Proton conduction in discotic mesogens

Article abstract of DOI:10.1039/c1cc10509d

In this communication, we show that liquid crystalline phases lower the activation energy barrier for proton transport. The liquid crystalline phases were obtained using a triphenylene core with alkyl chains bearing a triazole moiety at their termini.

Full text of DOI:10.1039/c1cc10509d

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COMMUNICATION
Proton conduction in discotic mesogensw
Dipankar Basak,a Scott Christensen,b Sravan K. Surampudi,a Craig Versek,c
Daniel T. Toscano,a Mark T. Tuominen,c Ryan C. Haywardb and D. Venkataraman*a
Received 25th January 2011, Accepted 23rd March 2011
DOI: 10.1039/c1cc10509d
In this communication, we show that liquid crystalline phases
lower the activation energy barrier for proton transport. The
liquid crystalline phases were obtained using a triphenylene core
with alkyl chains bearing a triazole moiety at their termini.
triphenylene aromatic cores may provide structural alignment
of the nitrogen heterocycles necessary for proton conduction.
Furthermore, the alkyl spacer may provide enough flexibility
for molecular reorientation. Therefore, we hypothesized that
the activation energy barrier for proton transport may be
lower in the LC phase compared to the liquid or crystalline
phases.
Recently, there is widespread interest in developing molecules
and macromolecules with high proton conductivities at
moderate temperatures (120 1C) under low humidity conditions
for proton exchange membranes in fuel cells.1–4 A common
approach that has been pursued, albeit with limited success,
is to design polymers with pendant nitrogen-containing
heterocycles that can mimic the thin film nanoscale phase
morphology of Nafions, which is the active component in
many commercial proton exchange membranes.5–9 Nafion
consists of hydrophobic (tetrafluoroethylene) and hydrophilic
(sulfonic acid) groups which phase segregate to result in proton
conducting channels with occluded water molecules. For
efficient anhydrous proton transport, it is imperative that the
proton conducting groups are organized into supramolecular
structures that provide a continuous path for the proton.10
Moreover, the proton conducting groups should have sufficient
flexibility for molecular reorientation to receive subsequent
protons.11–14 Based on earlier reports on ion transport,15–17
we believe that liquid crystalline phases may offer a balance
between the required supramolecular organization of the
proton-conducting groups and the flexibility for molecular
reorientation, thus providing a viable platform for developing
efficient proton transporting materials. Herein we report the
synthesis, structure and performance of liquid crystalline (LC)
proton conductors based on a triphenylene core with alkyl
chains, whose termini are covalently attached to the triazole
moiety.
2,3,6,7,10,11-Hexahydroxytriphenylene (3) was synthesized
in two steps starting from veratrol using well-established
literature procedures.21,22 Alkyl chains of varying length with
a terminal alkyne group were attached to 3 via Williamson’s
ether synthesis. The terminal alkyne groups were then converted
to pivolyloxymethyl protected 1,2,3-triazole moieties through
the azide–alkyne-Huisgen dipolar cycloaddition reaction.23,24
The deprotection of the pivaloyloxymethyl group was
accomplished using NaOH to furnish 9a–9c as sticky solids
in 62–69% yield (Scheme 1).25, 28
Thermogravimetric analysis (TGA) of the compounds
9a–9c showed no mass loss up to 300 1C, indicating thermal
stability of the triazole-functionalized triphenylene moieties
(Fig. S1, ESIw). The mesomorphic behavior of 9a–9c were
studied by polarized optical microscopy (POM), differential
scanning calorimetry (DSC), and small and wide angle X-ray
scattering. On the first heating cycle, each sample showed a
broad endotherm in DSC, corresponding to a strong first
order phase transition. This transition was irreversible for 9a
(64.3 1C, Fig. S2, ESIw) and 9b (60.4 1C, Fig. S3, ESIw). It was
quasi-reversible in 9c (68 1C, DHmelt = 44 kJ molÀ1; Fig. 1).
POM studies did not indicate any mesomorphic behavior or
recrystallization for 9a and 9b, which is consistent with
the DSC and X-ray scattering data. However, 9c exhibits
birefringence over a wide temperature range (25 1C to 140 1C),
with needle-like birefringence between 75 1C and 140 1C
(Fig. 2). Cooling of 9c below 70 1C yielded crystallites in
POM, indicating crystallization. However, no isotropic phase
was observed in this temperature range. The persistence of a
LC phase for such a wide temperature range for 9c is similar to
that observed for hexacarboxamidohexaazatriphenylene.19
Wide angle X-ray scattering of 9c at 25 1C exhibits peaks at
q values O3 and O4 times the primary peak q*, suggesting a
hexagonal arrangement of columns in the crystalline state
(Fig. 3). The characteristic spacing was found to be 23.5 A,
consistent with columnar packing of the triphenylene unit with
partially interdigitated side chains. Upon heating to 100 1C,
beyond the phase transition temperature, the primary peak at
It is well known that triphenylene-based molecules self-
assemble into structures that exhibit discotic LC phases.18–20
We reasoned that in the LC phase, the registry between the
a Department of Chemistry, University of Massachusetts Amherst,
710 N. Pleasant Street, Amherst, MA 01003, USA.
E-mail: dv@chem.umass.edu; Fax: +1 413 545 4490;
Tel: +1 413 545 2028
b Department of Polymer Science and Engineering,
University of Massachusetts Amherst, MA 01003, USA
c Department of Physics, University of Massachusetts Amherst,
MA 01003, USA
w Electronic supplementary information (ESI) available: Synthesis
and characterization of all the molecules, TGA, DSC, and powder
XRD data for 9a–9c. See DOI: 10.1039/c1cc10509d
c
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Fig. 3 (left) A schematic representation of the speculated columnar
arrangement of 9c in the liquid crystalline phase and (right) WAXS
data for 9c showing the crystalline phase at 25 1C, liquid crystalline
phase at 100 1C and the recovery of the crystalline phase at 50 1C.
The y-axis has been shifted for clarity.
formation of a liquid crystalline phase, although the absence
of higher order peaks in the X-ray scattering pattern suggests
it is poorly organized. An additional peak at 2.9 A also
sharpened upon heating; while the origin of this peak is
unknown, we note it is at too short of a distance to represent
p–p stacking. Upon cooling, the original peaks reappeared
indicating the reemergence of the initial morphology. Based on
DSC, POM and X-ray scattering data we conclude that 9c
exists in the crystalline phase below 68 1C and in the LC phase
above the melting point and until a temperature of at least
140 1C. Given the birefringence observed in the LC phase, the
indication of hexagonal packing in the crystalline phase and
prior literature,19 we speculate that there may be a columnar
arrangement of 9c in the LC phase. Samples 9a and 9b, upon
heating also showed a single peak around 20 A, similar to that
seen in 9c at elevated temperatures. Thus all three samples
exhibit some degree of mesomorphic organization, but the
ordering is too poorly defined in 9a and 9b for birefringence to
be observed in POM.
Scheme 1 Synthesis of the triphenylene-based molecules 9a–9c.
The proton conductivities of 9a–9c were obtained by AC
impedance spectroscopy.25–27 Vacuum dried samples were
placed into a cavity (length: 0.1 cm, cross-sectional area:
0.079 cm2) between two custom brass electrodes, forming
liquid seals at the ends of a PTFE tube. This setup was
placed in a chamber equipped with a variable temperature
controller. Conductivities were obtained after two heating
and cooling cycles (25–140 1C at a rate of 1 1C minÀ1) to
ensure homogeneity of the sample. In general, 9c showed
higher conductivity compared to 9a and 9b over the entire
temperature range (Fig. 4). The conductivity of 9c at 140 1C
was 1 Â 10À5 S cmÀ1 which was two orders of magnitude
higher than the conductivity at 25 1C (6 Â 10À7 S cmÀ1).
A plot of log(s) vs. 1/T of 9c showed three distinct features: a
linear increase in conductivity up to 68 1C, a sharp change in
slope from 68–78 1C, and a linear increase in conductivity
beyond 78 1C. DSC data also showed a phase transition at
68 1C. Therefore, the discontinuity in the slope from 68–78 1C
can be attributed to structural reorganization due to melting and
recrystallization. The conductivity before 68 1C emanated from
the crystalline phase and the conductivity after phase transition
emanated from the LC phase. The activation energy barrier for
proton conduction was calculated to be 46 kJ molÀ1 in the
crystalline phase and 11 kJ molÀ1 in the LC phase.
Fig. 1 Differential scanning calorimetric trace for compound 9c from
the first cooling/second heating cycle. The heating and cooling rates
were 10 1C minÀ1 and 5 1C minÀ1, respectively.
Fig. 2 Polarized optical microscopy images of 9c showing (a) crystal-
lization at 70 1C and (b) the persistence of birefringence at 85 1C even
after 8 h.
q = 2.67 nmÀ1 shifted to q = 3.14 nmÀ1 (corresponding to
a spacing of 20.0 A) and broadened. Together with the
birefringence observed in POM, this clearly indicates the
c
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Fig. 4 Comparison of temperature dependent proton conductivity of
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68 1C indicating a change in the activation energy. This temperature
also corresponds to a crystalline–LC phase transition. Molecules 9a
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We have shown that the LC phase can lower the activation
energy barrier for proton conduction. We believe that LC
materials are viable platforms for creating next generation
PEMs. Further studies to attach different proton transporting
functional groups and compare their proton conductivities are
currently under investigation.
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This work was primarily supported by Fueling the Future
Center for Chemical Innovation (CHE 0739227), sponsored
by National Science Foundation. This material is also based
upon 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 Foundation
Materials Research and Science Center on Polymers (DMR
0820506).
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5568 Chem. Commun., 2011, 47, 5566–5568
This journal is The Royal Society of Chemistry 2011