Enhancing the solubility of 1,4-diaminoanthraquinones in electrolytes for organic redox flow batteries through molecular modification

Article abstract of DOI:10.1039/d0ra06851a

1,4-Diaminoanthraquinones (DAAQs) are a promising class of redox-active molecules for use in nonaqueous redox flow batteries (RFBs) because they can have up to five electrochemically accessible and reversible oxidation states. However, most of the commercially available DAAQs have a low solubility in the polar organic solvents that are typically used in RFBs, in particular when supporting electrolyte salts are present. This significantly limits the energy densities that can be achieved. We have functionalized the amino groups in the DAAQ structure with three types of chains, namely alkyl chains, cationic alkyl chains and oligoethylene glycol ether chains, and measured the solubility of these derivatives in various organic solvents by quantitative UV-Vis absorption spectroscopy. The DAAQ derivatives with higher polarity exhibit a significantly higher solubility in commonly used organic electrolytes in comparison to apolar derivatives. Cyclic voltammetry was used to assess the viability of the DAAQs as redox-active species for RFBs. Although the cationic DAAQ derivatives have an enhanced solubility in the electrolytes, the cathodic redox reactions have a poor reversibility, most likely due to an internal decomposition reaction of their reduced forms. The oligoethylene-glycol-ether-functionalized DAAQs are the most promising compounds for use in organic RFB electrolytes because they have the optimal combination of high solubility and a high reversibility of the redox couples. This journal is

Full text of DOI:10.1039/d0ra06851a

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Enhancing the solubility of 1,4-
diaminoanthraquinones in electrolytes for organic
redox flow batteries through molecular
modification†
Cite this: RSC Adv., 2020, 10, 39601
a
a
a
b
Pieter Geysens,
Jorik Evers, Wim Dehaen,
Jan Fransaer
a
and Koen Binnemans
*
1,4-Diaminoanthraquinones (DAAQs) are a promising class of redox-active molecules for use in
nonaqueous redox flow batteries (RFBs) because they can have up to five electrochemically accessible
and reversible oxidation states. However, most of the commercially available DAAQs have a low solubility
in the polar organic solvents that are typically used in RFBs, in particular when supporting electrolyte
salts are present. This significantly limits the energy densities that can be achieved. We have
functionalized the amino groups in the DAAQ structure with three types of chains, namely alkyl chains,
cationic alkyl chains and oligoethylene glycol ether chains, and measured the solubility of these
derivatives in various organic solvents by quantitative UV-Vis absorption spectroscopy. The DAAQ
derivatives with higher polarity exhibit a significantly higher solubility in commonly used organic
electrolytes in comparison to apolar derivatives. Cyclic voltammetry was used to assess the viability of
the DAAQs as redox-active species for RFBs. Although the cationic DAAQ derivatives have an enhanced
solubility in the electrolytes, the cathodic redox reactions have a poor reversibility, most likely due to an
internal decomposition reaction of their reduced forms. The oligoethylene-glycol-ether-functionalized
DAAQs are the most promising compounds for use in organic RFB electrolytes because they have the
optimal combination of high solubility and a high reversibility of the redox couples.
Received 8th August 2020
Accepted 20th October 2020
DOI: 10.1039/d0ra06851a
rsc.li/rsc-advances
a high stability against power uctuations, a deep discharge
Introduction
6–8
ability and fast response times.
Many aqueous RFB chemistries such as all-vanadium,
9,10
Redox ow batteries (RFBs) are regarded as a promising elec-
11
12
13
trochemical energy storage (EES) technology that can facilitate iron–vanadium, iron-chrome, zinc–bromine etc. have been
the integration of intermittent renewable energy sources, such widely studied in the literature and new developments are still
1–3
as solar and wind, in the electrical grid. Because the energy is being reported frequently. However, the achievable cell voltage,
stored in redox couples that are dissolved in the electrolyte, as and therefore the energy density of aqueous RFB systems is
opposed to storage in solid electrodes (e.g. lithium-ion, lead- limited by the narrow electrochemical stability window of water.
acid), the power and capacity of RFBs scale independently This has caused a shi in RFB research towards non-aqueous
from each other. This makes RFBs highly versatile and conve- electrolytes such as ionic liquids, deep eutectic solvents, and
niently scalable EES systems, capable of storing power and molecular organic solvents that typically have a wider electro-
4
–6
14–18
energy on a multi-MW and multi-MW h scale, respectively.
chemical window, thus enabling higher cell voltages.
Addi-
Moreover, they have a good tolerance against over(dis)charging, tionally, using non-aqueous solvents opens the door to a wide
range of organic redox-active compounds that are insoluble in
aqueous electrolytes. An all-organic redox ow battery (AORFB)
KU Leuven, Department of Chemistry, Celestijnenlaan 200F, P.O. box 2404, B-3001 based on organic redox couples with a high cell potential and
a
Leuven, Belgium. E-mail: Koen.Binnemans@kuleuven.be
b
a high solubility in organic electrolytes can reach much higher
energy densities than any aqueous RFB. From an economical
KU Leuven, Department of Materials Engineering, Kasteelpark 44, P.O. box 2450, B-
3
001 Leuven, Belgium
and environmental point-of-view, organic redox-active species
also have an advantage over transition metal-based species such
†
Electronic supplementary information (ESI) available: Tables with detailed
solubility data of DAAQs. TLC experiment comparing the DAAQs. Optical
9,10,19
20,21
22
as vanadium,
chromium
and cobalt, that are oen
absorption spectra of DAAQs in different solvent systems. Comparative cyclic
ꢀ1
ꢀ1
voltammograms of 0.01 mol L DB-134 in 0.1 mol L TEATf
with and without IR drop correction. Blank cyclic voltammogram of 0.1 mol L
TEATf N in acetonitrile. See DOI: 10.1039/d0ra06851a
2
N in acetonitrile scarce and/or toxic.
ꢀ1
2
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An ideal organic redox-active species for RFBs should have the compound. The DAAQ structure was functionalized with
following properties: (1) it should either be commercially available various chains on the amino positions (Fig. 1). The inuence of
at low cost or require few steps to synthesize from inexpensive solvent polarity and supporting electrolyte concentration on the
precursors; (2) it should have a high solubility in polar organic solubility of these derivatives was studied by quantitative UV-Vis
solvents, also when high concentrations of supporting electrolyte absorption spectroscopy. The electrochemical behavior of the
salts are present; (3) the electron-transfer reactions between its DAAQ derivatives was characterized by cyclic voltammetry to
different oxidation states should be highly reversible and have fast evaluate their suitability for SRFB applications.
kinetics; (4) it should be chemically stable over time in all its
oxidation states to prevent capacity losses through side reactions;
Experimental
(5) it should have at least two electrochemically accessible redox
reactions at well separated potentials, as this would allow to Chemicals
construct a symmetric redox ow battery (SRFB), similarly to the
1,4-Dihydroxyanthraquinone (quinizarin, 96%), 2-ethylhexyl-
aqueous all-vanadium redox ow battery (VRB). Because they are
based on one single active species, SRFBs do not suffer from
electrolyte contamination through crossover which simplies the
choice of a separator. Many commercially available organic
compounds and their derivatives have shown promising reversible
electrochemical behavior for use as redox-active species, most
amine (99%), 2(2-ethoxyethoxy)ethanol (diethylene glycol mono-
ethyl ether, Et-DEG-OH, $98%), heptane (for HPLC, 99%),
methanesulfonyl chloride (MsCl, 99.5%), 2-butanone (methyl
ethyl ketone, MEK, $99%, extra pure), acetonitrile (MeCN, 99.9%,
extra dry, over molecular sieves) and dichloromethane (DCM,
99.8%, extra dry, over molecular sieves) were purchased from
23–25
notably 2,2,6,6-tetramethylpiperidinooxyl (TEMPO)
and
However, these compounds exhibit only one
reversible redox reaction, so they can only be used in one half-cell.
For symmetric RFB cell setup, 2-phenyl-4,4,5,5-
Acros Organics (Geel, Belgium). Sodium dithionite (Na S O ,
2 2 4
26,27
phenothiazines.
puried, $86%) and ethyl acetate (EtOAc, $99.7%, for HPLC)
were purchased from Honeywell Riedel-deHa ¨e n (Seelze, Ger-
many). Ammonia (NH ) 25 wt% aqueous solution (analytical
3
reagent grade) was purchased from VWR (Haasrode, Belgium).
Toluene (analytical reagent grade), sodium hydroxide (NaOH,
analytical reagent grade) and methanol (MeOH, $99.9%,
synthesis grade) were purchased from Fisher Scientic (Asse,
Belgium). Triethylamine (TEA, $99.5%, for synthesis) was
purchased from Carl Roth (Karlsruhe, Germany). Potassium
a
tetramethylimidazoline-1-oxyl-3-oxide (PTIO) is more interesting
because it is highly soluble in organic solvents, and has two
28
reversible redox reactions enabling a cell voltage of 1.7 V.
Unfortunately, the high cost and fast capacity fading during
cycling limits the usefulness of this compound.
Recently, 1,4-diaminoanthraquinones (DAAQs) have been
proposed as an exceptionally promising compound class that can
carbonate (K
2
CO
Cl, analytical reagent grade) were purchased from
(Zedelgem, Belgium). Lithium bis(tri-
3
, analytical reagent grade) and ammonium
29
be used as active species in a SRFB electrolyte. Anthraquinones
AQs) in general exhibit two subsequent cathodic one-electron
transitions in non-aqueous conditions, corresponding to the
chloride (NH
Chem-Lab
4
(

uoromethylsulfonyl)imide (LiTf N, 99%) was purchased from
2
30,31
well-known reduction of the quinone functional unit.
The
Iolitec (Heilbronn, Germany). Silver nitrate (AgNO , $99%) was
3
amino functional groups on the 1- and 4-positions of DAAQs are
sensitive to oxidation, resulting in two additional subsequent
anodic one-electron reactions that correspond to the formation
32
of a divalent diimine cation. The aromatic base structure serves
to stabilize the reduced and oxidized DAAQ species via electron
delocalization. Potash et al. studied the electrochemical behavior
of several commercially available DAAQs and observed that the
dye Disperse Blue 134 (DB-134, 1,4-bis(isopropylamino)anthra-
29
quinone) had the highest reversibility for all four redox couples.
A SRFB using DB-134 as redox-active component can store two
electrons per molecule of active species in both the anolyte and
catholyte, resulting in an impressive theoretical molar energy
ꢀ1
density of 120 W h mol at a cell voltage of 2.7 V. This is much
higher than the theoretical molar energy density of VRB electro-
ꢀ
1
lytes (34 W h mol ). However, the achievable energy density is
limited by the poor solubility of DB-134 in organic solvents
ꢀ1
ꢀ1
(
approx. 20 mmol L in acetonitrile and approx. 200 mmol L
29
in toluene).
We have recently reported a highly soluble DAAQ derivative, Fig. 1 Chemical structures of (a) 1,4-diaminoanthraquinone (DAAQ)
and the side chains (R) on the amino positions: (b) 2-ethylhexyl (Et-He),
1
9
,4-bis((2-(2-(2-methoxyethoxy)ethoxy)ethyl)amino)anthracene-
,10-dione (Me-TEG-DAAQ) that is miscible in any ratio with
(
c) N,N,N-trimethylethanaminium (Me
pan-1-aminium (Me N-Pr), (e) 2-(2-ethoxyethoxy)ethyl (Et-DEG), and
f) 2-(2-(2-methoxyethoxy)ethoxy)ethyl (Me-TEG). The dashed bonds
represent the connection point of the side chains on the amino
report the general strategy that was used to discover this groups.
3
N-Et), (d) N,N,N-trimethylpro-
3
many polar organic solvents, while also retaining the reversible
electrochemical behavior of DAAQs. In the present paper, we
(
33
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purchased from Alfa Aesar (Karlsruhe, Germany) and was dried potential (OCP). For the CV measurements, IR drop compen-
for 12 hours on a Schlenk line (0.6–1.6 mbar) at room temperature sation was applied by measuring the impedance of the setup by
prior to use. The synthesized DAAQ derivatives and tetraethy- electrochemical impedance spectroscopy (EIS) and manually
lammonium bis(triuoromethylsulfonyl)imide (TEATf N) were correcting the potential. EIS spectra were recorded in threefold
2
ꢁ
dried on a Schlenk line (0.6–1.6 mbar) at 70 C for 12 hours prior at OCP in a frequency range of 1 MHz–1000 MHz, using an
to use and were stored in an argon-lled glovebox with water and Autolab PGSTAT302N potentiostat and Nova 2.1 soware.
oxygen concentrations below 1 ppm. All chemicals were used as
received, unless stated otherwise.
Synthesis procedures
1,4-Bis[(2-ethylhexyl)amino]-9,10-anthracenedione (Et-He-
Characterization
DAAQ). 2-Ethylhexylamine (31.00 g, 9.6 equiv., 240 mmol) was
added to a stirring suspension of 1,4-dihydroxyanthraquinone
Melting points were measured on a Mettler-Toledo DSC-1
differential scanning calorimeter at
1
(
a
heating rate of
0 C min under a helium atmosphere. Samples were weighed
3 to 12 mg) inside aluminum crucibles and subsequently sealed
(
6.01 g, 1 equiv., 25.0 mmol) in water (35 mL) and the mixture
ꢁ
ꢀ1
ꢁ
was stirred at 100 C under N atmosphere for 20 h. The reac-
2
tion mixture was extracted 4 times with 50 mL of dichloro-
methane and the combined organic phases were washed 2
with an aluminum cap. The samples were cycled twice (heating

rst) between ꢀ70 ꢁC and an appropriate upper temperature
ꢀ1
times with 200 mL of a 1 mol L aqueous HCl solution and 5
times with 200 mL of water. The organic phase was dried over
limit to achieve complete melting. The melting point value
determined in the second cycle is reported. Infrared spectra were
recorded on a Bruker Vertex 70 FT-IR spectrometer in attenuated
total reectance (ATR) mode, equipped with the Bruker platinum
ATR module and a diamond ATR crystal. Each measurement
consisted of 32 scans at a resolution of 4 cm . FTIR spectra were
analyzed with OPUS 7.0 soware. NMR spectra were recorded on
either a Bruker 400 MHz Avance III HD spectrometer, operating
4
MgSO and concentrated on a rotary evaporator to give a dark
blue solid. This crude product was puried by column chro-
matography on silica gel with eluent 9 : 1 heptane/ethyl acetate.
The dark blue band (R ¼ 0.60) was isolated to give to give 1,4-bis
ꢀ1
f
[
2
(2-ethylhexyl)amino]-9,10-anthracenedione (2.37 g, 5.12 mmol,
1
0% yield) as a dark blue solid. H NMR (400 MHz, CD
2 2
Cl , d/
ppm): 0.93–0.97 (m, 12H), 1.36 (m, 8H), 1.47 (m, 4H), 1.53 (m,
1
13
at 400 MHz for H NMR and 100 MHz for C NMR or a Bruker
4
8
1
1
H), 1.72 (m, 2H), 3.35 (m, 4H), 7.30 (s, 2H), 7.69–7.70 (m, 2H)
1
300 MHz Avance spectrometer, operating at 300 MHz for H NMR
1
3
.33 (m, 2H), 10.97 (s, 2H). C NMR (400 MHz, CDCl
0.97, 14.11, 23.02, 24.52, 29.03, 31.27, 39.47, 45.98, 109.66,
23.62, 126.02, 131.82, 134.56, 146.52, 182.09. IR (ATR, nmax
3
, d/ppm):
1
3
and 75 MHz for C NMR. Tetramethylsilane (TMS) was used as
1
13
internal reference (0.00 ppm for H and C). NMR spectra were
analyzed with TopSpin 4.0.2 soware. UV-Vis absorption spectra
were recorded on a Varian Cary 6000i UV-Vis-NIR spectropho-
tometer at a scan rate of 10 nm s with 1 nm intervals. The
instrument was used in double-beam mode. Samples were
diluted to fall within the absorbance range from 0.1 to 1.0 and
were measured in quartz glass cuvettes with a path length of
/
ꢀ
1
cm ): 3080, 2958, 2922, 2871, 2859, 1645, 1605, 1578, 1554,
1
1
4
520, 1457, 1416, 1377, 1349, 1281, 1255 (n C–N), 1163, 1141,
076, 1041, 1010, 967, 920, 891, 814, 799, 768, 723, 668, 562, 518,
ꢀ
1
97, 458, 432. CHN found (calculated) for C30
H
42
N
2
O
ꢁ
2
: C 77.97
(77.88) %, H 8.63 (9.15) %, N 5.89 (6.05) %. Mp: 84 C.
1,4-Bis((2-(dimethylamino)ethyl)amino)anthracene-9,10-
1
2
cm. Blanks consisted of the supporting electrolyte (TEATf N)
dione. N,N-Dimethylethane-1,2-diamine (19.86 g, 9 equiv., 225
mmol) was added to a stirring suspension of 1,4-dihydroxyan-
dissolved in the same solvent system and at the same concen-
tration as the samples, and were measured in a matched pair of
cuvettes. The saturated samples were diluted in threefold and the
determined concentrations are given as an average of three
separate measurements.
thraquinone (6.01 g, 1 equiv., 25.020 mmol) in water (35 mL) and
ꢁ
the mixture was stirred at 100 C under N atmosphere for 24 h. A
2
dark blue precipitate was formed which was collected by vacuum

ltration and washed with water (200 mL). This crude product was
puried by recrystallization from acetonitrile to give 1,4-bis((2-
dimethylamino)ethyl)amino)anthracene-9,10-dione (6.10 g,
Electrochemistry
(
1
All the electrochemical measurements were performed inside 16.032 mmol, 64% yield) as a dark blue solid. H NMR (300 MHz,
an argon-lled glovebox with water and oxygen concentrations CDCl
, d/ppm): 2.35 (s, 12H), 2.67 (t, J ¼ 6.26 Hz, 4H), 3.48–3.53 (m,
below 1 ppm. The electrolyte solutions were purged for 10 min 4H), 7.24 (s, 2H), 7.67–7.68 (m, 2H), 8.35–8.36 (m, 2H), 10.74 (s,
3
13
with argon gas prior to the measurements in order to remove 2H). C NMR (300 MHz, CDCl , d/ppm): 41.11, 45.69, 58.61,
3
traces of dissolved oxygen. Cyclic voltammograms (CVs) were 110.13, 123.41, 126.11, 131.96, 134.47, 145.83, 182.60. IR (ATR,
ꢀ1
measured using an Autolab PGSTAT302N potentiostat and Nova
nmax/cm ): 3180, 3069, 2970, 2939, 2857, 2818, 2759, 1614, 1582,
2.1 soware. The working electrode for CV was a polished 1553, 1519, 1455, 1392, 1368, 1341, 1309, 1262, 1239, 1186, 1165,
platinum disk electrode (B ¼ 0.5 mm), the counter electrode 1152, 1125, 1099, 1062, 1045, 1021, 961, 904, 887, 849, 817, 794,
ꢁ
was a piece of platinum-coated silicon wafer (approx. surface 723, 667, 617, 566, 501, 463, 430, 416. Mp: 173 C.
2
ꢀ1
0
area ¼ 0.0025 dm ) and the scan rate was 50 mV s , unless
stated otherwise. The reference electrode consisted of a silver diyl))bis(N,N,N-trimethylethanaminium)iodide. Iodomethane
2,2 -((9,10-Dioxo-9,10-dihydroanthracene-1,4-diyl)bis(azane-
ꢀ
1
wire, submerged in a solution of silver nitrate (0.01 mol L
)
(4.56 g, 12 equiv., 32.126 mmol) was added to a stirring solution of
N (0.1 mol L ) in anhydrous acetonitrile, contained 1,4-bis((2-(dimethylamino)ethyl)amino)anthracene-9,10-dione
inside a fritted glass tube. The CVs were started at open circuit (1.00 g, 1 equiv., 2.628 mmol) in diethyl ether (15 mL) and the
ꢀ
1
and TEATf
2
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mixture was stirred under reux for 24 h. A dark purple/blue anthracenedione (3.54 g, 1 equiv., 8.667 mmol) in diethyl
precipitate was formed which was collected by vacuum ltration ether (53 mL) and the mixture was stirred under reux for 24 h.
and washed with cold diethyl ether. The solid was dried overnight at A dark purple/blue precipitate was formed which was collected
0
room temperature on a Schlenk line to give 2,2 -((9,10-dioxo-9,10- by vacuum ltration and washed with cold diethyl ether. The
dihydroanthracene-1,4-diyl)bis(azanediyl))bis(N,N,N-trimethylethan
solid was dried overnight at room temperature on a Schlenk line
0
aminium)iodide (1.463 g, 2.202 mmol, 84% yield) as a dark purple to give 3,3 -((9,10-dioxo-9,10-dihydroanthracene-1,4-diyl)bis(a-
1
solid. H NMR (300 MHz, DMSO-d
6
, d/ppm): 3.21 (s, 18H), 3.66 (t, J zanediyl))bis(N,N,N-trimethylpropan-1-aminium)iodide (4.74 g,
¼
5.8 Hz, 4H), 4.00–4.01 (m, 4H), 7.59 (s, 2H), 7.84–7.87 (m, 2H), 6.846 mmol, 79% yield) as a dark purple solid.
ꢀ
1
0
3,3 -((9,10-Dioxo-9,10-dihydroanthracene-1,4-diyl)bis(azane-
8
.24–8.26 (m, 2H), 10.64 (t, J ¼ 5.42 Hz, 2H). IR (ATR, nmax/cm ):
434, 3006, 2952, 1635, 1609, 1595, 1579, 1564, 1516, 1476, 1414, diyl))bis(N,N,N-trimethylpropan-1-aminium)
392, 1365, 1296, 1259, 1211, 1174, 1100, 1064, 1047, 1020, 1015, bis(triuoromethylsulfonyl)imide (Me N-Pr-DAAQ). Lithium
3
1
3
9
68, 952, 918, 866, 799, 727, 658, 625, 560, 545, 502, 471, 461, 451, bis(triuoromethylsulfonyl)imide (4.02 g, 2 equiv., 14.003 mmol)
0
431, 425, 417.
was added to a stirring solution of 3,3 -((9,10-dioxo-9,10-dihy-
droanthracene-1,4-diyl)bis(azanediyl))bis(N,N,N-trimethylpropan-
1-aminium)iodide (4.74 g, 1 equiv., 6.846 mmol) in water (350 mL)
Lithium and the mixture was stirred at room temperature for 2 h. A dark
0
2
,2 -((9,10-Dioxo-9,10-dihydroanthracene-1,4-diyl)bis(azane-
diyl))bis(N,N,N-trimethylethanaminium)bis(tri-
uoromethylsulfonyl)imide (Me N-Et-DAAQ).

3
bis(triuoromethylsulfonyl)imide (0.82 g, 1.9 equiv., 2.856 mmol) blue precipitate was formed which was collected by vacuum
0
was added to a stirring solution of 2,2 -((9,10-dioxo-9,10-dihy- ltration and washed with cold water. The solid was dried over-
ꢁ
0
droanthracene-1,4-diyl)bis(azanediyl))bis(N,N,N-trimethylethanami- night on a Schlenk line at 80 C to give 3,3 -((9,10-dioxo-9,10-
nium)iodide (1.00 g, 1 equiv., 1.505 mmol) in water (200 mL) and dihydroanthracene-1,4-diyl)bis(azanediyl))bis(N,N,N-
the mixture was stirred at room temperature for 2 h. A dark blue trimethylpropan-1-aminium)bis(triuoromethylsulfonyl)imide
1
precipitate was formed which was collected by vacuum ltration (6.19 g, 6.197 mmol, 91% yield). H NMR (400 MHz, DMSO-d
6
, d/
and washed with cold water. This crude product was puried by ppm): 2.09–2.17 (m, 4H), 3.10 (s, 18H), 3.41–3.46 (m, 4H), 3.51–
recrystallization from water/ethanol 7 : 3 and the crystals were dried 3.58 (m, 4H), 7.54 (s, 2H), 7.82–7.85 (m, 2H), 8.25–8.28 (m, 2H),
ꢁ
0
13
overnight on a Schlenk line at 80 C to give 2,2 -((9,10-dioxo-9,10- 10.79 (t, J ¼ 5.5 Hz, 2H). C NMR (400 MHz, DMSO-d
dihydroanthracene-1,4-diyl)bis(azanediyl))bis(N,N,N-trimethylethan 23.07, 52.32, 52.36, 52.40, 63.35, 108.89, 114.69, 117.89, 121.09,
aminium)bis(triuoromethylsulfonyl)imide (1.06 g, 1.092 mmol, 124.29, 124.41, 125,77, 132.62, 133.77, 145.66, 181.21. IR (ATR,
6
, d/ppm):
1
ꢀ1
7
7% yield). H NMR (400 MHz, DMSO-d , d/ppm): 3.32 (s, 18H), 3.64
n
max/cm ): 3044, 2880, 1640, 1598, 1575, 1523, 1482, 1421, 1327,
6
(
t, J ¼ 6.5 Hz, 4H), 3.99 (q, J ¼ 6.2 Hz, 4H), 7.57 (s, 2H), 7.83–7.87 (m, 1257, 1227, 1177, 1132, 1076, 1057, 1050, 997, 965, 954, 944, 910,
13
2
H), 8.23–8.26 (m, 2H), 10.64 (t, J ¼ 5.99 Hz, 2H). C NMR (300 885, 853, 819, 792, 763, 740, 725, 656, 615, 601, 569, 548, 509, 479,
MHz, CDCl , d/ppm): 36.62, 53.25, 64.29, 110.27, 124.65, 126.30, 458. CHN found (calculated) for C30 : C 35.79 (36.07)
33.40, 134.07, 145.44, 182.39. IR (ATR, nmax/cm ): 3044, 2959, %, H 4.03 (3.83) %, N 7.96 (8.41) %. Mp: 187 C.
883, 2825, 1639, 1613, 1598, 1583, 1564, 120, 1480, 1423, 1394, 2-(2-Ethoxyethoxy)ethyl methanesulfonate (Et-DEG-OMs). 2-
346, 1328, 1299, 1263, 1227, 1174, 1131, 1051, 1022, 967, 953, 916, (2-Ethoxyethoxy)ethanol (diethylene glycol monoethyl ether, Et-
65, 811, 788, 763, 739, 730, 652, 613, 598, 569, 533, 509, 471, 456, DEG-OH) (15.10 g, 1 equiv., 0.113 mol) and triethylamine (TEA)
3
38 12 6 10 4
H F N O S
ꢀ
1
ꢁ
1
2
1
8
ꢁ
4
50, 432, 405. Mp: 183 C.
1
(19 mL, 1.2 equiv., 0.136 mol) were dissolved in anhydrous
dichloromethane (DCM) (60 mL). The solution was stirred under
,4-Bis[[3-(dimethylamino)propyl]amino]-9,10-anthracene-
dione. N,N-Dimethylpropane-1,3-diamine (16.40 g, 9.7 equiv., dry nitrogen gas inside a 250 mL round bottom ask equipped
60.50 mmol) was added to a stirring suspension of 1,4-dihy- with a dropping funnel and placed inside an ice bath. Aer 5 min
1
droxyanthraquinone (3.98 g, 1 equiv., 16.569 mmol) in water (30 of cooling, a solution of methanesulfonyl chloride (MsCl) (12 mL,
ꢁ
mL) and the mixture was stirred at 100 C under N
2
atmosphere 1.4 equiv., 0.155 mol) in anhydrous DCM (20 mL) was added
for 27 h. The reaction mixture was extracted 4 times with 50 mL of dropwise to this solution and under vigorous stirring. The
dichloromethane and the combined organic phases were washed immediate formation of a white/pale yellow precipitate (triethy-
4
times with 200 mL of water. The organic phase was dried over lammonium chloride) was observed. The reaction mixture was
MgSO and concentrated on a rotary evaporator to give a dark further stirred under nitrogen atmosphere in the ice bath over-
blue solid. This crude product was puried by column chroma- night and was allowed to warm up to room temperature. A
tography on silica gel with eluent 9 : 1 DCM/methanol + 20 vol% saturated aqueous NaHCO solution (50 mL) was added and the
triethylamine. The dark blue band was isolated to give to give 1,4- mixture was stirred at room temperature for 30 min. The layers
4
3
bis[[3-(dimethylamino)propyl]amino]-9,10-anthracenedione
were separated and the organic phase was washed with
1
ꢀ1
(
3.54 g, 8.665 mmol, 52% yield) as a dark blue solid. H NMR (300 a 1 mol L aqueous HCl solution (2 ꢂ 50 mL). The acidic
MHz, CDCl , d/ppm): 1.86–1.95 (m, 4H), 2.26 (s, 12H), 2.43 (t, J ¼ washing phases were back-extracted with DCM (2 ꢂ 50 mL) and
3
7.0 Hz, 4H), 3.41–3.48 (m, 4H), 7.25 (s, 2H), 7.66–7.69 (m, 2H), the combined organic phases were dried over Na SO and
2 4
8
.32–8.35 (m, 2H), 10.81 (t, J ¼ 5.3 Hz, 2H).
concentrated on a rotary evaporator to give 2-(2-ethoxyethoxy)
0
3
,3 -((9,10-Dioxo-9,10-dihydroanthracene-1,4-diyl)bis(azane-
ethyl methanesulfonate (Et-DEG-OMs) (22.67 g, 0.107 mol, 95%
1
diyl))bis(N,N,N-trimethylpropan-1-aminium)iodide.
Iodo- yield) as an orange/yellow liquid. H NMR (300 MHz, CDCl
3
, d/
methane (19.93 g, 140.411 mmol) was added to a stirring ppm): 1.20 (t, J ¼ 7.0 Hz, 3H), 3.08 (s, 3H), 3.52 (q, J ¼ 7.0 Hz, 2H),
solution of
1,4-bis[[3-(dimethylamino)propyl]amino]-9,10- 3.57–3.60 (m, 2H), 3.65–3.68 (m, 2H), 3.77 (t, J ¼ 4.4 Hz, 2H), 4.38
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ꢀ
1
(
1
8
t, J ¼ 4.37 Hz, 2H). IR (ATR, nmax/cm ): 3022, 2975, 2935, 2871, Disperse Blue 14 (1,4-bis(methylamino)anthraquinone),
487, 1455, 1413, 1348, 1302, 1247, 1171, 1108, 1015, 972, 914, Disperse Blue 134 (1,4-bis(isopropylamino)anthraquinone),
34,35
34, 797, 733, 715, 526, 457, 407.
Solvent Blue 59 (1,4-bis(ethylamino)anthraquinone) etc.
2
-(2-Ethoxyethoxy)ethanamine (Et-DEG-NH ). Et-DEG-OMs Furthermore, because they have been shown to have biological
2
(
25.92 g, 1 equiv., 0.122 mol) was added to a stirring solution activity as anti-cancer agents, the synthesis procedures of many
of NH Cl (150 g) in 25 wt% aqueous NH (1000 mL, 62 equiv., DAAQ derivatives have been reported in medicinal chemistry
.450 mol NH ) inside a 1000 mL Erlenmeyer ask, and the journals.
4
3
36–38
6
3
The reaction of the commercially available orange
mixture was stirred at room temperature for two days. Subse- dye 1,4-dihydroxyanthraquinone (quinizarin, Solvent Orange
quently, NaCl (150 g) was added and the mixture was stirred 86) or its reduced form leucoquinizarin with primary amines
37,38
until complete dissolution. The reaction mixture was extracted will generally give the corresponding DAAQs in good yield.
with DCM (8 ꢂ 100 mL) and the combined organic phases were This offers a convenient strategy to functionalize the DAAQ
dried over Na SO and concentrated on a rotary evaporator to structure by using differently substituted amines with the goal
2
4
give 2-(2-ethoxyethoxy)ethanamine (Et-DEG-NH
2
)
(12.00 g, of increasing the solubility. For this study, three types of amino
1
0.090 mol, 74% yield) as a pale yellow liquid. H NMR (300 MHz, substituents were investigated: rstly, the branched alkyl chain
CDCl
3
, d/ppm): 1.22 (t, J ¼ 7.0 Hz, 3H), 2.31 (s, 2H), 2.88–2.91 2-ethylhexyl (Et-He) that has been used before to increase the
13
3
(m, 2H), 3.50–3.64 (m, 8H). C NMR (300 MHz, CDCl , d/ppm): solubility of organic compounds because of its high degree of
ꢀ
1
1
3
1
4.89, 41.28, 66.35, 69.52, 70.08, 72.65. IR (ATR, nmax/cm ): conformational freedom, thereby increasing the entropy of
372, 3298, 2974, 2864, 1598, 1485, 1455, 1375, 1349, 1323, dissolution; secondly, alkyl chains with terminal quaternary
39
+
296, 1243, 1175, 1107, 1051, 916, 842, 800, 515, 437.
,4-Bis((2-(2-ethoxyethoxy)ethyl)amino)anthracene-9,10-
ammonium groups, N,N,N-trimethylethanammonium (Me N -
3
+
1
Et) and N,N,N-trimethylpropan-1-aminium (Me N -Pr). Because
3
dione (Et-DEG-DAAQ). Et-DEG-NH
2
(9.84 g, 10 equiv., 74 mmol) these chains contain charged groups, the derived DAAQs are
was added to leucoquinizarin (1.80 g, 1 equiv., 7.4 mmol) and highly polar, increasing their solubility in polar solvents.
Na (1.71 g, 9.8 mmol) inside a 20 mL glass vial equipped Thirdly oligoethylene glycol ether chains were considered:
2 2 4
S O
with a magnetic stirring bar. The vial was brought under nitrogen diethylene glycol monoethyl ether (Et-DEG), and triethylene
atmosphere, sealed tightly with a screwcap lined with a silicone glycol monomethyl ether (Me-TEG). These chains are signi-
ꢁ
septum and with Paralm® and the mixture was stirred at 80 C cantly more polar than alkyl chains, while also having a high
overnight. Subsequently, the reaction mixture was cooled to degree of conformational freedom.
room temperature, the screwcaps were removed and stirring was
Fig. 2 gives an overview of the synthesis routes that were used
continued open to the air overnight. The reaction mixture was to prepare these derivatives. The alkyl (A) and cationic (B)
poured into deionized water (50 mL) aer which it was extracted derivatives were prepared by the reaction of quinizarin and the
with ethyl methyl ketone (MEK) (150 mL + 2 ꢂ 50 mL). The corresponding alkyl- or N,N-dimethylaminoalkylamines. For the
combined organic phases were washed with a 1 : 3 mixture of cationic derivatives, the terminal ternary amino groups were
ꢀ
1
1
mol L aqueous HCl and brine (2 ꢂ 50 mL), followed by pure quaternized with iodomethane, resulting in a cationic DAAQ
brine (2 ꢂ 50 mL). The dried organic phases were dried over iodide salt. Subsequently the iodide salt was converted to
ꢀ
MgSO and concentrated on a rotary evaporator to give 1,4-bis((2-
a
bis(triuoromethylsulfonyl)imide (Tf N ) salt via ion-
4
2
(2-(2-methoxyethoxy)ethoxy)ethyl)amino)anthracene-9,10-dione
exchange with lithium bis(triuoromethylsulfonyl)imide. This
as a dark blue solid (1.86 g, 3.95 mmol, 53% crude yield). The anion is electrochemically more stable than iodide and it is
crude product was puried by column chromatography on silica more hydrophobic, resulting in higher solubility in organic
gel with eluent EtOAc. The bright blue band (R
EtOAc) was isolated (1.42 g, 3.02 mmol, 41% nal yield). H NMR prepared via the leucoquinizarin route (C
f
¼ 0.58 with solvents. The oligoethylene glycol ether derivatives were
1
1
). In this case, the
(
400 MHz, CDCl
3
, d/ppm): 1.22 (t, J ¼ 7.0 Hz, 6H), 3.55 (q, J ¼ required primary amines are not commercially available and
40
7
5
1
1
1
3
.0 Hz, 4H), 3.60–3.65 (m, 8H), 3.69–3.72 (m, 4H), 3.81 (t, J ¼ were synthesized via a two-step procedure (C2). The hydroxyl
.8 Hz, 4H), 7.29 (s, 2H), 7.66–7.70 (m, 2H), 8.31–8.36 (m, 2H), group of the oligoethylene glycol monoalkyl ethers was rst
1
3
0.82 (t, J ¼ 5.2 Hz, 2H). C NMR (400 MHz, CDCl , d/ppm): converted to methanesulfonate by reacting with meth-
3
5.20, 42.73, 66.74, 69.88, 70.19, 70.91, 110.21, 123.49, 126.08, anesulfonyl chloride. Subsequently, the methanesulfonate
ꢀ
1
32.05, 134.51, 146.14, 182.69. IR (ATR, nmax/cm ): 3388, 3220, groups were converted to primary amine groups by reaction
068 (n N–H), 2972, 2866 (n C–H CH ), 1611 (n C]O), 1582, 1554 with an excess of aqueous ammonia solution. The detailed
2
(n C–C aromatic ring), 1522 1447, 1350, 1231, 1196, 1162, 1112, reaction conditions for the DAAQs and precursors can be found
1
050, 1022, 980, 940, 920, 841, 816, 797, 725, 656, 613, 565, 510, in the Experimental section. The synthesis of Me-TEG-DAAQ
33
458, 432. CHN found (calculated) for C H N O : C 66.38 (66.36) and its precursors was reported in our previous work.
26 34 2 6
ꢁ
%
, H 7.23 (7.28) %, N 5.58 (5.95) %. Mp: 50 C.
All the DAAQ derivatives are dark blue solids at room
temperature. The melting points of the alkyl and oligoethylene
glycol ether derivatives are signicantly lower than the melting
Results and discussion
Synthesis and physicochemical properties
ꢁ
point of DB-134 (170 C) due to the high degree of conforma-
tional freedom associated with these chains. This can also be
Many 1,4-diaminoanthraquinones are commercially available advantageous for achieving a high solubility. Another important
as dyes that are used extensively in the textile industry: e.g. parameter to consider for solubility is the polarity of the solute
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Fig. 2 Procedures for the synthesis of the DAAQ derivatives: (A) alkyl DAAQs, (B) cationic DAAQs, (C) oligoethylene glycol ether DAAQs.
compared to the solvent. For organic compounds, the polarity intense with a molar absorption coefficient in the 10
4
L
ꢀ
1
ꢀ1
33
can be conveniently compared by thin layer chromatography mol cm order of magnitude. Therefore, quantitative UV-
TLC) on silica gel plates (Fig. S1†). The aromatic core of the Vis absorption spectroscopy was used as an inexpensive and
(
DAAQ structure is rather apolar due to an absence of polar fast technique to determine the solubility of DAAQs with good
groups. For the alkyl DAAQ derivatives, this apolar character is precision. Several standards were prepared, typically between 10
ꢀ
1
ꢀ1
further increased by the alkyl chains on the amino groups. This mmol L and 70 mmol L , in order to construct calibration
is demonstrated by the increase in R
values (eluens ¼ EtOAc/ curves. The solubility of the DAAQs was determined by precisely
heptane 1/1) from DB-134 (R diluting saturated samples down to fall within the absorbance
¼ 0.74) to Et-He-DAAQ (R
.87). Contrarily, the oligoethylene glycol ether derivatives are range of the calibration curves. The inuence of solvent polarity
f
f
f
¼
0
signicantly more polar due to the many ether functionalities in on the solubility was determined by using mixtures of the polar
the side chains, as shown by the small R values: R ¼ 0.16 for Et- solvent acetonitrile (MeCN), and the apolar solvent toluene in
f
f
DEG-DAAQ and R ¼ 0.02 for Me-TEG-DAAQ. Like all ionic different volume ratios. Another parameter that was studied
f
compounds, Me N-Et-DAAQ and Me N-Pr-DAAQ exhibit very was the inuence of a supporting electrolyte salt on the solu-
3
3
slow migration over the silica TLC plates due to their strong bility of the active species, which is oen overlooked in the
interaction with the polar stationary phase.
literature on non-aqueous RFBs. This was achieved by
measuring the solubility in a 1 mol L solution of TEATf N in
2
ꢀ1
acetonitrile, which is a realistic supporting electrolyte used in
non-aqueous RFBs. The results of the solubility experiments are
summarized in Fig. 3. A logarithmic scale was used because the
solubility of the different DAAQs varied over several orders of
Solubility studied by UV-Vis absorption spectroscopy
All the DAAQs that are included in this study strongly absorb
electromagnetic radiation in the visible spectral region, result-
ing in an intense blue color. Typically, two maxima are observed
in the optical absorption spectra in the wavelength ranges 580–
ꢀ3
0
ꢀ1
magnitude (10
to 10 mol L ). The numerical solubility
values can be found in the ESI, Tables S1–S6.†
605 nm and 620–650 nm, depending on the solvent (Fig. S2†).
With the exception of Me N-Et-DAAQ, the latter peak is the most
3
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ꢀ
1
toluene is added to 0.97 mol L
in pure toluene. When
ꢀ
1
1
mol L TEATf N is added, the solubility of Et-DEG-DAAQ is
2
ꢀ
1
decreased to 1.3 mol L . Nevertheless, this is still a very
promising value for a redox-active compound for all-organic
RFBs. To summarize: the modication of the DAAQ structure
with a branched alkyl chain such as 2-ethylhexyl was successful
in increasing the solubility in apolar solvents. However, the use
of apolar solvents in RFB electrolytes is not recommended
because herein, supporting electrolyte salts are poorly soluble,
Fig. 3 Plot of the solubility (logarithmic scale) of the DAAQs in resulting in low electrical conductivity of the electrolyte. The
ꢀ
1
different solvent systems: (a) acetonitrile + 1 mol L TEATf
2
N, (b) modication with polar side chains did result in an increased
acetonitrile/toluene 1/0, (c) acetonitrile/toluene 3/1, (d) acetonitrile/
toluene 1/1, (e) acetonitrile/toluene 1/3, and (f) acetonitrile/toluene 0/
solubility in polar solvents, in particular for the oligo
ethyleneglycol-ether-functionalized DAAQs. Other than match-
ing the polarity of the DAAQ with the solvent/electrolyte, a long
chain length is also advantageous to achieve a high solubility.
This is most likely related to the high degree of conformational
freedom associated with these chains which increases the
entropy of dissolution.
1
. Error bars are indicated at the top of the columns.
The solubility of DB-134 (1,4-bis(isopropylamino)anthraqui-
none) in pure acetonitrile is 19 mmol L , as determined in our
previous work. When toluene is added, a gradual increase in
solubility is observed as the solvent mixture becomes increas-
ingly apolar. In pure toluene, DB-134 has a solubility of
44 mmol L , an eightfold increase compared to pure aceto-
2
nitrile. The addition of 1 mol L TEATf N to acetonitrile results
in a 20% decrease in solubility due to the increase in polarity
associated with the high salt concentration. Because of the 2-
ethylhexyl chains in Et-He-DAAQ, this DAAQ is more apolar
than DB-134. This results in a lower solubility in pure acetoni-
ꢀ
1
33
ꢀ
1
1
Cyclic voltammetry
ꢀ1
A high solubility in organic supporting electrolyte solutions is
only one of the desirable properties of redox-active compounds
for RFBs. The different amino substituents that are attached to
the DAAQ structure can also have a signicant inuence on the
29
reversibility of the electron-transfer reactions. Therefore, the
electrochemical behavior of the DAAQ derivatives was evaluated
by cyclic voltammetry (Fig. 4).
ꢀ1
trile (12 mmol L ), but the increasing trend as toluene is added
ꢀ1
is much more drastic, with a solubility of 471 mmol L in pure
ꢀ
1
toluene. Additionally, the presence of 1 mol L TEATf
2
N also
N-Et-
All CVs were compensated for solution resistance. An
example of the IR potential shi can be found in the ESI
results in a stronger decrease in solubility of 54%. For Me
DAAQ and Me N-Pr-DAAQ, a decrease in solubility is observed
as the fraction of toluene in the solvent mixtures is increased. In
pure toluene, the solubility of both DAAQs is below the detec-
tion limit of the spectrophotometer. This behavior is expected
because ionic compounds dissolve better in polar solvents.
3
3
(
Fig. S3†). In every CV, a small cathodic wave can be observed at
+
approx. ꢀ1.2 V vs. Ag /Ag (R1). This artefact was found to orig-
inate from the supporting electrolyte solution, as indicated in
the blank measurement (Fig. S4†). In the CV of the model
compound DB-134, the four expected highly reversible one-
electron transfer reactions are observed. The two subsequent
3
Interestingly, the solubility of Me N-Et-DAAQ in pure acetoni-
ꢀ
1
trile is only 46 mmol L , which is a signicant improvement
compared to DB-134, but still insufficient for a practical RFB
electrolyte. Remarkably, the slightly longer ionic side chains in
Me N-Pr-DAAQ result in a solubility of 601 mmol L in pure
acetonitrile, a thirteenfold increase compared to Me N-Et-
+
+
cathodic waves at ꢀ1.42 V vs. Ag /Ag (R2) and ꢀ1.80 V vs. Ag /Ag
R3) correspond to the reduction of the quinone functionality,
(
+
and the two subsequent anodic waves at +0.33 V vs. Ag /Ag (O1)
and +0.87 V vs. Ag /Ag (O2) correspond to the oxidation of the
ꢀ1
3
+
3
two amino functionalities. In the CV of Et-He-DAAQ, two
ꢀ1
2
DAAQ. In acetonitrile + 1 mol L TEATf N the solubility of
+
reversible cathodic waves can be observed at ꢀ1.43 V vs. Ag /Ag
both ionic DAAQs is decreased compared to pure acetonitrile,
most likely due to the common anion effect. In our previous
work, we reported on the complete miscibility of Me-TEG-DAAQ
with acetonitrile and 1,4-dimethoxyethane (DME), with and
without 1 mol L TEATf N added in both solvents. The same
behavior was observed with all the acetonitrile/toluene
mixtures. Because this compound is liquid close to room
+
(
R1) and ꢀ1.82 V vs. Ag /Ag (R2). Thus, the 2-ethylhexyl
substituents do not have a signicant inuence on the cathodic
behavior, as expected from their chemically inert nature. The
+
two anodic waves are present as well at +0.33 V vs. Ag /Ag (O1)
ꢀ1
2
+
and +0.80 V vs. Ag /Ag (O2). However, judging from the relative
anodic and cathodic peak current densities, these electron-
transfer reactions are less reversible, in particular O2.
The overall electrochemical behavior of Me-TEG-DAAQ and
Et-DEG-DAAQ is very similar to that of DB-134, with four
accessible and reversible redox couples at comparable poten-
ꢁ
temperature (mp ¼ 25 C), it readily forms homogeneous
mixtures with most organic solvents. The slightly shorter side
chain in Et-DEG-DAAQ results in a higher melting point (mp ¼
ꢁ
5
0 C), and this allowed for the determination of its maximum
tials. For Me-TEG-DAAQ, the redox couples are located at
solubility in the solvent mixtures. In pure acetonitrile, an
is observed for this
compound. Due to its high polarity, the solubility decreases as
+
ꢀ
1.40 V (R2), ꢀ1.76 V (R3), +0.35 V (O1), and +0.84 V vs. Ag /Ag
ꢀ1
impressive solubility of 1.7 mol L
(
O2), and for Et-DEG-DAAQ at ꢀ1.39 V (R2), ꢀ1.75 V (R3),
+
+
0.34 V (O1), and +0.86 V vs. Ag /Ag (O2). As discussed in our
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ꢀ1
Fig. 4 Cyclic voltammograms of 0.01 mol L DB-134 (a), Et-He-
DAAQ (b), Me N-Et-DAAQ (c), Me N-Pr-DAAQ (d), Et-DEG-DAAQ (e),
and Me-TEG-DAAQ (f) in 0.1 mol L TEATf
at ambient temperature on a platinum disk working electrode (B ¼ 0.5
3
3
ꢀ
1
2
N in acetonitrile, recorded
ꢀ1
Fig. 5 Cyclic voltammograms of 0.01 mol L Me
3
N-Et-DAAQ (a) and
ꢀ1
mm) with a scan rate of 50 mV s . The reference electrode was a silver
ꢀ1
Me
3
N-Pr-DAAQ (b) in 0.1 mol L TEATf
2
N in acetonitrile, recorded at
ꢀ1
wire immersed in 0.01 mol L AgNO
3
in acetonitrile. The counter
ambient temperature on a platinum disk working electrode (f ¼ 0.5
electrode was a piece of platinum-coated silicon wafer with a surface
ꢀ1
ꢀ1
mm) with scan rates varying from 10 mV s to 150 mV s . The
2
ꢀ1
area of approx. 25 mm . The dotted curve is a blank CV of 0.1 mol L
TEATf N in acetonitrile, recorded under identical conditions.
ꢀ1
reference electrode was a silver wire immersed in 0.01 mol L AgNO
in acetonitrile. The counter electrode was a piece of platinum-coated
silicon wafer with a surface area of approx. 25 mm .
3
2
2
previous work, O2 is less reversible than the other redox
couples, and this is also the case for Et-DEG-DAAQ.
33
ꢀ1
At a low scan rate (10 mV s ), the reversibility of the
The overall electrochemical behavior of the cationic DAAQ cathodic electron-transfer reactions is very poor, judging from
derivatives, and in particular the cathodic electron-transfer the low current density of the anodic peaks. As the scan rate is
reactions, is signicantly altered by the presence of the increased, the anodic peak current density is drastically
quaternary ammonium groups in the side chains. The two increased, indicating a higher overall reversibility. It also
anodic redox couples are accessible for both cationic DAAQs, becomes apparent that the anodic peak in the CV of Me N-Et-
3
but O2 is signicantly less reversible than O1. For Me
3
N-Et- DAAQ actually consists of two separate closely adjacent peaks,
DAAQ, the expected two cathodic peaks are observed at and that the more negative peak becomes more intense as the
+
+
ꢀ
1.27 V vs. Ag /Ag and ꢀ1.61 V vs. Ag /Ag. These potentials are scan rate increases. In the CV of Me N-Pr-DAAQ, the anodic
3
signicantly more positive compared to those of the alkyl peaks are more separated, but the same scan-rate-dependent
DAAQs, which is most likely a consequence of the electron- trend is observed. These results are a strong indication that
withdrawing quaternary ammonium groups that are located the reduced forms of the cationic DAAQs are chemically
relatively close to the aromatic core of the molecule. However, in unstable and decompose over time. Considering that the
the backwards scan, one single sharp anodic peak is observed at reduced anionic quinone functionality is closely located to the
+
ꢀ
1.17 V vs. Ag /Ag. The cathodic behavior of Me
3
N-Pr-DAAQ is cationic quaternary ammonium groups, the possibility of an
+
similar, with two cathodic peaks at ꢀ1.35 V vs. Ag /Ag and internal decomposition reaction occurring seems plausible.
+
ꢀ
1.65 V vs. Ag /Ag but here, a sharp double peak at ꢀ1.29 V vs. This makes the ionic DAAQs much less suitable for use in RFB
+
+
Ag /Ag and ꢀ1.40 V vs. Ag /Ag is observed in the backwards electrolytes, because considerable capacity losses would occur.
scan. The cathodic behavior of the cationic DAAQs was further In general, the oligoethylene-glycol-functionalized DAAQs are
studied by varying the scan rate (Fig. 5).
the most promising compounds for use in all-organic RFB
electrolytes, because they combine a high solubility in polar
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supporting electrolyte solutions with a high reversibility of their
inherent electron-transfer events. In previously reported cycling
experiments, we demonstrated fast capacity fading when both
sets of redox couples are utilized in a symmetric RFB setup, but
when only the inner set of redox couples (R2 and O1) is used,
4 W. Wang, Q. Luo, B. Li, X. Wei, L. Li and Z. Yang, Recent
progress in redox ow battery research and development,
Adv. Funct. Mater., 2013, 23, 970–986, DOI: 10.1002/
adfm.201200694.
5 P. Leung, X. Li, C. Ponce de Le ´o n, L. Berlouis, C. T. J. Low and
F. C. Walsh, Progress in redox ow batteries, remaining
challenges and their applications in energy storage, RSC
Adv., 2012, 2, 10125–10156, DOI: 10.1039/c2ra21342g.
6 C. Ponce de Le `o n, A. Fr ´ı as-Ferrer, J. Gonz ´a lez-Garc ´ı a,
D. A. Sz ´a nto and F. C. Walsh, Redox ow cells for energy
conversion, J. Power Sources, 2006, 160, 716–732, DOI:
33
good capacity retention can be achieved. Even with this limi-
ꢀ1
tation, a saturated electrolyte (1.29 mol L ) consisting of Et-
ꢀ
1
DEG-DAAQ in 1 mol L
a theoretical energy density of 60 W h L and a cell potential of
.73 V, which is much higher than any aqueous RFB.
2
TEATf N in acetonitrile, provides
ꢀ1
1
1
0.1016/j.jpowsour.2006.02.095.
Conclusions
7
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Several types of amino substituents, namely alkyl groups,
cationic groups and oligoethylene glycol ether groups, were
successfully introduced in the DAAQ structure via a convenient
one-step synthesis procedure starting from inexpensively
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lyte solutions, in contrast with the apolar alkyl DAAQs, which
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substituents had a strong inuence on the redox behavior of the
molecules. The reversibility of the quinone reduction reaction
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have the optimal combination of a high solubility in supporting
electrolyte solutions, and good retention of the reversibility of
the electron-transfer reactions. This enables to design
a symmetric nonaqueous RFB with a high cell potential and
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There are no conicts to declare.
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