Pyromellitic diimide-based donor-acceptor poly(phenylene ethynylene)s

Article abstract of DOI:10.1021/ma2009063

A series of donor-acceptor poly(phenylene ethynylene)s (PPEs) with N,N′-dialkylpyromellitic diimide (PMDI) as electron acceptor and various donor units are reported. Optoelectronic properties were investigated by UV/vis absorption and electrochemical measurements, revealing constant LUMO energies (～-3.6 eV) across the series with HOMO energy levels governed by the donor monomers and optical band gaps from 2.30 to 1.50 eV. With a high volume fraction of solubilizing side chains, the polymers are soluble in common organic solvents, but solution NMR measurements and thermochromism in solution indicate aggregation due to additive intermolecular interactions between donor and acceptor units.

Full text of DOI:10.1021/ma2009063

ARTICLE
pubs.acs.org/Macromolecules
Pyromellitic Diimide-Based DonorꢀAcceptor Poly(phenylene
ethynylene)s
Xugang Guo and Mark D. Watson*
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055, United States
S Supporting Information
b
ABSTRACT:
A
series of donorꢀacceptor poly(phenylene
ethynylene)s (PPEs) with N,N0-dialkylpyromellitic diimide (PMDI)
as electron acceptor and various donor units are reported. Optoelec-
tronic properties were investigated by UV/vis absorption and electro-
chemical measurements, revealing constant LUMO energies (∼ꢀ3.6
eV) across the series with HOMO energy levels governed by the donor
monomers and optical band gaps from 2.30 to 1.50 eV. With a high
volume fraction of solubilizing side chains, the polymers are soluble in
common organic solvents, but solution NMR measurements and
thermochromism in solution indicate aggregation due to additive intermolecular interactions between donor and acceptor units.
’ INTRODUCTION
polyimides for advanced applications ranging from communica-
tions to aerospace,37ꢀ39 to the best of our knowledge, the only
published example of a polymer with the benzene ring of PMDI
serving as part of the electronically “conjugated” polymer back-
bone is a poly(p-phenylene) with limited backbone conjugation
due to steric hindrance (Figure 1).40
Strongly electron-accepting imide-functionalized arylenes
have been widely studied as small-molecule n-type organic
semiconductors.1ꢀ4 The rylenes naphthalene diimide
(NDI)5ꢀ7 and perylene diimide (PDI)8ꢀ10 in particular have
been extensively studied. They are also attractive electron-
accepting building blocks for donorꢀacceptor (DꢀA) conju-
gated polymers11ꢀ17 for two reasons (among others): (1)
Optical energy gaps (Egopt) can be widely varied via electron-
donating substituents attached to the rylene core,18 translatable
to DꢀA polymers with constant lowest unoccupied molecular
Variably colored dyes resulting from core functionalization of
PMDI units with electron donors have been reported.42ꢀ46 In
hopes of translating this to variable band gap polymers, we
prepared DꢀA copolymers with thiophene derivatives as donor
and PMDI as acceptor (not shown). Perhaps due to intramole-
cular steric/electronic repulsion, conjugation seemed relatively
limited in those preliminary examples. Insertion of alkyne
linkages between successive rings along the backbone would
lead to donorꢀacceptor poly(phenylene ethynylene)s
(PPE)47,48 with more planarized backbones. PPEs have been
extensively studied in sensor applications.49ꢀ53 Their applica-
tions in OFET and OPV devices have achieved much less
success, which may be partially due to their large band gaps
and suboptimal frontier molecular orbital (FMO) energy levels.
Published PPEs usually have relatively broad band gaps of ∼2.5
eV and low lying highest occupied molecular orbital (HOMO)
energy levels of ꢀ5.9 to ꢀ6.3 eV.54 To advance their potential
applications in OFETs and OPVs, a primary concern is to
engineer the band gaps and FMO energy levels. Roy reported
an end-functionalized oligo(phenylene ethynylene), which de-
monstrated promising OFET performance with hole mobility of
opt
orbital (LUMO) energies but varied Eg via variable donor
units.12 (2) The imide nitrogens allow attachment of side chains
distal from the polymer backbone to manipulate solubility,
morphology, and solid-state packing without disrupting back-
bone electronic conjugation. However, with the exception of
polymers based on thiophene imide (thieno[3,4-c]pyrrole-4,6-
dione, TPD),19,20 DꢀA polymers with arylene imides electro-
nically conjugated along the backbone have only recently begun
to appear in the literature.11,12,21ꢀ23 A PDI DꢀA copolymer
yielded moderately good organic field-effect transistor (OFET)
and all-polymer organic photovoltaic (OPV) device
performance.11 We reported12 a series of DꢀA polymers with
NDI acceptor units with absorption profiles spanning the UV/vis/
near-IR. These and other NDI DꢀA copolymers have since been
shown to yield good to excellent device performance.13,14,21,24,25
Copolymersbasedon phthalimide22,26,27 and imide-functionalized
thiophenes23,28ꢀ35 have also very recently provided good to near
state-of-the-art device performance.
0.3 cm2 Vꢀ1 sꢀ1 55
Recently, solution-processable oligo-
.
(phenylene ethynylene)s reported by Marks showed hole mo-
bility up to 0.07 cm2 Vꢀ1 sꢀ1 in OFETs and power conversion
Pyromellitic diimide (PMDI) has been all but overlooked as a
building block for organic electronic materials. Recently, Katz
demonstrated small molecule PMDI-based OFETs with electron
mobility of 0.079 cm2 Vꢀ1 sꢀ1 (Figure 1).36 Although PMDI has
been widely used as a building block for insulating aromatic
Received: April 19, 2011
Revised:
July 11, 2011
Published: August 03, 2011
r
2011 American Chemical Society
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Figure 1. PMDI-based materials: polyimide (left),41 PMDI-based poly(p-phenylene) (middle),40 and PMDI-based n-type organic semiconductor
(right).36
Scheme 1. Synthetic Route to PMDI Monomer 1a
a Reagents and conditions: (i): (a) Br2, CCl4, 54 °C, dark; (b) Br2, CCl4, hv, reflux; (ii) 65% HNO3, NaVO3, reflux; (iii) 205 °C sublimation, vacuum;
(iv) HOAc, 2-ethylhexylamine, reflux.
Scheme 2. Synthesis of PMDI-Based Model Compounds M1ꢀM3a
a Reagents and conditions: (i) Pd2(dba)3, PPh3, CuI, DIPA, toluene 80 °C.
efficiency greater than 1% in OPVs.56 We report here a series of
DꢀA PPE’s with PMDI as acceptor to impart essentially con-
stant LUMO energies and different donor units to vary HOMO
energy and therefore Egopt. The polymers are all soluble but
appear to aggregate extensively in solution due to additive
intermolecular attraction between the strongly electron accept-
ing PMDI units and the donor units.
properties when donor groups are electronically conjugated to
the PMDI core through phenyl acetylene bridges and (b)
demonstrate that Sonogashira coupling with 1 proceeds cleanly
with conversions sufficiently high for analogous polymerizations.
This second point is particularly relevant given that some of the
polymers reported here, though soluble in organic solvents, seem
to aggregate so extensively as to prevent structure proof
via NMR.
All three model compounds were obtained in near quantitative
isolated yield (g94%) under standard Sonagashira coupling
conditions (Scheme 2). Highly electron-deficient 1 facilitates
oxidative addition, and the electron-rich aryl acetylenes favor
transmetalation. The model compounds are colored yellow
(M1), orange (M2), and red (M3) in the solid state. The
absorption maxima were bathochromatically shifted from M1
(377 nm) to M2 (407 nm) to M3 (477 nm) in chloroform. Each
absorption spectrum (Figure 2) contains a low-intensity
shoulder or tail on the low-energy side of the absorption profile.
The absorption onsets shift bathochromically from M1
(472 nm) to M2 (512 nm) to M3 (606 nm); therefore, the
HOMOꢀLUMO gaps were decreased with the increasing
electron-donating ability of donor counits, following the same
trend observed for NDI18 and PDI58 based chromophores.
Shinmyozu59 reported a compound similar to M1, differing
only in the alkyl chains attached to the imide nitrogens. Their
’ RESULTS AND DISCUSSION
Synthesis of PMDI monomer. The published procedure57 to
synthesize the key building block, 1, was simplified by combining
steps and minimizing purification (Scheme 1, see Supporting
Information for details). Durene was consecutively brominated
in near quantitative yield in one pot to 1,4-dibromo-2,3,5,6-
tetrakis(bromomethyl)benzene 2, which was oxidized to the
tetracarboxylic acid 3, and then directly dehydrated/sublimed
to afford dianhydride 4. Finally, 4 was reacted with 2-ethylhex-
ylamine in glacial acetic acid18 to afford key monomer 1 in 58%
isolated yield.
Model Study for Related Polymerizations and Engineer-
ing of Egopt. PMDI-based monomer 1 was functionalized, via
Sonagashira coupling, with phenylacetylene derivatives having
different electron-donating abilities (Scheme 2). Two purposes
for these studies are (a) demonstrate control over optical
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Figure 2. UVꢀvis absorption (left, 7 ꢁ 10ꢀ6 M in CHCl3) and photoluminescence spectra (right, 7 ꢁ 10ꢀ8 M in CHCl3) of M1ꢀM3.
Scheme 3. Synthesis and Properties of PMDI-Based Polymers P1ꢀP5; Properties of PMDI and Compound 5 Are Included for
Comparison
calculations indicated HOMO and LUMO+1 were delocalized
over the phenylene ethynylene scaffold, while LUMO was mainly
localized on the PMDI unit along the perpendicular axis. The
oscillator strength of the HOMO f LUMO transition should be
significantly lower than the HOMO f LUMO+1 (λmax), giving
rise to the low-intensity absorption tails seen here. Quantum
yields were not measured, but photoluminescence visible to the
naked eye dramatically decreases from M1 to M2 to M3. This
might be attributed to enhanced charge transfer (CT) character
when a stronger donor is coupled with the PMDI core, leading to
photoluminescence quenching.60 The starting materials for M3,
4-ethynyl-N,N0-dimethylaniline, and 1 produce a deep purple
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Figure 4. Thermochromism of P2 solution (7 ꢁ 10ꢀ6 M in o-DCB,
based on polymer repeat unit) at room temperature and 150 °C.
Figure 3. Solution (dashed line, 7 ꢁ 10ꢀ6 M in CHCl3, based on
polymer repeat unit) and film (solid line) absorption spectra of PPEs.
polymers: P1 containing dialkylbenzene displays some fine
structure, while dialkoxybenzene-containing P2 shows just a
weakly discernible shoulder, and finally aminobenzene-contain-
ing P3 and P5 have broad and featureless absorption profiles.
Polymer thin film Eg values are decreased as the electron
density on the donor monomers is increased: 2.31 eV (P1),
solution, suggesting intermolecular charge transfer,61 similar to
studies of the parent PMDI with various donor small
molecules.62 The high isolated yields and optical properties of
M1ꢀM3 establish respectively that well-defined PMDI-based
PPEs can be prepared cleanly and with broadly variable optical
energy gaps.
opt
2.12 eV (P2), 1.90 eV (P3), and 1.53 eV (P5). In comparison to
opt
Synthesis of PMDI-Based Poly(phenylene ethynylene)s
(PPEs). PPEs P1ꢀP5 were synthesized via Sonagashira coupling
(Scheme 3, see Supporting Information for details) with num-
ber-average molecular weights (Mn) ranging from 5.4 to
70.5 kDa (GPC vs polystyrene standards; eluent: THF). All of
the polymers, except P1, are readily soluble in common organic
solvents. The relative solubility of P1ꢀP5 depends on the side-
chain volume fraction, the relative placement of side chains along
the polymer backbone, and the symmetry of the side-chain
placement around the monomer units. An analogue built with
unsubstituted donor units (P1, with R = H) was completely
insoluble and therefore not included in this study. Polymer P5,
with a high side-chain volume fraction and great steric bulk in the
vicinity of the backbone, is understandably the most soluble. This
the relatively large Eg
reported for most PPEs in the
literature,47 these PMDI-based polymers demonstrated rather
narrow Eg and that of P5 seems to be the lowest reported
opt
for a PPE.
The absorption profiles change little on going from solution to
solid state, indicating little difference in electronic states. In other
words, the polymers are likely extensively aggregated in solution.
The small red shifts on going to the solid state might be
attributed to slightly increased intermolecular interaction and
rigidification in the film state. As seen in Figure 4, the “solution”
absorption profile of P2 (7 ꢁ 10ꢀ6 M in o-DCB) is relatively
sharp at room temperature but significantly broadened at 150 °C
with λmax blue-shifted by ∼50 nm, corresponding to a color
change from red to yellow. At higher temperature, the shift closer
to a molecularly dissolved solution should lead to broadened and
featureless absorption profile as a consequence of diminished
aggregation and/or more randomized thermal population of
states.63 The changes in color and absorption profile are com-
pletely reversible upon cooling to RT.
From photoluminescence (PL) measurements, the trend in
PL λmax versus donor strength followed the same trend as
observed for absorption (Supporting Information). PL could
not be measured at all from P5, and PL from any of the polymers
in solution or film state was barely visible to the naked eye. This
can be attributed to aggregation-induced64 and/or charge-trans-
fer induced quenching.60,65
The solution absorption λmax for P4 is red-shifted (31 nm)
relative to P3 (Figure 5), and even slightly red-shifted relative to
P5, which contains the more strongly electron-donating diami-
nobenzene units. In addition to obvious differences in side-chain
steric demands, this could be attributed to the ability of P4 to
form intramolecular H-bonds between neighboring secondary
amino and carbonyl substituents, thus increasing backbone
coplanarity and rigidity. Zhao66 proposed similar intramolecular
hydrogen bonding between pendant amide and ester groups in
monodisperse oligo(p-phenylene ethynylene)s.
1
allowed collection of the only reasonably well-resolved H and
13C NMR spectra at relatively low temperature (60 °C, Support-
1
ing Information). The signals within the H NMR spectra of
P1ꢀP4 were very broad even at high temperature (up to
130 °C), likely due to extensive aggregation. A low-molecular
weight version of P2 (P2a: Mn = 9.3 kDa; PDI: 3.9) was prepared
via monomer stoichiometric imbalance and postpolymerization
end-capping with phenyl acetylene (Supporting Information).
1
The H NMR spectrum collected from P2a at 160 °C (Figure
S19) is consistent with the expected structure.
Polymer Optical Properties. Polymers P1ꢀP5 all share
PMDI as the electron-accepting unit; therefore, the absorption
and emission profiles should vary with donor monomers. The
longest-wavelength absorption maxima shift to lower energies in
the order P1 f P2 f P3 f P5 (P4 discussed below) as follows
—490 nm (P1), 555 nm (P2), 566 nm (P3), and 591 nm (P5)—
with a similar trend in thin film spectra.
Increasing electron-donating ability of donor monomers from
dialkylarylene (P1) to dialkoxyarylene (P2) and monoaminoar-
ylene (P3) to diaminoarylene (P5) destabilizes HOMO, de-
creasing the HOMOꢀLUMO gap. There is also some similarity
between the absorption profiles of model compounds and
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’ ASSOCIATED CONTENT
S
Supporting Information. Synthesis and characterization
b
details, H and 13C NMR spectra, photoluminescence spectra,
and cyclic voltammograms of polymers. This material is available
free of charge via the Internet at http://pubs.acs.org.
1
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: mdwatson@uky.edu.
’ ACKNOWLEDGMENT
We thank NSF (CHE-0616759) for financial support and Dr.
Manfred Wagner (Max-Planck Institut fuer Polymerforschung)
for high-temperature NMR measurements of polymer P2a.
Figure 5. UVꢀvis absorption spectra of P3 and P4 in solution (dashed
line, 7 ꢁ 10ꢀ6 M in CHCl3, based on polymer repeat unit) and film
(solid line).
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Pyromellitic diimide (PMDI) units impart near constant
LUMO energies to PPEs, allowing facile tuning of optical energy
gap via variation of donor comonomers. The polymers reported
here are soluble in common organic solvents, but extensive
aggregation prevents full characterization of all the polymers.
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