Controlled catalyst transfer polycondensation and surface-initiated polymerization of a p-phenyleneethynylene-based monomer

Article abstract of DOI:10.1021/ja401740m

Herein, we describe a catalyst transfer polycondensation that enabled access to well-defined poly(p-phenyleneethynylene) (PPE), a prominent conjugated polymer. Treatment of a stannylated 4-iodophenylacetylene derivative with PhPd(t-Bu₃P)Br afforded the corresponding PPE in up to 94% yield. Under optimized conditions, the molecular weight of the polymer increased linearly with monomer consumption, and was controlled by adjusting the initial monomer-to-catalyst ratio. The chain-growth nature of the polymerization reaction was utilized to produce well-defined PPE-containing block copolymers, as well as to grow PPE brushes from silica nanoparticles via a surface-initiated polymerization process.

Full text of DOI:10.1021/ja401740m

Communication
pubs.acs.org/JACS
Controlled Catalyst Transfer Polycondensation and Surface-Initiated
Polymerization of a p‑Phenyleneethynylene-Based Monomer
Songsu Kang,‡ Robert J. Ono,‡ and Christopher W. Bielawski*
Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712, United States
S
* Supporting Information
catalyzed cross-coupling reactions; however, their distinguishing
ABSTRACT: Herein, we describe a catalyst transfer
polycondensation that enabled access to well-defined
poly(p-phenyleneethynylene) (PPE), a prominent con-
jugated polymer. Treatment of a stannylated 4-iodophe-
nylacetylene derivative with PhPd(t-Bu3P)Br afforded the
corresponding PPE in up to 94% yield. Under optimized
conditions, the molecular weight of the polymer increased
linearly with monomer consumption, and was controlled
by adjusting the initial monomer-to-catalyst ratio. The
chain-growth nature of the polymerization reaction was
utilized to produce well-defined PPE-containing block
copolymers, as well as to grow PPE brushes from silica
nanoparticles via a surface-initiated polymerization proc-
ess.
feature is that the oxidative addition of the catalyst occurs in an
intra-chain fashion that facilitates “living” chain-growth-like
behavior of the corresponding polymerization. Herein, we report
the first controlled chain-growth synthesis of PPE via a modified
Stille-type CTP.
As summarized in Scheme 1, initial efforts were directed
toward the polymerization of 1,4-bis(2-ethylhexyloxy)-2-ethyn-
a
Scheme 1. Chain-Growth Synthesis of PPE
oly(phenyleneethynylene)s (PPEs) are an important and
Pversatile class of conjugated polymers that have found use in
light emitting diodes,1 explosives detection,2 and molecular
wires,3 among other applications. The aforementioned polymers
are most commonly prepared by condensing a 1,4-dihaloarene
with a 1,4-diethynylbenzene using Sonogashira-type cross
coupling chemistry4 or by alkyne metathesis of a diyne,5
although variants of both methodologies are known. In all
cases, however, the corresponding polymerizations proceed via a
step-growth mechanism which precludes an ability to rigorously
control the important properties exhibited by the polymer
produced, including molecular weight, dispersity, and end-group
fidelity when semi- or hetero-telechelic polymers are desired.
Moreover, the lack of synthetic control can lead to ill-defined
materials or batch-to-batch inconsistencies. A controlled, chain-
growth polymerization of a phenyleneethynylene would not only
alleviate the aforementioned drawbacks, but also enable the
preparation of more complex PPE-containing macromolecular
structures, such as block copolymers and surface-grafted polymer
brushes, thereby facilitating the realization of applications long
envisioned for PPEs.4,5
We postulated that a step-growth polymerization of phenyl-
eneethynylene may be transformed into a chain-growth process
by developing a suitable catalyst transfer polycondensation
(CTP) method. Chain-growth CTP has been successfully
utilized for the controlled synthesis of conjugated polymers
including polythiophenes,6 polyfluorenes,7 and polyphenylenes,8
as well as for the preparation of more complex donor−acceptor-
type alternating copolymers.9,10 Mechanistically, it is generally
accepted that CTPs undergo the same oxidative addition−
reductive elimination cycles that are typical of transition metal-
a
Pd Catalyst = PhPd(t-Bu3P)Br; ligand: see Table 1.
yl-5-iodobenzene (1) using Sonogashira-coupling conditions.
Although a variety of Pd- and Ni-catalysts afforded polymeric
materials (results not shown), PhPd(t-Bu3P)Br gave high
molecular weight, monodisperse polymer in good yield. For
example, treatment of 1 ([1]0 = 0.020 M) with copper iodide (20
mol %), PPh3 (20 mol %), and PhPd(t-Bu3P)Br11 (2 mol %) in
THF at 25 °C afforded a PPE in 58% yield (Table 1, entry 1).
While the aforementioned reaction conditions did provide
polymer of a desirable number-averaged molecular weight (Mn)
and dispersity (Đ) (Mn = 10 800, Đ = 1.28), other conditions
were explored to increase the conversion of monomer 1 to
polymer. Attention was directed toward using a stannylated
monomer, ((2,5-bis(2-ethylhexyloxy)-4-iodophenyl)ethynyl)-
tributylstannane (2), as alkynyltin reagents under Stille-type
conditions have been shown to be more reactive than the
corresponding terminal alkynes.12,13 Indeed, under otherwise
identical reaction conditions, the polymerization of 2 proceeded
to high conversion (>99%) within 3 h, and a PPE was obtained
by precipitation from methanol as a yellow solid in 94% yield
(Table 1, entry 7). The isolated polymer exhibited a Mn of 14.4
kDa, a value close to the theoretically expected Mn of 17.8 kDa
assuming quantitative initiation and complete consumption of
monomer, and a low Đ of 1.47.14,15 Moreover, upon initiation,
the catalyst afforded polymers with a phenyl group at a chain
Received: February 17, 2013
Published: March 25, 2013
© 2013 American Chemical Society
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Journal of the American Chemical Society
Communication
a
the initial monomer to catalyst ratio (i.e., [2]0/[PhPd(t-
Bu3P)Br]0) while the dispersity remained constant (Figure
1B). Taken together, these results suggested to us that the
polymerizations proceeded in a controlled, chain-growth
manner.
According to the CTP mechanism, all of the propagating
polymer chains should contain the catalytically active metal at a
chain terminus and therefore display living polymerization
characteristics as long as chain termination is suppressed. Thus,
dormant polymer chains synthesized by CTP should act as
macroinitiators that provide access to chain extended polymers
or well-defined block copolymers. To test this hypothesis, a chain
extension experiment was carried out wherein a fresh batch of
monomer 2 was added, upon the complete consumption of the
initial bolus of monomer, to an unquenched solution of growing
PPE. As shown in Figure 2A, the SEC curve of the polymer
Table 1. Syntheses of PPEs under Various Conditions
c
c
CuI
(mol %)
Mn
Đ
yield
d
b
entry
ligand (mol %)
(kDa)
(Mw/Mn)
(%)
1
2
3
4
5
6
7
8
9
10
20
20
20
20
0
PPh3 (20)
PCy3 (20)
P(2-furyl)3 (20)
P(t-Bu)3
10.8
5.0
1.28
1.31
1.69
1.60
1.54
1.47
1.47
1.34
1.38
1.43
58
26
99
64
63
30
94
88
72
37
17.2
6.8
PPh3 (20)
none
7.6
20
20
10
10
2
5.6
PPh3 (20)
PPh3 (10)
PPh3 (15)
PPh3 (2)
14.4
11.0
10.0
3.6
a
Polymerization conditions: [monomer]0 = 0.020 M, Pd catalyst (2
mol %), THF, 25 °C, 2 to 3 h. Entry 1: monomer 1 was used; entries
2−10: monomer 2 was used. Determined by size exclusion
chromatography (SEC) in THF against polystyrene standards.
b
c
d
Isolated yields.
terminus,8,16 as determined by H NMR spectroscopy (see
1
Supporting Information (SI)).
A key feature of the polymerization of 2 with PhPd(t-Bu3P)Br
is that both CuI17 and additional phosphine ligand were
necessary to achieve high molecular weight polymer (see Table
1, entries 5 and 6). Thus, to further optimize the polymerization
reaction, the effect of using different phosphine ligands and
different ligand loadings was examined (Table 1). For example,
the use of P(2-furyl)3 afforded quantitative conversion of
monomer to polymer, albeit with a higher Đ (entry 3).
Subsequent efforts were directed toward using PPh3 as this
ligand provided comparably high reaction conversions while still
producing polymers that exhibited low dispersities. Although
lowering the loading of PPh3 and CuI as well as varying the ratio
of PPh3 to CuI did result in a narrowing of the measured Đ
(entries 8−10), the results were accompanied by decreases in
both Mn and yield of polymer. These data were consistent with a
controlled polymerization reaction that proceeded at a relatively
reduced rate. Ultimately, the conditions summarized in Table 1,
entry 7 were considered to be optimal and employed for
subsequent reactions.
The ability to synthesize a PPE with low dispersity and a
molecular weight in agreement with the expected value was
consistent with the polymerization proceeding in a chain-growth
manner. To test, the molecular weight of the polymer formed as a
function of monomer conversion was monitored over time. As
shown in Figure 1A, the Mn of the polymer increased linearly
with monomer conversion, while the Đ remained relatively
constant. Furthermore, when the polymerization of 2 was carried
out at varying initial catalyst loadings, the Mn of the polymer
(obtained at >99% monomer conversion) was proportional to
Figure 2. Size exclusion chromatograms of polymers obtained via chain
extension. (A) PPE homopolymer before (black line; Mn = 10 500 Da, Đ
= 1.40) and after (red line; Mn = 23 700 Da, Đ = 1.48) adding a second
feed of 2. (B) PPE macroinitiator (black line; Mn = 10 500 Da, Đ = 1.39)
and corresponding PPE-b-poly(fluorenylethynylene) block copolymer
(red line; Mn = 16 000 Da, Đ = 1.55).
obtained at the end of the reaction shifted toward higher
molecular weight than the PPE analyzed prior to the second
monomer addition (isolated yield: 86%). Furthermore, the curve
assigned to the chain extended polymer was monomodal and
exhibited a Đ similar to that displayed by the macroinitiator prior
to the introduction of additional monomer. These results
suggested to us that the macroinitiation efficiency was high and
enabled the synthesis of well-defined block copolymers. Indeed,
2 was sequentially polymerized with 2-tributylstannylethynyl-7-
iodo-9,9-dioctylfluorene, to afford a PPE-b-poly-
(fluorenylethynylene) copolymer in 92% yield (Figure
2B),14,18 which is a rare example of a fully conjugated
poly(aryleneethynylene) block copolymer expected to exhibit
useful properties for light emitting diode applications.
Having verified that the polymerization of 2 proceeded in a
controlled, chain-growth fashion, we reasoned that this method-
ology would be an excellent candidate for the synthesis of
surface-bound PPE brushes via surface-initiated polymerization.
The surface-initiated or “grafting-from” polymerization approach
represents one of the most powerful methods for attaching
polymers to surfaces because it offers a high degree of control
over polymer grafting density, thickness, and composition.19 To
date, relatively few examples describe the surface-initiated
polymerization of conjugated polymers from surfaces, all of
which were accomplished through the use of Kumada or Suzuki-
type CTP.20
We therefore turned our attention toward exploring the
surface-initiated polymerization of 2; our general approach is
outlined in Scheme 2. Silica nanoparticles represented an ideal
substrate for these studies due to their well-established surface
Figure 1. Mn (left Y-axis) and dispersity (right Y-axis) plotted (A) as a
function of monomer conversion and (B) as a function of initial
monomer/catalyst loading.
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Scheme 2. Surface-Initiated Polymerization of 2
chemistry and tendency to assemble into highly ordered close-
packed colloidal arrays,21 a feature which may be exploited for
optoelectronic applications20d,22 and/or the generation of
“smart” surfaces.23 Furthermore, the use of silica nanoparticles
offered a practical advantage as the immobilized polymers may be
detached from the solid surface for further analysis by treating the
silica−polymer composites with aqueous hydrofluoric acid (HF).
Spherical silica particles with an average diameter of ∼200 nm
Figure 3. STEM-EDX analysis of SiO2-PhBr (A, C, E) and SiO2-Pd (B,
D, F): Secondary electron (SE) image of (A) SiO2-PhBr and (B) SiO2-
Pd. Elemental mapping analysis showing the presence of (C) Si on
SiO2-PhBr, (D) Si on SiO2-Pd, (E) Br on SiO2-PhBr, and (F) Pd on
SiO2-Pd. Scale bar = 200 nm.
were prepared using the Stober process.23,24 To provide
̈
functional handles on the surface of the nanoparticles for catalyst
immobilization, silanization was performed using [2-(4-
bromophenyl)-ethyl]-triethoxysilane. Although scanning elec-
tron microscopy (SEM) images of the silica particles recorded
before and after silanization showed that the size of the particles
remained essentially unchanged (see SI), thermogravimetric
analysis (TGA) of the latter confirmed that organic residues were
immobilized on the silica surface to afford SiO2-PhBr. Using the
weight retention values obtained from the TGA data, a grafting
density of 3.6 μmol/m2 was calculated for the organosilane,
which corresponded to a cross-sectional area of 0.46 nm2/
organosilane (see SI). The aforementioned value was consistent
with literature reports23,25,26 and further verified by elemental
analysis, which gave a similar grafting density value of 4.19 μmol/
m2.
Having confirmed surface coverage of the nanoparticles using
the bromobenzene-containing organosilane, subsequent efforts
were directed toward the generation of a Pd(II)-containing
polymerization initiator. Recently, Kiriy reported the Pd-
catalyzed Suzuki CTP of a poly(9,9-dialkylfluorene) from
cross-linked poly(4-bromostyrene) films, having prepared the
polymerization initiator by treating the poly(4-bromostyrene)
film with a solution of Pd(t-Bu3P)2.20a Building on this
methodology, SiO2-PhBr was reacted with an excess of Pd(t-
Bu3P)2 in toluene at 70 °C for 3 h. The particles were then
subjected to numerous washings with THF to remove unbound
Pd species. After drying under vacuum, the Pd-bound silica
nanoparticles, SiO2-Pd, were recovered as a pale yellow-brown
powder, a distinct color change from the off-white hue of the
SiO2-PhBr precursor. As shown in Figure 3, SEM coupled with
energy dispersive X-ray spectroscopy (STEM-EDX) confirmed
the presence of Br and Pd on the SiO2-PhBr and SiO2-Pd
particles, respectively.27
The surface-initiated polymerization was accomplished by
stirring SiO2-Pd for 8 h in the presence of monomer 2, copper
iodide (20 mol %), and PPh3 (20 mol %) in THF at 50 °C under
an atmosphere of nitrogen. The resulting SiO2-PPE particles
were then purified by repeated centrifugation/redispersion
cycles in THF until the supernatant became colorless and was
later isolated as a yellow powder in 62% yield. We attributed the
modest percent recovery to material losses that occurred during
the purification process;28 however, the monomer was
Figure 4. TGA curves of (a) bare SiO2 particles, (b) SiO2-PhBr, (c)
SiO2-Pd, and (d) SiO2-PPE. The TGA experiments were performed
under an atmosphere of nitrogen at a heating rate of 20 °C min−1.
quantitatively consumed during the surface-initiated polymer-
1
ization, as determined by H NMR analysis against an internal
standard. Thermal analysis of SiO2-PPE revealed that the
isolated material lost 14.1% of its weight between 100 and 800
°C, an increase of 5.4% and 7.8% when compared to the percent
mass lost by SiO2-PhBr and virgin SiO2, respectively, over the
same temperature range (Figure 4). To further characterize the
PPE produced in the reaction, the grafted polymers were
detached from the silica surface by treating the nanoparticles with
aq HF, and analyzed. The detached polymeric material exhibited
a Mn of 24.5 kDa, as determined by SEC analysis, as well as 1H
NMR signals similar to those displayed by an analogous PPE
synthesized under homogeneous conditions.
To gain further insight into the surface-initiated polymer-
ization, a chain extension experiment similar to that previously
performed for the homogeneous polymerization of 2 using
PhPd(t-Bu3P)Br was carried out as follows: a fresh feed of 2 was
added to an unquenched solution of newly prepared SiO2-PPE,
allowed to react for 8 h, and then quenched with dilute aq HCl.
After purification by centrifugation, the resultant material, along
with a sample isolated from an aliquot taken before the
introduction of additional monomer, was treated with aq HF
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13156. (c) Elmalem, E.; Biedermann, F.; Johnson, K.; Friend, R. H.;
Huck, W. T. S. J. Am. Chem. Soc. 2012, 134, 17769.
(8) Yokozawa, T.; Kohno, H.; Ohta, Y.; Yokoyama, A. Macromolecules
2010, 43, 7095.
(9) Elmalem, E.; Kiriy, A.; Huck, W. T. S. Macromolecules 2011, 44,
9057.
to remove the silica core, and analyzed by SEC. The Mn of the
liberated PPE was effectively doubled (11.3 to 21.9 kDa) as
measured from aliquots quenched before and after the second
monomer addition, respectively (see SI).29 These results,
coupled with a shift in the monomodal SEC trace toward higher
molecular weight, suggested to us that the controlled, chain-
growth CTP mechanism was operative under heterogeneous
conditions.
In conclusion, we report the first catalyst transfer poly-
condensation of a p-phenyleneethynylene-based monomer to
afford a PPE of controlled molecular weight and low dispersity.
The polymerization methodology was determined to proceed in
a controlled, chain-growth fashion, which facilitated the
preparation of diblock copolymers by straightforward sequential
monomer addition. We also demonstrated the first surface-
initiated synthesis of surface-grafted PPE using Pd-functionalized
SiO2 nanoparticles as the solid substrate and polymerization
initiator, although we note that the presented methodology
should be applicable to a variety of solid substrates, both curved
and flat. We believe that the protocol described herein will create
new opportunities in optoelectronic and other applications for
PPEs and potentially other conjugated polymers. We are
particularly interested in understanding how surface-immobiliza-
tion influences the optical properties of PPE as well as the self-
assembly30 of composite nanoparticles such as SiO2-PPE.
(10) Ono, R. J.; Kang, S.; Bielawski, C. W. Macromolecules 2012, 45,
2321.
(11) Stambuli, J. P.; Incarvito, C. D.; Buhl, M.; Hartwig, J. F. J. Am.
̈
Chem. Soc. 2004, 126, 1184.
(12) Giardina, G.; Rosi, P.; Ricci, A.; Lo Sterzo, C. J. Polym. Sci., Part A:
Polym. Chem. 2000, 38, 2603.
(13) Suraru, S.-L.; Wurthner, F. Synthesis 2009, 11, 1841.
̈
(14) Using ICP-MS, less than 0.5 ppb of tin was detected in the isolated
PPE homopolymer and less than 1.54 ppb of tin was detected in the
PPE-b-poly(fluorenylethynylene) copolymer.
(15) Diyne defects were not detected by 13C NMR spectroscopy.
(16) Yokoyama, A.; Suzuki, H.; Kubota, Y.; Ohuchi, K.; Higashimura,
H.; Yokozawa, T. J. Am. Chem. Soc. 2007, 129, 7236.
(17) Farina, V.; Kapadia, S.; Krishnan, B.; Wang, C.; Liebeskind, L. S. J.
Org. Chem. 1994, 59, 5905.
(18) See SI for polymerization conditions and further characterization
details.
(19) Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. Rev.
2004, 33, 14.
(20) (a) Beryozkina, T.; Boyko, K.; Khanduyeva, N.; Senkovskyy, V.;
Horecha, M.; Oertel, U.; Simon, F.; Stamm, M.; Kiriy, A. Angew. Chem.,
Int. Ed. 2009, 48, 2695. (b) Tkachov, R.; Senkovskyy, V.; Horecha, M.;
Oertel, U.; Stamm, M.; Kiriy, A. Chem. Commun. 2010, 46, 1425.
(c) Senkovskyy, V.; Khanduyeva, N.; Komber, H.; Oertel, U.; Stamm,
M.; Kuckling, D.; Kiriy, A. J. Am. Chem. Soc. 2007, 129, 6626.
(d) Senkovskyy, V.; Tkachov, R.; Beryozkina, T.; Komber, H.; Oertel,
U.; Horecha, M.; Bocharova, V.; Stamm, M.; Gevorgyan, S. A.; Krebs, F.
C.; Kiriy, A. J. Am. Chem. Soc. 2009, 131, 16445. (e) Doubina, N.;
Jenkins, J. L.; Paniagua, S. A.; Mazzio, K. A.; MacDonald, G. A.; Jen, A. K.
Y.; Armstrong, N. R.; Marder, S. R.; Luscombe, C. K. Langmuir 2012, 28,
1900. (f) Sontag, S. K.; Sheppard, G. R.; Usselman, N. M.; Marshall, N.;
Locklin, J. Langmuir 2011, 27, 12033. (g) Marshall, N.; Sontag, S. K.;
Locklin, J. Macromolecules 2010, 43, 2137.
ASSOCIATED CONTENT
* Supporting Information
Additional details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
bielawski@cm.utexas.edu
■
Author Contributions
‡S.K. and R.J.O. contributed equally.
(21) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater.
1999, 11, 2132.
(22) Labastide, J. A.; Baghgar, M.; Dujovne, I.; Yang, Y.; Dinsmore, A.
D.; G. Sumpter, B.; Venkataraman, D.; Barnes, M. D. J. Phys. Chem. Lett.
2011, 2, 3085.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(23) Li, D.; Sheng, X.; Zhao, B. J. Am. Chem. Soc. 2005, 127, 6248.
This work was supported as part of the program “Understanding
Charge Separation and Transfer at Interfaces in Energy Materials
(EFRC:CST)”, an Energy Frontier Research Center funded by
the U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences, under Award DE-SC0001091.
(24) Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62.
̈
(25) Kim, J. W.; Kim, L. U.; Kim, C. K. Biomacromolecules 2006, 8, 215.
(26) Bartholome, C.; Beyou, E.; Bourgeat-Lami, E.; Chaumont, P.;
Zydowicz, N. Macromolecules 2003, 36, 7946.
(27) The presence of phosphine was also detected on SiO2-Pd using
STEM-EDX; see SI.
(28) Some ungrafted PPE was also produced during the reaction and
separated from the SiO2-PPE composite particles by centrifugation. A
control experiment ruled out nonspecific adsorption; see the SI for
additional details.
(29) Đ values of 2.5 and 3.8 were recorded for PPE samples isolated
before and after the second monomer addition step, respectively. We
surmise that slow initiation during the surface-initiated polymerization,
when compared to the corresponding homogeneous polymerization,
may contribute to broadening of the dispersity.
(30) For discussions on the ordering of PPE chains, see: (a) Bunz, U.
H. F.; Imhof, J. M.; Bly, R. K.; Bangcuyo, C. G.; Rozanski, L.; Vanden
Bout, D. A. Macromolecules 2005, 38, 5892. (b) Bunz, U. H. F.;
Enkelmann, V.; Kloppenburg, L.; Jones, D.; Shimizu, K. D.; Claridge, J.
B.; zur Loye, H.-C.; Lieser, G. Chem. Mater. 1999, 11, 1416. (c) Wilson,
J. N.; Steffen, W.; McKenzie, T. G.; Lieser, G.; Oda, M.; Neher, D.; Bunz,
U. H. F. J. Am. Chem. Soc. 2002, 124, 6830.
REFERENCES
■
(1) (a) Grimsdale, A. C.; Leok Chan, K.; Martin, R. E.; Jokisz, P. G.;
Holmes, A. B. Chem. Rev. 2009, 109, 897. (b) Pschirer, N. G.; Miteva, T.;
Evans, U.; Roberts, R. S.; Marshall, A. R.; Neher, D.; Myrick, M. L.;
Bunz, U. H. F. Chem. Mater. 2001, 13, 2691.
(2) (a) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chem. Rev. 2007, 107,
1339. (b) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864.
(3) Allara, D. L.; Arnold, J. J.; Bumm, L. A.; Burgin, T. P.; Cygan, M. T.;
Dunbar, T. D.; Jones, L., II; Tour, J. M.; Weiss, P. S. Science 1996, 271,
1705.
(4) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605.
(5) Bunz, U. H. F. Acc. Chem. Res. 2001, 34, 998.
(6) (a) Osaka, I.; McCullough, R. D. Acc. Chem. Res. 2008, 41, 1202.
(b) Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. J. Am. Chem. Soc. 2005,
127, 17542.
(7) (a) Yokoyama, A.; Suzuki, H.; Kubota, Y.; Ohuchi, K.;
Higashimura, H.; Yokozawa, T. J. Am. Chem. Soc. 2007, 129, 7236.
(b) Zhang, H.-H.; Xing, C.-H.; Hu, Q.-S. J. Am. Chem. Soc. 2012, 134,
4987
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