Furan-containing singlet oxygen-responsive conjugated polymers

Article abstract of DOI:10.1021/ma3025656

This paper describes conjugated polymers with 2,5-diarylfuran moieties as nonconjugated pendants that respond to singlet oxygen (¹O ₂) by fluorescence quenching. By oxidizing the diarylfurans to more electron-poor moieties such as en

Full text of DOI:10.1021/ma3025656

Article
pubs.acs.org/Macromolecules
Furan-Containing Singlet Oxygen-Responsive Conjugated Polymers
Esra Altınok, Simone Friedle, and Samuel W. Thomas, III*
Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford, Massachusetts 02155, United States
S
* Supporting Information
ABSTRACT: This paper describes conjugated polymers with 2,5-
diarylfuran moieties as nonconjugated pendants that respond to
singlet oxygen (1O2) by fluorescence quenching. By oxidizing the
diarylfurans to more electron-poor moieties such as enol esters,
singlet oxygen causes poly(fluorene-co-phenylene) conjugated
backbones to donate excited electrons to the oxidized pendants,
resulting in quenching of up to 93% of the initial fluorescence of
the polymer, while an analogous furan-substituted poly(phenylene−ethynylene) showed no quenching upon furan−1O2 reaction.
All furan derivatives reacted rapidly with 1O2 (k = 107−108 M−1 s−1), with more electron-poor furans reacting more slowly, but
yielding more efficient fluorescence quenching, than less electron-poor furans.
however, are only stable at low temperatures and react further
at room temperature to produce other products.14 Incorpo-
ration of furan moieties is becoming an increasingly important
design strategy in conjugated materials:15 Bendikov and co-
workers recently prepared a series of oligofurans with up to
nine rings16 and have also studied the semiconducting17 and
Diels−Alder reactivity18 of these materials. A number of groups
have prepared conjugated polymers with furan rings in the main
chain, generally to compare their properties to the more typical
thiophene-based polymers.19−30 Diels−Alder reactions between
furans and maleimides have been a design feature of functional
conjugated polymers,24,31 including work from the group of
Reynolds that used furan-substituted polyfluorenes to enable
solution-processed multilayer polymer-based light-emitting
devices.32 We are unaware of any reported 1O2-responsive
systems comprising a coupled furan derivative and fluorophore.
Herein we present conjugated polymers that respond to 1O2 by
fluorescence quenching induced by oxidation of 2,5-diarylfuran
pendant groups.
INTRODUCTION
■
Conjugated polymers have a number of advantages for
developing responsive materials. One of the main advantages
is the intramolecular and intermolecular mobility of excitons
and charge carriers. This mobility results in amplification of
signal in sensing applications because one mobile quasi-particle
can probe a large number of potential trap sites. General
approaches to fluorescent sensing include intensity-based
schemes, which can use fluorescence quenching (turn-off),
enhancement (turn-on), or a combination of both (ratiometric)
as well as lifetime-based schemes. Although systems that rely on
fluorescence enhancement by removing an exciton trap have
distinct advantages, such as preventing false positives from
bleaching and fewer interferents, the amplification mechanism
of CPs enables maximal amplification by introduction of a trap
for excitons, by either energy transfer or electron transfer
quenching.1 Therefore, it is advantageous to have a number of
related strategies that show different types of fluorescent
response that could be employed depending on the importance
of different performance-related characteristics.
RESULTS AND DISCUSSION
■
Our laboratory is interested in developing new materials that
respond to singlet oxygen (1O2).2 1O2 can be chemically
amplified through photosensitization, which uses photons and
ambient O2 as the only stoichiometric reactants, and is useful in
applications such as surface patterning,3 photodynamic
therapy,4 and protein inactivation.5−7 There are a number of
As shown in Scheme 1, endoperoxide intermediates from
cycloaddition between 1O2 and diarylfurans decompose to
several products resulting from fragmentation at room
temperature. Although the distribution of products can depend
strongly on both the structure of the starting material and the
conditions of the reaction, the final products are generally
oxidized, electron-poor molecules relative to the furan starting
material. Therefore, our hypothesis was that the products of the
1O2−diarylfuran reaction would be more likely to quench the
fluorescence of electron-rich fluorophores by photoinduced
electron transfer than the comparatively electron-rich diaryl-
furan derivatives.
1
known fluorescent sensing dyes that respond to O2 through
cycloaddition reaction between 1O2 and acenes. These
previously described dosimeters typically use anthracenes as
8−12
1
the O2-reactive moieties;
we, however, have developed a
tetracene-linked CP that shows a ratiometric fluorescent
1
response to O2 by interrupting energy transfer from the CP
backbone to the tetracene pendant.13 We have also
demonstrated a related approach using tetracene-doped CP
1
thin films for responding to O2 in aqueous environments.
Received: December 13, 2012
Revised: January 11, 2013
Published: January 25, 2013
In addition to acenes, furans also undergo facile cyclo-
addition reactions with 1O2; the resulting endoperoxides,
© 2013 American Chemical Society
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Scheme 1. The Intermediate Endoperoxide Product of 1O2−
2,5-diiodohydroquinone gave diiodide monomers substituted
with two diarylfurans 8a and 8c (Scheme 3).
Diphenylfuran Cycloaddition Opens at Room Temperature
a
To Give One of Several Oxidized Products
Scheme 3. Synthesis of Bis-Furan Monomers 8a and 8c
a
The oxidized products are potential fluorescence quenchers by
photoinduced electron transfer.
Scheme 2 illustrates the modular synthetic scheme of furan-
linked poly(fluorene-co-phenylene) (PF) polymers with a
variety of substituents on the 4-position of the terminal aryl
ring of the diarylfuran moiety. Deprotonation of furan in the 2-
position with n-BuLi followed by quenching with molecular
iodine yielded 2-iodofuran 1. Because of the instability of this
molecule, it was converted as soon as possible to a 2-arylfuran 2
by Suzuki coupling. Various conditions for bromination at the
5-position of the furan gave 2-bromo-5-arylfuran in only low
yields.33 We found, however, that deprotonation with n-BuLi
followed by quenching with I2 gave higher yields of the
iodofurans 3, which are common intermediates in the syntheses
of furan-containing monomers described here. Suzuki coupling
of the unstable 3 with arylboronic acids gave unsymmetrically
substituted 2,5-diarylfurans 4a−4d,33 which each had a p-
methoxyphenyl as one aryl ring and either a phenyl or
fluorinated phenyl ring as the other. Deprotection of the methyl
ether with either BBr3 or alkylthiolate followed by a sequence of
two alkylation reactions yielded the desired monomers furan-
substituted diiodide monomers 7a−7d. A similar strategy using
Furan-containing monomers 7 and 8 were amenable to
cross-coupling polymerizations for the synthesis of conjugated
polymers. We prepared PFa-d, which had one furan moiety per
repeat unit, as well as PF-8a and PF-8c, which had two furan
units per repeat unit, using Suzuki polymerizations. We also
prepared PF, a control poly(fluorene-co-phenylene) with no
furan derivatives, as well as furan-containing polymer PPEa and
control polymer PPE by Sonogashira polymerization. Typical
number-average molecular weights as determined by gel-
permeation chromatography were between 8000 and 25 000
g/mol with polydispersity (Table 1).
Figure 1 shows the normalized absorbance spectra of two
furan-containing conjugated polymers (PFa and PPEa), model
conjugated polymers that do not have furan groups as pendants
(PF and PPE), and a small molecule model of the furan
Scheme 2. Synthesis of Furan-Linked Poly(fluorene-co-phenylene)s
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CPs PFa and PPEa show features attributable to both the
conjugated polymer backbone and the furan pendants with no
additional signals. These observations are consistent with both
their chemical structures and the conclusion that no electroni-
cally significant interactions occur between the polymer chains
and furan pendant groups. Diarylfuran 4a, with a λmax of 332
nm, does not absorb light at wavelengths greater than 370 nm,
which enables selective irradiation of the CP backbones without
the furan moieties absorbing light. Diarylfurans with donor−
acceptor character due to fluorinated substituents (4c and 4d)
showed red-shifted absorbance spectra, with 4d having λmax of
344 nm.
Table 1. Properties of 2,5-Diarylfurans and Furan-Linked
Conjugated Polymers
a
a
Mn
λmax(ab)
[nm]
λmax(em)
[nm]
b
sample
X
[kg/mol] PDI
ΦF
4a
H
332
352
335
440
330
350
342
361
346
344
360
373
440
383
415
417
478
379
415
405
416
415
414
415
417
478
0.73
0.62
0.58
0.36
0.68
0.60
0.69
0.59
0.58
0.39
0.59
0.76
0.54
PFa
PF-8a
PPEa
4b
H
12
8.3
13
2.4
1.4
2.8
H
H
4-F
4-F
PFb
4c
8.0
1.5
4-CF3
4-CF3
4-CF3
3,5-diCF3
3,5-diCF3
H
PFc
PF-8c
4d
25
8.2
2.3
1.2
Figure 2 shows height-normalized emission spectra of furan-
containing CPs PFa and PPEa, while Table 1 summarizes their
PFd
PF
10.3
28
1.7
2.1
1.6
PPE
H
9.0
a
b
Measured in CH2Cl2. Determined relative to either quinine sulfate
in 0.1 N H2SO4 (ΦF = 0.54) or Coumarin 6 in ethanol (ΦF = 0.78).
Chart 1. Other CPs Investigated in This Study
Figure 2. Normalized emission spectra of furan-linked CPs PFa and
PPEa. The shapes of these spectra match those of analogous polymers
with no furan pendants.
emission maxima and fluorescence quantum yields. These
polymers have shapes of emission spectra that are indistin-
guishable from control polymers PF and PPE. Both excitation
spectra and emission spectra acquired with an excitation
wavelength of 320 nm revealed that excitation of the furan
pendants on these polymers leads to emission from the
polymer backbone. These observations are consistent with the
conclusion that energy transfer from the larger band gap furan
to the smaller band gap CP backbone and that the presence of
furan has at most a limited effect on the luminescence of the
CP upon direct excitation of the polymer backbone.
1
To understand the reactivity between O2 and the furan-
linked CPs, we studied the reaction between 1O2 and
diarylfuran derivative 4a, the π-conjugated network of which
is identical to the furan pendant bound to the PFa backbones,
as well as the known cycloaddition reaction between 9,10-
diphenylanthracene (DPA) and 1O2 (k = 3 × 106 M−1 s−1). We
exposed 4a to 1O2 by photosensitization using the dye
methylene blue (MB), a sensitizer that is soluble in moderately
polar organic solvents such as CH2Cl2. All kinetic experiments
were performed under ambient conditions with identical
concentrations of MB (OD = 0.87 at 653 nm) using either a
combination of high-pass filters and a Hg/Xe lamp or a 4.5 mW
laser diode at 635 nm to prevent direct excitation of any
chromophores other than MB. We followed the disappearance
of the furan as a function of irradiation time in each experiment
Figure 1. Normalized absorbance spectra (in CH2Cl2) of conjugated
polymers with and without furan pendants as well as that of the furan
pendant model compound 4a.
pendant, compound 4a. As Figure 1 and Table 1 illustrate, PPE
derivatives have absorbance spectra that are red-shifted by ca.
70 nm from PF derivatives because of their more highly
conjugated backbone. The shapes and spectral positions of
these polymers are consistent with those reported in the
literature.34 The shapes of absorbance spectra of furan-linked
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by absorbance spectrophotometry. Upon subtracting the
absorbance due to the MB in solution, the resulting absorbance
data fit semilogarithmic linear regressions versus time with
correlation coefficients of R ≥ 0.99 and yielded relative pseudo-
first-order rate constants. Negative control experiments in the
presence of MB with no irradiation showed no change in the
concentration of furan.
Figure 3 shows the response of the UV/vis spectra of furan
1
4a and polymer PFa during exposure to O2 over 5 min of
irradiation of MB using a 4.5 mW laser diode at 635 nm as well
as fits of the decrease in absorbance of the furan moiety to a
pseudo-first-order kinetic model. The spectral responses of 4a
and PFa are similar, with the lowest energy band of PFa after
the completion of the reaction resembling that of PF with a
λmax value of 367 nm. During the duration of the experiment,
the absorbance features due solely to the CP backbone (λ >
370 nm) did not change, which indicates that the polymer did
1
not degrade from exposure to O2 under these conditions;
PPEa showed similar resistance to degradation during the
1
reaction between O2 and the side-chain furan moieties. In
addition, there was not a statistically significant difference
between the pseudo-first-order kinetics for the disappearance of
the furan moieties of the small molecule 4a and the furan side
groups in polymer PFa. The rate of reaction of both furan
derivatives was 56
10 times higher than 9,10-diphenylan-
thracene (DPA). We therefore estimate that the bimolecular
rate constant of reaction between the diarylfuran moiety 4a and
1O2 is ∼1 × 108 M−1 s−1; this value is similar to the previously
published value of 8.6 × 107 M−1 s−1.35
1
We examined the rate of O2−furan cycloaddition for both
the parent 2,5-diarylfurans and the furan-substituted polymers
as a function of the electronic effect of substituents. In each
small molecule studied, one of the aryl rings was p-anisyl in
order to model the electronics of the furan side chains on the
polymers, which were bound to the polymer backbones
through ether linkages. Data for each reaction fit a pseudo-
first-order model and allowed us to estimate bimolecular rate
constants by comparing pseudo-first-order rate constants with
the DPA standard. As shown in Figure 4, increasingly strong
electron-withdrawing groups on the nonanisyl phenyl ring (4-
fluoro, 4-trifluoromethyl, and 3,5-bis(trifluoromethyl) slowed
1
the cycloaddition reaction between diarylfuran and O2, such
that the most electron-poor furan we investigated, 4d, reacted
1
with O2 4 times slower than 4a. In all cases, the rates of
reaction of furan moieties bound to conjugated polymers
differed by no more than 15% from the rates of reaction of the
analogous small molecule diarylfuran derivatives.
To establish that the reaction between the diarylfuran
moieties in these materials and 1O2 yielded the expected
oxidized products, we followed the MB-mediated photo-
oxidation of 4a by NMR in CDCl3. After selective irradiation
of methylene blue in the presence of 4a sample in for 15 min,
1H NMR revealed no remaining furan 4a. TLC analysis
indicated four major products. We identified 1O2-derived
Figure 3. Response of UV/vis spectrum of (a) diarylfuran 4a and (b)
furan-linked polyfluorene-co-phenylene PFa to 1O2 produced by
photosensitization with MB (1.0 OD) in CH2Cl2 during 5 min of
irradiation with a 4.5 mW laser diode at 635 nm. (c) Pseudo-first-order
kinetics of furan disappearance in 4a and PFa.
1
8.7 ppm, J = 12 Hz) enol esters.14 Similar to previously
reported observations,14 we found that the initial enol ester
product, which is nearly completely of cis configuration,
isomerized to the trans configuration at room temperature
within several days. Analogous photooxidation experiments
using conjugated polymers showed complete disappearance of
furan resonances and new signals consistent with the furan side
chains oxidized to both the enol esters and epoxides.
products through a combination of H chemical shift analysis,
1H−1H coupling in two-dimensional NMR, and mass
spectrometry. As shown in the Supporting Information and
Scheme 1, the major products identified were the cis-
dibenzoylethylene (olefinic protons give two coupled doublets
at δ 7.1−7.2 ppm), its epoxidized analogue (resonance at 4.5
ppm), and a mixture of the cis- (coupled doublets at δ 6.3 and
8.0 ppm, J = 8 Hz) and trans- (coupled doublets at δ 6.9 and
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Figure 5. Fluorescent response of furan-linked conjugated polymers
1
(a) PFa and (b) PPEa to photogenerated O2 (via irradiation of MB
Figure 4. (a) Comparison of the relative bimolecular rate constants for
reaction of diarylfuran derivatives 4a−4d as determined using
absorption spectrophotometry, with krel = 1 for the reaction between
under ambient conditions using a 200 W Hg/Xe lamp equipped with
high-pass filters) in CH2Cl2. The excitation wavelength was 380 nm,
which the furan moieties do not absorb. Control polymers that lacked
furan pendants showed less than a 3% change in fluorescence intensity
after 5 min of irradiation of MB.
1
9,10-diphenylanthracene (DPA) and O2. Heights of the bars are the
average relative rate constants from three independent kinetic
experiments; errors bars represent one standard deviation. (b)
Representative pseudo-first-order kinetic data for the consumption
of furans 4a−4d at steady-state [1O2].
photoexcited CP and oxidized pendants. During this experi-
ment, only the intensity of the emission changed; the shapes of
the emission spectra were the same at all irradiation times;
normalized fluorescence spectra of PFa at 0 and 300 s of
1
To test our principal hypothesis that O2-induced oxidation
of pendant furans on CPs would cause fluorescence quenching,
we monitored the fluorescence spectra of PFa (Figure 5a) and
1
exposure to O2 are nearly indistinguishable. An analogous
1
PPEa (Figure 5b) during photochemical generation of O2 by
polymer that contained only alkoxy side chains with no furan
moieties showed no change in fluorescence intensity upon
exposure to 1O2 under identical conditions (Supporting
Information).
selective irradiation of the photosensitizer MB using a 200 W
Hg/Xe lamp. The observations of fluorescence response of the
1
CPs to O2 are consistent with quenching of CP backbone
fluorescence by oxidized furan side chains via photoinduced
In contrast to the significant quenching of the fluorescence of
PFa, exposure of PPEa to 1O2 under identical conditions led to
no significant change of either the shape or intensity of the
emission spectrum of this polymer after 40 min. This lack of
fluorescent response is in spite of the complete reaction of the
furan pendant under these conditions after ca. 10 min, as
determined by absorbance spectrophotometry (Supporting
Information). The lack of a response from the PPEa derivative
is also consistent with this scenario, as the smaller band gap of
PPEs relative to poly(fluorene-co-phenylene)s has been
identified as a limitation in using them for responding to
weakly oxidizing potential quenchers.34
electron transfer. The polymer with the larger band gap of the
1
two, PFa, showed a dramatic response to O2, with a 12%
reduction in fluorescence intensity and quantum yield after only
1
1
5 s of exposure to O2. After 5 min of exposure to O2, the
degree of fluorescence quenching of the polymer reached a
plateau of ca. 73%; additional exposure to 1O2, up to 25 min in
duration, resulted in no further change in either the
fluorescence or absorbance spectra. We attribute the plateau
in quenching to favorable reduction potentials of the oxidized
products of the furan side chains (enol esters and epoxides) and
therefore leading to an electron transfer reaction from
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Substituents on the diarylfuran pendants had significant
effects on the quenching of CP fluorescence. While introducing
increasingly powerful fluorine-based electron-withdrawing
magnitude of the reduction potentials of the products of the
1O2−furan cycloaddition/fragmentation process, making them
more powerful electron acceptors and therefore more efficient
fluorescence quenchers by photoinduced electron transfer from
the CP backbone to the oxidized side chains, and (ii) a
potentially different distribution of products of epoxide and
ester side chainsat concentrations suitable for NMR analysis
(∼2 mg/mL), the trifluoromethyl-substituted diaryl furans of
PFc yielded a epoxide:ester ratio of 4:1, while the unsubstituted
diaryl furans of PFa yielded a 1:2 epoxide:ester ratio. In
addition, because of an increase in the local concentration of
1O2-induced quenchers, increasing the number of furan side
chains per repeat unit also increased the efficiency of
quenching: PF-8a, which has two diarylfurans per repeat unit,
showed 1.7-fold more quenching upon consumption of all
furans than PFa, which has one diarylfuran per repeat unit.
Finally, the effect the electron-withdrawing trifluoromethyl
group and increased concentration of potential quenchers with
doubly substituted monomers was roughly additive, with
1
substituents slowed the rate of reaction between O2 and the
2,5-diarylfuran (vide supra), poly(fluorene-co-phenylene)s with
fluorinated diarylfurans showed more efficient fluorescence
1
quenching upon reaction with O2 than analogous polymers
with nonfluorinated diarylfurans. To illustrate this effect, Figure
6 shows the ratio of initial polymer fluorescence intensity to
polymer PF-8c showing I0/If of 12.1
quenching).
1.89 (90−93%
In conclusion, we have prepared a series of new diarylfuran-
linked poly(fluorene-co-phenylene)s that show a fluorescence
1
quenching response to O2 by photoinduced electron transfer.
1
The rapid rate of reaction between O2 and diarylfurans (k ∼
108 M−1 s−1) enables a fast fluorescence quenching response of
these polymers, while variations in the electronic nature of
substituents on the furan moiety enables tuning both of the
rates of reactions and efficiency of fluorescence quenching.
Conjugated polymer backbones with a lower excited state
energy (PPEs) showed no fluorescence quenching in response
to oxidation of the furan side chains. These results advance our
1
capabilities for designing the components of luminescent O2-
responsive materials rationally.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details regarding the preparation and character-
ization of all new compounds and materials, UV/vis and
fluorescence spectra of all polymers and diarylfurans, and
results of control experiments. This material is available free of
charge via the Internet at http://pubs.acs.org.
Figure 6. (a) Ratio of fluorescence intensity of furan-substituted
poly(fluorene)s before 1O2 exposure to 1O2 (I0) to fluorescence
intensity after consumption of all furan side chains by O2 (I) as a
function of polymer structure. Heights of the bars represent the
average result of at least three trials; errors bars represent standard
deviations of those averages. (b) Fluorescent response of PF-8c, which
of all polymers we studied, showed the largest amount of fluorescence
quenching upon complete conversion of furans by 1O2. The excitation
wavelength was 390 nm.
AUTHOR INFORMATION
Corresponding Author
*Tel 1-617-627-3771; e-mail sam.thomas@tufts.edu.
Notes
■
1
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Acknowledgement is made to the donors of the American
Chemical Society Petroleum Research Fund for partial support
of this research (51028-DNI10). This work was also supported
by an NSF CAREER award (DMR-1151385) and Tufts
University.
polymer fluorescence intensity after all furan pendants had
reacted with 1O2 (when the UV/vis and fluorescence spectra no
longer changed in response to 1O2). For example, polymer PFa,
which has no fluorinated substituents, showed I0/If (ratio of
1
fluorescence intensity before exposure to O2 to fluorescence
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