Reversible interconversion between 11,11,12,12-tetraaryl-1,4-diaza/-1,4,5,8-tetraazaanthraquinodimethanes and their cationic species: Electrochromic and halochromic responses

Article abstract of DOI:10.1246/cl.150251

11,11,12,12-Tetrakis(4-methoxyphenyl)-1,4-diaza-2,3-diphenyl- and -1,4,5,8-tetraaza-2,3,6,7-tetraphenylanthraquinodimethanes (1 and 2) adopt a bent geometry. They undergo reversible redox interconversion with twisted dications 1²⁺ and 2^2+<

Full text of DOI:10.1246/cl.150251

Received: March 20, 2015 | Accepted: April 1, 2015 | Web Released: April 9, 2015
CL-150251
Reversible Interconversion between 11,11,12,12-Tetraaryl-1,4-diaza/-1,4,5,8-
tetraazaanthraquinodimethanes and Their Cationic Species:
Electrochromic and Halochromic Responses
Takanori Suzuki,* Yu Umezawa, Yuto Sakano, Hitomi Tamaoki, Ryo Katoono, and Kenshu Fujiwara
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, Hokkaido 060-0810
(E-mail: tak@sci.hokudai.ac.jp)
11,11,12,12-Tetrakis(4-methoxyphenyl)-1,4-diaza-2,3-di-
phenyl- and -1,4,5,8-tetraaza-2,3,6,7-tetraphenylanthraquinodi-
methanes (1 and 2) adopt a bent geometry. They undergo
reversible redox interconversion with twisted dications 12+ and
22+, exhibiting a vivid change in color (electrochromism).
Treatment with acid also induced drastic color change due to the
formation of protonated species H-1+ and H-2+ (halochromism),
thus demonstrating their two-way-input chromic behavior.
and H-2+ would be generated by protonation on N1 of the
pyrazine ring (N-form). Due to the proximity of the exocyclic
carbon (C11) to N1, the C-form could be also contributed, if
possibly, by proton transfer from N1 to C11. Halochromic
response is expected upon protonation. Thus, H-1+ and H-2+
would have red-shifted absorptions in the visible region since a
stronger CT interaction is expected between the electron-
donating exomethylene units and the more electron-accepting
pyrazinium in the N-form. When the C-form is contributed to
some extent, the absorption band characteristic to the diaryl-
methylium chromophore might be added in the visible region.
Here we report the preparation, properties, and X-ray
structures of 1 and 2, along with their electrochromic and
halochromic behaviors based on their interconversion with the
corresponding cationic species.
9,10-Anthraquinodimethane (AQD) is the dibenzo analogue
of p-quinodimethane, which is the representative cross-conju-
gated π-electron system. By the attachment of charge-stabilizing
end groups on the exocyclic carbons, AQD can serve as a
reversible redox system, in which the steric repulsion between
the end groups on the exomethylenes and the fused benzene
rings sometimes causes severe deformation both in the neutral
and the charged states.1 We previously reported2 that tetrakis(4-
methoxyphenyl)-substituted AQD 3 with a bent geometry is
By following the preparation scheme for
3 from
11,11,12,12-tetrabromo-AQD (4), diaza- and tetraazaanthraqui-
nones 58 and 69 were first treated with CBr4 and PPh3 to give
bis(dibromomethylene) precursors 710 and 810 in respective
yields of 89 and 69%. Then, by quadruple Suzuki-Miyaura
coupling11 with 4-methoxyphenylboronic acid, 110 and 210 were
obtained as stable yellow and red crystals in 91% and 94% yield,
respectively (Scheme 3). The first band of 1 [-max/nm: 358
(log ¾ 4.42) in CH2Cl2] and 2 [509 (4.26)] appeared in the
longer-wavelength region compared to that of 3 [309 (4.34)],
which can be accounted for by considering the lower LUMO
reversibly interconvertible with the corresponding dication 32+
,
in which the two diarylmethylium dye units are attached nearly
perpendicular3 to the planarized anthracene core (Scheme 1).
Such a drastic structural change4 is the reason for the facile one-
wave two-electron transfer in the voltammogram, and thus the
spectroelectrogram exhibited several isosbestic points during the
electrochromic response5 from colorless (3) to deep purple (32+).
In the course of our continuing studies on novel redox-based
response systems,6 we got interested in further exploitation of
the chromic behavior of 3/32+ by functional addition of
sensitivity toward external stimuli other than electrons (e.g.
H+), which prompted us to design the 1,4-diaza- and 1,4,5,8-
tetraaza-AQD derivatives 1 and 2. The diphenyl substituents
are attached on each of the pyrazine rings, so that the target
molecules as well as their precursors can gain enough
solubility.7 As shown in Scheme 2, the cationic species H-1+
Ar
Ar
X2
X2
Ar
Ar
X2
X2
H+
H
N
H
Y2
Y2
Ph
Ph
Y2
Y2
N
Ph
Ph
1, 2
N
N
H+
Ar
Ar
Ar
Ar
H-1+, H-2+ (N-form)
H-1+, H-2+ (C-form)
Scheme 2. Protonation of aza-AQDs into H-1+ and H-2+, and
the equilibrium of two protonated forms (N-form and C-form).
Ar
Ar
11
Ar
Ar
2e
2e
Y2
Y2
X2
Y2
Y2
X2
X1
Y1
Y1
X1
Y1
X1 Y1
8
1
Br
Br
X2
X2
X2
4
5
O
X2
X1
Y2
Y2
X2
X2
Y2
Y2
N
N
Ph
Ph
N
N
Ph
Ph
a
b
Ar
Ar
12+
1, 2
12
Ar
Ar
2+
1 - 3
-
3
O
1: X1 = N, X2 = CH, Y1 = Ph, Y2 = H;
2: X1 = X2 = N, Y1 = Y2 = Ph;
3: X1 = X2 = CH, Y1 = Y2 = H
Br
Br
[Ar = 4-MeOC6H4]
5, 6
7, 8
[1/5/7: X2 = CH, Y2 = H; 2/6/8: X2 = N, Y2 = Ph]
Scheme 1. Redox scheme for tetraaryl-AQD 3 and its diaza- and
tetraaza-analogues 1 and 2.
Scheme 3. Preparation of 1 and 2: a) CBr4, PPh3 in CH2Cl2;
b) 4-MeOC6H4B(OH)2, K2CO3, [Pd(PPh3)4] in PhMe-EtOH-H2O.
Chem. Lett. 2015, 44, 905–907 | doi:10.1246/cl.150251
© 2015 The Chemical Society of Japan | 905

Table 1. 1H NMR chemical shifts (¤) of 1 and its cationic species
12+ a and H-1+ b measured in CDCl3
12+
H-1+
(a)
(b)
1
Me (Me¤)
x (x¤)
y (y¤)
o
m
p
α
β
3.71, 3.81
6.74, 6.81
7.26, 7.36
7.06
4.10
7.40
7.93
6.99
7.11
7.23
7.54
7.49
3.85
6.91
7.28
6.80
7.29
7.47
7.21
6.99
7.15
7.21
7.06
6.80
aValues for 12+(PF6
)
salt. bGenerated in situ in degassed
¹
Figure 1. ORTEP drawings of (a) 2 [upper: top view; lower:
2
¹
side view] and (b) 12+ in 12+(SbCl6 )2 salt determined by X-ray
CDCl3 containing 10% TFA at 25 °C.
analyses measured at 150 K.
Me'
O
O
Me
x'
x
y'
y
level of 1 and 2 with respect to the electron-withdrawing
pyrazine ring(s). X-ray analyses12 at 150 K revealed that 1 and 2
adopt a bent geometry similar to that of 32 (Figures 1 and S1).
As shown by the comparisons of the bent angles (ª)13 in the
boat-shaped central hexagon, the degree of deformation de-
creases by replacing N for CH at the peri-positions [ª: 27.4(8),
38.9(8)° in 1; 18.8(1), 20.1(1)° in 2; 33.2(2), 34.1(2)° in 3].
According to the voltammetric analyses in CH2Cl2, both
1 and 2 undergo quasi-reversible 2e-oxidation at +0.58 V
(Figure S2).14 The lower oxidation potential as well as less-
separated redox waves compared to 3 must be related to the
more flattened geometry in 1 and 2 than in 3. Upon oxidation
p
α
m
N
o
β
Ar
Ar
Ar
Ar
N
N
Ph
Ph
oxdn
¹
of 1 and 2 with 2 equiv of (4-BrC6H4)3N•+PF6 in CH2Cl2,
¹
10
dicationic salts 12+(PF6
)
[-max/nm: 535 (log ¾ 4.96) in
[542 (4.83)] were obtained as deep
2
¹
10
CH2Cl2] and 22+(PF6
)
2
purple crystalline solids in respective yields of 100 and 96%.
Their strong absorptions in the visible region resemble that in
32+ [531 (5.20)]. From the dication salts, neutral donors 1 and 2
were regenerated in 100 and 87% yields, respectively, upon
treatment with Zn dust in MeCN.
Figure 2.
A continuous change in UV-vis spectra of 1
[9 © 10¹6 M] in MeCN containing 0.05 M Et4NClO4 upon
constant-current electrochemical oxidation (30 ¯A, every 10 min).
Due to the positive charges on the molecule, all the
resonances of dication 22+ are shifted to the lower field
compared to those of 2 in the 1H NMR spectra measured in
CDCl3 (Table S1). For example, a sharp singlet for MeO at ¤
¹
3.89 in 2 was shifted to ¤ 4.11 in 22+(PF6 )2. Similarly, two
singlets of 1 at ¤ 3.71 and 3.81 were transformed into a sole
¹
singlet at ¤ 4.10 in 12+(PF6
) (Table 1). The latter change
2
suggests that four 4-MeOC6H4 groups become equivalent in 12+
by adopting a twisted conformation with two diarylmethylium
attached nearly perpendicular to the planarized diazaanthracene
Figure 3. A change in UV-vis spectra of 1 [3 © 10¹5 M, 3 mL]
in CH2Cl2 upon treatment with acid and base: (a) before and after
addition of 20 ¯L of TFA; (b) after addition of 20 ¯L of TFA, with
or without further addition of Et3N (40 ¯L).
¹
skeleton. In fact, X-ray analysis12 on the 12+(SbCl6
)
salt15
2
revealed large twisting angles of 69.0(6) and 72.4(7)° around the
exocyclic bonds of C9-C11 and C10-C12 (Figure 1b), as in the
¹
case of 32+(PF6 )2 [76.3(3)°].2
A vivid change in color from yellow to deep purple was
observed upon electrochemical oxidation of 1 to 12+ in MeCN
with a continuous change in the UV-vis spectrum as shown
in Figure 2, which resembles the electrochromic behavior of
3/32+. A less drastic color change was observed in the case of 2
to 22+ since the neutral donor 2 has an absorption maxima close
to that of 22+ (Figure S3).
(Figure 3). The wavelength of the first band is close to that of
diarylmethylium, although the extinction coefficient is rather
small. The color change is reversible since the addition of Et3N
resulted in the disappearance of brown color and regeneration of
the original shape in the UV-vis spectrum (halochromism). To
1
identify the protonated species, the H NMR spectrum of 1 was
Drastic change in color was also observed upon addition of
TFA to a CH2Cl2 solution of 1: new broad absorptions appeared
at -max of 538 (log ¾ 4.05), 445 (4.27), and 384 nm (4.35)
measured in degassed CDCl3 containing 10% TFA (Figure S4).
A set of resonances were observed, which can be assigned to a
sole chemical species. Most of the resonances were shifted to the
906 | Chem. Lett. 2015, 44, 905–907 | doi:10.1246/cl.150251
© 2015 The Chemical Society of Japan

Ar
Ar
Ar
Ar
Ar
Ar
Ar
H
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Navarrete, Y. Li, W. Zeng, D. Kim, K.-W. Huang, R. D. Webster, J.
Casado, J. Wu, J. Am. Chem. Soc. 2012, 134, 14513.
Y. Sakano, R. Katoono, K. Fujiwara, T. Suzuki, Chem. Lett. 2014, 43,
1143.
The largest twisting angle ever reported for the two π-units in the
dications is 90.0°: T. Suzuki, H. Shiohara, M. Monobe, T. Sakimura, S.
Tanaka, Y. Yamashita, T. Miyashi, Angew. Chem., Int. Ed. Engl. 1992,
31, 455.
H
N
Ph
Ph
N
Ph
Ph
N
N
Ph
Ph
N
Ph
Ph
2
3
N
N
N
H
H
Ar
Ar
N1-adduct
N4-adduct
C12-adduct
C11-adduct
Scheme 4. Degenerated isomerism in H-1+.
4
5
Review on “dyrex” systems undergoing drastic structural changes
upon electron transfer: T. Suzuki, E. Ohta, H. Kawai, K. Fujiwara, T.
Fukushima, Synlett 2007, 851.
a) P. M. S. Monk, R. J. Mortimer, D. R. Rosseinsky, Electrochromism:
Fundamentals and Applications, Wiley-VCH, Weinheim, 1995.
doi:10.1002/9783527615377. b) P. M. S. Monk, R. J. Mortimer,
D. R. Rosseinsky, Electrochromism and Electrochromic Devices,
Cambridge University Press, Cambridge, 2007. doi:10.1017/
CBO9780511550959.
lower field (Table 1). By considering the lower degree of lower-
field-shifts compared to the case of 12+ as well as the weak
basicity of pyrazine (pKb = 13), it is likely that the monocationic
specie (H-1+) but not the diprotonated species was generated.
The spectrum indicated that the cationic species has C2v
symmetry, which is higher than that expected for the N-form
(C1) or the C-form (Cs) (Scheme 2). The equivalency of the four
4-methoxyphenyl groups can be accounted for only by consid-
ering the contribution from the C-form with the rapid equi-
librium of the C11- and C12-protonated species (Scheme 4).16 In
the case of 2, TFA addition caused a UV-vis spectral change
[-max of 660 (log ¾ 4.25), 557 (4.38), 425 nm (4.53) in CH2Cl2],
and the original spectrum was recovered upon addition of Et3N
(Figure S5). Well-averaged resonances assignable to H-2+ were
observed in the 1H NMR spectrum (1% TFA in CDCl3)17
(Table S1), showing very rapid equilibrium for degenerated
proton transfer in H-2+ within the pyrazine ring as well as
between the pyrazine rings.
6
a) T. Suzuki, H. Tamaoki, R. Katoono, K. Fujiwara, J. Ichikawa, T.
Fukushima, Chem. Lett. 2013, 42, 703. b) T. Suzuki, T. Kuroda, H.
Tamaoki, S. Higasa, R. Katoono, K. Fujiwara, T. Fukushima, H.
Yamada, Chem. Lett. 2013, 42, 706. c) T. Suzuki, Y. Tokimizu, Y.
Sakano, R. Katoono, K. Fujiwara, S. Naoe, N. Fujii, H. Ohno, Chem.
Lett. 2013, 42, 1001. d) T. Suzuki, Y. Hoshiyama, K. Wada, Y.
Ishigaki, Y. Miura, H. Kawai, R. Katoono, K. Fujiwara, T. Fukushima,
Chem. Lett. 2013, 42, 1004. e) T. Hamura, R. Nakayama, K. Hanada,
Y. Sakano, R. Katoono, K. Fujiwara, T. Suzuki, Chem. Lett. 2013, 42,
1244. f) T. Suzuki, Y. Sakano, T. Iwai, S. Iwashita, Y. Miura, R.
Katoono, H. Kawai, K. Fujiwara, Y. Tsuji, T. Fukushima, Chem.®Eur.
J. 2013, 19, 117. g) T. Suzuki, Y. Kuroda, K. Wada, Y. Sakano, R.
Katoono, K. Fujiwara, F. Kakiuchi, T. Fukushima, Chem. Lett. 2014,
43, 887. h) T. Suzuki, K. Hanada, R. Katoono, Y. Ishigaki, S. Higasa,
H. Higuchi, H. Kikuchi, K. Fujiwara, H. Yamada, T. Fukushima,
Chem. Lett. 2014, 43, 982. i) K. Tanino, T. Yamada, F. Yoshimura, T.
Suzuki, Chem. Lett. 2014, 43, 607. j) T. Suzuki, Y. Sakano, Y.
Tokimizu, Y. Miura, R. Katoono, K. Fujiwara, N. Yoshioka, N. Fujii,
H. Ohno, Chem.®Asian J. 2014, 9, 1841. k) Y. Ishigaki, S. Yoshida,
H. Kawai, R. Katoono, K. Fujiwara, T. Fukushima, T. Suzuki,
Heterocycles 2015, 90, 126. l) Y. Ishigaki, H. Kawai, R. Katoono, K.
Fujiwara, T. Suzuki, Heterocycles 2015, 90, 136. m) Y. Uchimura, T.
Takeda, R. Katoono, K. Fujiwara, T. Suzuki, Angew. Chem. Int. Ed.
2015, 54, 4010.
In summary, we succeeded in constructing two-way-input
molecular response systems based on the aza-derivatives of
11,11,12,12-tetraaryl-AQD. Studies on multi-output systems are
now under way.
T.S. thanks Prof. Takanori Fukushima at TITECH and the
Cooperative Research Program of “Network Joint Research
Center for Materials and Devices.” The authors are grateful to
financial support by the Hokkaido University Clark Memorial
Foundation to YS.
7
11,11,12,12-Tetracyano-1,4-diaza-AQD is less soluble than its 2,3-
diphenyl derivative (ref 9).
8
9
G. A. Efimova, L. S. Efros, Zh. Org. Khim. 1966, 2, 531.
Y. Yamashita, T. Suzuki, G. Saito, T. Mukai, Chem. Lett. 1986, 715.
Supporting Information is available electronically on J-STAGE.
10 Experimental detail and selected spectral data are given in Supporting
Information.
References and Notes
1
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11 N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457.
12 CCDC deposition numbers are as follows:
1 [P212121, Z = 4]
¹
1047047; 12+(SbCl6 )2(CH2Cl2)3 [P1, Z = 2] 1047048;
2
[P1,
ꢀ
ꢀ
Z = 2] 1047046. See also Figure S1.
13 The bent angle of 9,10-anthraquinodimethane is defined as the dihedral
angle (ª) between the two planes defined by C8a-C9a-C11 and C8a-
C9a-C4a-C10a [or C4a-C10a-C12 and C8a-C9a-C4a-C10a].
14 Cyclic voltammetry was conducted in CH2Cl2 containing 0.1 M
Bu4NBF4 as a supporting electrolyte (E/V vs. SCE, glassy carbon
electrode, scan rate 100mV s¹1).
¹
¹
15 Due to higher crystallinity than 1+(PF6 )2, 12+(SbCl6 )2 was used for
crystallographic work, which was obtained by oxidation of 2 with
¹
(4-BrC6H4)3N•+SbCl6
.
16 The sharp singlet for the MeO protons became broad upon lowering
the temperature, and two singlets at ¤ 3.92 and 3.82 appeared below
¹30 °C. From the value of Tc (ca. ¹25 °C), the energy barrier of
12.2 kcal was estimated, which corresponds to the degenerate
interconversion among the protonated species (Scheme 4).
17 The 1H NMR spectrum of 2 in CDCl3 containing 10% TFA showed
considerable broadening, probably due to additional formation of the
diprotonated species H2-22+
.
Chem. Lett. 2015, 44, 905–907 | doi:10.1246/cl.150251
© 2015 The Chemical Society of Japan | 907