Primary coloured electrochromism of aromatic oxygen and sulfur diesters

Article abstract of DOI:10.1039/c4ra02523g

Eleven aromatic diesters and thioic S,S′-diesters were synthesized and investigated using electrochemical (cyclic voltammetry and controlled potential electrolysis) and UV-vis spectroscopic techniques over a range of temperatures. Nine of the compounds exhibited vibrant colour changes from a colourless state in their neutral forms to brightly coloured upon one-electron electrochemical reduction in acetonitrile. The compounds were found to display either red, green or blue colours in their one-electron reduced states. The electrochromic properties of 3 of the compounds that displayed the most vibrant colour changes were examined in solution using a gold micro-mesh electrode laminated inside a polymer film.

Full text of DOI:10.1039/c4ra02523g

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PAPER
Primary coloured electrochromism of aromatic
oxygen and sulfur diesters†
Cite this: RSC Adv., 2014, 4, 18100
Xiuhui Xu and Richard D. Webster*
0
Eleven aromatic diesters and thioic S,S -diesters were synthesized and investigated using electrochemical
(cyclic voltammetry and controlled potential electrolysis) and UV-vis spectroscopic techniques over a
range of temperatures. Nine of the compounds exhibited vibrant colour changes from a colourless state
in their neutral forms to brightly coloured upon one-electron electrochemical reduction in acetonitrile.
The compounds were found to display either red, green or blue colours in their one-electron reduced
states. The electrochromic properties of 3 of the compounds that displayed the most vibrant colour
changes were examined in solution using a gold micro-mesh electrode laminated inside a polymer film.
Received 22nd March 2014
Accepted 7th April 2014
DOI: 10.1039/c4ra02523g
www.rsc.org/advances
classes of EC materials include the oxides of transition metals
1
Introduction
1,2,10
(
such as tungsten oxide and titanium oxide),
viologens,
metal coordina-
To be considered
reversibly and persistently in a certain wavelength range in potentially useful in commercial applications, the EC materials
11–14
Electrochromism is a phenomenon exhibited by certain mate- conjugated conducting polymers (PEDOT),
19–21
15–18
rials, which is the ability to change light absorption properties tion complexes
and Prussian blue.
1
–3
response to a small applied electrical voltage.
chromism was discovered as early as the nineteenth century,
but it was only in the late twentieth century that there was a
Electro- are preferred to have good cycle life, high contrast ratio, write-
2
2
erase efficiency and colouration efficiency.
It has been reported that some aromatic esters exhibit strong
substantial amount of research and reviews that detail the (intense) colour changes from colourless to red or blue upon
4
theory and application of electrochromic materials. According one-electron reduction at negative potentials in aprotic solvents
5
22–29
to the classication developed by Chang et al., electrochromic such as CH
3
CN or CH
2
Cl
2
.
Previously, it was reported that
(
EC) materials can be divided into three types. Type I consists of the anion radicals of para-positioned diesters have longer
EC materials that are soluble in both reduced and oxidised stabilities than the ortho- or meta-substituted forms due to
states, with one example being viologens. Type II refers to EC having higher electron-density within the aromatic ring,
materials which are soluble in one redox state, but form a solid thereby causing the –OR (or –SH) group to be less nucleophilic,

lm on the surface of an electrode upon electron transfer, e.g. hence less susceptible to reaction with H
bipyridilium compounds. Type III EC materials have two or oxygen.
2
O or molecular
29
0
more redox states that are solid. Usually they are studied as thin
In this project, eleven oxygen and sulfur (thioic S,S -) diesters

lms on electrode surfaces. Metal oxides, conjugated conduct- were synthesized and investigated using cyclic voltammetry,
ing polymers and Prussian blue are example of Type III EC controlled potential electrolysis and in situ electrochemical-UV-
materials.
Commercial applications of EC materials include smart
windows and antiglare car mirrors. Proposed applications
vis spectroscopy to determine which of the esters were long-
6–8
include protective eyewear, camouage materials and devices
9
for frozen-food monitoring. Some well-known, established
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical
Sciences, Nanyang Technological University, Singapore 637371, Singapore. E-mail:
webster@ntu.edu.sg
†
Electronic supplementary information (ESI) available: Compound
1
13
characterisation data ( H/ C NMR, FTIR and HRMS) cyclic voltammograms at
ꢀ1
ꢁ
varying scan rates (0.1–20 V s ) and temperatures (20 and ꢀ30 C), data from
controlled potential electrolysis experiments, in situ electrochemical-UV-vis
spectra at platinum mesh and gold micro-mesh electrodes and video les of
experiments performed during potential cycling experiments at
micro-mesh electrode. See DOI: 10.1039/c4ra02523g
a gold
Chart 1 Eleven esters synthesized.
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lived and formed brightly coloured radical anions upon one- experiments were conducted in a divided controlled potential
electron reduction (Chart 1). The goal of this work was to electrolysis cell separated with a porosity no. 5 (1.0–1.7 mm)
synthesize and study an extensive series of simple organic sintered glass frit. The glassy carbon working electrode and Pt
esters, which are reduced at relatively low negative potentials, mesh counter electrode were symmetrically arranged with
and to screen for an ester, which exhibited green colour upon respect to each other with an Ag wire reference electrode (iso-
one-electron reduction (to compliment the blue and red colours lated by a salt bridge) placed within 2 mm of the surface of the
already identied). The electrochromic (EC) properties of those glassy carbon electrode. Low temperature electrolysis experi-
long-lived reduced esters were studied in order to fabricate a ments were achieved in a Metrohm jacketed glass cell with the
multi-colour EC cell, which includes a colourless transparent temperature controlled with a Julabo FP89-HL ultralow refrig-
2
state.
erated circulator.
Experiments to electrochemically cycle the colour change
were performed using a gold micro-mesh electrode consisting
of 20 mm wide electrode strips separated by 20 mm (Buckbee-
Mears Co., St Paul, Minnesota). The gold micro-mesh was
laminated and sealed between two sheets of plastic with a 6 mm
diameter hole punched through to allow the electrode to come
in contact with the solution. A platinum wire was used to make
electrical contact to the gold micro-mesh.
2
Experimental section
2
.1 Chemicals and reagents
n-Bu NPF was synthesized and puried by a standard proce-
dure from the literature.
idinecarboxylic acid and ethanethiol (97%) were obtained from
4
6
30
Terephathalic acid, 2,5-pyr-
0
0
Alfa Aesar while 4,4 -biphenyldicarboxylic acid and 2,2 -bipyr-
idine-5,5 -dicarboxylic acid were obtained from Tokyo Chemical
0
Industries. Oxalyl chloride was purchased from Sigma Aldrich. 2.3 Spectroscopic experiments
Sulfuric acid (98%) was obtained from Quality Reagent Chem-
In situ electrochemical UV-vis spectra were obtained using an
optically semi-transparent thin layer electrochemical (OSTLE)
cell at low temperatures (0 and ꢀ30 C) controlled by a Thermo
Electron Neslab RTE 740 circulating bath and recorded on a
Perkin-Elmer Model Lambda 750 UV-vis-NIR spectrophotometer.
ical and used as received. Compounds 1,2 were prepared by
reuxing oxalyl chloride with the respective dicarboxylic acids
to form the acid chloride follow by ethanethiol to yield the
respective sulfur-diesters (Scheme 1). Compounds 3–11 were
prepared by reuxing the respective dicarboxylic acids in
sulfuric acid and in the corresponding alcohol to form the
diester. NMR spectroscopy, high-resolution mass spectrometry,
31
ꢁ
3
Results and discussion
TLC and melting points of the solids conrmed purity of the 3.1 Cyclic voltammetry
compounds.
The compounds were investigated by cyclic voltammetry in
CH CN, a solvent in which many carboxylate anion radicals
3
2
.2 Electrochemical procedures
have been shown to survive on the CV timescale over the large
ꢁ
temperature range (ꢀ45 to 82 C) with tetraalkylammonium
Cyclic voltammetry experiments were conducted with
a
22–28
hexauorophosphates as the supporting electrolytes.
computer controlled Metrohm Autolab PGSTAT302N potentio-
stat in a three-electrode cell where a 1 mm diameter planar
glassy carbon (GC) disk (Cypress Systems) working electrode
was used in conjunction with a Pt auxiliary electrode (Metrohm)
and an Ag wire miniature reference electrode (Cypress Systems)
connected to the test solution via a salt bridge containing 0.5 M
n-Bu NPF in CH CN. Solutions of the analyte with electrolyte in
CH CN were deoxygenated prior to voltammetric analysis by
purging with high purity argon gas. Accurate potentials were
obtained using ferrocene as an internal standard. Electrolysis
Compounds 10 and 11 are not soluble in CH CN at room
3
temperature and instead they were studied in dimethylforma-
mide (DMF) and CH Cl , respectively.
2 2
The cyclic voltammograms for all eleven aromatic diesters
ꢁ
(
AD) were performed at room temperature (22 ꢂ 2 C) (see Fig. 1
and 2). All 9 electrochromic esters (compounds 1 to 9) have
4
6
3
similar electrochemical properties, and are all able to be
3
reduced but not oxidized in CH CN according to the reactions
3
in eqn (1) and (2).
ꢀ
ꢀ
AD + e # AD c
(1)
(2)
ꢀ
ꢀ
2ꢀ
AD c + e # AD
The chemical reversibility of the reduction processes
depends on the nature of the individual compound. When the 9
compounds were subjected to one-electron reduction at nega-
+
tive potentials [ꢀ1.56 V to ꢀ2.15 V vs. Fc/Fc ] (Fc ¼ ferrocene),
all the compounds could be chemically reversibly oxidized back
to the starting materials, as evidenced by the appearance of a
reverse oxidation peak during CV measurements. However, not
all of the compounds were able to achieve chemical reversibility
Scheme 1 Esterification reaction routes.
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ꢀ1
Fig. 2 Cyclic voltammograms of diesters at scan rate ¼ 0.1 V s
obtained at a 1 mm diameter glassy carbon electrode in CH CN (0.2 M
Bu NPF ) for the 1-electron reduction (solid line) and 2-electron
reduction (dotted line) of ca. 2 mM analyte at ꢀ30 C.
3
ꢀ
1
4
6
Fig. 1 Cyclic voltammograms of diesters at scan rate ¼ 0.1 V s
ꢁ
3
obtained at a 1 mm diameter glassy carbon electrode in CH CN (0.2 M
Bu
4
NPF
6
) for the 1-electron reduction (solid line) and 2-electron
ꢁ
reduction (dotted line) of ca. 2 mM analyte at 20 C. Cyclic voltam-
mograms of diesters 10 and 11 were obtained in DMF and DCM,
respectively.
increased, which is reected by a more chemically reversible
second reduction process (Fig. 2).
Compound 11 displays a chemically irreversible reduction
ꢀ
1
+
(
on the CV timescale) at room temperature during the second process (at n ¼ 0.1 V s ) at ca. ꢀ2.2 V vs. Fc/Fc and multiple
+
reduction process (ꢀ2.0 to ꢀ2.4 V vs. Fc/Fc ) (see Fig. 1), where chemically irreversible processes at more negative potentials
ox red
ox
p
the i
current and i
p
/i
p
-ratio was <1 for some compounds (i
¼ anodic peak (Fig. 1), due to chemical instability of the reduced compound.
red
p
¼ cathodic peak current). It was deduced that Compound 10 exhibits a chemically reversible one-electron
ꢀ1 +
the chemically irreversibility is due to the chemical instability of reduction process (at n ¼ 0.1 V s ) at ca. ꢀ1.8 V vs. Fc/Fc , but
some of the dianions, which prevents them from being able to the second one-electron reduction is only partially chemically
ꢀ
1
undergo oxidation back to the original compound. It has been reversible (at n ¼ 0.1 V s ), probably also due to chemical
reported that for compounds where the radical anions formed instability of the dianion.
by a one-electron reduction are stable for long periods of time,
The voltammograms in Fig. 1 and 2 indicate that the
the radicals ultimately decompose via reaction with molecular benzene esters are more difficult to reduce than the analogous
24,25,28
oxygen (or H
2
O) to form carboxylate anions.
pyridine ester derivatives. The reduction potential becomes
When subjected to a second one-electron reduction, the slightly more negative as the length of the alkyl chain on the
dianions of compounds 3, 5 and 7 were observed to be relatively esters increases. The Me ester is easier to reduce than the Pr
i
short-lived as only a small oxidation peak was seen in their ester, and both are easier to reduce than the Pr ester. There was
i
respective voltammograms at room temperature when the scan no apparent difference in the CV behaviour of Me, Pr and Pr
direction was reversed (Fig. 1). CV experiments were repeated esters in terms of the stability of the anion radicals formed by
for the electrochromic compounds (1–9) at low temperature, one-electron reduction. A summary of the formal electrode
ꢁ
ꢀ
30 C in order to see if the lifetimes of the dianions could be reduction potentials are given in Table 1.
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.2 Controlled potential electrolysis
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3
It was discussed above that the radical anions of many of the
compounds were able to survive over the cyclic voltammetry
time scale (#a few seconds). Bulk controlled potential elec-
trolysis and coulometry experiments were performed in order to
determine the number of electrons transferred and to estimate
the lifetime of the reduced compounds over minutes to hours.
The applied potential for bulk reduction was set at a potential
100 mV more negative than the rst cathodic peak.
The grey solid line in Fig. 3(a) is the cyclic voltammogram
obtained at a 1 mm GC working electrode placed in the working
electrode compartment of the electrolysis cell before the elec-
trolysis of compound 5. The red dashed line is the cyclic vol-
tammogram obtained upon completion of the one-electron
reduction. Due to some decomposition of the anion radical over
the electrolysis timescale, the peak currents observed at the end
of the electrolysis were smaller than those obtained prior to the
electrolysis. During the electrolysis, the solution colour was
observed to change from colourless to bright red. Notably, the
position of zero current in the voltammograms shi between
the initial (grey solid line) and end of the electrolysis (red
dashed line), indicating that the reduced compound exists in a
charged state. The chemical instability is caused by reactions
ꢁ
Fig. 3 Voltammetric and coulometric data obtained at ꢀ30 C during
with residual water and dissolved oxygen that are present in the the controlled potential electrolysis of 2 mM compound 5 in CH CN
3
24,25,28
electrolysis cell.
4 6
Similar electrolysis experiments were with 0.2 M n-Bu NPF . (a) Cyclic voltammograms recorded at a scan
ꢀ1
rate of 0.1 V s with a 1 mm diameter planar GC electrode: (grey solid
line) before the bulk reduction of compound 5; (red dashed line) after
the exhaustive reduction of compound 5. (b) Current/coulometry vs.
Table 1 Spectroscopic and electrochemical data of the one-electron
reduced forms of the diesters
time data obtained during the exhaustive reduction of compound 5 at
+
ꢀ
2.25 V vs. Fc/Fc .
a
+
b
ꢀ1
ꢀ1
Compound
E/V vs. Fc/Fc
T/K
243
243
l/nm
3 /M cm
carried out for the rest of the diesters, with the results provided
in the ESI.†
1
2
ꢀ1.78; ꢀ2.12
ꢀ1.60; ꢀ2.04
399
18 300
10 100
27 100
15 600
22 000
10 100
5000
12 000
7000
7700
620
Solutions of compounds 1 to 9 all exhibit intense colour
changes when transitioning from their colourless neutral states
391
5
6
69
12
0
to coloured one-electron reduced states. The S,S -diesters
3
4
ꢀ2.11; ꢀ2.51
ꢀ1.88; ꢀ2.36
243
243
364
31
354
(
(
compounds 1 and 2) turned a bright blue colour, the O-diesters
compounds 3–8) turned a bright red colour and compound 9
5
turned bright green, upon one-electron reduction. Compound
10 showed a lime green colour just aer the commencement of
the one-electron electrolysis. However, the reduced compound
did not survive on the electrolysis time-scale, with the solution
turning to a brown colour approximately 20 minutes into elec-
trolysis process. Compound 11 did not form brightly coloured
compounds (not electrochromic) upon electrochemical reduc-
tion, presumably because its anion radical was too short-lived to
visually detect during the electrolysis process. The bright
colours of the reduced forms of compounds 1 to 9 were even
noticeable at the surface of the planar glassy carbon electrode
during cyclic voltammetry experiments.
4
5
82
11
5
6
ꢀ2.14; ꢀ2.55
ꢀ1.91; ꢀ2.34
243
243
365
49 900
18 300
23 300
20 700
10 800
14 270
14 700
8000
39 700
17 800
22 300
7900
5
5
08
33
355
4
5
84
13
7
8
ꢀ2.12; ꢀ2.53
ꢀ1.90; ꢀ2.35
243
243
365
33
357
5
4
5
81
11
9
ꢀ2.07; ꢀ2.44
273
440
4
6
6
7
77
08
63
32
6300
4200
11 300
17 300
3.3 In situ electrochemical-UV-vis spectroscopy
Further investigations using in situ electrochemical UV-vis
a
The formal potentials (1-electron and 2-electron respectively) were
spectroscopy showed that the one-electron reduced O-diesters
ꢁ
b
obtained at 20 C. The molar absorptivity values (3) were obtained
for the radical anions (generated during 1-electron reduction) at ꢀ30
0
(
red compounds), S,S -diesters (blue compounds) and naph-
ꢁ
ꢁ
C except for compound 9 at 0 C.
thalene O-diester (green compound) absorbed at ꢃ520 nm,
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ꢃ610 nm and 440/730 nm, respectively. Experiments using in values, but may be up to 25% higher since some decomposition
situ electrochemical UV-vis spectroscopy at low temperatures of the anion radicals occurred over the timescale of the in situ
31
performed in an OSTLE cell, (pathlength of 0.067 cm) electrochemical UV-vis experiment (approximately 30 minutes
conrmed that the reduced compounds could be oxidized back in total).
to the original compounds but with some decomposition
products. Representative UV-vis spectra of the different col- behaviour and the UV-vis spectra of each diester, one
oured species are shown in Fig. 4. compound of each colour was chosen which had the best
Aer comparing the cyclic voltammograms, electrolysis
In Fig. 4, the grey solid lines shows the spectra of the original potential properties for use in an electrochromic device.
compounds while the coloured solid line shows the spectra Compound 2 was chosen over compound 1 due to its higher
obtained during the forward one-electron reduction reaction, molar absorptivity and lower reduction potential. Compound 5
and the black dashed line shows the spectra obtained aer the was selected over the other O-diesters for its high molar
reduced compounds had been oxidized back to the starting absorptivity and the relatively long lifetime of its reduced state,
material. The similarity of the solid grey (before electrolysis) while compound 9 was the only compound which was green in
and dashed black (aer re-oxidation) lines show that the its one-electron reduced state.
compounds are able to achieve regeneration of the starting
Fig. 5 shows photographs that were obtained during poten-
tial cycling of a gold micro-mesh electrode in CH CN solutions
ꢁ
material aer one-electron reductive electrolysis (at ꢀ30 C).
3
Similar in situ electrochemical UV-vis experiments were containing either compounds 2, 5 or 9. The gold mesh electrode
performed for the other electrochromic diesters, with the was enclosed in a plastic laminating lm with a 6 mm diameter
spectra provided in the ESI.† The approximate extinction coef- circular hole through the plastic lm that allowed the mesh to

cients (molar absorptivity) for the nine electrochromic diesters be in electrical contact with the solutions. When the applied
are given in Table 1. The 3-values in Table 1 are the minimum potential was sufficiently negative to cause reduction of the
compounds, the solution directly adjacent to the mesh elec-
trode could be observed to turn blue, red or green for
compounds 2, 5 and 9, respectively.
When an oxidising potential was applied to the gold mesh
electrode (aer rst producing the radical anions), the solution
adjacent to the mesh electrode surface could be seen to change
back to an uncoloured state. However, the switch from coloured
to uncoloured occurred at a slower rate than that on the forward
cycle (uncoloured to coloured) due to some radical material
diffusing away from the electrode surface. The ESI† section
includes two video les that show the colour change in real time
for compounds 2 and 5 over 10 potential cycles.
All the compounds prepared in this study have very high
solubilities in aprotic organic solvents that are most suitable for
electrochemistry. Therefore, experiments that were performed
by attempting to spin coat or drop coat the compounds on a
transparent indium tin oxide (ITO) coated glass electrode were
not successful due to the compounds rapidly dissolving from the
surface of the ITO-coated glass. Experiments in protic solvents
(such as water) where the compounds have low solubility were
also not suitable for the potential cycling experiments because
the radical anions undergo immediate protonation reactions in
protic media resulting in rapid loss of the colour. Nevertheless,
the experiments performed at the gold micro-mesh electrode do
illustrate the rapid change to a coloured state on the forward
potential cycle as well as illustrating the high intensity of the
colours observed for relatively low concentration solutions.
The contrast ratios (CR) for the colour changes at the gold
micro-mesh electrode were estimated from the ratio of trans-
mission through the electrode in the uncoloured state (R ) to
0
the transmission in the coloured state (R ) at a xed wavelength
x
and at analyte concentrations of 1 mM in CH
3
CN containing
2,12
0
.2 M Bu NPF (eqn (3)) (see Table S1 in the ESI†).
4 6
Fig. 4 In situ electrochemical UV-vis spectra of 1 mM analytes in
ꢁ
CH
3
CN containing 0.2 M n-Bu
4
NPF
6
at ꢀ30 C. (a) 2, (b) 5, (c) 9. (—)
CR ¼ R
/R
0 x
(3)
Before electrolysis. (Coloured solid line) After one-electron reduction.
---) After re-oxidation back to starting material.
(
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are highly dependent on the analyte concentrations since the
measurements were performed in solution. Therefore, at a
concentration of 10 mM, the CR-values would be expected to
increase approximately 10-fold, to 60, 38 and 35 for compounds
2, 5 and 9, respectively. As a reference, a CR-value of 60 : 1
achieved for lms of the n-heptyl viologen cation radical is
2,32
considered to be high.
It is important to note that in this
work the coloured compounds remain completely in solution,
thus it is difficult to make direct comparisons with other studies
2,12
that use lms.
The relatively high CR-values for these
compounds (taking into account their low solution phase
concentrations) is due to the high molar extinction coefficients
of the electronic p–p* transitions observed in the reduced
states, compared to their effectively zero molar extinction
coefficients in the ground states at the appropriate wavelengths
(
p–p* bands in the neutral molecules occur outside the visible
26
0
wavelength range). Hence the R -values are determined largely
by the transmission through the gold micro-mesh electrode,
which remains constant as the concentration of the analyte
varies.
There is a report on using a combination of anthraquinone
and diphenylamine based compounds in a propylene carbonate
solution (containing 0.2
M
tetrabutylammonium tetra-

uoroborate) sandwiched between two ITO plates, where the
33
CR-values were estimated to be between 2 and 3.5. However,
the concentration of the anthraquinone and diphenylamine
derivatives were both extremely high at 0.1 M each, suggesting
that the measured CR-values of these solution phase species
were lower compared to those obtained in this work. The low
33
CR-values were possibly due to inefficiencies in the electro-
chemical cell design which meant that the oppositely charged
coloured electrochromes continuously and spontaneously
reacted with each other in the thin layer cell and converted back
33
to their uncoloured states.
The chromatic efficiency (h) was estimated from the ratio of
the change in absorbance in going from the resting to coloured
d
state (DA), to the charge injected per unit area (Q ) for 1 mM
2,12
solutions of analyte at a xed wavelength according to eqn (4).
h ¼ DA/Q
(4)
The values of charge were obtained from integration of the
current readings during linear sweep voltammetry measure-
ments (given in Fig. S42 in the ESI†). For these calculations, the
active area of the electrode needed to be calculated as it differs
from the geometric area. Because the electrode has a three-
dimension structure, the current values were much higher than
at a standard planar electrode. The electrode area was estimated
by performing cyclic voltammetry experiments on a solution of
Fig. 5 Photographs obtained of a gold micro-mesh electrode in
ꢀ1
CH
3
CN containing 0.2 M Bu
4
NPF
6
at a scan rate of 0.1 V s when the
ꢀ5
ferrocene (which has a known diffusion coefficient of 2.2 ꢄ 10
applied potential was 100 mV more negative than the formal reduction
potential of the compounds. (a) No applied potential, (b) 10 mM 2, (c)
2
ꢀ1 34
cm
s
)
and using the Randles-Sevcik equation (eqn (5)),
where i
p
is the peak current, n is the number of electrons
1
0 mM 5, and (d) 15 mM 9.
transferred, A is the electrode area, c is the concentration, D is
the diffusion coefficient and n is the scan rate.
For compounds 2, 5 and 9 the CR-values were calculated to
5
3/2
1/2 1/2
n
i ¼ 2.686 ꢄ 10 n AcD
(5)
p
be 6.0 (l ¼ 612 nm), 3.8 (l ¼ 533 nm) and 3.5 (l ¼ 732 nm),
respectively at a concentration of 1 mM. However, the CR values
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h-values for compounds 2, 5 and 9 were calculated to be approximately 0.05 cm (500 mm). This compares with a solid
2
ꢀ1
ꢀ1
2
ꢀ1
36
8
4
00 cm C (l ¼ 612 nm), 690 cm C (l ¼ 533 nm) and lm thickness of 0.12–0.15 mm used in a polymeric lm.
2
30 cm C (l ¼ 732 nm), respectively (positive values for h are Therefore, even though there is a large decrease in concentra-
2
used for reactions involving reduction). In this instance, the tion at the electrode surface in going from a solid lm to a
h-values are expected to be independent of the analyte solution phase electrochrome, the expected decrease in
concentration because both the absorbance and charge will observed colouration intensity is partly offset by the large
increase proportionally with concentration, therefore, the diffusion layer thickness of the solution phase species (leading
values reported are the maximum expected at the micro-mesh to an increase in the pathlength for absorption), which is at
3
electrode. The h-values obtained for compounds 2, 5 and 9 are least 10 times greater than the thickness of the solid lm (at
seemingly high when compared to polymeric organic based scan rates of 0.1 V s ).
electrochromes cast as lms on an ITO surface.
ꢀ1
2,12
UV-vis
The response time of the micro-mesh electrode on the
spectroscopic data obtained at the gold micro-mesh electrode forward (colouration) step was on the order of 2 s for a
are provided in the ESI as well as Table S2† which gives the concentration of 10 mM (as indicated in the video les in the
measured parameters used to calculate the h-values.
ESI†). However, for the reverse decolouration step the response
time was substantially slower due to diffusion of the electro-
2
Another solution phase study obtained a h-value of 377 cm
for the reaction of the electrochrome 4,4 -biphenyl dicar- chrome away from the electrode surface. Since the compounds
ꢀ1
0
C
boxylic acid diethyl ester (BDD), a compound of similar struc- themselves undergo fast heterogeneous electron transfer,
35
ture to those examined in this study. The electrochromic the limitation is given by the present cell design rather than the
properties of BDD were examined by incorporating it inside a electron transfer properties of the molecules. Furthermore, the
polymer gel electrolyte mixture sandwiched between semi- molecules and micro-mesh electrode display high stability to
35
transparent ITO and NiO electrodes. The h-value obtained for repeated cycles, with no loss in signal observed over at least
the BDD containing electrochromic cell is more easily 100 cycles (Fig. S43(a)–(c) in the ESI†). However, while there is
compared to the values obtained in this work rather than high reversibility in the responses over short time scales (a few
2,11
h-values obtained from solid lms, which in the case of the seconds), the lifetimes of the coloured states over long periods
0
0
polymer poly(1,4-bis(2-[3 ,4 -ethylenedioxy]thienyl)-2-methoxy- are strongly inuenced by the presence of trace contaminants.
00
5
6
,2 -ethylhexyloxybenzene has been reported to be as high as Thus in order for the coloured state to remain, the solution
2
ꢀ1 36
80 cm C
.
surrounding the micromesh must be completely free of oxygen
The micro-mesh electrode does have some signicant and trace water.
differences with respect to its electrochemical and optical
properties compared to an ITO electrode. One major difference
is that diffusion can occur to either side of the micro-mesh
electrode, which will result in high absorbance readings
4
Conclusions
The electrochemical and optical properties of some organic
diesters have been examined using cyclic voltammetry
controlled potential electrolysis and in situ electrolysis-UV-vis
spectroscopy. Importantly, a new colourless compound was
synthesised which exhibited a green colour upon one-electron
electrochemical reduction. A number of papers and mono-
graphs have previously reported on electrochromism of inor-
ganic materials, metals oxides and conjugated conducting
(
compared to an ITO surface where only one side is active).
2
ꢀ1
Therefore, the h-value of 800 cm C for compound 2 is not
disproportionately high when compared to the value of 377 cm
C
2
ꢀ1
35
for BDD since there is essentially twice the absorbance
measured for the same charge value (alternatively, it can be
considered that the h-values obtained at the micro-mesh should
be halved when compared with results from an ITO electrode).
Another reason why h-values may be expected to be relatively
high for solution phase electrochromic cells is that the solution
phase electrochromes are generally small in size and each
electron transferred (at least for one-electron processes) results
in the generation of a coloured compound. Therefore, solution
phase electrochromes that have high molar extinction coeffi-
cients and where the electrochemical generation of the coloured
state is very efficient should have relatively high chromatic
efficiencies. Nevertheless, more work is needed on other solu-
tion phase electrochromes to compare these h-values, since
2,12,13
polymers.
We are not aware of any publications to date that
have reported on 3 different small organic ester compounds
displaying RGB primary colours upon one-electron reduction
from their transparent neutral state. An investigation of elec-
trochromic lms made from these three compounds is in
progress, which can possibly be achieved by using two closely
spaced micro-mesh electrodes (one as the counter electrode and
one as the working electrode) and placing them between very
closely spaced glass or plastic slides. Additional experiments are
also in progress to try to synthesise compounds with lower
reduction potentials in order to make the reduced states less
reactive to trace water and oxygen.
31,37,38
oen only their molar extinction coefficients are reported.
A further factor that inuences the intensity of the colour
observed (visually) at the solution phase micro-mesh cell is the
thickness of the diffusion layer (d). Digital simulations using the
39–44
soware package DigiElch
for a stationary (unstirred) 1–10
Acknowledgements
mM solution of an electrochrome at a planar electrode with a
ꢀ
1
ꢀ5
scan rate of 0.1 V s and with a diffusion coefficient of 2 ꢄ 10
This work was supported by an A*Star SERC Public Sector
2
ꢀ1
cm
s , indicated that the diffusion layer thickness was Funding (PSF) Grant (112 120 2006).
18106 | RSC Adv., 2014, 4, 18100–18107
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