Face-to-face dimeric tetrathiafulvalenes and their cation radical and dication species as models of mixed valence and π-dimer states

Article abstract of DOI:10.1246/bcsj.20110224

Intramolecular interactions of face-to-face tetrathiafulvalenes (TTFs) in 1,8-bis(tetrathiafulvalenyl)naphthalene frameworks were investigated in neutral and cationic states. From X-ray analysis and NMR spectroscopy on neutral 1,8-bis(tetrathiafulvalenyl)

Full text of DOI:10.1246/bcsj.20110224

© 2012 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 85, No. 1, 51-60 (2012)
51
BCSJ Award Article
Face-to-Face Dimeric Tetrathiafulvalenes and Their Cation Radical and
Dication Species as Models of Mixed Valence and ³-Dimer States
Masashi Hasegawa,*1 Kota Daigoku,2 Kenro Hashimoto,3
Hiroyuki Nishikawa,4 and Masahiko Iyoda*3
1Department of Chemistry, School of Science, Kitasato University,
1-15-1 Kitasato, Minami-ku, Sagamihara, Kanagawa 252-0373
2Division of Chemistry, Center for Natural Sciences, College of Liberal Arts and Sciences, Kitasato University,
1-15-1 Kitasato, Minami-ku, Sagamihara, Kanagawa 252-0373
3Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University,
Hachioji, Tokyo 192-0397
4Faculty of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512
Received July 25, 2011; E-mail: masasi.h@kitasato-u.ac.jp, iyoda@tmu.ac.jp
Intramolecular interactions of face-to-face tetrathiafulvalenes (TTFs) in 1,8-bis(tetrathiafulvalenyl)naphthalene
frameworks were investigated in neutral and cationic states. From X-ray analysis and NMR spectroscopy on neutral 1,8-
bis(tetrathiafulvalenyl)naphthalenes, which exist as a mixture of syn- and anti-isomers in solution, there is steric repulsion
between TTF moieties. However, analysis of the cyclic voltammogram (CV) revealed that a strong attractive interaction
was present in the cation radical state. Electronic spectra of the cation radical state showed the presence of a mixed-
valence (MV) state with a strong charge resonance feature. ESR spectra of the 1,8-bis(tetrathiafulvalenyl)naphthalene
cation radical showed a typical signal of an MV state. Furthermore, a ³-dimer, which exhibits pronounced Davydov
splitting in the electronic spectra, was formed in the dication state. The observed absorption bands in the electronic spectra
were characterized by using quantum chemical calculations of the cation radical and dication species. NMR spectra of
both the ³-dimer and tetracation species show either a deshielding effect due to cationic charge or a shielding effect due to
1
diamagnetic ring currents. To our knowledge, this is the first reported example of a H NMR spectrum of TTF ³-dimer.
Tetrathiafulvalene (TTF) and its derivatives can be easily
oxidized to form stable cation radicals and dications, and
these cationic species have been employed extensively in
the development of new organic metals, superconductors,
and semiconductor devices.1,2 The ability of TTFs to readily
self-aggregate in the cation radical state and form columnar
structures, which act as conductive pathways in the solid
state,3 is of great interest from the viewpoint of a bottom-up
approach to constructing supramolecular structures and
nanoscale architectures.3,4 Thus, a detailed understanding of
the face-to-face interactions between TTFs is essential not
only for elucidating the electronic state of organic metals
but also for designing functionalized nanostructures based on
TTF.
interactions (A), whereas neutral TTFs easily dissociate in
solution.1-3 In contrast, ³-extended TTF derivatives and TTFs
having amphiphilic properties self-associate even in solution,
resulting in the formation of nanostructures.5-7 Furthermore,
stronger molecular interactions occur in mixed valence (MV)
states with strong charge resonance (CR) (B). Electronic
•+
coupling in the MV state of dimeric (TTF)2 shows Robin-
Day Class II-III behavior on the basis of Müllken-Hush
analysis of their intervalence charge-transfer (IV-CT) band.8,9
Although paramagnetic TTF•+ exists as a monomer in solution
at room temperature,9a diamagnetic ³-dimer C is formed at
lower temperatures in solution, except under certain conditions
when supramolecular cages are formed.10 As for C, Torrance
et al. have reported a large blue shift in the electronic spectra,
known as a Davydov blue shift.11,12 In the case of TTF
dications D, the two dications easily dissociate due to
Coulombic repulsion.
Face-to-face TTF dimers can be regarded as the simplest
model for columnar stacks of TTFs. Typical stacking modes for
face-to-face TTFs in neutral and/or cationic states are summa-
rized in Scheme 1. In the solid state, neutral TTFs weakly stack
to form dimers or columnar structures through S£S and ³-³
A variety of TTF dimers and oligomers have been reported
to form MV and ³-dimer states.3,12 However, there are some
Published on the web December 30, 2011; doi:10.1246/bcsj.20110224

52
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012) BCSJ AWARD ARTICLE
S
S
S
S
S
S
S
-
S
S
-
S
S
S
-
-2e
-e
-e
S
S
S
S
S
S
S
S
-
-
-
S
S
+e
+2e
+e
S
S
S
S
S
S
S
S
S
S
•+
•+
2+
(TTF )
(TTF)
(TTF)
2(TTF )
2
2
2
(MV Aggregate)
(π-Dimer)
A
B
C
D
Scheme 1. Interactions between TTFs in a face-to-face manner.
R
R
4
5
R
R
2a: R, R = SCH2CH2S
2b: R = SCH3
2c: R = H
S
S
S
S
R
R
S
S
S
S
S
S
S
S
1) n-BuLi, THF, -78 °C
2) ZnCl2, -65 °C
4'
R
R
ZnCl
I
S
S
S
S
1a: R, R = SCH
1b: R = SCH3
1c: R = H
2CH2S
[Pd(PPh3)4]
I
I
Chart 1.
[Pd(PPh3)4]
ambiguities in the reported MV and ³-dimer states of TTFs in
solution because the formation of those states are discussed on
the basis of various spectroscopic data.12c,13 In order to obtain
accurate spectroscopic information about the MV and ³-dimer
states of TTFs, we prepared 1,8-bis(tetrathiafulvalenyl)naph-
thalenes 1a-1c (Chart 1) as a model system. Since two TTF
units are attached at the 1,8-positions of the naphthalene ring in
1a-1c, the face-to-face interactions between two TTFs can be
determined very easily.
1a: R, R = SCH2CH2S
(78%)
1b: R = SCH3
(59%)
S
S
S
S
S
S
1c: R = H
3 (70%)
(85%)
We have previously reported the syntheses, structures, and
properties of 1a-1c.14 In this paper, we focus on the face-to-
face interactions between TTFs in the MV and ³-dimer states,
which can be utilized to construct supramolecular architectures.
Furthermore, the electronic spectra of aggregated (TTF)2•+ and
(TTF•+)2 in the MV and ³-dimer states, respectively, were
corroborated via excitation energy calculations based on a time-
dependent density functional theory (TD-DFT) method.
Scheme 2. Synthesis of 1a-1c and 3.
Molecular Structures of Neutral 1,8-Bis(tetrathiafulvale-
nyl)naphthalenes 1a-1c. 1,8-Bis(tetrathiafulvalenyl)naphtha-
lenes 1a-1c showed temperature-dependent NMR spectra due
to interconversion between the syn- and anti-isomers. The
1H NMR spectrum of 1a at 25 °C showed averaged signals due
to fast interconversion between syn- and anti-isomers, which
decoalesced at ca. ¹75 °C, and two sets of signals were ob-
served at ¹90 °C in an anti-/syn-isomer ratio of 93:7.16 The
values of the equilibrium constant (Ksyn-anti) and free energy
difference (¦E) between the syn- and anti-isomers are summa-
rized in Table 1. Since the ¦E values are in the order 1c < 1b <
1a, the steric hindrance of the substituent on the TTF moieties
dominates the molecular conformations in the neutral state.
Recrystallization of 1a from CH2Cl2 and i-Pr2O at 4 °C gave
single crystals large enough for X-ray crystallographic analysis
(Figure 1). The molecule has crystallographic C2 symmetry
with a twofold axis passing through the C(1)-C(2) bond to
form an anti-conformation. The mean deviations of C(1) and
C(6) from the naphthalene plane were determined to be 0.051
and ¹0.043 ¡, respectively, by using a least-squares method.
Results and Discussion
Synthesis. 1,8-Bis(tetrathiafulvalenyl)naphthalenes 1a-1c
were synthesized by using palladium-mediated coupling of
organozinc intermediates,12b,15 as shown in Scheme 2. For
example, the reaction of bis(alkylthio)-TTFs 2a and 2b or
TTF (2c) with n-BuLi in THF, followed by addition of
anhydrous ZnCl2 at ¹65 °C, yielded an organozinc species.
The reaction of the zinc species with 1,8-diiodonaphthalene
in the presence of [Pd(PPh3)4] produced the corresponding
coupling products 1a-1c in 78%, 59%, and 85% yields,
respectively. In a similar manner, the palladium-mediated
reaction of the zinc complex of 2a with iodobenzene afforded 3
in 70% yield.

M. Hasegawa et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012)
53
Table 1. Energy Difference between the syn- and the anti-
Isomersa)
R
R
S
R
R
R
R
R
R
S
S
S
S
S
S
S
S
S
S S
S
S
S
S
syn
anti
Ratiob)
(syn:anti)
¹1
Ksyn-anti
¦Esyn-anti/kcal mol
1a
1b
1c
7:93
1:5
2:7
13.3
5.0
3.5
0.94
0.58
0.45
a) In CDCl3 solution at ¹90 °C. b) The ratios of the isomers
1
were estimated from H NMR signals of each TTF proton of
1a and 1b and that at the 2-position of naphthalene of 1c,
respectively.
(a)
(b)
C(13*)
S(6*)
C(14)
S(6)
C(14*)
S(5*)
C(11*)
S(3*)
C(13)
S(5)
C(11)
C(12*)
S(4*)
C(12)
S(4)
S(3)
C(10) C(10*)
S(2*)
Figure 2. Cyclic voltammograms of 1a and 3 in PhCN.
Table 2. Redox Potentials of 1a-1c, 2a-2c, and 3a)
S(2)
C(9)
S(1)
C(9*)
S(1*)
E1
/V
E2
/V
E3
/V
E2 ¹ E1
1/2
1/2
1/2
1/2
1/2
b)
C(7*)
C(7)
Kc
/V
C(6)
C(6*)
C(1)
¹
¹
¹
1a ¹0.18 (1e ) ¹0.05 (1e ) +0.44 (2e )
1b ¹0.13 (1e ) ¹0.01 (1e ) +0.43 (2e )
1c ¹0.23 (1e ) ¹0.13 (1e ) +0.42 (2e )
0.13
0.12
0.10
®
158
107
50
®
®
C(5*)
C(4*)
C(5)
C(4)
¹
¹
¹
¹
¹
¹
C(2)
¹
¹
2a
2b
2c
3
0.01 (1e )
+0.35 (1e )
0.32 (1e )
0.28 (1e )
+0.35 (1e )
C(3*)
C(3)
¹
¹
¹0.01 (1e )
®
®
¹
¹
¹0.09 (1e )
®
®
Figure 1. ORTEP drawing of 1a: (a) top and (b) side views.
¹
¹
¹0.01 (1e )
®
Thus, molecular strain is released by the anti-conformation of
the TTF units and by the deformation of the naphthalene ring.
The dihedral angle between the naphthalene and TTF rings
was determined to be 54.6(3)°. The interatomic distances
C(6)£C(6*) and C(7)£C(7*) were determined to be 2.50 and
2.76 ¡, respectively, which are 3% and 8-9% shorter than those
of previously reported 1,8-diarylnaphthalenes.17 There are only
two intermolecular S£S interactions within the sum of the van
der Waals radii in the crystal.
a) Conditions: in PhCN containing 0.1 M n-Bu4NClO4 at 25 °C;
Ag/Ag+ reference electrode, Pt counter and working elec-
trodes, 100 mV s¹1; potentials referred to Fc/Fc+. b) Compro-
portionation constant of the monocationic species: See Ref. 17.
potential of 1a corresponding to the formation of the tetracation
1a4+ is much higher than the second potential of 3, owing to the
face-to-face interactions in the mono-, di-, and tetracations of
1a. The significant lowering of the first oxidation potential of 1a
compared with 3 is due to the formation of mixed-valence (MV)
state as a result of charge delocalization between the two TTF
moieties. The potential difference between the second oxidation
of 1a and the first oxidation of 3 may be attributed to the
weakened conjugation between TTF and naphthalene moieties
in 1a compared with the conjugation between TTF and benzene
moieties in 3.18 The higher third oxidation potential of 1a than
the second oxidation potential of 3 is due to the Coulombic
interaction between the two TTF cation radicals.
Electrochemistry.
In cyclic voltammograms of 1a-1c,
three reversible redox waves were observed, whereas in that of
3, two reversible redox waves were observed (Figure 2 and
Table 2). For 1a-1c, the first and second waves were two one-
electron processes, and the third wave was a two-electron pro-
cess. Compound 3 shows oxidation potentials at ¹0.01 and 0.35
V under identical conditions. The first and second oxidation
potentials of 1a corresponding to the formation of 1a•+ and 1a2+
are lower than the first potential of 3, whereas the third oxidation

54
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012) BCSJ AWARD ARTICLE
Figure 3. Color of (a) 1a, (b) 1a•+, (c) 1a2+, (d) 1a3+, and
(e) 1a4+ in 1:1 CH2Cl2-MeCN.
Figure 4. Electronic absorption spectra of 1a, 1a•+, 1a2+
,
1a3+, 1a4+, and 3•+ in a 1:1 mixture of CH2Cl2-MeCN.
The inset shows expanded electronic spectra of 1a2+
together with 3•+ for comparison.
Table 3. Experimental and Calculated (TD B3LYP/6-31G(d)// HF/6-31G(d)) Longest Wavelength Absorp-
tion Maxima for 1a and 3, and Their Cation Radicalsa),b)
Experimental -max/nm (¾)
Calculated -max/nm (oscillator strength)
1a
312 (34000)
286 (0.334)
1a•+ c)
1a2+
1a3+
1a4+
3
312 (26000), 816 (7600), 1850 (3500)
402 (23000), 732 (18000), 858 (13000)
400 (19000), 703 (19000), 860 (9700)
652 (30000)
314 (16000)
448 (12000), 836 (9100)
423, 715, 895
315 (0.104), 862 (0.120), 1678 (0.056)
344 (0.202), 794 (0.204), 969 (0.177)
673 (0.785)
282 (0.169)
372 (0.204), 852 (0.158)
3•+
3•+ (Solid)d)
32+
644 (11000)
651 (0.219)
a) In a 1:1 mixture of CH2Cl2-MeCN. b) HF-optimized structures were used for the anti-isomer of 1 and
14+, and syn-isomer of 12+ in the TD-DFT calculations. c) The structure obtained from X-ray analysis14b
was applied. d) The sample was cast on a quartz plate. See Supporting Information.
Similar redox behavior was observed in the cyclic voltam-
mograms of 1b and 1c, although the first and second oxidation
potentials were different. Relative thermodynamic stability of
the cation radicals 1a•+, 1b•+, and 1c•+ was evaluated via the
comproportionation constant Kc calculated from the difference
between the first and second oxidation potentials.19 The Kc
values of 1a, 1b, and 1c were determined to be 158, 107, and
50, respectively. The large values of Kc suggest the formation
of a stable cation radical with strong interactions in the MV
state, and the thioalkyl groups at the 4 and 5 positions stabilize
the MV state compared to the unsubstituted MV state.
Furthermore, coplanar ethylenedithio groups more effectively
stabilize the MV state than two flexible methylthio groups.
Electronic Spectra. The electronic spectra of the cations
of 1a-1c, prepared by oxidation with Fe(ClO4)3, were acquired
in 1:1 CH2Cl2-MeCN at room temperature.20 Neutral 1a
showed the longest maximum absorption at 312 nm. The
electronic spectra of 1b and 1c were similar, but oxidation of
1b and 1c with Fe(ClO4)3 produced unstable cation radicals.
Therefore, we focused on the oxidation of 1a for the
investigation of the cationic states. Sequential addition of 1,
2, 3 equiv and excess amounts of Fe(ClO4)3 to 1:1 CH2Cl2-
MeCN solutions of 1a afforded five different colored solutions
(Figure 3),7e,21 i.e., electrochromism depending on the oxida-
tion state.
Chemical titration of 1a with 1 equiv of Fe(ClO4)3 gave
¹
1a•+¢ClO4 . The electronic spectrum showed two absorption
maxima at 312 and 816 nm, together with a broad absorption in
the range of 1800-2200 nm (Figure 4). The absorption max-
imum at 816 nm was assigned to the intrinsic absorption of
the TTF cation radical (SOMO¹1 ¼ SOMO),22 and the broad
absorption in the range of 1800-2200 nm was assigned to the
CR band of 1a•+ due to the delocalization of the positive
charge between the two TTF units in the MV state (Table 3).
The CR band disappeared when 1a•+ was oxidized with a
second equivalent of Fe(ClO4)3, and new absorption maxima
were observed at 402, 732, and 860 nm. The face-to-face
interactions between the two TTF•+ units result in the splitting
of the longest absorption maximum around 750 nm. Thus, the
absorption at 732 nm was assigned to an intrinsic transition of
TTF•+ (D2), and the absorption at 858 nm (D3) was assigned to
the HOMO-LUMO transition of ³-dimer 1a2+. Furthermore,
the absorption maximum at 402 nm (D1) was blue-shifted by
46 nm compared with that of 3•+ (M1: -max = 448 nm). This
pronounced blue shift is known as a Davydov blue shift and is
due to the formation of a face-to-face ³-dimer.9-11 Interest-
¹
ingly, the electronic spectra of 3•+¢ClO4 in the solid state
showed the longest absorption maxima at 715 and 895 nm due
to the formation of the ³-dimer in the solid state (Figure S3c).
Treatment of 1a2+ with 1 equiv of Fe(ClO4)3 led to the

M. Hasegawa et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012)
55
(a)
(b)
(b)
(a)
#182
(SOMO)
CR
#181
#180
S2
1a2+
1a
(syn-isomer)
(anti-isomer)
#181β
#182β
(c)
Figure 6. (a) Illustration of electronic transition of 1a•+
.
(b) MO of 1a•+. The wave function value of the MO
¹3/2
surface is 0.02 bohr
.
In the case of 1a•+, the transition to the first excited state was
calculated to be at 0.74 eV (1678 nm) and corresponds to the
observed CR band (1850 nm), which involves a one-electron
excitation from SOMO¹1 (#181¢) to the lowest vacant orbital
(#182¢). As seen in Figure 6b, these two MOs can be viewed
as in-phase and out-of-phase combinations of the ³ orbitals of
the TTF moieties, respectively. The absorption at 816 nm was
attributed to the second excited state calculated at 1.44 eV
(862 nm, S2 in Figure 6a).27
1a4+
(anti-isomer)
Figure 5. Optimized structures of 1a, 1a2+, and 1a4+
.
formation of 1a3+, which showed absorption maxima at 400,
703, and 860 nm with a tail extending to 1600 nm. Addition of
1-3 equiv of Fe(ClO4)3 to 1a3+ formed the tetracation 1a4+
which showed the longest absorption maximum at 652 nm,
similar to that of other TTF dications, such as 34+ (-max
Transitions in 3•+ and 1a2+ and related MOs are illustrated in
Figure 7. For the syn-isomer of 1a2+, three states were obtained
at 1.28 (969 nm), 1.56 (794 nm), and 3.61 eV (344 nm), to which
the observed D3, D2, and D1 bands were assigned, respectively
(inset of Figure 4). D3, which is the lowest in energy, is a
HOMO-LUMO transition. The HOMO and LUMO involve in-
phase and out-of-phase mixing of the SOMO in 3•+ (#95¡ and
#95¢ in Figure 7), which is the TTF ³-orbital. The D2 band
corresponds to the electronic transition from HOMO¹2 (#179)
to the LUMO in 1a2+. HOMO¹2 involves negative overlap of
SOMO¹1 (#94¢) in 3•+, which is comprised of low-energy ³-
orbitals on the TTF moieties, whereas HOMO¹1 (#180), which
involves the naphthalene moiety, is not related to the D2 tran-
sition.27 In addition, calculations qualitatively reproduced the
observed Davydov blue shift. The energy of D2 is higher than
that of the M2 transition from SOMO¹1 (#94¢) to the SOMO
(#94¢) in 3•+. Similarly, the transition energy of D1 (3.61 eV) is
greater than that of M1 in 3•+ (3.34 eV). Although D1 involves a
transition from the HOMO to LUMO+3 (#185) in 1a2+, M1
involves a transition from the SOMO (#95¡) to LUMO+1
(#97¡). LUMO+3 (#185) and LUMO+1 (#183) in 1a2+ are the
out-of-phase and in-phase combinations of LUMO+1 (#97¡) in
3•+, respectively, supporting the Davydov blue shift observed in
the electronic spectra.27 LUMO+2 (#184), formed from orbital
#98¢ in 3•+, is not involved in the D1 transition.
,
=
644 nm).22
Theoretical Calculations.
Geometry optimization was
carried out at the RHF/6-31G(d) level for 1a, 1a2+, and 1a4+
,
as well as 3 and its cation radicals.23 For 1a, two local minima,
an anti-isomer with C2 symmetry and a syn-isomer with C1
symmetry, were found (Figure 5). Total energy of the former is
lower than that of the latter by 3.4 kcal mol¹1, and the geometry
of the anti-isomer is similar to that obtained from X-ray
crystallographic analysis. In contrast, as for diamagnetic 1a2+
,
the syn-isomer with Cs symmetry is more stable than the anti-
isomer with C2 symmetry by 3.2 kcal mol¹1. In syn-1a2+, the
two TTF units are stacked in a face-to-face manner forming a
stable ³-dimer.24 In tetracation 1a4+, only the anti-isomer was
an energy minimum. The two dithiolium cation rings in each
TTF moiety were twisted by 21.6°, and the central C-C length
of 1.447 ¡ is typical of those in the crystal structures of
dicationic TTF species.25
To characterize the absorption bands, time-dependent (TD)-
DFT calculations were performed at the B3LYP/6-31G(d)
level. The geometries optimized by using the HF/6-31G(d)
method were used,26 except for paramagnetic 1a•+, for which
the X-ray crystal structure (Figure 10a) was employed. Select-
ed excitation energies and oscillator strengths are summarized
in Table 3. For 1a, the anti-conformation was chosen, and the
calculation predicted an intense transition at 4.33 eV (286 nm),
which corresponds to the lowest absorption band at 312 nm in
the electronic spectra. Since the excitation to the ³*-type
LUMO extending onto the naphthalene moiety is mixed in the
wave function of the final state, the transition includes a
contribution from intramolecular charge transfer.27
Finally, a band at 1.84 eV (673 nm) was predicted for 1a4+
.
The transitions from HOMO¹2 (#178) to the LUMO (#181)
and from HOMO¹1 (#179) to LUMO+2 (#182) are respon-
sible for this band. All four orbitals are located on the TTF2+
moieties.27
¹
¹
ESR and NMR Spectra of 1a•+¢ClO4 and 3•+¢ClO4
.
ESR spectra of paramagnetic 1a•+¢ClO4 and 3•+¢ClO4 in
CH2Cl2-MeCN (1:1) are shown in Figure 8. The spectrum of
1a•+ showed hyperfine splitting with a binominal intensity ratio
¹
¹

56
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012) BCSJ AWARD ARTICLE
(a)
(b)
(LUMO+1)
#97
#96α
#97α
#98β
#185
#184
#183
#96
M1
D1
#182
(LUMO)
(SOMO)
#95
D3
D2
#94β
#95α
#95β
#181
(HOMO)
M2
#180
#179
#94
+
1a2+
3
(c)
#183
#184
#185
#179
#180
#181 (HOMO)
#182 (LUMO)
Figure 7. (a) Illustration of electronic transitions in 3•+ and 1a2+ and MOs of (b) 3•+ and (c) 1a2+. The wave function value of the
¹3/2
MO surface is 0.02 bohr
.
of 1:2:1 at g = 2.0066. The two essentially equivalent protons
of the EDT-TTF moieties in 1a•+ cause the hyperfine splitting,
and the hyperfine coupling constant (hfcc) was 0.063 mT,
which is almost half the value for the TTF monocation radical.
The spectrum of 3•+ showed only hyperfine splitting with an
intensity ratio of 1:1 (hfcc = 0.125 mT). Since both cation
radicals showed almost 100 mol % unpaired spin content in the
solution ESR spectra,28 the half value of hfcc in 1a•+ compared
with that of 3 is due to the stable MV state. In contrast to 1a•+,
³-dimer 1a2+ showed no ESR signal because it is diamagnetic.
(1a2+: ¤ 8.25; 1a: ¤ 7.85). To the best of our knowledge, this is
the first report of an NMR spectrum of a TTF ³-dimer.
Furthermore, the TTF proton of 1a4+ (¤ 8.94) was observed at a
lower field than that of 1a2+ owing to the higher positive
charge on the molecules. However, in comparison with that of
32+ (¤ 9.40), which shows a typical chemical shift for TTF
dications,25 the TTF proton of 1a4+ was shifted upfield by
0.46 ppm, reflecting the shielding effect from the face-to-face
aromatic 1,3-dithiolium cations. The neighboring ¢ proton of
naphthalene (¤ 8.50) in 1a4+ is deshielded due to the aromatic
1,3-dithiolium cations.
1
As shown in Figure 9, H NMR spectra for 1a2+, 1a4+, and
32+ in MeCN-d3 were obtained. The TTF proton of 1a2+
showed a broad peak at ¤ 7.45, showing the effect of the
positive charge on the 1,3-dithiolium ion (1a: ¤ 6.03 for the
TTF proton). A similar downfield shift was observed for the
peak corresponding to the neighboring ¢ proton of naphthalene
Solid-State Structure of (1a)2¢(I3)0.5¢I•¹(I2)0.5¢C6H5Cl (4).
Cationic species 4 was prepared by controlled electrolysis of 1a
at a constant potential in chlorobenzene in the presence of
n-Bu4NI3 and was characterized by X-ray analysis. As shown in
Figure 10a, the crystal lattice contains two different cation

M. Hasegawa et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012)
57
C(22a)
C(21a)
aH = 0.125 mT
(a)
(a)
C(8b)
C(7b)
C(20a)
C(17a)
C(15b)
C(16b)
C(26a)
C(25a)
C(10a)
S(8a)
C(6b)
C(14b)
C(5b)
C(13b)
C(9a)
C(1a)
C(2a)
S(7a)
S(2a)
S(1a)
C(11a)
C(12a)
C(4b)
C(3b)
C(12b)
C(3a)
C(4a)
S(10a)
C(11b)
S(3a)
C(5a)
S(9a)
S(4a)
C(13a)
a2H = 0.063 mT
C(2b)
C(14a)
S(12a)
(b)
C(25b)
C(1b)
C(9b)
C(6a)
S(11a)
C(15a)
C(10b)
C(17b)
S(5a)
S(6a)
C(26b)
C(21b)
C(7a)
C(22b)
C(8a)
C(16a)
A
B
C(20b)
Figure 8. ESR spectra of (a) 3•+ (5.0 © 10¹5 M) and
(b) 1a•+ (1.3 © 10¹4 M) in MeCN-CH2Cl2 at rt.
(b)
I
II
II
IV
III
V
VII
VIII
II
V
V
III
IV
(a)
a
Naphthalene protons
I
I
VI
VI
VI
δ = 7.45
(TTF proton)
I
II
2+
1a
IV
III
10
9
8
7
6
δ
c
0
(b)
Figure 10. ORTEP diagram of (1a)2¢(I3)0.5¢I•¹(I2)0.5
¢
Naphthalene
protons
δ = 8.94
(TTF proton)
C6H5Cl. (a) Molecular structure of 4. A and B are two
crystallographically independent molecules. Selected bond
lengths [¡] and angles [°]: C(1a)-C(2a) 1.36(1), C(3a)-
C(4a) 1.35(1), C(5a)-C(6a) 1.35(1), C(11a)-C(12a)
1.36(1), C(1b)-C(2b) 1.36(1), C(3b)-C(4b) 1.35(1),
C(5b)-C(6b) 1.38(1), C(11b)-C(12b) 1.37(1), C(17a)-
C(26a)-C(25a) 127.3, C(20a)-C(21a)-C(22a) 118.6,
C(17b)-C(26b)-C(25b) 125.8, C(20b)-C(21b)-C(22b)
119.8. (b) Packing diagram. Chlorobenzene and naphtha-
lene ring carbons except for C(17a) and C(25a) are omitted
for clarity. The solid lines indicate the intramolecular
interaction of two TTF units, the dashed lines correspond-
ing to the intermolecular interaction of two TTF units
(short contacts [¡], I in A: S(1a)£S(7a) 3.69 and
S(2a)£S(8a) 3.46 ¡; II in B: S(1b)£S(7b) 3.70 and
S(2b)£S(8b) 3.51; III: S(7a)£S(8b) 3.39 and S(7a)£
S(10b) 3.70; IV: S(2b)£S(12a) 3.67; V: S(1a)£S(5b) 3.66,
S(1b)£S(3a) 3.56, and S(1b)£S(5a) 3.66; VI: S(8a)£
S(12b) 3.56, S(8a)£S(10b) 3.58, and S(12a)£S(8b) 3.69;
VII: S(3a)£S(4a) 3.60; VIII: S(3b)£S(7b) 3.64 and
S(5b)£S(7b) 3.53.
4+
1a
10
9
8
7
6
δ
(c)
δ = 9.40
(TTF proton)
Phenyl
protons
2+
3
10
9
8
7
6
δ
Figure 9. 1H NMR spectra of (a) 1a2+, (b) 1a4+, and
(c) 32+
.

58
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012) BCSJ AWARD ARTICLE
¹
¹
radicals, I3 , I2, I , and chlorobenzene. Taking into account that
the ratio of the molecules of 1a and the anionic charges based on
formation of stable ³-dimers. To the best of our knowledge,
this is the first report of NMR spectra of a diamagnetic TTF
³-dimer. Our results provide detailed information about the
interactions in the MV and ³-dimer states of TTF derivatives
and will be useful in the design of supramolecular structures
and functional nanostructures.
¹
¹
I3 and I is 4:3, cation radical 4 possesses an average charge
per molecule of 1a of +0.75. The two cation radicals (1a2
1.5+
)
adopt a syn-conformation (Figure 10a), reflecting the stability
of the face-to-face stacking structure in the cation radical and
dication states. Interestingly, these dimeric TTFs are aligned in
an edge-to-face mode to form a two-dimensional sheet structure
(Figure 10b). The dihedral angles between the naphthalene and
TTF rings were determined to be 52°-56°. Although one
TTF ring adopts a planar structure with a maximum deviation
of 0.1 ¡ from the least-squares plane, the other three TTF
rings have bent 1,3-dithiole rings. The syn arrangement of
the TTF moieties causes the 1,3-dithiole rings to form envelope-
like structures, and the intramolecular distances between
S(1a)£(7a), S(2a)£(8a), S(1b)£(7b), and S(2b)£(8b) are shorter
than the sum of the van der Waals radii (3.70 ¡). Although there
is only one short S£I contact in 4 (I2£S(9b) 3.53 ¡), there are
a number of short intermolecular S£S contacts, as shown in
Figure 10b, and these S£S contacts result in the formation of
a sheet structure. In spite of the ¬-like structure,29 electric
conductivity of 4 was not very high (5.0 © 10¹5 S cm¹1), which
is thought to be due to inhibition of the b axis conductivity
pathway by the naphthalene moiety. Moreover, 4 in DMSO
underwent a charge recombination, and in the UV-vis-NIR
spectrum of 4 in DMSO, absorptions at 836 (SOMO¹1 ¼
SOMO) and 1800 nm (CR band) were observed, similar to those
in the spectrum of 1a•+ shown in Figure 4.27 Finally, X-ray
analysis of 4 experimentally confirmed that the syn-conforma-
tion in the cation radical and dication states of 1a was
much more stable than the corresponding anti-conformation,
although neutral 1a existed as a mixture of two isomers in
solution in an anti-syn ratio of 93:7 (Table 1) and adopted the
anti-form in the solid state (Figure 1) in order to avoid steric
crowding.
Experimental
1,8-Bis[(4,5-ethylenedithio)tetrathiafulvalenyl]naphtha-
lene (1a). To a solution of 2a (770 mg, 2.6 mmol) in THF
(40 mL) was added n-C4H9Li (1.65 mL, 2.6 mmol) dropwise
at ¹78 °C under N2 atmosphere. The mixture was stirred for
90 min. Then, a suspension of anhydrous ZnCl2 (442 mg,
3.2 mmol) in THF (3 mL) was added to the reaction mixture at
¹65 °C. The mixture was stirred for 1 h at the same temperature
and warmed up to ¹20 °C. A solution of 1,8-diiodonaphthalene
(355 mg, 0.9 mmol) in THF (2 mL) and [Pd(PPh3)4] (216 mg,
0.19 mmol) was added. The mixture was stirred for 1 h at
¹20 °C and warmed up to 45 °C. After stirring for 18 h,
aqueous NH4Cl solution was added to the mixture and
extracted with CS2. The combined organic layer was washed
with saturated brine and dried over MgSO4. The solution was
filtered, and solvent was removed in vacuo. The residue was
purified by column chromatography on silica gel (activity V)
with CS2 as the eluent. Recrystallization from CS2-diisopropyl
ether gave red crystals of 1a (513 mg, 78%). 1a: mp 199.3-
200.4 °C; LDI-TOF-MS m/z = 712 (M+); 1H NMR (CDCl3:
CS2 = 1:1, 500 MHz): ¤ 7.85 (dd, J = 7.5 and 1.2 Hz, 2H),
7.56 (dd, J = 7.5 and 1.2 Hz, 2H), 7.46 (t, J = 7.5 Hz, 2H),
6.03 (s, 2H), 3.29 (s, 8H); 13C NMR (CDCl3:CS2 = 1:1,
125 MHz): ¤ 135.2, 133.2, 131.0, 130.4, 129.5, 125.4, 118.4,
114.8, 114.2 (2C), 105.4 (2C), 30.5; UV-vis-NIR (4.2 ©
10¹5 M, CH2Cl2): -max (¾) 312 (30000), 338sh (22000) nm; IR
(KBr): 2957, 2923, 2853 cm¹1; Anal. Calcd for C26H16S12: C,
43.79; H, 2.26%. Found: C, 43.63; H, 2.25%.
1,8-Bis[4,5-bis(methylthio)tetrathiafulvalenyl]naphtha-
lene (1b). The synthesis of 1b was carried out from 2b and
1,8-diiodonaphthalene in 59% yield in a similar manner as
described for 1a. 1b: a red powder; mp 86.9-87.8 °C; MALDI-
TOF-MS m/z = 716 (M+); 1H NMR (CDCl3, 500 MHz):
¤ 7.88 (dd, J = 7.5 and 1.2 Hz, 2H), 7.58 (dd, J = 7.5 and
1.2 Hz, 2H), 7.48 (t, J = 7.5 Hz, 2H), 6.07 (s, 2H), 2.47
(s, 6H), 2.44 (s, 6H); 13C NMR (CDCl3, 125 MHz): ¤ 135.1,
133.5, 130.9, 130.3, 129.4, 127.9, 126.7, 125.4, 117.8, 115.4,
106.8 (2C), 19.3 (2C); UV-vis-NIR (2.5 © 10¹5 M, CH2Cl2):
Conclusion
We studied in detail the interactions between two TTF units
by using 1,8-bis(tetrathiafulvalenyl)naphthalenes 1a-1c as a
model. From X-ray analyses and molecular structural calcu-
lations, the anti-isomer of neutral 1a-1c is more stable than
the syn-isomer due to steric effect, although 1a-1c exist as a
mixture of anti- and syn-isomers in solution. CV measurements
revealed that 1a-1c underwent two one-electron and one two-
electron redox processes. Large Kc values were obtained due to
a stable mixed-valence (MV) state in 1a-1c, and the thioalkyl
groups at the 4 and 5 positions of the TTF moieties stabilize the
MV state. Cationic species 1a•+, 1a2+, and 1a4+ were prepared
by chemical and electrochemical oxidation methods, and their
electronic structures were elucidated from absorption spectra.
In the case of 1a•+, a strong charge resonance (CR) band was
observed arising from the MV state due to the face-to-face
arrangement of the TTF moieties. For 1a2+, the lowest energy
absorption band was split, indicating a Davydov blue shift due
to the formation of a ³-dimer. The experimentally observed
transitions could be reproduced by TD-DFT calculations. From
an ESR spectrum of 1a•+, a hyperfine coupling constant (hfcc)
consistent with that for an MV state was obtained. The
1H NMR spectra of 1a2+ and 1a4+ were consistent with the
-
(¾) 308 (33000), 390sh (5800) nm; IR (KBr): 2918,
max
2853 cm¹1; Anal. Calcd for C26H20S12: C, 43.54; H, 2.81%.
Found: C, 43.34; H, 3.05%.
1,8-Bis(tetrathiafulvalenyl)naphthalene (1c). The syn-
thesis of 1c was carried out from tetrathiafulvalene and 1,8-
diiodonaphthalene in 85% yield in a similar manner as
described for 1a. 1c: an orange powder; mp 145.6-146.7 °C;
1
LDI-TOF-MS m/z = 532 (M+); H NMR (CDCl3, 400 MHz):
¤ 7.86 (dd, J = 8.0 and 1.0 Hz, 2H), 7.60 (dd, J = 8.0 and
1.0 Hz, 2H), 7.46 (t, J = 8.0 Hz, 2H), 6.30 (d, J = 6.8 Hz, 2H),
6.28 (d, J = 6.8 Hz, 2H), 6.05 (s, 2H); 13C NMR (CDCl3,
100 MHz): ¤ 135.2, 133.5, 130.9, 130.3, 129.8, 125.5, 119.2
(2C), 118.9, 118.1, 111.1, 110.7; UV-vis-NIR (4.2 © 10¹5 M,
CH2Cl2): -max (¾) 310 (30000), 400 (3100) nm; IR (KBr):

M. Hasegawa et al.
Bull. Chem. Soc. Jpn. Vol. 85, No. 1 (2012)
59
3061, 2924, 2853 cm¹1; Anal. Calcd for C22H12S8: C, 49.59; H,
2.27%. Found: C, 49.31; H, 2.47%.
This work was partly supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology. We thank Prof. Yasuhiro
Mazaki (Kitasato University), Prof. Tohru Nishinaga (Tokyo
Metropolitan University), and Dr. Yoshiyuki Kuwatani (VSN
Inc.) for helpful discussions, Prof. Gaku Yamamoto (Kitasato
University) for the lineshape analysis, and Prof. Fujiko Iwasaki
(the University of Electro-Communications) for the X-ray
analyses. Theoretical calculations were partly performed at the
Research Center for Computational Science, Okazaki, Japan.
4,5-Ethylenedithio-4¤-phenyltetrathiafulvalene (3). 3 was
synthesized from 2c and iodobenzene in 70% yield in a similar
procedure as described for 1a. 3: an orange powder; mp 125.4-
126.3 °C; MALDI-TOF-MS: m/z = 370 (M+); 1H NMR
(CDCl3, 500 MHz): ¤ 7.37 (d, J = 8.3 Hz, 2H), 7.34 (t, J =
8.3 Hz, 2H), 7.29 (t, J = 8.3 Hz, 1H), 6.48 (s, 1H), 3.28 (s, 4H);
13C NMR (CDCl3, 125 MHz): ¤ 136.0, 132.3, 128.9, 128.6,
126.3, 117.6, 114.0 (2C), 113.1, 105.4, 30.2 (2C); IR (KBr):
3063, 2957, 2923, 2852 cm¹1; UV-vis-NIR (7.1 © 10¹5 M,
CH2Cl2): -max (¾) 301sh (16500), 313 (16900), 336 (14264),
388sh (4150) nm; Anal. Calcd for C14H10S6: C, 45.37; H,
2.72%. Found: C, 45.18; H, 2.74%.
Supporting Information
1
Figures of H and 13C NMR, MS, and UV spectra of 1a-1c
and 3, cyclic voltammograms of 1a-1c and 3, electronic
absorption spectra of oxidized 1a and 3, and Cartesian
coordinates of 1a, 1a2+, and 1a4+ and the molecular orbital
phase and energy diagram for them. This material is available
free of charge via the Internet at http://www.csj.jp/journals/
bcsj/.
X-ray Crystallographic Study.
X-ray crystallographic
measurements were made by using graphite-monochromated
Mo K¡ (- = 0.71069 ¡) radiation. The intensity data were
collected with a Rigaku AFC-7R four-circle diffractometer at
rt for 1a and with a Rigaku RAXIS-RAPID Imaging Plate
diffractometer at ¹80 °C. The structures were solved by direct
method (SIR 9230) and refined by full-matrix least-squares
methods. All non-hydrogen atoms were refined anisotropically,
and hydrogen atoms were refined isotropically. Crystal data
for 1a: C26H16S12, MW: 713.18, space group C2/c (# 15),
References
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¹
¹
Crystal data for 4: 2(C26H16S12)¢(C6H5Cl)¢(I3 )¢(I )¢(I2),
3
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32+ as well as UHF/6-31G(d) for 3•+. For 1a, 1a2+, and 1a4+
Geometry optimization was
,
both syn- and anti-conformations were elucidated. All opti-
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by harmonic vibrational analysis. Time-dependent (TD) DFT
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1a•+, the geometry obtained from X-ray crystallographic
analysis 4 was used.
6
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26 The geometry optimizations of cation radicals at the
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structures of EDT-TTF moiety with good reproducibility, whereas
the DFT method gave only poor structures at EDT-TTF moieties.
Similar results were found in the literature. See: E. Demiralp,
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27 See Supporting Information.
28 The spin contents of cation radicals were evaluated through
double integration of the ESR spectra and comparison with
2,2,6,6-tetramethyl-1-piperidine-N-oxyl (TEMPO) radical and Mn
marker under identical conditions.
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16 The activation free-energy (¦Gº) from anti to syn of 1a is
¹1
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0.36 and 0.38 V vs. SCE, respectively, and the conjugation with
aryl moiety increases the E11/2-value of TTF: see Ref. 15a.
29 For recent examples, see: a) F. Schödel, U. Tutsch, F.
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19 Comproportionation constant (Kc) is defined as follow:
Kc
1 + 12+ ꢀ 2(1•+
)
The Kc value was calculated from the difference between the first
(E11/2) and the second (E21/2) oxidation potentials. See also: C.