Enhanced anhydrous proton conduction in binary mixtures of 1H-imidazole-1H-1,2,3-triazole based compounds

Article abstract of DOI:10.1039/c2jm32835f

What is the impact of mixing two proton-conducting heterocycles on proton conductivity? Herein we answer this question through our investigations on two linear rod-like compounds 2-(4-(dodecyloxy)phenyl)-1H-imidazole (4) and 5-(4-(dodecyloxy)phenyl)-1H-1,

Full text of DOI:10.1039/c2jm32835f

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PAPER
Enhanced anhydrous proton conduction in binary mixtures of
1H-imidazole–1H-1,2,3-triazole based compounds†
Dipankar Basak,a Craig Versek,b Jacob A. Harvey,a Scott Christensen,c Jibben Hillen,‡a Scott M. Auerbach,a
b
Mark T. Tuominen and D. Venkataraman
a
*
Received 5th May 2012, Accepted 10th August 2012
DOI: 10.1039/c2jm32835f
What is the impact of mixing two proton-conducting heterocycles on proton conductivity? Herein we
answer this question through our investigations on two linear rod-like compounds 2-(4-(dodecyloxy)
phenyl)-1H-imidazole (4) and 5-(4-(dodecyloxy)phenyl)-1H-1,2,3-triazole (10). We have found that
mixtures of molecules 4 and 10 at certain compositions show enhanced proton conductivity compared
to their pure components. We attribute the increased conductivity in these materials to the increased
charge density due to facile co-ionization and increased mobility due to the incorporation of long alkyl
chains, which prevent crystallization of protogenic groups while maintaining the required hydrogen
bonded network. Our results suggest a new strategy for enhancing intrinsic proton conductivity in
heterocyclic systems.
protonic charge carriers exists, with enough flexibility for
molecular reorientation of the hydrogen-bonded network.9–12
This can be achieved via immobilization of the heterocycles with
flexible linkers (such as long alkyl chains), which satisfies both
the requirements.13,14 At high temperature (above Tg for poly-
mers and above Tm for small molecules), where the mobility of
the heterocycles is high it is the number of intrinsic charge
carriers that guides the resulting conductivity.15,16 For N-based
heterocycles, however, this intrinsic charge carrier concentration
is typically low (ꢀ0.1%), resulting from self-dissociation of the
protogenic groups.7,17 Extrinsic doping with strong acids has
been carried out to increase this concentration,6,18,19 but this
approach often suffers from potential leaching at high temper-
atures and a reduction in the proton mobility due to the per-
turbed hydrogen bonded network upon acid-doping.7,17 Our
initial findings on certain binary mixtures of 1H-imidazole and
1H-1,2,3-triazole show a significant enhancement in proton
conductivity compared to either of the pure compounds.20 This
1
Introduction
Proton transporting materials that have high intrinsic proton
conductivity are highly desirable for fuel cell applications.1,2 For
this purpose high boiling organic heterocycles such as 1H-imid-
azole or 1H-1,2,3-triazole have been considered due to their
ability to form hydrogen bond networks similar to water.3–5
Proton conduction in these heterocycles is believed to occur
through the Grotthuss mechanism wherein the protons are
transported through non-diffusive pathways.6,7 In general, the
magnitude of ion conductivity is described as:
P
s(T) ¼ nimiqi
(1)
where s(T) is the conductivity, T is the temperature and qi, ni(T)
and mi(T) are charge, concentration of the intrinsic charge
carriers, and mobility of the charge, respectively.8 Eqn (1) further
indicates that both the charge carrier mobility and the concen-
tration of intrinsic charge carriers will have a profound impact
on the overall proton conductivity. Furthermore, for efficient
proton transport, it is imperative that the proton-transferring
groups organize in such a way that a continuous pathway of
result suggests that mixing slightly basic 1H-imidazole (pKa1
¼
7.18, pKa2 ¼ 14.52)21 with slightly acidic 1H-1,2,3-triazole
(pKa1 ¼ 1.17, pKa2 ¼ 9.26)5,22 facilitates co-ionization, generating
additional charge carriers consisting of imidazolium triazolate. It
appears, therefore, that a viable strategy to increase charge
carrier density and mobility is to incorporate complementary
weak-acid and weak-base into a supramolecular structure.
As model systems we investigate linear rod-like compounds
with a long alkyl chain at one end and heterocyclic functionality
such as 1H-imidazole (4) or 1H-1,2,3-triazole (10) at the other
end (Fig. 1). These systems have been designed such that inter-
actions between the long alkyl chains and the protogenic groups
aDepartment of Chemistry, University of Massachusetts Amherst, 710 N.
Pleasant Street, Amherst, MA, 01003, USA. E-mail: dv@chem.umass.
edu; Fax: +1 4135454490; Tel: +1 4135452028
bDepartment of Physics, University of Massachusetts Amherst, MA,
01003, USA
cDepartment of Polymer Science and Engineering, University of
Massachusetts Amherst, MA, 01003, USA
† Electronic supplementary information (ESI) available: DSC, POM,
WAXS and NMR spectra. See DOI: 10.1039/c2jm32835f
‡ Presently a student at Amherst Regional High School, Amherst, MA
01002, USA.
will yield
a nanoscale organization allowing continuous
hydrogen bond pathways, which, coupled with dynamic
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sample chamber at a distance of 139 mm. The whole system is
evacuated. The actual scattering angles are calibrated using the
accurately known reflections from silver behenate and tricosane.
Mass spectroscopic methods were performed by the Mass
Spectrometry Center at the University of Illinois, Urbana-
Champaign. Impedance data were obtained using a Solartron
1260 frequency response analyzer in the range of 0.1 Hz to 10
MHz with a sinusoidal excitation voltage of 0.1 V root-mean-
square amplitude. Vacuum dried samples were placed into a
cavity (L ¼ 0.029 cm long by A ¼ 0.079 cm2 cross-sectional area)
of a PTFE tape spacer between two gold-coated stainless steel
electrodes. The sample’s resistance to proton transport, R, was
estimated by fitting a constant value to the plateau of impedance
magnitude, which occurred within this frequency range.
Conductivity was computed as s ¼ L/(RA). Measurements were
conducted in an ESPEC SH-241 temperature control chamber
Fig. 1 Molecular structures of rigid rod-like compounds based on
1H-1,2,3-triazole and 1H-imidazole.
behaviour, may furnish high proton conductivities. Experiments
with several composites of 4 and 10 (complementary mixtures of
triazole and imidazole) were performed to understand the impact
of effective charge carrier density on the overall proton
conductivity. Additionally, by changing the protogenic group
from 1H-1,2,3-triazole to 1H-imidazole in these small molecule
systems, we are able to investigate the impact of each heterocycle
in a nearly identical structural environment. This systematic
study investigating the role of charge carrier density in a system
with nanoscale domains is an important step towards setting the
design criteria for co-polymers with complimentary heterocycles
for next generation polymer electrolyte membranes.
ꢂ1
ꢁ
ꢁ
ꢁ
between 40 C and 150 C with a sweep rate of 0.25 C min
.
All reactions were carried out in oven-dried glassware (160
ꢁC). The chemicals were purchased from major chemical
suppliers (Acros, Sigma-Aldrich, Alfa-Aesar and TCI America)
and were used without further purification unless otherwise
noted. Triethylamine and dichloromethane were redistilled over
ꢀ
anhydrous calcium hydride and stored over 4 A molecular sieves
under argon. Flash chromatography was performed using silica
ꢀ
gel from Sorbent Technologies (Standard Grade, 60 A, 230–400
mesh). Analytical thin layer chromatography was performed on
a pre-coated silica gel glass plate with visualization under UV
light. N-Bromosuccinimide (NBS) was recrystallized from water.
4-(Dodecyloxy)benzaldehyde
(2),23
1-(dodecyloxy)-4-ethy-
nylbenzene (8),24 and azidomethyl pivalate25 were synthesized
2
Experimental section
according to the reported literature procedures, respectively.
2.1 Materials and characterization
1
All H and 13C NMR spectra were recorded on a Bruker DPX
2-(4-(Dodecyloxy)phenyl-4,5-dihydro-1H-imidazole (3). Com-
pound 3 was synthesized from compound 2 according to the
procedure reported in the literature.26 Pale yellow solid. Yield:
400 MHz spectrometer. Chemical shift (d) and coupling constant
(J) are reported in parts per million and Hertz, respectively. The
abbreviations for splitting patterns are s, singlet; bs, broad
singlet; d, doublet; t, triplet; q, quartet; dd, doublet of doublets,
and m, multiplet. TGA was carried out using a TA Instruments
TGA 2950 thermogravimetric analyzer with a heating rate of 10
ꢁC minꢂ1 from room temperature to 600 ꢁC under nitrogen.
Differential scanning calorimetry (DSC) was performed on a
Mettler–Toledo model DSC 822c differential scanning calorim-
eter, where temperature and enthalpy were calibrated with In
(430 K, 3.3 J molꢂ1) and Zn (692.7 K, 12 J molꢂ1) standard
samples using sealed Al sample pans. Cooling and heating
profiles were recorded and analyzed using the Mettler–Toledo
STARc software system. Optical microscopy was performed on a
Zeiss Axiotech Vario reflected light microscope under crossed
polarizers, equipped with an Instec HCS62N hot stage. X-Ray
scattering was done using an in-house setup from Molecular
Metrology Inc. (presently sold as Rigaku S-Max3000). It uses a
30 W microsource (Bede) with a 30 ꢃ 30 mm2 spot size matched
to a Maxfluxꢀ optical system (Osmic) leading to a low-diver-
gence beam of monochromatic CuKa radiation (wavelength l ¼
0.1542 nm). After passing beam defining and guard pinholes, the
beam of about 0.4 mm diameter enters the sample chamber.
Samples were placed within a hot stage connected to an Omega
CN76000 temperature controller. WAXS is performed using an
image plate (maximum resolution 50 mm) positioned in the
1
69%. H NMR (CDCl3, 400 MHz) d 7.74 (d, J ¼ 9.2 Hz, 2H),
6.88 (d, J ¼ 9.2 Hz, 2H), 4.86 (bs, 1H), 3.96 (t, J ¼ 6.8 Hz, 2H),
3.76 (s, 4H), 1.78 (quintet, J ¼ 6.8 Hz, 2H), 1.46–1.26 (m, 18H),
0.87 (t, J ¼ 6.8 Hz, 3H); 13C NMR (CDCl3, 101 MHz) d 164.4,
161.1, 128.5, 122.7, 114.2, 68.1, 31.9, 29.7, 29.66, 29.63, 29.60,
29.57, 29.39, 29.35, 29.18, 26.0, 22.7, 14.1; HRMS (ESI) m/z
calcd for C21H35N2O [M + H]+: 331.2749; found: 331.2746.
2-(4-(Dodecyloxy)phenyl)-1H-imidazole (4). Compound 3 was
converted to compound 4 according to the procedure reported in
the literature.26 Pale yellow solid. Yield: 78%. 1H NMR (CDCl3,
1
400 MHz) H NMR (CDCl3, 400 MHz) d 7.75 (d, J ¼ 8.8 Hz,
2H), 7.11 (s, 2H), 6.96 (d, J ¼ 9.2 Hz, 2H), 3.97 (t, J ¼ 6.8 Hz,
2H), 1.79 (quintet, J ¼ 6.8 Hz, 2H), 1.47–1.26 (m, 18H), 0.88 (t,
J ¼ 6.8 Hz, 3H); 13C NMR (CDCl3, 101 MHz) d 159.7, 146.9,
126.6, 122.9, 114.9, 68.1, 31.9, 29.67, 29.65, 29.61, 29.59, 26.41,
29.36, 29.2, 26.0, 22.7, 14.1; HRMS (ESI) m/z calcd for
C21H33N2O [M + H]+: 329.2593; found: 329.2587.
(5-(4-(Dodecyl)phenyl)-1H-1,2,3-triazol-1-yl)methyl pivalate
(9). In a single-necked round bottom flask a mixture of 8 (0.86 g,
3.0 mmol) and CuI (0.29 g, 1.5 mmol) were taken together under
argon. To the mixture a solution of N,N-diisopropylethylamine
(1.60 mL, 9.0 mmol), 0.72 g (4.50 mmol) of azidomethyl pivalate
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in 40 mL of deoxygenated THF were added and stirred at 50 ꢁC
for 12 h. The reaction mixture was cooled down to room
temperature after 12 h and 60 mL of water was added. The
organic layer was extracted with dichloromethane (2 ꢃ 60 mL),
washed with water (2 ꢃ 50 mL), brine (2 ꢃ 50 mL), dried over
anhydrous Na2SO4 and concentrated under reduced pressure.
The crude product was purified by column chromatography
(4 : 0.5 hexane–ethyl acetate) to afford 1.2 g of 9 (90%) as a white
solid.
1
1H NMR (CDCl3, 400 MHz) H NMR (CDCl3, 400 MHz) d
Scheme 2 Synthetic route for DOPhTz (10).
7.93 (s, 1H), 7.76 (d, J ¼ 8.8 Hz, 2H), 6.95 (d, J ¼ 8.8 Hz, 2H),
6.26 (s, 2H), 3.98 (t, J ¼ 6.6 Hz, 2H), 1.79 (quintet, J ¼ 6.8 Hz,
2H), 1.47–1.26 (m, 18H), 1.19 (s, 9H), 0.88 (t, J ¼ 6.8 Hz, 3H);
13C NMR (CDCl3, 101 MHz) d 178.0, 159.4, 148.3, 127.1, 122.5,
120.0, 114.9, 69.7, 68.1, 38.8, 31.9, 29.67, 29.64, 29.60, 29.59,
26.41, 29.36, 29.25, 26.8, 26.0, 22.7, 14.1; HRMS (ESI) m/z calcd
for C26H42N3O3 [M + H]+: 444.3226; found: 444.3233.
under mild basic conditions. The alkyne group was then con-
verted to pivolyloxymethyl protected 1,2,3-triazole moiety (9)
using azide-alkyne-Huisgen dipolar cycloaddition reaction.30,31
The deprotection of the pivolyloxymethyl group in the presence
of 0.5 M NaOH furnished compound 10 as a white solid.25
5-(4-(Dodecyloxy)phenyl)-1H-1,2,3-triazole (10). In a single-
necked round bottom flask 9 (1.1 g, 2.48 mmol) was dissolved in
30 mL of 0.5 M NaOH/THF (1 : 1 v/v mixture) and the mixture
3.2 Thermal analysis
Thermal stability of 4, 10 and their mixtures was tested by
ꢁ
thermogravimetric analysis (TGA) with a heating rate of 10 C
ꢁ
was heated at 50 C for 12 h. The reaction mixture was cooled
minꢂ1 under a nitrogen atmosphere. Solid samples were kept in
vacuo for 12 h before recording TGA data. DOPhIm (4) and
DOPhTz (10) were mixed at different compositions and heated to
140 ꢁC for 1 h and then cooled (5 ꢁC minꢂ1) back slowly to room
temperature to ensure intimate mixing. TGA plots of DOPhIm
(4), DOPhTz (10) and their mixture of different compositions
showed minimal weight loss before 230 ꢁC, indicating the
thermal stability of the components at least up to 230 ꢁC (Fig. 2).
A continuous weight loss was observed after 250 ꢁC, presumably
due to decomposition of the hydrocarbon.
down to room temperature, 20 mL water was added to it and pH
was adjusted to 7.0 by dropwise addition of 1 M HCl. The
mixture was filtered off, washed with water and dried under
vacuum. The product was further purified by eluting through a
small pad silica column (3% MeOH/CHCl3) to afford 10 as a
white solid (quantitative yield). 1H NMR (DMSO-d6, 400 MHz)
d 14.95 (bs, 1H), 8.23 (s, 1H), 7.79 (d, J ¼ 8.4 Hz, 2H), 7.03 (d,
J ¼ 8.4 Hz, 2H), 4.02 (t, J ¼ 6.4 Hz, 2H), 1.794 (quintet, J ¼ 6.4
Hz, 2H), 1.44–1.27 (m, 18H), 0.88 (t, J ¼ 6.8 Hz, 3H); 13C NMR
(DMSO-d6, 101 MHz) d 159.1, 127.4, 115.3, 67.9, 31.8, 29.5,
29.49, 29.46, 29.2, 29.19, 29.12, 25.9, 22.6, 14.1; HRMS (ESI) m/z
calcd for C20H32N3O [M + H]+: 330.2545; found: 330.2549.
Thermal properties of DOPhTz (10) and DOPhIm (4) and
their different compositions were studied by differential scanniꢁng
calorimetry (DSC). DSC traces were recorded between ꢂ30 C
and 200 ꢁC with a heating and cooling rate of 10 ꢁC minꢂ1 and 5
ꢁC minꢂ1, respectively. The composites were heated up to 200 ꢁC
to remove any thermal history (first heating), followed by slow
cooling (first cooling cycle) and subsequent heating (second
heating cycle) (Fig. 3). Upon second heating DOPhTz (10) shows
a single endotherm at 138.7 ꢁC, corresponding to the crystal (K)
to isotropic (I) transition. The transition is fully reversible as one
exotherm corresponding to the crystallization arrives at 130.7 ꢁC
3
Results and discussions
3.1 Synthesis
2-(4-(Dodecyloxy)phenyl)-1H-imidazole (4) was synthesized
from 4-hydroxybenzaldehyde (1) using the well-established
literature procedure (Scheme 1).26 1 was alkylated to 2 via Wil-
liamson’s ether synthesis; the latter was then converted to imi-
dazoline compound (3) by reacting with ethylenediamine.27
Swern oxidation28 of 3 furnished the desired imidazole derivative
4 as a pale yellow solid in 78% yield.
5-(4-(Dodecyloxy)phenyl)-1H-1,2,3-triazole (10) was obtained
from 4-iodophenol in five steps (Scheme 2). 1-(Dodecyloxy)-4-
ethynylbenzene (8) was obtained using Sonogashira coupling29
between compound 6 and TMS-acetylene using Cu(I)/Pd(0) in
the presence of triethylamine as base, followed by TMS-cleavage
Fig. 2 Thermogravimetric analysis (TGA) of DOPhIm(4)–DOPhTz(10)
composites (heating rate 10 ꢁC minꢂ1), the composites show minimal
weight loss at least up to 230 ꢁC.
Scheme 1 Synthetic route for DOPhIm (4).
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Fig. 4 Phase diagram for different DOPhIm(4)–DOPhTz(10) compos-
ites. The data points were extracted from the second heating cycle in DSC.
Except some intermediate compositions, most of the composites show two
distinct phases: the crystalline state is labeled as K1, the isotropic state is
labeled as I and the intermediate state is labeled as K1 + I.
3.3 Anhydrous proton conduction
The temperature dependence of proton conductivities for various
DOPhIm(4)–DOPhTz(10) composites were studied by imped-
ance spectroscopy and are shown in Fig. 5. All samples were
heated to 140 ꢁC, the highest temperature recorded, and
conductivities were measured during slow cooling of the samples
at the rate of 0.25 ꢁC minꢂ1. The anhydrous proton conductivity
Fig. 3 DSC profiles for different DOPhIm(4)–DOPhTz(10) composites.
(a) 2nd heating cycle (10 ꢁC minꢂ1) and (b) 1st cooling cycle (5 ꢁC minꢂ1).
All the transitions were completely reversible in the heating and cooling
cycles.
of DOPhIm (4) was 1.3 ꢃ 10ꢂ4 S cmꢂ1 at 140 C, at least three
ꢁ
orders of magnitude higher than that of DOPhTz (10) (7.4 ꢃ
10ꢂ7 S cmꢂ1) at the same temperature. A sharp slope change in
log(s) vs. 1/T plot was observed for both DOPhIm (4) (at 123 ꢁC)
and DOPhTz (10) (at 134 ꢁC), corresponding to the isotropic to
crystalline phase transitions, correlating well with the transitions
observed in DSC. This drastic drop in the conductivity curve
further indicates that dynamic hydrogen bonding of a locally
liquid-like state is critical for anhydrous proton conduction, in
agreement with our previous report on discotic liquid crystalline
proton conductors.
during the cooling cycle. Polarized optical micrographs (POM)
of DOPhTz (10) above 140 ꢁC show no birefringence indicating
an isotropic phase, while cooling the sample below 130 ꢁC results
in birefringence, indicating crystallization (see Fig. S2 in the
ESI†). Upon heating, DOPhIm (4) shows a broad endotherm at
131.3 ꢁC, corresponding to the crystal (K) to isotropic (I) phase
transition, which was further supported by POM (Fig. S1 in the
ESI†). Similar to DOPhTz (10), an exotherm arises at 119.3 ꢁC,
indicating a fully reversible phase transition. Most of the
DOPhTz(10)–DOPhIm(4) composites exhibit two endotherms
during heating, corresponding to two-phase transitions (K1 to K2
and K2 to I), which are fully reversible during the subsequent
cooling cycles. However, the composites DOPhTz(10)–
DOPhIm(4) ¼ 40/60, 50/50 and 33.3/66.7 exhibit one endotherm
during the heating cycle, corresponding to only one K to I
transition. Details of transition temperatures and associated
enthalpy changes are summarized in Table T1 of the ESI.†
The phase diagram of DOPhIm(4)–DOPhTz(10) composites is
shown in Fig. 4. All the compositions are solid at room
temperature and the melting points (Tm) of the composites are
lower than the Tm of either DOPhTz (10) or DOPhIm (4). As the
mole fraction of DOPhIm (4) increases, the Tm of the composites
decreases until they form an equimolar mixture (50/50
DOPhIm(4)–DOPhTz(10)). Furthermore, two other composites
DOPhIm(4)–DOPhTz(10) ¼ 60/40 and 66.7/33.3 show similar
Tm, indicating a range of eutectic compositions. When the mole
fraction of DOPhIm (4) increases further, the Tm of the
composites again increases. The second melting peaks remain
invariant with composition, located close to the Tm of the
eutectic compositions.
Fig. 6 shows the composition dependence of isothermal proton
conductivities of different DOPhIm(4)–DOPhTz(10) compos-
ites. Neat DOPhTz (10) shows the lowest conductivity (e.g. 7.4 ꢃ
10ꢂ7 S cmꢂ1 at 140 ꢁC) of any composition investigated, and
increasing the mole fraction of DOPhIm (4) increases the
proton conductivity until it reaches a local maximum at
Fig. 5 Anhydrous proton conductivities of different DOPhIm(4)–
DOPhTz(10) composites.
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Fig.
6 Isothermal anhydrous proton conductivities of different
DOPhIm(4)–DOPhTz(10) composites as a function of DOPhIm (4) mole
fraction.
Fig. 7 (a) DFT optimized at the B3LYP/6-31 G level of DOPhTz (10)
estimating the molecular length of ꢀ2.3 nm; (b) SAXS profile of DOPhTz
(10) at different temperatures: samples were initially heated to 160 ꢁC and
subsequently cooled down to 80 ꢁC and 25 ꢁC. I(q) values have been
shifted vertically for clarity; (c) an illustration of the proposed structure
of DOPhTz (10), which self-assembles into a head-to-head bilayer type
packing.
DOPhTz(10)–DOPhIm(4) ¼ 50/50. At this 50/50 composition, a
conductivity at 140 ꢁC of 1.9 ꢃ 10ꢂ4 S cmꢂ1 is observed, prior to
a drop at DOPhTz(10)–DOPhIm(4) ¼ 40/60. A similar trend is
followed in the DOPhIm(4) rich range, wherein an increase in
conductivity is observed at DOPhTz(10)–DOPhIm(4)
¼
12.5/87.5 (2.0 ꢃ 10ꢂ4 S cmꢂ1 at 140 ꢁC) prior to a decrease
at DOPhTz(10)–DOPhIm(4) ¼ 25/75. Among all the composi-
tions, the highest conductivity is observed at DOPhTz(10)–
DOPhIm(4) ¼ 33.3/66.7, which is in the range of eutectic
ꢂ4
ꢁ
compositions. At 140 C, the peak conductivity is 2.7 ꢃ 10
S
cmꢂ1, which is three orders of magnitude higher than the
conductivity of neat DOPhTz (10) and slightly higher than neat
DOPhIm (4) (1.3 ꢃ 10ꢂ4 S cmꢂ1) at the same temperature.
3.4 Powder X-ray scattering
Structures of neat DOPhTz (10), DOPhIm (4) and their equi-
molar composite DOPhTz(10)–DOPhIm(4) (50/50) were char-
acterized using small- and wide-angle X-ray scattering. A distinct
scattering peak is observed for neat DOPhTz (10) in SAXS, while
neat DOPhIm (4) remains featureless in this range, necessitating
wide-angle scattering for this sample. Further investigation of
DOPhTz (10) was performed using variable temperature SAXS,
with Fig. 7b showing results recorded upon slow cooling from
160 ꢁC to 25 ꢁC. Upon heating to 160 ꢁC, beyond the phase
transition temperature, a diffuse broad peak arises at q ¼ 1.61
nmꢂ1, indicating a poorly ordered, liquid-like material with a
characteristic length scale of 3.91 nm. The lack of birefringence
observed in POM at this temperature furꢁther indicates poorly
defined organization. Upon cooling to 80 C, the primary peak
shifts to q ¼ 1.49 nmꢂ1, corresponding to a characteristic spacing
of 4.22 nm, which was further sharpened upon cooling to 25 ꢁC.
Although the structure cannot be determined unambiguously
from these data, it is notable that the fully extended length of
DOPhTz (10) as calculated by DFT optimized at the B3LYP/6-
311 G(d,p) level is 2.3 nm (Fig. 7a). The estimated thickness of
‘head-to-head’ stacking of the triazole-functionalized dodecyl
alkyl chains will therefore be 4.6 nm, very close to the observed
spacing of d ¼ 4.22 nm. This analysis suggests the presence of
bilayer domains in 10 (Fig. 7c). The increase in length scale
observed (from d ¼ 3.91 nm to d ¼ 4.22 nm) upon cooling likely
reflects the increased mobility of the alkyl chains at high
Fig. 8 (a) DFT optimized at the B3LYP/6-31 G level of DOPhIm (4)
estimating the molecular length of ꢀ2.3 nm; (b) WAXS profile of
DOPhIm (4) at different temperatures: samples were initially heated to
150 ꢁC and subsequently cooled down to 80 ꢁC and 50 ꢁC. I(q) values
have been shifted vertically for clarity; (c) an illustration of the proposed
structure of DOPhIm (4), which self-assembles into an interdigitated
bilayer type packing.
temperature, causing them to become less extended, decreasing
layer thickness and ordering in the system.
Fig. 8b shows wide-angle X-ray scattering (WAXS) of
DOPhIm (4) recorded at variable temperatures. For comparison
purposes, only the primary peak is shown in Fig. 8 (full WAXS
spectrum is shown in the ESI Fig. S3†). Upon heating to 150 ꢁC
(beyond the phase transition temperature) no scattering peak
was observed, indicating an isotropic phase, consistent with the
DSC observation. Upon subsequent cooling to 80 ꢁC, a well-
resolved scattering peak appeared at q ¼ 1.91 nmꢂ1, indicating
the emergence of a characteristic length scale of 3.29 nm. This
value is much lower than that for head-to-head packing of fully
stretched dodecyl chains with imidazole functionality (ꢀ4.6 nm –
Fig. 8a), and as such suggests an interdigitated structure
obtained through side-to-side hydrogen bonding of adjacent
imidazole groups (Fig. 8c). The single crystal X-ray structure of
2-[4-(decyloxy)-phenyl]-1H-imidazole reported in the literature26
exhibits extended chains of hydrogen bonding as well as
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Fig. 9 SAXS profile of DOPhTz(10)–DOPhIm(4) composite 50/50 at
different temperatures: samples were initially heated to 130 ꢁC and
subsequently cooled down to 25 ꢁC. I(q) values have been shifted verti-
cally for clarity.
interdigitation of the imidazole rings, further supporting our
speculation of an interdigitated bilayer type packing for
DOPhIm (4) from variable temperature WAXS observations.
The structure of DOPhIm(4)–DOPhTz(10) composite (50/50)
was further investigated using small angle X-ray scattering at
various temperatures (Fig. 9). Upon heating to 130 ꢁC, a diffuse
broad peak arises at q ¼ 1.45 nmꢂ1, indicating a poorly ordered,
liquid-like material with a characteristic length scale of 4.34 nm.
Fig. 10 Illustration of hypothetical proton conduction pathways for
DOPhTz (10) (a) and (b); (c) DOPhIm (4) and (d) equimolar composite of
DOPhIm(4)–DOPhTz(10) – 50/50. While extended hydrogen bonding
exists in DOPhIm (4) and in the composite, the predominant presence of
hydrogen bond clusters results in poor performance for DOPhTz (4).
ꢁ
Upon cooling to 90 C, a broad primary peak falls at q ¼ 1.34
nmꢂ1 (d ¼ 4.69 nm) and a second, diffuse peak centers on the
original peak position at 130 C (q ¼ 1.45 nmꢂ1). Based on the
packing observed in DOPhTz (10) presumably supports the
predominant existence of pyrazole-like pathways, resulting in
hydrogen-bonded clusters with low charge mobility, yielding a
low overall conductivity. Conversely, as X-ray scattering data
support, DOPhIm (4) forms interdigitated structures through
side-to-side hydrogen bonding between adjacent imidazole rings
(Fig. 10c).26 Therefore, while DOPhTz (10) forms local hydrogen
bonds, DOPhIm (4) forms extended one-dimensional hydrogen
bonded networks, increasing protonic charge mobility and
favouring long-range Grotthuss-type proton transport.
Although the exact mechanism of proton transfer is still unclear
and requires further attention, this result confirms that a change
in the protogenic group from 1H-imidazole to 1H-1,2,3-triazole
has a significant influence on charge mobility in supramolecular
systems. While the nanoscale organization of protogenic groups
has previously proven beneficial to proton conductivity, care
must be taken to achieve a packing of heterocycles, which yields
an extended hydrogen-bonded network versus locally hydrogen-
bonded clusters.
ꢁ
persistence of the secondary peak, we speculate that a portion of
the composite does not crystallize completely at this temperature
(DSC cooling curve of 50/50 composite also shows a broad
temperature regime of crystallization) and the peak location and
breadth are due to the liquid-like nature of the aliphatic chains.
Upon further cooling to 50 ꢁC, a single peak persists at q ¼ 1.34
nmꢂ1, suggesting complete crystallization. The characteristic
length scale of 4.69 nm further indicates that the composite
maintains a bilayer type ‘head-to-head’ stacking as observed in
the packing of DOPhTz (10) component. It is also notable that
the characteristic spacing (d ¼ 4.69 nm) of the composite is
higher than either DOPhIm (4) (d ¼ 3.29 nm) or DOPhTz (10)
(d ¼ 4.2 nm), suggesting swelling of the head-to-head DOPhTz
(10) domains via incorporation of interdigitating DOPhIm (4)
(vide infra).
Combining these X-ray scattering results with our conduc-
tivity measurements provides insight into the nature of proton
transport in both imidazole-(DOPhIm (4)) and triazole-con-
taining (DOPhTz (10)) supramolecular architectures. It is known
in the literature that in the crystal state, 1,2,3-triazole can exist in
equal proportions of two tautomeric forms – 1H-1,2,3-triazole
and 2H-1,2,3-triazole (in the gas phase 2H-1,2,3-triazole is the
predominant form while 1H-1,2,3-triazole becomes increasingly
more stable with an increase in the polarity of the medium).32
Based on these and further studies elucidating the possible
proton transfer mechanisms in 1,2,3-triazole, we envision that
DOPhTz (10) can exhibit two types of hydrogen bonding –
extended bonds in the same layer using two side N atoms of the
triazole ring (so-called imidazole-like pathway33 – Fig. 10b), and
localized bonds using the middle and one of the side N atoms (so-
called pyrazole like pathway33 – Fig. 10a). The head-to-head
Based solely on this argument of extended vs. local hydrogen
bonding, DOPhIm (4), with the most imidazole-like hydrogen
bonds, would have the highest conductivity of any composition
measured. This is clearly not the case, however, as incorporating
up to 50 percent DOPhTz (10) increases measured conductivity
while presumably decreasing the occurrence of these favorable
pathways. X-ray data for the equimolar mixture of
DOPhTz(10)–DOPhIm(4) does suggest that DOPhIm (4) inter-
penetrates into the hydrogen bonded network of DOPhTz (10)
and facilitates the formation of imidazole-like bonding in the
composite (Fig. 10d), yet this alone cannot account for the
enhanced conductivity of the composite mixtures. In order to
understand the effect of complementary weak-acid–base
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correct. These Keq values were referenced to the result for imid-
azole auto-ionization (Reaction 1 below) and are plotted on a
(1)
logarithmic scale i.e., Keq values are shown as log10(Keq(n)/Keq
)
(n)
where Keq is the Keq value for the nth reaction.
The standardized reaction Keq values are shown in Fig. 12. We
note that the auto-ionization of imidazole is predicted to be more
likely than that of triazole (Reactions 2a/2b). This is consistent
with the experimental result that the conductivity of DOPhIm is
greater than that of DOPhTz. In addition, we observe the
particularly interesting result that Keq for Reaction 3, in which
triazole protonates imidazole, is the largest value computed
herein. This suggests that the charge carrier density is expected to
be greatest in an imidazole–triazole mixture, which may lead to
an increase in conductivity, all other things being equal.
Although Reactions 4a and 4b also pertain to the mixture, they
are rather unlikely because imidazole acts as the acid, which in
contrast to the pKa values of imidazole and triazole as described
above. These results suggest that the charge carrier density is
expected to be greatest in mixtures of these functional groups,
which is consistent with the experimentally obtained enhanced
conductivity values.
Fig. 11 Auto-ionization and co-ionization of imidazole, triazole and
imidazole–triazole composites.
interactions between 1H-1,2,3-triazole and 1H-imidazole, we
initiated a computational study.
3.5 Computational studies
To determine the trend in charge carrier density, a thermo-
chemistry analysis was performed using the Gaussian09 software
package.34 Structures of neutral, protonated, and deprotonated
imidazole and 1,2,3-triazole were optimized using a 6-311G(d,p)
basis set and the B3LYP hybrid density functional. Triazole may
be protonated at adjacent or opposite positions, producing a
total of six distinct auto- and cross-ionization reactions, listed
below (Fig. 11). The standard Gibbs free energy for each isolated
reactant under fictitious, ideal-gas conditions at STP was deter-
4
Conclusion
We have synthesized two linear rod-like compounds based on
1H-imidazole and 1H-1,2,3-triazole proton transferring func-
tionalities and investigated their anhydrous proton conductivi-
ties. The 1H-1,2,3-triazole-based compound DOPhTz (10)
exhibited anhydrous proton conductivity at least three orders of
magnitude lower than the conductivity of a 1H-imidazole-based
compound DOPhIm(4) at the same temperature. The thermal
behavior of these two compounds was found to be nearly iden-
tical, however, X-ray data support head-to-head bilayer type
packing for DOPhTz (10) while an interdigitated structure
through sideways hydrogen bonding is observed for DOPhIm
(4). The enhanced conductivity of DOPhIm (4) is attributed to
the formation of extended hydrogen bonded networks through
an imidazole-like arrangement, while DOPhTz (10) primarily
exists in pyrazole-like locally hydrogen bonded clusters. Mixing
of the two materials in complementary acid–base composites
demonstrates a potential increase in charge carrier density
associated with co-ionization of 1H-imidazole and 1H-1,2,3-tri-
azole, with maximum conductivity observed at a DOPhTz(10)–
DOPhIm(4) ratio of 1/2. We conclude that to increase charge
carrier density and mobility in anhydrous supramolecular
systems, it is favourable to select complementary protogenic
groups, which self-assemble into global hydrogen bonded
networks. As a result, we are currently investigating new co-
polymers containing complementary acid–base pairs, which we
hope to publish in due course.
mined from
a
subsequent frequency calculation using
Gaussian09. Standard reaction DG values for the six distinct
reactions below were then computed and used to calculate each
reaction Keq, which is, in principle, proportional to the charge
carrier density notwithstanding the environmental effects of
solid-state packing. Because the solids synthesized and studied
herein are likely to be low-dielectric materials, we do not expect
solvation effects to control the actual ionization thermochem-
istry, and thus the gas-phase trends should remain qualitatively
Acknowledgements
This work was initiated through a support from the NSF Fueling
the Future Center for Chemical Innovation (CHE 0739227),
sponsored by the National Science Foundation. This material is
also based upon the work supported in part by the U.S. Army
Research Laboratory and the U.S. Army Research Office under
grant number 54635CH, and by the National Science
Fig. 12 Standardized Keq for auto-ionization and co-ionization reac-
tions. Reaction 3 has the largest Keq value, indicating facile co-ionization
of imidazole (weak base) and triazole (weak acid).
20416 | J. Mater. Chem., 2012, 22, 20410–20417
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20 C. Versek and M. T. Tuominen, unpublished results.
21 J. Catalan, J. L. M. Abboud and J. Elguero, Adv. Heterocycl. Chem.,
1987, 41, 187–274.
22 J. Catalan, R. M. Claramunt, J. Elguero, J. Laynez, M. Menendez,
F. Anvia, J. H. Quian, M. Taagepera and R. W. Taft, J. Am.
Chem. Soc., 1988, 110, 4105–4111.
Foundation Materials Research and Science Center on Polymers
(DMR 0820506). We sincerely thank Prof. Ryan C. Hayward for
his valuable input in interpreting the X-ray data.
23 T. Cardinaels, J. Ramaekers, P. Nockemann, K. Driesen, K. Van
Hecke, L. Van Meervelt, S. Lei, S. De Feyter, D. Guillon,
B. Donnio and K. Binnemans, Chem. Mater., 2008, 20, 1278–
1291.
24 M. B. Goldfinger and T. M. Swager, J. Am. Chem. Soc., 1994, 116,
7895–7896.
Notes and references
1 K. D. Kreuer, Chem. Mater., 1996, 8, 610–641.
2 L. Carrette, K. A. Friedrich and U. Stimming, Fuel Cells, 2001, 1, 5–39.
3 M. F. H. Schuster, W. H. Meyer, M. Schuster and K. D. Kreuer,
Chem. Mater., 2004, 16, 329–337.
25 J. C. Loren, A. Krasinski, V. V. Fokin and K. B. Sharpless, Synlett,
2005, 2847–2850.
26 K. Goossens, S. Wellens, K. Van Hecke, L. Van Meervelt,
T. Cardinaels and K. Binnemans, Chem.–Eur. J., 2011, 17, 4291–4306.
27 H. Fujioka, K. Murai, Y. Ohba, A. Hiramatsu and Y. Kita,
Tetrahedron Lett., 2005, 46, 2197–2199.
28 K. Omura and D. Swern, Tetrahedron, 1978, 34, 1651–1660.
29 K. Sonogashira, J. Organomet. Chem., 2002, 653, 46–49.
30 R. Huisgen, Proc. Chem. Soc., London, 1961, 357.
31 H. C. Kolb, M. G. Finn and K. B. Sharpless, Angew. Chem., Int. Ed.,
2001, 40, 2004–2021.
4 H. G. Herz, K. D. Kreuer, J. Maier, G. Scharfenberger,
M. F. H. Schuster and W. H. Meyer, Electrochim. Acta, 2003, 48,
2165–2171.
5 Z. Zhou, S. W. Li, Y. L. Zhang, M. L. Liu and W. Li, J. Am. Chem.
Soc., 2005, 127, 10824–10825.
6 M. E. Schuster and W. H. Meyer, Annu. Rev. Mater. Res., 2003, 33,
233–261.
7 K. D. Kreuer, S. J. Paddison, E. Spohr and M. Schuster, Chem. Rev.,
2004, 104, 4637–4678.
8 A. R. West, Basic Solid State Chemistry, Wiley Chichester, West
Sussex, UK, 1988.
32 Z. Zhou, R. Liu, J. H. Wang, S. W. Li, M. L. Liu and J. L. Bredas, J.
Phys. Chem. A, 2006, 110, 2322–2324.
33 C. Nagamani, C. Versek, M. Thorn, M. T. Tuominen and
S. Thayumanavan, J. Polym. Sci., Part A: Polym. Chem., 2010, 48,
1851–1858.
9 D. Basak, S. Christensen, S. K. Surampudi, C. Versek, D. T. Toscano,
M. T. Tuominen, R. C. Hayward and D. Venkataraman, Chem.
Commun., 2011, 47, 5566–5568.
10 C.-F. Chow, V. A. L. Roy, Z. Ye, M. H. W. Lam, C. S. Lee and
K. C. Lau, J. Mater. Chem., 2010, 20, 6245–6249.
11 K. D. Kreuer, Solid State Ionics, 1997, 94, 55–62.
12 J. A. Harvey, D. Basak, D. Venkataraman and S. M. Auerbach, Mol.
Phys., 2012, 110, 957–966.
13 M. Schuster, W. H. Meyer, G. Wegner, H. G. Herz, M. Ise,
K. D. Kreuer and J. Maier, Solid State Ionics, 2001, 145, 85–92.
14 J. C. Persson and P. Jannasch, Chem. Mater., 2003, 15, 3044–3045.
15 R. C. Woudenberg, O. Yavuzeetin, M. T. Tuorninen and
E. B. Coughlin, Solid State Ionics, 2007, 178, 1135–1141.
16 C. A. Alabi, Z. Chen, Y. S. Yan and M. E. Davis, Chem. Mater., 2009,
21, 4645–4652.
34 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li,
H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery, Jr, J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin,
V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari,
A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi,
N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski,
R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth,
P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels,
17 M. Schuster, T. Rager, A. Noda, K. D. Kreuer and J. Maier, Fuel
Cells, 2005, 5, 355–365.
18 S. Granados-Focil, R. C. Woudenberg, O. Yavuzcetin, M. T. Tuominen
and E. B. Coughlin, Macromolecules, 2007, 40, 8708–8713.
19 S. Martwiset, R. C. Woudenberg, S. Granados-Focil, O. Yavuzcetin,
M. T. Tuominen and E. B. Coughlin, Solid State Ionics, 2007, 178,
1398–1403.
€
O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and
D. J. Fox, Gaussian 09, Gaussian, Inc., Wallingford CT, 2009.
This journal is ª The Royal Society of Chemistry 2012
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