Azulene methacrylate polymers: Synthesis, electronic properties, and solar cell fabrication

Article abstract of DOI:10.1021/ja504670k

We report the synthesis of novel azulene-substituted methacrylate polymers by free radical polymerization, in which the azulene moieties represent hydrophobic dipoles strung pendant to the polymer backbone and impart unique electronic properties to the polymers. Tunable optoelectronic properties were realized by adjusting the azulene density, ranging from homopolymers (having one azulene group per repeat unit) to copolymers in which the azulene density was diluted with other pendant groups. Treating these polymers with organic acids revealed optical and excitonic behavior that depended critically on the azulene density along the polymer chain. Copolymers of azulene with zwitterionic methacrylates proved useful as cathode modification layers in bulk-heterojunction solar cells, where the relative azulene content affected the device metrics and the power conversion efficiency reached 7.9%.

Full text of DOI:10.1021/ja504670k

Article
pubs.acs.org/JACS
Azulene Methacrylate Polymers: Synthesis, Electronic Properties, and
Solar Cell Fabrication
†
†
†
‡
†
Egle Puodziukynaite, Hsin-Wei Wang, Jimmy Lawrence, Adam J. Wise, Thomas P. Russell,
‡
,†
Michael D. Barnes, and Todd Emrick*
†
‡
Polymer Science and Engineering Department and Department of Chemistry, University of Massachusetts Amherst, 120 Governors
Drive, Amherst, Massachusetts 01003, United States
*
S Supporting Information
ABSTRACT: We report the synthesis of novel azulene-
substituted methacrylate polymers by free radical polymerization,
in which the azulene moieties represent hydrophobic dipoles
strung pendant to the polymer backbone and impart unique
electronic properties to the polymers. Tunable optoelectronic
properties were realized by adjusting the azulene density, ranging
from homopolymers (having one azulene group per repeat unit) to
copolymers in which the azulene density was diluted with other
pendant groups. Treating these polymers with organic acids
revealed optical and excitonic behavior that depended critically on
the azulene density along the polymer chain. Copolymers of
azulene with zwitterionic methacrylates proved useful as cathode
modification layers in bulk-heterojunction solar cells, where the relative azulene content affected the device metrics and the
power conversion efficiency reached 7.9%.
1
. INTRODUCTION
well-defined azulene density. Okawara and co-workers reported
an attempted polymerization of azulene-substituted acrylates, in
which azulene was thought to be responsible for inhibiting
homopolymerization and retarding copolymerization with
Developments in organic electronics are enabled by novel
materials, architectures, and interfacial engineering
1
,2
and
benefit from multicomponent polymers that integrate electroni-
cally active structures into otherwise insulating backbones.
Azulene, a vibrant-blue 10-π-electron isomer of naphthalene,
gains stabilization from resonance contributions of cyclo-
pentadienyl anion and tropylium cation; this “donor−acceptor”
resonance structure affords a dipole moment (>1 D) that
distinguishes azulene from conventional fused benzenoids.
The optoelectronic characteristics of azulene can be manipu-
lated by (reversible) protonation with strong acids, altering its
absorption profile and leading to S excited state emission.
Because of their unique optical and electronic properties,
azulene-based structures are useful in charge-transport,
nonlinear optics,
18
styrene. Our polymerization of azulene-substituted meth-
acrylates hinged on a new monomer design and thus did not
suffer from such complications, providing straightforward
access to both homopolymers and copolymers with tunable
azulene density along the backbone. Adjusting the azulene
density with interspersed polar or apolar comonomers dictated
the photophysical properties, electronic interactions, and
exciton migration between pendant units. While the azulene-
based polymers we describe may have numerous applications in
electronic materials, we chose to examine their role as
interlayers in bulk-heterojunction solar cells, specifically as
cathode modification layers intended to improve device
efficiency.
3
−6
7
−10
1
1
1,12
1
3,14
15
and sensor applications. The combina-
tion of their dipolar nature, high polarizability, and ion/metal
complexation capability
16,17
renders azulene derivatives attrac-
2
. RESULTS AND DISCUSSION
.1. Synthesis. Azulene-based methacrylate homopolymers
tive candidates for functional composite architectures and self-
assembled structures. However, the relative lack of azulene-
containing polymer materials limits their availability in
numerous materials applications.
To date, the preparation of azulene-based polymers has
centered on conjugated structures with azulene as the repeat
unit, in which the polymer backbone extends through the five-
or seven-membered ring, as seen for example in the reports of
2
as well as copolymers with methyl methacrylate and
sulfobetaine methacrylate were synthesized from the novel
azulene methacrylates as shown in Figures 1 and 2. 2-
Hydroxyazulene (4), the common starting material for
monomers 5 and 10, was synthesized by condensation of 2-
chlorotropone (2) with diethyl acetone-1,3-dicarboxylate
8
12
Hawker and Lai. To our knowledge, reports on chain-growth
polymers with pendant azulene groups are lacking, despite the
potential utility of such structures as processable materials with
Received: May 13, 2014
©
XXXX American Chemical Society
A
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Figure 1. Preparation of azulene methacrylates 5 and 10.
Figure 2. Homo- and copolymerization of azulene methacrylates 5 and 10 and representative gel-permeation chromatography (GPC) traces (far
right).
9
followed by decarboxylation. Azulen-2-yl methacrylate (5) was
isolated as a deep-purple solid in 75% yield by the reaction of 4
with methacryloyl chloride in the presence of triethylamine and
butylated hydroxytoluene (BHT) as a free radical scavenger. 2-
Bromoazulene (6), our chosen precursor to monomer 10, was
obtained in 73% yield by the reaction of 4 with PBr39.
Sonogashira coupling of 6 with trimethylsilylacetylene gave 7,
and subsequent in situ TMS deprotection and copper-catalyzed
azide−alkyne cycloaddition with 3-azido-1-propanol (note
safety comment in Supporting Information) afforded triazole
Table 1. GPC-Estimated M and PDI Values of Azulene-
Containing Polymers with Variable Mole Percentages of 5
and 10
n
azulene (mol %)
a
polymer
PA
feed
incorporated
M (kDa)
PDI
n
100
14
25
25
50
100
14
25
25
50
75
100
22
40
25
45
100
19
36
24
49
76
13.5
17.2
22.0
33.5
18.9
13.6
20.5
13.9
38.9
28.4
25.4
2.5
1.8
2.0
1.5
1.6
2.2
1.7
2.6
1.7
1.7
1.7
PAMMA-1
PAMMA-2
PASB-1
9
1
. The reaction of 9 with methacryloyl chloride gave monomer
0 as a blue solid. Each monomer synthesis proved to be easily
PASB-2
PAT
scalable to gram quantities.
PATMMA-1
PATMMA-2
PATSB-1
PATSB-2
PATSB-3
Azulene methacrylates 5 and 10 were amenable to free
radical polymerization using azobis(isobutyronitrile) (AIBN)
initiation, affording the respective polymers in 73 and 82% yield
(
Figure 2 and Table 1). These azulene (PA) and azulene-
a
1
triazole (PAT) methacrylate homopolymers with molecular
weight (MW) values of <15 kDa had sufficient solubility for
solution characterization. Copolymerization of azulenes 5 and
As determined by H NMR spectroscopy.
(PDIs) of 1.7−2.6. Copolymerization of the azulene monomers
with sulfobetaine (SB) methacrylate afforded polymers with Mn
values of 19−39 kDa and PDIs of 1.5−1.7. While incorporation
of monomer 5 into the SB-based copolymers was limited by the
solubility of the resulting polymers, PATSB copolymers with
high azulene content (>70 mol %) retained favorable solubility,
leading to their facile implementation into devices (described
1
0 with methyl methacrylate (MMA) gave polymers with a
tunable average interchromophore distance and good solubility
in organic solvents for characterization. PAMMA and
PATMMA copolymers containing 20−40 mol % azulene
were obtained with number-average molecular weight (M )
n
values ranging from 14 to 22 kDa and polydispersity indices
B
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1
1
21
later). All of the polymers in the PA and PAT series were
obtained as purple or blue solids, respectively. For PA, the
350 nm corresponds to an S ( A ) → S ( A ) transition.
0 1 2 1
These visible and UV absorptions represent transitions
protons of the pendant azulene units were identified as broad
polarized perpendicular and parallel to the C axis, respectively,
2
1
1
1
H NMR (CDCl ) signals at 7.88 (4- and 8-positions), 7.49 (6-
and the very small oscillator strength of the S ( A ) → S ( B )
0 1 1 1
21
3
transition is due to small differences in the electron densities
of the two states (particularly between C9 and C10). Thus,
excitation with visible wavelengths lead to primarily non-
position), and 7.06 ppm (1-, 3-, 5-, and 7-positions). For PAT,
the azulene proton signals appeared at 8.01 (4- and 8-
positions), 7.51 (1- and 3-positions), 7.36 (6-position), and
2
3,24
radiative decay,
and only weak photoluminescence (PL)
6.97 ppm (5 and 7-positions), whereas the proton resonance of
25
originating from the S state (in violation of Kasha’s rule ) is
observed for neutral azulenes (Figure S2 in the SI).
the triazole ring appeared at 7.85 ppm. Polymerizations were
conducted in anisole, freshly distilled 2,2,2-trifluoroethanol
2
2
6,27
Treatment with acid (such as trifluoroacetic acid, TFA)
(TFE), or mixtures of the two, and maintaining solubility
+
protonates azulene at C1 or C3, forming [Az + H ] and
during the polymerization was crucial to achieving high
monomer conversion. In addition, two azulene acetates, A-M
and AT-M (Figure 3), were prepared as model systems
generating a new absorption with a transition energy
intermediate between those of the Az S → S and S → S
0
1
0
2
28,29
bands, appearing at 355 nm (Figure S3 in the SI).
Figure 5
[
Scheme S1; detailed syntheses are provided in the Supporting
Information (SI)].
Figure 3. Azulene model compounds A-M and AT-M.
Azulene exhibits a ground-state dipole moment (μ) of 1.08
3
D. To determine the impact of substituents, the model
compounds A-M and AT-M were examined by density
functional theory calculations at the B3LYP/6-311G+(d,p)//
B3LYP/6-311G+(d,p) level [μcalc(azulene) = 1.08 D]. A-M and
AT-M were calculated to possess larger dipole moments than
azulene itself (5.8 and 4.0 D, respectively), suggesting a
substantive effect of these substituents on the electronic
properties of azulene resulting from extension of the structure
11,19
through the 2-position (along the dipole).
The calculated
molecular orbital (MO) distributions for A-M and AT-M
further indicate a direct effect of 2-substitution on the lowest
4
,5,20
unoccupied MO (LUMO) level (Figure S1 in the SI).
.2. Optical Properties. The photophysical features
Figure 5. UV−vis absorption spectra of dilute chloroform solutions of
(A) A-M and (B) PA upon addition of TFA. Gray lines indicate the
evolution (growth) of the absorption intensity with continuous
addition of TFA.
2
+
reported for azulene (Az) and azulinium cation [Az + H ]
2
1,22
are unique among fused aromatic hydrocarbons
and useful
for comparison to the polymer structures and model
compounds we have prepared. Figure 4 shows solution
absorption spectra of azulene, A-M, and AT-M. The visible
shows absorption spectra of A-M, PA, before and after
treatment with TFA. For A-M, the absorption associated with
1
1
absorption (∼575 nm) corresponds to an S ( A ) → S ( B )
0
1
1
1
+
electronic transition, while the much stronger absorption near
[Az + H ] dominates the spectrum at high TFA concentration.
+
For PA, the same [Az + H ] absorption (slightly red-shifted) is
observed as well as a new strong absorption near 600 nm. This
feature is also seen for PAMMA-1 and PAMMA-2 solutions as
well as thin A-M films treated with TFA vapor (Figures S4 and
S5 in the SI). Such differences between the solution absorption
spectra of the model compound and the polymers may arise
from complexation with TFA, σ-dimer formation, or a Stark
1
2,30−32
effect.
The latter hypothesis is supported by the close
proximity of this absorption to the neutral Az S → S band
0
1
(
and the associated 0.13 eV red-shift), suggesting a
perturbation of the Az absorption signature. In the PA
polymer, a neutral Az moiety situated near (<1 nm) a
protonated pendant Az group may experience a strong Stark
effect from the direct-current (DC) field generated by the
1
localized proton. The resultant mixing of the B excited state
1
with states of different (b-type) symmetry, depending on the
precise orientation of the two adjacent Az moieties, would
enhance the S → S transition moment for the neutral Az,
Figure 4. Normalized solution absorption spectra of azulene, A-M, and
1
1
AT-M in chloroform. The inset shows the weak S ( A ) → S ( B )
0
1
1
1
absorption normalized for clarity.
0
1
C
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giving rise to the strong absorption in the visible spectral
region.
We were intrigued to note that the spectral properties of AT-
M and PAT evolve differently with addition of TFA. Figure 6
Figure 7. (A) Normalized emission spectra of azulene, A-M, and AT-
M in their protonated states (S → S ) as dilute chloroform solutions.
1
0
(
B) Normalized PL spectra of AT-M and PAT (S → S ) as dilute
Figure 6. UV−vis absorption spectra of a dilute chloroform solution of
PAT upon addition of TFA, demonstrating sequential protonation of
the triazole (red curves to cyan curves in A and B) and azulene (cyan
curve to black curve in B) moieties. Gray lines in (B) indicate the
growth in absorption intensity with addition of TFA. “1” denotes the
shift due to triazole protonation, and “2” denotes changes in
absorption intensity with azulene doping.
1 0
chloroform solutions with equal absorption intensities at the excitation
wavelength, indicating that the azulinium luminescence is significantly
quenched in the PAT system.
and Figure S6 in the SI show representative absorption spectra
of PAT and AT-M, respectively, with increasing TFA
concentration. Two distinct spectral features were observed:
(1) a rapid blue shift in the near-UV absorption, attributed to
protonation of the triazole ring and an alteration of the
aromaticity in the triazole, and (2) a slower growth in the
azulene absorption intensity near 400 nm. The red shift of the
azulinium absorption of PAT relative to those of A-M and PA
is attributed to the increased conjugation length. The absence
of the strong absorption at 600 nm in the protonated PAT
system is noteworthy and can be rationalized by considering
the weaker electronic interactions between the charged
(
protonated) triazole and neighboring azulenes due to their
larger separation distance.
Photoluminescence quenching was used to probe the
electronic interactions between protonated and neutral pendant
azulene moieties in the PA and PAT polymer families. Because
of the low oscillator strength of the S → S transition in
0
1
neutral azulene, the neutral system is essentially nonfluorescent
i.e., nonradiative decay dominates). On the other hand, visible
(
PL from the azulinium cation is readily observed. Figure 7A
shows PL spectra of azulene, A-M, and AT-M upon addition of
TFA, excited near the azulinium absorption maximum. Small
shifts in the peak PL wavelength for the different species were
observed: both the A-M and AT-M PL spectra were red-shifted
with respect to that of (pure) azulinium, while the peak of AT-
M was red-shifted by 50 nm (250 meV) with respect to A-M,
consistent with the extended conjugation length of protonated
AT-M.
Figure 8. Time-resolved PL spectra and the respective exponential fits
of (A) A-M, PA, and PAMMA-2 and (B) AT-M, PAT, and
PATMMA-1 in dilute chloroform solutions upon the addition of TFA.
Interestingly, while the PL decay of the PA family is
characterized by a single-exponential decay of azulinium, the
PAT family shows biexponential decay with a fast-decay time
constant of 150 ps for PAT and 260 ps for PATMMA-1 (Table
S1 in the SI). The slow-decay time constant (∼2 ns) was
identified as that for the normal azulinium cation decay.
Comparison of the PL spectra of AT-M and PAT under
conditions of roughly equal absorbance shows that the PL in
protonated PAT is strongly quenched (Figure 7B), and thus,
the fast PL decay in the time-resolved measurements reflects
the quenching rate rather than a new radiative process.
Time-resolved photoluminescence of the azulene monomers
and polymers shows evidence of polymer-specific chromophore
interactions. Figure 8A shows PL decay profiles obtained from
A-M, PA, and PAMMA-2 upon addition of TFA and excited in
the azulinium band at 405 nm (20 ps pulse width; 40 MHz
repetition rate). The protonated molecules show similar single-
exponential decays with time constants of ∼2.2 ns. Figure 8B
shows the PL decay profiles of AT-M, PAT, and PATMMA-1.
D
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A plausible mechanism for the PL quenching observed for
the PAT family is energy transfer from protonated pendant
azulene triazole moieties to neighboring neutral (or singly
protonated) species. Figure 9 shows the scaled azulinium
onto the PTB7:PC BM active layer, which was prepared on an
71
ITO/PEDOT:PSS anode, followed by thermal evaporation of a
Ag cathode. Control devices featuring poly(sulfobetaine
methacrylate) (PSBMA) interlayers as well as those containing
bare silver electrodes were also constructed and characterized.
Key characteristics of the fabricated solar cells are summarized
in Figures 10 and 11.
Figure 9. (Left) Emission (blue) and S → S absorption (black)
0
1
spectra of triazole azulinium and neutral azulene species (for AT-M).
The integrated area between the two (shaded) defines the Forster
̈
radius for energy transfer (R ). (Right) Schematic illustration of
0
Figure 10. J−V characteristics of solar cells with a bare Ag cathode and
with polymers interlayers containing azulene and sulfobetaine.
energy transfer from triazole azulinium to neutral azulene with
associated time constants (for PAT).
(
donor) PL spectrum and the unperturbed S → S absorption
0
1
for neutral azulene of AT-M. We computed a Fo
̈
rster radius of
1
.5 nm for energy transfer between neighboring azulinium and
6
azulene moieties (see the SI). From the relation τ ≈ τ (r/R ) ,
0
0
̈
where R is the Forster radius, r is the interchromophore
0
separation, and τ is the azulinium natural radiative lifetime, we
0
compute a quenching lifetime (inverse rate) of 190 ps at an r
value of 1 nm. Thus, the observed fast-decay component (150
ps) in the PAT photoluminescence is consistent with an energy
transfer mechanism. To explain why energy transfer occurs in
PAT and not PA, we suggest that the 130 meV red shift
observed for the S0 → S1 transition (possibly a Stark
̈
perturbation) in the protonated PA system reduces the Forster
radius and effectively turns off the energy transfer decay
pathway. Moreover, we speculate that the probability of finding
an energy transfer partner (i.e., a neutral azulene) is greater in
the PAT family because of the presence of the triazole moiety.
We note that such unique aspects found for these azulene-
based polymers are not observed for naphthalene-based
systems, which exhibit S → S emission, excimer formation,
1
0
5
,33,34
and no fluorescence switching in the presence of acid.
.3. Solar Cells. We examined whether the integration of
2
azulene into polymers would have any ramifications for
conducting materials and/or devices. Polyelectrolytes and
polymer zwitterions can modify the work function (WF) of
metals, affording composite electrodes that represent a route to
improved power conversion efficiency (PCE) in solar
Figure 11. Characteristics of solar cells with a bare Ag cathode and
with cathode modification layers containing azulene and sulfobetaine.
1
,2,35,36
cells.
Polymer zwitterions represent particularly attrac-
Device performance generally increased with azulene density
in the interlayer polymers. The average PCE values for bare Ag,
PSBMA, PATSB-1, and PATSB-2 were 2.4%, 3.7%, 5.1%, and
7.2%, respectively. Notably, the PATSB-3 copolymers, having
highest azulene content (∼75 mol % azulene), gave the highest
efficiencies: the average PCE was 7.6%, and the best-performing
device reached a PCE of 7.9%. PASB-1 and PASB-2 polymers
were also found to be effective as interlayers, enabling organic
photovoltaic (OPV) device efficiencies of 6.0% and 7.1%,
respectively (Table S2, Figure S7, SI). As shown in Figure 11,
the increase in efficiency was mainly due to a higher open-
tive platforms because of their orthogonal solubility with active-
layer polymers and the absence of counterions that can degrade
device performance. PA- and PAT-based copolymers with
sulfobetaine methacrylate having two types of dipoles, one
hydrophobic (azulene) and one hydrophilic (SB), were thus
examined in devices having an active layer of PTB7 and
PC BM, with Ag chosen as the cathode for its excellent
7
1
stability and amenability to WF modification. In a typical solar
cell architecture, thin (≤5 nm) zwitterionic azulene meth-
acrylate copolymer interlayers were deposited by spin-coating
E
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Journal of the American Chemical Society
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circuit voltage (V ), with smaller contributions from the short-
effective as cathode modification layers in OPV devices, giving
PCE values approaching 8%.
oc
circuit current density (J ) and the fill factor (FF). It is
sc
noteworthy that azulene-functionalized sulfobetaine methacry-
lates, when used as interlayers, were found to exhibit
comparable or superior performance to the respective
ASSOCIATED CONTENT
Supporting Information
Experimental procedures and additional synthesis, photo-
physical, and device characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
*
S
3
6,37
conjugated-polymer-based structures.
Additionally, devices
fabricated with these interlayers outperformed the commonly
used LiF cathode modifier while offering the additional benefits
of air stability and compatibility with solution and roll-to-roll
3
6
processing. Voc enhancements in devices with cathode
modifiers may arise from the respective WF reduction at the
electrode, creating a built-in potential gradient across the active
layer between the electrodes. In addition, such electrode
modifications allow for increases in Jsc and FF via charge
extraction control and recombination reduction. To gain
additional insight into the performance mechanism of
zwitterionic azulene-containing polymers, thin films of the
respective polymers on Ag substrates were studied by
ultraviolet photoelectron spectroscopy (UPS). UPS was used
to measure the WF of the substrates, revealing an interfacial
dipole as a result of the polymer coatings (Figure 12).
AUTHOR INFORMATION
Corresponding Author
tsemrick@mail.pse.umass.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge support from the National Science
Foundation (NSF-CHE-1152360, T.E.) associated with poly-
mer synthesis and the Department of Energy-supported Energy
Frontier Research Center (EFRC) at UMass Amherst
(
H.W.W., T.P.R.) for solar cell fabrication and characterization.
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SUMMARY
́
̧
ziorek, M.; Jurczak, J. Chem.Eur. J. 2008, 14,
■
A new functional polymer platform was realized through the
preparation of azulene-substituted methacrylate monomers and
their successful use in chain-growth polymerization. Controlling
the density of pendant azulene groups along the polymer
backbone dictated the optical properties and excitonic
interactions upon acid doping, distinguishing this polymer
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