Water-soluble fluorescent polymers that respond to singlet oxygen

Article abstract of DOI:10.1002/pola.28130

Although acenes with more than three fused rings can both fluoresce efficiently and react with singlet oxygen (¹O₂) rapidly, their hydrophobic nature presents a challenge to their use in aqueous environments. Herein we report a series of fluorescent, water-soluble random copolymers that each comprise (oligoethylene glycol) methacrylate (OEGMA) and one of several diarylacene methacrylates, including a tetracene methacrylate and a tetraceneothiophene methacrylate. Exposure to ¹O₂ in water oxidizes the pendant acenes, resulting in diminution of their fluorescence intensities. The observed rate of oxidation of the tetracene-containing polymers compares favorably with a commercial ¹O₂-sensitive dye. Polymers that also include energy-donating coumarin side chains show ratiometric fluorescence changes in response to ¹O₂.

Full text of DOI:10.1002/pola.28130

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Water-Soluble Fluorescent Polymers That Respond to Singlet Oxygen
Esra Altinok, Fanny Frausto, Samuel W. Thomas III
Department of Chemistry, Tufts University, 62 Talbot Avenue, Medford, Massachusetts 02155
Correspondence to: S. W. Thomas (E-mail: sam.thomas@tufts.edu)
Received 16 February 2016; accepted 22 March 2016; published online 00 Month 2016
DOI: 10.1002/pola.28130
ABSTRACT: Although acenes with more than three fused rings
can both fluoresce efficiently and react with singlet oxygen
(1O2) rapidly, their hydrophobic nature presents a challenge to
their use in aqueous environments. Herein we report a series of
fluorescent, water-soluble random copolymers that each com-
prise (oligoethylene glycol) methacrylate (OEGMA) and one of
several diarylacene methacrylates, including a tetracene methac-
rylate and a tetraceneothiophene methacrylate. Exposure to 1O2
in water oxidizes the pendant acenes, resulting in diminution of
their fluorescence intensities. The observed rate of oxidation of
the tetracene-containing polymers compares favorably with a
commercial 1O2-sensitive dye. Polymers that also include
energy-donating coumarin side chains show ratiometric fluores-
1
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cence changes in response to O2. 2016 Wiley Periodicals, Inc.
J. Polym. Sci., Part A: Polym. Chem. 2016, 00, 000–000
KEYWORDS: acene; fluorescence; photochemistry; water-soluble
polymers; singlet oxygen
includes a commercially available probe, Singlet Oxygen Sen-
sor Green.22
INTRODUCTION Due to the large driving forces and rapid
rates for its formation from excited triplet states, many dif-
ferent chromophores can generate the lowest energy elec-
tronic excited state of O2, singlet oxygen (1O2), upon
irradiation with light under oxygenated conditions.1,2 1O2 is
a key reactive oxygen species (ROS) for a number of rea-
sons.3,4 When generated upon irradiating a photosensitizer
in vivo, its cytotoxic nature makes it the key ROS in Type II
photodynamic therapy.5,6 1O2 is an important ROS when
human cells are exposed to oxidative stress,7,8 and when
photosynthetic organisms are exposed to excess light.9,10 Its
readily amplified nature—one sensitizer molecule can yield
Our group has used acenes other than anthracene as dienes
in luminescent materials that respond to 1O2.23 The broad
palette of available acenes offers highly tunable emission
wavelengths as well as a wide range of reactivity with 1O2,
including some with bimolecular rate constants of cycloaddi-
tion that approach ꢀ109 M21 21 24,25
s
.
We have used in sev-
eral instances tetracene derivatives as 1O2-reactive energy
acceptors, together with energy donating conjugated poly-
mers, to yield ratiometric fluorescent responses to 1O2.26–28
Other examples of ratiometric luminescent responses to 1O2
are also reported in the literature.29 The hydrophobic nature
of acenes, however, presents a challenge for our approach of
using large acenes in aqueous environments. We therefore
present herein oligo(ethylene glycol) (OEG) side-chain acrylic
polymers as an alternative construct for rendering acenes
1
many O2 molecules—also makes it a key secondary analyte
used for transmitting information regarding the binding of
proteins to surfaces of beads in luminescent oxygen channel-
ing assays.11,12
In terms of specificity, the phosphorescence of 1O2 in the
infrared region of the spectrum is a particularly useful ana-
lytical tool in a variety of studies.13–16 Alternatively, a num-
ber of luminescent probes for 1O2 are reported in the
literature;3,17 these reactive dyes often rely upon [4 1 2]
1
soluble in water for O2-responsive fluorophores.
EXPERIMENTAL
All synthetic manipulations were performed under standard
air-free conditions under an atmosphere of argon gas with
magnetic stirring unless otherwise mentioned. Flash chroma-
tography was performed using silica gel (230–400 mesh) as
the stationary phase. NMR spectra were acquired on a
Bruker Avance III 500 or Bruker DPX-300 spectrometer.
Chemical shifts are reported relative to residual protonated
1
cycloaddition reactions between O2 and a diene (such as an
anthracene derivative) to slow the rate of photoinduced elec-
tron transfer quenching between an excited state of a
coupled chromophore and the anthracene.18–21 The resulting
“unquenching” yields increased quantum yield of lumines-
cence of the fluorophore. This class of 1O2-reactive dyes
Additional Supporting Information may be found in the online version of this article.
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solvent for CHCl3. Molecular weight distribution measure-
ments of the polymers were conducted with a Shimadzu gel
permeation chromatography (GPC) system equipped with a
Tosoh TSKgel GMHhr-M mixed-bed column and guard col-
umn using either UV or refractive index detectors. The col-
umn was calibrated with low polydispersity poly(styrene)
standards (Tosoh, PSt Quick Kit) with THF as the mobile
phase eluting at 0.75 mL/min. All reactants and solvents
were purchased from commercial suppliers and used with-
out further purification, unless otherwise noted.
9,10-Bis-(4-methoxyphenyl)-anthracene (1)
4-methoxyphenylboronic acid (525 mg, 3.45 mmol, 2.2
equiv) and 9,10-dibromoanthracene (525 mg, 1.56 mmol, 1
equiv) was placed in a 100-mL two-neck flask and evacuated
and refilled with argon three times. Na2CO3 (415 mg, 3.92
mmol, 2.5 equiv) was dissolved in 5 mL of DI H2O and then
added to reaction mixture followed by the addition of 30 mL
of a THF/toluene (1:1) mixture. The suspension was then
deoxygenated by sparging with argon for 30 min followed by
the addition of Pd(PPh3)4 (53 mg, 0.045 mmol, 0.03 equiv)
under an argon atmosphere. The mixture was heated to 85
8C and stirred overnight. The mixture was then cooled to
room temperature and water was added. Organics were
extracted twice with Et2O, and combined organic phases
were washed with brine and dried over MgSO4, filtered, and
concentrated using rotary evaporation. The crude product
was purified via flash chromatography using hexanes and
dichloromethane (3:1, v/v) to yield 1.
Optical Experiments
All solution optical spectra were acquired of samples in
quartz cuvettes (NSG Precision Cells). Electronic absorbance
spectra were acquired with a Varian Cary-100 instrument in
double-beam mode using a solvent-containing cuvette for
background subtraction spectra. Fluorescence emission spec-
tra were obtained using a PTI Quantum Master 4 equipped
with a 75-W Xe lamp. All fluorescence spectra are corrected
for the output of the lamp and the dependence of detector
response to the wavelength of emitted light. Fluorescence
spectra were acquired using sample absorbances less than
0.1 OD, corresponding to concentrations of acene moieties
between 2 and 10 lM. Fluorescence quantum yields were
determined relative to either quinine sulfate in 0.1 M H2SO4
or Coumarin 6 in ethanol. Irradiation of the photosensitizer
to generate 1O2 was performed with a 200-W Hg/Xe lamp
(Newport-Oriel) equipped with a condensing lens, water fil-
ter, shutter, and colored glass filter.
Yield: 235 mg (60%). 1H and 13C NMR of this compound is
in good agreement with the same compound reported in the
literature.30
4-(10-(4-Methoxyphenyl)anthracen-9-yl)phenol (2)
Compound 1 (215 mg, 0.55 mmol, 1.0 equiv) was suspended
in 35 mL of dry CH2Cl2, followed by the addition of BBr3
(0.55 mL, 1.0 M in CH2Cl2, 0.55 mmol, 1 equiv) at 278 8C
under argon and stirred overnight at room temperature. The
reaction was stopped by adding 10% HCl(aq). Organics were
extracted twice with CH2Cl2, and combined organic phases
were washed with water and brine, dried over MgSO4, and
filtered. The crude product was purified via flash chromatog-
raphy using pure dichloromethane to yield 2.
Fluorescence Response to Singlet Oxygen
A cuvette containing the test sample solution was irradiated
for numerous timed intervals. Both absorbance and fluores-
cence spectra were taken after each interval of irradiation.
The excitation wavelengths used for emission measurements
are: 285 nm (P1), 300 nm (P2), 350 nm (P3), 340 nm (P4),
340 nm (P5). The absorbance for both methylene blue (MB)
and acene chromophores were approximately 0.1 OD. A typi-
cal 120-min irradiation resulted in an approximate ꢀ15%
decrease in absorbance at 663 nm.
1
Yield: 55 mg (27%). H NMR (500 MHz, CDCl3): d 7.77–7.76
(m, 4H), 7.43–7.41 (m, 2H), 7.38–7.35 (m, 6H), 7.18–7.16
(m, 2H), 7.11–7.09 (m, 2H), 4.89 (s, 1H), 3.99 (s, 3H). 13C
NMR (125 MHz, CDCl3): d 159, 154.9, 132.6, 132.4, 131.4,
131.1, 130.3, 127.1, 126.9, 124.9, 124.8, 115.4, 113.9, 55.4.
4-(10-(4-Methoxyphenyl)anthracen-9-yl)phenyl
Methacrylate (3)
Protein Labeling with Eosin
Compound 2 (55 mg, 0.15 mmol, 1.0 equiv) and DMAP (cata-
lytic amount) was placed in a 25-mL two-neck flask and evac-
uated and refilled with argon three times. Then, 0.1 mL of
dry Et3N and 3.6 mL of dry THF was added to the flask, fol-
lowed by the addition of freshly distilled methacryloyl chlo-
ride (14 mL, 0.15 mmol, 1 equiv) at 0 8C under argon and
stirred overnight at room temperature. The reaction was
stopped by adding 5% aqueous NaHCO3. Organics were
extracted twice with CH2Cl2, and combined organic phases
were washed with water and brine, dried over MgSO4, and fil-
tered. The crude product was purified via flash chromatogra-
phy using hexanes and dichloromethane (1:1, v/v) to yield 3.
Neutravidin biotin-binding protein (10 mg, Thermo Scien-
tific) was dissolved in 1.0 mL of 0.10 M carbonate buffer at
pH 9.4 in an Eppendorf tube. In another Eppendorf tube,
eosin-5-isothiocyanate (5 mg) was dissolved in 0.5 mL of
DMF and 100 mL of it immediately added to the protein solu-
tion, with continuous stirring for an hour at room tempera-
ture. A Sephadex column was prepared to separate the
neutravidin–eosin conjugate from any excess Eosin. Approxi-
mately 5 g of Sephadex G-50 was allowed to swell overnight
in excess distilled water. The Sephadex gel filtration column
was then prepared using 0.10 M pH 9.4 carbonate buffer as
the eluent. An absorbance spectrum of the fraction contain-
ing the conjugate was acquired. The concentration of the
labeled protein was determined as 0.6 lM at 280 nm by
taking into account additional absorbance at 280 nm from
the MB.
1
Yield: 50 mg (75%). H NMR (500 MHz, CDCl3): d 7.79–7.74
(m, 4H), 7.53 (d, J 5 8.5 Hz, 2H), 7.43–7.40 (m, 4H), 7.38–
7.36 (m, 4H), 7.18 (d, J 5 8.5 Hz, 2H), 6.49 (s, 1H), 5.86 (s,
1H), 3.99 (s, 3H), 2.18 (s, 3H). 13C NMR (125 MHz, CDCl3): d
2
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165.9, 159.1, 150.4, 137.2, 136.6, 136, 135.9, 132.37, 132.36,
131.1, 130.2, 130, 127.4, 127.1, 126.9, 125.1, 124.9, 121.7,
113.9, 55.4, 18.4. HRMS calcd for C31H24O3 (M 1 H)1,
445.1798, found, 445.1787.
4-(12-(4-Methoxyphenyl)tetracen-5-yl)phenyl
Methacrylate (7)
Compound 6 (160 mg, 0.38 mmol, 1.0 equiv) and DMAP
(4.8 mg, 0.04 mmol, 0.1 equiv) was placed in a 25-mL two-
neck flask and evacuated and refilled with argon three times.
Then, 0.25 mL of dry Et3N and 6 mL of dry THF was added
to the flask, followed by the addition of freshly distilled
methacryloyl chloride (38 mL, 0.38 mmol, 1 equiv) at 0 8C
under argon and stirred overnight at room temperature. The
reaction was stopped by adding 5% aqueous NaHCO3. Organ-
ics were extracted twice with CH2Cl2, and combined organic
phases were washed with water and brine, dried over
MgSO4, and filtered. The crude product was purified via flash
chromatography using hexanes and ethyl acetate (4:1, v/v)
to yield 7.
5,12-Bis(4-methoxyphenyl)-5,12-dihydrotetracene-
5,12-diol (4)
About 30 mL of dry THF was added to 1-bromo-4-
methoxybenzene (6.8 mL, 54 mmol, 7 equiv), followed by
dropwise addition of n-butyllithium (29 mL, 47 mmol, 6
equiv, 1.6 M in hexanes) at 278 8C. The reaction mixture
was stirred at 278 8C for 1 h and then transferred via can-
nula to 5,12-naphthacenequinone (2.0 g, 7.8 mmol, 1 equiv),
which had been dissolved in 100 mL of dry THF and cooled
to 278 8C. Upon completion of transfer, the reaction mixture
was allowed to warm to room temperature and stirred over-
night under argon. It was then washed with 10% aqueous
HCl and extracted with CH2Cl2. The organic layer was
washed with brine and dried over MgSO4. The crude product
was purified via flash chromatography using hexanes and
EtOAc (1.5:1, v/v) to yield 4.
Yield: 162 mg (86%). 1H NMR (500 MHz, CDCl3): d 8.39 (s,
1H), 8.36 (s, 1H), 7.86––7.83 (m, 2H), 7.77–7.75 (m, 1H),
7.73–7.71 (m, 1H), 7.61–7.59 (m, 2H), 7.51–7.46 (m, 4H),
7.35–7.33 (m, 2H), 7.31–7.28 (m, 2H), 7.24–7.23 (m, 2H),
6.51 (s, 1H), 5.89 (t, 1H), 4.04 (s, 3H), 2.2 (s, 3H). 13C NMR
(125 MHz, CDCl3): d 166, 159.2, 150.5, 137.1, 136.8, 136,
135.8, 132.6, 131.3, 131.1, 130.9, 129.6, 129.5, 129.4, 129.2,
128.43, 128.4, 127.5, 127.1, 126.9, 125.9, 125.5, 125.3,
125.2, 125.1, 124.9, 124.7, 121.8, 114, 55.4, 18.5. HRMS
calcd for C35H28O3 (M 1 H)1, 495.1955, found, 495.1944.
Yield: 2.8 g (75%). 1H and 13C NMR of this compound is in
good agreement with the same compound reported in the
literature.28
5,12-Bis(4-methoxyphenyl)tetracene (5)
Compound 4 (2.77 g, 5.8 mmol, 1.0 equiv) and potassium
iodide (4.62 g, 28 mmol, 4.8 equiv) was dissolved in 115 mL
of acetic acid at room temperature. The reaction mixture
was then heated to 115 8C for 2 h. After cooling to room
temperature, 300 mL of deionized H2O was added to the
reaction mixture, and the resulting orange solid was col-
lected via vacuum filtration and washed with deionized H2O.
The crude product was used without further purification.
2,3-Bis(1,3-dioxolan-2-yl)thiophene (S1)
2,3-Thiophenedicarboxaldehyde (2.0 g, 14 mmol, 1.0 equiv),
p-toluenesulfonic acid monohydrate (6 mg, 0.03 mmol, 0.002
equiv), and ethylene glycol (3.9 mL, 70 mmol, 5 equiv) were
dissolved in 25 mL of benzene. The mixture was refluxed
and water was collected in a Dean-Stark trap. After cooling,
the reaction mixture was washed with 10% NaOH, then H2O
and dried over MgSO4. The crude product was used directly
in the next step without further purification. 1H and 13C
NMR of this compound is in good agreement with the same
compound reported in the literature.31
Yield 2.3 g (92%). 1H and 13C NMR of this compound is in
good agreement with the same compound reported in the
literature.28
5-butyl-2,3-Thiophenedicarboxaldehyde (S2)
2,3-Bis(1,3-dioxolan-2-yl)thiophene (3.0 g, 13 mmol,
4-(12-(4-Methoxyphenyl)tetracen-5-yl)phenol (6)
1
Compound 5 (500 mg, 1.13 mmol, 1.0 equiv) was dissolved
in 56 mL of dry CH2Cl2, followed by the addition of BBr3
(1.13 mL, 1.0 M in CH2Cl2, 1.13 mmol, 1 equiv) at 278 8C
under argon and stirred overnight at room temperature. The
reaction was stopped by adding 10% HCl(aq). Organics were
extracted twice with CH2Cl2, and combined organic phases
were washed with water and brine, dried over MgSO4, and
filtered. The crude product was purified via flash chromatog-
raphy using pure dichloromethane to yield 6.
equiv) was dissolved in 27 mL of dry THF under argon and
cooled to 278 8C. n-BuLi (10.8 mL, 17 mmol, 1.3 equiv,
1.6 M in hexane) was then added dropwisely. After 75 min
of stirring, 1-bromobutane (2.1 mL, 18 mmol, 1.4 equiv)
was added. The solution was left to warm to room temper-
ature and stirred overnight. Organics were extracted twice
with ether, and combined organic phases were washed
with water and brine, dried over MgSO4, and filtered. The
crude product was purified via flash chromatography using
hexanes and ethyl acetate (2:1, v/v) to yield 2,3-bis(1,3-
dioxolan-2-yl)-5-butylthiophene. Yield: 2.0 g (54%). The
crude product was used directly in the next step without
further purification.
Yield: 200 mg (41%). 1H NMR (500 MHz, CDCl3): d 8.38 (s,
1H), 8.37 (s, 1H), 7.84–7.82 (m, 2H), 7.77–7.74 (m, 2H),
7.51–7.49 (m, 2H), 7.46–7.44 (m, 2H), 7.34–7.32 (m, 2H),
7.31–7.28 (m, 2H), 7.24–7.22 (m, 2H), 7.17–7.15 (m, 2H),
4.95 (s, 1H), 4.04 (s, 3H). 13C NMR (125 MHz, CDCl3): d
159.1, 155, 136.7, 136.5, 132.8, 132.6, 131.7, 131.4, 130.94,
130.93, 129.7, 129.51, 129.49, 128.44, 128.41, 127.1, 127,
125.8, 125.7, 125.14, 125.11, 124.7, 124.6, 115.5, 114, 55.4.
A solution of 1.4 g of 2,3-bis(1,3-dioxolan-2-yl)-5-butylthio-
phene in 50 mL of THF:3M HCl (1:1) was stirred at room
temperature for 4 h. The mixture was extracted with ether.
The combined organic phases was washed with water and
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brine, and dried over MgSO4. The crude product was used
directly in the next step without further purification.
Yield: 430 mg (58%). 1H NMR (500 MHz, CDCl3): d 8.91 (s,
2H), 8.34 (s, 3H), 8.23 (s, 1H), 7.91–7.89 (m, 2H), 7.57–7.54
(m, 2H), 7.06 (s, 1H), 6.51 (m, 4H), 6.17–6.15 (m, 4H), 3.57
(s, 6H), 2.98 (t, 3H), 1.84–1.78 (m, 2H), 1.53–1.45 (m, 2H),
1.01 (t, 3H). 13C NMR (125 MHz, CDCl3): d 158, 148, 140,
139.9, 139.7, 138.9, 138.1, 137.4, 135, 134.9, 132.6, 129,
128.9, 128.1, 126.2, 124.78, 124.77, 120.6, 119.9, 119.3,
112.6, 75.99, 75.97, 54.9, 33.2, 30.7, 22.3, 13.9.
1H NMR (500 MHz, CDCl3): d 10.4 (s, 1H), 10.3 (s, 1H), 7.35
(s, 1H), 2.91 (t, 2H), 1.76–1.7 (m, 2H), 1.47–1.39 (m, 2H),
0.99 (t, 3H) 13C NMR (125 MHz, CDCl3): d 184.8, 182.2,
156.3, 145, 143.9, 126.9, 33.2, 30.2, 22.1, 13.7.
1,4-Dihydroxyanthracene (S3)
2-Butyl-5,12-bis(4-methoxyphenyl)tetraceno[2,3-
b]thiophene (9)
1,4-Anthraquinone (850 mg, 4.1 mmol, 1.0 equiv) and
Na2S2O4 (2.3 g, 15.5 mmol, 3.8 equiv) was placed in a 100-
mL round bottom flask and evacuated and refilled with
argon three times. A solution of 40 mL of 1,4-dioxane:H2O
(1:1) was then added to the flask and stirred overnight at
room temperature. The precipitate was collected by filtration
and filtrate was diluted with EtOAc. Organic phase were
washed twice with water and brine, dried over MgSO4, and
filtered. The crude product was used directly in the next
step without further purification. 1H and 13C NMR of this
compound is in good agreement with the same compound
reported in the literature.32
Compound 8 (430 mg, 0.7 mmol) was dissolved in 15 mL of
THF. Then, 20 mL of 10% HCl aqueous solution saturated
with tin(II) chloride dihydrate was added to the reaction
mixture. The solution was stirred for 3 h at room tempera-
ture. Organics were diluted with CH2Cl2, washed with water
and brine, dried over MgSO4, and filtered. The crude product
was purified via flash chromatography using hexanes and
dichloromethane (2:1, v/v). Recrystallization from hexanes
and dichloromethane yielded 260 mg (64%) of pure 9.
1H NMR (500 MHz, CDCl3): d 8.37 (s, 2H), 8.13 (s, 1H), 8.02
(s, 1H), 7.82–7.80 (m, 2H), 7.54–7.52 (m, 4H), 7.30–7.27 (m,
6H), 6.89 (s, 1H), 4.06 (s, 3H), 4.05 (s, 3H), 2.87 (t, 2H),
1.76–1.72 (m, 2H), 1.47–1.42 (m, 2H), 0.98 (t, 3H). 13C NMR
(125 MHz, CDCl3): d 159.2, 159.1, 149.5, 139.8, 138.2, 136.4,
135.2, 132.8, 132.7, 131.9, 131.7, 130.72, 130.7, 128.9,
128.8, 128.5, 128.1, 125.6, 125.5, 124.91, 124.9, 119.8,
119.2, 118.8, 114.1, 114, 55.5, 32.3, 31.1, 22.2, 13.8.
2-Butyltetraceno[2,3-b]thiophene-5,12-dione (TMT
Quinone) (S4)
5-Butyl-2,3-thiophenedicarboxaldehyde (142 mg, 0.72 mmol,
1 equiv) and 1,4-dihydroxyanthracene (152 mg, 0.72 mmol,
1 equiv) were dissolved in 6 mL of ethanol, and the mixture
was vigorously stirred for 1 h at room temperature followed
by the addition of 1 mL of 10% aqueous NaOH. After stirring
at room temperature for 1 h, the precipitate was collected
via vacuum filtration. The crude product was used directly
in the next step without further purification.
4-(2-Butyl-12-(4-methoxyphenyl)tetraceno[2,3-
b]thiophen-5-yl)phenol (10)
Compound 9 (227 mg, 0.41 mmol, 1.0 equiv) was dissolved
in 21 mL of dry CH2Cl2, followed by the addition of BBr3
(0.41 mL, 1.0 M in CH2Cl2, 0.41 mmol, 1 equiv) at 278 8C
under argon and stirred overnight at room temperature. The
reaction was stopped by adding 10% HCl(aq). Organics were
extracted twice with CH2Cl2, and combined organic phases
were washed with water and brine, dried over MgSO4, and
filtered. The crude product was purified via flash chromatog-
raphy using pure dichloromethane to yield 10.
Yield: 225 mg (85%). 1H NMR (500 MHz, CDCl3): d 8.91 (s,
2H), 8.83 (s, 1H), 8.69 (s, 1H), 8.14–8.12 (m, 2H), 7.72–7.7
(m, 2H), 7.27 (s, 1H), 3.01 (t, 2H), 1.85–1.79 (m, 2H), 1.53–
1.47 (m, 2H), 1.01 (t, 3H). 13C NMR (125 MHz, CDCl3): d
183.3, 182.9, 154.1, 144.7, 144.3, 135.24, 135.19, 130.8,
130.3, 130.1, 129.5, 129.3, 122.4, 122.3, 121.7, 32.9, 30.9,
22.3, 13.8.
Yield: 35 mg (16%). 1H NMR (500 MHz, CDCl3): d 8.36 (s,
2H), 8.12 (s, 1H), 8.00 (s, 1H), 7.81–7.80 (m, 2H), 7.53–7.52
(m, 2H), 7.49–7.47 (m, 2H), 7.30–7.25 (m, 4H), 7.20–7.18
(m, 2H), 6.89 (s, 1H), 5.09 (s, 1H), 4.06 (m, 3H), 2.86 (t, 2H),
1.77–1.71 (m, 2H), 1.48–1.40 (m, 2H), 0.97 (t, 3H).
2-Butyl-5,12-bis(4-methoxyphenyl)-5,12-
dihydrotetraceno[2,3-b]thiophene-5,12-diol (8)
About 16 mL of dry THF was added to 1-bromo-4-
methoxybenzene (0.73 mL, 5.8 mmol, 4.6 equiv), followed by
dropwise addition of n-butyllithium (3.5 mL, 46.5 mmol, 5.7
equiv, 1.6 M in hexanes) at 278 8C. The reaction mixture
was stirred at 278 8C for 1 h and then transferred via can-
nula to S4 (470 mg, 1.26 mmol, 1 equiv), which was dis-
solved in 16 mL of dry THF and cooled to 278 8C. Upon
completion of transfer, the reaction mixture was allowed to
warm to room temperature and stirred overnight under
argon. After removal of solvent, organics were dissolved in
CH2Cl2 and then washed with 10% aqueous HCl. The com-
bined organic layer was washed with brine and dried over
MgSO4. The crude product was purified via flash chromatog-
raphy using hexanes and EtOAc (2:1, v/v) to yield 8.
4-(2-Butyl-12-(4-methoxyphenyl)tetraceno[2,3-
b]thiophen-5-yl)phenyl methacrylate (11)
Compound 10 (33 mg, 0.06 mmol, 1.0 equiv) and DMAP (ꢀ
1 mg) were placed in a 25-mL two-neck flask and evacuated
and refilled with argon three times. Then, 45 mL of dry Et3N
and 1.5 mL of dry THF was added to the flask, followed by
the addition of freshly distilled methacryloyl chloride (6 mL,
0.06 mmol, 1 equiv) at 0 8C under argon and stirred over-
night at room temperature. The reaction was stopped by
adding 5% aqueous NaHCO3. Organics were extracted twice
with CH2Cl2, and combined organic phases were washed
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with water and brine, dried over MgSO4, and filtered. The
crude product was purified via flash chromatography using
hexanes and dichloromethane (1:1, v/v) to yield 11.
MgSO4, and filtered. The crude product was purified by
recrystallization from ethanol.
1
Yield: 520 mg (79%). H NMR of this compound agrees with
the literature.33
Yield: 21 mg (56%). 1H NMR (500 MHz, CDCl3): d 8.33 (s,
1H), 8.29 (s, 1H), 8.09–8.05 (m, 1H), 7.97–7.93 (m, 1H),
7.79–7.77 (m, 2H), 7.62–7.59 (m, 2H), 7.52–7.45 (m, 4H),
7.28–7.21 (m, 3H), 6.87–6.85 (m, 1H), 6.5 (s, 1H), 5.86 (s,
1H), 4.03 (s, 3H), 2.83 (t, 2H), 2.18 (s, 3H), 1.73–1.67 (m,
2H), 1.44–1.37 (m, 2H), 0.93 (t, 3H). HRMS calcd for
C41H34O3S (M 1 H)1, 607.2301, found, 607.2310.
15%D-2%TET Polymer (P4)
Monomer 13 (43.7 mg, 0.15 mmol, 15 equiv), monomer 7
(10 mg, 0.02 mmol, 2 equiv), oligo(ethylene glycol) methyl
ether methacrylate comonomer (251.7 mg, 0.84 mmol, 83
equiv), and azobisisobutyronitrile (AIBN) [3 mg, 1% (w/w)]
were dissolved in 2 mL of toluene and sparged with argon
for 30 min. The reaction was then heated to 65 8C and
stirred overnight. The reaction mixture was precipitated into
hexanes. The collected polymer was dissolved in THF and
precipitated into diethyl ether twice.
General Polymerization Procedure
Monomers 3, 7, or 11 (0.8 mol %), oligo(ethylene glycol)
methyl ether methacrylate comonomer, and 1% (w/w) azobi-
sisobutyronitrile (AIBN) were dissolved in toluene and
sparged with argon for 30 min. The reaction was then
heated to 65 8C and stirred overnight. The reaction mixture
was precipitated into diethyl ether. The collected polymer
was dissolved in THF and precipitated into diethyl ether
twice.
1H NMR (500 MHz, CDCl3): d 7.5 (broad), 6.98 (broad), 6.15
(broad), 4.35–4.22 (broad), 4.1 (2H, broad), 3.67 (16H,
broad), 3.57 (2H, broad), 3.4 (3H, broad), 2.4 (broad), 1.8
(2H, broad), 1.04–0.9 (3H, broad). Mn (g/mol): 34k, Mw (g/
mol): 50k (GPC).
P1: 1H NMR (500 MHz, CDCl3): d 4.11 (2H, broad), 3.69–
3.67 (16H, broad), 3.57 (2H, broad), 3.41 (3H, broad), 1.8
(2H, broad), 1.05–0.89 (3H, broad). Mn (g/mol): 36k, Mw (g/
mol): 55k (GPC).
15%D-2%TMT Polymer (P5)
Monomer 13 (32 mg, 0.11 mmol, 15 equiv), monomer 11
(9 mg, 0.015 mmol, 2 equiv), poly(ethylene glycol) methyl
ether methacrylate comonomer (185 mg, 0.62 mmol, 83
equiv), and azobisisobutyronitrile (AIBN) [2.3 mg, 1% (w/
w)] were dissolved in 1.5 mL of toluene and sparged with
argon for 30 min. The reaction was then heated to 65 8C and
stirred overnight. The reaction mixture was precipitated into
hexanes. The collected polymer was dissolved in THF and
precipitated into diethyl ether twice.
1
P2: H NMR (500 MHz, CDCl3): d 4.1 (2H, broad), 3.67 (16H,
broad), 3.57 (2H, broad), 3.40 (3H, broad), 1.81 (2H, 3H,
broad), 1.03–0.89 (broad). Mn (g/mol): 47k, Mw (g/mol):
92k (GPC).
P3: 1H NMR (300 MHz, tetrahydrofuran-d8): d 4.06 (2H,
broad), 3.58 (16H, broad), 3.45 (2H, broad), 3.28 (3H,
broad), 1.77 (2H, broad), 1.05–0.91(3H, broad). Mn (g/mol):
41k, Mw (g/mol): 58k (GPC).
1H NMR (500 MHz, CDCl3): d 7.5 (broad), 6.9 (broad), 6.15
(broad), 4.35–4.27 (broad), 4.11 (2H, broad), 3.67 (16H,
broad), 3.57 (2H, broad), 3.4 (3H, broad), 2.43 (broad),
1.91–1.82 (2H, broad), 1.05–0.90 (3H, broad). Mn (g/mol):
37k, Mw (g/mol): 55k (GPC).
7-(2-Hydroxyethoxy)-4-methylcoumarin (12)
A mixture of 17-hydroxy-4-methylcoumarin (1.0 g, 5.7 mmol,
1 equiv), 2-bromoethanol (0.44 mL, 6.3 mmol, 1.1 equiv)
and potassium carbonate (2 g, 14.3 mmol, 2.5 equiv) in
15 mL of ethanol was heated under reflux for overnight.
Organics were diluted with diethylether and washed with
water and brine, dried over MgSO4, and filtered. The crude
product was sufficiently pure to use in the next step without
further purification. Yield: 1.1 g (88%)
RESULTS AND DISCUSSION
Our overall approach for incorporating acenes as side chains
into hydrophilic acrylic polymers required the synthesis of
methacrylate monomers with acene functionality. Based on
our previous work examining how acene structural cores
1
and substituents influence their reactivity with O2, we chose
three different diarylacenes as side-chains for our poly-
mers—anthracene, tetracene, and tetraceneothiophene
(TMT). The monomers were available through the syntheses
summarized in Scheme 1.24 Suzuki coupling of 4-
methoxyphenylboronic acid and 9,10-dibromoanthracene
gave dianisylanthracene 1, which we were able to singly
deprotect with BBr3 to give phenol 2. Esterification of 2
with methacryloyl chloride yielded the desired anthracene-
substituted monomer 3.
7-(2-Methacryloyloxyethoxy)-4-methylcoumarin (13)
Compound 12 (500 mg, 2.27 mmol, 1.0 equiv) and DMAP
(28 mg, 0.2 mmol, 0.1 equiv) was placed in a 100-mL two-
neck flask and evacuated and refilled with argon three times.
Then, 2.5 mL of dry Et3N and 37 mL of dry THF was added
to the flask, followed by the addition of freshly distilled
methacryloyl chloride (220 mL, 2.3 mmol, 1 equiv) at 0 8C
under argon and stirred overnight at room temperature. The
reaction was stopped by adding 5% aqueous NaHCO3. Organ-
ics were extracted twice with CH2Cl2, and combined organic
phases were washed with water and brine, dried over
Analogous strategies gave the tetracene and TMT methacry-
lates: addition of
a
slight excess of the 4-
methoxyphenyllithium, to either 5,12-tetracenequinone or 2-
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TABLE 1 Molecular Weight Distributions and Optical Properties
of Polymers P1–P3 in Water
Polymer
Mn (PDI)
kmax (0,0)
UF
P1
P2
P3
36 kDa (1.5)
47 kDa (2.0)
41 kDa (1.4)
397 nm
496 nm
554 nm
0.75
0.52
0.44
ent with those known for linear acenes, having intense absorb-
ance in the ultraviolet and a lower energy band with vibronic
resolution. The TMT-based long wavelength absorbance band
between 475 and 575 nm is the furthest red-shifted of the
three. The emission spectra of all molecules follows a similar
pattern to that found in absorbance, with P3 red-shifted by
62 nm from P2, and P2 red-shifted by 85 nm from P1. The
spectral positions of absorbance and fluorescence bands in the
visible portion of the spectrum for all three of these polymers
in water are nearly identical to those reported for similarly
substituted, small molecule acenes in organic solvent.24 In
addition, polymers P1–P3 are fluorescent with quantum yields
of greater than 0.4 (Table 1). These observations indicate that
attachment of these hydrophobic acenes to the POEGMA back-
bone minimizes their aggregation in water.
We next determined the relative rates of reaction of these
polymers with 1O2 by monitoring the absorbance and fluo-
rescence of each acene chromophore as a function of irradia-
tion time of the 1O2 sensitizer MB. Figure 2 shows both the
comparison in relative rates of reactivity determined by UV–
vis spectrophotometry, as well as the diminution of fluores-
cence intensity of each of these three polymers during expo-
sure to 1O2 under identical irradiation conditions, using an
Hg/Xe lamp and a 630-nm long-pass filter. Overall, these
SCHEME 1 Synthesis of diarylacene-linked P1–P3, with x ꢀ
99y.
butyltetraceno[2,3-b]thiophene-5,12-dione27
followed
by
1
polymers reacted with O2 more slowly in water than analo-
gous small molecules in organic solvent, which we ascribe to
two factors: (i) the low solubility of O2 in water and (ii) the
either potassium iodide or tin(II)-mediated reduction of the
resulting diols gave the dianisyltetracene 5 and dianisyl-TMT
9. Single deprotection of each of these compounds with BBr3,
followed by esterification with methacryloyl chloride, yielded
the corresponding acene methacrylates, 7 and 11. For the
synthesis of TMT quinone, we followed a strategy similar to
that previously published, through
a condensation 5-
butylthiophene-2,3-dicarbaldehyde and anthracene-1,4-diol
under basic conditions.34 Finally, AIBN-initiated free radical
polymerizations of monomers 3, 7, and 11 with oligo(ethy-
lene glycol) methyl ether methacrylate (OEGMA) comonomer
gave polymers P1, P2, and P3, respectively, after precipitation
into diethyl ether multiple times. In all three polymerization
reactions, the hydrophobic acene monomer was included at
0.8 mol %. All polymers had similar molecular weight distri-
butions as determined by GPC, with number-average molecu-
lar weights of 36–47 kDa and polydispersity indices between
1.4 and 2.0 (Table 1). Polymers P1–P3 were soluble in water
at concentrations of at least 25 mg/mL, even without dissolu-
tion in polar organic solvents before dilution into water.
FIGURE 1 Height-normalized absorbance spectra of acene-
substituted POEGMA polymers P1–P3 in water. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
We characterized each acene-containing polymer by elec-
tronic absorbance spectrophotometry and fluorescence
spectroscopy in H2O (Figs. 1 and 2). These spectra are consist-
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FIGURE 2 Demonstrations of the relative reactivity of acene pendants of P1, P2, and P3: (a) Comparison of pseudo first-order rates
of disappearance of acenes bound to P1–P3 upon exposure to 1O2 in water. (b–d) Diminution of fluorescence intensity of P1–P3
under identical conditions. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
short lifetime (ꢀ3 ls) of 1O2 in water.35 Nevertheless,
although the differences in observed rates were compressed
relative to those observed in organic solvent, the order of
reactivities of these polymers was consistent with our previ-
ous study of analogous acenes in CH2Cl2: the diaryltetraceno-
thiophene polymer P3 reacted the fastest and the
diarylanthracenene polymer P1 reacted the slowest, with a
difference of approximately 103 in their observed rates.
A unique characteristic of 1O2 is that its lifetime is approxi-
mately 10-fold longer in D2O than in H2O, because vibrations
of the excited-state O2 couple more efficiently to O-H bonds
FIGURE 3 Faster diminution of acene emission intensity of P2 and P3 upon exposure to 1O2 in D2O when compared identical irradia-
tion conditions as used in Figure 2. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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crylic polymers P1–P3 presented above do not have distinct
energy donors and acceptors. We therefore chose to
integrate 1O2-inert energy-donating chromophores as addi-
tional pendant groups in random copolymers of OEGMA and
acene-substituted methacrylates (Scheme 2). We prepared
these polymers in analogous fashion to polymers P1–P3,
using AIBN as initiator, 15 mol % coumarin methacrylate,
and 2 mol % of monomer 7 (tetracene methacrylate) or 11
(tetracenothiophene methacrylate). The balance of monomer
in each polymerization was OEGMA. The resulting isolated
polymers P4 (Mn 5 34 kDa, PDI 5 1.5) and P5 (Mn 5 37 kDa,
PDI 5 1.5) had molecular weight distributions similar to P1-
P3. Although both P4 and P5 were somewhat soluble in
water, the solubility of P5 was limited to less than 1 mg/mL.
Regardless of their limited solubility in water, these poly-
mers still showed optical spectra that responded to 1O2 as
expected. Although both polymers P4 and P5 showed evi-
dence of energy transfer from the coumarin pendants to the
acenes, the recovery of donor emission upon extended expo-
sure times to 1O2 was smaller (ꢀ30% of the original inten-
sity) for P5 (see Supporting Information) than for P4 (100–
150% of the original intensity). We attribute this observation
to better spectral overlap of the fluorescence of coumarin
donors (kem 5394 nm) with the absorbance of tetracenes in
P4 than with the tetraceneothiophenes in P5. We therefore
focused on the performance of P4 due to its more robust
ratiometric fluorescent response to 1O2. Polymers prepared
using smaller feed ratios of acene-based monomers
(z 5 0.01) showed insufficient energy energy transfer for a
robust ratiometric response, while higher concentration of
acene (where z 5 0.05) showed self-quenching of acene fluo-
SCHEME 2 Design of donor–acceptor copolymers P4 and P5
for ratiometric fluorescent response to 1O2 in water. In all
cases, x 5 0.83, y 5 0.15, z 5 0.02, based on feed ratios of mono-
mers. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
than to O-D bonds.35 To support the role of 1O2 in these
observed spectroscopic changes, we compared the rates of
acene disappearance upon irradiation of MB in H2O, as
described above, to rates in D2O. The acene pendants of both
P2 and P3 (we did not further investigate P1 in this context
because of its slow oxidation) oxidized more rapidly in D2O
than in H2O, as shown in Figure 3. For example, after 10 min
of irradiation of sensitizer in H2O, I0/I for P2 and P3 were
1.1 and 1.3, respectively, while identical exposure in D2O for
10 min gave I0/I values of 5.8 and 25.
1
Chemical probes that show ratiometric responses to target
analytes are generally preferable to those that show changes
of intensity at only one wavelength.36 Our previously
reported design, using acene-linked conjugated polymers,
naturally fits this design, as the conjugated polymer back-
bones are light-harvesting energy donors while the acene
pendants are 1O2-reactive energy acceptors.28 The metha-
rescence. Therefore, we performed O2 exposure experiments
with z 5 0.02.
A number of important applications of photogenerated 1O2
require that sensitizers are bound and kept in close proxim-
ity to proteins, including targeted delivery systems for pho-
todynamic therapy and luminescent oxygen channeling
FIGURE 4 (a) Chemical structure of P4 used in these experiments. (b) Ratiometric response of P4 to selective irradiation of eosin-
labeled neutravidin in water. (c) Comparison of rates of oxidation of the acenes in P4 and Singlet Oxygen Sensor Green (SOSG)
under identical conditions. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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assays.5,11 That proteins can physically and chemically
quench 1O2 can present challenges for its productive use in
these types of applications.37 We therefore prepared a conju-
gate of the biotin-binding protein neutravidin, which is useful
in a variety of in vitro bioassay applications due to its reduced
non-specific binding and near-neutral isoelectric point, with
the brominated xanthene dye eosin, a sensitizer of 1O2 for
which an isothiocyanate derivative is commercially available.
Selective irradiation of this protein-eosin conjugate in the pres-
ence of P4 gave a ratiometric response of fluorescence, analo-
gous to that observed for P4 upon irradiation of MB (Fig. 4
and Supporting Information Fig. S2). The protein-bound eosin
yielded oxidation of acenes in these polymers that was 3–10
fold slower than irradiation of solvated MB, which we attribute
at least in part to a combination of quenching of both sensi-
tizer excited states and 1O2 by readily oxidized amino acid resi-
dues. Interestingly, attempts to oxidize P4 using a similar MB–
neutravidin conjugate failed to yield significant oxidation,
which we suspect may be due to efficient quenching of the MB
excited state by the covalently bound protein.
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In addition to this fluorescence response, we also monitored
tetracene oxidation as a function of irradiation time of this
eosin-labeled protein using absorbance spectrophotometry.
The rate of acene oxidation of P4 compared favorably to that
observed for the commercially available Singlet Oxygen Sen-
sor Green (SOSG), in which an alkylated anthracene moiety
reacts with 1O2, under identical irradiation conditions [Fig.
4(c)]. This comparison highlights the potential utility of
endoperoxidation of acenes with more than three fused aro-
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CONCLUSIONS
The use of long, hydrophobic acenes for fluorophores that
respond to 1O2 holds promise due to the rapid rates of oxi-
dation and the potential for ratiometric fluorescent
responses. This work demonstrates the potential for harness-
ing these features in aqueous environments as pendants of
water-soluble OEGMA-based polymers. Our overall synthetic
approach is applicable for preparing water-soluble counter-
parts of highly hydrophobic fluorescent molecules. A key dis-
advantage of our approach is the low density of
chromophores, which slows energy transfer and limits the
range of ratiometric responses available using this design.
Current work in our laboratory is focusing on maintaining a
high chromophore density in water for maximally sensitive
1O2-responsive aqueous materials.
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27 D. Koylu, S. Sarrafpour, J. Zhang, S. Ramjattan, M. J.
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The authors thank the National Science Foundation (CHE-
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