Synthesis and electronic structure of highly electron-accepting radiaannulene and its reduced species

Article abstract of DOI:10.1016/j.tetlet.2012.07.109

Highly electron-accepting radiaannulene (1) was synthesized. CV measurements revealed a high electron accepting ability and strong electronic coupling in the anionic species. Spectroelectrochemical analysis revealed a very low-energy absorption band in th

Full text of DOI:10.1016/j.tetlet.2012.07.109

Tetrahedron Letters 53 (2012) 5385–5388
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Tetrahedron Letters
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Synthesis and electronic structure of highly electron-accepting radiaannulene
and its reduced species
Masashi Hasegawa a, , Yuya Takatsuka a, Yoshiyuki Kuwatani b, Yasuhiro Mazaki a,
⇑
⇑
a School of Science, Kitasato University, 1-15-1 Kitasato. Minami-ku, Sagamihara, Kanagawa 252-0373, Japan
b VSN Inc., 2-1-3 Nishimiyahara, Yodogawa-ku, Osaka 532-0004, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Highly electron-accepting radiaannulene (1) was synthesized. CV measurements revealed a high electron
Received 12 June 2012
Revised 19 July 2012
Accepted 25 July 2012
Available online 1 August 2012
accepting ability and strong electronic coupling in the anionic species. Spectroelectrochemical analysis
revealed a very low-energy absorption band in the spectra of 1ÅÀ and 12À
.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Radiaannulene
Aromaticity
Mixed-valence
Charge resonance absorption
A large number of redox-active molecules that contain quinoi-
dal p-systems have been synthesized for their potential application
Although the incorporation of redox groups into a radiaannu-
lene framework seems to be an efficient design for electron-donat-
in nonlinear optical chromophores and organic electronic devices,
such as semiconductors in organic field effect transistors.1,2 The
most remarkable property of these compounds is the definitive
proaromaticity; that is, the ability of the compound to form aroma-
tized mesomeric structures as a result of oxidation or reduction.
Recently, radiaannulenes,3,4 which are acetylene-extended quinoi-
dal structures, that have electron-donating/accepting groups were
reported as a hybrid molecule composed of a cross-conjugated
radialene and a linearly-conjugated dehydroannulene.5–7 Their
core framework possesses proaromaticity that can lead to a larger
ing/accepting molecules having a large p-system, there has been
little study of the detailed electronic structures of the oxidized/re-
duced counterparts of these molecules. Quite recently, parallel to
the completion of our present study, Nielsen and co-workers re-
ported the synthesis of TTF-fused radiaannulene as a multiple re-
dox system.8 Their radiaannulene exhibited a strong intervalence
charge transfer (CT) band in its oxidized state.
To aid in the development of more practical applications of re-
dox-active radiaannulenes in electron-accepting materials, we de-
signed benzo-annulated radiaannulene derivative 1 containing two
dicyanomethylenes.9 Because the benzo-annulated radiaannulene
framework was first synthesized from 2 as a precursor of Swor-
ski-type dehydro[14]-annulenes,10 1 can undergo more facile
(4n + 2)p electron system upon redox. Previously, Zhao and co-
workers reported tetrathiafulvalene analogues that were extended
with the radiaannulene core.5 In 2009, Tykwinski and co-workers
synthesized a radiaannulene containing two fluorenylidene units
at the terminal position. Their compound showed two well-re-
solved reversible redox waves in cyclic voltammetry (CV) analy-
sis.6 Regarding the push-pull type, Diederich and co-workers
recently reported radiaannulenes containing electron-donating
and electron-accepting substituents at each terminal alkylidene
position.7 These compounds had aromatized mesomeric characters
in the neutral, and the resulting charge separation led to a very
small HOMO–LUMO gap.
reduction. In addition, the extended
p-conjugation may reduce
the on-site Coulomb repulsion, which is a parameter related to
hole/electron transportation in the solid state of conducting mate-
rials. In this communication, we examine the detailed electronic
structures of the neutral and reduced forms of 1.
NC
NC
CN
CN
O
NC
CN
⇑
Corresponding authors. Tel.: +81 42 778 8158; fax: +81 42 778 9961 (M.H.); tel.:
NC
CN
O
+81 42 778 8313; fax: +81 42 778 9961 (Y.M.).
E-mail addresses: masasi.h@kitasato-u.ac.jp (M. Hasegawa), mazaki@kitasato-
u.ac.jp (Y. Mazaki).
1
2
TCNQ
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.07.109

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M. Hasegawa et al. / Tetrahedron Letters 53 (2012) 5385–5388
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In the previous literature, syntheses of radiaannulenes were carried
out by macrocyclization of the precursor having redox active units
by Sonogashira coupling reaction.5–7 Therefore, we chose a Knoeve-
nagel condensation of 2 with malononitrile because the CN groups
may not be suitable for cyclization (Scheme 1). Compound 2 was
prepared from 4 in seven steps by using a modified literature pro-
cedure. Knoevenagel condensation with Al2O3 afforded 1 in accept-
able yield (8%).11 Similarly, reference compound 3 was synthesized
from 1,5-diphenyldiphenyl-1,4-pentadiyn-3-one.
350
M-
80
60
40
20
0
MM-
m/z = 350 (M-)
The structure was characterized by 1H NMR, 13C NMR, TOF-MS,
IR, and elemental analysis. The 1H NMR signal of the olefinic proton
in 1 was found to be at d 6.56 ppm, indicating a formal quinoidal
structure. The stretching frequency of mCN of 1 in IR spectra is ob-
served in 2198 cmÀ1, while the frequency of 3 is found in
MMM-
M4
-
-
M5
-
-
M6
M7
2195 cmÀ1
.
These values are lower than those of TCNQ
CN = 2226 cmÀ1), because of a conjugation effect with the acety-
500
1000
1500
Mass/Charge
2000
2500
3000
(m
lene extended skeleton. Interestingly, the TOF-MS spectrum of 1
obtained with negative ionization clearly indicated a strong molec-
ular association in the gaseous phase (Fig. 1). This result implies a
facile molecular association of the anionic species.
Figure 1. LDI-TOF-MS spectrum of 1 in negative ionization mode.
Recrystallization of 1 from CH2Cl2–MeOH solution at 4 °C gave
single crystals suitable for X-ray analysis (Fig. 2).12 The molecule
adopted an almost planar structure in which all atoms showed
only small mean deviations from the least-squared plane
(0.081 Å to À0.098 Å). The structure of the core cyclic framework
and alkylidene unit exhibited typical features of cross-conjugation.
The bond length of C(4)–C(9), found to be 1.411(3) Å, is slightly
longer than the other Csp2–Csp2 bonds in the annulated benzene
moiety. The cyclic outline of the radiaannulene segment showed
a bond length alternation (BLA), and its index (dBLA) is calculated
to be 0.121 Å (for 18 bonds). However, a clear difference in the in-
dex was found between the diethynylbenzene part, that is, the 11
bonds through C(1)–C(5)–C(8)–C(12), and the other part (C(12)–
C(15)–C(16)–C(1), for 7 bonds). The dBLA of the former (0.091 Å)
was less than that of the latter (0.170 Å), suggesting a higher de-
gree of delocalization through the benzene unit. As seen in Fig-
ure 2b, the molecules stacked uniformly along with the a-axis in
slipped-stack manner, where the dicyano-methylene units overlap
with the central cyclic segment. The face-to-face distance between
the C(1) and the neighboring molecular plane (1 + x, y, z) was cal-
culated to be 3.304(3) Å. The columns formed a herring-bone array,
in which one CN group was close to the alkylidene of the other
molecule in the neighboring column.
CV analysis of 1 in THF showed two one-electron reversible
redox waves assigned to 1/1ÅÀ and 1ÅÀ/12À and additional quasi-
reversible reduction waves (Fig. 3), whereas CV of 3 showed two
irreversible reduction waves at lower potentials under similar con-
ditions. Differential pulse voltammetry (DPV) measurements also
corroborated this reduction profile and gave potentials at À0.50,
À0.87 and À1.65 V (vs Fc/Fc+), respectively (Table 1). Although a
negative shift of the first reduction potential (E1red) was found in
1
compared with the first reduction potential of TCNQ
(E1red = À0.25 and Er2ed = À0.99 V), 1 exhibited more facile reduction
than previously reported radiaannulenes. In contrast, the E2red value
of 1 was found at À0.87 V, a potential that was more positive than
that of TCNQ. This E2red value clearly indicated the considerable de-
crease in the on-site Columbic repulsion between the two redox
sites. Finally, the third reduction wave was obtained at À1.65 V
as a further electronic reduction. The difference in
D
(E1ÀE2) sug-
gests a certain electronic coupling that may be attributed to an aro-
matized resonance stabilization of the [14]annulene core. The
13
obtained comproportionation constant value Kc was determined
to be 1.8 Â 106, indicating strong electronic coupling in the stable
anion radical of 1.
We calculated nucleus-independent chemical shift (NICS)
values after geometry optimization at (U)B3LYP/6-311++G(d,p) le-
vel.14 Table 2 summarized calculated BLA and the NICS of 1, 1ÅÀ and
1
2À. As for neutral 1, relatively small BLA value and the positive
NICS(0), at the ring central position, value (d = +1.16 ppm) was ob-
tained, and they support a formal quinoidal structure of 1. This
NICS value is comparable to the electron poor radiaannulenes
calculated previously.7 On the contrary, the dBLA and the NICS(0)
values of 1ÅÀ and 12À clearly indicate an aromaticity of the central
radiaannulene core. In addition, the calculations suggested that the
divalent 12À possess a higher degree of the aromatic structures.
To elucidate details of the electronic structure in the reduction
species of 1, we attempted spectroelectrochemistry of 1 and TCNQ
in THF under Ar atmosphere. In addition, we also have calculated
the absorption maxima by the time dependent Coulomb-attenuat-
ing DFT method (TD-CAM-B3LYP), which generally shows better
performance in long-range charge transfer transitions.15 Figure 4
shows the electronic spectra of 1 in each reduction state generated
by applying constant voltage that correspond to the formation of
1ÅÀ and 12À, respectively.16 The electronic spectra of 1ÅÀ exhibited
mainly four absorption maxima at 633, 672, 740, and 1292 nm
Scheme 1. Synthesis of 1 and 3.

M. Hasegawa et al. / Tetrahedron Letters 53 (2012) 5385–5388
5387
(a)
(b)
i
ii
iii
O
a
b
Figure 2. ORTEP drawing of 1 (a) Top view. (b) Packing diagram. The dash lines indicate the intermolecular interactions along the columnar direction. (short contacts, (i)
C(1)–C(12)1 3.298(3) Å [1 = 1+x, y, z], (ii) N(2)–C(22)2 3.244(3) Å [2 = 3/2Àx, 1/2+y, 1/2Àz], (iii) N(2)–C(12)2 3.229(3) Å).
E1
E2
pa
pa
E3
pa
12-
10 µA
M2
1•-
1
D1
E1
pc
E2
pc
M1
E3
pc
-2.5
-2.0
-1.5
-1.0
-0.5
0
Potentials (V vs. Fc/Fc+)
Figure 3. Cyclicvoltammogram of 1.
400
600
800
1000
1200
1400
Table 1
Redox potentials of 1 and related compoundsa
wavelength (nm)
Figure 4. Electronic Spectra of 1 and its reduced species.
E1red [V]
E2red [V]
E3red [V]
Compound
D
(E1ÀE2) [V]
Kc
1
À0.50
À0.25
À1.05
À0.87
À0.99
À1.41
À1.65
À1.59
—
0.37
0.74
—
1.8 Â 106
(MV) state. Since the CR band of TCNQÅÀ generated under similar
manner was observed at 852 nm,17 such a low-energy-lying CR
TCNQ
3
3.3 Â 1012
band in 1ÅÀ may be attributed to the highly
p-extended framework
a
All potentials were obtained from DPV measurements in THF containing 0.1 M
nBu4NPF6 at 25 °C with glassy carbon working electrode. Potentials were measured
of the aromatized central core. The calculation also reproduced M2
band qualitatively as the second transition at 1.84 eV (674 nm)
which involves a one-electron transition from the SOMO-1. In
addition, the CR band disappeared when dianion 12À was gener-
ated, and a new absorption maximum, D1 at 713 nm, whose edge
was observed up to 1500 nm, was generated. In calculation, tran-
sitions are found at 1.89 eV (656 nm) and 2.14 eV (580 nm) as a
against Ag/Ag+ electrode and converted to the values vs Fc/Fc+.
(Table 3). In TD-CAM-B3LYP/6-311++G(d,p) calculation, the transi-
tion to the first excited state is calculated to be at 1.18 eV
(1051 nm) and can be assigned to the observed charge resonance
(CR) band, M1, which involves mainly a one-electron transition
from the SOMO to the LUMO, located on whole the radiaannulene
framework. Thus, it clearly indicates the delocalization of the an-
ionic charge between two (CN)2C@ units in the mixed-valence
Table 3
Experimental and calculated (TD-CAM-(U)B3LYP/6-311++G(d,p)//(U) B3LYP/6-
311++G(d,p)) absorption maxima of 1 and its reduced species
Compound
Experimentala
Theoretical
Table 2
1
304, 434
633, 672
740, 1292
371, 713
351 (0.159), 413 (1.131)
393 (0.188), 589 (0.098)
674 (0.588), 1051 (0.126)
371 (0.157), 580 (0.160)
656 (0.662)
1ÅÀ
Calculated bond length of C@C (exo), dBLA of the central ring unit and isotropic NICS(0)
values in vacuum at GIAO HF/6-311++G(d,p)a
12À
Compound
Csp2 = sp2(exo) [Å]
dBLA [Å]
NICS(0) [ppm]
1
1.387
1.418
1.448
0.104
0.084
0.066
+1.16
À3.30
À9.79
TCNQ
402
398 (1.174)
348 (0.460), 735 (0.404)
1ÅÀ
12À
TCNQÅÀ
750, 769
831, 852
326
TCNQ2À
326 (0.5315)
Optimized geometries at restricted B3LYP/6-311++G(d,p) for 1and 12À, and
a
unrestricted B3LYP/6-311++G(d,p) for 1ÅÀ were used.
In THF solution. 25 °C, under Ar atmosphere.
a

5388
M. Hasegawa et al. / Tetrahedron Letters 53 (2012) 5385–5388
132, 10453–10466; (c) Zhu, X.; Tsuji, H.; Nakabayashi, K.; Ohkoshi, S.;
Nakamura, E. J. Am. Chem. Soc. 2011, 133, 16343–16345. For selected
examples, see:.
lower oscillator strength. The lowest energy band mainly consists
of HOMO–LUMO transition. The absorption spectra of TCNQ2À
exhibits only in higher energy region (kmax = 326 nm), and hence
12À possesses much smaller HOMO–LUMO gap due to their extre-
mely extended aromatized framework.
2. (a) Abboto, A.; Beverina, L.; Bradamate, S.; Facchetti, A.; Klein, C.; Pagani, G. A.;
Redi-Abshiro, M.; Wortmann, R. Chem. Eur. J. 2003, 9, 1991–2007; (b) Aliás, S.;
Andreu, R.; Blesa, M. J.; Cerdán, M. A.; Franco, S.; Garín, J.; López, C.; Orduna, J.;
Sanz, J.; Alicante, R.; Villacampa, B.; Allain, M. J. Org. Chem. 2008, 73, 5890–
5898.
3. (a) Mitzel, F.; Boudon, C.; Gisselbrecht, J.-P.; Seiler, P.; Gross, M.; Diederich, F.
Helv. Chim. Acta 2004, 87, 1130–1157; (b) Mitzel, F.; Boudon, C.; Gisselbrecht,
J.-P.; Seiler, P.; Gross, M.; Diederich, F. Chem. Commun. 2003, 1634–1635; (c)
Gholami, M.; Melin, F.; McDonald, R.; Ferguson, M. J.; Echegoyen, L.; Tykwinski,
R. R. Angew. Chem., Int. Ed. 2007, 46, 9081–9085.
4. (a) Gholami, M.; Tykwinski, R. R. Chem. Rev. 2006, 106, 4997–5027; (b) Kivala,
M.; Diederich, F. Acc. Chem. Res. 2009, 42, 235–248; (c) Diederich, F.; Kivala, M.
Adv. Mater. 2010, 22, 803–812.
5. (a) Chen, G.; Dawe, L.; Wang, L.; Zhao, Y. Org. Lett. 2009, 11, 2736–2739; (b)
Chen, G.; Wang, L.; Thompson, D. W.; Zhao, Y. Org. Lett. 2008, 10, 657–660.
6. Gholami, M.; Chaur, M. N.; Wilde, M.; Ferguson, M. J.; McDonald, R.; Echegoyen,
L.; Tykwinski, R. R. Chem. Commun. 2009, 3038–3040.
In conclusion, we have synthesized a novel radiaannulene 1 that
has two dicyanomethylene groups as electroactive units, and we
determined its structure by X-ray analysis. CV measurements re-
vealed facile reductive nature of the radiaannulene and the forma-
tion of stable anionic species of 1ÅÀ and 12À. The difference in
D
(E1ÀE2) indicated a significant decreasing of the on-site Columbic
repulsion. Their electronic structures were studied electrospectro-
chemically, and the CR band was observed in 1ÅÀ, whose electronic
charge is delocalized for the whole of the radiaannulene unit. The
electronic spectra of dianion exhibited a low-energy absorption
band due to the aromatized p-framework.
7. Wu, Y.-L.; Bureš, F.; Jarowski, P. D.; Schweizer, W. B.; Boudon, C.; Gisselbrecht,
J.-P.; Diederich, F. Chem. Eur. J. 2010, 16, 9592–9605.
8. Lincke, K.; Frellsen, A. F.; Parker, C. R.; Bond, A. D.; Hammerich, O.; Nielsen, M.
B. Angew. Chem., Int. Ed. 2012, 51, 6099–6102.
Acknowledgments
9. Hopf, H.; Kreutzer, D.-C. M. Angew. Chem., Int. Ed. 1990, 29, 393–395; (b)
Moomen, N. N. P.; Bouden, C.; Gisselbrecht, J.-P.; Seiler, P.; Gross, M.; Diederich,
F. Angew. Chem., Int. Ed. 2002, 41, 3044–3047; (c) Moonen, N. N. P.; Gist, R.;
Bouden, C.; Gisselbrecht, J.-P.; Seiler, P.; Kawai, T.; Kishioka, A.; Gross, M.; Irie,
M.; Diederich, F. Org. Biomol. Chem. 2003, 1, 2032–2034.
10. Kuwatani, Y.; Ueda, I. Angew. Chem., Int. Ed. 1995, 17, 1892–1894.
11. Moonen, N. N. P.; Pomerantz, W. C.; Gist, R.; Boudon, C.; Gisselbrecht, J.-P.;
Kawai, T.; Kishioka, A.; Gross, M.; Irie, M.; Diederich, F. Chem. Eur. J. 2005, 11,
3325–3341.
This work was supported by the Mazda Foundation and by a
Kitasato University Research Grant for young researchers. We are
also grateful to Prof. Mao Minoura (Kitasato University) for helpful
discussion, Prof. Masahiko Iyoda (Tokyo Metropolitan University)
for LDI-TOF MS measurements and Prof. Yohji Misaki (Ehime
University) for the spectroscopic measurements. The calculations
were performed at the Research Centre for Computational Science,
Okazaki, Japan.
12. Crystal data for 1: C24H6N4, Mw = 350.06, light yellow block crystal. Space
group P21/n (#14), a = 6.3276(6), b = 14.6503(14), c = 19.2518(19) Å,
b = 96.7180(10),
V = 1772.4(3) Å3,
Z = 4,
= 0.081 mmÀ1, 6743 reflections measured, 2349 unique (Rint = 0.0280), final
indices [I > 2 (I)]: R1 = 0.0350, wR2 = 0.0578, GOF = 1.115. CCDC 869428
Dc = 1.313 gcmÀ3
,
173 K,
l
R
r
Supplementary data
contains the supplementary crystallographic data.
13. The Kc value of 1 is defined as the comproportionation constant of 1 and 12À to
21ÅÀ, and it is calculated from
D
(E1ÀE2) in DPV measurements.
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.
07.109.
14. Frisch, M. J. et al. GAUSSIAN 09, Revision B. 01; Gaussian Inc.: Pittsburgh, PA, 2009.
15. Iikura, H.; Tsuneda, T.; Yanai, T.; Hirao, K. J. Chem. Phys. 2001, 115, 3540–3544.
16. As for 1, the voltages at À0.55 and À1.1 V (Epc values vs Fc/Fc+, Pt mesh
working electrode), corresponding to the formation of 1ÅÀ and 12À based on the
CV measurement in that conditions, were employed. As for TCNQ, the voltages
at À0.3 and À1.2 V were employed.
References and notes
17. Jonkman, H. T.; Kommandeur, J. Chem. Phys. Lett. 1972, 15, 496–499. Observed
electronic spectra of reduced TCNQ are similar to those in literature. See:.
1. (a) Janzen, D. E.; Burand, M. W.; Ewbank, P. C.; Pppenfus, T. M.; Higuchi, H.;
Fiho, D. A. S.; Young, V. G.; Brédas, J.-L.; Mann, K. R. J. Am. Chem. Soc. 2004, 126,
15295–15308; (b) Suzuki, Y.; Miyazaki, E.; Takimiya, K. J. Am. Chem. Soc. 2010,