Self-assembly of radially π-extended tetrathiafulvalene tetramers for visible and near infrared electrochromic nanofiber

Article abstract of DOI:10.1246/bcsj.20190283

The self-assembly and electrochromic nanofiber formation of radially 7pi;-extended tetrathiafulvalene (TTF) tetramers anchored to 1,2,4,5-tetraethynylbenzene were investigated. The tetramer with SBu-substituents underwent self-assembly in solution. Cationic species of the tetramer, obtained by chemical oxidation with Fe(ClO₄)₃, exhibited a marked electrochromism in the solution. Their electronic spectra revealed absorption bands corresponding to intermolecular mixed-valence aggregation based on (TTF//TTF)+, and π-aggregation based on (TTF?+//TTF?+) due to the strong molecular association in the cationic species. Furthermore, the tetramer formed an entangled nanoscale fibrous material from CHCl₃ hexane. Electrochemical oxidation of the nanofiber on an indium tin oxide electrode revealed a repeatable redox profile. The nanofiber displayed remarkable electrochromic behavior: The color of the fiber changed from purple (neutral) to brown/brownish green (dication and trication) and green (tetracation). These color changes of the nanofiber are similar to those in solution, and the electronic spectra of the oxidized nanofibers reflected the stacked TTF units in the cationic nanofibers.

Full text of DOI:10.1246/bcsj.20190283

Advance Publication Cover Page
Self-assembly of Radially π-Extended Tetrathiafulvalene Tetramers for Visible and
Near Infrared Electrochromic Nanofiber
Masashi Hasegawa* and Masahiko Iyoda*
Advance Publication on the web November 27, 2019
doi:10.1246/bcsj.20190283
© 2019 The Chemical Society of Japan
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Publication manuscripts.

Self-assembly of Radially π-Extended Tetrathiafulvalene Tetramers for Visible and
Near Infrared Electrochromic Nanofiber
Masashi Hasegawa*1 and Masahiko Iyoda*2
1Department of Chemistry, Graduate School of Science, Kitasato University, Sagamihara, Kanagawa 252-0373, Japan
2Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan
E-mail: masasi.h@kitasato-u.ac.jp
Masashi Hasegawa
Masashi Hasegawa received his Ph.D. from Tokyo Metropolitan University in 2004. He started his academic carrier as
postdoctoral research in Tokyo Metropolitan University, Massachusetts Institute of Technology, and Ehime University. In
2007, he moved to Kitasato University as a research associate. His current research interests focus on creating novel
π-conjugated compounds.
E-mail: iyoda@tmu.ac.jp
Masahiko Iyoda
Masahiko Iyoda received a Ph.D. degree (1974) in Chemistry from Osaka University. After postdoctoral work at Osaka
University and the University of Cologne (with Prof. Emanuel Vogel), he returned to Osaka University in 1977 and
became an Assistant Professor in 1978. In 1988, he became an Associate Professor in the Department of Chemistry,
Osaka University. In 1991, he moved to Tokyo Metropolitan University (TMU) as a Full Professor. Since retirement from
TMU in 2011, he has been working as a research professor at TMU. His research interests include the synthesis of
functional π-electron systems and conjugated macrocycles, the noncovalent synthesis of functional supramolecular
structures, and their applications in Materials Science.
Abstract
The self-assembly and electrochromic nanofiber
TTF•+ (radical cation) and TTF2+ (dication) at low oxidation
potentials. These redox processes are repeatable, and hence
formation of radially π-extended tetrathiafulvalene (TTF)
tetramers anchored to 1,2,4,5-tetraethynylbenzene were
investigated. The tetramer with SBu-substituents underwent
self-assembly in solution. Cationic species of the tetramer,
obtained by chemical oxidation with Fe(ClO4)3, exhibited a
marked electrochromism in the solution. Their electronic
spectra revealed absorption bands corresponding to
intermolecular mixed-valence aggregation based on
(TTF//TTF)•+, and π-aggregation based on (TTF•+//TTF•+) due
to the strong molecular association in the cationic species.
Furthermore, the tetramer formed an entangled nanoscale
fibrous material from CHCl3–hexane. Electrochemical
oxidation of the nanofiber on an indium tin oxide electrode
revealed a repeatable redox profile. The nanofiber displayed
remarkable electrochromic behavior: the color of the fiber
changed from purple (neutral) to brown/brownish green
(dication and trication) and green (tetracation). These color
changes of the nanofiber are similar to those in solution, and
the electronic spectra of the oxidized nanofibers reflected the
stacked TTF units in the cationic nanofibers.
TTF can be a trustworthy module for redox-triggered
functional materials in host-guest chemistry,11-14 positive
electrode materials,15,16 electrochromic (EC) devices,17,18 and
chiroptical switching materials.19-24 Furthermore, the molecular
self-assembly of TTF and its derivatives has become a superior
vehicle leading to complex nanoscopic structures that are
difficult to fabricate using electron beam lithography
techniques as
a
top-down approach.25-29 The strong
intermolecular stacking by π‒π and electrostatic force between
TTF0//TTF0, (TTF//TTF)•+ (mixed-valence, MV), and
(TTF//TTF)2+ (π-dimer) can be expected both in solution and in
solid state.30-32 Thus,
a
variety of electroconducting
one-dimensional (1D) nanofibers based on TTF derivatives
have been developed through molecular self-assembly so
far.33-37
When dealing with doped nanofibers (oxidized
nanofibers), there are two pivotal stages that should be
overcome. The first is the method for molecular assembly that
can provide a long-range 1D ordering of cationic species in
nanostructures. Highly adequate molecular design should be
required for ordered nanofibers, because coulombic repulsion
between cationic species prevents desirable molecular
stacking.38-40 The second is the doping method to generate
unfilled open-shell electronic structures responsible for
electrical conductivity. Exposure of neutral fibers to iodine
vapor is commonly applied for the emergence of
conductivity.41-49 Nevertheless, the cationic state generated by
adsorption of iodine vapor is usually unstable due to the easy
desorption of iodine. Previously, we found that
hexakis(tetrathiafulvalenylethynyl)benzene (2a in Figure 1)
underwent strong molecular association in solution both in
neutral and oxidized states.50 The addition of excess amounts of
Keywords:
electrochromism,
tetrathiafulvalene,
nanofiber
1. Introduction
Apart from the initial research interest in electrical
conductivity and superconductivity, tetrathiafulvalene (TTF)
and its derivatives have been extensively utilized as building
resources in the design of supramolecular chemistry and
materials science.1–10 TTF undergoes definite oxidation to give

poor solvent (less polar solvent) into the solution containing
self-assembled cationic species readily led to the electrical
conducting nanofibers. However, such a successful example is
still limited due to the fundamental difficulty in the formation
of ordered structures consisting of cationic species. Therefore,
our requirements of doped nanofibers forced us to develop
alternate methods of preparing ordered cationic species.
self-assembly and nanofiber formation that is stable enough to
be treated under ambient conditions. In addition, facile
formation of the MV nanofibers by electrical stimuli results in
clear multi-step electrochromism and conventional regulation
of the oxidation state in the nanofibers.
2. Results and Discussion
Synthesis. Synthesis of 1a and 1b was summarized in
Scheme 1. Four TTF units were anchored by the Sonogashira
coupling reaction of 1,2,4,5-tetraethynylbenzene, prepared
from 1,2,4,5-tetrakis(trimethylsilylethynyl)benzene (5), with a
iodo-TTF derivative (6a and 6b). Compounds 1a and 1b were
obtained as deep purple solids in moderate to good yields. The
1
structures of 1a and 1b were fully characterized by H and 13C
NMR, MS, IR, and UV-Vis spectra. Because of the extremely
low solubility of octaethylthio derivative 1b, further studies on
the self-assembly and nanostructure formation were performed
only on the octabutylthio derivative 1a. Compound 1a is much
stabler than 2a both in solution and solid state, and hence we
can treat under ambient conditions without particular treatment.
As
a
reference
compound
for
1a
and
1b,
1,2-bis(tetrathiafulvalenylethynyl)benzene 4c was prepared in
90% yield by a similar method.
Structural Analysis. Although single crystals of 1a and
1b suitable for X-ray analysis were not obtained, we have
succeeded in X-ray structural determination of 4c as a model
for the half-unit of radially π-extended tetramer (Figure 2).
Compound 4c crystallizes in the monoclinic chiral space group
P21. Compound 4c has an almost planar structure except for the
MeS-groups, and the two TTF units are aligned side by side
with a tilting angle. One of two TTF units is slightly twisted
toward the central benzene ring, while the other TTF unit and
benzene ring are almost coplanar. There are two intramolecular
S••H short contacts between the two TTF units: S(1)•••H(12)
2.811 Å and S(8)•••H(6) 2.994 Å, which are shorter than the
sum (3.05 Å) of van deer Waals radii of sulfur (1.85 Å) and
hydrogen (1.2 Å). Thence, the enantiomeric propeller-like
structure in the crystal was induced by these interactions.
Furthermore, 4c stacks along the b-axis closely (Figure 2b) to
form a one-dimensional columnar structure. The face-to-face
distance between the S(1) atom and the neighboring least
squares plane of TTF is found to be 3.62 Å. In addition, there
are two additional S•••H interactions between stacked
molecules.
Figure 1. Chemical structures of radially π-extended
tetrathiafulvalene oligomers (1–3)
Another possible approach is post-doping of the
nanofibers by electric stimuli. The application of a proper
voltage to the electrode where the nanofibers are deposited can
easily lead to MV or higher oxidation states. Such
electrochemical doping may be accompanied with multi-color
EC behavior. Thus far, EC films based on electro-active organic
or inorganic compounds have been developed because of their
potential applicability in displays and smart electronics.51-56 In
1979, Kaufman et al. achieved EC films based on polymer
compounds having TTF moieties as
a
redox-active
chromophore.17 However, few studies have focused on EC
materials composed of flexible nanofibers, gels, and other soft
materials,57–60 although they can be a promising component for
wearable electronics in the future.
We
have
newly
designed
tetrakis(tetrathiafulvalenylethynylbenzene 1a and 1b for the
core structure of self-assembled nanofibers. Although
numerous TTF oligomers, extended with acetylene scaffolds
such as 2a, exhibit strong self-assembly leading to
nanofibers,61–67 they occasionally decompose under ambient
conditions in solution owing to the intrinsic thermal instability
of the acetylene scaffold. However, the previously reported 3a
and 3b are stable despite weak self-assembly and no fiber
formation.68 Thus, the decrease in the number of ethynyl-TTF
units from 2a to that in the present system, could encourage
both self-assembly and stability. We report herein,
Scheme 1. Synthesis of 1a, 1b, and 4c.

Figure 3. Calculated molecular structure of (a) 1c-i and (b)
1c-ii (r = 3.12 Å, 1 = 19.7°, 2 = 3.91°). (c) Frontier molecular
orbitals of 1c-ii calculated by B3LYP/6–31G(d,p).
Figure 2. Synthesis of (a) ORTEP drawing of 4c. Thermal
ellipsoids are drawn at the 50% probability, (b) Crystal packing
structure along the b-axis; hydrogen atoms are omitted for the
sake of clarity, and (c) Stacking motif of 4c (upper molecule:
filled with orange and lower molecule: filled with light yellow).
Self-assembly in Solution. Radial tetramer 1a displays
self-assembly in solution, and their properties were studied by
1H NMR spectra. The spectrum in CDCl3 solution exhibited
both concentration and temperature dependence. The signal
assigned to the aromatic proton Ha of 1a varied from  7.49 to
7.30 ppm as the concentration changed from 0.55 to 67.2 mM
at 20 °C. In 15.9 mM solution, the chemical shift of Ha varied
from  7.43 to 7.33 ppm as the temperature changed from 40
to –30 °C. This obvious upfield shift is due to the shielding
effect of the benzene ring in the neighboring molecules when
1a stacks in a face-to-face geometry. However, the TTF proton
Hb did not show remarkable concentration dependence. Only
small temperature dependence of a downfield shift less than
0.07 ppm was observed from 30 to –30 °C, presumably owing
to the low shielding effect of the 1,3-dithiole ring associated
with the intramolecular S•••H interactions.
A brief screening of geometry optimization of 1c with
SMe groups at B3LYP/6-31G(d,p) level of theory for several
point groups suggested that both a conformation with D2
symmetry (1c-i) and a propeller-like quasi-planar conformation
with Ci symmetry (1c-ii) are relatively stable (Figure. 3). The
former conformation has four TTF units anchored with an
ethynylbenzene core in the parallel direction. However, the
latter propeller-like conformation has two TTFs that tilt slightly
from the central benzene ring so as to approximate a
neighboring H atom (r = 3.12 Å in Figure. 3b). The dihedral
angles 1 and  2 between the central benzene ring and TTFs are
19.7° and 3.91°, respectively. This conformation is similar to
the molecular structure of 4c found in the molecular packing in
the crystal. Although the total energy of 1c-ii (+0.9 kJ mol–1) is
slightly higher than that of 1c-i in the gas phase, this
conformation could be favorable for molecular packing in the
solid state as found in the crystal structure of 4c. The calculated
HOMO and LUMO based on 4c-ii geometry are illustrated in
Figure 3c. The HOMO and almost degenerated HOMO–1 are
mainly located on four TTF moieties, while the LUMO and
almost degenerated LUMO+1 are located on the
tetraethynylbenzene core. As for other conformations,
essentially similar HOMO and LUMO descriptions were
obtained. This is a typical tendency for extended TTF with an
ethynyl–π scaffold.50, 61–68
Assuming
a
monomer-oligomer equilibrium
(oligomeric association model) for the self-assembly of 1a, the
association constant (Ka) was determined to be 21.5 ± 3.1 M–1
at 293 K in CDCl3.69 Furthermore, thermodynamic parameters
were also estimated from the results of variable temperature 1H
NMR experiments: H = –16.7 (kJmol–1) and S = –31.3
(JK–1mol–1). The self-assembly appears to be driven by
enthalpy, but opposed by entropy. The negative enthalpy value
is due to the π-π stacking, van der Waals interactions, and
S•••H interactions in the molecular stacking.
Redox Properties. The electrochemical properties of 1a
and 4c were investigated by cyclic voltammetry (CV), and the
redox potentials are summarized in Table 1 together with those
of the related trimer 3a.68 As shown in Figure. 4a, the CV
profile of 1a in dilute solution (c = 1.0 x 10–5 M) displayed a
1
The H NMR spectrum of 1a suggests an intramolecular
S•••H contact in solution: chemical shift assigned to the Hb
proton in 1a (Figure. 1) was found at lower field of  6.74 ppm
(CDCl3, 20 °C) similar to the TTF proton of 4c at  6.70 ppm
(CDCl3, 20 °C), while the corresponding protons of 6a and 3a
were observed at  6.40 and 6.57 ppm, respectively. Such S•••H
interactions could give a pseudo-planar geometry as a favored
conformation despite the free rotation of the terminal TTF
moieties.
broad reversible response centered at E11/2 = 0.04 V and E2
=
1/2
0.40 V (vs. Fc/Fc+). The first anodic peak was found to be
broad. Because the CV profile of 4c exhibited similar redox
waves at comparable potentials, the CV profile of 1a revealed
few intermolecular or intramolecular interactions between the
TTF
units
through
the
acetylene
scaffold
(–ethynylene–phenylene–ethynylene–).61 Hence, the two
oxidation peaks in Figure. 4a correspond to the formation of
1a4+ and 1a8+, respectively. On the contrary, CV under
concentrated conditions (c = 1.2 x 10–3 M) resulted in three
pairs of well-dissolved reversible redox waves at E1 = –0.06,
1/2

E2 = 0.09, and E3 = 0.40 V (V vs. Fc/Fc+). Judging from
1/2
1/2
the peak current ratio and molecular structure, the first and
second oxidations can be attributed to the oxidation to 1a2+ and
1a4+ formation, respectively. The highest oxidation wave
should correspond to the formation of 1a8+. As the
intramolecular electronic interactions among the TTF moieties
are negligible in this scaffold, 61 the formation of 1a2+, before
the oxidation to 1a4+ implies intermolecular MV stacking based
on (TTF//TTF)•+ or its larger aggregates.70 Thus, the MV
species (1a2+)2 were predominantly formed at E1 in the
concentrated conditions, following which the aggregates may
undergo further oxidation at E2. Such a redox profile was found
in a series of TTFs in the radial oligomers of 2a50 , 3a68 and
also found in trimeric or tetrameric stacking TTF system.31
Figure 5. (a) Colors and (b) electronic spectra of 1a (2.1 x 10–5
M), 1a•+ (2.8 x 10–5 M), 1a2+ (2.8 x 10–5 M), 1a3+ (4.2 x 10–5
M), 1a4+ (3.4 x 10–5 M) and 1a8+ (1.5 x 10–5 M) in
CH2Cl2-MeCN (v/v = 4:1) solution. The inset shows enlarged
spectra of 1a•+ and 1a4+.
Figure 4. CV charts of 1a in CH2Cl2 at 23 °C. (a) 1a: 1.0 x 10–5
M and (b) 1a: 1.2 x 10–3 M. Conditions: CH2Cl2, 0.1 M
Bu4ClO4, all potentials were measured against an Ag/Ag+
reference electrode and converted to vs. Fc/Fc+.
Radical cation 1a•+ exhibited very broad absorption at
approximately 1800-2600 nm together with a prominent
absorption at 783 nm (Figure 5b). This spectral feature is quite
similar to that of the cationic species derived from 2a50 and
3a,68 which exhibit strong self-association in solution. The
broad absorption can be attributed to the charge resonance (CR)
in an MV state based on a (TTF//TTF)•+ stacking unit. On the
contrary, the electronic spectra of radical cation 1c•+ did not
exhibit such a broad absorption in long wavelength region
(Figure S9). On the basis of previous investigations of
face-to-face arranged TTFs,30,31 the two remarkable absorption
bands are interpreted as a combination of stacked TTF units in
an MV aggregate (Figure. 6a). They are assigned to the
electronic transition from the HOMO to the SOMO (S1, CR
nature) and from the HOMO–1 to the SOMO (S2) in stacked
species of 1a•+. These spectral outlines did not change even in
dilute solution, indicating large association constants for the
aggregation of dications and trications in solution.
Table 1. Redox potentials of 1a, 4c, and 3a (V vs. Fc/Fc+)
Conc.
(mM)
E11/2 (V)
E21/2 (V)
E31/2 (V)
0.04
(4e–)
–0.06
(2e–)
0.06
(1e–)
–0.04
(3e–)
0.04
(4e–)
0.09
1a a
1a a
4c a
3a b
0.01
–
0.40
(4e–)
1.2
0.42
5.4
(2e–)
0.37
(1e–)
–0.14
(3e–)
–0.47
(6e–)
a Measured in CH2Cl2. b Measured in PhCN (ref. 68).
Electronic Spectra of Cationic Species. We performed
the chemical oxidation of 1a with Fe(ClO4)3 in CH2Cl2-MeCN
(v/v = 4:1) to afford cationic species. The addition of 1, 2, 3, 4,
and 10 (excess) equivalents of Fe(ClO4)3 formed 1a•+, 1a2+,
1a3+, 1a4+ and 1a8+, respectively (Figure 5). During the
chemical oxidation, 1a displayed clear electrochromism shown
in Figure 5a.
Table 2. Absorption maxima of 1a and its cationic species in
CH2Cl2-MeCN (v/v = 4:1) solution a
Conc.
(M)
Color
max (nm)
1a
purple
dark-
brown
21
320 (S2), 514 (S1)
321, 550, b 783 (S2),
ca. 2060 (S1)
314, 550,b 778 (S2),
ca. 1900 (S1)
311, 385, 767 (S2),
ca. 1800 (S1)
400, 750 (S2), ca. 900 b (S1)
The electronic spectrum of 1a exhibits two
1a•+
28
28
42
absorption maxima at 514 and 320 nm (Table 2). These are
typical absorption bands found in an acetylene-extended TTF
framework.48,50,61,68 The absorption band at 514 nm is
associated with charge transfer electronic transition from the
HOMO of the TTFs to the LUMO of the acetylene moieties,
while the absorption at 320 nm involves the electronic
transition within the TTF frameworks. The time-dependent
1a2+
1a3+
brown
brownish
green
green
1a4+
1a8+
34
15
(TD) DFT study of model compound
1c at
blue
307, 640
a Spectra were measured in CH2Cl2-MeCN (v/v = 4:1). S1 and
S2 denote electronic transitions described in Figure 6.
b Shoulder absorption.
CAM-B3LYP/6-31G(d,p) level reproduced these transitions
qualitatively (Figure S3).

Figure 7. Scanning electron micrograph of the nanofibers of 1a
on an Si-wafer. (b) XRD profile of the nanofibers of 1a. Inset
shows appearance of the fibril material obtained from cloudy
suspensions.
The longest absorption maximum of π-π* transition of 1a
fiber exhibited a red-shift reaching 530 nm (Table 3). The
observed bathochromic shift compared to the absorption
maximum of 1c in CHCl3 solution (514 nm) is attributed to the
J-type aggregation (off-set geometry), which can be a similar
arrangement found in the X-ray crystal packing of 4c. The
out-of plane X-ray powder diffraction (XRD) of as-cast 1a
fibers on the Al plate displayed a mostly amorphous halo with
relatively weak peaks at 2 = 3.10, 7.58° (d = approximately
28.5 and 11.6 Å) and some other ambiguous peaks (Figure. 7b).
The observed d-spacing of the smallest 2  is comparable to the
molecular size estimated from PM3-optimization of 1a (35 Å)
(Figure. S11). Therefore, compound 1a was slip-stacked with
face-to-face stacking to give a one-dimensional nanofiber. The
addition of poor solvents may promote effective phase
separation of aggregated 1a.
Figure 6. Possible electronic transitions of (a) MV-aggregate
(TTF//TTF)•+ in 1a•+ and 1a2+, and (b) π-aggregate
(TTF//TTF)2+ in 1a4+.
In contrast, 1a4+ exhibited no CR band in the NIR region.
The electronic spectra exhibited absorption maxima at 750 and
400 nm. The former band tails with a shoulder peak
(approximately 900 nm) up to 1400 nm are shown (Figure 5b,
inset). The absorption maximum at 752 nm was blue-shifted
compared to the S2 band in 1a•+ (783 nm). This hypsochromic
shift can be associated with the Davydov blue-shift because of
the face-to-face stacking of TTF•+ in π-dimer or larger
π-aggregates.30-32 The broad shoulder is related to the
HOMO-LUMO transition, which is essentially forbidden in a
π-aggregate of TTF•+ (Figure 6b). As these experiments were
performed under dilute conditions, 1a4+ was stacked strongly in
solution. Finally, the spectrum of 1a8+ exhibited only a sharp
absorption band at 640 nm, suggesting no association behavior
of 1a8+ due to the repulsion between the TTF2+ units.
Doping of the nanofibers, which are formed as a pellet
shape, with I2 vapor resulted in an inky-black nanofiber without
collapse. The bulk electrical conductivities of the resultant
fibers reached moderately high conductivities of rt = 5.5 x 10–4
Scm–1, reflecting the molecular stacking in the fiber structure
(Figure. S12).
On the other hand, the electronic spectra of the radical
cations of 4cn+ (n = 1, 2, and 4), which were prepared via
oxidation using Fe(ClO4)3 under similar conditions, exhibited
spectra based only on the oxidation of the TTF scaffold (Figure.
S9). Therefore, there are neither intramolecular nor
intermolecular interactions in the cations derived from 4c in
solution.
Electrochromic Properties of Nanofiber. The nanofibers
of 1a are thin enough, as observed in SEM measurements
(Figure 7a), and hence we envisage that the nanofibers undergo
oxidation by post-electrical doping on the electrode surface. We
first measured redox properties of the nanofibers on an indium
tin oxide (ITO) electrode (Figure 8a). The electrode, where the
nanofibers were deposited, was immersed in MeOH containing
0.1 M Bu4NClO4 as the electrolyte. CV profiles between –0.4
and 0.8 V (vs. Fc/Fc+) gave only irreversible anodic peaks
(Figure S13), and nanofibers completely dissolved in MeOH
beyond the second oxidation (> 0.2 V). However, repeatable
redox cycles were observed without dissolution during the
scanning of a narrower potential range (–0.4-0.18 V).
CV profiles were strongly affected by the scan rate. When
faster scan rate of 100 mV/s was applied, a couple of
semi-reversible peaks with wider peak-to-peak separation are
recorded without remarkable color changes (Figure S15). In the
fast-scanning condition, electron transfer might occur only on
the surface, and hence the nanofiber could not undergo
complete oxidation. On the contrary, when slow scanning of 1
mV/s was applied, several intense peaks were found (Figure
8a). In the oxidation process, a weak current response at
approximately –0.15 V and an intense peak at 0.13 V were
observed. In the cathodic scan, on the contrary, there are two
peaks at 0.02 and –0.1 V. These redox profiles were repeatable
without particular decomposition at least 10 times.
Nanofiber Formation. Compound 1a formed a fibrous
structure in CHCl3-hexane solution. When hexane was added to
a self-assembled solution of 1a in CHCl3, dark purple fibrous
material appeared from the resultant cloudy suspension after
several hours. Similar fibrous material was obtained by an
ordinal reprecipitation procedure using CHCl3 or CH2Cl2 with a
poor solvent such as MeOH, EtOH, ether, or pentane. Scanning
electron microscopy (SEM) measurements revealed that the
fibril material from CHCl3-hexane is composed of a slender
and frizzled fiber structures (Figure 7a). The fibrils are
approximately 100–200 nm thick and the length was above 20
nm. There is almost no concentration or solvent dependence of
the morphology.

Table 3. Absorption maxima of self-assembled nanofibers of
1a and its cationic species at various oxidation stages
obtained by post-doping a
Ox. Stage b
(Potential)
I (–0.40 V)
Conc.
(mM)
1a
color
max
Purple
530 (S1)
550,c 818 (S2),
ca.1800 (S1, CR)
1an+
(0 < n < 4)
Brownish
green
II (0.00 V)
c
418, 750 (S2),
ca. 1010 (S1) c
III (0.18 V)
1a4+
Green
a
Spectra of the nanofibers were measured on an ITO
electrode, which was immersed in MeOH containing
b
Bu4NClO4 as the electrolyte. Each oxidation stage was
depicted in Figure 8a.
c Shoulder absorption.
Figure 8. (a) Cyclic voltammograms of the nanofiber of 1a
with potential scan at 1 mV/s. (b) Photographs of the fibers of
1a at I (–0.4 V), II (0.0 V), III (0.13 V) and IV (0.18 V) in the
CV profile.
Figure 9. Electronic spectra of neutral and cationic nanofibers
on quartz plate (1a) or ITO electrode (1an+ and 1a4+).
As shown in Figure 8b, the nanofiber on the electrode
exhibited various colours. For instance, the colour gradually
changes from purple to brown around 0.0 V (Stage II, in Figure
8a), brownish green around 0.13 V (Stage III, in Figure 8a),
and then vivid green around 0.18 V (Stage IV, in Figure 8a)
along the oxidation process. Opposite colour change –– from
green, brownish green, brown, to purple –– was observed along
the reduction process (Figure S16). These colour changes are
almost coincident with those of cationic species in solution
(Figure 5a). When the voltage is kept constant, the colour is
also stationary without remarkable decomposition.73 Thus, the
electronic spectra can be collected at the selected stage (Table 3
and Figure 9). The electronic spectra at the constant voltage of
0.00 V on the anodic scan (Stage II, in Figure 8a) possess
absorption maxima at 818 nm, together with a weak shoulder
(Table 3). The former absorption band, assigned to the S2
transition, exhibited a remarkable bathochromic shift compared
to that of 1an+ (0 < n < 4) in solution (767–787 nm, Table 2). It
is presumably due to the molecular stacking within the J-type
arrangement that was solidified as the nanofiber in the neutral
state before doping. The broad shoulder absorption (S1),
featured by CR nature in the stacked TTF units, was slightly
weakened compared with that in solution. An off-set stacking
based on molecular assembly in neutral solution may result in
the decrement of this absorption band. When the constant
voltage of 0.18 V (Stage IV in Figure 8) was applied, the green
nanofiber exhibited characteristic absorption bands at 750 (S2),
together with shoulder absorption (S1, max = ca. 1010 nm) tails
to 2500 nm. This spectrum curvature is quite similar to that of
By comparison with the redox potentials of 1a in solution,
we can conclude that the first broad current response and the
latter intense peak in the oxidation process can be attributed to
the sequential formation of MV cationic species of 1an+ (n = 1,
2, and 3) and tetracation 1a4+ in the nanofibers, respectively.In
the range below 0.00 V, oxidation of 1a to 1a•+ first occurs on
the ITO electrode. Thereby, the injected hole can be quickly
delocalized though π-π stacking in the nanofiber, although the
diffusion of the anion into the fiber might be slow to lead to a
weak current response. Then, the fiber continuously undergoes
oxidation with the low anodic current until 1a is oxidized to
form 1a3+ (< ca. 0.00 V). After that, oxidation corresponding to
1a3+/1a4+ occurs with larger peak current. Similar oxidation
profile was found in highly self-assembled species of 2a in
solution50 and in π-conjugated polymer thin film deposited on
the electrode.71,72 However, in the reduction process, the first
step at 0.02 can be assigned to the process from 1a4+ to 1a3+,
correlating with the corresponding peak at 0.13 V in the
oxidation process. The second peak at –0.1 V may correspond
to the process from 1a3+ to 1an+ (n = 2, 1, and 0), in which the
reduction of 1a3+ to 1a2+ is the most energetically favorable
process, exhibiting an intense peak. Such a large difference
between the oxidation and reduction profile is presumably due
to the difference of the rate in electrical doping/dedoping in the
nanofibers on the electrode surface.

typical dimeric TTFs in π-dimer form found in the
literature.30,31 While the S2 band (750 nm) was found at a
similar wavelength to that in solution of 1a4+ (S2, 752 nm), a
bathochromic shift and clear enhancement were observed in S1
transition as compared to that in solution (approximately 900
nm), presumably due to its tight aggregation in the solid state.
2.
3.
4.
TTF Chemistry: Fundamental and Applications of
Tetrathiafulvalene, (Eds.: J. Yamada, T. Sugimoto),
Kodansha-Springer, Tokyo, 2004.
A. Jana, S. Bähring, M. Ishida, S. Goeb, D. Canevet, M.
Sallé, J. O. Jeppesen, J. L. Sessler, Chem. Soc. Rev. 2018,
47, 5614.
V. Croué, S. Goeb, M. Sallé, Chem. Commun. 2015, 51,
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4. Conclusion
5.
6.
F. Pop, N. Avarvari, Chem. Commun. 2016, 52, 7906.
S. Bähring, H. D. Root, J. L. Sessler, J. O. Jeppesen, Org.
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We have synthesized TTF tetramers (1a and 1b) anchored
to a 1,2,4,5-tetraethynylbenzene scaffold. DFT calculations of
1c and X-ray crystallographic analysis of 4c revealed a stable
molecular structure having intramolecular S••H interactions
7.
1
between two neighboring TTF moieties. H NMR experiments
8.
9.
of 1a at various temperatures and concentrations in CDCl3
suggest molecular self-assembly in solution, and the
association constant (Ka) was determined (21.5 ± 2.0 M–1 at
20 °C). CV of 1a in CH2Cl2 displayed two two-electron and one
four-electron reversible redox responses in concentrated
solution, although two four-electron redox waves were
observed in dilute conditions. These differences are indicative
of the intermolecular interactions between radical cations;
furthermore, electronic spectra of radical cations, prepared by
the chemical oxidation with Fe(ClO4)3, revealed their electronic
structures. The oxidized species of 1an+ (0 < n < 4) exhibited
the intrinsic absorption of TTF•+ (S2) and characteristic CR
bands (S1) in the NIR/IR region (1200-2500 nm), attributed to
MV states among the intermolecular TTF stacks. Moreover,
1a4+ exhibited blue-shifted maximum absorption because of
face-to-face stacked TTF•+ units in solution. One-dimensional
nanofibers of 1a were prepared from CHCl3-hexane solution.
The redox and electronic properties of the nanofibers were
investigated using CV technique on an ITO electrode.
Repeatable color changes were observed between 1a (purple),
1an+ (0 < n < 4) (brown and brownish green), and 1a4+ (green),
and each color is associated with their oxidation stage. While
the color of the nanofibers of 1a at each oxidation stage is
almost similar to that in solution, the electronic spectra of the
fibers are affected by the molecular packing in the nanofibers.
Although an approach to stationary doped-nanofibers without
continuous electrical injection is still developing, our approach
using the electrochemical oxidation of nanofibers on ITO
electrode shows a new application of post-doping nanofibers.
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Acknowledgement
We thank Prof. Dr. Kenro Hashimoto (The Open University of
Japan), Mr. Jun-ichi Takano (Tokyo Metropolitan University),
Dr. Kota Daigoku (Kitasato University), Dr. Yoshiyuki
Kuwatani (VSN Inc.), and Prof. Dr. Yasuhiro Mazaki (Kitasato
University) for the helpful discussion. This work was partially
supported by the JSPS KAKENHI grants JP04J04136,
JP16K17871, and JP18K05092. All calculations were
performed at the Research Center for Computational Science,
Okazaki (Japan).
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Supporting Information
Instrumentation and materials, experimental procedures, NMR
spectra, crystal data, cyclic voltammograms, and DFT studies.
This material is available on http://dx.doi.org/10.1246/bcsj.
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