A new type of tricolor electrochromic system based on the dynamic redox properties of hexaarylethane derivatives

Article abstract of DOI:10.1039/a806037a

Electrochemical interconversion between 9,9,10,10-tetra-aryldihydrophenanthrene 1 and 2,2′-bis(triarylmethylium) 2²⁺ has been proven to proceed via open form cation radical 2⁺ as a stable intermediate; using these three species, a no

Full text of DOI:10.1039/a806037a

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A new type of tricolor electrochromic system based on the dynamic redox
properties of hexaarylethane derivatives
Takanori Suzuki,*† Jun-ichi Nishida and Takashi Tsuji
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
Electrochemical interconversion between 9,9,10,10-tetra-
aryldihydrophenanthrene 1 and 2,2A-bis(triarylmethylium)
22+ has been proven to proceed via open form cation radical
2+· as a stable intermediate; using these three species, a novel
tricolor electrochromic system that exhibits hysteretic color
change (color 1 ? color 2 ? color 3 ? color 1) is formed.
Molecular systems whose properties can be controlled electro-
chemically are attracting much attention because they might be
applicable to the construction of molecular devices such as
redox switches.1 From this point of view the redox couples of
hexaarylethanes 1d,e and bis(triarylmethane) dyes 2d,e2+ are
Y-C6H4
+
Y-C6H4
Y-C6H4
C6H4-X
C6H4-X
C6H4-Y
Fig. 1 Molecular geometry of ethane 1b determined by X-ray analysis. The
bipheny skeleton is nearly planar (the dihedral angle between planes 1 and
2: 16.2°), and the central six-membered ring adopts a pseudo-half chair
conformation (planes 3 and 6: axial; 4 and 5: equatorial). The xanthene
moiety is deformed slightly into the butterfly shape (the dihedral angle
between planes 3 and 4: 15.2°). There is no disorder around the ethane bond
in 1b, unlike the symmetric ethane 1e (ref. 2).
X-C6H4
+
1
C6H4-X
22+
Y-C6H4
OH
C6H4-Y
a X = p-Me2N; Y = p-MeO
b X = p-Me2N; Y–Y = o-O-o
c X–X = Y–Y = o-O-o
revealed that 1b has a very long C–C bond [1.643(6) Å] (Fig.
1).¶ Thus, oxidation of 1a,b with
2
equiv. of
(p-BrC6H4)3N+·SbCl62 in CH2Cl2 led to the fission of the weak
bonds to regenerate the dications 2a,b2+, which were isolated as
SbCl62 salts‡ in 82 and 79% yield, respectively. C2-Symmetric
ethane 1c‡ [lmax(MeCN)/nm (log e) 282 (4.27), 230 sh (4.71)]
and orange-colored 2c2+(BF42)2‡ [495 sh (3.62), 460 (3.79),
382 (4.56), 263 (4.82)] containing two xanthene moieties were
also prepared from diol 3c‡ for comparisons.
d X = Y = p-Me2N
e X = Y = p-MeO
X-C6H4
HO
C6H4-X
3
The cyclic voltammogram of 1c is quite similar to those of
1d,e; it shows irreversible 2e oxidation peak at +1.42 V, and the
corresponding cathodic peak is largely shifted to +0.50 V,
which was assigned to the 2e reduction peak of 2c2+ (Table 1).
In contrast, unsymmetric ethanes 1a,b behave rather differently
from 1c–e. Although their oxidation potentials are close to that
of the tetrakis(dimethylamino) derivative 1d, in the return cycle
of the voltammograms there appeared two cathodic peaks
(Fig. 2). From independent measurements on 2a,b2+ it was
confirmed that these peaks are due to two-stage 1e reduction
interesting;2 their noteworthy features include electrochromic
behaviour with vivid changes in color and very high bistability
due to reversible C–C bond making/breaking3 upon two-
electron transfer (dynamic redox properties). During the course
of our mechanistic investigation on the interconversion between
1 and 22+, we have found that a unique tricolor electrochromic
system could be achieved using 1a,b and 2a,b2+ possessing two
different triarylmethane moieties, whose properties are reported
herein. Although several multi-color electrochromic systems
have been reported (color 1 Ô color 2 Ô color 3 Ô etc.),4 the
hysteretic chromicity of the present systems (color 1 ? color 2
? color 3 ? color 1) is unprecedented.
Table 1 Redox potentials of ethanes 1 and dications 22+ in CH2Cl2
a
Unsymmetric diols 3a‡ and 3b‡ were prepared in 12 and 8%
yield, respectively, by the reaction of 2,2A-dilithiobiphenyl5
with a mixture of the corresponding two ketones followed by
chromatographic separation on SiO2 from the symmetric diols
3c–e. Deeply colored dication salts 2a2+(BF42)2‡ [lmax-
(MeCN)/nm (log e) 632 (4.93), 519 (4.72), 319 (4.25), 272
(4.24)] and 2b2+(BF42)2‡ [632 (4.92), 488 sh (3.76), 425 (4.22),
377 (4.46), 321 (4.23), 261 (4.63)] were obtained in 91 and 93%
yield, respectively, by treating these diols with 42% HBF4–
(EtCO)2O. Reduction of the dication salts with SmI2 in THF
gave colorless ethanes 1a‡ [lmax(MeCN)/nm (log e) 268 (4.61)]
and 1b‡ [270 (4.58)] in 97 and 78% yield. X-Ray analysis§ has
E/V vs. SCE
Compound
Eox (1)
E1red (22+
)
E2red(22+
)
a X = p-Me2N, Y = p-MeO
b X = p-Me2N, Y–Y = o-O-o
c X–X = Y–Y = o-O-o
dd X = Y = p-Me2N
+0.83b,c
+0.76b,c
+1.39b,c
+0.74b,c
+1.44b,c
+0.10
+0.24
+0.53b,c
20.42b,c
+0.21b,c
20.45c
20.19c
ed X = Y = p-MeO
0.1 mol dm23 Bun4NBF4, Pt electrode, scan rate 100 mV s21
electron process. c Irreversible wave, values were calculated as Eox = Epeak
2 0.03 and Ered = Epeak + 0.03, respectively. d Ref. 2.
.
Two-
a
b
Chem. Commun., 1998
2193

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Fig. 2 Cyclic voltammogram of ethane 1a (1023 mol dm23) in CH2Cl2 (E/V
vs. SCE 0.1 mol dm23 Bun4NBF4, Pt electrode, scan rate 500 mV s21). The
reduction peaks are absent when the voltammogram is first scanned
cathodically. As shown by the dotted line, the first reduction wave at +0.07
V is reversible when the scanning was reversed at 20.10 V.
processes of the dications; the first one corresponding to the
Y-C6H4-C+-C6H4-Y moiety is completely reversible. Fur-
thermore, the anodic peak due to the oxidation of 1a,b appears
in the voltammograms of 2a,b2+ after scanning the irreversible
second 1e reduction wave. Such redox properties can be
accounted for only by assuming the reaction mechanism shown
in Scheme 1, in which the weakened C–C bond of hexaaryl-
ethane 1 is cleaved after 1e oxidation∑ to 1+· whereas two-fold
1e reduction of 22+ to 22· is necessary before the ring closure to
1. It should be noted that 2+· produced from 1+· is more easily
oxidized than 1 [Eox(1a,b) is much more positive than
Eox(2a,b+·) = E1red(2a,b2+)], thus the steady-state concentra-
tion of 2+· is negligible during the electrochemical oxidation of
1, although the same specimen is a long-lived intermediate in
the reduction process of 22+.
e
Y-C6H4
C–C bond
making
e
22•
C6H4-Y
•
E2red(22+
)
E1red(22+
)
22+
1
Eox(1)
X-C6H4
Fig. 3 Changes in the UV–VIS spectra of (a) 1a (3.6 ml; 4.1 3 1025
mol dm23 in MeCN containing 0.04 mol dm23 Bun4NBF4) upon
electrochemical oxidation (15 mA) at 10 min intervals, and 2a2+ (3.6 ml
soln; 6.6 3 1026 mol dm23 in MeCN containing 0.05 mol dm23 Bun4NBF4)
upon electrochemical reduction (70 mA): (b) stage 1, at 0.5 min intervals; (c)
stage 2, at 1 min intervals
+
C–C bond
breaking
1+•
e
C6H4-X
2+•
Scheme 1
e
Thanks to the hysteretic interconversion between 1, 2+· and
22+, novel tricolor electrochromic systems could be constructed
using the unsymmetric derivatives. Thus, upon electrochemical
oxidation of colorless 1a, both the blue (X = p-Me2N) and red
(Y = p-MeO) triarylmethylium chromophores grow simultane-
ously to develop the violet color of 2a2+ [Fig. 3(a)]. On the other
hand, the red chromophore disappears first upon reduction of
2a2+ [Fig. 3(b), stage 1], and next the blue cation radical 2a+· is
converted to colorless 1a [Fig. 3(c), stage 2] even under the
constant-current electrolytic conditions. Similar behaviour but
with a different color was observed for the interconversion of
2b2+ (green) (isosbestic points: 248, 270, 300 nm) ? 2b+· (blue)
(296 nm) ? 1b (colorless) (309 nm) ? 2b2+ (green), showing
the generality of the unprecedented pattern of color change.
This work was supported by the Ministry of Education,
Science, and Culture, Japan (No. 08640664 and 10146101). We
thank Professor Tamotsu Inabe (Hokkaido University) for the
use of X-ray analytical facilities. Elemental analyses were
carried out by Ms Akiko Maeda at the Center for Instrumental
Analysis (Hokkaido University).
§ Crystal Data for 1b: C42H36N2O, M 584.76, P212121, a = 14.216(4),
b = 20.806(6), c = 10.483(3) Å, V = 3100(1) Å3, Dc (Z = 4) = 1.253
g cm21, Rw = 0.025. CCDC 182/1003.
¶ Hexaphenylethanes of [3.3.n]propellane-type were recently reported to
have long C–C bonds [1.611(3)–1.621(3) Å]: G. Dyker, J. Körning, P.
Bubenitschek and P. G. Jones, Liebigs Ann./Recl., 1997, 203.
∑ Activation energies for the mesolytic C–C bond fission of diarylethane
cation radicals were reported to be lowered by an average of 23 kcal mol21
with respect to homolysis: P. Maslak, W. H. Chapman, Jr., T. M.
Vallombroso, Jr. and B. A. Watson, J. Am. Chem. Soc., 1995, 117,
12380.
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36, 1329.
3 M. Horner and S. Hünig, J. Am. Chem. Soc., 1977, 99, 6122; W. Freund
and S. Hünig, J. Org. Chem., 1987, 52, 2154; T. Suzuki, M. Kondo, T.
Nakamura, T. Fukushima and T. Miyashi, Chem. Commun., 1997, 2325;
T. Suzuki, H. Takahashi, J. Nishida and T. Tsuji, Chem. Commun., 1998,
1331.
4 P. M. S. Monk, R. J. Mortimer and D. R. Rosseinsky, Electrochromism:
Fundamentals and Applications, VCH, Weinheim, 1995, p. 185.
5 N. Neugebauer, A. J. Kos and P. von R. Schleyer, J. Organomet. Chem.,
1982, 228, 107.
Notes and References
† E-mail: tak@science.hokudai.ac.jp
‡ All new compounds gave satisfactory spectral data and analytical
values.
Received in Cambridge, UK, 3rd August 1998; 8/06037A
2194
Chem. Commun., 1998