Dual-mode electrochromism switched by proton transfer: Dynamic redox properties of bis(diarylmethylenium)-type dyes

Article abstract of DOI:10.1039/b104742f

Upon oxidative dimerization of pale yellow Ar₂C=CHPh 1 (Ar = 4-Me₂NC₆H₄), deep blue 1,4-dication 2²⁺ was obtained as a stable salt, which was transformed into 1 by reductive C-C bond fission; deproton

Full text of DOI:10.1039/b104742f

Dual-mode electrochromism switched by proton transfer: dynamic
redox properties of bis(diarylmethylenium)-type dyes†
Takanori Suzuki,* Hiroki Higuchi, Masakazu Ohkita and Takashi Tsuji
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan.
E-mail: tak@sci.hokudai.ac.jp
Received (in Cambridge, UK) 31st May 2001, Accepted 28th June 2001
First published as an Advance Article on the web 3rd August 2001
Upon oxidative dimerization of pale yellow Ar2CNCHPh 1
(Ar
=
4-Me2NC6H4), deep blue 1,4-dication 22+ was
obtained as a stable salt, which was transformed into 1 by
reductive C–C bond fission; deprotonation of 22+ gave
intense yellow diene 3, which was interconvertible with
violet dication 42+ by two-electron transfer, thus exhibiting
two distinct modes of electrochromism before and after
proton transfer.
Recently, much attention has been focused on electrochromic
dyes1,2 for use as optical memories or electrochemical
switches.3 The representative examples are a series of open-
chain violenes4 shown by the redox couple of C and D in
Scheme 1. Diene C can be prepared by oxidative dimerization
of olefin A,2a and the similar procedure has been widely used to
synthesize novel TTF vinylogues.5,6 Although the oxidation
reactions are believed to proceed via unconjugated 1,4-dication
B, this intermediate has never been isolated nor even detected
spectroscopically probably due to its instability by rapid
deprotonation to C under the reaction conditions.
Scheme 2
In our continuing efforts to develop new electrochromic
systems endowed with bistability by reversible C–C bond
making/breaking7 or drastic structural changes8 upon electron
transfer (dynamic redox properties), we have found here that the
oxidation. In the present case, path (a) is plausible because 22+
(I32)2 was obtained in 92% yield by the reaction of 1 with 1.5
eq. I2 under the presence of Ph2CNCHPh (2 eq.) which remained
intact and was recovered quantitatively.
dication
B
possessing two dye chromophores [X
=
The 1,4-dication 22+ is surprisingly stable with no sign of
spontaneous deprotonation; its methine proton appears at 6.02
(4-Me2NC6H4)2C] and two aryl groups (R = aryl) can be
isolated as stable salts. By the selective transformation of B to
A by reduction and B to C by deprotonation, novel dual-mode
electrochromism shown in Scheme 1 could be realized, where
proton transfer alters the mode from one to another.
1
ppm as a sharp singlet in the H NMR spectrum measured in
CD3CN. Upon reduction of this salt with Zn powder in MeCN,
olefin 1 was regenerated in 100% yield. Such high-yield
interconversion indicates that 1 and 22+ can be considered as a
‘reversible’ redox pair even though C–C bond making and
breaking are accompanied by two-electron transfer.7,10
The voltammetric analysis‡ indicated that olefin 1§ under-
goes irreversible oxidation at +0.55 V vs. SCE in MeCN. The
corresponding reduction wave appeared in the far cathodic
region (20.38 V), which is rather close to the reduction
potential of (4-Me2NC6H4)2CH+ (20.54 V). Such a large shift
of redox peaks can be accounted for by assuming oxidative
dimerization of 1 (Scheme 2).
When 22+ (I32)2 was treated with Bun NF in THF–MeCN,
4
deprotonation proceeded smoothly to give intense yellow diene
3§ in 75% yield, which is a stronger donor [Eox +0.18 V (2e,
rev.)]‡ than 1. Attempted deprotonation of 22+ (I32)2 with Et3N
[Eox +0.85 V (irrev.)] resulted in formation of olefin 1 in
quantitative yield by electron transfer, and deprotonation with
pyridine was very slow.
Upon treatment of 3 with 3 eq. of I2 in CH2Cl2, deep violet
dication salt 42+ (I32)2§ was obtained in 98% yield, which
regenerated diene 3 in 100% yield upon reduction with Zn in
MeCN. According to the X-ray analyses,¶ the diene unit in 3
adopts a nonplanar geometry with a large torsion angle of 59.1°
around the C2–C3 bond (Fig. 1a), which is undoubtedly due to
steric congestion among six aryl groups. It is likely that lack of
effective conjugation in diene 3 as well as steric shielding
against the base in the hindered C–H acid 22+ are the reasons for
reluctant deprotonation of 22+ to 3. On the other hand, all of the
diene carbons lie nearly on the same plane in 42+ (Fig. 1b), thus
confirming drastic geometrical changes by twisting motions2,6,8
during interconversion between 3 and 42+.
In fact, dication 22+ was isolated as deep blue I32 salt§ in 97%
yield by the reaction of yellow olefin 1§ with 1.5 eq. of I2 in
CH2Cl2. Mechanistically, there can be two pathways to produce
22+ from 1: (a) dimerization of 1+· to 22+, and (b) reaction of 1+·
with neutral 1 to form 2+· followed by further one-electron
In this way, two types of dynamic structural changes are
presented by the redox couples of 1–22+ and 3–42+, which can
be switched by proton transfer. Furthermore, the novel dual-
mode electrochromism can be realized by these couples since
each pair exhibits distinct spectral changes upon electrolyses
Scheme 1
† Electronic supplementary information (ESI) available: spectral data for
new compounds. See http://www.rsc.org/suppdata/cc/b1/b104742f/
1574
Chem. Commun., 2001, 1574–1575
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b104742f

scan rate 100 mV s21. Values for Eox and Ered were calculated as Ep 2 0.03
and Ep + 0.03 V, respectively, in the cases of irreversible processes.
§ Olefin 1 was prepared in 95% yield by Wittig–Horner reaction of PhCHO
and Ar2CH-PO(OMe)2/BunLi in THF, and its physical data are identical to
those reported in ref. 9. Stereochemistry of 22+ was determined to be dl by
X-ray analysis of I32 salt.¶ All new compounds show satisfactory analytical
values. lmax (MeCN)/nm (log e): 1, 345 sh (4.38), 322 (4.43), 266 (4.30),
255 (4.28); 22+ (I32)2, 619 (4.83), 595 (4.84), 423 (4.47), 361 (4.80), 293
(5.04); 3, 397 sh (4.27), 345 sh (4.59), 312 (4.71), 277 (4.65); 42+ (I3 )2, 679
2
(4.86), 545 (4.91), 426 (4.48), 358 (4.83), 292 (5.14).†
¶ Crystal data for 22+ (I32)2: C48H52N4I6, M 1446.39, monoclinic, P21/n, a
= 10.054(2), b = 17.393(3), c = 28.880(5) Å, b = 92.970(9)°, U =
5043(1) Å3, Dc (Z = 4) = 1.905 g cm21, m(Mo-Ka) = 37.43 cm21, T =
123 K. The final R value is 0.062 for 4842 independent reflections with I >
3sI and 263 parameters. For 3: C48H50N4, M 682.95, monoclinic, P21/c, a
= 14.612(3), b = 16.076(2), c = 17.1807(7) Å, b = 91.792(1)°, U =
4034.0(10) Å3, Dc (Z = 4) = 1.124 g cm21, m(Mo-Ka) = 0.66 cm21, T =
110 K. The final R value is 0.065 for 3614 independent reflections with I >
3sI and 469 parameters. For 42+ (I32)2: C48H50N4I6, M 1444.38,
monoclinic, P21/n, a = 9.6884(8), b = 23.432(1), c = 22.8948(4) Å, b =
107.5794(4)°, U = 4954.9(5) Å3, Dc (Z = 4) = 1.936 g cm21, m(Mo-Ka)
= 37.99 cm21, T = 97 K. The final R value is 0.026 for 7707 independent
reflections with I > 3sI and 526 parameters. CCDC/164961–164963. See
http://www.rsc.org/suppdata/cc/b1/b104742f/ for electronic files in .cif or
other electronic format.
Fig. 1 (a) ORTEP drawing of diene 3 determined by X-ray at 2163 °C.
Torsion angle of diene unit is 59.1°. Two aryl groups are arranged in a face-
to-face manner (dihedral angle 7.9°). Short intramolecular C…C contacts
(3.21 and 3.29 Å) are shown by dotted line. (b) ORTEP drawing of dication
42+ determined by X-ray at 2176 °C on I32 salt. The four carbon atoms of
C+–CNC–C+ lie nearly on the same plane (the largest deviation from the
least-squares plane, 0.09 Å).
∑ Similarly,
2,2A-bis[2,2-bis(4-dimethylaminophenyl)ethenyl]biphenyl
undergoes oxidative cyclization to give another isolable 1,4-dication framed
in a 9,10-dihydrophenanthrene skeleton, which will be reported in a full
paper.
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Fig. 2 Two modes of electrochromism. a) Mode-1; changes in the UV-VIS
spectrum of 1 (3.5 mL, 1.1 3 1024 mol dm23 in MeCN) upon constant-
current electrochemical oxidation (32 mA, 4 min interval) to 22+. (b) Mode-
2; changes in the UV-VIS spectrum of 3 (3.5 mL, 2.6 3 1025 mol dm23 in
MeCN) upon constant-current electrochemical oxidation (31 mA, 2 min
interval) to 42+. Note the presence of isosbestic points in both trans-
formation.
(Fig. 2), which opens up a way to construct a molecular device
where two independent inputs (e2 and H+) are transduced into
a unified two-dimensional output (e vs. l in UV-VIS).
This work has revealed that the 1,4-dication B in Scheme 1
can exist as stable species (as in 22+)∑ when deprotonation is not
feasible due to steric shielding in B and/or prohibited full-
conjugation in diene C by twisted geometry. In such an
appropriate case, all of the species of A–D (as in 1–42+) are
stable enough to realize the dual-mode optical response. The
present electrochromism modulated by proton transfer provides
a new successful entry into the proton–electron cooperating
functions.11
This work was supported by the Ministry of Education,
Science, and Culture, Japan (No. 13440184). Financial support
from the Iwatani Naoji Foundation is gratefully acknowledged.
We thank Professor Tamotsu Inabe (Hokkaido University) for
use of the X-ray structure analysis system.
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Notes and references
‡ All of the redox potentials shown in the text were measured under the
same conditions: E/V vs. SCE, 0.1 mol Et4NClO4 in MeCN, Pt electrode,
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