Novel bipyridinium ionic liquids as liquid electrochromic devices

Article abstract of DOI:10.1002/chem.201304451

Novel mono and dialkylbipyridinium (viologens) cations combined with iodide, bromide, or bis(trifluoromethanesulfonyl)imide [NTf₂] as anions were developed. Selective alkylation synthetic methodologies were optimized in order to obtain the desired salts in moderate to high yields and higher purities. All prepared mono- and dialkylbipyridinium salts were completely characterized by ¹H, ¹³C, and ¹⁹F NMR spectroscopy, Fourier-transform IR spectroscopy, and elemental analysis (in the case of NTf₂ salts). Melting points, glass transition temperatures by differential scanning calorimetry (DSC) studies, and decomposition temperatures were also checked for different prepared organic salts. Viscosities at specific temperatures and activation energies were determined by rheological studies (including viscosity dependence with temperature in heating and cooling processes). Electrochemical studies based on cyclic voltammetry (CV), differential pulsed voltammetry (DPV), and square-wave voltammetry (SWV) were performed in order to determine the redox potential as well as evaluate reversibility behavior of the novel bipyridinium salts. As proof of concept, we developed a reversible liquid electrochromic device in the form of a U-tube system, the most promising dialkylbipyridinium-NTf₂ ionic liquid being used as the electrochromic material and the room-temperature ionic liquid (RTIL), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)- imide [EMIM][NTf₂], as a stable and efficient electrolyte. Novel mono and dialkylbipyridinium cations combined with halides and bis(trifluoromethanesulfonyl)imide anions were prepared and characterized. Thermal, rheological, and electrochemical properties were studied for different bipyridinium salts. Reversible liquid electrochromic devices composed of dialkylbipyridinium-NTf₂ ionic liquids (see figure; IL=ionic liquid) have been successfully developed.

Full text of DOI:10.1002/chem.201304451

DOI: 10.1002/chem.201304451
Full Paper
&
Electrochromic Materials
Novel Bipyridinium Ionic Liquids as Liquid Electrochromic Devices
[
a]
Noꢀmi Jord¼o, Luis Cabrita, Fernando Pina, and Luꢁs C. Branco*
Abstract: Novel mono and dialkylbipyridinium (viologens)
cations combined with iodide, bromide, or bis(trifluorome-
were determined by rheological studies (including viscosity
dependence with temperature in heating and cooling pro-
cesses). Electrochemical studies based on cyclic voltammetry
(CV), differential pulsed voltammetry (DPV), and square-wave
voltammetry (SWV) were performed in order to determine
the redox potential as well as evaluate reversibility behavior
of the novel bipyridinium salts. As proof of concept, we de-
veloped a reversible liquid electrochromic device in the form
of a U-tube system, the most promising dialkylbipyridin-
ium-NTf2 ionic liquid being used as the electrochromic
material and the room-temperature ionic liquid (RTIL),
1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)-
thanesulfonyl)imide [NTf ] as anions were developed. Selec-
2
tive alkylation synthetic methodologies were optimized in
order to obtain the desired salts in moderate to high yields
and higher purities. All prepared mono- and dialkylbipyridin-
1
13
ium salts were completely characterized by H, C, and
19
F NMR spectroscopy, Fourier-transform IR spectroscopy,
and elemental analysis (in the case of NTf salts). Melting
2
points, glass transition temperatures by differential scanning
calorimetry (DSC) studies, and decomposition temperatures
were also checked for different prepared organic salts. Vis-
cosities at specific temperatures and activation energies
imide [EMIM][NTf ], as a stable and efficient electrolyte.
2
Introduction
the three common redox states of disubstituted bipyridinium
salts.
Bipyridinium (bpy) salts are prepared by mono- or diquaterni-
zation of 4,4’- and 2,2’-bipyridine scaffolds by using appropri-
ate halide alkylating agents. The dication salts are electroactive
For example, alkyl groups can promote a blue-violet color,
whereas aryl groups generally impart a variety of colors to the
radical cation. The desired final color can be tuned according
to the type of substituent at the N-position of the bipyridinium
[
1]
species with relevant electrochromic properties. In general,
the bipyridinium salts can be used as herbicides, redox indica-
tors, functional organic electro-
[6]
cation and the type of anion.
chromic materials, or coordina-
[
2–4]
tion polymers.
In general, dialkyl viologen
cation salts (1,1’-dimethyl-4,4’-bi-
pyridinium, for example) under-
Figure 1. Three common viologen redox states.
go two successive electron-
transfer processes and thus ex-
hibit three redox states: the dication, the radical cation, and
the neutral compound. Of these three redox states, it is known
that the dication is the most stable; it is also a colorless spe-
cies, unless optical charge transfer with the counter anion
In last years, the reported discoveries of novel and efficient
electrochromic materials have been based mainly on metal
[7–9]
oxides, polymers, viologens, among others.
Recently, our
group described the development of intrinsically electrochro-
mic ionic liquids based on ethylenediaminotetraacetic (EDTA)
[
1]
occurs. On the other hand, the radical cation is intensely col-
ored, exhibiting high molar absorption coefficients related
with an intense intramolecular optical charge transfer. The
colors of radical cations are dependent of the substituents
[
10]
metal complexes ([cation][Co(EDTA)])
and vanadate oxide
[11]
([cation][VO ]).
3
[12]
Ionic liquids (ILs)
are salts with a melting point below
[
5]
(
type and length) on the nitrogen atoms. Figure 1 illustrates
1008C, (some are liquids at room temperature) where at least
one of the elements of the salt is organic. This type of com-
pound possesses negligible volatility combined with other pe-
culiar properties that can be tuned by choosing the right com-
bination of cation and anion. This possibility allowed the use
[a] N. Jord¼o, Dr. L. Cabrita, Prof. Dr. F. Pina, Dr. L. C. Branco
REQUIMTE, Departamento de Quꢀmica
Faculdade de CiÞncias e Tecnologia, Universidade Nova de Lisboa
Campus da Caparica, 2829-516 Caparica (Portugal)
E-mail: l.branco@fct.unl.pt
[13]
of ionic liquids in a wide range of applications, such as or-
[14,15]
[16,17]
ganic reaction media,
extractive and separation media,
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304451.
Chem. Eur. J. 2014, 20, 3982 – 3988
3982
ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[
18,19]
functional materials,
and active pharmaceutical ingredients
[
20]
(APIs).
Herein, we present the development of different ILs based
on monoalkyl- and dialkylbipyridinium cations combined with
iodide, bromide, and bis(trifluoromethanesulfonyl)imide, [NTf2],
anions (Figure 2). The thermal, physical, and electrochemical
studies on these compounds have been performed in order to
elucidate the potential of these salts for electrochromic appli-
cations.
Scheme 1. Preparation of monoalkylbipyridinium salts.
In the case of alkylbipyridinium cations combined with
[
NTf ] anion, it was observed that longer alkyl chains (hexyl
2
and decyl) engender lower melting points and higher thermal
stability.
Synthesis and characterization of dialkylbipyridinium salts
Figure 2. Derivatives of mono and bipyridinium salts prepared herein.
It is known that different symmetric dialkylbipyridinium salts
can be prepared by dialkylation reaction by using appropriate
alkyl iodide in excess. However, in our case a simple separation
of mono- from dication species, as previously described, al-
lowed the complete isolation of the desired symmetric dialkyl-
bipyridinium salts in high purity levels.
Results and Discussion
The synthetic methodologies were optimized for the prepara-
tion of different alkyl- and dialkylbipyridinium salts. All pre-
pared mono- and dialkylbipyridinium salts were completely
Asymmetric dialkylbipyridinium salts were prepared by alky-
lation of previous prepared 1-alkyl-4,4’-bipyridinium iodides (as
shown in Scheme 1) with different alkyl iodides or bromides in
excess, as described in Scheme 2. Using this experimental ap-
proach, novel asymmetric dialkylbipyridinium salts were ob-
tained in moderate to high yields (55–90%) using acetonitrile
as solvent.
1
13
19
characterized by H, C, and F NMR spectroscopy, Fourier-
transform IR spectroscopy, and elemental analysis (in the case
of NTf2 salts). Melting points, glass transition temperatures,
and decomposition temperatures were also determined for dif-
ferent prepared organic salts.
In order to lower the melting point of these iodide salts,
[
NTf ] anion was combined with different dialkylbipyridinium
2
Synthesis and characterization of alkylbipyridinium salts
cations in methanol or aqueous media, as previously described
(see Scheme 2). [C C bpy][NTf ] , [C C bpy][NTf ] , and
Monocation bipyridinium salts were obtained by monoalkyla-
tion of the 4,4’-bipyridine unit by using an appropriate alkyl
iodide followed by a second step based on anion ex-
change reaction by using lithium bis(trifluoro-
6
6
2 2
1
10
2 2
[C C bpy][NTf ] as electrochromic ionic liquids were obtained
2
6
2 2
methanesulfonyl)imide (LiNTf ) in methanol or aque-
2
Table 1. Alkylbipyridinium salts prepared in this work.
ous media. Ethyl acetate or acetonitrile was used as
the reaction medium, at temperatures from room
temperature to 608C, for the monoalkylation synthet-
ic step. Under these reaction conditions, different al-
kylbipyridinium iodide salts were obtained in moder-
ate to high yields (62–89%). After recrystallization
from the appropriate solvent, the salts were obtained
in higher levels of purity (separation of mono from
dication species). The synthesis of monocation bipyri-
dinium salts is described in Scheme 1.
Salts
Yield
[%]
Physical
state
l
m.p. (T )
[8C]
g
Td
[8C]
[a]
[b]
[c]
[nm]
1
[C bpy][I]
89
71
62
90
64
94
orange solid
white solid
orange solid
brown solid
yellow solid
brown liquid
249
262
250
264
250
262
240
70
190
56 (ꢀ40.58)
124
240
>210
190
>235
>155
>240
[C
1
6
6
bpy][NTf
bpy][I]
bpy][NTf
2
]
]
[C
[C
[C
[
d]
2
10bpy][I]
[
e]
[f]
[C bpy][NTf ]
1
0
2
RTIL (ꢀ38.39)
ꢀ
5
[
a] Maximum wavelength (lmax) of each salt in acetonitrile solution (10 m) was deter-
mined by UV/Vis spectra at room temperature. [b] Melting point (m.p.) was measured
using a melting point apparatus; glass transition temperature (T ) measured by DSC
analysis. [c] Decomposition temperature (T ) measured by visual observation of the
color change by using a melting point apparatus. [d] Melting point was determined
The optimized anion exchange reactions for the
g
exchange of iodide for NTf allowed a significant re-
d
2
duction in the final melting point of alkylbipyridinium
using a melting point apparatus; glass transition temperature (T
DSC analysis with heating/cooling rate of 108Cmin
g
) was determined by
salts. [C bpy][NTf ], [C bpy][NTf ], and [C bpy][NTf ]
ꢀ1
ꢀ1
1
2
6
2
10
2
.
[e] Density: 1.216 g cm
were obtained in high-purity grade as water immisci-
ble ionic liquids. Table 1 summarizes the different al-
kylbipyridinium salts prepared herein.
(26.58C), measured by picnometer. [f] RTIL (room temperature ionic liquid); glass tran-
sition temperature (T
g
) was determined by DSC analysis with heating/cooling of
ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ꢀ
1
2
08Cmin .
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3983

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Rheological properties
Detailed rheological studies for ionic liquids [C bpy][NTf ] and
10
2
[
C C bpy][NTf ] were performed. These studies allowed the
1 10 2 2
evaluation of their Newtonian fluid behavior, as well as the de-
termination of both their viscosity at specific temperatures and
viscosity dependence with temperature (heating and cooling
processes). Viscosities were measured at 258C ([C bpy][NTf ])
10
2
and 608C ([C C bpy][NTf ] ) to ensure that both salts are in
1
10
2 2
Scheme 2. Preparation of dialkylbipyridinium salts.
the liquid state. From the steady-state flow measurements of
both salts analyzed under a wide range of shear conditions,
Newtonian behavior was observed.
Table 2. Dialkylbipyridinium salts prepared in this work.
Table 3 summarizes the viscosity values at different tempera-
tures as well as the activation energy obtained from the fitting
of the heating and cooling profiles.
Salts
Yield Physical
%] state
l
m.p. (T
[8C]
g
)
T
d
[8C]
[
a]
[b]
[c]
[
[nm]
[
d]
[C
[C
[C
[C
[C
[C
[C
[C
[C
[C
1
1
6
6
C
C
C
C
1
1
6
bpy][I]
bpy][NTf
bpy][I]
bpy][NTf
10bpy][I]
10bpy][NTf
2
n.d.
68
n.d.
86
n.d.
95
90
83
55
97
orange solid 247
white solid 257
>290
>290
>250
256
>250
290
2
]
]
2
2
130
256
75
290
161
225
46 (ꢀ26.45)
241
48 (ꢀ28.27)
2
red solid
yellow solid 261
red solid 247
yellow solid 262
red solid 248
brown solid 259
red solid 253
brown solid 259
248
2 1 2 2
Table 3. Rheological properties of [C10bpy][NTf ] and [C C10bpy][NTf ] .
6
2
10
10
C
C
2
Salts
Viscosity
Activation energy
2
]
2
230
225
ꢀ1 [a]
ꢀ1 [b]
[
Pa.s
]
[c]
[kJmol ]
1
1
2
2
C
10bpy][I]
10bpy][NTf
2
[c]
258C
608C
808C
E
h
E
c
[
[
e]
f]
C
C
C
2
]
2
>250
241
>250
[
d]
[e]
[f]
[i]
6
6
bpy][I][Br]
bpy][NTf
[C10bpy][NTf
2
]
0.118
n.d.
0.027
0.625
0.012
0.194
45.96
58.35
51.50
59.34
[g]
[d]
[h]
2
]
2
[C
1
C
10bpy][NTf
2
]
2
ꢀ
5
[a] Maximum wavelength (lmax) in acetonitrile solution (10 m) was deter-
[a] Viscosity values were measured by using a rheometer with control of
mined by UV/Vis at room temperature. [b] Melting point (m.p.) was mea-
sured by using a melting point apparatus; glass transition temperature
temperatureꢁ0.58C. [b] Activation energy calculated with Arrhenius
2
model, with r >0.999. [c] Viscosity values were determined by single
(
T
g
) determined by DSC analysis. [c] Decomposition temperature (T
d
) de-
point of temperature profile. [d] Newtonian fluid. [e] Heating profile
ꢀ
1
termined by visual observation of the color change by using a melting
point apparatus. [d] Not determined (n.d.); these salts were obtained by
recrystallization of monocation. [e] Melting point and glass transition
(258C to 808C) with heating rate of 0.98Cmin . [f] Cooling profile (808C
1
ꢀ
to 258C) with cooling rate of 0.98Cmin . [g] Not determined. [h] Heating
ꢀ
1
profile (608C to 808C), with heating rate of 1.08Cmin . [i] Cooling profile
ꢀ
1
temperature (T
g
ꢀ
) were determined by DSC analysis with heating/cooling
(808C to 608C), with cooling rate of 1.08Cmin .
1
rate of 58Cmin . [f] Melting point and glass transition temperature were
determined by DSC analysis with heating/cooling rate of 108Cmin
ꢀ
1
.
As expected, [C C bpy][NTf ] is significantly more viscous
1
10
2 2
than [C bpy][NTf ] at 608C and 808C ( Table 3). It is known
10
2
in high yields. Table 2 summarizes the different dialkylbipyridi-
nium salts prepared in this work.
that the incorporation of NTf tends to reduce viscosity owing
2
to the presence of fluorine atoms. However, the stronger hy-
drophobic nature of [C C bpy][NTf ] may justify its higher vis-
Asymmetric dialkylbipyridinium salts showed lower melting
points but higher levels of thermal stability than symmetric
salts, as indicated in Table 2.
1
10
2 2
cosity. Consequently, for [C C bpy][NTf ] , the activation ener-
1
10
2 2
gies for the heating and cooling processes (Eh and E ) are
c
Our approach to obtain electrochromic ionic liquids by
using a significant difference in the alkyl chain units was suc-
higher than those for [C bpy][NTf ]. Figure 3 illustrates the de-
10
2
pendence of viscosity with temperature through heating and
cooling processes. In the case of [C C bpy][NTf ] , both pro-
cessfully achieved. In particular, [C C bpy][NTf ]
and
1
10
2 2
1
10
2 2
[
C C bpy][NTf ] are examples of ionic liquids with lower melt-
cesses follow the same profile and the fluid returns to its initial
state. On the other hand, the levels of viscosity of [C bpy]-
2
6
2 2
ing points (468C and 488C) as well as characteristic glass tran-
sition temperatures (ꢀ26.458C and ꢀ28.278C). In general, all
halide salts (iodide and bromide) are water miscible, whereas
the corresponding NTf2 salts are not. The final salts were
1
0
[NTf ] during the cooling process are higher than those found
2
in the heating process.
tested for the presence of halides by using AgNO aqueous so-
3
Electrochemical properties
lution and the results were negative.
Absorbance measurements were performed for all salts, in
The electrochemical behavior of the mono- and dialkylbipyridi-
nium salts combined with iodide [I], bromide [Br] and [NTf2]
anions was studied by cyclic voltammetry (CV), differential
pulsed voltammetry (DPV) and square-wave voltammetry
(SWV). Table 4 summarizes the redox potentials of mono- and
dialkylbipyridinium salts.
ꢀ
5
acetonitrile solutions at the same concentrations (10 m), to
determine the wavelength of maximum absorbance. The ex-
change of iodide for NTf for a particular cation leads to a red
2
shift in the wavelength of maximum absorbance (a shift vary-
ing from 10 to 15 nm).
Additional experimental details are included in the Experi-
mental Section and the Supporting Information.
In general, the reduction potentials were more negative in
the case of monoalkyl cations (ꢀ1.68/ꢀ0.98 V) than for the cor-
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Table 4. Electrochemical properties of mono and dialkylbipyridinium
[
a]
salts.
Salts
Ep1 [V]
E
p2 [V]
[C
[C
[C
[C
[C
[C
[C
[C
[C
[C
6
6
bpy][I]
bpy][NTf
10bpy][I]
10bpy][NTf
ꢀ0.86
ꢀ0.98
ꢀ0.96
ꢀ0.98
ꢀ0.44
ꢀ0.44
ꢀ0.47
ꢀ0.40
ꢀ0.43
ꢀ0.44
–
–
–
–
2
]
2
]
1
1
2
2
1
1
C
C
C
C
C
C
1
1
6
6
bpy][I]
bpy][NTf
bpy][I][Br]
bpy][NTf
10bpy][I]
10bpy][NTf ]
2 2
2
ꢀ0.85
ꢀ0.85
ꢀ0.89
ꢀ0.84
ꢀ0.86
ꢀ0.86
2
]
2
2
2
]
2
[
a] Square-wave voltammetry (SWV) and cyclic voltammetry (CV) studies
were performed in order to check the redox peaks of mono and dialkylbi-
pyridinium salt in dry acetonitrile (1 mm) with 0.1m TBAP. Acquisition pa-
ꢀ
1
rameters: SWV at 20 Hz, CV at 200 mVs scan rate; working, auxiliary,
and reference electrodes of Pt, Pt wire, and SCE, respectively.
Figure 3. Profile of the dependence of viscosity with temperature: (a) Heat-
Figure 4. Square Wave Voltammetry (SWV) of 1 mm solutions of ILs based
on bipyridine salts with NTf anions, in acetonitrile containing 0.1m TBAP, at
2
a platinum working electrode vs SCE, 6 mV step potential, 60 mV pulse am-
plitude, 20 Hz.
ꢀ
1
ing and cooling profiles of [C
1
C
10bpy][NTf
10bpy][NTf from 608C to 258C (5.08Cmin ).
c) Heating and cooling profile of [C10bpy][NTf ] from 258C to 608C
2
]
2
from 608C to 808C (1.08Cmin
)
ꢀ
1
(
(
(
b) Cooling profile of [C
1
C
2 2
]
2
ꢀ
1
0.98Cmin ).
responding dialkyl dications, as expected. Dialkylbipyridinium
salts were studied in more detail in accordance with their
higher electrochemical stability as well as their potential elec-
trochromic applications.
Figure 4 shows the two well-defined redox waves
(
ꢀ0.86 V/ꢀ0.44 V vs SCE) upon reduction of the 4,4’-bipyridin-
ium moiety in the dialkylpyridinium salt.
The first reduction wave of the dialkylbipyridinium salt was
studied in detail and it was shown to be fully reversible, as
shown in Figure 5.
According to the electrochemical data, symmetrical and
asymmetrical dialkylbipyridinium cations combined with [NTf2]
anion can be used for electrochromic cell devices operating at
potentials close to the first reduction wave (ꢀ0.5 V). Our re-
sults are comparable with some voltammetry studies already
reported in the literature regarding the use of methyl viologen
Figure 5. Cyclic voltammetry (CV) of 1 mm solutions of ILs based on bipyridi-
nium salts with NTf
num working electrode vs SCE, at 200 mVs , showing the reversibility of
2
anions in acetonitrile containing 0.1m TBAP, at a plati-
ꢀ1
[
21]
dissolved in methylimidazolium ionic liquids.
first reduction process (dication!radical cation) in detail.
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Detailed electrochemical data can be found in the Support-
ing Information in the form of additional CV and SWV figures.
chromic ionic liquids. Dialkylbipyridinium salts exhibited higher
viscosities and were more electrochemically reversible in com-
parison to the corresponding monoalkyl salts. In this context,
we developed a reversible liquid electrochromic device com-
Electrochromic applications
posed of dialkylbipyridinium NTf salts as the electrochromic
2
After electrochemical studies, [C C bpy][NTf ] and [C C bpy]-
material and another conventional RTIL as a stable and effi-
cient electrolyte.
1
10
2 2
2 6
[
NTf ] were selected for application as liquid electrochromic
2 2
devices in the form of a U-tube system. These ionic liquids
were tested as novel electrochromic materials dissolved in dry
The thermal, physical, and electrochromic properties of the
salts could be tuned by varying the N-alkyl groups of the bi-
pyridinium cation and the anion.
acetonitrile (0.025m) or conventional RTIL [EMIM][NTf ] (0.01m
2
with w/w 5 and 34% of acetonitrile). In the acetonitrile system,
the dialkylbipyridinium salt is working as the electrochromic
species and the electrolyte simultaneously, whereas, in the Experimental Section
case of the RTIL [EMIM][NTf ] system, [EMIM][NTf ] is working
2
2
General remarks
as the electrolyte. Two-sides of a U-tube were filled with the
reported mixtures and platinum wires placed at each side
were connected to a commercial battery (potential>ꢁ0.5 V).
Figure 6 illustrates a reversible liquid electrochromic device
by using [C C bpy][NTf ] as electrochromic ionic liquid dis-
All solvents were used as supplied; acetonitrile was purchased
from J.T. Baker; methanol and ethyl acetate were purchased from
Carlo Erba Reagents; dichloromethane and acetone were pur-
chased from Sigma–Aldrich; 4,4’-bipyridine was purchased from
Alfa Aesar (98%); iodomethane was purchased from B.D.H labora-
tory reagent; iododecane, bromoethane, iodohexane were pur-
chased from Sigma–Aldrich (98%); lithium bis(trifluoromethanesul-
fonyl)imide was purchased from Sigma–Aldrich. RTIL [EMIM][NTf2]
was used as purchased from Solchemar. For electrochemical stud-
ies, acetonitrile (J.T.Baker) and methanol (Carlo Erba) were of gradi-
2
6
2 2
solved in conventional RTIL [EMIM][NTf ] as electrolyte. This
2
ent-HPLC grade and acetonitrile was dried over CaH and distilled
2
under Argon. Electrochemical-grade tetrabutylammonium perchlo-
rate (TBAP) was purchased from Fluka, and was dried overnight at
8
08C before use. Type I water was obtained from a Watermax pu-
rification station (Diwer Technologies). All glassware was cleaned
with a mixture of concentrated H SO /H O (1:1), thoroughly
2
4
2
2
rinsed, oven dried, and cooled in a desiccator prior to use. NMR
spectra were obtained by using a Bruker AMX 400 instrument op-
1
13
19
erating at 400.13 MHz ( H), 100.61 MHz ( C), and 376.50 MHz ( F).
Elemental analysis was carried out by using a Thermofinnigan
Flash EA 1112 Series instrument and was performed by the Labora-
tꢃrio de Anꢄlises at REQUIMTE. UV/Vis spectroscopy was carried
out by using a Varian Cary 100 Bio spectrophotometer. Melting
points were determined by using capillaries in an Electrothermal
melting point apparatus (uncorrected). DSC analysis was carried
out by using a TA Instruments Q-series TM Q200 DSC with a refri-
gerated cooling system. The sample is continuously purged with
Figure 6. A liquid electrochromic cell containing [C
2 6 2 2
C bpy][NTf ] in [EMIM]-
[
NTf ] (0.01m) (with w/w 34% CH CN). Two-sides of U-tube were submitted
2
3
to > ꢁ0.5 V potential using two platinum wires connected to a commercial
battery. Reversible switch between a transparent or pale-yellow-colored so-
lution to a blue-colored solution.
ꢀ
1
device can reversibly switch between being transparent or as
a pale yellow solution to a blue solution, a process that can be
tuned by varying the viscosity of the media and applied redox
potential (the time taken for the initial color switch can be
varied from an order of seconds to a few minutes).
50 mLmin nitrogen gas. About 5–10 mg of salt was crimped into
an aluminum standard sample pan with lid. And the melting point
and glass transition temperature were determinate in the second
heating. The density measurement was obtained by using a micro-
pycnometer. The density of [C bpy][NTf ] was measured by an in-
10
2
direct method in the presence of n-heptane to complete the
volume of the micropycnometer. IR spectra were recorded on
a Buker Tensor 27, by using a KBr pellet for the solids and NaCl
cells for the liquids. The viscosity measurements were done using
a Rheometer (RS-300, Haake, Germany). Steady-state flow measure-
ments were carried out by using a controlled-stress rheometer
fitted with a PP20 sensor. The torque amplitude was imposed by
using a logarithmic ramp of shear stress increasing in 30 min from
Conclusion
Novel mono- and dialkylbipyridinium cations combined with
iodide, bromide, and bis(trifluoromethanesulfonyl)imide [NTf2]
as anions were developed. These salts were obtained in mod-
erate to high yields and higher purities by using optimized
synthetic methodology. Longer alkyl chains (hexyl and decyl)
in the case of monoalkylbipyridinium salts and asymmetric (in-
stead of symmetric) alkyl chains in the case of dialkylbipyridin-
ium salts lead to lower final melting points as well as the ob-
servation of glass transition temperatures. Six bipyridinium
0
.01 to 1000 Pa. The temperature of [C bpy][NTf ] and [C C bpy]-
10 2 1 10
[
NTf ] were maintained at 25ꢁ0.58C and 60ꢁ0.58C, respectively,
2
2
by using a circulating water bath. Another study made was the
temperature dependence of viscosity. [C bpy][NTf ] and
[
1
0
2
ꢀ1
C C bpy][NTf ] were heated from 25 to 808C (0.918Cmin ) and
1 10 2 2
1
ꢀ
60 to 808C (1.008Cmin ), respectively, by using a constant shear
salts based on the NTf anion were obtained as novel electro-
stress of 20 Pa. All the measurements were performed in duplicate
2
Chem. Eur. J. 2014, 20, 3982 – 3988
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3986
ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
and the viscosity values were determinate based on the average of
these two measurements.
General procedure for the anion exchange of monocation
salts
Additional experimental details can be found in the Supporting In-
formation.
Monocation salts were dissolved in water (or methanol) and lith-
ium bis(trifluoromethanesulfonyl)imide, LiNTf2 (1.2 equiv) was
added. After addition, the reaction was stirred for 24 h at room
temperature. The product was recovered by filtration (precipitate)
or extraction (biphasic solution) with water/dichloromethane. In
the case of extracted product, the organic layer was dried over an-
hydrous magnesium sulfate, filtered, and concentrated to give the
desired product. In order to eliminate residual inorganic salts, all
General procedure for synthesis of monoalkylbipyridinium
salts
To a solution of 4,4’-bipyridine in acetonitrile (or ethyl acetate), was
added slowly the appropriate iodoalkane in excess (1–5 equiv). The
reaction mixture was stirred (25–808C) for 15–72 h. After this
period of time, the crude product was precipitated and washed
with diethyl ether (3ꢅ15 mL). After purification by recrystallization
from acetone (or ethanol), the desired product was isolated and
dried in vacuum.
final NTf salts were completely washed with water. The presence
2
of halides was tested by the addition of an aqueous solution of
AgNO3.
[
C bpy][NTf ]: 1-decyl-4,4’-bipyridinium bis(trifluoromethanesulfo-
10 2
nyl)imide was obtained as a brown liquid (0.582 g, 94%); Tg
1
ꢀ
38.398C; H NMR (400.13 MHz, DMSO): d=9.24 (d, J=6.5 Hz, 2H),
[
C bpy]I: 1-decyl-4,4’-bipyridinium iodide was obtained as yellow
8.88 (d, J=5.7 Hz, 2H), 8.65 (d, J=6.5 Hz, 2H), 8.05 (d, J=5.9 Hz,
2H), 4.63 (t, J=7.3 Hz, 2H), 2.03–1.90 (m, 2H), 1.38–1.15 (m, 14H),
0.85 ppm (t, J=6.5 Hz, 3H); C NMR (100.61 MHz, DMSO): d=
10
1
solid (0.877 g, 64%). M.p. 1248C; H NMR (400.13 MHz, MeOD,
58C): d=9.15 (d, J=6.6 Hz, 2H), 8.82 (d, J=6.1 Hz, 2H), 8.53 (d,
J=6.4 Hz, 2H), 8.01 (d, J=6.1 Hz, 2H), 4.70 (t, J=7.6 Hz, 2H), 2.16–
13
2
152.72, 151.45, 145.76, 141.36, 125.87, 122.38, 60.89, 31.74, 31.15,
1
9
2
.00 (m, 2H), 1.50–1.20 (m, 14H), 0.88 ppm (t, J=6.5 Hz, 3H);
C NMR (100.61 MHz, MeOD, 258C) d=155.50, 151.79, 146.54,
43.66, 127.20, 123.61, 62.80, 33.03, 32.49, 30.59, 30.50, 30.39,
0.14, 27.21, 23.71, 14.43 ppm; FTIR (KBr): n˜ =3038, 2920, 2853,
29.33, 29.25, 29.11, 28.86, 25.89, 22.55, 14.41 ppm; F NMR
1
3
(376.50 MHz, MeOD): d=ꢀ80.64 ppm; FTIR (NaCl): n=3072, 2928,
˜
1
3
1
2856, 1643, 1607, 1549, 1524, 1464, 1348, 1188, 1055, 816, 789,
ꢀ1
741 cm ; elemental analysis calcd (%) for C H F N O S : C 45.75,
22
29
6
3
4 2
ꢀ1
641, 1543, 1462, 1412, 1217, 1178, 812, 727 cm
.
H 5.07, N 7.27; found: C 45.52, H 4.50, N 6.99.
General procedure for the anion exchange of dication salts
General procedure for synthesis of symmetric
dialkylbipyridinium salts
Dication salts were dissolved in water (or methanol) and lithium
bis(trifluoromethanesulfonyl)imide (2.3 equiv) was added. After ad-
dition, the reaction was stirred for 24 h at room temperature. The
product was recovered by filtration (precipitate) or extraction (bi-
phasic solution) with water/dichloromethane. In the case of ex-
tracted product, the organic layer was dried over anhydrous mag-
nesium sulfate, filtered, and concentrated to give the desired prod-
Symmetric dialkylbipyridinium salts were recovered from the crude
mixture by recrystallization of the monocation salt from acetone or
ethanol, the desired product was isolated and dried in vacuum.
[
C C bpy]I : 1,1’-didecyl-4,4’-bipyridinium diiodide was obtained
10 10 2
1
as red solid. M.p. 2908C (dec.); H NMR (400.13 MHz, MeOD, 258C):
d=9.29 (d, J=5.7 Hz, 4H), 8.69 (d, J=5.2 Hz, 4H), 4.75 (t, J=
uct. In order to eliminate residual inorganic salts, all final NTf salts
2
were completely washed with water. The presence of halides was
tested by the addition of an aqueous solution of AgNO3.
7
6
1
2
1
.4 Hz, 4H), 2.15–2.03 (m, 4H), 1.51–1.22 (m, 28H), 0.88 ppm (t, J=
1
3
.8 Hz, 3H); C NMR (100.61 MHz, MeOD, 258C): d=151.23, 147.27,
28.29, 128.29, 63.33, 33.03, 32.57, 30.61, 30.52, 30.41, 30.15, 27.24,
3.72, 14.44 ppm; FTIR (KBr): n˜ =3011, 2920, 2853, 1634, 1555,
[
C C bpy][NTf ] :
1’-methyl-1-decyl-4,4’-bipyridinium
di-
1
10
2 2
[
bis(trifluoromethanesulfonyl)imide] was obtained as brown solid
1
ꢀ
1
(0.195 g, 83%); M.p. 468C; T ꢀ26.458C; H NMR (400 MHz, DMSO,
g
454, 1371, 1230, 1175, 831, 723 cm
.
2
58C): d=9.36 (d, J=6.8 Hz, 2H), 9.27 (d, J=6.7 Hz, 2H), 8.75 (dd,
J=6.8 Hz, J=6.9 Hz, 4H), 4.66 (t, J=7.4 Hz, 2H), 4.43 (s,3H), 2.05–
1
.90 (m, 2H), 1.38–1.15 (m, 14H), 0.84 ppm (t, J=6.7 Hz, 3H);
C NMR (100.61 MHz, MeOD, 258C) d=148.59, 148.23, 146.63,
45.75, 126.54, 126.08, 121.06, 117.86, 60.94, 48.01, 31.26, 30.76,
13
General procedure for synthesis of asymmetric
dialkylbipyridinium salts
1
2
1
9
8.87, 28.79, 28.64, 28.39, 25.43, 22.08, 13.90 ppm; F NMR
The prepared monocation salts were dissolved in dried acetonitrile,
and the appropriate haloalkane (iodide or bromide) was slowly
added to the mixture. The reaction mixture was vigorously stirring
at 25–608C for a period of 44–144 h under an argon atmosphere.
Upon completion of the reaction, the product was precipitated
from acetonitrile media, filtered, washed with diethyl ether, and
dried in vacuum.
(
3
8
376.50 MHz, DMSO, 258C): d=ꢀ78.75 ppm; FTIR (NaCl) n˜ =3128,
070, 2930, 2860, 1641, 1564, 1508, 1456, 1350, 1192, 1140, 1057,
ꢀ1
31, 793, 735 cm
;
elemental analysis calcd (%) for
C H F N O S : C 34.40, H 3.70, N 6.42; found: C 35.20, H 3.26, N
6
25
32 12
4
8 4
.18.
Electrochemical studies
[
C C bpy]I : 1’-methyl-1-decyl-4,4’- bipyridinium diiodide was ob-
1 10 2
1
tained as red solid (1.030 g, 90%). M.p. 2258C (dec.); H NMR
400.13 MHz, DMSO, 258C): d=9.40 (d, J=6.6 Hz, 2H), 9.30 (d, J=
General remarks: Cyclic voltammetry (CV), differential pulse vol-
tammetry (DPV), and squared-wave voltammetry (SWV) measure-
ments were performed on an Autolab PGSTAT 12 potentiostat/gal-
vanostat, controlled with GPES software version 4.9 (Eco-Chemie),
by using a cylindrical three-electrode cell of 10 mL. A platinum
electrode (MF-2013, f=1.6 mm, BAS inc.) was used as the working
electrode and a Pt wire as the auxiliary electrode. All potentials
refer to a SCE (3m KCl) reference electrode (Metrohm). Prior to use,
(
6
.6 Hz, 2H), 8.79 (dd, J=6.7 Hz, 4H), 4.69 (t, J=7.3 Hz, 2H), 4.44 (s,
H), 2.03–1.89 (m, 2H), 1.39–1.13 (m, 14H), 0.85–0.82 ppm (m, 3H);
C NMR (100.61 MHz, DMSO, 258C): d=148.43, 148.08, 146.56,
45.70, 126.52, 126.06, 60.81, 48.05, 31.23, 30.71, 28.84, 28.77,
8.61, 28.37, 25.38, 22.05, 13.94 ppm; FTIR (KBr): n˜ =3015, 2922,
3
1
3
1
2
2
ꢀ
1
853, 1637, 1555, 1456, 1364, 1273, 1178, 829, 719 cm .
Chem. Eur. J. 2014, 20, 3982 – 3988
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ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
the working electrode was polished in aqueous suspensions of 1.0
and 0.3 mm alumina (Beuhler) over 2–7/8“ micro-cloth (Beuhler)
polishing pads, then rinsed with water and methanol. Subsequent-
ly, the electrode was sonicated in acetonitrile for 1 min. This clean-
ing procedure was always applied before any electrochemical mea-
surement. Electrochemical cleaning was also employed by submit-
ting the electrode to ꢀ1/+1 V currents for 60 s.
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PTDC/CTM-NAN/120658/2010, PTDC/CTM/103664/2008, and
PEst-C/EQB/LA0006/2013) and Solchemar. The NMR spectrome-
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Published online on February 27, 2014
Chem. Eur. J. 2014, 20, 3982 – 3988
www.chemeurj.org
3988
ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim