Molecular design and preparation of bis-isophthalate electrochromic systems having controllable color and bistability

Article abstract of DOI:10.1021/ol802208x

(Chemical Equation Presented) New electrochromic systems based on bis-isophthalates were designed and prepared. They showed electrochromism behavior by two-electron reduction. The displayed colors showed greatly enhanced bistability and were dependent on

Full text of DOI:10.1021/ol802208x

ORGANIC
LETTERS
2008
Vol. 10, No. 23
5365-5368
Molecular Design and Preparation of
Bis-isophthalate Electrochromic
Systems having Controllable Color and
Bistability
W. Sharmoukh,† Kyoung Chul Ko,† So Yeon Park,† Ju Hong Ko,† Ji Min Lee,‡
Changho Noh,‡ Jin Yong Lee,*,† and Seung Uk Son*,†
Department of Chemistry, Sungkyunkwan UniVersity, Suwon 440-746, Korea, and
Display Laboratory, Samsung AdVanced Institute of Technology (SAIT),
Suwon 440-600, Korea
sson@skku.edu
Received September 22, 2008
ABSTRACT
New electrochromic systems based on bis-isophthalates were designed and prepared. They showed electrochromism behavior by two-electron
reduction. The displayed colors showed greatly enhanced bistability and were dependent on the length of the conjugated bridge between the
two isophthalate groups. We believe that the connection of two electrochromic systems with a conjugated bridge is a good basic strategy to
obtain new bistable electrochromic systems.
Recently, the electrochromism of organic molecules has
become an academically and industrially important issue.1
During the past decade, diverse organic electrochromic
systems have been developed, including organic conducting
polymers in which the cationic radical species formed via
oxidation have usually shown color in the visible light range.2
Compared to oxidation-inducd electrochromism, reduction-
induced electrochromism is relatively rare.3 Most studies
have focused on N,N′-disubstituted viologen derivatives,
which have been applied for industrial purposes such as smart
windows and diverse mirror devices.4 Usually, they have
displayed a blue color via one-electron reduction to form
radicals.
Two-electron-based electrochromic systems also have been
developed.5 Usually, the examples in the literature have
focused on the oxidative breaking of covalent bonds via two-
electron oxidation to form dications. It has been suggested
† Sungkyunkwan University.
‡ Samsung Advanced Institute of Technology.
(1) (a) Gra¨tzel, M. Nature 2001, 409, 575. (b) Nishida, J.-I.; Miyagawa,
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Y.; Zheng, Y.; Song, N.; Wan, X.; Wang, Z. Y. Org. Lett. 2008, 10, 641.
(d) Deng, J.; Song, N.; Zhou, Q.; Su, Z. Org. Lett. 2007, 9, 5393. (e)
Rastegar, M. F.; Todd, E. K.; Tang, H.; Wang, Z. Y. Org. Lett. 2004, 6,
4519. (f) Perepichka, D. F.; Bryce, M. R.; Mclnnes, E. J. L.; Zhao, J. P.
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(3) Nakamura, K.; Oda, Y.; Sekikawa, T.; Sugimoto, M. Jpn. J. Appl.
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(4) Cummins, D.; Boschloo, G.; Ryan, M.; Corr, D.; Rao, S. N.;
Fitzmaurice, D. J. Phys. Chem. B 2000, 104, 11449.
(2) (a) Mortimer, R. J.; Dyer, A. L.; Reynolds, J. R. Displays 2006, 27,
2. (b) Gaupp, C. L.; Zong, K.; Schottland, P.; Thompson, B. C.; Thomas,
C. A.; Reynolds, J. R. Macromolecules 2000, 33, 1132. (c) Kumar, A.;
Reynolds, J. R. Macromolecules 1996, 29, 7629.
(5) (a) Suzuki, T.; Ohta, E.; Kawai, H.; Fujiwara, K.; Fukushima, T.
Synlett 2007, 6, 0851. (b) Suzuki, T.; Nishida, J.; Tsuji, T. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 1329. (c) Suzuki, T.; Nishida, J.; Ohkita, M.; Tsuji,
T. Angew. Chem., Int. Ed. 2000, 39, 1804.
10.1021/ol802208x CCC: $40.75
Published on Web 11/08/2008
2008 American Chemical Society

that the more stable (bistable) color species could be obtained
in these two-electron systems because dications are believed
to be more stable than the cationic or anionic radical species.
The stability of colored species via redox reaction is another
important issue because if the displayed color can be
maintained after power is off, electricity and the cost of
operating devices can be saved.6 Thus, to obtain more diverse
colors with bistability, more diverse organic electrochromic
systems need to be developed.
Scheme 2. Synthesis of Bis-isophthalate Derivative IS1
Our research group has studied the electrochromism of
isophthalates via one-electron reduction to form anionic
radicals.7 However, the displayed color immediately disap-
peared after power off as a result of the instability of radical
species, like the known case of dimethyl terephthalate
(Scheme 1). In our ongoing efforts to develop new electro-
coupling between 5-pinacolatoborane-substituted isophthalate
and 5-iodo-isophthalate. The 5-pinacolatoborane-substituted
isophthalate could be prepared in situ via coupling between
the 5-iodo-isophthalate and bis(pinacolato)diborane.10 Ter-
minal double bond groups were introduced in each ester
group for possible further modification.
Scheme 1
.
Electrochromism of Dimethyl Terephthalate and
Bis-isophthalate Derivatives
For the longer conjugated bridge groups between two
isophthalates, the benzene (IS2), biphenyl (IS3), thienyl
(IS4), and dithienyl groups (IS5) were introduced by
coupling of two 5-iodo isophthalates with the corresponding
diboronic acids. (Scheme 3) The synthesized bis-isophtha-
Scheme 3
.
Synthesis of Bis-isophthalates IS2, IS3, IS4, and IS5
chromic systems that are more bistable and can be tailored
to control the displayed colors, we designed two-electron
systems based on bis-isophthalates with the help of density
functional theory (DFT) calculation.8 If two isophthalate
groups were connected by a conjugated bridge, the generated
radicals in each part could be coupled to form a more stable
quinoid dianion, like Chichibabin’s hydrocarbon.9 In the case
of the singlet diradical form, it was theoretically anticipated
that the dihedral angle (39.6°) between two arenes was
decreased to be coplanar via two-electron reduction.8 The
color can be dependent on the length of the conjugated
systems. In this regard, bis-isophthalate systems having
diverse conjugated bridges were designed and prepared.
Scheme 2 shows the synthetic route for the preparation
of simple bis-isophthalates; IS1 was prepared via Suzuki
1
lates were fully characterized by H NMR, 13C NMR, and
high resolution mass spectroscopy.
A p-terphenyl group was introduced into the bridge by
coupling between 5-pinacolatoborane isophthalate (Scheme
4) and diiodo-p-terphenyl, which was prepared via iodation
of p-terphenyl using iodine in acidic solution.11
(6) This has been described as “bistability” or “memory effect” in
electrochromism.
(7) Sharmoukh, W.; Ko, K. C.; Ko, J. H.; Nam, H.-J.; Jung, D.-Y.; Noh,
C.; Lee, J. Y.; Son, S. U. J. Mater. Chem. 2008, 18, 4408.
(8) See Supporting Information for detail.
(9) Porter, W. W., III.; Vaid, T. P.; Rheingold, A. L. J. Am. Chem. Soc.
2005, 127, 16559.
(10) Mazik, M.; Ko¨nig, A. Eur. J. Org. Chem. 2007, 20, 3271.
(11) (a) Zauhar, J.; Bandrauk, A. D.; Truong, K. D.; Michel, A. Synthesis
1995, 703. (b) Unroe, M. R.; Reinhardt, B. A. Synthesis 1987, 981.
5366
Org. Lett., Vol. 10, No. 23, 2008

Scheme 4. Synthesis of Bis-isophthalates IS6
The two-electron reduction processes of IS1, 2, 3, and 6
were studied by cyclic voltammetry. Monoisophthalate shows
reversible one-electron reduction.7 Each compound (5 mM)
was dissolved in N-methylpyrrolidone (NMP) containing 0.2
M tetrabutylammonium hexafluorophosphate (TBAPF6). As
shown in Figure 1, IS1 showed two separated reduction
Figure 2. UV-visible spectra and photos of displayed colors before
and after voltage supply.
Table 1. Redox Potentials of Compounds in Cyclic
Voltammetry and Maximum UV-vis Absorption Peak
redox potentiala vs
compounds
Ag/Ag+ (V)
max abs (nm)
IS1
IS2
IS3
IS4
IS5
IS6
-2.24, -2.48
-2.46
-2.36
-2.06, -2.36
-2.18, -2.49
-2.36
533
542
572
590
674
573
a Redox potential vs Ag/Ag+ (reference electrode) determined in a
conventional three-electrode cell by using 0.2 M tetrabutylammonium
hexafluorophosphate as supporting electrolyte in N-metylpyrrolidone, ITO
glass as the working electrode, and platinum as the counter electrode.
Figure 1. Cyclic voltammograms of bis-isophthalate derivatives
IS1 (a), IS2 (b), and IS3 (c).
reduction of IS3 showed a dark blue color, which was shifted
to violet and blue colors as the bridge became longer.
Interestingly, IS6 showed nearly the same UV absorption
peak (573nm) as IS3 (572 nm), indicating the limitation in
controlling the color via bridge-length control. However, we
could move the absorption peaks in the red direction by
introducing a thienyl group in the bridge.
Interestingly, IS4 showed a deep blue color with a
maximum absorption peak at 592 nm. In cyclic voltammetry,
IS4 showed two separated reduction peaks, indicating a facile
electronic communication between the two isophthalate
groups (Table 1 and Figure S1, Supporting Information). It
is noteworthy that the thiophene ring is popular in designing
organic electronic systems due to its excellent performance
in electron transfer.12 The two-electron reduction of IS5
showed a display of green color with a maximum UV-vis
absorption peak at 674 nm. According to the cyclic volta-
mmogram of IS5, the two isophthalate groups were still
electronically communicating well (Table 1 and Figure S1,
Supporting Information). According to these observations,
peaks, indicating that the second reduction needs higher
energy than first one and, therefore, that the two isophthalate
rings are electronically well connected. With increasing
length of the conjugated bridge, the two reduction peaks
became more overlapped. In case of IS3, which has biphenyl
as bridge, one redox peak was observed. The height of
the reduction peak indicated a correspondence to the two-
electron reduction process. With increasing length of the
connector, reduction of each isophthalate ring occurred at
nearly the same position. As expected, the cyclic voltam-
mogram of IS6, which has a terphenyl bridge, showed nearly
the same shape as that of IS3. (table 1).
To investigate the bridge-length-dependent electrochromic
behavior of the bis-isophthalates, each compound (0.20 M)
was dissolved in NMP containing 0.20 M TBAPF6 as a
supporting electrolyte. This solution was injected into the
sandwich-type ITO glass cells. The DC voltage was gradually
applied to these cells. The displayed color was characterized
by UV-vis absorption spectroscopy. As shown in Figure 2,
IS1 showed a reddish color with a maximum absorption peak
at 533 nm. The color displayed by IS2 can be described as
violet with a maximum absorption peak at 542 nm. The
(12) Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Yum, J.-H.; Fantacci, S.;
Angelis, F. D.; Censo, D. D.; Nazeeruddin, M. K.; Gra¨tzel, M. J. Am. Chem.
Soc. 2006, 128, 16701.
Org. Lett., Vol. 10, No. 23, 2008
5367

it was concluded that the displayed color is highly dependent
on the length of the conjugated bridge and the ability of
electronic communication.
Next, we tested the stability of the colored species of bis-
isophthalates. We gradually supplied -3.2 V to the ITO
working electrode and then suddenly stopped the voltage
supply. IS1 exhibited a promising memory effect as shown
in Figure 3a, which shows the time-dependent color intensity
change of IS1.
reduced bis-isophthalates and oxidized ferrocene species
through electrolyte solution. Thus, we believe that the
efficient separation of working and counter materials by
heterogenization will greatly extend the lifetime of colored
species. To remove that displayed color, we applied a positive
potential (+1.0V) to the working electrode, which resulted
in the immediate disappearance of color. Figure 3b shows
the reversible switching of color with the help of the
contemporary redox reaction by ferrocene.13 The memory
effect of the displayed color after power off was also
dependent on the bridge length. With increasing conjugated
bridge length from IS1, through IS2 to IS3, the memory
effect was shortened (see Supporting Information).
In conclusion, new electrochromic systems based on bis-
isophthalates were designed and prepared. They showed
electrochromism behavior by two-electron reduction. The
displayed colors showed greatly enhanced bistability and
were dependent on the length of the conjugated bridge
between the two isophthalate groups. The color change was
reversible in the presence of the contemporary redox system,
ferrocene. The study results suggested that the connection
of two electrochromic systems with a conjugated bridge is
a good basic strategy to obtain bistable electrochromic
systems. We believe that this method can be applied to the
design of more diverse electrochromic systems.
Acknowledgment. This work was supported by the
Samsung Research Fund, Sungkyunkwan University, 2008.
W.S. is grateful for a grant from the Korea Research
Foundation (MOEHRD) (KRF-2005-005J11901). J.Y.L.
acknowledges financial support by the Korea Science and
Engineering Foundation (KOSEF) grant funded by the Korea
government (MEST) (no. R11-2007-012-03002-0) (2008).
Figure 3. Change of color intensity of IS1 after power off (a) and
Supporting Information Available: Experimental pro-
cedure for the preparation of new compounds and their
characterization data, and manipulation procedure of two ITO
electrodes cells. This information is available free of charge
via the Internet at http://pubs.acs.org.
with switching voltage between -3.2 V and +1.0 V for 22 s,
respectively (b).
It took around 300 s for half-intensity to be obtained. In
sharp contrast, the similar color as was formed by the one-
electron reduction of dimethyl terephthalate to form an
anionic radical disappeared rapidly. The gradual color
quenching process resulted from the recombination of
OL802208X
(13) Kobayashi, N.; Miura, S.; Nishimura, M.; Urano, H. Solar Ener.
Mater. Solar Cells 2008, 92, 136.
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