Synthesis of donor-acceptor chromophores by the [2 + 2] cycloaddition of arylethynyl-2H-cyclohepta[b]furan-2-ones with 7,7,8,8-tetracyanoquinodimethane

Article abstract of DOI:10.1039/c2ob06931h

Arylethynyl-2H-cyclohepta[b]furan-2-ones reacted with 7,7,8,8- tetracyanoquinodimethane (TCNQ) in a formal [2 + 2] cycloaddition reaction, followed by ring opening of the initially formed cyclobutene derivatives, to afford the corresponding dicyanoquinodi

Full text of DOI:10.1039/c2ob06931h

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PAPER
Synthesis of donor–acceptor chromophores by the [2 + 2] cycloaddition of
arylethynyl-2H-cyclohepta[b]furan-2-ones with 7,7,8,8-
tetracyanoquinodimethane†
Taku Shoji,*a Junya Higashi,b Shunji Ito,c Tetsuo Okujima,d Masafumi Yasunamie and Noboru Moritab
Received 17th November 2011, Accepted 9th January 2012
DOI: 10.1039/c2ob06931h
Arylethynyl-2H-cyclohepta[b]furan-2-ones reacted with 7,7,8,8-tetracyanoquinodimethane (TCNQ) in a
formal [2 + 2] cycloaddition reaction, followed by ring opening of the initially formed cyclobutene
derivatives, to afford the corresponding dicyanoquinodimethane (DCNQ) chromophores in excellent
yields. The intramolecular charge-transfer (ICT) interactions between the 2H-cyclohepta[b]furan-2-one
ring and DCNQ acceptor moiety were investigated by UV/Vis spectroscopy and theoretical calculations.
The redox behavior of the novel DCNQ derivatives was examined by cyclic voltammetry (CV) and
differential pulse voltammetry (DPV), which revealed their multistep electrochemical reduction properties
depended on the number of DCNQ units in the molecule. Moreover, a significant color change was
observed by visible spectroscopy under electrochemical reduction conditions.
tetracyanobutadienes (TCBDs) and dicyanoquinodimethanes
(DCNQs) by the formal [2 + 2] cycloaddition, followed by ring-
Introduction
Charge-transfer (CT) materials composed of an organic donor–
acceptor system have been intensively studied because of their
potential for applications in organic electronic devices in the
next generation. Cyano-based acceptor is one of the most prom-
ising units for application in organic electronic devices within a
large number of organic acceptors previously reported.1 Tetra-
cyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethane
(TCNQ) are well known as powerful electron acceptors to form
CT complexes with a variety of electron-rich organic and orga-
nometallic compounds.2 The CT complexes are revealed to
exhibit a number of interesting properties such as electric con-
ductivity,3 which is a very promising feature for application in
optoelectronic devices (e.g., organic light-emitting diodes and
solar cells).4
opening cycloreversion of the initially formed cyclobutene
derivatives, of TCNE5 and TCNQ6 with a variety of alkynes
bearing an electron-donating substituent (e.g., N,N-dialkylani-
line, ferrocene, and tetrathiafulvalene substituents). These com-
pounds are revealed to show the intense ICT absorptions on their
UV/Vis spectra. Furthermore, the ICT chromophores are
expected to show optical non-linearities7 and their utilities as
organic-base magnets and conductive materials are proved.5,6
Li et al. have also reported the synthesis and properties of
donor-acceptor substituted π-conjugated system by cyclo-
addition-reversion sequence reaction of acetylene with TCNE
and TCNQ.8
Michinobu et al. have extended this class of chemistry to the
polymer science.9 They reported the TCBD and DCNQ-
embedded polymer materials to show the ICT absorption bands
with the low energy band gap. Moreover, these polymers are
expected to be promising materials for semiconductors in
organic photovoltaic devices,9a nonlinear optical applications9c
and ion sensors.9i
Recently, we have reported the reaction of mono-, bis-, and
tris(1-azulenylethynyl)benzene and thiophene derivatives with
TCNE10 and TCNQ11 to give the corresponding azulene-substi-
tuted TCBD and DCNQ derivatives as redox active ICT chromo-
phores, which revealed their electrochromic behavior to show
significant color changes under electrochemical reduction con-
ditions. Therefore, from the viewpoint of the development of
organic electronics, construction of a new series of donor–accep-
tor systems composed of cyano-based acceptors is one of the
most attractive targets in current organic chemistry.
Diederich et al. have reported the synthesis of intramolecular
charge-transfer (ICT) chromophores, donor-substituted 1,1,2,2-
aDepartment of Chemistry, Faculty School of Science, Shinshu
University, Matsumoto 390-8621, Japan. E-mail: tshoji@shinshu-u.ac.
jp; Fax: +81 263 2476; Tel: +81 263 2476
bDepartment of Chemistry, Graduate School of Science, Tohoku
University, Sendai 980-8578, Japan
cGraduate School of Science and Technology, Hirosaki University,
Hirosaki 036-8561, Japan
dDepartment of Chemistry and Biology, Graduate School of Science and
Engineering, Ehime University, Matsuyama 790-8577, Japan
eDepartment of Materials Chemistry and Engineering, College of
Engineering, Nihon University, Koriyama 963-8642, Japan
†Electronic supplementary information (ESI) available: The experimen-
tal, UV/Vis spectra, cyclic voltammograms and theoretical calculations
of reported compounds. See DOI: 10.1039/c2ob06931h
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2H-Cyclohepta[b]furan-2-one is well known as a heteroazu-
lene, a versatile precursor for azulene derivatives.12 For the
extension of this chemistry, we have focused our attention on the
synthesis of 2H-cyclohepta[b]furan-2-one-substituted TCBD
derivatives by the reaction of the corresponding alkynes with
TCNE.13 The study revealed the TCBD derivatives feature a
strong ICT absorption in their UV/Vis spectra, which is charac-
terized by time-dependent density functional theory (TD-DFT)
calculations. Reaction of arylethynyl-2H-cyclohepta[b]furan-2-
ones 1–9 with TCNQ is expected to show high reactivity toward
the formal [2 + 2] cycloaddition–cycloreversion sequence to
afford the novel DCNQ derivatives. Moreover, 2H-cyclohepta[b]
furan-2-one-substituted DCNQs should also exhibit multistage
redox behavior, similar to the TCBD derivatives. Moreover, a
significant color change is also expected during the electroche-
mical reduction of the DCNQ derivatives.
Herein, we describe the synthesis of the 2H-cyclohepta[b]
furan-2-one-substituted DCNQs by the reaction of arylethynyl-
2H-cyclohepta[b]furan-2-ones 1–9 with TCNQ. Electronic prop-
erties of the novel donor–acceptor systems are characterized by
absorption spectroscopy, theoretical calculations, cyclic voltam-
metry (CV), and differential pulse voltammetry (DPV). The data
confirm the ICT characters of the novel donor–acceptor systems
with their electrochromic property.
The electronically more deficient alkyne 2, which has p-nitro-
substituent on the benzene ring, also reacted with TCNQ to
afford 11 in 90% yield (Scheme 2, Table 1, entry 1), although
the relatively longer reaction period was required in this case
(24 hours). Recently, we have reported the reaction of 2 with
TCNE, which completed within 2 hours.13 Diederich et al. have
also reported that TCNQ shows lower reactivity, compared with
TCNE, towards the formal [2 + 2] cycloaddition reaction with
electron-deficient cyanoalkynes.6b Furthermore, they have also
reported that excess of TCNQ, prolonged reaction period, and
elevated temperature are required to complete the reaction with
the alkynes substituted with a strong electron-withdrawing
group. Therefore, the relatively longer reaction period in the
reaction of 2 also reflects the lower reactivity of TCNQ toward
such electron deficient alkynes. The new chromophore 12 was
also obtained in 92% yield by the formal [2 + 2] cycloaddition–
cycloreversion sequence between TCNQ and electron-rich
alkyne 3 (Table 1, entry 2). The reaction of 4 with TCNQ in
refluxing ethyl acetate afforded the corresponding product 13 in
91% yield (Table 1, entry 3).
For the reaction of 5 with TCNQ, compound 14, with different
regiochemistry shown in Scheme 3, was obtained in 87% yield
(Table 2, entry 1). The reaction of ferrocene- and azulene-
Results and discussion
Synthesis
The arylethynyl-2H-cyclohepta[b]furan-2-ones 1–9 were pre-
pared by Sonogashira–Hagihara reactions, according to the pro-
cedure reported by us, recently.13 To the synthesis of the novel
DCNQ derivatives, the [2 + 2] cycloaddition–cycloreversion
sequence was applied to the acetylene derivatives 1–9 with
TCNQ under the conditions described in the literature.6,11 Thus,
the reaction of 1 with TCNQ in refluxing ethyl acetate afforded
10 in 91% yield as a sole product (Scheme 1). Proposed reaction
sequence is also shown in Scheme 1. The reaction commences
with the formal [2 + 2] cycloaddition between the exocyclic
CvC double bond of TCNQ and the alkyne moiety of 1, fol-
lowed by the ring opening of the intermediately formed strained
cyclobutene derivative to give 10 as shown in the parentheses.
Scheme 2 Reaction of 2H-cyclohepta[b]furan-2-one derivatives 2–4
with TCNQ.
Table 1 Synthesis of TCNQ-adducts with 2H-cyclohepta[b]furan-2-
one substituents 11–13
Entry
1
Substrate
Ar
Product
Yield [%]
90
2
11
2
3
3
4
12
13
92
91
Scheme 1 Presumed reaction mechanism for [2 + 2] cycloaddition of
1 with TCNQ.
Scheme 3 Reaction of 2H-cyclohepta[b]furan-2-one derivatives 5–7
with TCNQ.
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Table 2 Synthesis of TCNQ-adducts with 2H-cyclohepta[b]furan-2-
one substituents 14–16
Entry
1
Substrate
Ar
Product
Yield [%]
87
5
14
2
3
6
7
15
16
98
97
Scheme 5 Reaction of 2H-cyclohepta[b]furan-2-one derivative 9 with
TCNQ.
Scheme 4 Reaction of 2H-cyclohepta[b]furan-2-one derivative 8 with
TCNQ.
substituted alkynes 6 and 7 with TCNQ afforded DCNQ deriva-
tives 15 (98%) and 16 (97%), respectively (Table 2, entries 2
and 3). The difference in the regiochemistry of 14, 15 and 16
might be reflected by the higher electron-donating property of N,
N-dimethylaniline, ferrocene, and azulene moieties than that of
the 2H-cyclohepta[b]furan-2-one unit.6f
Thiophene and bithiophene substituted DCNQ chromophores
17 and 18 were also obtained by the reaction of alkynes 8 and 9
with TCNQ in excellent yields (17: 92% and 18: 96%), although
a relatively longer reaction period was required to complete the
reactions (Schemes 4 and 5). The lower reactivity of 8 and 9
might be explained by the deactivation of the alkyne moiety by
the DCNQ unit generated by an addition of a TCNQ molecule
within the two acetylene units in the molecule. These new
DCNQ chromophores 10–18 are stable, deep-colored crystals,
and storable in crystalline state at ambient temperature under
aerobic conditions, likewise the corresponding 2H-cyclohepta[b]
furan-2-one-substituted TCBD chromophores.
Fig. 1 UV/Vis spectra of 10 in dichloromethane and in dichloro-
methane–hexane.
DCNQs 10–18 are shown in Table 3. Most of the DCNQ chro-
mophores showed intense ICT absorption bands in the visible
region. Solvatochromism was also observed as a characteristic
feature of these molecules.14 The distinct absorption band of 10
at 615 nm in CH2Cl2 exhibits blue shift by 27 nm in the less
polar 10% CH2Cl2–hexane, which suggests the ICT nature of
this band (Fig. 1). The shift value of 10 (27 nm) is larger than
that of the corresponding TCNE-adduct of 1 (6 nm).13 These
results assume that the first excited-state has a larger dipole
moment compared with that in the ground state, due to the effec-
tive ICT character from 2H-cyclohepta[b]furan-2-one to the
DCNQ unit.
The solvent dependence of the absorption maxima and coeffi-
cients (log ε) of 10 are summarized in Table 4. The UV/Vis
spectra of DCNQs 10 in each solvent are shown in the ESI.†
The largest solvent effect was observed when the solvent was
changed from acetonitrile (λmax = 617 nm) to 10% CH2Cl2–
hexane (λmax = 588 nm).
UV/Vis spectroscopy
UV/Vis spectra of DCNQs 10 in dichloromethane and hexane
including dichloromethane are shown in Fig. 1. UV/Vis spectra
of the other DCNQs 11–18 are summarized in the ESI.† The
absorption maxima (λmax) and their coefficients (log ε) of
Similar with 10, compounds 11 and 14 showed a broad ICT
absorption band at 626 nm and 718 nm in CH2Cl2, respectively.
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Table 3 Absorption maxima [nm] and their coefficients (log ε) of
DCNQ chromophores 10–18 in dichloromethane and in 10% CH2Cl2/
hexane
Table 5 Electronic transitions for 10′ and 12′ derived from the
computed values based on B3LYP/6-31G** method and experimental
values from 10 and 12
Sample
λmax (log ε) in CH2Cl2
λmax (log ε) in hexanea
Computed value
Experimental
Sample λmax (log ε)
10
11
12
13
14
15
16
17
18
615 (4.44)
626 (4.33)
624 (4.49)
607 (4.48)
496 (4.43), 718 (4.46)
590 (4.40)
653 (4.35)
597 (4.77)
602 (4.70)
588 (4.35)
597 (4.32)
590 (4.49)
585 (4.39)
480 (4.41), 657 (4.46)
573 (4.36)
604 (4.31)
635 (4.53)
600 (4.59)
λmax Strength Composition of banda,b
10′
10, 615 (4.44) 559
0.4374
0.1888
H − 1 → L (−0.54)
H → L (0.76)
H → L + 2 (−0.27)
H − 1 → L (0.63)
H → L (−0.50)
12′
12, 624 (4.49) 585
H → L + 1 (−0.56)
a H = HOMO; L = LUMO. b The values in the parentheses are the
configuration interaction (CI) coefficients.
a Dichloromethane (10%) was included to keep the solubility of these
compounds.
absorption band (λmax = 597 nm, log ε = 4.77 in CH2Cl2). The
coefficient of 17 was almost twice as large as that of 13 (λmax
=
Table 4 Solvatochromic data for the longest wavelength absorption of
607 nm, log ε = 4.48 in CH2Cl2), although the absorption
maximum showed blue-shift by 10 nm compared with that of
13. Bis-adduct 18 also exhibited an intense and broad ICT
absorption band (λmax = 602 nm, log ε = 4.70 in CH2Cl2) as
similar with 17, but exhibited bathochromic shift by 5 nm rela-
tive to that of 17 probably due to the results on the extension of
π-electron system by bithiophene unit.
10
Solvent
λmax (log ε)
Solvent
λmax (log ε)
MeCN
CH2Cl2
THF
617 (4.29)
615 (4.33)
615 (4.32)
CHCl3
AcOEt
10% CH2Cl2/hexane
610 (4.33)
605 (4.27)
588 (4.35)
To elucidate the nature of the absorption bands of 10 and 12,
TD-DFT calculation at the B3LYP/6-31G** level15 are carried
out as model compounds 10′ and 12′, where isopropyl groups in
10 and 12 are replaced by H-groups. The frontier Kohn–Sham
orbitals of these compounds are shown in the ESI.† Judging
from the comparison between the experimental and the theoreti-
cal UV/Vis spectra, absorption maxima of 10 and 12 were
assignable to overlaps of some transitions (Table 5).
The ICT of 10′ occurred from the HOMO located on 2H-
cyclohepta[b]furan-2-one and HOMO − 1 located at the benzene
ring to the LUMO located on DCNQ moiety and LUMO + 2
which are mainly located on the dicyanovinyl (DCV) group,
although the contribution of the transitions for HOMO − 1 →
LUMO and HOMO → LUMO + 2 is relatively low. Whereas,
the ICT of 12′ confirmed that the longest absorption band arisen
from the overlapping of HOMO − 1 → LUMO, HOMO →
LUMO and HOMO → LUMO + 1. Different from the results on
10′, contribution of HOMO → LUMO (i.e., transition between
attached 2H-cyclohepta[b]furan-2-one and DCNQ moieties), is
less effective than that of detached 2H-cyclohepta[b]furan-2-one
to DCNQ. The red-shift of the absorption for 12 might be
attributable to the expansion of π-electron system by the cross-
conjugated DCV moiety.
Scheme 6 Presumed resonance structure of 14.
In these cases, the longest wavelength absorption maxima also
exhibited hypsochromic shift in 10% CH2Cl2–hexane (11:
597 nm and 14: 657 nm). Although the longest wavelength
absorption of 10 and 11 are observed in an almost similar spec-
tral region, compound 14 in CH2Cl2 showed bathochromic shift
by about 100 nm compared with those of 10, 11 and 13. The
large shift might be ascribed to the considerable contribution of
the conjugation between N,N-dimethylaniline and DCNQ moi-
eties as illustrated in Scheme 6.
The DCNQ chromophores 12, 15 and 16 exhibited a strong
and broad ICT absorption band at 624 nm, 590 nm and 653 nm
in CH2Cl2, respectively. Whereas the longest wavelength absorp-
tion maximum of 12 in the spectral region is resembled with
those of 10 and 11, absorption maxima of 15 and 16 in CH2Cl2
exhibit considerable hypsochromic and bathochromic shifts,
respectively. These results might be attributed to the effective-
ness of the ICT between the DCNQ unit and directly conjugated
ferrocenyl and 1-azulenyl groups, rather than the cross conju-
gation with the 2H-cyclohepta[b]furan-2-one ring.
Electrochemistry
To clarify the electrochemical properties, the redox behavior of
DCNQ chromophores 10–18 was examined by CV and DPV.
Measurements were carried out with a standard three-electrode
configuration. Tetraethylammonium perchlorate (0.1 M) in ben-
zonitrile was used as a supporting electrolyte with platinum wire
and disk as auxiliary and working electrodes, respectively. All
measurements were carried out under an argon atmosphere, and
potentials were related to an Ag/Ag+ reference electrode and
The larger absorption coefficient was displayed by the ICT
absorption bands of bis-adducts 17 and 18, compared with that
of mono-adduct 13. The spectrum of 17 displayed an ICT
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Table 6 Redox potentials of DCNQ chromophores 10–18a
Sample
10
Method
E1red [V]
E2red [V]
E3red [V]
CV
(DPV)
CV
(DPV)
CV
(DPV)
CV
(DPV)
CV
(DPV)
CV
(DPV)
CV
(DPV)
CV
−0.37
−0.53
(−0.35)
−0.29
(−0.51)
−0.43
11
(−0.27)
−0.38
(−0.41)
−0.49
(−1.46)b
(−1.92)
12
(−0.36)
−0.42
(−0.47)
−0.50
Fig. 2 TCNQ-adducts with 1-azulenyl substituent 19 and 20.
13
(−0.40)
−0.47
(−0.48)
−0.58
14
(−0.45)
−0.49
(−0.58)
−0.62
(−1.73)
15
(−0.47)
−0.42
(−0.60)
−0.57
16
(−0.40)
−0.26
(−0.55)
−0.37
(−1.92)
17
−0.52 (2e)
(DPV)
CV
(DPV)
(−0.24)
−0.50 (4e)
(−0.36)
(−0.35)
(−0.50)c
18
(−0.48)
a Redox potentials were measured by CV and DPV [V vs. Ag/AgNO3,
1 mM in benzonitrile containing Et4NClO4 (0.1 M), Pt electrode
(internal diameter: 1.6 mm), scan rate = 100 mV s−1, and Fc/Fc+ =
+0.15 V]. In the case of reversible waves, redox potentials measured by
CV are presented. The peak potentials measured by DPV are shown in
parentheses. b The E4 was observed at −1.80 V on DPV. c The E4
red
red
was observed at −1.77 Von DPV.
Scheme 7 Presumed electrochemical behavior of 16.
Fc/Fc+ as an internal reference, which discharges at +0.15
V. The redox potentials (in volts vs. Ag/AgNO3) of 10–18 are
summarized in Table 6.
cyclohepta[b]furan-2-one substituent contributes less effectively
for electrochemical reduction compared with 1-azulenyl moiety
as shown in Scheme 7. Thus, the first reduction potential of
these compounds might depend on the 1-azulenyl substituent.
Similar to the electrochemical reduction of compounds 10–12
and 14–16, that of DCNQ with thiophene substituent 13 showed
a reversible two-stage reduction wave on CV (−0.42 V and
−0.50 V) attributed to the formation of a dianionic species. A
reversible three-stage wave was observed in bis-adduct 17 on CV
(−0.26 V, −0.37 V and −0.52 V), in which the third reduction
wave could be concluded to be a two-electron transfer in one
step to form a tetraanionic species (Fig. 3). The electrochemical
reduction of bithiophene derivative 18 showed a reversible one-
stage broad reduction wave on CV centered at −0.50 V, which
was identified as two waves at −0.36 V and −0.48 V by DPV,
probably due to the formation of a tetraanionic species. For the
series of electrochemical reductions of thiophene derivatives 13,
17 and 18, introduction of the two DCNQ units into the thio-
phene core results in large shifts of the first reduction potentials
(i.e., 13: −0.40 V, 17: −0.24 Vand 18: −0.36 Von DPV).
On the whole, the novel DCNQ chromophores 10–18 exhibit
negative reduction potential compared with those of the corre-
sponding TCBD chromophores, except for 11. These results are
attributable to the higher electron-accepting nature of the DCNQ
moieties than that of the corresponding TCBD derivatives
reported by us, recently.13
Electrochemical reduction of DCNQs 10, 11 and 14 on CV
exhibited a two-stage wave, but the first reduction potentials are
varied to each other. The DCNQ with phenyl substituent 10
showed the reduction waves at −0.37 V and −0.53 V on CV. A
positive shift of the first reduction potential was observed in 11
(−0.29 V) because of the substitution of the electron-withdraw-
ing p-nitrophenyl group, which directly reflects the LUMO level
of the molecules. In contrast, electrochemical reduction of 14
was featured by a negative shift of the first reduction potential
(−0.47 V) owing to the N,N-dimethylaniline substituent with
strong electron-donating nature.
The compounds 12, 15 and 16 showed a reversible two-stage
wave composed of two one-electron transfer processes. The
reduction potentials of 12 were identified as −0.38 V and −0.49
V on CV. Ferrocene-substituted DCNQ 15 also showed a revers-
ible two-step wave at the half-wave potentials of −0.49 V and
−0.62 V on CV, probably due to the formation of a radical
anionic and a dianionic species, respectively. Reversible one-step
oxidation wave centered at +0.46 V was also observed on CV,
which can be ascribed to the oxidation of the ferrocene moiety.
The oxidation potential of 15 is much more positive compared
with that of the parent ferrocene (+0.15 V). It could be ascribed
to effects on the electron-withdrawing DCNQ moiety substi-
tuted. The azulene-substituted DCNQ 16 exhibited a reversible
two-step reduction wave, the potentials of which were identified
by CV as −0.42 V and −0.57 V, to form a radical anionic and a
dianionic species. Recently, we have reported the reduction
We have reported the synthesis of redox-active chromophores
with the aim of the creation of stabilized electrochromic
materials.16 As a part of this study, we have also reported 2H-
cyclohepta[b]furan-2-one-substituted TCBDs13 and azulene-sub-
stituted TCBDs10 and DCNQs,11 in which we identified some
red
potentials of the compounds 19 and 20 (19: E1 = −0.43 V,
red
red
red
E2 = −0.59 V and 20: E1 = −0.44 V, E2 = −0.58 V)
(Fig. 2).11 Since the values are consistent with those of 16, 2H-
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Fig. 4 Continuous change in the visible spectrum of 17: constant-
current electrochemical reduction (100 μA) in benzonitrile containing
Et4NClO4 (0.1 M) at 30 sec intervals.
Fig. 3 Cyclic voltammograms of the reduction of 17 (1 mM) in benzo-
nitrile containing Et4NClO4 (0.1 M) as a supporting electrolyte; scan
rate = 100 mV s−1
.
the absorption of 12, completely. The greenish-blue color of the
solution of 15 changed to yellow during electrochemical
reduction, and reverse oxidation of the yellow-colored solution
regenerated the visible spectra of 15. The color of the solution of
16 changed from green to yellow during the electrochemical
reduction with the decrement of an original absorption band in
the visible region. However, reverse oxidation of the yellow sol-
ution generated did not regenerate the parent spectrum of 16,
completely.
Recently, we have reported the electrochromic behavior of the
thiophene-substituted TCNE-adducts, which show a reversible
color change in the electrochemical redox sequence.13 The rever-
sibility of the color changes might be ascribed to the formation
of a stabilized closed-shell dianionic species with thienoquinoid
structure in the two-electron reduction. Although the thiophene-
substituted TCNQ-adducts 13, 17 and 18 also exhibited a revers-
ible multistage reduction wave on CV, these adducts did not
exhibit reversible color changes in redox sequence under the
conditions of the spectral measurements (Fig. 4).
novel hybrid structures of violenes and cyanines17 with the
redox activities. Bis(2H-cyclohepta[b]furan-2-one-substituted)
and bis(azulene-substituted) TCBDs, which exhibit reversible
color changes during the electrochemical reaction, are successful
examples of an extension of the violene–violene hybrid struc-
ture.17 Similar to 2H-cyclohepta[b]furan-2-one-substituted
TCBDs,
2H-cyclohepta[b]furan-2-one-substituted
DCNQs
10–18 might exemplify the redox system for the extensions of
the violene–violene hybrid system with multiple electron trans-
fer. Thus, visible spectra of DCNDs 10–18 were monitored to
clarify the color changes during the electrochemical reactions.
A constant-current reduction was applied to the solutions of
10–18 with a platinum mesh as the working electrode and a wire
counter electrode. Visible spectra were measured in degassed
benzonitrile containing Et4NClO4 (0.1 M) as a supporting elec-
trolyte at room temperature under electrochemical reduction con-
ditions (see the ESI†). The longest absorption of 10 at around
600 nm gradually decreased and thus the color of the solution
gradually changed from greenish-blue to pale yellow during the
electrochemical reduction, but reverse oxidation of the pale
yellow-colored solution did not regenerate the spectrum of 10,
although good reversibility was observed in the two-step
reduction on CV. The poor reversibility of the color changes
might be attributable to the instability of the presumed dianionic
species under the conditions for spectral measurements.
It is well known that the radical anion of DCNQ derivatives
readily dimerizes to form σ-bond between the two DCNQ moi-
eties.18 Thus, the irreversibility of the color changes of DCNQ
derivatives 10–18 should be reasoned by the σ-bond formation
caused by the coupling of the intermediary forming radical
anion exhibiting dimerization and/or polymerization of the
DCNQ moiety by the electrochemical reduction.
The longest absorption of 14 gradually decreased, and the
color of the solution changed from dark-purple to yellow during
the electrochemical reduction. However, reversible oxidation of
the yellow-colored solutions did not regenerate the spectra of the
corresponding original compound, completely. Vis spectra of 11
were measured under electrochemical reduction conditions,
absorption of 11 in the visible region gradually decreased along
with color change from green to yellow. Reverse oxidation did
not regenerate the original absorption of 11, although the com-
parative TCNE-adduct of 2 shows good reversibility on the spec-
tral changes. When Vis spectra of 12 were measured under
electrochemical reduction conditions, absorption of 12 in the
visible region at around 620 nm gradually decreased. Reverse
oxidation decreased the new absorptions, but did not regenerate
Conclusions
In conclusion, we have described the synthesis and electronic
properties of 2H-cyclohepta[b]furan-2-one-substituted DCNQs
by absorption spectroscopy, theoretical calculation, CV and DPV,
and electrochromic analysis.
The novel 2H-cyclohepta[b]furan-2-one-substituted DCNQs
10–18 were synthesized in a one-step procedure consisting of the
formal [2 + 2] cycloaddition reaction of the arylethynyl-2H-
cyclohepta[b]furan-2-ones 1–9 with TCNQ, followed by the ring
opening reaction of the initially formed cyclobutene derivatives.
Strong ICT absorption bands with large solvent dependence in
2436 | Org. Biomol. Chem., 2012, 10, 2431–2438
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4111–4123; (d) S. Kato, M. Kivala, W. B. Schweizer, C. Boudon, J.-
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their UV/Vis spectra featured these 2H-cyclohepta[b]furan-2-
one-substituted DCNQs. Furthermore, their transition state was
characterized by TD-DFT calculations. An analysis by CV and
DPV revealed that these compounds 10–18 exhibit a reversible
multistep reduction wave dependent on the number of DCNQ
units in the molecule. In contrast to the results on the TCBD
derivatives, DCNQs 10–18 showed poor reversibility of electro-
chromic behavior, although a significant color change was
observed during the electrochemical reduction.
These results could lead to progress of donor–acceptor
systems for applications in organic electronics and optoelectronic
materials. For the creation of advanced organic materials, prep-
aration of TCBDs and DCNQs with different π-electron systems
is now in progress in our laboratory.
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I. Biaggio, Adv. Mater., 2008, 20, 4584–4587.
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Acknowledgements
This work was partially supported by a Grant-in-Aid for
Research Activity Start-up (Grant 22850007 to T.S.) from the
Ministry of Education, Culture, Sports, Science, and Technology,
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