As a result, the Fermi surface calculated by the tight-binding method is a typical eclipse characteristic of 2D metals. Among these salts, the PF₆⁻ and AsF₆⁻ salts exhibited metallic conductivity down to 4.2 K, although maximum resistivity was observed around 80 K. In contrast, an increase in resistivity below 50 K has been observed for the SbF₆⁻ salt. Although the low-temperature behavior of the SbF₆⁻ salt is not clear, it is plausible that the largest anion of SbF₆⁻ among those three salts induces a strong electron correlation.

Figure 19 Overlap modes of donor molecules in (CPTM-TTP)₄AsF₆.

TTP analogs including non-TTF moiety

Various TTP donors possessing non-TTF moieties have been synthesized (see scheme 9) [83–125]. Molecular packing patterns and electrical properties of molecular conductors based on these TTP analogs are summarized in table 3.

Table 3 Molecular packing patterns and electrical properties of conducting salts based on TTP donors containing non-TTF moieties (scheme 10).

Material	Donor packing	σrt (S cm^−1)	Conducting behavior^a	Ref
(DTEDT)3Au(CN)2	Uniform β-type	15	SC, Tc=4 K	[83, 84]
(DSEDS)3TaF6	Uniform β-type	9	M	[86]
(ET-PDT)4PF6(cn)^b	λ-type	50	M′	[89]
(TM-TPDS)2AsF6	Windmill type	240	TMI=100 K	[92]
(SM-PDT)PF6(PhCl)x	Pseudo β-type	4.8	S	[93]
(SM-PDT)TCNQ(PhCl)	D-A stack	3.9×10^−3	S	[126]
(BDH-TTP)2PF6	κ-type	102	M	[97]
(BDH-TTP)4[Fe(NO)(CN)5]	κ-type		M	[127]
(BDH-TTP)Br0.88	θ-type	98	M	[128]
(BDA-TTP)2A	β-type	1.5–3.8	SC, Tc=4.4–5.9 K	[98]
(A=PF6, AsF6, SbF6)
(BDA-TTP)2A	β-type	9.4–53	SC, Tc=2–8 K	[129–131]
(A=GaCl4, FeCl4, I3)
(DHDA-TTP)2A	β-type	8.1–39	M	[99]
(A=PF6, AsF6)
(DHDA-TTP)4AuI2	κ-type	7.4	M	[99]
(EDDH-TTP)2AuI2	κ-type	14	M	[96]
(EDDH-TTP)3	κ-type	100	M	[132]
[Cr(phen)(NCS)4]·2CH2Cl2^b
(C2TDA-TTP)2PF6	Uniform β-type	200	M	[37]
(C2TDA-TTP)2DMTCNQ	DDAA stack	< 10^−6	S	[37]
(MeDTET)3PF6(TCE)x	κ-type	440	M	[134]
(CPDTET)Ax	κ-type	4.2–27	M (A=AsF6)	[135]
(A=AsF6, SbF6,x=0.72, 0.88)			TMI=200K
			(A=SbF6)
(MeDTDM)2AsF6	β-type	94	M  20 K	[101]
(CPDT-STF)(TCNQ)	β-type	40	M	[102]
(MeDH-TTP)2AsF6	κ-type		TMI=165–205 K	[103]
(MeDA-TTP)2A	β-type	1.1–5.6	TMI=165–205 K	[104]
(A = PF6, AsF6, SbF6)
(MeDA-TTP)2ClO4	β-type	1.2	S	[104]
Ni(ptdt)2	3D	7	S	[138]
Ni(tmdt)2	3D, β′-type	400	M	[120]
Au(tmdt)2	3D	3	S	[139]
Zn(tmdt)2	3D	0.05	S	[140]
Cu(dmdt)2	3D, β″-type	15	S	[141]
Co(dmdt)2	Dimeric structure	0.012	S	[142]
Pd(dt)2				[143]
Co(dt)2	Dimeric structure	19		[144]
Pd(C3-tdt)2		10–^2		[121]

^a M: metallic down to low temperature (4.2 K), M′: increase of resistivity at low temperature, but no metal-to-insulator transition, S: semiconductor, T_MI: metal-to-insulator transition temperature, SC: superconductor, T_c: superconducting transition temperature.

^b cn=1-chloronaphthalene, phen = 1, 10-phenanthroline.

Scheme 9 Molecular structures of analogous TTP derivatives.

Scheme 10 Molecular structures of analogous TTP derivatives listed in table 3.

Radical cation salts based on vinylogous TTP donors, DTEDT and DSEDS

The BDT-TTP molecule has a stripe-like structure, and all sulfur atoms have the same phase in the HOMO (figure 20). On the contrary, vinylogous TTP donor 2-(1,3-dithiol-2-ylidene)-5-(2-ethanediylidene-1,3-dithiole)-1,3,4,6-tetrathiapentalene (DTEDT) has a crooked structure, and the π-orbitals of sulfur atoms at the terminal of the vinylogous 1,3-dithiole ring in the HOMO are out of phase with those of the other sulfur atoms (see also figure 20). Thus, DTEDT conductors are expected to show both sterically and electronically less effective lateral interactions than BDT-TTP salts. However, similar to BDT-TTP, DTEDT has afforded many metallic radical cation salts regardless of the size and shape of the counter anions. Furthermore, (DTEDT)₃Au(CN)₂ has exhibited a superconducting transition at 4 K under ambient pressure (figure 21) [83, 84]. The donors form conducting sheets parallel to the ac-plane as shown in figure 22. The array of the donors is close to that of most BDT-TTP salts of the β-type with an almost uniform stack (figure 23). In contrast to most β-type salts based on unsymmetrical donors, DTEDT molecules are arranged in such a way that the stacking and lateral directions are parallel in (DTEDT)₃Au(CN)₂. Because DTEDT has a crooked structure and the phase of the sulfur atoms in the HOMO is reversed at the terminal vinylogous 1,3-dithiole ring, such a parallel array results in the most effective intermolecular interaction both sterically and electronically. On the other hand, a selenium analog of DTEDT, DSEDS, has afforded a metallic TaF₆⁻ salt, in which the array of donors is identical to the average structure of (DTEDT)₃Au(CN)₂ [86]. Therefore, DTEDT may have a strong tendency to form the β-type donor arrangement similar to BDT-TTP. A tight-binding band calculation suggests that (DTEDT)₃Au(CN)₂ has a closed Fermi surface, although the one-dimensional character along the stacking direction is stronger because the stacks are almost uniform.

Figure 20 HOMO of DTEDT (left) and BDT-TTP (right).

Figure 21 Temperature dependence of resistivity of (DTEDT)₃Au(CN)₂. (Reprinted with permission from [83].)

Figure 22 Crystal structure of (DTEDT)₃Au(CN)₂. (Reprinted with permission from [83].)

Figure 23 Donor-sheet structure of (DTEDT)₃Au(CN)₂. The intermolecular overlap integrals are c1=−19.2, c2=−19.9, c3=−10.8, a1=−1.5, a2=−1.3, a3=−0.7, p1=−3.8, p2=−3.4 and p3=−6.9× 10⁻³.

Radical cation salts based on (thio)pyran analogs

Three-dimensional (3D) organic conductors are of significant interest from the viewpoint of the search for molecular superconductors with higher critical temperatures (T_c), because M₃C₆₀ (M=alkali metal) in which C₆₀ molecules are arranged in a 3D fashion show superconductivity with the highest T_c ∼ 40 K among molecular materials [144]. It has also been pointed out that an increase of 3D character through an insulating anion layer results in higher T_c in BEDT-TTF superconductors [145, 146]. However, a 3D Fermi surface has not yet been observed in solids based exclusively on organic molecules, but it has been theoretically suggested by a tight-binding band calculation in a metal dithiolene system, Me_xH_4−xN[Pd(dmise)₂]₂ (x=2, 3) [147]. TTP donors containing the pyran-4-ylidene moiety, 2-(1,3-dithiol-2-ylidene)-5-(pyran-4-ylidene)-1,3,4,6-tetrathiapentalene (PDT-TTP) [89], and its thiopyran analog (TPDT-TTP) [90], are potentially interesting for the preparation of 3D conductors, because a significant intermolecular overlap along the long molecular axis is expected through chalcogen atoms in the (thio)pyran ring, in addition to the π–π stacking and lateral chalcogen–chalcogen interactions.

The ethylenedithio derivative of PDT-TTP (ET-PDT) has yielded a 4 : 1 salt with the PF₆⁻ anion (ET-PDT)₄PF₆(cn) (cn=1-chloronaphthalene) [89]. The ET-PDT molecules form conducting sheets, each being divided by insulating layers composed of the anions and solvent molecules. The array of the donors is classified as the so-called λ-type (figure 24). Thus, two crystallographically independent ET-PDT molecules A and B form a face-to-face stack with a four-folded period AABB similar to (CPTM-TTP)₄A (A=PF₆, AsF₆, SbF₆) [17]. The molecules are stacked in a head-to-tail manner for molecules AA′ and BB′ and in a head-to-head manner for AB (figure 25). This salt has an almost uniform stack from the viewpoint of interplanar distances (3.60–3.62 Å). However, the head-to-tail overlap between the unsymmetrical π-electron frameworks of PDT-TTP prevents effective intrastack overlaps, because there is little consistency in the phases of the molecular coefficients between the outer 1,3-dithiole ring and the pyran ring. The calculated overlap integrals of the head-to-tail stack, p1 (8.6× 10⁻³) and p3 (4.9× 10⁻³), are only about 30–50% of the head-to-head overlap p2 (−16.4× 10⁻³), indicating that the ET-PDT molecules are strongly tetramerized along the stacking direction. Such difference in the manner of overlap does not affect the overlap integrals in the (CPTM-TTP)₄A (19.7–21.3× 10⁻³ for A=PF₆), which has a symmetrical π-electron framework of BDT-TTP. On the other hand, relatively large interstack overlaps a1 (4.4× 10⁻³) and c2 (−4.5× 10⁻³) are coupled with the largest intrastack overlap integral p2 (figure 26). Thus, the overlap along the c-direction is much larger than that along the a-direction. As a result, the calculated Fermi surface is open along k_a.

Figure 24 Donor-sheet structure of (ET-PDT)₄PF₆ (cn) (Reprinted with permission from [89].)

Figure 25 Overlap modes of the donor molecules in (ET-PDT)₄PF₆(cn). (Reprinted with permission from [89].)

Figure 26 A schematic drawing of overlaps between the donor molecules in (ET-PDT)₄PF₆(cn); bars and broken lines denote the donor molecules projected along the long molecular axis and relatively large intra- and interstack interactions, respectively.

Radical cation salts of bis(methylthio)-derivative TM-TPDS have quite a different donor packing from that of (ET-PDT)₄PF₆(cn). The crystal structure of (TM-TPDS)₂AsF₆ is shown in figure 27. This salt does not form a 2D conducting sheet in contrast to the typical TTP conductors. Thus, the TM-TPDS molecules are arranged in a ‘windmill’ manner in the bc-plane, whereas they form a conduction column along the a-axis [92]. The anions are located in a cavity formed in the center of the windmill and are surrounded by hydrogen atoms of the methylthio groups and of the thiopyran rings in the donors. The unsymmetrical TM-TPDS molecules are stacked in a head-to-tail manner along the a-axis and are slightly dimerized. Several sulfur–sulfur contacts are shorter than the sum of van der Waals radii between the central tetrathiapentalene moiety and the thiopyran ring or methylthio groups. The calculated overlap integrals between the columns are as large as ∼10% of the intracolumnar overlaps, thanks to the relatively large molecular orbital coefficient of the sulfur atom in the thiopyran ring and to the short sulfur–sulfur contacts. However, the calculated Fermi surface is closed only along the k_a direction because intrastack interactions are still dominant in this material. (TM-TPDS)₂AsF₆ has high conductivity of σ_rt=240 S cm⁻¹ and metal-like conducting behavior down to 100 K. At lower temperatures, the resistivity gradually increased. This metal-to-semiconductor transition should not be a Peierls transition derived from highly 1D character, because no obvious decrease of magnetic susceptibility has been observed below T_MI.

Figure 27 Crystal structure of (TM-TPDS)₂AsF₆ projected onto the bc-plane. Dotted lines show the intermolecular S···S contacts.

Exchange of sulfur (or oxygen) atoms with selenium atoms on tetrathiapentalene, methylthio and (thio)pyran-4-ylidene moieties is expected to form a windmill structure with a higher 3D character. For this reason, bis(methylseleno) derivatives were investigated. SM-PDT has yielded single crystals of PF₆⁻ and AsF₆⁻ salts suitable for x-ray structure analysis and electrical conductivity measurement [93]. Unexpectedly, these salts adopt a fully oxidized state with a composition of (SM-PDT)A(PhCl)_x (A=PF₆ and AsF₆), in which the arrangement of donors is quite different from that of (TM-TPDS)₂AsF₆. Thus, the donor columns are completely divided by the counter anions and by disordered solvent molecules along the a-axis (figure 28), indicating the absence of lateral interaction. Instead, the donor columns are bridged by short selenium–selenium contacts within the sum of the van der Waals radii to form a 2D sheet-like structure. The Se···Se contacts between selenomethyl groups may be more preferable in this salt than the edge-to-side interactions through the tetrathiapentalene moiety, as observed in (TM-TPDS)₂AsF₆. The donors are dimerized in the column with slip distances along the donor long axis of 1.4 and 4.7 Å. On the other hand, the interstack overlap integrals through the selenomethyl groups are ∼1/4 of the intrastack ones (figure 29), and they are significantly larger than the longitudinal interaction. Calculation suggests that this salt is a band insulator because of its fully oxidized state and dimerized stack. However, it has relatively high conductivity of σ_rt=4.8 S cm⁻¹. Considering the high conductivity (in spite of being a 1 : 1 salt) and the relatively large edge-to-edge interaction, the 2 : 1 salts with the same donor array are expected to exhibit metallic conductivity down to low temperature.

Figure 28 Crystal structure of (SM-PDT)PF₆(PhCl)_x viewed along the c-axis. Disordered solvent molecules are located in the cavity.

Figure 29 Donor column structure viewed along the a-axis and definition of the overlap integrals of (SM-PDT)PF₆(PhCl)_x. The intermolecular overlap integrals are c1=6.3, c2=8.1, p1=−2.3, p2=0.00 and b=−0.8× 10⁻³. (Reprinted with permission from [93].)

TTP donors with reduced π-electron system

TTP-type donors have yielded many molecular metals stable down to low temperatures, but only one superconductor. This is probably due to excessive stabilization of the metallic state by the large bandwidth as well as reduced on-site Coulomb repulsion [148].

Yamada et al have developed molecular conductors based on BDH-TTP and 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene (BDA-TTP) [97, 98]¹ whose π-electron systems have been reduced by the exchange of double bonds in the outer 1,3-dithiole rings of BDT-TTP with a saturated ethylene or propylene bridge. Cyclic voltammetry revealed that the ΔE value becomes larger than that in BDT-TTP, suggesting an increase of the on-site Coulomb repulsion by reduction of the π-electron system. Among the conducting materials based on BDH-TTP and BDA-TTP, most BDH-TTP conductors are metallic down to low temperatures. X-ray structure analysis of (BDH-TTP)₂PF₆ reveals that the donors adopt the κ-type arrangement [97]. In this material, intradimer overlap is much smaller than those of κ-type BEDT-TTF superconductors due to steric hindrance at the terminal sp³ carbons. In contrast, interdimer interaction should be larger than in BEDT-TTF salts, because all the rings are five-membered. As a result, the split of the HOMO bands characteristic of κ-type conductors should not occur in this system. Thus, the metallic state of this material is stabilized considerably by the realization of a quarter-filled band instead of the half-filled one as observed in κ-type BEDT-TTF superconductors [5]. On the other hand, BDA-TTP conductors have a strong tendency to take β-type molecular packing. In these materials, intrastack interaction is smaller compared with that of typical β-type conductors and the degree of dimerization is very large, because the six-membered rings at the terminal of BDA-TTP largely protrude from the molecular plane as shown in figure 30. It is noteworthy that BDA-TTP affords several superconductors with various anions of octahedral (PF₆⁻, AsF₆⁻, SbF₆⁻), tetrahedral (FeCl₄⁻, GaCl₄⁻) and linear (I₃⁻) shapes (figure 31) [98, 129–131].

Figure 30 Molecular structure of BDA-TTP in (BDA-TTP)₂SbF₆; the dihedral angles φ1 and φ2 are 47.9 and 33.2°, respectively. (Reprinted with permission from [98].)

Figure 31 Temperature dependence of normalized resistivity in BDA-TTP superconductors. (Reprinted with permission from [98].)

The author and co-workers have developed many reduced TTP derivatives, in which one of the 1,3-dithiole rings is removed [43, 100–103]. An ethylenedithio derivative, MeDTET yields many metallic radical cation salts stable down to 0.6–1.5 K regardless of the counter anions. Among metallic MeDTET salts, x-ray structure analysis of the PF₆⁻ salt revealed that this salt has the (MeDTET)₃PF₆(TCE)_x composition as well as κ-type packing of donors [134]. No observation of superconductivity, even though it is a κ-type salt, is probably owing to reduced on-site Coulomb repulsion derived from a partially filled band rather than a half-filled band. A cyclopentane analog CPDTET also yields κ-type salts, (CPDT-TTF)₂(AsF₆)_0.72 and (CPDT-TTF)₂(SbF₆)_0.88 [135], in which two donor molecules are strongly dimerized with each other in a ‘head-to-head’ manner (figure 32). However, the transport properties are very different between these two salts. (CPDT-TTF)₂(AsF₆)_0.72 exhibits metallic temperature dependence down to 4.2 K. In contrast, (CPDT-TTF)₂(SbF₆)_0.88 shows a metal-to-semiconductor transition at 200 K, and it retains metallic behavior down to low temperatures under a high pressure of 5.6 kbar. The different transport properties are probably due to different occupancies of the anion sites. The band structure of the SbF₆⁻ salt is very close to a half-filled band, leading to a metal–semiconductor transition due to strong electron correlation. In contrast, the band filling of the AsF₆⁻ salt is far from half-filled because there are more anion defects than in the SbF₆^- salt, which hampers the semiconducting transition. On the other hand, a selenium analog CPDT-STF forms a TCNQ complex with 2D β-type donor packing. (CPDT-STF)(TCNQ) exhibits metallic conductivity down to low temperatures [102]. A dimethyl-substituted derivative, MeDTDM, yields β-type salts. They show metallic conductivity down to 20 K, below which the resistivity increases [101].

Figure 32 Overlap mode in a dimer of (CPDT-TTF)₂(AsF₆)_0.72.

Nishikawa et al investigated TTP analogs, MeDH-TTP and MeDA-TTP, with more reduced π-conjugation [103, 104]. MeDA-TTP salts with octahedral and tetrahedral anions are isostructural and have β-type donor packing similar to that in BDA-TTP superconductors [104]. They exhibit metallic behavior down to 165–205 K, whereas the ClO₄⁻ salt is a semiconductor with a small activation energy of 0.028 eV. (MeDH-TTP)₂AsF₆ crystallizes in a κ-type structure [103]. A tight-binding calculation suggests the dimerization gap in this salt is much smaller than in typical κ-type BEDT-TTF conductors, leading to an almost 3/4-filled band (figure 33). This salt shows semiconducting behavior below room temperature with a small activation energy of 0.032 eV. ESR measurements suggest an antiferromagnetic transition at 40 K [149]. A detailed study of the dependence of resistivity on T-P revealed simple metallic behavior above P_c=2.4 GPa [150], which is much larger than the critical pressure for a typical Mott transition in κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl (P_c=0.03 GPa) [151] and is similar to that observed in the charge-ordered insulator (DIDCNQ)₂Ag (P_c=2.2 GPa) [152]. Considering this salt has a quarter-filled band and a large P_c, the most plausible origin of the insulating state for (MeDH-TTP)₂AsF₆ is the charge-ordering state [150].

Figure 33 Band structure and Fermi surface of (MeDH-TTP)₂AsF₆. (Reprinted with permission from [149].)

Higher homologs of TTP

The higher homologs of TTP donors are also of interest, not only as a promising component for molecular metals but also as a candidate for molecular electronics. A tris-fused TTF, 2,2′-bi(1″,3″-dithiol-2″-ylidene-1,3,4,6-tetrathiapentalene-5-ylidene) (BDT-TTPY, scheme 8), has been synthesized by the P(OEt)₃-mediated coupling reaction of 1,3-dithiol-2-one fused with TTFs (5) [107, 108]. The cyclic voltammograms of BDT-TTPY derivatives measured in benzonitrile-carbon disulfide are composed of three pairs of redox waves, each corresponding to a two-electron transfer [107]. The solubility of BDT-TTPYs in common organic solvents is extremely low compared with the corresponding BDT-TTP derivatives. Therefore, the usual electrochemical oxidation technique cannot be adopted for the preparation of conducting materials. Instead, chemical doping of the BDT-TTPY suspension with appropriate oxidants such as iodine or NO₂X (X=BF₄, SbF₆) has been carried out. Among the conducting materials obtained, several iodine complexes have high conductivity of σ_rt=0.7–16 S cm⁻¹ on compressed pellet samples [107, 108], and several have very small activation energies of 0.017–0.033 eV. Therefore, these materials are expected to exhibit metallic conductivity if conductivity measurements are performed on single crystals. A crystal of TTC₃-TTPY doped with iodine vapor exhibits metal-like conductivity down to 1.3 K, as shown in figure 34 [110].

Figure 34 Temperature dependence of resistivity of TTC₃-TTPY after 7 min iodine doping followed by annealing at 75 °C. (Reprinted with permission from [110].)

Analogous TTPY donors containing a π-electron framework other than TTF have also been synthesized. In particular, extended TTPYs, in which an aromatic ring, such as benzene, thiophene or furan, is inserted into the central TTF moiety, exhibit higher solubility in common solvents [112]. As a result, CT complexes have been formed by the usual mixing technique in organic solvent. The TCNQF₄ complexes based on 14–16 exhibit relatively high conductivity of σ_rt=10⁻¹–10⁰ S cm⁻¹ on compressed pellets. Higher homologues of TTPY possessing up to seven donor units have also been synthesized thanks to increased solubility using a thiophene-inserted TTF unit [113].

Analogous metal complexes of BDT-TTPY, M (dt)_>2: highly conducting single-component molecular conductors

The derivatives of metal bis(TTF-dithiolato) (M(dt)₂), where dt is 2-(1,3-dithiol-2-ylidene)-1,3-dithiole-4,5-dithiolate, designed by exchange of the central double bond of BDT-TTPY with a metal ion, have received significant attention as candidates for single-component molecular metals, because both the donor (TTF) and acceptor (metal bis(dithiolato)) moieties are present in the molecule. Schumaker and Engler have reported, the synthesis of metal complexes with TTF-dithiolato ligands M(dt)₂ as analogs of BDT-TTPY [8]. Unfortunately, most anionic M(dt)₂ complexes are readily oxidized to insoluble neutral complexes due to the presence of strongly electron-donating TTF moieties. The first synthesis and characterization of anionic M(dt)₂ complexes have been reported independently by the groups of Underhill (M=Hg) and Matsubayashi (M=Au) [114, 115]. Several M(dt)₂ derivatives have been prepared since then: however, the synthesis of single crystals of neutral complexes remains difficult.

Kobayashi and co-workers have succeeded in growing single crystals of various M(dt)₂ derivatives using an electrocrystallization technique and have studied their structures and physical properties [119–123, 138–141]². Among them, Ni(tmdt)₂ exhibits very high conductivity of σ_rt=400 S cm⁻¹ on a single crystal. The temperature dependence of the resistivity and magnetic susceptibility reveals metallic conducting behavior down to 0.6 K [120]. Figure 35 shows the crystal structure of Ni(tmdt)₂. The packing pattern of the molecules viewed along the long molecular axis resembles the so-called β′-type as observed in BEDT-TTF conductors. On the other hand, half of the molecules are overlapped in the stack: namely, one molecule is stacked to extend over two molecules. A tight-binding band calculation, assuming a HOMO-LUMO (lowest unoccupied molecular orbital) gap of 0.1 eV, suggests that this material has a 3D Fermi surface. This result is consistent with the Fermi surface experimentally obtained by measuring the de Haas–van Alphen effect [151]. A gold complex, Au(tmdt)₂, is isostructural with Ni(tmdt)₂. The ESR, nuclear magnetic resonance, and magnetic susceptibility measurements revealed an antiferromagnetic transition at 110 K in this complex (figure 36) [139]. Electrical conductivity of single-crystal Au(tmdt)₂ revealed metallic behavior down to low temperatures, indicating coexistence of itinerant electrons and localized magnetically ordered spins. Thus, Au(tmdt)₂ is a new antiferromagnetic molecular metal with a critical temperature T_N above 100 K.

Figure 35 Crystal structure of Ni(tmdt)₂ viewed along the molecular (a) short and (b) long axis. (Reprinted with permission from ref. [137].)

Figure 36 Resistivities and susceptibilities of Au(tmdt)₂: (a), resistivity of compacted pellet (ρ_rt=0.067Ω cm); (b), spin susceptibility χ_s by ESR; (c)–(e), magnetic susceptibilities by SQUID at 30 kOe (blue, <300 K), 10 kOe (red, <200 K) and 3 kOe (green, <200 K). (Reprinted with permission from [139].)

Summary and outlook

In summary, BDT-TTP derivatives and their analogs have yielded numerous radical cation salts and charge-transfer complexes that retains metallic conductivity down to low temperatures (4.2 K). Over 100 TTP-type molecular metals stable down to liquid helium temperatures have been obtained thus far. Stabilization of the metallic state in TTP conductors is mainly achieved by increasing dimensionality, thanks to lateral interaction through chalcogen atoms in the tetrathiapentalene moiety. It is noteworthy that a TTP vinylog, DTEDT, has yielded many molecular conductors exhibiting metallic conductivity down to 4.2 K regardless of the counter anions, although its half-unit of vinylogous TTF itself has never yielded any metallic material down to low temperatures. Furthermore, highly conducting metals with unusual oxidization states (+1, +5/3 and neutral) have also been developed, thanks to the long molecular structure with large π-electron conjugation of TTP-type molecules. The fusion of two (or more) donor units, in other words, the insertion of the tetrathiapentalene moiety into π-electron donors, is a reliable guiding principle to design stable molecular metals. However, the metallic state is too stabilized to cause superconducting transitions in many cases, with the exception of BDA-TTP systems. Furthermore, various chemical modifications of TTP molecules are important to control the bandwidth, band-filling and on-site Coulomb repulsion in their conducting materials and to develop new superconductors with higher T_c and novel molecular metals with multifunctionality, such as magnetic molecular metals or photoinduced highly conducting materials.

Acknowledgments

The author is grateful to Dr T Shirahata and Professor H Nishikawa for valuable discussions. This work was partially supported by Grants-in-Aid for Scientific Research (Nos. 15073216, 18GS0208 and 20110006) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, from the Japan Society for the Promotion of Science, and from the Ehime University COE incubation program.

Notes

For a overview of the reduced TTP conductors see [136].

For a overview of molecular conductors based on M(dt)₂ derivatives see [137].