Preparation, redox properties, and x-ray structures of electrochromic 11,11,12,12-tetraarylanthraquinodimethane and its bianthraquinodimethane analogue: Drastic geometrical changes upon interconversion with dicationic dyes

Article abstract of DOI:10.1246/cl.140393

Tetrakis(4-methoxyphenyl)anthraquinodimethane 1(b) with a bent geometry undergoes reversible redox interconversion with twisted dication 1(t) ²⁺ exhibiting a vivid change in color (electrochromism) accompanied by a drastic structural change. Electrochemical oxidation of bianthraquinodimethane 2(b) with a doubly bent structure to bianthrylidene-type twisted dication 2(t)²⁺ proceeded smoothly, whereas the reverse conversion was less effective because 2(t)₂ generated upon reduction of 2(t)²⁺ is a long-lived species to undergo side reactions.

Full text of DOI:10.1246/cl.140393

Received: April 17, 2014 | Accepted: April 28, 2014 | Web Released: May 3, 2014
CL-140393
Preparation, Redox Properties, and X-ray Structures
of Electrochromic 11,11,12,12-Tetraarylanthraquinodimethane
and Its Bianthraquinodimethane Analogue:
Drastic Geometrical Changes upon Interconversion with Dicationic Dyes
#
Yuto Sakano, Ryo Katoono, Kenshu Fujiwara, and Takanori Suzuki*
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo, Hokkaido 060-0810
(
E-mail: tak@mail.sci.hokudai.ac.jp)
Tetrakis(4-methoxyphenyl)anthraquinodimethane 1(b) with
a bent geometry undergoes reversible redox interconversion
with twisted dication 1(t)2 exhibiting a vivid change in color
analyses demonstrated outstanding difference in geometry,
namely the bent form in pQDs 1 and 2 and the twisted form
+
2+
2+
in the dications 1 and 2
.
(
electrochromism) accompanied by a drastic structural change.
As in the case of dibenzo-Ar4oQD, the 4-methoxyphenyl
group was chosen as an aryl group on the exocyclic carbons to
attain sufficient stability in the dicationic state. By conducting
Electrochemical oxidation of bianthraquinodimethane 2(b) with
a doubly bent structure to bianthrylidene-type twisted dication
2
less effective because 2(t) generated upon reduction of 2(t) is
a long-lived species to undergo side reactions.
2+
7
(t) proceeded smoothly, whereas the reverse conversion was
the Suzuki­Miyaura coupling of 11,11,12,12-tetrabromo-9,10-
2
•
2+
8
anthraquinodimethane (3) with 4-methoxyphenylboronic acid,
9
1 was obtained as colorless crystals in 99% yield. By using
1
1
1
1,11,11¤,11¤-tetrabromo-10,10¤-bianthraquinodimethane (4)
9
instead of 3, π-extended analogue 2 was obtained as pale
yellow crystals in 81% yield (Scheme S1).
Upon oxidation of 1 with 2 equiv of (4-BrC6H4)3N PF6 in
1
0
The o- and p-quinodimethanes (oQD and pQD) are
representative cross-conjugated π-electron systems. Attachment
of charge-stabilizing end groups on the exocyclic carbons
provides a useful protocol to construct reversible redox
•+
¹
2+
¹ 9
CH2Cl2, dicationic salt 1 (PF6 )2 was obtained as green
lustrous crystals in 91% yields. The dication dye exhibits a
strong absorption at 531 nm (log ¾ 5.20) in CH2Cl2, a character-
istic of the diarylmethylium unit. A broad NIR absorption [710
(3.96)] was also observed, which is assignable to the intra-
molecular CT absorption from the anthracene moiety to the
diarylmethylium units. Upon treatment of 1 (PF6 )2 with Zn
dust in MeCN, 1 was regenerated in 96% yield. Because of steric
repulsion between the bulky aryl groups and the peri-hydrogens
1
­3
systems,
where generation of the Clar’s sextet facilitates
their redox reactions (Scheme 1). As a successful example, we
previously reported tetraaryl-substituted oQDs fused with two
benzene rings (dibenzo-Ar4oQDs), which undergo facile 2e-
4
2+
¹
oxidation to transform into the corresponding dications.
5
In the course of our continuing studies on novel organic
cationic dyes, we planned further exploitation of QD-based
electrochromic systems, including the study on the pQD-type
6
2+
on the anthracene unit, both 1 and 1 would adopt severely
isomer 1 of dibenzo-Ar oQD (Scheme 2). Unlike the oQD
deformed geometries; e.g., a butterfly-shaped bent structure in 1
and a largely twisted structure around the exocyclic bonds in 1 .
4
2+
counter part, π extension of the system could be attained easily
in 1 by inserting an anthrylidene unit, as in bianthraquinodi-
methane 2. Here, we report the preparation and redox properties
of 1 and 2, along with their electrochromic behavior based on
Previously, the X-ray analyses on the molecules structurally
related to 1 had revealed the bent geometry for the 9,10-
1
2,13
anthraquinodimethane skeleton,
and we confirmed here that
2+
the interconversion with the corresponding dications 1 and
1 actually suffers from a similar deformation by conducting its
2+
14
15
2
. We successfully isolated the dicationic salts, whose X-ray
X-ray analysis [bent angle: 33.2(2) and 34.1(2)°] (Figure 1a).
On the other hand, there had been no report on the isolation and
X-ray structure of 9,10-anthrylidenebis(diarylmethylium) deriv-
X
X
e−
X
X
e−
X
e−
X
X
e−
X
X
1
6,18
2+
atives.
Here, we found that the anthracene core in 1 is
nearly planer (the largest deviation of atoms from the mean
plane is 0.02 ¡), to which the two diarylmethylium dye units are
e−
e−
e−
e−
X
1
4
pQD
attached with a large torsion angle [76.7(3)°] (Figure 1b). Such
a geometrical change is related to the decrease in the π-bond
order of the exocyclic C=C bonds upon oxidation. The bond
Scheme 1. Redox scheme of a pQD-based system.
(
a)
(b)
(Ar = 4-MeOC6H4)
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
Ar
2
Ar
dibenzo-Ar oQD
1
2
4
Figure 1. ORTEP drawings of (a) 1 in benzene solvate and (b) 12
+
+
¹
Scheme 2. Molecular design based on the QD skeletons.
in 12 (PF6 )2 salt determined by X-ray analyses measured at 150 K.
Chem. Lett. 2014, 43, 1143–1145 | doi:10.1246/cl.140393
© 2014 The Chemical Society of Japan | 1143

a)
b)
2e
1(b)2+
Ar
Ar
Ar
Ar
Ar
Ar
fast
unstable
fast
Ar
Ar
Ar
Ar
1
(t)2
2e
1(t)2+
1
(b)
unstable
2e
2(b)2+
Ar
Ar
fast
unstable
Figure 2. Cyclic voltammograms of (a) 1/12+ and (b) 2/22+
measured in CH2Cl2 (0.1 M Bu4NBF4, E/V vs. SCE, Pt electrode,
2e
¹1
Ar
1
00 mV s ).
Ar
slow
2
e
(
a)
(b)
Ar
Ar
(b)
Ar
Ar
2
side
reactions
2(t)2+
2
(t)2 metastable
Ar
Ar
Scheme 3. Redox scheme for a) 1/12 and b) 2/22+ accompanied
+
by geometrical changes.
(
a)
(b)
Figure 3. A continuous change in UV­vis spectrum a) upon
¹6
constant current electrochemical oxidation of 1(b) (8.1 © 10 M)
50 ¯A, every 4 min) and b) upon constant current electrochemical
(
2
+
reduction (50 ¯A, every 4 min) of as-prepared 1(t) generated from
in situ oxidation of 1(b) in CH2Cl2 containing 0.05 M Bu4NBF4.
lengths in 1 [1.354(6) and 1.352(6) ¡] and 12+ [1.444(14) ¡]
clearly show that the bonds in 1 are typical double bonds,
whereas those in 1 are single bonds, thus switching the way to
reduce steric repulsion.
Because of the drastic structural change upon redox
interconversion, the cyclic voltammogram19 of 1 exhibited a
two-electron oxidation peak at +1.00 V vs. SCE in CH2Cl2,
whereas the return peak appeared in the far cathodic region
2
+
Figure 4. ORTEP drawings of (a) 2 in ether solvate and (b) 22+ in
2+
¹
2
(PF6 )2 salt MeCN solvate determined by X-ray analyses measured
at 150 K.
1
6
member of 10,10¤-(9,9¤-bianthrylidene)bis(diarylmethylium)s.
1
4
2+
¹
The X-ray analysis of 2 and 2 (PF6 )2 clearly showed that a
drastic geometrical change occurred upon oxidation of 2(b) to
2(t) (Figure 4). In the doubly bent bianthraquinodimethane
2
+
(
(
+0.42 V) that corresponds to the 2e-reduction process of 1
Figure 2a). Such a separation of redox peaks is characteristic
2+
20
11b
of the “dynamic redox systems,” in which the steady-state
concentration of the intermediary radical cation 1 is negligible.
In fact, upon electrochemical oxidation of 1 in CH Cl , a
2, the central [1.354(4) ¡] and two terminal exocyclic bonds
•+
[1.349(3) and 1.351(4) ¡] are almost planar. They have bond
lengths typical for C=C. The bent angles in the two 9,10-
dihydroanthracene units are 35.9(1)­38.9(1)°, which are close
2
2
continuous change was observed, as shown in Figure 3 exhibit-
ing several isosbestic points. The final spectrum is identical to
2+
to those in 1. The twisted deformation observed in 2 is
outstanding. The two anthracene units are arranged nearly
2+
that of isolated 1 salt. Reversibility of the chromic behavior
was confirmed by the electrochemical reduction of as-prepared
2
1
perpendicular to each other [dihedral angle: 86.5(2)°]. The
diarylmethylium units are attached on both ends of the
bianthrylidene core with a large torsion angle of 63.4(2)° in
2+
1
by switching the polarity of the electrodes. By considering
that the open-shell species in general exhibit high reactivity to
•
+
2+
undergo side reactions, the low concentration of 1 in the
interconversion is favored to attain reversibility of this electro-
chromism. In this way, the redox cycle of 1/12 , including the
geometrical change, is shown in Scheme 3a [(b): bent vs. (t):
twisted], and only the two species 1(b) and 1(t)2+ are apparently
involved in the chromic behavior.
2(t) . Such twisting deformation was realized by the reduced
bond order in the central [1.500(4) ¡] and exocyclic [1.489(4) ¡]
bonds, as deduced from their bond lengths.
+
1
9
2+
As shown in Figure 2b, the voltammogram of 2/2 pair
2+
exhibited a significant difference from that of 1/1 . Oxidation
of 2(b) proceeded at +1.08 V vs. SCE in CH2Cl2 to transform
2+
A similar redox scheme can be drawn for bianthraquinodi-
methane 2 (Scheme 3b). Upon oxidation of 2 with 2 equiv of
into 2(t) . However, in the return cycle, a pair of reversible
redox peaks was observed at +0.17 V that corresponded to the
•
+
¹
2+
¹
9
2+
2•
(
[
4-BrC6H4)3N PF6 in CH2Cl2, dicationic salt 2 (PF6 )2
­max/nm: 530 (log ¾ 5.22), 728 (4.16) in CH Cl ] was obtained
interconversion between 2(t) and 2(t) while maintaining their
twisted geometry. Thus, the electrochemically generated twisted
2
2
2•
as green lustrous crystals in quantitative yield as the first isolable
diradical 2(t) is a long-lived metastable species because of slow
1144 | Chem. Lett. 2014, 43, 1143–1145 | doi:10.1246/cl.140393
© 2014 The Chemical Society of Japan

relaxation11,22 into the doubly bent structure in bianthraquinodi-
methane 2(b). Although 2(b) was recovered in 98% yield upon
Huang, R. D. Webster, J. Casado, J. Wu, J. Am. Chem. Soc. 2012,
34, 14513. b) The doubly bent geometry of the structurally related
tetraarylbianthraquinodimethane (Ar = 4-t-BuC6H4) was previously
reported in ref 11a.
1
2+
¹
reduction of 2(t) (PF6 )2 with Zn dust in MeCN­CH2Cl2 (1:1),
reversibility in transformation under electrochemical conditions
could not be attained easily (Figure S1).10 Thus, upon elec-
trochemical oxidation of 2(b) in CH2Cl2, a continuous change
in UV­vis­NIR spectrum to that of 2(t)2 was observed with
1
2
a) U. Schubert, S. Hünig, A. Aumüller, Liebigs Ann. Chem. 1985,
1
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+
2+
several isosbestic points. However, upon reduction of 2(t)
,
9
3. e) K. Maruyama, H. Imahori, K. Nakagawa, N. Tanaka, Bull.
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shell intermediate 2(t)2• underwent some side reaction(s)23
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2+
before regenerating 2(b). Thus, 1/1 system was proven to be
more promising than 2/22 in constructing reversible electro-
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tration of open-shell species. Further studies are now in progress
on the structurally related quinodimethanes.
+
13
14
15
This work was supported by Grant-in-Aid for Scientific
Research on Innovative Areas: “Organic Synthesis Based on
Reaction Integration” (No. 2105) and by JSPS Grant-in-Aid for
Challenging Exploratory Research on “Maximum Function on
Minimum Skeleton (MFMS)” (No. 25620050) from MEXT,
Japan. T.S. thanks Prof. Takanori Fukushima (TITEC) for the
Cooperative Research Program of “Network Joint Research
Center for Materials and Devices.”
1
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CCDC deposition numbers are as follows: 1¢(PhH)2 [P21/c, Z = 4]
2+
¹
9
91887; 1 (PF6 )2 [P4/m, Z = 2] 996246; 2¢(ether)2 [P21/c, Z = 4]
991888; 22 (PF6 )2¢(MeCN)2 [C2/c, Z = 4] 991886. See also
+
¹
Figure S2 (ref 10).
The bent angle of pQD is defined as the dihedral angle between the two
planes defined by C1­C2­(C6/C7) and C2­C3­C5­C6 [or C3­C5­
(
C4/C8) and C2­C3­C5­C6].
References and Notes
16 Tetraarylanthraquinodimethanes,
tetraarylbianthraquinodimethanes,
#
This paper is dedicated to the memory of Dr. Yoshinori Nishizawa
1951­2014).
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in this study.
(
1
(
DCNQI) (ref 2b) are the most successful examples of this molecular
design. The oQD­pQD hybrid-type electron acceptor was also reported
ref 3).
(
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2
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5
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similar to 1 was reported (ref 13).
19 Cyclic voltammetry was conducted in CH2Cl2 containing 0.1 M
Bu4NBF4 as a supporting electrolyte (E/V vs. SCE, Pt electrode, scan
¹1
rate 100mV s ). For the irreversible waves, the oxidation potentials
ox
red
ox
(E ) and reduction potentials (E ) were estimated as
E
=
peak
red
peak
E
¹ 0.03 V and
E
= E
+ 0.03 V, respectively. Ferrocene
undergoes one-electron oxidation at +0.53 V under the similar
conditions.
1
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nodimethane (ref 11a).
7
8
9
2+
¹
23 In the electrochemical reduction of 2(t) (PF6 )2 in MeCN, 4,4¤-
dimethoxybenzophenone (18%) was obtained in addition to 2(b),
suggesting that the reaction with O2 is one of the major side reactions
10
11
2
•
of 2(t) .
Chem. Lett. 2014, 43, 1143–1145 | doi:10.1246/cl.140393
© 2014 The Chemical Society of Japan | 1145