Acene-linked conjugated polymers with ratiometric fluorescent response to 1O2

Article abstract of DOI:10.1039/c0cc05770c

This communication describes new conjugated polymers that bear diarylanthracene or diaryltetracene pendants and respond to singlet oxygen by interrupting energy transfer resulting in blue-shifted fluorescence.

Full text of DOI:10.1039/c0cc05770c

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COMMUNICATION
Acene-linked conjugated polymers with ratiometric fluorescent response
1
to O2w
Jingjing Zhang, Syena Sarrafpour, Robert H. Pawle and Samuel W. Thomas III*
Received 24th December 2010, Accepted 19th January 2011
DOI: 10.1039/c0cc05770c
This communication describes new conjugated polymers that
bear diarylanthracene or diaryltetracene pendants and respond
to singlet oxygen by interrupting energy transfer resulting in
blue-shifted fluorescence.
Fluorescent conjugated polymers (CPs) have applications in
emerging technologies such as sensors,1 organic light-emitting
displays,2 and photovoltaic devices.3,4 These technologies
harness the ability of CPs to conduct excitons, holes, and
Scheme 1 Interruption of linear acene conjugation via cycloaddition
of 1O2 with diaryltetracene. Only one of two regioisomeric products is
electrons. CP sensors rely on intra- and inter-chain exciton
mobility to amplify fluorescence quenching or energy transfer
in response to specific analytes, such as single-stranded
DNA,5–7 nitroaromatics,8,9 or oxidants such as hydrogen
peroxide.10 This amplifying property of CPs increases their
sensitivity over comparable small molecule fluorescent
sensors, which makes CPs exceptional sensing materials,
including for sensing of biological analytes.11–13
shown.
linear acene bound via an insulating four-carbon linker. The
pendant acenes are acceptors in an energy transfer process
with the CP backbone. We anticipated that the linear acene
would: (i) alter the emission of the CP backbone through
electron transfer, energy transfer, or both, and (ii) undergo a
1
[4 + 2] cycloaddition with O2 to shut these pathways down
We are interested in developing CPs that respond
specifically to analytes that can be amplified chemically.
Singlet oxygen (1Dg),14,15 which has been used as a chemical
trigger in chemiluminescence-based bioassays,16,17 is generated
readily by triplet energy transfer from excited dye molecules to
and restore CP backbone fluorescence (Scheme 1).
In addition to a CP that contained only octyloxy chains on
the phenylene rings with no 1O2-reactive linear acenes (P1), we
1
prepared O2-responsive CPs P2 and P3 (Scheme 2). Suzuki
1
coupling between 9-bromo-10-phenylanthracene27 and
O2.18 In addition, O2 is an important reactive oxygen species
4-methoxyphenylboronic
acid
gave
anisyl-substituted
in photodynamic therapy and has been implicated in the
genotoxicity of UV light.19,20 A number of molecules that
sense 1O2 specifically through changes in intensity of fluorescence,
phosphorescence, or chemiluminescence have been reported.21–26
These examples often comprise a luminescent dye linked to a
1O2-reactive quencher (an anthracene derivative); upon [4 + 2]
cycloaddition, quenching slows dramatically to allow
observation of fluorescence of the dye. To our knowledge, CPs
designed to respond to 1O2 have not been reported. In this
communication, we describe fluorescent CPs substituted with
1
linear fused acenes that respond to O2 in a ratiometric manner
through interruption of energy transfer pathways.
Our 1O2-responsive conjugated polymers comprise
a
poly(phenylene-co-fluorene) backbone and
a
1O2-reactive
Tufts University Department of Chemistry, 62 Talbot Avenue,
Medford, MA 02155, USA. E-mail: sam.thomas@tufts.edu;
Fax: +1 617-627-3443; Tel: +1 617-627-3771
w Electronic supplementary information (ESI) available: Experimental
details, synthetic procedures and additional supportive data. See DOI:
10.1039/c0cc05770c
Scheme 2 Synthesis of acene-linked diiodo monomers and structures
of resulting conjugated poly(fluorene-co-phenylene)s P2 and P3.
c
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excited the polymer backbone (373 nm) or the tetracene
pendant. In addition, the excitation spectrum of P3 resembles
its UV/vis spectrum. This evidence taken together strongly
supports the conclusion that efficient energy transfer takes
place from the polymer backbone to the tetracene pendant
groups. The significant overlap between the backbone
emission (lmax = 417) and the tetracene absorbance between
410 and 510 nm also support this conclusion.
Covalent linkage between these two chromophores is
necessary for energy transfer to occur in solution: when each
component (P1 and 4) was dissolved in the same solution in an
equimolar (per polymer repeat unit) ratio, fluorescence from
P1 dominated upon excitation at 373 nm. Anthracene-
substituted polymer P2 participated in analogous intramolecular
energy transfer from the CP backbone to the pendant,
although the effect was subtler because of the small difference
in emission maxima between the pendant and the backbone
(5 nm).
Fig.
1 Normalized UV/vis (left, black lines) and fluorescence
To examine the response of these molecules to 1O2, we
irradiated the photosensitizer methylene blue (MB) in a
CH2Cl2 solution containing the fluorophore(s) of interest.
(right, blue lines) spectra of P3 (solid lines) and 5,12-dianisyltetracene
4 (dotted lines) in CH2Cl2.
1
MB generates O2 with a quantum yield of B0.5,29 is soluble
anthracene 1, which we demethylated with BBr3 to yield
phenol 2. Alkylation of 2 with known28 monomer precursor
3 gave the anthracene-functionalized diiodo monomer M2.
Synthesis of tetracene-substituted CP P3 followed a different
strategy, which involved addition of 4-methoxyphenyllithium
to 5,12-napthacenequinone followed by reduction to the fully
aromatized dianisyltetracene 4, deprotection of the methoxy
groups, and stepwise alkylation of the resulting diphenol 5
with 3 and CH3I gave tetracene-containing diiodo monomer
M3. Step-growth Suzuki polymerizations of M2 or M3 with
commercially available 9,9-dioctylfluorene-2,7-diboronic acid
bis(1,3-propanediol) ester using standard biphasic conditions
[Pd(PPh3)4, K2CO3, toluene/water, phase transfer catalyst, 90 1C]
gave the target polymers P2 and P3. These polymers had
number average molecular weights (Mn) of 10 000–13 000 g molÀ1
as determined by size exclusion chromatography versus
polystyrene standards after precipitation into methanol and
Soxhlet extraction with hexanes.
in organic solvents, and has an absorbance band that is
strongly red-shifted (lmax = 654 nm) from the molecules
and polymers studied here, which allows for its selective
irradiation using a 590 nm high pass filter. Upon exposing
model acenes 1 and 4 to identical conditions of 1O2 photo-
generation, the bimolecular rate constant for cycloaddition
(as determined by the dependence of UV/vis spectra on
irradiation time)30 for tetracene 4 was 20 times faster than
for anthracene 1 in CDCl3. It is known that longer linear
1
acenes generally react faster via cycloaddition with O2 than
similarly substituted shorter acenes.31 NMR analyses of these
reactions on a preparative scale in CDCl3 were consistent with
the endoperoxides as the products of these reactions.
We observed two key changes in the optical spectra of
tetracene-substituted P3 upon exposure to photogenerated
1O2. First, the tetracene-based absorbance bands at 283 nm
and between 410 and 510 nm decreased in intensity (ESIw).
Fig. 1 shows the electronic absorbance and fluorescence
spectra of P3 (solid lines) in dichloromethane, as well as that
of the small molecule model tetracene 4 (dotted lines). The
absorbance spectrum of P3 contains both the characteristic
bands of tetracene derivatives (a sharp band at 283 nm and the
well-resolved vibronic bands between 400 and 500 nm) as well
as a broad featureless band between 300 and 400 nm
characteristic of poly(fluorene) derivatives. No bands appear
in the UV/vis spectrum of P3 that are not present in the
spectra of the individual chromophores (4 and P1), which
indicates that there are no significant ground-state interactions
between the polymer backbone and the pendant tetracene. In
addition, the anthracene-substituted polymer P2 shows similar
characteristics, with the difference that the vibronic bands of
the anthracene pendant overlap strongly with the absorption
of the polymer backbone.
In contrast to differences between their absorbance spectra,
P3 and tetracene 4 show nearly complete overlap of emission
spectra, with maxima at 512 nm (B100 nm red-shifted from P1).
This overlap occurred whether the excitation wavelength
Fig. 2 Ratiometric fluorescence response of P3 in O2-saturated
CH2Cl2 as a function of time of irradiation of methylene blue.
c
3446 Chem. Commun., 2011, 47, 3445–3447
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Table 1 Fluorescence maxima, quantum yields, and relative rates of
1O2 cycloaddition of acene-linked polymers and small molecule acenes
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lmax (emission)/nm
FF
krel (1O2)
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1
P2
4
422
422
512
512
0.60
0.58
0.64
0.44
1
0.6
21
10
P3
Second, the tetracene emission at 510 nm decreased, while
polymer backbone emission at 417 nm increased (Fig. 2). The
absorbance of the CP backbone at 365 nm did not decrease
during exposure to 1O2. The initial rate of reaction of P3 with
1O2 was of the same order of magnitude (two times smaller) as
the rate of reaction between tetracene 4 and 1O2 (Table 1).
Although P2 had a qualitatively similar response to photo-
generated 1O2, the shift in emission spectra was 5 nm, and the
rate of reaction was slower, as expected from the studies with
small molecules. The rate of reaction between P2 and 1O2 was
slow enough to allow observation of significant decomposition
of the CP backbone.32
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These observations are consistent with formation of the
tetracene-endoperoxide. Tetracene and rubrene are known to
form endoperoxides in high yield via 1O2 cycloaddition.33,34
Upon endoperoxide formation, energy transfer is no longer
competitive with fluorescence from the CP backbone. Two
negative control experiments were conducted: (i) irradiating a
solution of P3 at l > 590 nm in the absence of MB, and
(ii) allowing a solution containing MB and P3 to remain in the
dark. Both experiments showed no appreciable change in
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optical spectra. Finally, the H NMR of P3 showed changes
consistent with endoperoxide formation upon exposure to 1O2.
The tetracene resonance at 8.3 ppm disappeared while a
characteristic bridgehead resonance at 6.1 ppm appeared.
In conclusion, we have developed new acene-linked CPs that
respond to 1O2 through interruption of an energy transfer
pathway by cycloaddition of the analyte with energy accepting
linear acenes. To our knowledge, this is the first example of a
conjugated polymer designed to respond to 1O2. These
materials have a ratiometric response, which can improve
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tion, the fluorescence enhancement of P3 is against a dark
background, which yields a large contrast (200-fold) in the
fluorescence intensity between ‘‘off’’ and ‘‘on’’ states. These
characteristics are important for the development of fluorescent
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1
response to O2 should be possible by optimizing the stoichio-
metry of the reactive pendant. Ongoing work is focused on
determining the specificity of the response, demonstrating this
1
effect with solid-state CPs, and developing O2-mediated assays.
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The authors acknowledge support from a DARPA Young
Faculty Award (N66001-09-1-2116), Tufts University, and the
U.S. Department of Education for a GAANN fellowship
(R.H.P.).
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Chem. Commun., 2011, 47, 3445–3447 3447