Well-defined alternating copolymers of oligo(phenylenevinylene)s and flexible chains

Article abstract of DOI:10.1021/ma300430e

A series of alternating copolymers containing oligomeric bis(2-ethylhexyl)-p-phenylenevinylene (BEH-PPV) chromophores and conformational-flexible n-decyl or tetraethylene glycol chains were prepared. The polymerization was carried out using Sonogashira co

Full text of DOI:10.1021/ma300430e

Article
pubs.acs.org/Macromolecules
Well-Defined Alternating Copolymers of Oligo(phenylenevinylene)s
and Flexible Chains
Xinju Zhu,‡ Matthew C. Traub,† David A. Vanden Bout,*,† and Kyle N. Plunkett*,‡
†Department of Chemistry and Biochemistry and the Center for Nano and Molecular Science and Technology, University of Texas at
Austin, Austin, Texas 78712, United States
‡Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901, United States
S
* Supporting Information
ABSTRACT: A series of alternating copolymers containing
oligomeric bis(2-ethylhexyl)-p-phenylenevinylene (BEH-PPV)
chromophores and conformational-flexible n-decyl or tetra-
ethylene glycol chains were prepared. The polymerization was
carried out using Sonogashira coupling conditions between
monomers composed of an iodo-terminated PPV oligomer
(trimer, pentamer, or septamer) and a bis(phenylacetylene)-
containing flexible chain. Polymers containing the n-decyl
chain attained higher molecular weights compared to the tetraethylene glycol-containing polymers. 4-Ethynylanisole-capped
oligomers (trimer, pentamer, or septamer) were prepared, and their solution photophysical properties were compared to the
analogous polymeric materials. The solution optical properties of the polymers were primarily determined by chromophore
length of the constituent oligomers. In contrast, the thin film fluorescence spectra of the polymers showed substantial differences
between n-decyl and tetraethylene glycol containing materials, suggesting significant changes in the degree of interchain coupling
in the solid state. The control of effective conjugation length afforded by these materials makes them a promising system for
understanding electronic trap states in conjugated polymers.
ultralong-range energy transfer.9−11 In this work, we report
the synthesis and photophysical properties of a new series of
polymerized oligomers, providing even finer synthetic control
of both structure and optical properties.
INTRODUCTION
■
The morphology of conjugated polymers plays a critical role in
determining the electronic properties of the thin films utilized
in devices such as organic photovoltaics, organic light-emitting
diodes, and field effect transistors.1,2 The transition from a
single polymer molecule into a useful polymer film depends on
the macromolecule’s structural characteristics including molec-
ular weight, backbone composition, and solubilizing-chain
structure. To develop rational control over morphology of
bulk materials, it is imperative to understand the fundamental
factors that determine structure in conjugated polymer
materials. To accomplish this goal, we set out to create and
study well-defined polymers whose structure and energetics can
be systematically controlled at the synthetic level.
Recently, we synthesized a series of polymers that can access
predetermined conformations in the single polymer chain.3 Our
initial strategy utilized the random incorporation of morphol-
ogy directing (“morphon”) chemical groups into the backbone
of the prototypical conjugated polymer poly(2-methoxy-5-(2′-
ethylhexyloxy)-p-phenylenevinylene) (MEH-PPV). By using
single molecule spectroscopy (SMS),4,5 we found that
morphons that re-enforce the linear nature of the PPV6 chain
actually improve the alignment of chromophores in the
polymer7 while, consistent with previous work,8 morphons
that direct the polymer chain into a bent structure create
globular architectures. Control over single chain structure was
an important step toward ground-up control of film properties,
as highly aligned single polymer nanodomains facilitate
Although the polymers we described previously provided
access to varying morphologies, the Horner−Wadsworth−
Emmons polymerization that was utilized has several
disadvantages. The polymerization leads to low molecular
weights and a regiorandom copolymer with a broad distribution
of effective chromophore lengths. Here we report an updated
strategy that utilizes a Sonogashira polymerization to create
high molecular weight polymers with known oligomeric bis(2-
ethylhexyl)-p-phenylenevinylene (BEH-PPV) chromophores
between morphons (Figure 1). We have investigated two
flexible morphons (alkyl and tetraethylene glycol) that provide
conformational freedom and may allow single polymer chains
to fold into ordered sheets. Solution absorption and emission
measurements revealed that the photophysics of the polymers
were primarily determined by the conjugation length of the
constituent oligomers, allowing for rational control of their
optical as well as their structural properties.
Received: March 1, 2012
Revised: May 31, 2012
Published: June 12, 2012
© 2012 American Chemical Society
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with the phosphonate 9. Deprotection of the acetal provided
the aldehyde PPV dimer 12, which was converted into
pentamer 13 through coupling with 8. Alternatively, 12 could
be further elongated to create aldehyde PPV trimer 15 and
ultimately 16 through analogous chemistry. Similar to 11, the
higher oligomers were capped with 4-ethynylanisole to access
pentamer (14) and septamer (17) chromophores. All
oligomers prepared via the Horner reaction were refluxed in
toluene with catalytic iodine and then purified by silica gel
chromatography to ensure all vinyl groups adopted the more
stable trans configuration.17
Two flexible morphons based on an n-decyl chain (C10H20)
and a tetraethylene glycol (CH2CH2O)4 chain with terminal
phenylacetylenes were chosen to complement the BEH-PPV
oligomers for a Sonogashira polymerization (Scheme 2). Such a
strategy was previously employed with PPV trimers of varying
compositions.18,19 Here we have also employed the tetra-
ethylene glycol chain that may allow for greater conformational
flexibility in solution and the solid state by way of the preferred
gauche conformation around the C−C bond in ethylene
glycols.20,21 The synthesis of the alkyl-chain morphon began
with a Friedel−Crafts acylation of sebacoyl chloride (18) with
bromobenzene to provide the diketone 19. Wolff−Kishner
reduction using hydrazine and KOH in diethyelene glycol at
180 °C gave the completely aliphatic 20. Sonogashira coupling
of trimethylsilylacetylene (TMS-acetylene) followed by depro-
tection with aqueous KOH provided the desired bis-
(phenylacetylene) with alkyl linker 21. The synthetic procedure
for the tetraethylene glycol morphon utilized a nucleophilic
substitution of tosylated tetraethylene glycol 22 with 4-
bromophenol. Similar Sonogashira coupling utilizing TMS-
acetylene followed by deprotection provided the desired
bis(phenylacetylene) with ethylene glycol linker 24.
Figure 1. Flexible-morphon BEH-PPVs prepared in this work.
RESULTS AND DISCUSSION
■
Synthesis. The synthesis of polymerizable BEH-PPV
oligomers12 (10, 13, and 16) was carried out through repetitive
Horner−Wadsworth−Emmons reactions13 according to
Scheme 1. The oligomer series consisted of a trimer (10),
pentamer (13), and septamer (16) with terminal iodo
functionality and were easily synthesized owing to their
symmetry. The sequence began with the synthesis of previously
known monomers 7, 8, and 9 (Supporting Information).14−16
10 was prepared by the Horner coupling of 2 equiv of aldehyde
7 and the diphosphonate 8 with stoichiometric KOtBu. To
create an oligomeric chromophore that better resembles the
subsequent polymeric chromophores, the iodos were sub-
stituted with 4-ethynylanisole via a Sonogashira coupling to
produce 11. To access the higher oligomers, an elongation of
the aldehyde chromophore was accomplished by coupling of 7
Our interest in incorporating defined oligomers and
morphons into the polymer backbone limited our strategy to
step-growth polymerization conditions, which can lead to low
molecular weights without exact stoichiometric control.22
a
Scheme 1. Synthesis of BEH-PPV Oligomers
a
Reagents and conditions: (i) KOtBu, THF, 0 °C; (ii) 4-ethynylanisole, Pd(PPh3)4, HNiPr2, toluene, 85 °C; (iii) HCl, THF, H2O, 50 °C.
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a
Scheme 2. Synthesis of Flexible Morphons 21 and 24
a
Reagents and conditions: (i) bromobenzene, AlCl3, CH2Cl2, 40 °C; (ii) NH2NH2, KOH, diethylene glycol, 180 °C; (iii) TMS-acetylene,
PdCl2(PPh3)2, CuI, PPh3, NEt3, THF, 60 °C; (iv) 20% KOH(aq), THF, methanol; (v) 4-bromophenol, K2CO3, acetone, reflux.
a
Scheme 3. Synthesis of Polymers 1−6
a
Reagents and conditions: (i) Pd(PPh3)4, CuI, HNiPr2, toluene, 85 °C, 3 days.
Table 1. Summary of Molecular Weight and Photophysical Data
a
b
Mn
PDI
oligomer length
no. chromophores
λabs,max (nm)
λem,max (nm)
Φ
τ (ns)
polymer
1
78 300
61 600
87 500
42 400
34 100
46 000
1.7
1.5
1.2
1.4
1.4
1.3
3mer
5mer
7mer
3mer
5mer
7mer
56
29
31
29
16
16
442
467
482
441
463
479
504
539
550
506
539
547
0.75
0.27
0.27
0.62
0.27
0.18
1.167
0.557
0.393
0.509
0.526
0.442
2
3
4
5
6
oligomer
11
1 312
2 029
2 746
1.0
1.0
1.0
3mer
5mer
7mer
1
1
1
443
475
485
504
535
544
0.42
0.23
0.20
1.251
0.851
0.412
14
17
a
b
Average based on the Mn and weight of repeat unit. All properties were measured in toluene. Quantum yields were calculated relative to
Rhodamine 123 in ethanol (Φ = 0.96) and are accurate to 10%.
However, it has been shown that polymerizations based on the
Sonogashira coupling reaction can produce extremely high
molecular weights (>1500 kDa).23 The key to accessing these
high degrees of polymerization depends on the use of a slight
excess (1.03 equiv) of acetylene monomer to compensate for
the minor, yet unavoidable, Glaser coupling between alkynes.
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1
Figure 2. H NMR spectra (CD2Cl2) of trimer oligomers 10 (A) and 11 (B) and trimer polymers 4 (C) and 1 (D).
We polymerized the BEH-PPV oligomers (10, 13, and 16) with
both flexible morphons (21 and 24) to create six different
polymers (1−6) by employing the catalytic system of
Pd(PPh3)4, CuI, and HNiPr2 in toluene at 85 °C for 3 days
(Scheme 3). The molecular weights, PDIs, and average number
of BEH-PPV chromophores per polymer chain are summarized
in Table 1. As a consequence of the large, predefined
monomers that are used in the reaction, the PDIs are lower
than expected from a step-growth polymerization. It is
interesting to note that the alkyl morphon led to higher
molecular weights (almost double) for each of the respective
oligomer polymerizations. It is unclear at this time if the
reduction in the degree of polymerization is due to a less
efficient Sonogashira coupling of a para-alkoxy substituent (24)
compared to a para-alkyl substituent (21) on the phenyl-
acetylene or if the catalyst activity is reduced due to chelation
from the tetraethylene glycol functionality.
NMR Analysis. Figure 2 shows a representative example of
the BEH-PPV oligomers before and after polymerization. The
top spectrum (A) is of the iodo-terminated trimer 10, and the
aromatic region shows three singlets (7.11, 7.18, and 7.32 ppm)
for the aryl hydrogens and an AB quartet at 7.49 ppm for the
internal alkene protons. The methylene hydrogens adjacent to
the oxygen do not provide well-resolved signals; rather, the
slightly different protons are clustered around 3.91 ppm.
Capping the trimers with 4-ethynylanisole leads to 11 (B),
which displays a reduced coupling constant of the AB quartet,
two new aromatic doublets at 6.90 and 7.46 ppm, and a new
singlet at 3.83 ppm for the methoxy group. Upon incorporation
into polymer 4 (C), the aromatic region remains relatively
unchanged from 11; however, new signals arise between 3.5
and 4.2 ppm owing to the new tetraethylene glycol linker.
Integration shows that the trimer oligomers and flexible
morphons are incorporated into the polymer in a 1:1 ratio.
For polymer 1 (D), the aromatic region again stays relatively
unchanged except for a shift of the flexible linker aromatic
group doublets, which now overlap at 7.20 ppm. The alkyl
region shows the alkoxy protons associated with the trimer at
3.91 ppm as well as the aliphatic protons associated with the
hydrocarbon flexible linker at 2.62 ppm. Again the integration
between the alkyl flexible linker and the trimer show a 1:1
polymer backbone incorporation. Although not shown here, the
NMR analysis of polymers 2, 3, 5, and 6 shows the correct 1:1
incorporation of each building block into the polymer repeat
unit (Supporting Information). Finally, it is important to note
the lone signal at 3.91 ppm for the alkoxy protons. As
previously shown,24,25 multiple signals in this region suggest the
presence of cis-alkenes in the conjugated polymer backbone. In
our previous work utilizing the Horner polymerization
method,3 we found that large cis-alkene content was present
upon polymerization. Even after refluxing in toluene with
catalytic iodine, not all of the cis-alkene was removed. This
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highlights the value of the presently described system in that all
chromophores are identical and maintain the all-trans alkene
arrangement upon polymerization.
Solution UV−vis and Fluorescence. The absorption
spectra of capped oligomers (11, 14, 17) as well as polymers
(1−6) were obtained in toluene. The active chromophore of
the oligomer and its corresponding polymers (i.e., 11 vs 1 and
4) are very similar, and only small changes in the λmax were
observed between the materials (Table 1). Figure 3 shows an
pentamers or septamers. Previous reports have found that the
relationship of oligomer conjugation length on quantum yield
depends on the specific structure of the side chains and end-
caps, with some compounds displaying a similar drop in Φ with
increasing conjugation length30 and others displaying Φ values
which are essentially independent of conjugation length.31
Additionally, there were significant differences between 1, 4,
and 11 (Φ = 0.75, 0.62, and 0.42, respectively), likely a result of
the shortest chromophores being the most susceptible to
nonradiative decay channels that result from changes in the
periphery of the compound. While there are differences in the
lifetimes and quantum yields between the oligomers and
polymer, the excited state energies of the polymers are
essentially identical to those of the oligomers. Thus, the
oligomers−morphon approach provides the opportunity to
explore the origin of changes in excited state energies observed
in the solid state and develop improved control of conjugated
polymer materials.
Thin Film Fluorescence. The linked oligomers described
above allow us to probe the chain interactions governing film
electronic properties in systems with well-defined conjugation
length. The septamer compounds 3 and 6 in particular provide
a useful comparison point to the well-characterized PPV
derivatives, as their solution absorption peak is close to that of
MEH-PPV and BEH-PPV.32,33 The fluorescence spectrum of
MEH-PPV is well-known to undergo a substantial bath-
ochromic peak shift between solution and the solid state due
to the formation of low-energy traps.32,34,35 Electronic traps
present a critical performance issue for devices based on
conjugated polymer materials, limiting the efficiency of exciton
harvesting and charge transport in devices.36−38
Several possible mechanisms exist for the formation of low-
energy sites and red-shifted fluorescence spectra in conjugated
polymers. Red-shifted emission in the β-phase of poly(9,9-
dioctyl)fluorene relative to solution or amorphous phases is
ascribed to planarization of polymer chains due to interchain
steric interactions that lead to a corresponding increase in
effective conjugation length or hyperextended conjugation.39,40
Closely packed polymer chains can also lead to the formation of
excited state interactions (excimers).41 Additionally, recent
spectroscopic studies of aggregates of random MEH-PPV/
terphenyl copolymers have suggested that ground state
electronic interactions are the source of low-energy red sites.7
By studying materials with controlled conjugation lengths, we
can distinguish between these different scenarios because the
limited conjugation of the oligomer units eliminates the
possibility of hyperextended conjugation.
To probe the effects of the flexible morphons on the solid
state properties of these materials, we measured the
fluorescence spectra of thin films of 3 and 6 prepared by
spin-coating on glass (Figure 4). Despite the identical
conjugation lengths of the two polymers, there were substantial
differences in the spectra. The fluorescence spectrum of 3 could
be well fit to a Franck−Condon model with a single 0−0
energy and 3 quanta for the CC progression of Gaussian
broadened vibronic lines:
Figure 3. Absorbance (top) and emission (bottom) of n-decyl
morphon copolymers 1, 2, and 3 in toluene.
overlay of the absorption spectra of the n-alkyl morphon
polymers 1−3 while the remaining spectra can be found in the
Supporting Information. As expected,26,27 the λabs,max showed a
bathochromic shift upon increasing the conjugation length
from trimer (1, λabs,max = 442 nm) to pentamer (2, λabs,max = 467
nm) to septamer (3, λabs,max = 482 nm). The λabs,max of 3 is
slightly lower than native BEH-PPV (λmax = 505 nm).28 The
emission spectra displayed a similar bathochromic shift from
trimer (1, λem,max = 504 nm) to pentamer (2, λem,max = 539 nm)
to septamer (3, λem,max = 550 nm).
Consistent with the expected trends for phenyleneviny-
lenes,29,30 the fluorescence lifetimes of the compounds
generally decreased with increasing conjugation length due to
the stronger oscillator strength of the longer chromophores. An
exception to this general trend was found for 4, which had a
substantially shorter lifetime (0.509 ns) than other trimer
materials (1 = 1.167 ns and 11 = 1.251 ns). Similarly, both
pentamer polymers (2 and 5) showed shorter lifetimes than the
corresponding oligomer 14.
n
e−S Si
⎛
⎜
⎞
⎠
i
i
2
− (ω−E +n ℏω)/σ
Iem(ω) = Aω3
e
[
]
0
i
i
⎟
⎟
∑ ∏
⎜
ni!
⎝
ni=0
i
(1)
The quantum yields (Φ), measured against a reference
standard of Rhodamine 123 in ethanol, did not show the same
strong correlation with oligomer length. The trimer compounds
displayed substantially higher quantum yields than the
where ω is the energy of the emitted light, A is the amplitude of
the 0−0 transition, E0 is the energy of the 0−0 transition, and
ni, Si, ℏωi, and σ are the quanta, Huang−Rhys factor vibrational
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broad emission peaked at ∼645 nm. Such a fit does not agree
with the two transitions centered at 623 and 669 nm in the
experimentally obtained spectrum. This fitting suggests that
while some portions of the film behave similarly to 3, others
exhibit even stronger interchromophore interactions that yield
the red-shifted emission. This result is consistent with the
hypothesis that the greater length or flexibility of the alkoxy
linkers allows more extensive chain folding in the solid state,
facilitating these interchromophore interactions.
Broad, red-shifted fluorescence peaks can result from both
excimers or a distribution of overlapping Franck−Condon
progressions. These two sources of red-shifted emission can be
distinguished by their fluorescence lifetime, with excimers
typically being much longer lived (τ > 1 ns).46 Fluorescence
lifetime measurements of films of 6, fit to biexponential decay
functions, showed that greater than 97% of the signal came
from a relatively short-lived component (τ = 0.580 ns).
Repeating this measurement with a 670 nm long pass filter in
front of the detector yielded a similar result (τ = 0.670 ns for
97% of the decay curve). As these lifetimes are too short for
excimers, we assign this broad red-shifted emission to a
heterogeneous distribution of low-energy peaks. This distribu-
tion in energies for emissive sites is likely a result of differences
in interchromophore coupling that arise from heterogeneity of
the packing within the films. Studies of aggregated oligomers in
solution have observed species that are composed of varying
degrees of both monomer-like and aggregated species.45,47 The
predominance of these lower energy sites in the polymers with
the alkoxy linkers is evidence that they are folding in the solid-
state into structures that allow the oligomers to interact at
closer distances.
Figure 4. Thin film fluorescence spectrum of 3 (solid) and fit to single
CC progression of vibronic peaks (red-dashed) (top). Thin film
fluorescence spectrum of 6.
In summary, we have successfully synthesized a series of well-
defined BEH-PPV polymers with flexible morphons composed
of either a n-decyl or tetraethylene glycol chain. High molecular
weight polymers were obtained using a step-growth polymer-
ization based on the Sonogashira coupling. Owing to the
presynthesized PPV oligomers, each chromophore in the
polymer backbone was identical and contained no cis-alkene
linkages. The solution photophysical properties of 4-ethynyla-
nisole-capped PPV oligomers and polymer-incorporated PPVs
were very similar and demonstrate that the polymer
chromophores behave similarly to the free oligomers. By
varying the flexibility of the morphon linkers, the extent of
interchain interactions could be varied in the solid state. These
materials provide a powerful new platform for continuing
studies in the evolution of structure and properties from single
molecules to thin films.
frequency of the ith mode, and peak width, respectively.42−44
The fit values for Si, ℏωi, and σ (Table 2) are consistent with
Table 2. Best Fit Parameters for Thin Film Fluorescence
Spectra of 2 and 3
polymer
E0 (cm−1
)
S
σ (cm−1
)
ℏωi (cm−1
)
2
3
18 160
17 700
0.988
0.939
840
866
1500
1503
emission from a relatively uniform set of chromophores. The
0−0 transition for 3 at 17 700 cm−1 (565 nm) is bathochromi-
cally shifted 472 cm−1 from the solution peak. The magnitude
of this shift is comparable to that observed in MEH-PPV and
BEH-PPV. Thin films of 2 could also be well fit by a single
Franck−Condon progression with a 0−0 energy at 18 160
cm−1, which represents a bathochromic shift of only 393 cm−1
relative to solution (Supporting Information). The larger
bathochromic shift for the longer oligomers could be indicative
of better coupling between the polymers as a result of stronger
interactions of the individual chromophores.
In contrast to the n-alkyl morphon-containing polymers, the
spectrum of 6 is not well fit by either one or two Franck−
Condon progressions (similar to 5, Supporting Information).
Nor does the spectrum appear to be a single progression in
which the 0−0 peak has been suppressed as has been observed
previously for solution aggregates of PPV-oligomers.45 The first
peak in the spectrum is at 565 nm, which is the same energy as
the 0−0 transition in the spectrum of 3. If a single Franck−
Condon progression with a 0−0 transition at this energy is
constrained in the fit, the remainder of the spectrum is a very
ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed experimental procedures are available, including
experimental equipment, synthetic procedures, and NMR
characterization; solution absorption and fluorescence spectra
of compounds 4, 5, 6, 11, 14, and 17 as well as the thin film
fluorescence of 2 and 5. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: davandenbout@mail.utexas.edu (D.A.V.B.);
kplunkett@chem.siu.edu (K.N.P.).
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Notes
The authors declare no competing financial interest.
(28) Palacios, R. E.; Chang, W.-S.; Grey, J. K.; Chang, Y.-L.; Miller,
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J. Phys. Chem. B 2009, 113, 14619−14628.
ACKNOWLEDGMENTS
■
This work was supported as part of the program “Molecular
Tools for Conjugated Polymer Analysis and Optimization”, a
Center for Chemical Innovation (CCI, phase 1) under NSF
Award CHE-0943957 as well as the Department of Energy
through the Solar Photochemistry Program under Award DE-
FG02-05ER15746. K.N.P. also acknowledges startup funds
supplied by Southern Illinois University.
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