Highly efficient benzannulation of poly(phenylene ethynylene)s

Article abstract of DOI:10.1002/anie.201206964

Put a ring on it: Sterically congested polyarylenes can be synthesized by benzannulation reactions at each C≡C bond of a poly(phenylene ethynylene) (see picture), one of the most easily synthesized and versatile classes of conjugated polymers. The benzannulation reaction is highly specific and efficient, as determined by an isotopic labeling study and several complementary spectroscopic methods. Copyright

Full text of DOI:10.1002/anie.201206964

Angewandte
Chemie
DOI: 10.1002/anie.201206964
Conjugated Polymers
Highly Efficient Benzannulation of Poly(phenylene ethynylene)s**
Hasan Arslan, Jonathan D. Saathoff, David N. Bunck, Paulette Clancy,* and William R. Dichtel*
Macromolecules linked by single bonds between aromatic
substituents, that is, polyphenylenes, have attracted great
interest for use in organic light-emitting diodes (OLEDs)[1]
and other optoelectronic devices,[2] as structurally rigid
dendrimers,[3] for isolating[4] or organizing chromophores,[5]
and as soluble precursors of nanostructured forms of gra-
phene.[6] Para- and meta-linked polyphenylenes with high
molecular weight are synthesized through transition-metal-
catalyzed cross-coupling approaches, some of which proceed
through chain-growth mechanisms.[7] Only a few oligomeric
ortho-linked phenylenes have been reported[8] as a result of
poor cross-coupling efficiency during the polymerization.
Longer o-polyphenylenes are interesting because they can
adopt specific helical conformations induced by steric inter-
actions along the polymer backbone[8,9] and can be oxidized to
soluble, structurally precise graphene nanoribbons.[10] New
strategies for accessing o-polyphenylenes, particularly those
that tolerate additional aromatic substituents, are therefore of
great utility.
Here we report a versatile and efficient approach to
prepare polymers of high molecular weight containing o-
substituted arylene linkages through a cycloaddition reaction
at each alkyne function of a poly(phenylene ethynylene)
(PPE). PPEs are attractive precursors because of their
structural versatility, functional-group tolerance, and effi-
ciency of polymerizations. However, their alkyne moieties are
notably unreactive and have only been reduced to methyl-
ene[11] or cis-vinylene groups prior to this study.[12] This
benzannulation strategy (Scheme 1) is the first reported
efficient cycloaddition performed along a conjugated polymer
backbone,[13] and it tolerates bulky aromatic substituents
adjacent to the alkyne moieties, thus suggesting that it will be
applicable to a broad range of crowded aromatic architec-
tures.
Scheme 1. Benzannulation of each alkyne of a substituted poly(pheny-
lene ethynylene) 1 provides polyphenylene 3.
Yamamoto and co-workers reported a Cu(OTf)2-cata-
lyzed benzannulation of diphenylacetylene with o-(phenyl-
ethynyl)benzaldehyde 2 to produce 2,3-diphenylnaphtha-
lene,[14] but this transformation has not previously been
employed for polymer modification. We first evaluated its
feasibility for PPE benzannulation using dialkyne
4
(Figure 1), a model compound similar in structure to PPE 1.
Compound 4 reacts rapidly (in as little as 30 min) and
efficiently with 2 in the presence of catalytic amounts of
Cu(OTf)2 and excess trifluoroacetic acid (CF3CO2H) at
1008C to provide the doubly benzannulated product 5 in
98% yield of isolated product. Yamamoto and co-workers
found that some alkyne-containing substrates form more than
10% of a naphthyl ketone side product (Figure 1b, inset)
[*] H. Arslan, D. N. Bunck, Prof. Dr. W. R. Dichtel
Department of Chemistry and Chemical Biology
Cornell University, Baker Laboratory
Ithaca, NY, 14853 (USA)
E-mail: wdichtel@cornell.edu
J. D. Saathoff, Prof. Dr. P. Clancy
School of Chemical and Biomolecular Engineering
Cornell University
Ithaca, NY 14853 (USA)
[**] This research was supported by the National Science Foundation
(CHE-1124754). We also made use of the Cornell Center for
Materials Research (CCMR) facilities with support from the NSF
Materials Research Science and Engineering Centers (MRSEC)
program (DMR-0520404). D.N.B. acknowledges the award of
a Graduate Research Fellowship from the NSF. We acknowledge
Fernando J. Uribe-Romo for help in preparing the graphical abstract.
Figure 1. Model study of the benzannulation strategy. a) Subjection of
terphenyl-containing dialkyne 4, a molecular compound similar in
structure to PPE 1, to the Cu(OTf)2-catalyzed benzannulation.
b) MALDI-TOF mass spectra of dialkyne 4 and benzannulated product
5. Inset: structure of a possible naphthyl ketone side product based on
previous reports of the benzannulation reaction,[14] observed only as
a trace impurity in the mass spectrum.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206994.
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under the Cu(OTf)2-catalyzed benzannulation conditions,
although this side reaction was not observed for the substrate
diphenyl acetylene.[14] If present in our system, this side
reaction would induce defects along the polyphenylene
backbone. Fortunately, 4 forms little or no naphthyl ketone
side products (< 1%), as judged by the high yield of isolated
product 5, as well as by NMR and Fourier transform infrared
(FTIR) spectroscopy (see the Supporting Information), and
thin-layer chromatography.
A comparison of the MALDI-TOF mass spectra of 4 and
5 also indicates the high efficiency of the reaction. After
benzannulation, a strong signal for the doubly benzannulated
product 5 is observed. The weak signal in the spectrum at
773.6 mass units corresponds to a fragmentation product of 5.
The only signals corresponding to benzannulation side
products are weaker: the naphthyl ketone at 990.6 mass
units and monoannulated product at 784.6 mass units. These
by-products were formed in trace amounts and were not
observed by other analytical methods. In summary, this model
study confirms the potential of this reaction for polymer
modification.
The p-terphenyl-alt-phenyl PPE 1 was obtained by
copolymerizing the appropriate terphenyl dialkyne monomer
and 1,4-diiodobenzene under Sonogashira cross-coupling
conditions. A substoichiometric amount of a monofunctional
aryl iodide, 4-iodoanisole, was used to control the molecular
weight and the identity of the end-group of the polymer.
Nevertheless, at molecular weights approaching 100 kDa, 1 is
insoluble in the reaction mixture and partially precipitates
during the polymerization. As a result, we obtained a bimodal
molecular-weight distribution (Mn = 7.85 kDa; PDI = 3.53;
Dp = 12) by size-exclusion chromatography (SEC), consisting
of a main peak for polymers of low molecular weight with
a shoulder for polymers of high molecular weight (Figure 2).
making it an appropriate system in which to evaluate our
synthetic approach.
PPE 1 was benzannulated under similar conditions as the
model compound 5 (3 equiv of 2 per alkyne, 3 equiv
CF3CO2H, 0.05 equiv Cu(OTf)2), after which polyphenylene
3 was isolated by precipitation from CH2Cl2 into acetone,
followed by Soxhlet extraction using acetone as the liquid
phase. SEC analysis of the acetone-insoluble material showed
a monomodal distribution of chain sizes, the molecular weight
and polydispersity (Mn = 39.6 kDa, PDI = 1.65, Dp = 49) of
which correspond to the benzannulation of the higher-
molecular-weight portion of 1. Steric hindrance along the
polymer backbone prevents 3 from adopting a planar con-
formation and it is far more soluble in organic solvents than 1.
Despite its higher molecular weight, polyphenylene 3 is
retained longer by the SEC columns relative to its PPE
precursor 1, which we attribute to the polymer adopting
a more compact solvated structure as a consequence of its
steric demands (Figure 2). As a result, the polymer chains of 3
adopt smaller hydrodynamic volumes than random-coil
polymer chains of comparable molecular weight. This explan-
ation is consistent with recent findings that oligomeric o-
phenylenes adopt specific helical conformations as a conse-
quence of their crowded structure.[8,15]
According to a molecular dynamics (MD) simulation,
polyphenylene 3 adopts a compact structure in organic
solvent. MD provides an atomically explicit representation
of the dynamics of the system under the influence of inter-
and intramolecular forces. A 24-unit polymer chain of 3 was
simulated along with 11,740 1,2-dichlorobenzene solvent
molecules using the OPLS-AA force field[16] within the
LAMMPS software package[17] (see the Supporting Informa-
tion for complete details). The resulting structure of the
polymer was highly contorted with several distinct conforma-
tions at each monomer unit, which contribute to the compact
structure of the solvated polymer in solution. In each
monomer unit, we determined eight dihedral angles that
affect the local conformation of the polymer and thus
investigated their disposition. Preferred angles were found
by locating peaks in histograms generated by the data;
Gaussian distributions were fitted to each peak to estimate an
average angle value as well as a standard deviation (Support-
ing Information, Table S2). From left to right in Figure 3, the
three most common conformations of polymer 3 have lengths
of 5.8, 9.6, and 7.2 ꢀ per repeat unit, respectively. Even
though 3 does not fold into a single or small number of well-
defined conformers, as do o-phenylene oligomers, many of
the rotations of its aromatic rings are correlated, as evidenced
by their specific ranges of energetically favorable dihedral
angles. These simulations are consistent with the otherwise
anomalous shift to higher SEC retention times when 1 is
benzannulated to 3.
Figure 2. Differential refractive index (DRI) response of SEC traces of
PPE 1 (blue) and polyphenylene 3 (red).
These molecular weights were determined by multi-angle
light scattering, a direct measure of the polymer mass that
does not rely on comparisons to polymer standards of
questionable applicability. Despite its broad molecular-
weight distribution, we did not separate 1 into fractions of
high and low molecular weight because we found that the
shorter chains were easily removed by precipitation after the
benzannulation step. We estimate the Dp of the high-
molecular-weight fraction of 1 to be approximately 59,
The efficiency of the benzannulation of PPE 1 was
characterized using a full complement of spectroscopic
measurements. The inability of adjacent aromatic rings in 3
to adopt coplanar conformations induces a significant blue
shift in its UV/Vis absorption spectrum relative to its PPE
precursor 1. The lmax of 3 is blue-shifted by 140 nm compared
to that of 1 (Figure 4a) and its spectrum is strikingly similar to
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Figure 3. Illustration of the three most common conformations in the
simulation of several repeat units of polymer 3 in the presence of 1,2-
dichlorobenzene solvent. Truncated polymer structures are shown with
their n-nonyl side-chains omitted for clarity.
that of benzannulated model compound 5 (Figure 4a). In
addition, the absorption spectrum of model compound 4
shows an intense band with defined vibrational structure from
325–375 nm (Figure 4a). If the benzannulation were ineffi-
cient, residual alkyne-containing subunits would cause 3 to
absorb in this region or at longer wavelengths, yet 3 does not
absorb strongly within this spectral range.
Trends observed in the photoemission spectra of 1, 3, 4,
and 5 are also consistent with the interpretation that efficient
benzannulation of 1 occurred (Figure 4b). The emission of
PPE 1 shows well-known components of both isolated and
aggregated PPE with local maxima at 426 nm and 447 nm,
respectively.[18] Upon benzannulation, the maximum emission
blue-shifted by 22 nm but also broadens significantly. Emis-
sion over a similarly broad wavelength range was observed by
Hartley and co-workers in a series of o-phenylene oligomers,
which was attributed to emission from two different con-
formational states.[8a] FTIR spectroscopy of
3 showed
2
À1
À
increased C(sp ) H stretches at 3030 cm from the newly
installed naphthalene rings (Supporting Information, Fig-
ure S27), as well as significant changes in the 500–1500 cmÀ1
region that suggest major modifications to the structure of 3
relative to its PPE precursor 1. In summary, UV/Vis,
fluorescence, and FTIR spectroscopy are each consistent
with efficient benzannulation of PPE 1.
Figure 4. Characterization of the benzannulation of PPE 1. a) UV/
visible absorption, and b) photoemission spectra of PPE 1, benzannu-
lated polymer 3, dialkyne 4, and its benzannulation product 5. The
steric congestion of 3 reduces the effective conjugation length, shifting
the absorbance of the polymer to higher energy.
Finally, an isotopic-labeling study also provided direct
evidence for the high efficiency of the benzannulation
reaction. A sample of PPE 1 with 13C-enriched alkyne
moieties was prepared by employing [13C2]trimethylsilyl-
acetylene in the synthesis of the dialkyne monomer. The
most intense resonances in 13C NMR spectrum of this sample
(Figure 5a) are centered at 92.8 ppm and correspond to the
13C-enriched alkyne moieties. The three small resonances at
128.3, 129.3, and 131.5 ppm correspond to aromatic carbon
atoms with naturally abundant amounts of 13C, and occur at
identical chemical shifts as the three most intense aromatic
resonances in the spectrum of unlabeled 1 (Supporting
Information, Figure S10). After benzannulation, the signals
of the 13C-enriched alkyne moieties were shifted cleanly to
a broad peak centered at 139.5 ppm, and no residual alkyne
resonances are visible above the baseline (Figure 5b). Both
the 1H and 13C NMR spectra of the 13C-labeled and unlabeled
benzannulated polymer obtained at room temperature show
peak broadening as a consequence of hindered rotation about
the newly installed 2,3-disubstituted naphthalene moieties,
consistent with the above MD simulations. 13C NMR spectra
obtained in C2D2Cl4 sharpen at higher temperatures, such that
the nonequivalent 13C-enriched carbon signals are resolved.
These resonances are shown in a partial spectrum obtained at
1358C as an inset of Figure 5b, and complete spectra obtained
in 108 increments between 25–1358C are provided in the
Supporting Information (Figures S24 and S25). These reso-
nances are coupled to one another, as determined by a 2D-
INADEQUATE experiment (Supporting Information, Fig-
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Keywords: conjugation · cycloaddition · graphene ·
.
isotopic labeling · polymers
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Figure 5. Partial 13C NMR spectra (CDCl3, 100 MHz, RT) of a) 13C-
enriched 1, which was benzannulated to provide b) 13C-enriched 3. The
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benzannulation reaction tolerates additional aromatic sub-
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polyphenylene with limited conformational freedom, as
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Received: August 28, 2012
Published online: October 25, 2012
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