Designed synthesis of multi-electrochromic systems bearing diaryl ketone and isophthalates

Article abstract of DOI:10.1021/jo101350k

New multi-electrochromic systems have been developed through the combination of a diaryl ketyl radical system with isophthalate-based electrochromic materials. The location of the isophthalate group in compounds is very critical to obtaining different colors in the multi-electrochromism.

Full text of DOI:10.1021/jo101350k

pubs.acs.org/joc
than two monomers can be incorporated into conducting
polymers to induce multi-electrochromism.4
Designed Synthesis of Multi-Electrochromic Systems
Bearing Diaryl Ketone and Isophthalates
Recently, single molecular electrochromic materials have
attracted significant attention given their ability to be chemi-
cally engineered onto nanosized electrode materials.5 For ulti-
mate full-color displays, the combination of several independent
molecular electrochromic materials displaying the different
colors is needed. Usually, the related devices have been fabri-
cated based on a multilayer structure in which each layer con-
tains distinct colors.6 Comparatively, if the materials can show
multicolors depending on applied potentials, the structure of the
device can be more effectively designed. One can easily speculate
that the combination of several different molecular electrochro-
mic systems could generate more advanced systems. However,
compared to the conducting polymer-based electrochromic sys-
tems, the single molecular engineering of electrochromic systems
has been sparsely explored.
W. Sharmoukh,† Kyoung Chul Ko,† Changho Noh,‡
Jin Yong Lee,*,† and Seung Uk Son*,†
†Department of Chemistry and Department of Energy
Science, Sungkyunkwan University, Suwon 440-746, Korea,
and ‡Display Lab, Samsung Advanced Institute of Technology
(SAIT), Yongin 446-712, Korea
sson@skku.edu
Received July 14, 2010
The ketyl radical system, with an anionic radical of the
general structure [C-O] -, has been well recognized in orga-
3
nic chemistry for a long time.7 One of the most famous ketyl
radical systems can be formed by the one-electron reduction
of benzophenone. Our research group has studied the re-
ductive inorganic and organic electrochromic systems in-
cluding isophthalate derivatives.8 In this study, we report on
the molecular engineering of multi-electrochromic systems
through combination of the diaryl ketyl radical and iso-
phthalate systems (Scheme 1).
A series of new molecular electrochromic systems bearing the
diaryl ketones and isophthalates have been designed. As shown
in Chart 1, the diaryl ketyl radical system was connected to
the isophthalate unit. The target electrochromic compounds in
Chart 1 were prepared following Scheme 2. Although, in addi-
tion to the compounds in Chart 1, the bis(isophthalate) com-
pounds, bearing 2,6-anthracene-9,10-dione, 3,30-benzil, and
5,50-di(2-thienyl)methanone as bridges, were also prepared;
they were excluded from the study due to solubility problems.
First, EC1 was prepared by coupling with isophthalic acid
and successive esterification9 (Scheme 2). The central methylene
group was then oxidized to a carbonyl group using CrO3.10 For
improvement of solubility in organic media and ultimate attach-
ment of compounds to electrode materials through olefin
New multi-electrochromic systems have been developed
through the combination of a diaryl ketyl radical system
with isophthalate-based electrochromic materials. The
location of the isophthalate group in compounds is very
critical to obtaining different colors in the multi-electro-
chromism.
Organic electrochromic materials have attracted spe-
cial attention due to their unique advantages over their
inorganic counterparts.1 First, organic materials possess
the flexibility necessary for flexible device fabrications.
Second, compared to inorganic materials, they display
diverse and clear colors with a relatively narrow absorp-
tion band.
During the past decade, the related studies have focused
on polymeric organic materials.2 Usually, redox reactions of
conducting organic polymers result in a change in the ab-
sorption band in the visible light range. By changing the
electronic structure of the monomers, the electrochromic
properties of the polymers are tuned.2,3 Furthermore, more
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Mater. Solar Cells 2009, 92, 136.
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Schmittel, M.; Wenner, H.; Perepichka, I. F.; Peters, K. Eur. J. Org. Chem.
2001, 1391. (c) Porter, W. W.; Vaid, T. P. J. Org. Chem. 2005, 70, 5028.
(7) Selected references: (a) Thum, C. C. L.; Khairallah, G. N.; O’Hair,
R. A. J. Angew. Chem., Int. Ed. 2008, 47, 9118 and references therein.
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M.; Cai, X.; Fujitsuka, M.; Majima, T. Chem.;Eur. J. 2006, 12, 1610.
(8) (a) 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.
(b) Sharmoukh, W.; Ko, K. C.; Park, S. Y.; Ko, J. H.; Lee, J. M.; Noh, C.;
Lee, J. Y.; Son, S. U. Org. Lett. 2008, 10, 5365. (c) Park, S. Y.; Lee, J. M.; Son,
S. U. J. Mater. Chem. 2009, 19, 7959. (d) Sharmoukh, W.; Ko, K. C.; Ko,
J. H.; Jung, I. G.; Noh, C.; Lee, J. Y.; Son, S. U. Org. Lett. 2010, 12, 3226.
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Published on Web 09/09/2010
DOI: 10.1021/jo101350k
r
2010 American Chemical Society

Sharmoukh et al.
JOCNote
SCHEME 1. Formation of the Ketyl Radical (a); Reduction of
Isophthalate Derivatives (b); Design of New Electrochromic
Compounds through Combination of Both Systems (c)
SCHEME 2. Synthetic Pathways for EC1-EC8a
CHART 1. Electrochromic Compounds Used in This Study
a(a) CH2O, 20% Oleum, 120 °C, 6 h; (b) MeOH, HCl, reflux, 0.5 h;
(c) CrO3, Ac2O, rt, overnight; (d) NaOH, MeOH, reflux, 8 h, then HCl;
(e) SOCl2, reflux, 6 h; (f) 5-Hexen-1-ol, CH2Cl2, pyridine, rt, 6 h;
(g) NaNO2, 10% HCl, H2O, KI, rt, overnight; (h) KMnO4, I2, AcOH,
Ac2O, H2SO4, 70 °C, 4 h; (i) Bis(pinacolato)diborane, K2OAc, Pd(OAc)2,
DMF, 90 °C, 4 h; (j) Pd(OAc)2, K2CO3, DMF, 90 °C, 4-12 h.
3,30-Diiodobenzophenone and 2,7-diiodofluoren-9-one were
prepared by direct iodation of benzophenone and fluorenone.13
Then, these iodoarenes were coupled with 5-pinacolatoboron-
isophthalates by palladium catalysis. Moreover, to elucidate
the role of the carbonyl group in the bridge of EC3, EC4 was
prepared by coupling with bis(4-iodophenyl)methane.
Next, electrochemical and electrochromic properties of the
prepared compounds (EC1-EC8) were investigated by cyclic
voltammetry (CV) and in situ UV/visible absorption spectros-
copy (Table 1; Figures 2, S1, and S2 in the Supporting Informa-
tion). Compound EC1 showed simple reversible one-electron
reduction behavior at -1.76 V (versus Ag/Agþ). The expected
second reduction peak by isophthalate groups was not observed
in the electrochemical test range of the solvent, implying that the
second reduction needs a much higher voltage due to the close
electronic communication between the central carbonyl group
and two isophthalates. Compared to benzophenone, showing
a one-electron reduction peak at -2.14 V (versus Ag/Agþ), the
reduction peak of EC1 at -1.76 V was significantly shifted in a
more positive direction due to the electron-withdrawing effect of
the four ester groups. As has been well documented, the ketyl
radical of benzophenone is quite sensitive and unstable.7 Ben-
zophenone showed a pale blue-green color via reduction; the
metathesis chemistry,11 5-hexenyl groups were introduced
into the ester groups instead of methyl groups. Actually, EC1
can be regarded as a derivative of both benzophenone and
diisophthalates. To test the effects of the carbonyl groups in
the bridge of the two isophthalate groups, EC2, possessing a
methylene bridge, was also prepared.
As shown in Scheme 2, EC3-EC8 bearing 4,40-benzophe-
none, 4-benzophenone, 3,30-benzophenone, 3-benzophenone,
and 2,7-fluoren-9-one were prepared. First, 4,40-diiodobenzo-
phenone, 4-iodobenzophenone, and 3-iodobenzophenone were
prepared by respective iodination of 4,40-diaminobenzophe-
none, 4-aminobenzophenone, and 3-aminobenzophenone.12
(11) Noh, C. H.; Son, S. U. U.S. Patent US-2009097096, 2009.
(12) Takalo, H.; Hanninen, E.; Kankare, J. Acta Chem. Scand., Ser. B:
Org. Chem. Biochem. 1988, B42, 662.
(13) (a) Lulinski, P.; Krassowska-Swiebocka, B.; Skulski, L. Molecules
2004, 9, 595. (b) Xu, J.; Zheng, H.; Liu, H.; Zhou, C.; Zhao, Y.; Li, Y.; Li, Y.
J. Phys. Chem. C 2010, 114, 2925.
J. Org. Chem. Vol. 75, No. 19, 2010 6709

Sharmoukh et al.
JOCNote
TABLE 1. Electrochemical and Electrochromic Properties
reduction potentiala
UV/vis max. abs.
(nm)
(V, vs Ag/Agþ)
compounds
first (ON1)
second (ON2)
first
second
EC1
EC2
EC3
EC4
EC5
EC6
EC7
EC8
-1.76
-2.28b
-1.95
-2.30b
-2.05
-2.03
-2.14
-1.51
-
462
434
589
507
567
519
529
530
-
-
-
-2.34
-
-2.47
-2.43
-2.41b
-2.06b
507
-
510
505
501
575
aRedox potential vs Ag/Agþ(reference electrode), 0.20 M tetrabuty-
lammonium hexafluorophosphate as supporting electrolyte in acetoni-
trile, ITO glass as the working electrode, and platinum as the counter
electrode. bQuasi-reversible peaks.
FIGURE 2. Photographs of the displayed colors of EC1-8 before
and after the power supply.
peak at -2.30 V and a color change from colorless to red with
maximum absorption peaks at 507 nm. This confirmed again
that the existence of the carbonyl group played a key role in the
multi-electrochromic performance.
EC5 displayed very similar multi-electrochromism with
EC3. Due to the existence of one isophthalate group, it
showed two-step one-electron reduction peaks with a color
change from colorless to vivid blue (max. absorption peak at
567 nm) and then red (max. absorption peak at 510 nm).
Compounds EC6-EC8 demonstrated the two-step reduc-
tion process, and the displayed colors are quite dependent on
the chemical structure of compounds14 (Table 1 and Figure 2).
The color difference between the first and the second reduced
species will be an important factor in multi-electrochromism. In
this regard, a comparison of the displayed colors by EC3 with
those by EC6 was interesting. Compound EC6, containing a
3,30-benzophenone as a bridge instead of 4,40-benzophenone,
showed very similar electrochemical behavior to that of EC3
with one-electron reduction at -2.03 V and a two-electron re-
duction process at -2.43 V. However, the difference between
the displayed colors by EC6 was not as big as that by EC3; first,
violet-red (max. absorbance at 519 nm) and, second, dark-red
(the maximum absorbance at 505 nm), respectively (Figure 3).
It is worth noting that the second colors of EC3 (507 nm) and
EC6 (505 nm) were nearly identical and, furthermore, very
similar to those of the reduced EC4 (507 nm) and 5-phenyl-
isophthalate (506 nm)15(Figure S3).
FIGURE 1. Cyclic voltammograms of benzophenone (a) and EC3 (b).
displayed color, however, immediately disappeared. In compar-
ison, EC1 showed a stable color change from colorless to yellow-
orange (max. absorbance at 462 nm) through reduction, due to
the stabilization effect of the anionic radical by the four electron-
withdrawing ester groups.
Compound EC2, having a methylene bridge between two
isophthalates, showed a quasi-reversible two-electron reduc-
tion peak at -2.28 V (versus Ag/Agþ) and a color change from
colorless to yellow (max. absorbance at 434 nm), which corre-
sponds to the typical isophthalate-based electrochromism.8a
Considering the electrochemical properties of EC1 and EC2,
it is noteworthy that the benzophenone analogue underwent
reduction more easily than the isophthalates. Although multi-
electrochromism was not observed, it was clear that the exis-
tence of the carbonyl group had a crucial effect on the electro-
chromic properties of EC1.
Compounds EC3 and EC5-EC8 showed interesting multi-
electrochromic behaviors. According to the CV studies pre-
sented in Figure 1, EC3 demonstrated the two-step reduction
process with the first one-electron reversible reduction peak at
-1.95 V and the second two-electron reversible reduction peak
at -2.34 V. Interestingly, the corresponding color changes were
from colorless to vivid blue (max. absorption peak at 589 nm)
and then red (max. absorption peak at 507 nm) (Figure 2). The
potential difference between the two reduction processes was
0.39 V, and the clear color difference enables EC3 as a promis-
ing multi-electrochromic material. Compared to EC3, EC4,
containing the 4,40-biphenylmethane as a bridge instead of
4,40-benzophenone, showed a two-electron reversible reduction
To understand the color difference of the first reduced
species of EC3 and EC6, a computational calculation was
conducted on the reduced forms by one electron. As shown
in Figure 3c, the natural bond orbital (NBO) charge in a
model compound of the reduced EC3 by one electron is more
(14) In the case of EC8, the multielectrochromism resulted from the two-
step reduction of the central fluoren-9-one moiety.
(15) 5-Phenyl-isophthalate showed the maxium absorption peak at 506
nm by one-electron reduction in ref 8a.
6710 J. Org. Chem. Vol. 75, No. 19, 2010

Sharmoukh et al.
JOCNote
FIGURE 4. Multi-electrochromic performance of EC3; blue color
display (589 nm) with 2.4 (DC)V (a); red color display (507 nm) with
3.0 (DC)V.
molecular multi-electrochromic systems can be designed
through the combination of isophthalate with other redox
systems.
Experimental Section
Synthesis of EC3. To a DMF (20 mL) solution of di-5-hexenyl
5-pinacolatoboron-isophthalate8 (0.30 g, 0.66 mmol), 4,40-
diiodobenzophenone12 (0.12 g, 0.28 mmol), palladium acetate
(12 mg, 0.053 mmol), and potassium carbonate (0.50 g, 3.6
mmol) were added. The reaction mixture was heated at 90 °C for
4 h. Then, the reaction mixture was extracted using ethylacetate
and brine and then dried over MgSO4. After the solvent was
evaporated, the product was isolated with 42% yield by column
chromatography using a mixture of hexane and ethylacetate (10:1)
as eluent. 1H NMR (300 MHz, CDCl3, ppm) δ 8.70 (s, 2H), 8.51 (s,
4 H), 7.97 (d, J = 8.4 Hz, 4H), 7.80 (d, J = 8.4 Hz, 4H), 5.80 (m,
4H), 5.03 (d, J = 14 Hz, 4H), 4.98 (d, J = 6.9 Hz, 4H), 4.41 (t, J =
6.9 Hz, 8H), 2.14 (q, J= 7.2 Hz, 8H), 1.82 (q, J= 7.2 Hz, 8H), 1.58
(q, J = 7.2 Hz, 8H); 13C NMR (75 MHz, CDCl3, ppm) δ 195.6,
165.8, 143.3, 140.8, 138.4, 137.2, 132.5, 131.9, 130.9, 130.2, 127.4,
115.1, 65.7, 33.4, 28.2, 25.4; HRMS (FAB) calcd for [M þ H]þ,
C53H59O9, 839.4159; found, 839.4163.
FIGURE 3. UV/vis absorption spectra of the first and second
reduced species of EC3 (a) and EC6 (b) by one and three electrons.
The calculated NBO charge distribution in the model compounds of
the reduced EC3 and EC6 by one electron (c).
effectively delocalized than that of a model compound of the
reduced EC6.16 This resulted in a red shift of the absorption
band of the reduced EC3 by one electron, compared to the
reduced EC6. Actually, the more effective delocalization of
anionic radicals is an efficient strategy to obtain the red-
shifted absorption bands in diverse reductive electrochromic
systems.8a,b Figure 4 shows the reversible multi-electrochro-
mic performances of EC3 in a sandwich-type device,8a,17
displaying blue and red color with a supply of 2.4 and 3.0 DC
voltages respectively.
In conclusion, this work verified that the combination of
the ketyl radical system with the isophthalate electrochromic
system is a successful strategy to obtain a multi-electrochro-
mic system. Interestingly, the location of the isophthalate
group is very critical to obtaining different colors in the
multi-electrochromism. Also, we believe that more diverse
Acknowledgment. This work was supported by the
Samsung Research Fund-2010 and Grant R31-2008-000-
10029-0 (WCU program) through the National Research
Foundation of Korea funded by the Ministry of Education,
Science and Technology. W.S. is thankful for Grant NRF-
2009-0094024 (Priority Research Centers Program).
Supporting Information Available: Synthetic procedures
and characterization data of new compounds, details of calcula-
tion, and additional UV and CV data. This information is
available free of charge via the Internet at http://pubs.acs.org.
(16) See the Supporting Information for a detailed procedure of
calculation.
(17) Unfortunately, the reduced species have no bistability.
J. Org. Chem. Vol. 75, No. 19, 2010 6711