Improved stability of a metallic state in benzothienobenzothiophene-based molecular conductors: an effective increase of dimensionality with hydrogen bonds

Article abstract of DOI:10.1039/c7cc00784a

A dihydroxy-substituted benzothienobenzothiophene, BTBT(OH)₂, was synthesized, and its charge-transfer (CT) salt, β-[BTBT(OH)₂]₂ClO₄, was successfully obtained. Thanks to the introduced hydroxy groups, a hydrogen-bonded chain structure connecting the BTBT molecules and counter anions was formed in the CT salt, which effectively increases the dimensionality of the electronic structure and consequently leads to a stable metallic state.

Full text of DOI:10.1039/c7cc00784a

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Improved stability of a metallic state in
benzothienobenzothiophene-based molecular conductors: An
effective increase of dimensionality with hydrogen bonds
Received 00th January 20xx,
Accepted 00th January 20xx
Toshiki Higashino,* Akira Ueda,* Junya Yoshida, and Hatsumi Mori*
DOI: 10.1039/x0xx00000x
www.rsc.org/
A dihydroxy-substituted benzothienobenzothiophene, BTBT(OH)2, exploration of such new classes of molecular conductors.6
was synthesized, and its charge-transfer (CT) salt, β- These BTBT salts, however, change to an insulator at low
[BTBT(OH)2]2ClO4, was successfully obtained. Thanks to the temperatures, due to an abrupt resistivity jump resulting from
introduced hydroxy groups, a hydrogen-bonded chain structure 1D nature of the crystal/electronic structures.5 Unfortunately,
connecting the BTBT molecules and counter anions was formed in this 1D instability has not completely been suppressed by
the CT salt, which effectively increases the dimensionality of the substituting the sulphur atoms with selenium ones with a
electronic structure and consequently leads to a stable metallic larger atomic orbital.7 Therefore, another kind of chemical
state.
modification is required to realize a stable metallic state in the
BTBT-based molecular conductors.
In the field of functional molecular materials, increasing the
dimensionality of the electronic structures has been an
important issue for realization not only of stable metallic or
superconducting states,1 but also of next generation electronic
Hydrogen bonds (H-bonds) are frequently used as a
powerful tool to control and regulate the crystal structures of
molecule-based functional materials.8–12 In some TTF-based
conductors, the H-bonding interactions also enabled to control
the CT states and conducting properties, in addition to the
materials/devices.2
(TTF)(TCNQ)
A
charge-transfer (CT) complex
tetrathiafulvalene, TCNQ:
molecular arrangements.9–11
Moreover, our group has
(TTF:
tetracyanoquinodimethane) is known as the first organic metal,
however, its one-dimensional (1D) character causes a metal-
insulator transition at 53 K.3 This so-called 1D instability has
successfully been overcome by the chemical modification of
the TTF skeleton, such as the introduction of additional
chalcogen substituents, leading to stable metallic states and
even superconducting states.4
recently demonstrated that catechol-fused TTF derivatives
(Cat-TTF, Chart 1) provide several kinds of unique H-bonded
molecular conductors by taking advantage of strong H-bonding
ability of the catechol moiety.12
From these aspects, in this study, we have designed a new
13
BTBT molecule with a catechol substructure, BTBT(OH)2
(Chart 1), and consequently succeeded in the synthesis of its
CT salt β-[BTBT(OH)2]2ClO4. Thanks to the two hydroxy groups
introduced at the 2,3-positions, a H-bonded chain structure
connecting the BTBT molecules and counter anions was
formed in the CT salt, which effectively increases the
Recently, as a new class of molecular conductor without
the TTF skeleton,
a
CT salt (BTBT)2PF6 (BTBT:
[1]benzothieno[3,2-b][1]benzothiophene, Chart 1) and its
analogues were developed.5 Interestingly, in spite of lesser
sulphur atoms in the π-conjugated skeleton, these BTBT-based
Previous work
CT salts exhibit metallic behaviour with a significantly high
room temperature conductivity (σr.t. ~ 4100 S cm−1) and also
large thermoelectric power factor with a large bandwidth.5b
These results have reminded us of the importance of the
TTF
CatꢀTTF
strong
Hꢀbonding ability
This work
The Institute for Solid State Physics, The University of Tokyo, Kashiwanoha,
Kashiwa, Chiba 277-8581, Japan.
10
9
8
1
2
3
E-mail: thigashino@issp.u-tokyo.ac.jp, a-ueda@issp.u-tokyo.ac.jp, hmori@issp.u-
tokyo.ac.jp
† Electronic Supplementary Information (ESI) available: Additional information for
synthesis, electrochemical measurement, theoretical calculation, X-ray analysis,
bond length analysis, and resistivity measurement. CCDC 1529671-1529675
7
6
5
4
BTBT
BTBT(OH)2
contain the supplementary crystallographic information for 5a, 5b, BTBT(OMe)2,
[BTBT(OH)2]2·H2O, and β-[BTBT(OH)2]2ClO4. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/x0xx00000x
Chart 1 Molecular design of Cat-TTF12 and BTBT(OH)2 from TTF
and BTBT
.
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molecule in the general position and a half of the ClO4 anion
located on the two-fold axis are crystallographically
independent (Fig.1a). Since the unit cell contains eight BTBT
molecules and four anions, the donor:anion stoichiometry is
deduced to be 2:1. This means that the BTBT molecule is in a
partially oxidized state with a +0.5e charge, as also supported
by the bond length analysis (Table S2). Therefore, BTBT(OH)2
is expected to form a 3/4-filled band structure (discussed
below). As shown in Fig. 1b, the BTBT molecule is almost
planar and forms a head-to-tail-type uniform stack along the b-
axis with the interplanar spacing of 3.326 Å (corresponding to
the half-length of the b-axis). It is noteworthy that two kinds
of [O−H···O]-type H-bonding interactions were observed
between the hydroxy groups of the donor and the ClO4 anion
(O1−O4 and O2−O3 in Fig. 1c); the O···O distances are 2.809(7)
Å (O1−O4) and 2.783(5) Å (O2−O3), which are much shorter
than the sum of the van der Waals radii of oxygen atoms (3.04
Å). Consequently, an infinite 1D H-bond-chain structure is
formed along the c-axis (Fig. 1c).12a In this arrangement, very
weak C−H···S interactions (H···S distances: ~ 3.05 Å, green lines
in Fig. 1c) are also found in the side-by-side direction of the
i
iv
1
4
v
iii
ii
2
3
5a
+
vii
BTBT(OH)2
viii
vi
5b
BTBT(OMe)2
ClO4
electrocrystallization
2
βꢀ[BTBT(OH)2]2ClO4
Scheme 1 Synthesis of BTBT(OH)2 and
β-[BTBT(OH)2]2ClO4.
Reagents and conditions: (i) H5IO6, I2, MeOH, reflux, 9 h, 95%; (ii)
PdCl2(PPh3)2, CuI, Et3N, TMSꢀacethylene, THF, reflux, 2 h, 96%; (iii)
K2CO3, MeOH, CH2Cl2, r.t., 3h, 92%; (iv) PdCl2(PPh3)2, CuI, Et3N,
THF, reflux, 9 h, 83%; (v)
THF, 78 °C ꢀ r.t., overnight, 14% for 5a, 15% for 5b; (vi) Cu,
250 °C, 20 min, 84%; (vii) BBr3, CH2Cl2, 0 °C ꢀ r.t., 6 h, 92%, (viii)
ꢀBu4N ClO4, CH2Cl2, r.t., 2 ꢁA, 48 h.
tꢀBuLi, sulphur powder, K3[Fe(CN)6],
−
donor molecule.
As a result of these intermolecular
n
·
interactions, BTBT(OH)2 produces a sheet-type molecular
arrangement (Fig. 1d). In this sheet, BTBT(OH)2 stacks in a
“ring-over-bond” manner (represented as the black and gray
molecules in Fig. 1a and 1d) along the b axis, leading to a so-
called β-type molecular arrangement19 (thus termed β-
(BTBT(OH)2)2ClO4). The transfer integrals, which correspond
to intermolecular interactions between the neighbouring
molecules,20 are largest in the stacking direction b (91.0 meV)
and relatively smaller in the diagonal directions p (–2.77 meV),
q (–13.6 meV), and r (–13.1 meV), as shown in the caption of
dimensionality of the electronic structure and consequently
leads to a stable metallic state
.
The synthetic route for the donor molecule BTBT(OH)2 is
depicted in Scheme 1 (see Supplementary Information for
details). The asymmetric unit
conventional Sonogashira cross-coupling reaction of the
iodobenzene derivative with the phenyl acetylene derivative
. Treatment of with t-BuLi followed by sulphur powder and
4 was prepared by the
1
3
4
potassium hexacyanoferrate(III) afforded the thieno[3,2-
Fig. 1d.
Also, there are no meaningful donor-donor
c][1,2]dithiin compounds as an isomeric mixture of 5a and 5b
interactions between the sheets, due to the presence of the
anion layer.
in
dechalcogenation of the mixture of 5a and 5b provided a
single product BTBT(OMe)2 in 84% yield. Finally,
moderate
yields.14,15
The
copper-mediated
(a)
(b)
c
demethylation of the methoxy groups with boron tribromide
gave the target compound BTBT(OH)2 in 92% yield.
Surprisingly, such BTBT derivatives functionalized at the 2,3-
positions are unknown,16 although a lot of BTBT derivatives
have been designed and synthesized so far.17 Therefore, our
present synthetic strategy will be effective to explore a new
class of functionalized BTBT derivatives.
3.326 Å
3.326 Å
b
o
a
a
o
(c)
(d)
o
c
a
In cyclic voltammetry, BTBT(OH)2 showed one irreversible
oxidation wave (Fig. S1 in Supplementary Information). The
onset oxidation potential (Eonset) is 0.88 V (vs. SCE), which is
lower than that of the unsubstituted BTBT (1.18 V). This
means that electron-donating ability is significantly enhanced
by the introduction of the hydroxy groups into the BTBT
backbone, as suggested by DFT calculations.18
b
O1
O3
O2
O4
O4
O2
r
O3
q
b
p
O1
c
O1———O4: 2.807(8) Å
O2———O3: 2.780(6) Å
o
a
The electrocrystallization of BTBT(OH)2 in the presence of
tetra-n-butylammonium perchlorate gave needle-like black
Fig. 1 Crystal structures of
β-[BTBT(OH)2]2ClO4, viewed along (a)
the axis (the black and gray molecules: overlap mode), (b) the
b
c
crystals of the ClO4 salt, namely β-[BTBT(OH)2]2ClO4
.
The
axis, (c) the stacking direction (blue broken line: O−H···O Hꢀbonding
patterns, green broken line: C−H···S interactions), and (d) the
molecular long axis (ClO4 anions are omitted for clarity). Transfer
crystallographic data are listed in Table S1, and the crystal
structure is depicted in Fig. 1. This salt crystalizes in a
monoclinic system, space group C2/c, in which one BTBT
integrals:
b = 91.0, p = −2.77, q = −13.6, r = −13.1 meV.
2 | J. Name., 2012, 00, 1-3
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substantial interstack interactions (q, r ≈ 13 meV), in addition
to the strong intrastack one (b = 91 meV), as described before.
Thus, the intrastack/interstack anisotropy is significantly
decreased from 60 (= 87/1.4) in (BTBT)2PF6 to 7 (= 91/13) in β-
(BTBT)2PF6
βꢀ[BTBT(OH)2]2ClO4
Conducting column
Conducting layer
(a)
(c)
(BTBT(OH)2)2ClO4
. A similar decrease is observed in the 1D
and Q1D TTF conductors, that is, 100 for (TTF)(TCNQ) and 10
for (TMTSF)2PF6, respectively.23 These results clearly indicate
that the dimensionality of the electronic structure is
successfully enhanced by the introduction of the H-bonding
interactions.
C−H———F Hꢀbonds
O−H———O Hꢀbonds
Q1D metal
(b)
(d)
0.2
The increase of the dimensionality of the electronic
structure significantly influenced the electrical conducting
properties. Figure 3 shows temperature dependence of the
electrical resistivity in β-(BTBT(OH)2)2ClO4, together with that
in (BTBT)2PF6, where the values are normalized with that at
room temperature (ρr.t.). The crystal of β-(BTBT(OH)2)2ClO4
shows a relatively low room-temperature electrical resistivity
(ρr.t. ≈ 5.5 × 10−3 Ω cm), which decreases with decreasing
temperature down to 135 K (the inset of Fig. 3). This
temperature dependence indicates that this salt is metallic
above 135 K, and more importantly, this metallic state is more
stable than that of (BTBT)2PF6. This is because this salt does
not show an abrupt resistivity jump, as seen in (BTBT)2PF6 at
150 K.5 Therefore, we have proved that the increase of the
dimensionality caused by the H-bond interactions brings about
the stabilization of metallic state in BTBT-based conductors.
On further cooling, this salt finally undergoes a metal-
insulator-like transition around 60 K, after entering the
semiconducting state at 135 K. A similar transition without
hysteresis has also been observed in (BTBT)2PF6 at around 50
K.5 Investigation on the low-temperature insulating state as
well as the realization of a more stabilized metallic state are
currently underway.
kb
Y
A
Z
EF
kc
1D metal
Γ
–0.2
Γ
Y
A
Γ
ZV
Γ
Fig. 2 (a, c) Molecular arrangements and (b, d) electronic band
structures in (BTBT)2PF65 and
-[BTBT(OH)2]2ClO4, respectively.
β
The effect of the H-bond interactions in the crystal of β-
(BTBT(OH)2)2ClO4 is further disclosed by comparing the crystal
structures of β-(BTBT(OH)2)2ClO4 and the parent salt
(BTBT)2PF6. As shown in Fig. 2a, the BTBT molecules in
(BTBT)2PF6 form windmill-type columnar structures with
effective
π-π interactions. There are, however, no effective
interactions between the columns, due to the existence of
C−H···F contacts between the donor molecule and the PF6
anion.5 As a result, the columnar arrangement produces a
typical 1D electronic structure with a flat Fermi surface (Fig.
2b).21 On the other hand, the
π-stacking columns in the
present salt β-(BTBT(OH)2)2ClO4 (Fig. 1b) are connected with
the O−H···O H-bonding interactions through the ClO4 anions
(Fig. 2c). A resultant Fermi surface is warped in the 3/4-filled
band structure (Fig. 2d), which means the formation of a quasi-
In conclusion, we have successfully synthesized a new
benzothienobenzothiophene (BTBT) derivative with 2,3-
dihydroxy groups, BTBT(OH)2, and realized a stable metallic
one-dimensional (Q1D) electronic structure in β-
22
(BTBT(OH)2)2ClO4
.
This enhancement of the electronic
structure from 1D to Q1D is also evidenced by comparing the
anisotropy of the transfer integrals. (BTBT)2PF6 has a strong
state in
a
quasi-one-dimensional charge-transfer salt β-
Strong H-bonding ability of the catechol-
(BTBT(OH)2)2ClO4
.
interaction within the π-stacking column (87 meV), however,
type hydroxy groups has played a crucial role in the formation
of an infinite one-dimensional H-bonded chain structure,
which leads to the increase of the dimensionality of the
electronic structure and the stable metallic state. These
results demonstrate that functionalized BTBT derivatives are
promising electron donors in molecular conductors. We
believe that this study will pave a new way to design and
develop high-dimensional BTBT-based materials/devices24 with
interesting conducting properties (e.g. superconductivity and
high carrier mobility).
the interstack interaction is negligibly small (≈1.4 meV).5 On
the other hand, the present β-(BTBT(OH)2)2ClO4 has the
106
105
1.00
ρ
104
0.90
ρ
0.80
103
(BTBT)2PF6
100 150 200 250 300
102
ρ
T (K)
ρ
101
100
The authors gratefully acknowledge Prof. T. Mori, Tokyo
Institute of Technology, and Dr. T. Kadoya, University of Hyogo,
for the fruitful discussions. This work was partially supported
10ꢀ1
βꢀ[BTBT(OH)2]2ClO4
10ꢀ2
0
50
100 150 200 250 300
by
a Grant-in-Aid for Research Activity Start-up (No.
T (K)
15H06135) and Grants-in-Aid for Scientific Research (Nos.
15H00988, 16K05744, 24340074, 26610096, 16H04010) from
Japan Society for the Promotion of Science (JSPS), Japan, and
the Canon Foundation.
Fig. 3 Temperature dependence of the electrical resistivity for β-
[BTBT(OH)2]2ClO4 (black) and (BTBT)2PF6 (gray)5, measured along
the molecular stacking direction. The values are normalized with that
at room temperature (ρr.t.). Inset: magnified data in the metallic
region of -[BTBT(OH)2]2ClO4
β
.
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is
2,3-dihydroxy-[1]benzothieno[3,2-
4
5
7
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15 We found that the dithiin isomers can be isolated by recrystallization.
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structure analysis (See Supplementary Information).
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18 Taking the SCE energy level to be −4.44 eV from the vacuum level,
,
the HOMO energy level is estimated to be −5.32 eV for BTBT(OH)2
and −5.62 eV for BTBT. The theoretical calculation approximately
reproduces these values (−5.35 eV for BTBT(OH)2 and −5.58 eV for
BTBT, respectively, see Fig. S2).
7
8
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