Modular synthesis of polymers containing 2,5-di(thiophenyl)-N-arylpyrrole

Article abstract of DOI:10.1002/pola.28990

A modular and facile route has been developed to synthesize functionalized 2,5-di(thiophen-2-yl)-1-H-arylpyrroles from readily available starting materials. These units are compatible with various polymerization conditions and are versatile building blocks for conjugated polymers. The polymers show high thermal stability and solubility in a number of solvents. Characterization of the polymers reveals a correlation between molecular packing, controllable by polymer design, and charge carrier mobility.

Full text of DOI:10.1002/pola.28990

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Modular Synthesis of Polymers Containing 2,5-Di(Thiophenyl)-N-
Arylpyrrole
1
Tran N. B. Truong,1 Suchol Savagatrup,1 Intak Jeon,2 Timothy M. Swager
1Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
2Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Correspondence to: T. M. Swager (E-mail: tswager@mit.edu)
Received 24 January 2018; accepted 19 February 2018; published online 00 Month 2018
DOI: 10.1002/pola.28990
ABSTRACT: A modular and facile route has been developed to
synthesize functionalized 2,5-di(thiophen-2-yl)-1-H-arylpyrroles
from readily available starting materials. These units are com-
patible with various polymerization conditions and are versatile
building blocks for conjugated polymers. The polymers show
high thermal stability and solubility in a number of solvents.
Characterization of the polymers reveals a correlation between
molecular packing, controllable by polymer design, and charge
C
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carrier mobility.
2018 Wiley Periodicals, Inc. J. Polym. Sci.,
Part A: Polym. Chem. 2018, 00, 000–000
KEYWORDS: conjugated polymers; co-polymer; 2,5-di(thiophen-
2-yl)-1-H-arylpyrrole; organic electronics; Paal–Knorr pyrrole
synthesis
regioregular thiophenes.13,14 However, the issue of regioregu-
larity can be avoided with the use of symmetrical
monomers.
INTRODUCTION Conjugated polymer semiconductors afford
many advantages over the inorganic counterparts such as
the ability to be processed through roll-to-roll fabrication,1
robust mechanical properties,2 and tunability through molec-
ular design and synthesis.3 The ability to tune various prop-
erties of conjugated polymers is critical to performance in
specific applications, namely organic photovoltaic (OPV),4
organic light emitting diodes (OLED),5,6 and organic field-
effect transistors (OFET).7 The customization and molecular
engineering can, however, require extensive synthetic
sequences that result in low yields and rely on environmen-
tally unfriendly methods.8,9 Thus there is a strong need for
simple, robust, and scalable routes to synthesize modular
building blocks that can be readily incorporated into a vari-
ety of conjugated polymers.10
We have been interested in 2,5-dithiophenylpyrrole as a core
building block to create an expanded family of polymers.
Our interest is motivated by the fact that previous studies of
2,5-dithiophenylpyrrole-derived materials display similar
conductivity to polymers containing tert-thiophenyl units,
while providing an additional handle for functionalization of
the pyrrole rings at the 3,4 and N-positions.15 N-
functionalization is particularly attractive, as it can be done
in a modular manner and retain the symmetry of the unit
and regioregularity of the polymer, which in turn can influ-
ence polymer packing and performance in devices.16 McLeod
et al. first accessed 2,5-dithiophenylpyrrole derivatives via
the Paal–Knorr pyrrole synthesis using 1,4-dithienyl-buta-
none and methyl amine starting materials.17 This synthetic
route generally involves double condensation reactions,
using a catalytic amount of acid or an acidic solvent, which
are not compatible with some functional groups. Further
efforts have extended this method to attach different func-
tional groups to the polymer backbone, including alkyl,18
phenyl,19–22 and pyridinyl groups.23 Aryl rings can also be
attached to the 2,5-dithiophenylpyrrole unit via a linker.24–26
We are especially interested in the direct incorporation of N-
aryl group, which allows for electronically coupled function-
ality that can introduce organization by increased p-stacking
A common approach to fine-tune the physical properties of
semiconducting polymers is to attach sidechains and func-
tional groups to the polymer backbone. This approach can
be performed via post-polymerization modifications or by
polymerization of a functionalized monomer.11 Alkylated pol-
ythiophenes are among the most popular semiconducting
polymer building blocks; and, in cases with a single alkyl
chain on each repeating unit, regioregularity is a major fac-
tor that affects polymers’ solid-state structure and device
performance.12 Site-selective Grignard metathesis (GRIM)
polymerization or direct arylation polymerization can pro-
vide regioselectivity up to 99.3% in the production of
Additional Supporting Information may be found in the online version of this article.
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or enhance light absorption, both of which are of interest for
the design of active materials in photovoltaic devices.19 Fur-
thermore, the aryl groups incorporated via this route are
capable of bearing solubilizing side chains without affecting
the symmetry of the monomers.25
m/z): [M 1 H]1 calcd. for C54H83Br2NO3S2, 1018.4252; found
1018.4250.
Synthesis of Polymers
General Synthesis of Polymers via Stille Polycondensation
Reaction (P1, P2, P3)
Herein, we explore a modular synthetic scheme to synthesize
2,5-di(thiophen-2-yl)-1-H-arylpyrroles as building blocks for
conjugated polymers. The monomers are produced in modu-
lar and robust synthetic operations: attachment of alkoxyl
side chains onto an aryl ring followed by a modified Paal–
Knorr reaction with a 1,4-diketone. By employing solubilizing
side chains on the monomer, we are able to incorporate dif-
ferent types of co-monomers without the need for additional
solubilizing side chains.
Monomer 5 (1 equiv.), the bis(trimethylstannyl) aryl co-
monomer (1 equiv.), tris(dibenzylideneacetone)dipalla-
dium(0) (0.05 equiv.), and tri(o-tolyl)phosphine (0.2 equiv.)
were dissolved in dry toluene. The mixture was degassed via
3 freeze–pump–thaw cycles and then stirred at 100 8C for
24 h. The reaction was quenched with concentrated potas-
sium fluoride solution. The reaction mixture was added
dropwise into methanol under vigorous stirring. The residue
was collected by filtration and washed with methanol, ace-
tone, and hexanes. More details can be found in the Support-
ing Information.
EXPERIMENTAL
General
Synthesis
of
Polymer
P4
via
Suzuki–Miyaura
The reagent-grade starting materials were obtained from
commercial sources and used without further purification.
For general experimental procedures and instrumentation,
see Supporting Information.
Polycondensation Reaction
Monomer 5 (204 mg, 0.2 mmol, 1 equiv.) and 4,7-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)benzo[c][1,2,5]-thiadiazole
(78 mg, 1 equiv.) were added to a dry Schlenk flask equipped
with a magnetic stir bar. To this mixture, 2 mL of toluene, a
drop of Aliquat 336 and 20 equiv. of Na2CO3 in 2 M aqueous
solution were added. The mixture was then degassed via 3
freeze–pump–thaw cycles, and 4 mol % of Pd2dba3 and 32 mol
% of P(o-tolyl)3 were added against a flow of argon. The mix-
ture was stirred at 105 8C for 24 h. The reaction mixture was
then diluted with chloroform, precipitated in methanol and fil-
tered. The residue was washed with methanol, acetone, and
hexanes to give a purple solid product (70% yield, m.p. 197.3
8C, molecular weight Mn 7 kg/mol (GPC), UV–vis (chloroform):
Synthesis
Synthesis of Monomers
Synthesis
of
3,4,5-Tris(Dodecyloxy)Benzenaminium
Chloride Salt (3)
A suspension of 2 (6.1 g, 9 mmol) and tin(II) chloride dihy-
drate (8.4 g, 45 mmol) in ethanol (450 mL) and concen-
trated hydrochloric acid (180 mL) was refluxed for 16 h.
The reaction mixture was cooled to room temperature and
the white precipitate was collected by filtration, washed with
concentrated hydrochloric acid (3 3 15 mL), and dried in
vacuum to give product 3 as a white solid (6.0 g, 98%). 1H
NMR (400 MHz, CDCl3) d 10.36 (bs, 3H), 6.70 (s, 2H), 3.95
(t, J 5 6.4 Hz, 4H), 3.90 (t, J 5 6.5 Hz, 2H), 1.75 (m, 8H),
1.52–0.97 (m, 52H), 0.88 (t, J 5 6.6 Hz, 9H); 13C NMR (100
MHz, CDCl3) d 154.03, 138.35, 124.57, 101.54, 73.55, 69.41,
31.94, 30.28, 29.75, 29.72, 29.70, 29.68, 29.65, 29.61, 29.43,
29.40, 29.38, 29.26, 26.11, 22.70, 14.12. HRMS (ESI, m/z):
[M 2 Cl]1 calcd. for C42H80NO3, 646.6133; found 646.6123.
kmax 5 606 nm).
Synthesis of Polymer P5 via Nickel-Catalyzed Grignard
Metathesis (GRIM) Polymerization Reaction
iPrMgCl.LiCl (1 equiv.) was added dropwise to a solution of
5 in dry tetrahydrofuran at 0 8C. The solution was stirred at
room temperature for 2 h. Ni(dppe)Cl2 was then added. The
resulting solution was stirred at room temperature for 24 h.
The reaction was quenched with concentrated potassium
fluoride solution. The reaction mixture was added dropwise
to a mixture of methanol and 1 M HCl. The bright orange
oligomer collected is highly soluble in common organic sol-
vents (70% yield, m.p. 149.3 8C, molecular weight 8.6 kg/
mol (GPC), UV–vis (chloroform): kmax 5 473 nm).
Synthesis of 2,5-Bis(5-Bromothiophen-2-yl)-1-(3,4,5-
Tris(Dodecyloxy)Phenyl)-1H-Pyrrole (5)
Compounds 3 (1 mmol) and 4 (1 mmol) were dissolved in
dry pyridine (5 mL) under nitrogen. Trimethylsilyl chloride
(5 mmol) was added. The reaction was run on reflux for
16 h. The solvent was removed in vacuo. The crude mixture
was purified by flash column chromatography with hexanes:
dichloromethane 1:9 as the eluent to give a light yellow solid
(75% yield), m.p. 82.9 8C; 1H NMR (400 MHz, CDCl3) d 6.78
(d, J 5 3.9 Hz, 2H), 6.48 (s, 2H), 6.46 (s, 2H), 6.42 (d, J 5 3.9
Hz, 2H), 4.05 (t, J 5 6.5 Hz, 2H), 3.87 (t, J 5 6.6 Hz, 2H), 1.75
(m, 8H), 1.52–0.97 (m, 52H), 0.88 (t, J 5 6.6 Hz, 9H); 13C
NMR (100 MHz, CDCl3) d 153.45, 139.33, 136.12, 132.28,
129.73, 129.71, 124.07, 110.68, 109.40, 108.63, 73.66, 69.33,
31.96, 31.94, 30.24, 29.78, 29.76, 29.72, 29.70, 29.67, 29.63,
29.41, 29.38, 29.10, 26.13, 25.97, 22.70, 14.13. HRMS (ESI,
RESULTS AND DISCUSSION
Synthesis of Materials
Synthesis of Monomers
Monomers were synthesized in a modular fashion via the
Paal–Knorr pyrrole synthesis as outlined in Scheme 1. The
1,4-diketone component 4 was synthesized according to a
literature procedure.27 Based on modified literature proce-
dures, the aniline component 3 is produced in three simple
steps. First, pyrogallol was alkylated via a nucleophilic sub-
stitution reaction with the respective bromoalkane under
2
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SCHEME 1 Synthesis of monomers.
basic conditions to give 1.28 This method is useful for attach-
ing a variety of side chains to the respective phenol, enabling
customization of the monomer in a modular manner. Com-
pound 1 undergoes facile nitration to afford the nitro com-
pound 2 by stirring with silica gel that has been previously
treated with concentrated nitric acid.29,30 Reduction with tin
(II) chloride under acidic condition gives the anilinium salt
in high yields. The isolation of 3 as a salt is important as the
free base form of this molecule is mildly air-sensitive. These
reduction conditions provide higher yield and more stable
products than those in neutral conditions with hydrogen and
palladium on activated carbon or hydrazine. The need for
reduction under acidic conditions is most pronounced on
substrates with multiple electron-donating groups such as 3.
converts inexpensive starting materials into the desired con-
jugated polymers via a set of modular, robust reactions, and
simple purification steps, mainly via recrystallization. Hence,
this method constitutes a scalable, economical, and environ-
mentally friendly synthesis of conjugated polymers.9
Synthesis of Polymers
Cross-coupling polymerization of monomer 5 was explored
using Stille, Suzuki–Miyaura, and nickel-catalyzed Grignard
metathesis (GRIM) reactions, as shown in Scheme 2. Poly-
mers P1, P2, and P3 were synthesized via Stille polyconden-
sation reaction and displayed molecular weights 13–22 kDa.
Polymers P4 and P5 were synthesized by Suzuki–Miyaura
and Grignard metathesis (GRIM) polymerization reactions,
respectively with somewhat lower molecular weights. This
flexibility in the choice of polymerization method facilitates
the future construction of polymer libraries based on com-
mercially available co-monomers with different functional
groups and chemical compatibilities. The resulting polymers
are soluble in dichloromethane, chloroform, tetrahydrofuran,
and dichlorobenzene. We observed that the polymers tend to
assemble in aggregates at room temperature and break up
upon heating. This feature is reflected in additional shoulder
Condensation of compound 3 (R 5 C12H25) with 4 provides
pyrrole-containing monomer 5. Previously reported examples
of similar Paal–Knorr pyrrole synthesis employ acidic condi-
tions, with a catalytic amount of acid or with an acid as a
co-solvent.15,19,27 An extensive survey by Lacaze and cow-
orkers showed that although an acid is required to activate
the diketone, an excess of acid might lead to formation of
side products and decomposition of the main product.31 We
found that acidic conditions afforded some product with our
alkoxyl anilines; however, the yields of about 20% were
unacceptably low. It is likely that the low yields are the
result of decomposition of the electron-rich aniline substrate
as well as hydrolysis of the ether linkages with the acidic
conditions. To alleviate this problem, we developed a proce-
dure using pyridine as a solvent and trimethylsilyl chloride
as a dehydrating agent. Under this new condition, the anili-
nium salt is activated in situ and affords higher yields in the
condensation reaction, 65–80%. Pyridine also captures the
hydrochloric acid generated and prevents hydrolysis of the
ether linkages.
Synthetic Scheme 1 provides access to 2,5-di(thiophen-2-yl)-
1-H-arylpyrrole with a variety of side chains and function
groups as shown in Figure 1. The milder non-acidic conden-
sation conditions are critical for the derivatives containing
polyethylene glycol as a side chain, which opens up a possi-
bility of synthesizing conducting polymers that are water-
soluble. It is also worth noting that Scheme 1 quickly
FIGURE 1 Chemical structures of 2,5-di(thiophen-2-yl)-1-H-aryl-
pyrrole with a variety of side chains and functional groups.
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SCHEME 2 Polymer synthesis from monomer 5.
peaks in UV–vis absorption spectra (Fig. 2) as well as Gel
Permeation Chromatograms (GPC) (Fig. S16, Supporting
Information). To prevent this aggregation, we measured the
molecular weights of the polymers using a heated GPC col-
umn at 60 8C in chlorobenzene. In these studies, we were
not able to maintain heating at the injection point and
upstream of the columns. It is possible that polymers with
higher molecular weight may have been aggregated and
eliminated upstream of the heated columns, leading to an
underestimation of the bulk molecular weights (Table 1).
energy absorption is caused by aggregation-induced planari-
zation of the polymer backbones and potentially p-stacking
interactions. The shoulder peak is the most prominent in P1,
suggesting that a higher density of alkoxyl groups leads to
an increase in polymer chain aggregation.
From Figure 2, polymer P3 absorbs at a slightly shorter
wavelength than P1 and P2. The integration of thieno[3,2-
b]thiophene to polymer backbones has been known to have
diverse effects on the electronics of the polymers.32,33 In this
case, a possible explanation is that the delocalization of elec-
trons from this fused aromatic unit into the backbone is less
favorable than from a single thiophene ring, due to the
larger resonance stabilization energy of the fused ring over
the single thiophene ring.33 This reduced delocalization
along the backbone results in an increased bandgap and a
more blue-shifted absorption of P3.
Optical Properties
Figure 2 shows the UV–visible absorption spectra of mono-
mer 5 and the polymers containing 5 in chloroform solu-
tions. The polymer absorption peaks are substantially red-
shifted relative to 5 (kmax 5 340 nm). Polymers P1, P2, and
P3 have similar absorption spectra, with absorption peaks
from 526 to 536 nm and a low-energy shoulder peak caused
by aggregation of the polymer chains in solution. This lower
In addition to the p–p* transition peak at around 405 nm, poly-
mer P4 has also a low-energy peak which could be attributed
to the internal charge transfer between the donor and acceptor
units. This peak with the onset at 750 nm corresponds to a low
bandgap of 1.65 eV. These types of low-bandgap polymers are
especially useful in photovoltaic applications.34
Figure 3 shows the comparative absorption spectra of the
polymers in solution versus the thin film. The absorption
spectra of P1, P2, and P3 films were very similar to those of
the corresponding solutions, whereas for polymers P4 and
P5, we observed a more significant red shift in the solid
state. This smaller red shifts for P1–P3 polymer chains may
be related to their greater tendency to aggregate in solution,
possibly as a result of their higher molecular weights. See
Fig. S17 for complete optical characterization data.
Electrochemical Properties
The redox properties of the polymers were studied via
cyclic voltammetry. The measurements were performed on
FIGURE 2 UV–vis absorption spectra of monomer 5 and poly-
mers in chloroform solutions. [Color figure can be viewed at
wileyonlinelibrary.com]
4
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TABLE 1 Polymerization Method, Molecular Weight Distribution, Photophysical, and Electrical Properties of Polymers
Mn (kDa)a/Ð
kmax
(nm)
kmax
(nm)
Egopt (eV)
Eoxc (eV)
HOMOd
LUMOe
(s)b
(f)b
(f)
konset (nm)
b
Polymer
Method
P1
P2
P3
P4
P5
Stille
Stille
Stille
Suzuki
GRIM
22/1.9
13/1.8
15/1.8
7/2.9
530
526
505
606
473
536
529
526
627
530
620
618
615
750
610
2.00
2.00
2.02
1.65
2.03
0.32
0.34
0.26
0.31
0.32
25.12
25.14
25.06
25.11
25.12
23.12
23.14
23.04
23.46
23.09
8.6/1.6
a
c
Molecular weights were measured relative to polystyrene standard in
heated column at 60 8C.
Eox was estimated from cyclic voltammetry of the polymer film.
d
HOMO was calculated according to the formula:
EHOMO5
(s) and (f) refer to solution and film.
b
2ðEox14:8ÞðeVÞ.
e
The optical bandgap Egopt was estimated from the onset of the absorp-
LUMO was calculated as HOMO 1 Egopt
.
tion of thin film.
polymer films with 0.1 M TBAPF6/acetonitrile as the electro-
lyte. The results are summarized in Figure 4. The highest
occupied molecular orbital (HOMO) was calculated based on
the formula HOMO 5 2ð4:81 EoxÞ eV, where Eox was the oxi-
dation potential relative to ferrocene.
other dithiophenylpyrrole polymers.15,17,20 P3 has the lowest
oxidation potential of the polymers investigated, which likely
reflects the presence of electron-rich thieno[3,2-b]thiophene
groups in the backbone.
Thermal Properties
All the polymers showed a reversible oxidation peak at 0.2–
0.34 V relative to ferrocene, corresponding to HOMO levels
of 25.06 to 5.12 eV. The reversibility and Coulombic effi-
ciency of the voltammograms indicate that the polymers are
stable in their oxidized states. The polymers have lower oxi-
dation potential than polythiophene and are comparable to
Thermal gravimetric analysis (TGA) (Fig. 5) indicates that
most of the polymers have high decomposition temperatures
at above 400 8C. The homopolymer P5 has the lowest
decomposition temperature at 300 8C. Differential scanning
calorimetry results (DSC) as represented by P2 (Fig. 6)
revealed that most of the polymers have similar melting
points of about 200 8C (See Fig. S18 for more information).
Polymer P2 showed the most prominent glass transition
peak, followed by P1 and P3. Homopolymer P5 has the low-
est melting point and no observable glass transition peak.
This result suggests that the high density of alkyl chains
reduces crystallinity in the polymers. The high thermal sta-
bility and lower melting temperatures of these materials pre-
sent opportunities to create melt-processed devices,
reducing the need for solvent processing.35
FIGURE 4 Cyclic voltammograms of polymer films dropcast on
ITO-coated glass substrates with 0.1 M n-Bu4PF6 in acetonitrile
as the electrolyte, scan rate 100 mV/s. [Color figure can be
viewed at wileyonlinelibrary.com]
FIGURE 3 Absorption spectra of polymer chloroform solutions
(solid) and spin-coated thin films on glass substrates (dashed).
[Color figure can be viewed at wileyonlinelibrary.com]
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TABLE 2 d-Spacing Distance Obtained from XRD and Hole
Mobility from OFET (N ꢀ 4)
Polymer
d-Spacing (nm)
l (1023cm2V21s21
)
P1
P2
P3
2.68
2.58
2.66
0.021 6 0.002
0.031 6 0.003
0.013 6 0.001
lamellar structure (vide infra). However, we cannot eliminate
the possibility that there may be other interchain correla-
tions with this periodic spacing. The most prominent peaks
are at low angle and are consistent with what is usually
associated with a lamellar periodic pattern driven by side-
chain/main-chain segregation.37 These d-spacing distances
range from 2.6 to 2.7 nm (Table 2). Among these side-
stacking peaks, P2 has the sharpest XRD peak and the short-
est d-spacing distance, followed by P3 and P1. It appears
that the greater spacing between alkyl chains in P2 results
in a more regular packing. This trend is in agreement with
the thermal behaviors of the polymers as seen in the DSC
results. Both the thermal and XRD results suggest that P2
has the highest crystallinity, followed by P3 and P1. The lack
of a melting transition peak and any clear XRD peaks implies
that P5 is amorphous. The trends suggest that the extended
spacing between the adjacent dithiophenylpyrrole units is
necessary for efficient interdigitation of the alkoxyl groups
and stacking of the polymer chains.
FIGURE 5 Thermogravimetric analysis (TGA) results of poly-
mers, from 50 8C to 700 8C, scan rate 10 8C/min. [Color figure
can be viewed at wileyonlinelibrary.com]
X-Ray Diffraction Studies
The solid-state organization of the polymers was investigated
by X-ray diffraction (XRD). The polymers were dropcast onto
silicon substrates and annealed above their glass transition
temperatures (150 8C) to produce XRD diffraction peaks
with the highest intensity.
The XRD diffractograms (Fig. S19, Supporting Information)
give diffraction patterns with three peaks, suggestive of a
lamellar structure, similar to that of poly(3-alkylthiophene)s.
All the polymers share a common peak with 2h 5 208, corre-
sponding to a d-spacing of 4.5 Å, as commonly observed for
p–p stacking of aromatic polymer backbones.36 These peaks
are relatively broad and hence the regularity of the lateral
interchain associations appears to be less than usually
observed. In addition, we observe considerable diffuse scat-
tering from the disordered alkyl sidechains that are con-
volved with these signals. We also observed small peaks
centered about 1.2 nm, which in some cases are at the
proper location to be a second-order diffraction from the
Charge Mobility in Organic Field-Effect Transistors
Charge-transport properties of conjugated polymers are gen-
erally highly dependent on the degree of interchain interac-
tion in polymer thin films. Hole mobilities have been
correlated with the strength of aggregation as observed spec-
troscopically or by X-ray diffraction. To compare the hole
mobilities of the polymer in this study, we fabricated
bottom-gate, bottom-contact thin-film transistors (Fig. S20,
Supporting Information).
The hole mobilities of the polymer films are summarized in
Table 2. Refer to Figures S21-S23 for more details. The mobi-
lities of these polymers are lower than that of polythiophene,
but comparable to other polymers containing 2,5-di(thio-
phen-2-yl)-1-H-arylpyrrole.15,38 The trends in the mobilities
are consistent with the XRD data. Polymers with shorter d-
spacing and more pronounced diffraction peaks afforded
higher mobility. We attributed this effect to the stronger
interchain interactions as a result of better interdigitation
due to the alkoxyl chains.11 In future studies, it is possible
that shorter alkoxyl chains will further improve charge
mobility while maintaining polymer solubility.39
CONCLUSIONS
We have developed a facile, modular, and convergent synthetic
route to synthesize 2,5-di(thiophen-2-yl)-1-H-arylpyrroles
from simple starting materials and robust reactions. The
monomers were compatible with different polymerization
FIGURE 6 Representative differential scanning calorimetry
(DSC) thermogram for P2, from 25 8C to 350 8C, scan rate 10
8C/min.
6
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ACKNOWLEDGMENTS
Electrochim. Acta 2013, 90, 295.
This work was sponsored by Eni S.p.A. as part of the efforts of the
MIT-Eni Solar Frontiers Alliance. T.N.B.T acknowledges the sup-
port of the Agency of Science, Technology and Research (A*STAR),
Singapore for the graduate fellowship. S.S. was supported by an
F32 Ruth L. Kirschstein National Research Service Award.
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