Conjugated polymers that respond to oxidation with increased emission

Article abstract of DOI:10.1021/ja1019063

Thioether-containing poly(para-phenylene-ethynylene) (PPE) copolymers show a strong fluorescence turn-on response when exposed to oxidants in solution as a result of the selective conversion of thioether substituents into sulfoxides and sulfones. We propose that the increase in fluorescence quantum yield (κ_F) upon oxidation is the result of both an increase in the rate of fluorescence (k_F), as a result of greater spatial overlap of the frontier molecular orbitals in the oxidized materials, and an increase in the fluorescence lifetime (τ_F), due to a decrease in the rate of nonradiative decay. Contrary to established literature, the reported sulfoxides do not always act as fluorescence quenchers. The oxidation is accompanied by spectral changes in the absorption and emission of the polymers, which are dramatic when oxidation causes the copolymer to acquire a donor-acceptor interaction. The oxidized polymers have high fluorescence quantum yields in the solid state, with some having increased photostability. A turn-on fluorescence response to hydrogen peroxide in organic solvents in the presence of an oxidation catalyst indicates the potential of thioether-containing materials for oxidant sensing. The reported polymers show promise as new materials in applications where photostability is important, where tunability of emission across the visible spectrum is desired, and where efficient emission is an advantage.

Full text of DOI:10.1021/ja1019063

Published on Web 05/14/2010
Conjugated Polymers That Respond to Oxidation with
Increased Emission
Eric L. Dane, Sarah B. King, and Timothy M. Swager*
Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts AVenue,
Cambridge, Massachusetts 02139
Received March 6, 2010; E-mail: tswager@mit.edu
Abstract: Thioether-containing poly(para-phenylene-ethynylene) (PPE) copolymers show a strong fluo-
rescence turn-on response when exposed to oxidants in solution as a result of the selective conversion of
thioether substituents into sulfoxides and sulfones. We propose that the increase in fluorescence quantum
yield (ΦF) upon oxidation is the result of both an increase in the rate of fluorescence (kF), as a result of
greater spatial overlap of the frontier molecular orbitals in the oxidized materials, and an increase in the
fluorescence lifetime (τF), due to a decrease in the rate of nonradiative decay. Contrary to established
literature, the reported sulfoxides do not always act as fluorescence quenchers. The oxidation is
accompanied by spectral changes in the absorption and emission of the polymers, which are dramatic
when oxidation causes the copolymer to acquire a donor-acceptor interaction. The oxidized polymers
have high fluorescence quantum yields in the solid state, with some having increased photostability. A
turn-on fluorescence response to hydrogen peroxide in organic solvents in the presence of an oxidation
catalyst indicates the potential of thioether-containing materials for oxidant sensing. The reported polymers
show promise as new materials in applications where photostability is important, where tunability of emission
across the visible spectrum is desired, and where efficient emission is an advantage.
be used to make dangerous explosive devices.4 On the other
hand, hydrogen peroxide plays important biological roles,
Introduction
Living in an oxidizing environment is essential for most
life, but it has generally been an impediment to the use of
fluorescent conjugated polymers (CPs) in practical applica-
tions. Fluorescent CPs are an important class of functional
materials that have found use in applications ranging from
organic light emitting diodes (OLEDs)1 to the detection of
TNT vapor at ppb levels via amplified fluorescence quench-
ing.2 Because most conjugated polymers become less emis-
sive when oxidized, photobleaching attributed to reactions
involving oxygen has limited their use.3 To better understand
how to synthesize materials that are robust in our oxygen-
rich atmosphere, we became interested in CPs that upon
oxidation showed productive changes in emission, such as a
large increase in emission, a significant shift in emission
wavelength, or greater photostability.
including being a disease marker and acting as a signaling
molecule.5 Therefore, compounds that show an increase in
fluorescence when oxidized by peroxides are important to the
development of new sensing materials.
Previous studies on the photobleaching of fluorescent CPs
have focused on understanding the mechanism. For example,
fluorenone defects in polyfluorene act as low-energy trap sites
that cause decreased emission intensity.6 In poly(para-phe-
nylene-vinylene) (PPV) polymers, it has been suggested that
the main degradation pathway involves the reaction of singlet
oxygen with the alkenyl group, resulting in the formation of
carbonyl defects that eventually result in chain scission.3 In
poly(para-phenylene-ethynylene) (PPE)7 polymers, the mech-
anism of photobleaching has not been elucidated, but studies
suggest that in model systems alkynes are less prone to reaction
with singlet oxygen than alkenes.8 However, it is not clear that
the observed photobleaching results solely from covalent bond
Materials that show an increase in emission when oxidized
are also candidates for the turn-on fluorescence sensing of
peroxide oxidants. Being able to detect organic peroxides, such
as triacetone triperoxide (TATP), is important because they can
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A. E.; Swager, T. M. Chem. ReV. 2000, 100, 2537–2574.
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7758 J. AM. CHEM. SOC. 2010, 132, 7758–7768
10.1021/ja1019063  2010 American Chemical Society

CPs That Respond to Oxidation with Increased Emission
A R T I C L E S
formation between the CPs and oxygen species, because in some
cases it has been shown to be reversible.9 Although the specific
mechanism is not known, CPs show greater photostability when
irradiated under anaerobic conditions indicating that the presence
of oxygen is important to photobleaching.10 Previous work from
our group has found that antioxidants and triplet quenchers, such
as cyclooctatetraene derivatives, can be added to thin films of
CPs to attenuate photobleaching under irradiation.11 Addition-
ally, we have shown that polymers with high ionization
potentials, due to the presence of electron-withdrawing per-
fluoroalkyl substituents, showed resistance to photobleaching.12
Building on these previous investigations, this report focuses
on designing CPs that respond positively to oxidation.
Previous examples of conjugated polymers where it was
shown that oxidation did not cause a decrease in emission
include polyfluorenes that contain small amounts of either a
phosphafluorene or phosphafluorene oxide comonomer.13 Com-
pared to the phosphine polymer’s emission, the phosphine oxide
polymer’s emission was red-shifted in the solid state, but not
in solution. However, the oxidized polymers had roughly the
same emission efficiency as the unoxidized polymers, and in
this example, the oxidation was carried out on the monomer
before polymerization. We chose to append sulfur atoms, in
the form of thioethers, directly to the polymer backbone because
thioethers can be selectively oxidized in the presence of alkynes.
In organic compounds, sulfur can have a formal oxidation
state ranging from -2 to +6.14 In this investigation, we were
interested in what effect changing the formal oxidation state
from a thioether (-2) to that of a sulfoxide (0) or a sulfone
(+2) would have on the photophysical properties of a PPE.
Additionally, we hypothesized that the electron-withdrawing
nature of the oxidized thioethers would lead to changes in
absorption and emission wavelengths due to donor-acceptor
interactions.
We designed a monomer that contains four thioethers divided
between two dithiolane rings. Because the sulfur atoms of 1
(Scheme 1) are not members of an aromatic ring system, it was
anticipated that they would behave differently than a sulfur atom
in a thiophene ring system. The five-member rings have the
advantage of tying back the sulfur atoms (θCCS ) 123°-124°;
see Scheme 1, X-ray structure of 1) and minimizing their steric
interference with the reactive positions used in polymerization.
Additionally, they allow for the facile introduction of solubi-
lizing groups from readily available symmetrical and asym-
metrical ketones via acid-catalyzed condensation. It was an-
ticipated that the branching of the aliphatic solubilizing groups
would aid in solubility and prevent aggregation in the solid state.
Monomers and polymers with a similar geometry with oxygen
in place of sulfur have been recently described.19
Results
Synthesis. Monomer 1 is available in three steps from
inexpensive starting materials without the need for chromatog-
raphy. Compound 2 was synthesized via complete nucleophilic
aromatic substitution of 1,2,4,5-tetrachlorobenzene with sodium
2-methylpropane thiolate in refluxing dimethylformamide, fol-
lowed by recrystallization from refluxing methanol (59%
yield).20 The tert-butyl groups were deprotected in situ, and the
resulting thiols were condensed with 5-nonanone in the presence
of HBF4 ·Et2O in refluxing toluene. Compound 3 could not be
successfully brominated or iodinated via electrophilic substitu-
tion, presumably due to nucleophilic attack by the thioethers
on the electrophilic reagents and subsequent side reactions.
Rather than pursue a method of sequential lithiations, we used
an excess of lithium tetramethylpiperidine and chlorotrimeth-
ylsilane to drive the double deprotonation of 3 to completion
in one flask.21 The resulting ditrimethylsilyl product was carried
through to be iodinated with ICl in CH2Cl2. Monomer 3 was
purified by recrystallization from hexane and obtained in an
overall yield of 38% as an air-stable yellow solid.
Conjugated polymers that contain sulfur atoms are ubiquitous
in materials chemistry, but almost exclusively the sulfur atom
is contained in a thiophene ring.15 Surprisingly, we can find
relatively few examples of the incorporation of thioethers into
CPs, though a review by Gingras16 summarizes how chemists
have exploited the properties of persulfurated aromatic com-
pounds. Lehn17 synthesized a series of diacetylene-linked
poly(phenylthio)-substituted benzenes that showed the ability
to be multiply reduced, taking advantage of sulfur’s ability to
stabilize a negative charge. Additionally, several examples of
thioether-containing PPV CPs have been reported.18 In general,
aromatic sulfones, and especially aromatic sulfoxides, have not
been extensively utilized in materials chemistry.
Model compounds (MC1 and MC2) and polymers (P1 and
P2) were synthesized via Sonagashira cross-coupling in good
yield. P1 was isolated as a high molecular weight polymer (Mn
) 236 000 g/mol) according to gel permeation chromatography
(GPC) and was soluble in common organic solvents such as
CH2Cl2 and THF. Under the same conditions, P2 formed high
molecular weight polymers that became insoluble, either because
with increased chain length the polymers become insoluble or
because of the presence of cross-linking. To isolate soluble
polymers, the temperature of the polymerization was lowered
from 65 to 45 °C. At the lower reaction temperature, the
molecular weight of P2 (Mn ) 18 900 g/mol) decreased and
the isolated polymer was soluble. In addition to GPC, the
(9) Park, S.-J.; Gesquiere, A. J.; Yu, J.; Barbara, P. F. J. Am. Chem. Soc.
2004, 126, 4116–4117.
1
polymers were characterized by H and 13C NMR and FT-IR.
(10) (a) Yan, M.; Rothberg, L. J.; Papadimitrakopoulos, F.; Galvin, M. E.;
Miller, T. M. Phys. ReV. Lett. 1994, 73, 744–747. (b) Kocher, C.;
Montali, A.; Smith, P.; Weder, C. AdV. Funct. Mater. 2001, 11, 31–
35.
Oxidation of Model Compounds. Model compounds MC1
and MC2 were oxidized with 1 and 2 equiv of m-chloroper-
(11) Andrew, T. L.; Swager, T. M. Macromolecules 2008, 41, 8306–8308.
(12) (a) Kim, Y.; Swager, T. M. Chem. Commun. 2005, 372–374. (b) Kim,
Y.; Whitten, J. E.; Swager, T. M. J. Am. Chem. Soc. 2005, 127, 12122–
12130.
(18) (a) Yoon, C.-B.; Kang, I.-N.; Shim, H.-K. J. Polym. Sci., Part A:
Polym. Chem. 1997, 35, 2253–2258. (b) Shim, H. K.; Yoon, C. B.;
Ahn, T.; Hwang, D. H.; Zyung, T. Synth. Met. 1999, 101, 134–135.
(c) Reisch, H. A.; Scherf, U. Macromol. Chem. Phys. 1999, 200, 552–
561. (d) Hou, J.; Fan, B.; Huo, L.; He, C.; Yang, C.; Li, Y. J. Polym.
Sci., Part A: Polym. Chem. 2006, 44, 1279–1290. (e) Gutierrez, J. J.;
Luong, N.; Zepeda, D.; Ferraris, J. P. Polym. Prepr. (Am. Chem. Soc.,
DiV. Polym. Chem.) 2004, 45, 172–173.
(13) Chen, R.-F.; Zhu, R.; Fan, Q.; Huang, W. Org. Lett. 2008, 10, 2913–
2916.
(14) McNaught, A. D.; Wilkinson, A. IUPAC Compendium of Chemical
Terminology, 2nd ed.; Blackwell Science, 1997.
(15) Roncali, J. Chem. ReV. 1992, 92, 711–738.
(16) Gingras, M.; Raimundo, J.-M.; Chabre, Y. M. Angew. Chem., Int. Ed.
2006, 45, 1686–1712.
(19) Dutta, T.; Woody, K. B.; Parkin, S. R.; Watson, M. D.; Gierschner, J.
J. Am. Chem. Soc. 2009, 131, 17321–17327.
(20) Reddy, T. J.; Iwama, T.; Halpern, H. J.; Rawal, V. H. J. Org. Chem.
2002, 67, 4635–4639.
(17) Mayor, M.; Lehn, J.-M.; Fromm, K. M.; Fenske, D. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 2370–2372.
(21) Krizan, T. D.; Martin, J. C. J. Am. Chem. Soc. 1983, 105, 6155–6157.
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A R T I C L E S
Dane et al.
Scheme 1. Preparation of 1, MC1, MC2, P1, and P2
benzoic acid, and the mono- and disulfoxides of each
(MC1_1ox, MC1_2ox, MC2_1ox, and MC2_2ox) were iso-
lated (Scheme 2). The mono- and disulfoxides were character-
ized with 1H and 13C NMR, FT-IR, and HRMS. For MC2_1ox
and MC1_2ox_cis, X-ray crystallography was used to ensure
the identity of the compound (see Supporting Information for
ORTEP diagrams). For each of the disulfoxides, four diaster-
eomers were obtained (Scheme 2). The cis-isomers, in which
the polar sulfoxide bonds point in the same direction and
increase the overall dipole of the molecule, had a significantly
longer retention time on silica gel as compared to the trans-
isomers, in which the dipoles point in opposite directions. The
two isomers were easily separated via chromatography. As
expected, the directing influence of the thioethers favored a
meta-orientation for the sulfoxides over a para- or an ortho-
relationship, although a small amount of para-compound was
observed in each case. The meta- and para-isomers show
diagnostic 1H and 13C NMR, with the para-isomer having fewer
unique 1H and 13C signals because of its higher symmetry. For
MC1_2ox, the cis- and trans-products contained less than 10%
of the para-isomer according to integration of the proton NMR.
For MC2_2ox, the isolated cis- and trans-products contained
the meta- and para-isomer in a ratio of roughly 3:1. The
selectivity for the second oxidation to occur at the meta-position
as opposed to the para-position results from the first sulfoxide
withdrawing electron density from the sulfur atoms in an ortho-
and para-orientation more effectively than from the sulfur in a
meta-orientation. The ortho-compound was not observed. The
oxidation of the second sulfur in dithioacetals is often much
slower as compared to the oxidation of the first, with the
oxidation of the sulfoxide to sulfone as a competing process.22
Optical and Photophysical Properties of the Model
Compounds in Solution. Both MC1 and MC2 have similar
absorbance and emission properties (see Table 1, Figure 1) in
solution (CH2Cl2). MC1 and MC2 have strong absorbance peaks
between 300 and 400 nm, with a weaker absorbance between
400 and 500 nm. The absorbance maxima for MC2 (375 nm)
is red-shifted as compared to MC1 (355 nm), as a result of the
electron-donating methoxy groups at the 1- and 4-positions of
the outer aryl rings. However, the same effect is not seen in the
less intense band in the visible region, where the absorbance
maximum of MC1 (434 nm) is nearly the same as that of MC2
(435 nm). MC1 and MC2 are relatively nonemissive, with
quantum yields (ΦF) of less than 0.01, and they show broad,
featureless emission centered near 490 nm.
Interestingly, when thoroughly degassed with bubbling
nitrogen, both MC1 and MC2 show an additional sharp, red-
shifted emission peak (MC1, 560 nm; MC2, 580 nm) and a
vibronic shoulder above 600 nm (see Figure S1 in Supporting
Information). Exposure of the samples to air causes the peak to
completely disappear. Quenching of the emission by oxygen
indicates that the emissive process involves an excited triplet
state rather than a singlet state. Room temperature phospho-
rescence is unusual for organic compounds of this type, and it
indicates that the four thioether substituents on the central
(22) Aggarwal, V. K.; Davies, I. W.; Franklin, R.; Maddock, J.; Mahon,
M. F.; Molloy, K. C. J. Chem. Soc., Perkin Trans. 1 1994, 2363–
2368.
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CPs That Respond to Oxidation with Increased Emission
A R T I C L E S
Scheme 2. Oxidation of MC1 and MC2
is accompanied by a blue shift in emission, which is more
pronounced for MC1. The emission maximum of MC1 shifts
from 489 nm for the unoxidized version to 458 nm for the
monosulfoxide (MC1_1ox). The emission maximum of MC2
shifts from 493 nm for the unoxidized version to 467 nm for
the monosulfoxide (MC2_1ox). Further oxidation of MC1_1ox
to MC1_2ox does not cause a significant change in ΦF; however
it is accompanied by a shift in emission maximum from 458 to
403 nm and a change in the structure of the emission.
Conversely, further oxidation of MC2_1ox to MC2_2ox results
in a 10-fold increase in fluorescence quantum yield and is
accompanied by a small red shift of the emission maximum
from 467 to 474 nm. As discussed in the previous section, cis-
and trans-isomers of MC1_2ox and MC2_2ox were isolated
and characterized separately. There appears to be no significant
difference in the photophysical properties of the cis- and trans-
isomers. However, it should be noted that in all cases we isolated
the cis- and trans-isomers as a mixture of regioisomers. Because
the compounds were not separated, we cannot comment on how
the meta- versus para-orientation of the sulfoxides affects the
photophysical properties.
To investigate the effect of further oxidation, we observed
the absorbance and emission changes caused by adding increas-
ing amounts of m-CPBA to dilute solutions of MC1 and MC2
(see Figure S2 in Supporting Information). Increasing oxidation
of MC1 is accompanied by blue-shifted absorbance and
emission and a general increase in emission. Comparison of
the spectrum with those of isolated compounds indicates that,
after the initial formation of MC1_1ox, a significant amount
of MC1_2ox is formed. Additionally, a peak emerges at 422
nm that we attribute to sulfone-containing compounds with
significantly higher quantum yields than the sulfoxides.24 When
MC2 is titrated with oxidant, there is an initial blue shift in
absorbance and emission, also accompanied by a substantial
increase in emission. However, as the compound is further
oxidized the donor-acceptor interaction that develops between
the electron-rich terminal aromatic rings and the increasingly
electron-poor center ring causes a red shift in absorbance and
emission. The red shift in emission is accompanied by a decrease
in emission efficiency; however the resulting fluorophores are
still substantially more emissive than the parent compound.
Oxidation of Polymers. Polymers P1 and P2 were oxidized
with 2.10, 3.15, 4.20, 6.30, and 8.40 equiv of m-CPBA overnight
at room temperature in a CH2Cl2 solution (Figure 2, P1A-D
and P2A-D). The oxidation products of P1 did not show
substantial changes in color; however the reaction mixtures
became noticeably green fluorescent under ambient light.
Conversely, the color of P2 changed from yellow to red with
increasing amounts of oxidation. The effect of the oxidation
on molecular weight was monitored with GPC and is sum-
marized in Table 2. The Mn of the polymers decreased with
greater amounts of oxidation. The addition of 8.4 equiv of
oxidant caused the molecular weight of P1 to decrease by half.
This suggests that oxidation at the alkyne linkages becomes
more competitive with sulfur oxidation with higher amounts of
oxidant. It has been reported that diphenylacetylene slowly reacts
aromatic ring promote intersystem crossing from the singlet
excited state to triplet excited state.23
For both MC1 and MC2, oxidation to the monosulfoxide
causes an increase in ΦF relative to the parent compound, with
the effect being more dramatic for MC2. This increase in ΦF
(24) To understand the effect of sulfone formation, MC1 was treated with
8.0 equiv of m-CPBA in refluxing CH2Cl2 overnight. The least polar
fractions were separated by chromatography. The mixture obtained
showed strong sulfone signals in FT-IR, an absorption maximum at
392 nm, and a broad emission with a maximum near 414 nm. The
sulfone-containing compounds are also much more emissive than the
sulfoxides, with an average quantum yield of 0.40 measured at several
excitation wavelengths between 350 and 380 nm.
(23) Turro, N. J. Modern Molecular Photochemistry; University Science
Books: Sausalito, CA, 1991.
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A R T I C L E S
Dane et al.
Table 1. Model Compound Photophysics in CH2Cl2
a
ΦF
e
compound
λmax (nm)/ε (M-1 ·cm-1
)
λem (nm)
τF (ns)
kF (ns-1
)
MC1
MC1_1ox
MC1_2ox_cis
MC1_2ox_trans
355 (28 000), 372 (24 000), 434 (4600)
304 (35 000), 353 (26 000), 422 (7600)
308 (38 000), 358 (22 000)
489
458
403
403
0.0030
0.029
0.038
0.022
0.094b
0.032
<0.4b c
>0.07c
>0.08c
>0.06c
,
<0.5b c
,
<0.4b c
,
309 (38 000), 360 (21 000)
MC2
MC2_1ox
MC2_2ox_cis
MC2_2ox_trans
375 (22 000), 435 (4500)
308 (23 000), 381 (20 000)
310 (25 000), 387 (18 000)
310 (26 000), 387 (17 000)
493
467
473
473
0.0030
0.055
0.62
0.099b
2.7d
0.030
0.020
0.21
2.9d
0.52
2.9d
0.18
a Absolute quantum yields were determined by comparison with 9,10-diphenylanthracene in cyclohexane (ΦF ) 0.90). b Fluorescence lifetimes were
measured using a pulsed-femtosecond laser (350 nm) and were fit to monoexponential decays with R2 > 0.99. c Obtained lifetimes could be fit to
monoexponential decays, but the emission profile suggested that photodegradation via deoxygenation to the sulfide occurred during excitation. Therefore,
only an upper bound is placed on the lifetime, and a minimum kF is calculated. d Fluorescence lifetimes were measured using a pulsed-picosecond laser
(415 nm) and were fit to monoexponential decays with R2 > 0.99. e ΦF/τF.
Figure 1. Model compound oxidation. (a and c) The absorption spectra of MC1, MC2, and oxidation products. (b and d) The fluorescence spectra of MC1,
MC2, and oxidation products. Within each plot, the fluorescence intensity is scaled to reflect the relative emissivity of each fluorophore as determined by
its absolute quantum yield (see Table 1 for values). λex: 420 nm (MC1, MC2), 390 nm (MC1_1ox, MC2_1ox, MC2_2ox), 370 nm (MC1_2ox). All spectra
were taken in CH2Cl2, and all fluorescence spectra are corrected for PMT response.
with m-CPBA in chloroform at room temperature to give a
variety of oxidation products resulting from the initial formation
of an oxirene.25 Some of the oxidation products reported result
in cleavage of the diphenylacetylene into two molecules, while
others contain linkages that are susceptible to cleavage.25
The presence and relative abundance of sulfoxides and
sulfones in the oxidized samples of P1 and P2 were evaluated
with infrared spectroscopy and compared to similar spectra for
the model compounds (see Figures S3 and S4 in the Supporting
Information). Oxidation causes significant changes in the spectra
of both polymers between 1600 and 850 cm-1 (Figure 3).
Initially, P1 has a relatively featureless spectrum. Oxidation with
2.1 equiv of m-CPBA causes a strong absorption band at 1077
cm-1 to appear, indicating sulfoxide formation. This band
continues to grow and broaden with further oxidation. Beginning
with P1C and continuing in P1D, two peaks at 1335 and 1160
cm-1 belonging to sulfones appear. The FT-IR spectrum of P2
is more challenging to interpret, because signals between 1150
and 1000 cm-1 from the aryl ether groups interfere with the
observation of the sulfoxide band in the oxidized polymers. P2A
(25) Stille, J. K.; Whitehurst, D. D. J. Am. Chem. Soc. 1964, 86, 4871–
4876.
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CPs That Respond to Oxidation with Increased Emission
A R T I C L E S
Figure 2. Polymer oxidation. (a and b) The oxidation of P1 and P2 with increasing amounts of oxidant is shown. Photographs show solutions of the
polymers in CDCl3 (5 mg/mL) under ambient light (left) and irradiated at 365 nm with a hand-held lamp (right). (c and d) The absorbance spectra of dilute
solutions of P1, P1A-D, P2, and P2A-D in CH2Cl2 are shown. (e and f) The fluorescence spectra of dilute solutions of P1, P1A-D, P2, and P2A-D in CH2Cl2
are shown and are corrected for PMT response. λex: 440-450 nm (P1, P1A-D), λmax minus 10 nm (P2, P2A-D).
shows a strong sulfoxide band at 1081 cm-1 but does not show
sulfone bands. Sulfone bands appear in P2B and are ac-
companied by a decrease in the initial signal belonging to
sulfoxides, which suggests that some of the sulfoxides have been
oxidized to sulfones. Beginning with P2C and continuing in
P2D, the sulfone peaks at 1335 and 1161 cm-1 continue to grow
as the sulfoxide signal weakens.
In summary, FT-IR shows that both P1 and P2 are first
oxidized to sulfoxides and then oxidized to sulfones. P2 appears
to be more easily oxidized to sulfones relative to P1, because
of its more electron-rich comonomer. It is difficult to comment
quantitatively on the level of oxidation using FT-IR; however
the spectra support some important qualitative observations. The
P1A and P2A spectra suggest that most of the thioether-
containing aromatic rings have been oxidized to the disulfoxide,
which is in accord with the reactivity seen in the model
compounds. The weak sulfone bands seen in P1D indicate
minimal sulfone formation with 8.40 equiv of oxidant, which
suggests that most of the thioether repeating units have four or
less oxygen atoms. This is in contrast with the strong sulfone
bands observed in P2D, which suggest that each thioether
repeating unit contains at least four oxygen atoms. In both P1
and P2, the oxidized polymers have complex structures.
Although this extra dimension of polydispersity makes char-
acterization challenging, it likely aids in solubility and reduces
crystallinity.
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A R T I C L E S
Dane et al.
Table 2. Polymer Photophysics
λmax (nm)/
a
)
d
e
ε (M-1 ·cm-1
λem (nm)
CH2Cl2
ΦF
τF (ns)
kF (ns-1
)
f
Mw (kDa)
polymer
CH2Cl2
film
film
CH2Cl2b
filmc
CH2Cl2
CH2Cl2
Mnf (kDa)
PDIg
P1
444 (44 000)
457
512
517
0.0096
<0.01
0.13
0.074
236h
1990
8.40
P1A
P1A′
P1B
P1C
P1D
459 (38 000)
463 (50 000)
463 (54 000)
468 (69 000)
468 (48 000)
464
n.d.
467
469
468
492
480
479
483
483
500
n.d.
498
495
495
0.35
0.61
0.53
0.59
0.48
n.d.
n.d.
0.57
0.70
n.d.
0.67
0.68
0.73
0.71
0.80
0.52
0.90
0.72
0.83
0.60
265
n.d.
215
112
102
2710
n.d.
2010
587
10.2
n.d.
9.35
5.24
3.76
384
P2
457 (43 000)
489
526
503
0.0085
<0.01
0.17
0.050
18.9h
31.1
1.65
P2A
P2A′
P2B
P2C
P2D
494 (29 000)
497 (30 000)
503 (25 000)
513 (29 000)
524 (23 000)
500
n.d.
503
512
520
532
540
556
568
579
551
n.d.
580
594
599
0.49
0.42
0.28
0.25
0.19
0.56
n.d.
<0.10
<0.10
<0.10
1.35
1.34
1.51
1.66
1.72
0.36
0.31
0.19
0.15
0.11
18.8
n.d.
15.9
13.0
14.3
32.3
n.d.
23.6
20.3
22.4
1.72
n.d.
1.48
1.56
1.57
a Molar absorptivity based on molecular weight of repeat unit. b Absolute quantum yields were determined for P1, P1A-D, P2, and P2A-B by
comparison with coumarin-6 in ethanol (ΦF ) 0.76). P2C and P2D were compared to rhodamine B in ethanol (ΦF ) 0.49). c Thin-film quantum yields
of P1B, P1C, and P2A were determined using an integrating sphere and monochromatic light of several wavelengths near the λmax. Thin-film quantum
yields of P1, P2, P2B, P2C, P2D were determined by comparison with thin films of perylene in PMMA (ΦF ) 0.78). d Fluorescence lifetimes were
measured using a pulsed-femtosecond laser (350 nm, P1, P2) or pulsed-picosecond (415 nm, P1A-D, P2A-D) laser and were fit to monoexponential
decays with R2 > 0.99. e ΦF/τF. f Determined with GPC(THF) against polystyrene standards. g Mw/Mn. h Degree of polymerization (DP) based on polymer
repeat unit is 260 for P1 and 21 for P2.
Optical and Photophysical Properties of the Polymers in
Solution. Our primary method of evaluating the effect of
oxidation on the polymers is UV-vis and fluorescence spec-
troscopy (Figure 2, Table 2). P1 is relatively nonemissive, with
a quantum yield less than 0.01. P1A’s absorbance (λmax ) 459
nm) is red-shifted compared to that of P1 (λmax ) 444 nm).
P1A’s emission maximum is blue-shifted from 512 to 492 nm
relative to the parent polymer, and its quantum yield increases
dramatically (35-fold). When P1 is treated with 4.2 equiv of
oxidant (P1B), the absorbance red-shifts and the emission blue-
shifts. The spectral changes for P1B are accompanied by an
increase in quantum yield to 0.53, but the effect of further
oxidation is minor.
Similar to P1, P2 is relatively nonemissive in solution, with
a quantum yield ΦFof less than 0.01. The quantum yield of P2A
is 0.49, which represents a more than 49-fold increase in
emission when the polymer is treated with 2.1 equiv of oxidant.
Additional oxidation causes continued red-shifting in absorbance
and emission, as can be seen in P2B, P2C, and P2D. However,
greater oxidation is accompanied by a decrease in quantum yield,
which appears to be at least partially a result of a decrease in
the rate of fluorescence (kF).
Response to Hydrogen Peroxide in Solution. The model
compounds and polymers did not react with hydrogen peroxide
in solution at room temperature. To facilitate a reaction, the
oxidation catalyst methylrheniumtrioxide (MTO)26 was added
as described by Malashikin and Finney.4a The absorbance and
emission of dilute solutions of MC2 and P2 in quartz cuvettes
were recorded after the addition of catalyst ([MTO] ) 0.5 mM)
but before the addition of oxidant.27 Following the addition of
hydrogen peroxide, the stirred solutions were monitored at 15
min, 3 h, and 20 h. Initially, three solvents were screened (THF,
EtOH, DCM), and the extent of reaction was greatest in DCM.
Ethanol showed the same response at a slower rate, but no
reaction was observed in THF (unstabilized).
The response at 3 h of a 5.00 µM solution of P2 to a range
of H2O2 concentrations is shown in Figure 4. At 3 h, fluores-
cence increases in the 1.00 mM, 100 µM, 10.0 µM, 1.00 µM,
and 100 nM samples but not in the 10.0 nM and 1.0 nM H2O2
solutions. The response continues to develop over time, and it
can be seen by eye under UV illumination after 20 h (Figure 4,
inset picture) for all concentrations of H2O2 except 1.00 nM
(see Figure S5 in the Supporting Information).
To better understand and characterize the response of P2 to
hydrogen peroxide, a 10.0 µM solution of MC2 was subjected
to similar conditions (see Figure S6 in the Supporting Informa-
tion). At 15 min, a fluorescence increase is observed in the
samples with a H2O2 concentration of 1.00 mM, 100 µM, and
10.0 µM. Based on the absorbance spectrum, it appears that
after 15 min MC2 has been transformed to MC2_2ox in the
1.00 mM H2O2 solution. After 3 h there is no further reaction,
indicating that the disulfoxide is a stopping point under these
conditions. At 15 min, the 100 µM H2O2 solution, which
represents 10 equiv of H2O2 per molecule of MC2, has been
oxidized to the monosulfoxide based on the absorbance spec-
trum. After 3 h, the sample shows further oxidation, though it
has not yet reached the same point as the 1.00 mM H2O2
solution. The 10.0 µM H2O2 solution shows a significant
increase in fluorescence at 15 min, but its absorbance spectrum
is relatively unchanged. After 3 h, the absorbance spectrum of
the 10.0 µM H2O2 appears more like that of the monosulfoxide,
and the fluorescence has continued to increase. In the 1.00 µM
H2O2 sample, there is a significant increase in fluorescence,
presumably from the formation of some amount of the mono-
sulfoxide, but the absorbance spectrum changes only slightly.
The 100 nM H2O2 solution shows no response.
Thin-Film Photophysics of Polymers. Thin films of polymers
P1, P1A-D, P2, and P2A-D were prepared by spin-casting a
chloroform solution (1.0 mg/mL) onto silanized glass slides.
The films were annealed at 120 °C for 10 min. The films show
behavior similar to that of the dilute solutions, with no signs of
(26) (a) Herrmann, W. A.; Ku¨hn, F. E. Acc. Chem. Res. 1997, 30, 169–
180. (b) Vassell, K. A.; Espenson, J. H. Inorg. Chem. 1994, 33, 5491–
5498. (c) Yamazaki, S. Bull. Chem. Soc. Jpn. 1996, 69, 2955–2959.
(27) MC1 and P1 were not investigated because it was anticipated they
would respond more slowly to hydrogen peroxide than the more
electron-rich MC2 and P2.
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7764 J. AM. CHEM. SOC. VOL. 132, NO. 22, 2010

CPs That Respond to Oxidation with Increased Emission
A R T I C L E S
Figure 4. Fluorescence response of P2 to hydrogen peroxide. The response
of P2 to H2O2 after 3 h at room temperature is plotted for a series of
hydrogen peroxide concentrations. The inset shows the same solutions under
UV irradiation (365 nm) at 20 h. For all solutions: λex ) 460 nm, [P2] )
5.00 µM, [MTO] ) 0.5 mM, [urea] ) [H2O2] in CH2Cl2 with <5% ethanol.
Figure 5. Solid-state emission of P1C versus P3. The fluorescence emission
at an angle of 22.5° to the face of the film is shown for P3 and P1C. The
two samples had nearly identical optical densities at the excitation
wavelength (420 nm). Representative samples of P3 and P1C under UV
irradiation (365 nm) are shown in the inset.
shift in absorption when moving from solution to the solid state;
however in both cases the emission is red-shifted.
The films made from the oxidized forms of P1, specifically
P1B and P1C, were highly emissive, with quantum yields above
0.50. We compared films of P1C to films of a previously
reported nonaggregating PPE (P3) with a fluorescent quantum
yield of 0.33 in the solid state.2a In Figure 5, the emission of
P1C is significantly greater than that of a film of P3 when both
are excited with a monochromatic light of 420 nm, a wavelength
at which the two films have nearly identical optical densities.
Films of P2A are also highly fluorescent with a quantum yield
of 0.56. Comparatively, films of the more highly oxidized
polymers (P2B-D) are less emissive; however the fluorescence
can be clearly seen by eye under UV irradiation as shown in
Figure 6.
Figure 3. Polymer infrared spectra. A portion of the FT-IR spectra of P1,
P1A-D, P2, and P2A-D is shown with blue lines indicating sulfone bands
and red lines indicating sulfoxide bands. Samples were drop-cast on KBr
plates from concentrated CH2Cl2 solutions.
aggregation (Table 2, Figure S7 in the Supporting Information).
For P1, P1A, and P1B, the thin-film absorption and emission
maxima are red-shifted compared to the solution values. For
P1C and P1D, the absorption maxima are relatively unchanged
going from solution to thin film; however the emission maxima
are red-shifted in the solid state. For P2 and P2A, the thin-film
absorption and emission maxima are red-shifted compared to
the solution values. P2B displays no change in absorption
maximum in moving from solution to the solid state; however
its emission does red-shift. P2C and P2D show a slight blue
Thin-Film Photobleaching Studies. Films of P1, P1A-D, P2,
and P2A-D were tested for photostability by comparing the
emissions before and after the films were irradiated at their
absorbance maxima for 30 min, using monochromatic light
generated from a 450 W steady-state Xe lamp as the irradiation
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A R T I C L E S
Dane et al.
Figure 6. Photostability of P2 thin films. Thin films of P2, P2A-D, and
P3 of the same optical density (OD ) 0.05 ( 0.01) were irradiated for 30
min at their absorbance maxima with monochromatic light, and the ratio
of fluorescence at 30 min to the initial fluorescence was measured.
Representative samples under UV irradiation (365 nm) are shown at the
bottom of the plot.
source under aerobic conditions.11 The approximate area of
irradiation was 0.25 cm2 and the average power density of
irradiation was estimated to be 6 mW/cm2. Films of P1 and
P1A-D had an optical density (OD) of 0.10 ( 0.01. Films of
P2, P2A-D, and P3 had an OD of 0.05 ( 0.01. The films were
not moved during the process to ensure that the measurements
reflect the effect of irradiation on one area of the film. All
measurements were done in triplicate, and the error is reported
as the standard deviation. Films of P1 and P1A-D did not show
improved photostability as compared to P3 under identical
conditions11 (see Figure S8 in the Supporting Information).
However, films of P2A-D proved resistant to photobleaching
(Figure 6). P2A-D retained more than 95% of their original
fluorescence after 30 min of irradiation, with P2B-P2D even
showing a slight increase in emission. They are more photostable
than P3, which retains only 60% of its initial emission when
irradiated at its absorbance maximum (440 nm) at the same OD,
in accordance with previously reported results.11
Figure 7. HOMO-LUMO Computations. (a and b) Frontier orbital
topologies (B3LYP6-311+G*, Gaussian03) for structures S1 and S2.
Structures S1 and S2 represent MC1 and MC1_2ox_trans, respectively,
with the butyl groups being replaced with methyl groups for ease of
computation.
Upon oxidation of MC1 to MC1_1ox, the ICT band blue
shifts, and with oxidation to the disulfoxide, the ICT band
disappears. The computed HOMO and LUMO for S2 are in
agreement with this observation, as it appears that the spatial
overlap of the HOMO and LUMO is significantly greater for
S2 as compared to S1 (Figure 7). The situation is more complex
in the case of MC2 and its mono- and disulfoxides because the
oxidation of the thioethers introduces a donor-acceptor interac-
tion between the electron-rich outer rings and the electron-poor
middle ring. However, comparing the absorption spectrum of
MC2 to that of MC2_1ox reveals that oxidation to the
monosulfoxide causes a blue shift similar to that seen in
MC1_1ox. The origin of the weak red-shifted absorbance
present in both isomers of MC2_2ox may be elucidated by
future computational studies.
Discussion
Understanding the Photophysical Behavior of the Model
Compounds. Because of its relatively weak intensity, the
absorbance band of MC1 and MC2 centered at approximately
435 nm (Figure 1) suggests that the thioethers on the center
ring impart intramolecular charge transfer (ICT) character to
the HOMO-LUMO optical transition. The ICT character of
the transition results from poor spatial overlap of the frontier
molecular orbitals (FMOs), which we examined with quantum
mechanical calculations (Figure 7a, Supporting Information
Figure S9).19 Qualitatively, for S1 (a simplified version of MC1
for ease of computation) the HOMO is primarily located on the
center ring with most of the orbital density on the sulfur atoms,
while the LUMO is delocalized throughout the molecule’s π-system
with minimal density on the sulfur atoms. The presence of ICT
character has been observed in simple thioether containing aromatic
compounds, such as phenyl-n-propyl sulfide.28 Additionally, ICT
character is observed in compound 719 (Chart 1), which is an
oxygen analogue of the thioethers studied here.
For MC1 and MC2, the fluorescence quantum yield is low
in solution (ΦF ) 0.003, Table 1), for two reasons. First, MC1
and MC2 have short fluorescence lifetimes of approximately
100 ps, which indicates that another relaxation process is
competing with fluorescence. Based on previous studies of
thioethers we suggest that intersystem crossing (ISC) from the
(28) Becker, R. S.; Jordan, A. D.; Kolc, J. J. Chem. Phys. 1973, 59, 4024–
4028.
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CPs That Respond to Oxidation with Increased Emission
A R T I C L E S
Chart 1. Literature Compounds
because of nonradiative decay pathways involving the sulfoxide.
For example, pyrene has a quantum yield of 0.32 in CH2Cl2,31
and phenyl 1-pyrenyl sulfide has a quantum yield of 0.34 in
CH2Cl2.4a Oxidation of the sulfur in phenyl 1-pyrenyl sulfide
to a sulfoxide causes the quantum yield to decrease to 0.02,
whereas further oxidation causes the quantum yield to increase
to 0.76.4a A similar trend, in which the presence of a sulfoxide
attached directly to an aromatic core quenches a fluorophore
that is relatively unaffected by the presence of a thioether or a
sulfone in the same position, is observed for naphthalene,
biphenyl, and anthracene sulfides.32 However, in the compounds
we have examined sulfoxides do not appear to act as fluores-
cence quenchers except in MC1_2ox (and perhaps in
MC1_1ox), in which case the emission is blue-shifted toward
the near-UV. This suggests that when irradiated at higher
energies (shorter wavelengths) sulfoxides may quench fluores-
cence via a mechanism that is not operative at lower energies
(longer wavelengths). The behavior of our model compounds
and polymers is not unprecedented. Within a previously reported
series of 21 tetracene derivatives (Chart 1), 15 of which are
sulfoxides (9) and 6 of which are sulfones (10), the tetracene
derivatives with sulfoxides are more emissive than the tetracene
sulfones in nearly every case.33
a R1, R2, R3 ) H, OMe, t-Bu, Cl, Br.
singlet excited state to the triplet excited state accounts for the
shortened lifetime. In compounds as simple as phenyl-n-propyl
sulfide, ISC is believed to be the predominant deactivation
pathway of the excited state.28 An example of an aromatic
chromophore containing more than one thioether is 2,3,6,7,10,11-
hexakis(n-hexylsulfanyl)triphenylene (Chart 1, Compound 8),
which has a reduced fluorescence quantum yield compared to
the parent triphenylene (0.015 vs 0.066) but displays significant
phosphorescence at 77 K.29 Compound 8 has a significantly
shortened lifetime (0.7 ns) as compared to the parent triph-
enylene (36.6 ns) or the same compound with the sulfur atoms
replaced by oxygen atoms (8.0 ns). For MC1 and MC2, the
observation of room temperature phosphorescence (Figure S1,
Supporting Information) in degassed samples further supports
the argument that ISC is a main deactivation pathway for the
singlet excited state.
The second reason that the unoxidized model compounds
have low emission is that they have lower rates of fluorescence
(kF) as compared to the oxidized versions. Oxidation of MC1
to MC1_1ox and both isomers of MC1_2ox is accompanied
by an increase in kF. The increase in kF in the disulfoxide as
compared to the unoxidized compound appears to be a
consequence of better spatial overlap between the HOMO and
LUMO (see Figure 7).23,30 The oxidation of MC2 to MC2_1ox
causes the fluorescence lifetime to increase substantially from
0.099 to 2.7 ns but does not increase kF. Further oxidation to
MC2_2ox causes an order of magnitude increase in kF from
0.020 ns-1 to ∼0.20 ns-1 and a minor increase in the lifetime.
MC2 shows a more than 150-fold increase in ΦF when
oxidized to the disulfoxide, but the increase is more modest (7-
to 13-fold based on the isomer) when MC1 is oxidized to the
disulfoxide. We contend that the sulfoxides in MC1_2ox are
behaving differently than the sulfoxides in MC2_2ox. It has
been reported that the fluoresence of aromatic sulfoxides is low
Understanding the Photophysical Behavior of the Polymers.
In contrast to the model compounds, the HOMO-LUMO
optical transitions of P1 and P2 appear to be allowed, thus
suggesting that the HOMO is partially delocalized along the
polymer’s π-backbone and has better spatial overlap with the
LUMO. In contrast, when the oxygen-containing monomer 7
is incorporated into PPEs the HOMO remains localized.19 P1
and P2, which like MC1 and MC2 are not very emissive, show
large increases in ΦF when oxidizied due to an increase in both
kF and τF (Table 2). For example, when P1 is oxidzied to P1A
with 2.1 equiv of oxidant, kF increases from 0.074 ns-1 to 0.52
ns-1 and τF increases from 0.13 to 0.67 ns. When P2 is oxidized
to P2A, kF increases from 0.050 ns-1 to 0.36 ns-1 and the
lifetime increases from 0.17 to 1.35 ns. Based on the trends
observed for the oxidized model compounds, it appears that the
increase in kF is due to greater spatial overlap of the FMOs due
to delocalization of the HOMO along the π-backbone and that
the increase in τF is due primarily to a decrease in the rate of
ISC.
Understanding the Behavior of the Polymers in Thin
Films. Because oxidation in solution causes P1 and P2 to
become emissive, thin films of the thioether-containing polymers
were expected to show a fluorescence turn-on response when
irradiated in air. However, this phenomenon was not observed.
Instead, the already weak emission further decreased with
prolonged irradiation (Figure 6, P2), even though thioethers are
known to react with singlet oxygen.34 There are two possible
explanations for this behavior. The first is that the thioethers
are not being oxidized to sulfoxides or sulfones under the
conditions. The second is that oxidation is so minimal that any
oxidized moieties are effectively quenched by the remaining
unoxidized segments.
(29) (a) Baunsgaard, D.; Larsen, M.; Harrit, N.; Frederiksen, J.; Wilbrandt,
R.; Stapelfeldt, H. J. Chem. Soc., Faraday Trans. 1997, 93, 1893–
1901. (b) Marguet, S.; Markovitsi, D.; Millie´, P.; Sigal, H. J. Phys.
Chem. B 1998, 102, 4697–4710.
(31) Berlman, I. B. Handbook of Fluorescence Spectra; Academic Press:
New York, 1965.
(32) (a) Lee, W.; Jenks, W. J. Org. Chem. 2001, 66, 474–480. (b) Jenks,
W. S.; Matsunaga, N.; Gordon, M. J. Org. Chem. 1996, 61, 1275–
1283.
(30) Zucchero, A. J.; Wilson, J. N.; Bunz, U. H. F. J. Am. Chem. Soc.
2006, 128, 11872–11881.
(33) Lin, Y.; Lin, C. Org. Lett. 2007, 9, 2075–2078.
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A R T I C L E S
Dane et al.
Blue-Shifted versus Red-Shifted Fluorescence Turn-on
Response to H2O2. As discussed in the Results section, both
MC2 and P2 show a fluorescence turn-on response to oxidation
by hydrogen peroxide in the presence of a catalyst (MTO).
However, they differ in that MC2’s emission blue-shifts and
P2’s emission red-shifts under these oxidation conditions. As a
conjugated polymer, P2 offers the opportunity for exciton
migration along the polymer backbone to the most highly
oxidized, and therefore red-shifted, emissive sites.2,35 In contrast,
each molecule of MC2 in dilute solution behaves independently.
However, MC2 offers the opportunity for a dark field fluores-
cence turn-on response.36 A dark field turn-on is when the
emissive species is blue-shifted relative to the initial species
and, therefore, avoids overlapping emission from the initial
species’ vibronic manifold of transitions so that there is no initial
background signal to subtract. In this situation, the fluorescence
turn-on is limited more by the inherent noise in the detector
rather than the emissivity of the fluorophore. For example, our
instrumentation shows an average turn-on of 900-fold between
430 and 440 nm for MC2 in 1.00 mM [H2O2] after 15 min (see
Supporting Information, Figure S6), even though the emission
maximum is above 450 nm. In contrast, when P2 is oxidized
there is still substantial emission from the parent polymer to
subtract, even though P2 does show a slight blue shift in
emission upon initial oxidation. Therefore, MC2 offers an
advantage in situations where the increase in fluorescence
intensity is most important, and P2 offers an advantage in
situations where a change in emission color is desired.
increases in fluorescence quantum yield when oxidized. We
demonstrate that oxidation is accompanied by a red shift in
emission when the compounds contain electron-rich aromatic
rings that act as donor groups. Oxidation with hydrogen peroxide
in the presence of an organometallic catalyst points to the
potential application of these systems in peroxide sensing. Thin
films of the oxidized polymers are shown to be very emissive
and, in some cases, to have improved photostability. We
conclude that oxidation of the sulfur atoms causes an increase
in ΦF by both increasing the rate of fluorescence (kF) and
decreasing the rate of nonradiative decay, thereby increasing
the fluorescence lifetime (τF). Based on photophysical studies
and computation, we propose that the increase in kF is caused
by greater spatial overlap of the frontier molecular orbitals in
the oxidized compounds as compared to the unoxidized
compounds.
Acknowledgment. We thank Dr. Peter Mu¨ller for collecting
and solving X-ray crystal structures, Phil Reusswig for help
obtaining solid-state quantum yields, and Dr. Jing Zhao of Prof.
Moungi G. Bawendi’s research group and Casandra Cox of Prof.
Daniel G. Nocera’s research group for help obtaining fluorescence
lifetimes.
Supporting Information Available: Methods, materials, and
experimental details. X-ray crystallographic details of com-
pounds 1, MC2_1ox, and MC1_2ox_cis. Additional UV/vis and
fluorescence spectra, including results of H2O2 oxidation of MC2
and P2, and spectra of polymer thin films. FT-IR spectra for
MC1, MC2, and their oxidation products. P1 photostability
Conclusions
In summary, we have efficiently synthesized model com-
pounds and polymers containing thioethers that show large
1
results. H and 13C NMR spectra for all previously unreported
(34) Clennan, E. L. Acc. Chem. Res. 2001, 34, 875–884.
(35) Kim, T.-H.; Swager, T. M. Angew. Chem., Int. Ed. 2003, 42, 4803–
4806.
compounds and polymers. This material is available free of
charge via the Internet at http://pubs.acs.org.
(36) Thomas, S. W., III; Venkatesan, K.; Mu¨ller, P.; Swager, T. M. J. Am.
Chem. Soc. 2006, 128, 16641–16648.
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7768 J. AM. CHEM. SOC. VOL. 132, NO. 22, 2010