Synthesis, crystal structure, and charge-transfer complexes of TTF derivatives having two imidazole hydrogen-bonding units

Article abstract of DOI:10.1016/j.physb.2009.10.022

New hydrogen-bond functionalized tetrathiafulvalene (TTF) derivatives (1 and 2) having two imidazole moieties, which are attached to TTF at the 2- and 4-positions in the imidazole ring, respectively, were synthesized. Electrochemical measurements indicated that the introduction of imidazole moieties at 2- or 4-positions slightly reduced or enhanced the electron-donating ability of TTF. In the crystal structure of 1, N-H···N hydrogen-bonds of the imidazole ring formed a two-dimensional sheet, and further π-stacks on the TTF skeleton built up a one-dimensional column. Mixing 1 with electron-acceptors afforded fully ionic charge-transfer complexes having a 1:1 donor-acceptor ratio, while complex formation of 2 with tetracyanoquinodimethane and p-chloranil yielded partial charge-transfer complexes showing semiconducting behaviors (room temperature conductivities = 10^-3-10^-2Scm^-1).

Full text of DOI:10.1016/j.physb.2009.10.022

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Physica B 405 (2010) S41–S44
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Physica B
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Synthesis, crystal structure, and charge-transfer complexes of TTF derivatives
having two imidazole hydrogen-bonding units
Ã
Tsuyoshi Murata a, Yasushi Morita a, , Yumi Yakiyama a, Kazuhiro Nakasuji b
a Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
b Fukui University of Technology, Fukui, 910-8505, Japan
a r t i c l e i n f o
a b s t r a c t
New hydrogen-bond functionalized tetrathiafulvalene (TTF) derivatives (1 and 2) having two imidazole
moieties, which are attached to TTF at the 2- and 4-positions in the imidazole ring, respectively, were
synthesized. Electrochemical measurements indicated that the introduction of imidazole moieties at
2- or 4-positions slightly reduced or enhanced the electron-donating ability of TTF. In the crystal structure
of 1, N–H?N hydrogen-bonds of the imidazole ring formed a two-dimensional sheet, and further
Keywords:
Charge-transfer complex
Hydrogen-bond
Tetrathiafulvalene
Imidazole
p
-stacks on the TTF skeleton built up a one-dimensional column. Mixing 1 with electron-acceptors
afforded fully ionic charge-transfer complexes having a 1:1 donor–acceptor ratio, while complex
formation of
2 with tetracyanoquinodimethane and p-chloranil yielded partial charge-transfer
complexes showing semiconducting behaviors (room temperature conductivities=10À3–10À2 S cmÀ1).
& 2009 Elsevier B.V. All rights reserved.
1. Introduction
of oligo(imidazole)s constructed well-ordered structures to yield
highly conducting complexes [14–17].
The combination between TTF-Im and oligo(imidazole)s is
expected to give novel strategy to realize molecular materials
exhibiting intriguing properties based on the electronic modula-
tion effect and multi-dimensional H-bond network. In the present
study, we newly synthesized two H-bond functionalized TTF
derivatives having two imidazole moieties (1 and 2, Chart 1), in
which imidazole rings are attached to TTF at the 2- and
4-positions, respectively. Their electrochemical measurements
and crystal structure of 1 were investigated. Physical properties
of their CT complexes are also presented in this article.
Hydrogen-bond (H-bond), a robust and directional intermole-
cular interaction, is widely utilized as a powerful tool to construct
supramolecular assemblies in the recent development of mole-
cule-based materials [1,2]. In the research field of conducting
charge-transfer (CT) complexes [3,4], introduction of H-bond
functional groups into tetrathiafulvalene (TTF) [5] and incorpora-
tion of H-bonding counter-parts [6–8] have been achieved to
regulate molecular arrangement in the solid state.
To explore new conducting CT complexes whose crystal and
electronic structures are modulated by H-bonds, we have
previously designed an imidazole-substituted TTF derivative
(TTF-Im) [9–11]. It was demonstrated for the first time that
H-bonds between TTF-Im and electron-acceptor molecules can
control the ionicity of CT complexes by site-selective molecular
recognition and modulation of redox abilities of donor and
acceptor molecules [11]. These electronic effects of H-bond
provided highly conducting CT complexes. The imidazole-anne-
lated TTF derivatives, which have been investigated from the
viewpoint of cooperation between proton-transfer and CT, also
construct H-bonded self-assemblies [12,13]. In addition to the
investigations on H-bond functionalized TTF derivatives, we have
also studied the synthesis of oligo(imidazole)s and CT complexes
of their protonated species with tetracyanoquinodimethane
(TCNQ) radical anions, in which the multi-dimensional H-bonds
2. Experimental
2.1. Synthesis
Preparations of 1 and 2 were performed by the Stille-type cross
coupling reactions between bis(tributylstannylated)TTF and
N-[2-(trimethylsilyl)ethoxy]methyl (SEM)-protected 2- or 4-io-
doimidazole, respectively, followed by the deprotection using
tetrabutylammonium fluoride (Scheme 1) [18,19]. E- and
Z-isomers were yielded in the reaction, and were not isolated
since they change to each other due to rotation at the central
CQC bond. Single crystals of 1 suitable for X-ray structure
analysis were obtained by the vapor diffusion method using
DMSO and MeCN [20]. CT complexes were prepared by mixing the
solutions of donor and acceptor (TCNQ, p-chloranil (QCl4), and
Ã
Corresponding author. Tel.: +816 6850 5393; fax: +816 6850 5395.
E-mail address: morita@chem.sci.osaka-u.ac.jp (Y. Morita).
0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2009.10.022

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T. Murata et al. / Physica B 405 (2010) S41–S44
Table 1
S42
Oxidation potentials (Eox) of 1, 2, and TTF (V vs. Fc/Fc+)a.
Eox1 (V)
Eox2 (V)
D
E (V)
1
À0.05
À0.12
À0.09
+0.25
+0.15
+0.15
0.30
0.27
0.24
2
TTF
a
Conditions: solvent, DMF (0.1 M Et4NClO4); counter/working/reference
electrodes, Pt wire/glassy carbon/Ag/AgNO3 (0.01 M in MeCN).
Chart 1. Molecular structures of TTF-Im, oligo(imidazole)s, 1, and 2.
Fig. 1. Crystal structure of 1 viewed nearly along the a-axis showing the two-
dimensional H-bond network and p-stacking column.
of TTF with imidazole moieties at the 2- and 4-positions resulted
in the slight positive and negative shifts from that of TTF,
respectively. This behavior was also observed in the mono-
substituted derivatives [9] (Table 1).
Scheme 1. Synthetic procedures of 1 and 2, and molecular structures of E- and
Z-isomers of 1.
3.2. Crystal structure of 1
2,3-dichloro-5,6-dicyanobenzoquinone (DDQ)) in PhCN–DMF–
CH2Cl2 for 1 and in THF for 2 [21,22].
Crystal structure of 1 consisted only of the E-isomer. A half of
the molecule was crystallographically independent, and the
inversion center located at the central CQC bond. The TTF
skeleton was planar, and the imidazole ring showed a twisted
configuration to the TTF moiety by 12.61. The imidazole moiety
2.2. Measurements
Cyclic voltammetry measurements were made with an ALS
Electrochemical Analyzer model 612A in the DMF solution
containing tetraethylammonium perchlorate at room tempera-
ture. The results were calibrated with ferrocene/ferrocenium
couple (Fc/Fc+). X-ray crystallographic measurement was per-
˚
was connected through N–H?N H-bonds (2.97 A) along the c-axis
to form a two-dimensional sheet (Fig. 1). The molecules were
˚
stacked with a face-to-face distance of 3.50 A to construct a one-
dimensional column along the a-axis.
formed on a Rigaku Saturn CCD area detector with a graphite
˚
monochromated Mo K
a radiation (l=0.71070 A). Structure was
3.3. Properties of CT complexes
determined by a direct method (SIR-92 [23]), and refined with a
full-matrix least squares on F2. All non-hydrogen atoms were
refined anisotropically. Positional parameters of hydrogen atoms
were calculated and included in the final refinement with
isotropic thermal factors. Direct current electrical conductivity
measurements were made on compressed powder pellets by a
conventional two- or four-probe method using gold paint and gold
wires.
Fig. 2 presents the electronic spectra of CT complexes of 1 and
2 with TCNQ, DDQ and QCl4. Table 2 summarizes donor–acceptor
(D:A) ratios estimated by elemental analyses, the lowest CT
absorption bands, and transport properties of CT complexes
[21,22]. The protonation on the imidazole moiety in the CT
complex formation is hardly expected because the proton-
accepting abilities of 1 and 2 should be too weak to withdraw
proton from the reaction system (H2O, etc.) considering the small
pKa value (3.6) of protonated species of TTF-Im [24]. The
3. Results and discussions
complexes of
1 were characterized as 1:1 fully ionic CT
complexes from the elemental analyses and intermolecular CT
absorption bands at 9000–13 000 cmÀ1 (Fig. 2a, gray lines)
[25–27]. Fully ionic states of the acceptor moieties were also
suggested by the resemblances in IR spectra with their radical
anion salts. The complexes were insulators with room
temperature conductivities
activation energies (Ea) of 0.35–0.53 eV. From the formation of
D–A–D 2:1 H-bond triad in the CT complexes of mono-substituted
3.1. Electrochemical measurements
Both 1 and 2 exhibited reversible two-stage redox processes in
the cyclic voltammetry measurements. Table 1 summarizes the
oxidation potentials (Eox) of 1, 2 and TTF. Eox1 of 1 and 2 (À0.05
and À0.12 V, respectively) indicate that these molecules possess
comparable electron-donating abilities to that of TTF. Substitution
(sRT)
of 10À7–10À6 S cmÀ1 and

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T. Murata et al. / Physica B 405 (2010) S41–S44
S43
combined molecules are potential electron-donors for the devel-
opment of organic conductors with structural and electronic
modulations.
Acknowledgments
This work was in part supported by Grant-in-Aid for Scientific
Research (20550051) and that on Innovative Areas (20110006).
Y.Y. is a recipient of research fellowships of the Japan Society for
the Promotion of Science.
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Fig. 2. Electronic spectra of CT complexes of 1 and 2 in KBr.
Table 2
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(2008) 24.
1–TCNQ
1–DDQ
1–QCl4
2–TCNQ
2–DDQ
2–QCl4
1:1
1:1
1:1
5:2
1:1
3:1
11.7
12.6
9.0
6.4 Â10À7
2.9 Â10À6
1.8 Â 10À6
4.5 Â10À2
2.7 Â10À4
1.9 Â10À3
0.53
0.35
0.47
0.12
0.23
0.19
ꢀ3.0
9.0
ꢀ3.0
a
Estimated by elemental analysis.
Measured for KBr pellets.
b
c
Measured for compressed pellets.
[13] J. Wu, N. Dupont, S. Liu, A. Neels, A. Hauser, S. Decurtins, Chem. Asian J.
4 (2009) 392.
TTF-Im [11], it can be presumed that H-bond formation of 1 with
acceptors on both sides of the molecule establishes a one-
dimensional D–A–D–A 1:1 H-bond chain, resulting in the 1:1
D–A ratios of the complexes. Furthermore, the fully ionic states of
the complexes even for QCl4 complex having a relatively weak
electron-accepting ability is assumed to be caused by the
modulation of redox abilities of component molecules of
H-bond as seen in the TTF-Im CT complexes [11].
[14] Y. Morita, T. Murata, S. Yamada, M. Tadokoro, A. Ichimura, K. Nakasuji,
J. Chem. Soc. Perkin Trans. 1 (2002) 2598.
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H. Kitagawa, H. Yamochi, G. Saito, K. Nakasuji, J. Org. Chem. 70 (2005) 2739.
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K. Nakasuji, Cryst. Growth Des. 8 (2008) 3058.
[18] Physical data of 1: mp. 185–186 1C (dec); 1H NMR (270 MHz, DMSO-d6, 80 1C)
d
6.98 (s, 2), 7.15 (s, 2), 7.28 (s, 2); IR (KBr) 3300–2500, 1590, 1104 cmÀ1; EI-
MS, m/z 336 (M+, 64%); Anal. Calcd. for (C12H8N4S4)(H2O)0.3: C, 42.16; H, 2.54;
N, 16.39%. Found: C, 42.38; H, 2.32; N, 16.08%. Due to the hygroscopic nature
(probably Z-isomer), inclusion of water molecule was considered in the
elemental analysis.
Similar to 1-DDQ, DDQ complex of 2 was also characterized as
a 1:1 fully ionic complex from the elemental analysis and optical
spectra, while TCNQ and QCl4 complexes were partial CT
complexes showing intermolecular CT bands at around
3000 cmÀ1 (Fig. 2b). Since the acceptor moieties of these
complexes are assignable as full ionic species from the resem-
blances with their radical anion salts in IR spectra (1572 and
1505 cmÀ1 (CQC stretching modes) for TCNQ complex and
1532 cmÀ1 (CQO stretching mode) for QCl4 complex), the donor
molecules possessed the partial CT state as average. Both
[19] Physical data of 2: mp. 163–165 1C (dec); 1H NMR (270 MHz, DMSO-d6, 80 1C)
d
6.84 (s, 2), 7.37 (s, 2), 7.66 (s, 2); IR (KBr) 3200–2200, 1624, 1099 cmÀ1; EI-
MS, m/z 336 (M+, 100%). Anal. Calcd. for (C12H8N4S4)(H2O)2: C, 38.69; H, 3.25;
N, 15.04%. Found: C, 38.87; H, 2.78; N, 14.65%. Since donor 2 was poorly
soluble in organic solvents and unstable in the solution state, elemental
analyses did not give the appropriate value even if the inclusion of water was
considered.
[20] Crystal data of 1: C12H8N4S4, Fw=336.46, monoclinic, P21/c, a=3.894(2),
3
˚
˚
b=18.02(1), c=10.033(5) A,
cmÀ3
(Mo K
b
=94.54(1)1, V=701.9(7) A , Z=2, Dcalcd.=1.592 g
,
m
a
)=6.69 cmÀ1, T=93 K, 1591 unique reflections. The structure
was refined to R1=0.044, RW=0.084 for 958 reflections with I4 3s(I) and 91
complexes exhibited semiconductive behaviors with
10À2 S cmÀ1 and Ea=0.12–0.19 eV (Table 2).
s –
RT=10À3
parameters, GOF=1.787. CCDC 751690.
[21] Physical data of CT complexes of 1. 1–TCNQ: IR (KBr) 3200–2600, 1624,
1572, 1505 cmÀ1
UV (KBr) 284, 762, 852 nm; Anal. Calcd. for
;
(C12H8N4S4)(C12H4N4)(H2O)3.5: C, 47.75; H, 3.17; N, 18.56%. Found: C, 48.02;
H, 2.49; N, 17.92%; 1–DDQ: IR (KBr) 3200–2800, 2215, 1623, 1546 cmÀ1; UV
(KBr) 386, 796 nm; Anal. Calcd. for (C12H8N4S4)(C8Cl2N2O2)(H2O)2: C, 40.07;
H, 2.02; N, 14.02%. Found: C, 39.95; H, 1.86; N, 13.70%; 1–QCl4: IR (KBr) 3100–
2700, 1690, 1623, 1532 cmÀ1; UV (KBr) 288, 838, ꢀ1100 nm; Anal. Calcd. for
(C12H8N4S4)(C6Cl4O2)(C3H7NO): C, 38.72; H, 2.47; N, 10.87%. Found: C, 38.37;
H, 2.12; N, 11.29%. Due to the small contamination by complexes having
different component ratios, the elemental analyses did not give the
appropriate value even if the inclusion of solvent and water was considered.
[22] Physical data of CT complexes of 2. 2–TCNQ: IR (KBr) 3200–2600, 1624, 1572,
4. Conclusions
We have synthesized two kinds of new H-bond functionalized
TTF derivatives having two imidazole moieties, 1 and 2. The two-
dimensional H-bond network and one-dimensional
were well determined by X-ray crystallographic analysis. CT
complexes of 1 with acceptors having a wide range of acceptor
strengths were 1:1 fully ionic complexes, implying the formation
of H-bond self-assemblies and the modulation of redox abilities of
component molecules by donor–acceptor H-bonds, while
2 afforded partial CT complexes showing semiconducting beha-
viors. These results demonstrate that the TTF–oligo(imidazole)s
p-stack of 1
1505 cmÀ1
; UV (KBr) 284, 762, 852 nm; Anal. Calcd. for (C12H8N4S4)5
(C12H4N4)2: C, 48.26; H, 2.31; N, 18.76%. Found: C, 48.48; H, 2.41; N,
18.27%; 2–DDQ: IR (KBr) 3200–2800, 2215, 1623, 1546 cmÀ1; UV (KBr) 386,
796, ꢀ1200 nm; Anal. Calcd. for (C12H8N4S4)(C8Cl2N2O2)(H2O): C, 41.31; H,
1.73; N, 14.45%. Found: C, 41.56; H, 1.77; N, 14.27%; 2–QCl4: IR (KBr) 3100–
2700, 1690, 1623, 1532 cmÀ1
;
UV (KBr) 288, 838 nm; Anal. Calcd. for
(C12H8N4S4)3(C6Cl4O2)(H2O)3: C, 38.53; H, 2.31; N, 12.84%. Found: C, 38.26;

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T. Murata et al. / Physica B 405 (2010) S41–S44
H, 2.44; N, 12.89%. Due to the small contamination by complexes having
[24] The estimation of pKa of protonated TTF-Im will be published elsewhere.
Those of 1 and 2 have not been measured.
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not give the appropriate value even if the inclusion of solvent and water was
considered.
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