Dendritic mixed-valence dinuclear ruthenium complexes for optical attenuation at telecommunication wavelengths

Article abstract of DOI:10.1021/ma021018s

Novel dendritic mixed-valence dinuclear ruthenium complexes were prepared and chemically cross-linked to form thin films on ITO electrodes. The cross-linked films exhibited an excellent electrochromism in the near-infrared (NIR) region and optical attenua

Full text of DOI:10.1021/ma021018s

3
146
Macromolecules 2003, 36, 3146-3151
Dendritic Mixed-Valence Dinuclear Ruthenium Complexes for Optical
Attenuation at Telecommunication Wavelengths
Yin gh u a Qi a n d Zh i Yu a n Wa n g*
Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa,
Ontario, Canada K1S 5B6
Received J une 28, 2002; Revised Manuscript Received February 21, 2003
ABSTRACT: Novel dendritic mixed-valence dinuclear ruthenium complexes were prepared and chemically
cross-linked to form thin films on ITO electrodes. The cross-linked films exhibited an excellent
electrochromism in the near-infrared (NIR) region and optical attenuation of 5.4 dB/µm in thickness at
1
550 nm with a switching time of 2 s. A variable optical attenuator based on these NIR electrochromic
ruthenium complex materials could be envisioned.
In tr od u ction
hydrazido (DCH) and 2,2′-bipyridine (bpy) ligands
Scheme 1) are highly electrochromic in the NIR re-
gion. When the dinuclear metal center is oxidized from
(
A variable optical attenuator (VOA) is an essential
component for gaining control of optical signals in
wavelength-division multiplexing networks and for
dynamic channel power regulation and equalization in
cross-connected nodes. Various types of VOAs have been
explored, such as microelectromechanical devices, slid-
ing-block mechanical devices, side-polished fiber devices,
and thermooptic systems.1-4 At present, a major chal-
lenge in the telecommunication industry is to develop
inexpensive planar integrated VOA devices in response
to the ever increasing data transmission and processing
rates as well as lowering fabrication costs. Organic
materials that are electrically, optically, or thermally
active in the near-infrared (NIR) region, specifically at
the telecommunication wavelengths (e.g., 1300 and 1550
nm), can well serve this purpose, owing to their unique
electrical and optical properties, low-cost fabrication,
and feasibility for use in a monolithically integrated
optical device. The NIR-active organic materials are of
low band gap and commonly contain an extended
1
0
II
II
II
the Ru /Ru state to the mixed-valence state (Ru /
III
Ru ), the DCH-Ru complexes display an intense NIR
absorption between 1000 and 2000 nm. At the Ru /Ru
state or fully oxidized Ru /Ru state, it shows no
absorption in the NIR region. Complexes 1 are chemi-
cally stable in air and water at elevated temperatures
II
II
III
III
(
e.g., 200 °C). On the basis of the NIR electrochromism,
a new electrochromic VOA or ECVOA can be envisioned
using these dinuclear ruthenium complexes. ECVOA
works in the same principle as electrochromic devices
and employs EC materials that exhibit reversible and
stable optical changes at the telecommunication wave-
lengths. If under one applied voltage the EC material
in ECVOA exists in a redox state that is NIR absorbing,
the same material should be in an opposite redox state
that is NIR inactive at another voltage. When the NIR
light is directed through an ECVOA device, the light
will either pass through or be absorbed at two distinct
redox states and thus can be attenuated variably by
controlling the redox process within the EC material
through the applied voltages. Accordingly, basic require-
ments for organic materials to be used in an ECVOA
should include high coloration efficiency, fast switching
speed at low voltages, good film-forming property, and
stability.
5
,6
π-conjugation
or mixed-valence organometallic sys-
7
,8
tem. Among the conducting polymers, poly(3,4-ethyl-
enedioxythiophene) (PEDOT) and its derivatives have
exhibited the most promising electrical-optical proper-
ties for use in electrochromic (EC) devices, including
rapid switching speed with a high color contrast and
6
excellent electrochemical stability upon cycling. In
Extensive studies on the novel dinuclear ruthenium
complexes (DCH-Ru) have been carried out in our
group in the past few years. Several synthetic routes to
various ligands have been established, and a series of
DCH-Ru complexes having various donor to acceptor
groups and DCH-Ru linear polymers have been pre-
addition, these polymers are reasonably stable at el-
evated temperatures and can be easily electrochemically
deposited as thin films on substrates. Much research
effort has been devoted to the applications of PEDOT
and its derivatives in the visible region of the electro-
magnetic spectrum rather than at the telecommunica-
tion wavelengths. The mixed-valence metal complexes
are generally difficult for use in a device, as the low
molecular weight complexes do not form stable thin
films by common film-making techniques such as spin
coating and vapor deposition. To date, the application
of organic materials in VOA has not been fully ex-
1
1
pared. DCH-Ru complexes with weak electron-donat-
ing strength of substituents (e.g., phenyl, alkyl, phen-
ylene ether) are proposed to be the most promising
materials for use in VOA, as they have a strong
electrochromism within the telecommunication bands
9
of 1500-1600 nm.
In this paper, the synthesis and characterization of
two first-generation, dendritic DCH-Ru complexes 2
and 3 are presented (Scheme 2). These ruthenium
complexes contain the hydroxy groups, which allows for
chemical cross-linking with an isocyanate material to
form thin films on an electrode and fabrication of a
concept device for ECVOA.
7
a,9
plored.
The early work by Kaim et al. shows that the
dinuclear ruthenium complexes 1 with 1,2-dicarbonyl-
*
To whom correspondence should be addressed. E-mail: wangw@
ccs.carleton.ca.
1
0.1021/ma021018s CCC: $25.00 © 2003 American Chemical Society
Published on Web 04/04/2003

Macromolecules, Vol. 36, No. 9, 2003
Mixed-Valence Dinuclear Ru Complexes 3147
Sch em e 1
Sch em e 2
Exp er im en ta l Section
Syn th esis of 6-Hyd r oxyh exa n oic Hyd r a zid e. ꢀ-Capro-
lactone (5.5 mL, 0.05 mol) was added dropwise into a solution
of hydrazine hydrate (5 mL, 0.1 mol) in N,N-dimethylforma-
mide (DMF, 5 mL) at room temperature. After 1 h, the
resultant white precipitate was collected by filtration and
Ma ter ia ls. p-Hydroxybenzoic hydrazide was purified by
washing with tetrahydrofuran (THF). 1,3,5-Benzenetricar-
bonyl trichloride was distilled prior to use. cis-Ru(bpy)
H
2
2 2
Cl ‚
2
O was prepared according to the literature method.12 Other
washed with hot acetone twice to give a white powder (5.8 g,
chemicals were purchased from Sigma-Aldrich Canada Ltd.
and were used as received.
1
8
1% yield); mp 115-116 °C. H NMR (200 MHz, DMSO-d
6
):
δ 8.9 (s, 1H), 4.3 (t, 1H, J ) 6.6 Hz), 4.1 (s, 2H), 3.3 (m, 2H),
Mea su r em en ts. Cyclic voltammograms (CV) were per-
formed on a BAS 100 B/W electrochemical station interfaced
and monitored with a PC using tetra-n-butylammonium
hexafluorophosphate (TBAH, 0.1 M) as supporting electrolyte
in acetonitrile. A standard three-electrode configuration was
used with a platinum or tin-doped indium oxide (ITO) as
working electrode, a platinum wire as counter electrode, and
a silver wire as reference electrode. The silver pseudo-reference
electrode was calibrated with a ferrocene/ferrocenium redox
couple, and the potentials are with respect to the normal
hydrogen electrode (NHE), unless otherwise noted. All the
solutions were deoxygenated with nitrogen prior to electro-
chemical measurements. Spectroelectrochemical measure-
ments were carried out using the electrochemical station
together with a Perkin-Elmer Lambda 900 UV/vis/NIR spec-
trometer. Spectroelectrochemical measurements in solutions
were conducted using an OTTLE cell. The NMR spectra were
recorded at room temperature on a Bruker AMX-400 or Varian
Gemini 200. The IR spectra were performed on a Bomem
Michelson-100 FT-IR spectrometer. Thermogravimetric analy-
ses were carried out on a Seiko TG/DTA SSC 5200 under a
nitrogen atmosphere. Mass spectrometry was carried out by
the University of Ottawa Regional Mass Spectrometry Center.
Melting points were obtained with a Fisher-J ohns apparatus
and are uncorrected.
-
1
1
.9 (t, 2H, J ) 7.5 Hz), 1.4 (m, 6H). IR (KBr, cm ): 3311.3
(
C
υ
N-H), 1641.6 (υCdO), 1536.6 (υN-H). MS (ESI, m/z) for
+
6
H
14
N
2
O
2
: 147 (M + H ).
Liga n d 2′. A solution of 1,3,5-benzenetricarbonyl trichloride
0.56 g, 2.1 mmol) in DMF (5 mL) was added dropwise into a
(
solution of p-hydroxybenzoic hydrazide (1.2 g, 7.8 mmol) and
triethylamine (0.9 mL) in DMF (15 mL). The solution was
stirred at 0 °C for 1 h and then stirred at room temperature
for 0.5 h. The hydrochloric salt of triethylamine was removed
by filtration. The filtrate was poured into distilled water,
resulting in the formation of the desired product as white
solids. The crude product was recrystallized from methanol
1
to give ligand 2′ in 64% yield; mp 298-300 °C. H NMR (400
MHz, DMSO-d ): δ 10.7 (s, 3H), 10.4 (s, 3H), 10.1 (s, 3H), 8.6
6
(
s, 3H), 7.8 (d, 6H, J ) 8.8 Hz), 7.3 (d, 6H, J ) 8.8 Hz). IR
-
1
(
KBr, cm ): 3248.5 (υN- H+ ), 1657.4 (υCdO). MS (ESI, m/z) for
: 613 (M + H ).
Liga n d 3′. A solution of 1,3,5-benzenetricarbonyl trichloride
30 24 6 9
C H N O
(0.4 g, 1.5 mmol) in DMF (5 mL) was added dropwise into a
solution of 6-hydroxyhexanoic hydrazide (0.78 g, 5.3 mmol) and
triethylamine (0.7 mL) in DMF (10 mL). The solution was
stirred at 0 °C for 1 h and then stirred at room temperature
for 0.5 h. The reaction mixture was filtered to remove the
hydrochloric salt of triethylamine formed. The filtrate was
evaporated under reduced pressure, and the resultant viscous
yellow liquid was boiled with a mixture of ethyl acetate/hexane
(4:1 v/v) for 1 h. The ethyl acetate/hexane layer was decanted,
and the residual liquid was dried under vacuum to give a pale
white solid. The solid was washed with distilled water to give
Optical switching and spectroelectrochemical studies of
DCH-Ru complex films on ITO were performed using an UV
quartz cuvette which contains a platinum wire as the counter
electrode and a silver wire as the reference electrode and
electrolyte in solution. The cuvette was fitted with a Teflon
cap through which the reference electrode and counter elec-
trode were inserted via predrilled holes.
1
a white powder (0.3 g, 38% yield); mp 218-219 °C. H NMR
(400 MHz, DMSO-d ): δ 10.6 (s, 3H), 10.0 (s, 3H), 8.5 (s, 3H),
6

3
148 Qi and Wang
Macromolecules, Vol. 36, No. 9, 2003
4
6
.4 (t, 3H, J ) 5.1 Hz), 3.4 (dd, 6H, J ) 6.2 Hz, 5.2 Hz), 2.2 (t,
H, J ) 7.2 Hz), 1.6 (m, 6H), 1.4 (m, 6H), 1.3 (m, 6H). IR (KBr,
Ta ble 1. Electr och em ica l Da ta for Den d r itic DCH-Ru
Com p lexes
-
1
cm ): 3200.1 (υN-H), 1693.7 (υCdO). MS (ESI, m/z) for
1
E1/2a
2
E1/2a
+
C
27
H
42
N
6
O
9
: 595 (M + H ).
Gen er a l P r oced u r e for th e Syn th esis of Den d r itic
DCH- Ru Com p lexes. A solution of cis-Ru(bby) Cl ‚2H
0.2 g, 0.38 mmol), dendritic ligand (0.063 mmol), and sodium
complex (Ru /Ru fRu /RuIII) (Ru /Ru fRuIII/RuIII
II
II
II
II
III
∆Eb
)
2
3
780
817
1431
1451
651
634
2
2
2
O
(
a
carbonate (50 mg, 0.5 mmol) in a mixture of water/ethanol (5/1
v:v, 80 mL) were heated to reflux for 48 h under argon. After
cooling to room temperature, a solution of ammonium hexa-
fluorophosphate (1.5 g in 60 mL of water) was added. The red
dark precipitate was filtered off and redissolved in acetone (20
mL). The ruthenium complex was then precipitated out with
diethyl ether and dried under vacuum at room temperature.
From cyclic voltammetry performed at 100 mV/s scan rate.
Potentials E in mV vs NHE. b ∆E ) E1/2 - E1/2.
2
1
then placed in a glovebox to allow a complete gelation for 3
days before testing.
Resu lts a n d Discu ssion
Complex 2: 69% yield. IR (KBr, cm-1): 1604.4 (υCdN), 843.2
P-F).
Complex 3: 66% yield. IR (KBr, cm-1): 1606.4 (υCdN), 841.8
P-F).
Syn th esis of Den d r itic DCH-Ru Com p lexes. It
is known that the branched molecular structure of
dendrimers provides well-defined surface functionality
and specific geometrical sizes, which give rise to some
unique properties not accessible in linear polymers.15
In our case, the dendritic ruthenium complexes should
have a larger free volume than the linear analogue,
which favors a rapid ion transportation and thus may
result in rapid response time with a high optical
attenuation. Furthermore, they should have better
solubility in common organic solvents than the linear
analogue, thus allowing for easy film fabrication by spin
coating or casting. For the ECVOA application, the
dendritic ruthenium complexes can also be readily cross-
linked with a cross-linker, if an appropriate reactive
group is introduced into the dendrimers. Thus, DCH-
Ru complexes 2 and 3 with reactive hydroxy groups
rather than linear polymers were designed and prepared
in this study.
(υ
(υ
Deter m in a tion of th e Ru th en iu m Con ten t in Com -
p lexes 2 a n d 3. The quantitative analysis of the ruthenium
content in complexes 2 and 3 was carried out according to a
method reported by Balcerazk et al. Because of the absence
of other interfering metals such as osmium, UV-vis absorption
spectra were used directly for the determination of ruthenium.
1
3
3
-1
The reported extinction coefficient (ꢀ445 nm ) 2.7 × 10 L mol
-
1
cm ) of the analyte solutions with the ruthenium concentra-
tion in the range 0.5-45 µg/mL was used for calculation. A
typical method is described as follows: A 50 mL, two-neck,
round-bottomed flask equipped with a nitrogen inlet and a
distillation head was charged with complex 2 (13.4 mg) and
4 2 4
HClO /H SO (1/3 v/v, 20 mL). A gentle stream of nitrogen gas
was introduced into the flask and bubbled out at the distilla-
tion end into two connected traps, each containing 20 mL of
aqueous HCl solution (2 N) at a speed of 2-3 bubbles/s. The
sample solution was heated at 120 °C until it turned into
colorless. The aqueous HCl solution in two traps were com-
bined and diluted to the volume of 100 mL in a volumetric
flask. A small portion (2 mL) of this solution was taken out
from the volumetric flask into another flask, followed by
The synthesis of dendritic ligands 2′ and 3′ was
achieved via a nucleophilic substitution reaction of 1,3,5-
benzenetricarbonyl trichloride with the corresponding
hydrazides at low temperatures (Scheme 2). It is critical
to control the reaction temperature in order to avoid the
side reaction of the hydroxy group with acyl chloride.
The dendritic ruthenium complexes 2 and 3 were
prepared by reacting the ligands 2′ and 3′ with a
2
addition of 0.5 mL of SnCl (1 M) in 2 M aqueous HCl solution
and 7.5 mL of 6 M aqueous HCl solution. The resulting
solution in the flask was heated on a boiling water bath for
4
5 min, and a sample solution was taken out for UV-vis
analysis. The ruthenium content was calculated from the given
ꢀ
value and the measured absorbance at 445 nm.
II
II
ruthenium exchange agent and obtained in the Ru /Ru
Complex 2: Ru % found: 14.0 ( 0.3%; calcd: 15.3%.
Complex 3: Ru % found: 13.7 ( 0.4%; calcd: 15.4%.
P r ep a r a tion of Cr oss-Lin k ed DCH-Ru F ilm s on ITO
for Op tica l Atten u a tion Stu d ies. Complex 2 (0.06 g, 0.015
mmol) and triisocyanate (trimethylolpropane, carbamate with
xylylene diisocyanate) (0.01 g, 0.015 mmol) were dissolved in
state as purple solids. The ruthenium content in com-
plexes 2 and 3 was determined according to a method
1
3
reported by Balcerazk et al. and was found to be in a
reasonably good agreement with the calculated values.
The other methods such as NMR and mass spectroscopy
(
by electron, chemical, and electrospray ionizations)
0
.7 mL of acetonitrile that contained 1,4-diazabicyclo[2.2.2]-
were found to be futile in quantification of the ruthe-
octane (DABCO, 1 wt % relative to 2). The prepared solution
was immediately filtered through a syringe filter with a pore
size of 0.2 µm, and the filtrate was used to spin-coat the films
on ITO. The film on ITO was dried at room temperature for
.5 h and then thermally cured under nitrogen at 120 °C for
h.
II
II
nium content. Complexes 2 and 3 in the Ru /Ru state
are found to be stable to the air, water, and strong bases
and exhibit good thermal stability as assessed by
thermogravimetry for the onset temperature for 5%
weight loss (ca. 348 °C for complex 2 and 315 °C for
complex 3). The orange-colored complexes 2 and 3 in
the mixed-valence state can be obtained by chemical
0
2
The cross-linked film based on complex 3 was prepared by
the same procedure as for complex 2, except that the cure
temperature was at 140 °C.
(e.g., H2O2) or electrochemical oxidation and have good
III
stability in air. However, the complexes in the Ru
Ru state are highly labile and cannot be obtained in
high purity.
/
F a br ica tion of a P r ototyp e ECVOA Device. The sand-
wich-type test devices were assembled using two ITO glass
plates coated with two different EC materials and a gel
electrolyte based on poly(ethylene oxide) (PEO). The composi-
III
E lect r och em ica l a n d Sp ect r oelect r och em ica l
Stu d ies. The electrochemical and electronic absorption
data for the DCH-Ru dendrimers are summarized in
Tables 1 and 2. Figure 1 (trace a) shows the cyclic
voltammogram of complex 2. As expected, there are two
quasi-reversible, one-electron oxidation processes in the
positive potential region vs NHE, associated with the
4
tion of gel electrolyte was LiClO :PEO:propylene carbonate:
ethylene carbonate in a ratio of 9:20:30:41 by weight. The
working electrode was coated with the cross-linked DCH-Ru
complex 2. The counter electrode was coated with tungsten
oxide by electrochemical deposition according to the literature
1
4
method. The gel electrolyte was cast onto the counter
electrode and dried in air for 15 min, before it (ca. 0.1 mm in
thickness) was covered with the working. The devices were
II
III
successive Ru /Ru couples. The first oxidation wave

Macromolecules, Vol. 36, No. 9, 2003
Ta ble 2. Absor p tion Da ta for Den d r itic
Mixed-Valence Dinuclear Ru Complexes 3149
DCH-Ru Com p lexes
complex
oxidation state
λmax (log ꢀ)a
II
II
2
Ru /Ru
354 (4.60); 518 (4.49)
434 (4.34); 1593 (4.26)
476 (4.17); 804 (4.01)
348 (4.64); 515 (4.54)
434 (4.39); 1549 (4.15)
479 (4.00); 806 (4.16)
II
III
Ru /Ru
III
III
Ru /Ru
II
II
3
Ru /Ru
II
III
Ru /Ru
III
III
Ru /Ru
a
Wavelength in nm; molar extinction coefficient ꢀ in M-1 cm-1
.
F igu r e 3. UV-vis-NIR spectra of cross-linked film of
complex 2 on ITO glass.
3
show a larger MMCT extinction coefficient (ꢀ) at 1550
III
III
nm (Table 2). Upon further oxidation to the Ru /Ru
state, a new band at 800 nm appears, which is assigned
to a ligand-to-metal charge-transfer (LMCT) transition.
Op tica l Atten u a tion Stu d ies. To fabricate a test
cell for optical attenuation study, thin films of DCH-
Ru complexes need to be formed on ITO-coated glass.
Since complexes 2 and 3 contain a hydroxy group, a
triisocyanate compound was used as a cross-linker to
form a polymer network via the urethane bonds. The
cross-linking or cure reactions for complexes 2 and 3
were carried out under nitrogen at 120 and 140 °C,
respectively, and monitored by IR. The characteristic
F igu r e 1. Cyclic voltammograms of (a) complex 2 and (b)
cross-linked film of complex 2 on ITO in acetonitrile/0.1 M
TBAH (in reference to NHE).
-1
IR band of the isocyanate group (2260 cm ) decreased
with the increase of cure time and completely disap-
peared after 2 h. The cured films tightly adhered to the
ITO surface even after being soaked in organic solvents
such as DMF, acetone, acetonitrile, and chloroform over
days. The electrochemical and spectroelectrochemical
behaviors of the cross-linked films are almost identical
to those of DCH-Ru complexes in solution (Figures
1
-3).
The studies on dynamic attenuation or optical switch-
ing were performed using a quartz cuvette cell, which
contained an ITO plate coated with a layer of cured
DCH-Ru film with a thickness of 400 nm, a solution of
TBAH in acetonitrile, a silver reference electrode, and
a platinum counter electrode. The film can be switched
on and off in less than 2 s, and optical attenuation was
recorded at 1550 nm. The films of complexes 2 and 3
displayed optical attenuation of 2.2 and 2.0 dB at 1550
nm with a switching time of 2 s, respectively. Accord-
ingly, their attenuation powers are 5.4 and 4.9 dB/µm
of film in thickness.
To probe the effect of the applied potential on the
redox process and optical attenuation, optical attenua-
tion of complexes 2 and 3 was measured at various
potentials. Figure 4 depicts the optical attenuation as
a function of applied potential. The film-coated ITO
electrode was stepped between its reduced (-100 mV
vs silver electrode) and oxidized state with a switching
time of 2 s. At the same time, the transmittance at 1550
nm was monitored with UV-vis-NIR spectroscopy in
order to determine the optical attenuation. Both films
underwent 200 switching cycles for each stepping
potential and displayed an excellent reproducibility of
attenuation during the repeated redox switching. For
complex 2, optical attenuation increased dramatically
with the increase of applied potential. It reached a
plateau at a potential of 850 mV until a potential of 1200
mV. In the more positive range, optical attenuation
gradually decreased due to the partial oxidation of the
F igu r e 2. UV-vis-NIR spectra of complex 2 in three
oxidation states.
II
II
corresponds to the oxidation of the Ru /Ru metal
II
III
center to the Ru /Ru state, while the second corre-
II
III
sponds to the further oxidation of the Ru /Ru state
III
III
to the Ru /Ru
state. The gap between the two
oxidation potentials (∆E) is 0.65 V. This value is larger
8
-11
than those of other dinuclear metal complexes,
offering a better control in voltage for the ECVOA
application. Multisweep experiments showed a negli-
gible change after 20 cycles of scanning, which indicates
a good reversibility of the electrochemical process of
these dendritic ruthenium complexes.
II
II
Complexes 2 and 3 in the Ru /Ru state display two
intense absorptions around at 350 and 520 nm associ-
ated with the MLCT transition (metal-to-ligand charge
II
III
transfer). Oxidation to the Ru /Ru state results in the
appearance of a strong and broad intervalence transi-
tion band centered at 1593 nm for complex 2 (Figure 2)
and at 1549 nm for complex 3. These bands are
attributed to the MMCT transition (metal-metal charge
transfer) formed in the mixed-valence state. Compared
to small DCH-Ru complexes like 1,9b complexes 2 and

3
150 Qi and Wang
Macromolecules, Vol. 36, No. 9, 2003
F igu r e 4. Optical attenuation of films of complexes 2 and 3
on ITO glass as a function of applied potential with a switching
time of 2 s and a stepping potential (-100 mV vs silver
electrode).
F igu r e 6. Electroactivity of film of complex 2 on ITO as a
function of switching cycles.
Long-term stability of complexes 2 and 3 as films on
ITO was further determined by probing the ratio of
Q/Q0. The ratio of Q/Q0 represents the charge involved
in the redox process after a given number of switching
cycles to the initial switching charge. Figure 6 shows a
representative graph of electroactivity of complex 2 with
a switching time of 2 s. An initial increase of the
electroactivity was observed. Like some conjugated
polymers, this phenomenon is not surprising as more
electroactive sites of the film were activated after the
1
7
short break-in period. After 2000 switching cycles, the
film of complex 2 displayed a retention of 73% of its
original electroactivity. The drop of electroactivity was
found to be mainly due to the film delamination from
ITO in the electrolyte solution. Thus, a preliminary test
using 3-aminopropyltrimethoxysilane as the first layer
on the ITO electrode before coating complex 2 or 3
indicated an improvement in switching stability. The
films of complexes 2 and 3 on the treated ITO electrode
reached 19 000 cycles with a 15% drop of attenuation
and 18 000 cycles with a 19% drop of attenuation,
respectively.
F igu r e 5. Dependence of optical attenuation on switching
time for films of complexes 2 and 3 on ITO glass.
II
III
state to the RuIII/RuIII state. The optical
Ru /Ru
attenuation of complex 3 reached a maximum value at
about 900 mV. However, unlike complex 2, the attenu-
ation of complex 3 did not decrease until the potential
was higher than 1900 mV, which may be attributed to
III
the slow oxidation reaction of complex 3 to the Ru
/
III
Ru state. Clearly, variable optical attenuation could
be achieved and corresponded almost linearly to the
applied voltage within the range 500-2000 mV vs a
silver electrode.
P r ototyp e ECVOA Device. Since the cross-linked
DCH-Ru films exhibited good attenuating properties
at 1550 nm, a prototype ECVOA device was fabricated
for further evaluation using anodically NIR-coloring
complex 2 and cathodically NIR-coloring tungsten oxide
To study the effect of switching time on optical
attenuation at two telecommunication wavelengths, the
film on ITO was switched between -100 and +1100 mV
vs silver electrode with different switching time. Both
complexes 2 and 3 exhibited a gradual increase in
attenuation with the longer switching time (Figure 5).
In comparison with complex 2, complex 3 displayed a
lower optical attenuation at 1550 nm but a higher
optical attenuation at 1300 nm.
(WO3). A gel electrolyte based on poly(ethylene oxide)
was sandwiched in between two ITO glass plates coated
with complex 2 and WO3 films. The overall electro-
chemical process of this device may be written as
follows:
Coloration efficiency (CE) is an important wavelength-
dependent parameter for evaluating electrochromic
materials. It is determined as the variation of optical
density as a function of injected/ejected charge per unit
electrode area. The ideal EC material or device for the
ECVOA application should show a large transmittance
variation with a small amount of charge or has a high
CE. The CE values of cross-linked films of complexes 2
and 3 were assessed in solution according to the known
Such a device demonstrated a rapid response in the
NIR region to the applied potentials and reached an
attenuation of 5.4 dB at 1550 nm with a switching time
of 3 s (Figure 7). However, an increase in the switching
time (up to 10 s) did not further expand the attenuation
range.
6
c,16
method.
The relatively high CE values of 206 and
38 cm /C were obtained at 1550 nm for complexes 2
and 3, respectively.
2
2

Macromolecules, Vol. 36, No. 9, 2003
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