One-step synthesis of a thienylenepyridazinylenethienylene-based coil-rod-coil copolymer with enhanced emission and improved fluorescence stability

Article abstract of DOI:10.1002/pola.26535

A new coil-rod-coil copolymer is synthesized via Sonogashira coupling using one-step methodology. The copolymer PEG-OEPETPT-PEG constitutes of poly(ethylene glycol) (PEG) as the coil block, and oligo[p- (ethynylenephenyleneethynylene)-alt-(thienylenepyridazinylenethienylene)] (OEPETPT) as the rod segment. The conjugated polymer PEPETPT with the same conjugated building blocks is also synthesized for comparison. The structures of both polymers are confirmed by NMR, combined with other characterizations. PEG-OEPETPT-PEG has a 12 nm blue-shift in the emission maximum compared with that of PEPETPT, and a higher quantum yield of fluorescence in THF. PEG-OEPETPTE-PEG tolerates up to 20% water content in H₂O/THF mixed solvent without significantly changing the emission wavelength and intensity, while the fluorescence of PEPETPT is dramatically quenched by a very small quantity of water. Further photophysical studies about these two polymers indicate that the introduction of PEG coils onto the conjugated block retards the water-induced-aggregation and therefore improves the fluorescence stability of PEG-OEPETPT-PEG. Copyright

Full text of DOI:10.1002/pola.26535

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One-Step Synthesis of a Thienylenepyridazinylenethienylene-Based
Coil-Rod-Coil Copolymer with Enhanced Emission and Improved
Fluorescence Stability
Wei Wu, Haibo Xu, Dezhi Shen, Tian Qiu, Li-Juan Fan
Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and
Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Jiangsu 215123,
People’s Republic of China
Correspondence to: L.-J. Fan (E-mail: ljfan@suda.edu.cn)
Received 20 November 2012; accepted 15 December 2012; published online 9 January 2013
DOI: 10.1002/pola.26535
ABSTRACT: A new coil-rod-coil copolymer is synthesized via
Sonogashira coupling using one-step methodology. The copol-
ymer PEG-OEPETPT-PEG constitutes of poly(ethylene glycol)
2
water content in H O/THF mixed solvent without significantly
changing the emission wavelength and intensity, while the flu-
orescence of PEPETPT is dramatically quenched by a very
small quantity of water. Further photophysical studies about
these two polymers indicate that the introduction of PEG coils
onto the conjugated block retards the water-induced-aggrega-
tion and therefore improves the fluorescence stability of
PEG-OEPETPT-PEG. VC 2013 Wiley Periodicals, Inc. J. Polym.
(
PEG) as the coil block, and oligo[p-(ethynylenephenyleneethy-
nylene)-alt-(thienylenepyridazinylenethienylene)] (OEPETPT) as
the rod segment. The conjugated polymer PEPETPT with the
same conjugated building blocks is also synthesized for com-
parison. The structures of both polymers are confirmed by
NMR, combined with other characterizations. PEG-OEPETPT-
PEG has a 12 nm blue-shift in the emission maximum com-
pared with that of PEPETPT, and a higher quantum yield of flu-
orescence in THF. PEG-OEPETPTE-PEG tolerates up to 20%
Sci., Part A: Polym. Chem. 2013, 51, 1636–1644
KEYWORDS: aggregation; block copolymers; conjugated poly-
mers; fluorescence; oligomers
INTRODUCTION Conjugated polymers have received contin-
especially useful in creating new sensing systems, used as
19–23
ued research interest for the past 30 years due to their wide
photovoltaic materials or in other applications.
Combin-
1
–4
application in many fields.
Developing new soluble conju-
ing pyridazine with thiophene, a typical electron-donating ar-
2
4,25
gated polymers with certain specific building blocks will
bring new properties as well as broaden the applications.
Heterocyclic aromatic structures always displayed unique
features in photochemistry and photophysics, coordination
omatic unit, provides the charge transfer structure.
In
6–28
2
addition, this structure also provides the SꢀꢀꢀN interaction,
which may lead to certain unique coordinating and assem-
bling properties. The thiophene-pyridazine-thiophene (TPT)
structure has been synthesized as small molecules and the
5
–7
chemistry, self-assembly, and so forth.
Pyridazine, a six-
member aromatic ring with two nitrogen atoms neighboring
to each other, has been widely studied as the ligand in pre-
paring the many metal complexes as well as the supramolec-
24
properties have been studied. To our best knowledge, only
one literature reported such a structure in conjugated poly-
25
mer systems. However, the solubility always poses chal-
6
,8–13
ular metal coordinating units.
In addition, pyridazine is
lenges, which resulted in difficulty in the study of solution
properties of these polymers, or building-up the supramolec-
ular structure. In addition, it was very difficult to control
and tune the solid state morphology, which actually is a key
factor influencing the final performance of certain
applications.
often used as a structural component of compounds for bio-
1
4
related studies, such as antinociceptive drugs, antibacterial
1
5
16
agents, anti-inflammatory compounds, and antihyperten-
1
7
sive medicines. Besides the metal cation coordinating tend-
ency, pyridazine also exhibits electron-withdrawing features;
thus, it has also been incorporated with a certain electron-
donating structure to build up the intramolecular charge
transfer (ICT) molecules. The ICT character for the mole-
cules, such as strongly stokes-shift additional fluorescence
bands and facility for further charge separation, makes them
Sonogashira coupling has been demonstrated to be a power-
1
8
29–31
ful synthetic tool toward poly(aryleneethynylene)s.
Bisalkyne and dihaloarene monomers can be used to obtain a
linear polymer (Scheme 1). By selecting different monomers
Additional Supporting Information may be found in the online version of this article.
2013 Wiley Periodicals, Inc.
V
C
1
636
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32–39
ing such triblock copolymers,
several advantages are
obvious for this one-step approach. First, the reaction
sequence is greatly shortened. Second, this approach circum-
vents the post-polymerization reaction. This is desirable since
reactions that take place on the polymer chain usually are
not as simple compared to small molecular systems and
sometimes the reactions cannot be completed. Third, the
length of the PEG chains can be fixed by selecting the com-
mercial compound according to the requirements. In addi-
tion, the length of the conjugated rod can be adjusted by the
feeding ratio, due to the polycondensation nature of polymer-
ization under Sonogashira coupling protocol, which will be
discussed in detail later.
SCHEME 1 General synthetic strategy for poly(aryleneethyny-
lene)s using Sonogashira coupling protocol.
as the building blocks, a variety of poly(aryleneethynylene)s
with different backbones and pendant groups can be synthe-
sized. Generally, the molar ratio between the two monomers
should be 1:1 to achieve high molecular weight, according to
the polycondensation theory. Sonogashira coupling was also
employed in this study for the synthesis of the conjugated
polymers. To solve the solubility problem, ethynylene phenyl-
ene ethynylene (EPE) building block, which has long alkoxyl
groups, copolymerize the TPT building block through Sonoga-
This coil-rod-coil polymer is expected to have certain unique
properties in various applications, since it combines many inter-
esting characteristics, as mentioned above, in one molecule. As
an initial study of its properties, the investigation of photophy-
sics was carried out for the polymer in dilute solution. The
results showed that the PEG-OEPETPT-PEG polymer has
enhanced emission and improved fluorescence stability over the
PEPETPT polymer. The relationship between the fluorescence of
polymers in dilute solution and the composition of mixed sol-
vent was also investigated. Different aggregation behaviors have
been observed from the emission of the two polymers, which
can be attributed to the difference in the polymer structure.
shira coupling to obtain
a new conjugated polymer
(
PEPETPT) (Scheme 2). To further improve the solubility,
flexible poly(ethylene glycol) (PEG) chains were introduced
to both ends of the conjugated segment to form a coil-rod-
coil triblock polymer (PEG-OEPETPT-PEG). After the modifi-
cation of the commercial PEG, the coil-rod-coil polymer was
synthesized through the same Sonogashira coupling polymer-
ization, in one-step, by adding the modified PEG (PEG-I) and
the conjugated polymer building blocks in the reaction sys-
tem (Scheme 2). Compared with other methods for synthesiz-
SCHEME 2 Synthetic strategies for the small molecules and polymers in this study.
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EXPERIMENTAL
mL of dry pyridine under argon. A solution of toluene-p-sul-
fonyl chloride (1.40 g, 7.30 mmol) in 15 mL of pyridine was
then added dropwisely to the mixture. After being stirred for
Materials
3
-Methylthiophene, 3,6-dichloropyridazine, 1,3-bis(diphenyl-
phosphinol)propane nickel(II) chloride (NiCl (dppp)), 4-iodo-
phenol, tetrakis(triphenylphosphine) palladium ((PPh ) Pd),
24 h at room temperature under argon, the resulting solu-
2
tion was poured into water and extracted with methylene
chloride. The organic layer was washed with water, dried
over anhydrous magnesium sulfate. After filtration, the sol-
vent in the filtrate was removed in a rotary evaporator to
obtain the crude product, which was further purified by
silica gel column chromatography (ethyl acetate as eluent) to
3
4
cuprous iodide (CuI), and PEG monomethyl ether (M ¼ 1900,
n
PDI ¼ 1.05) were purchased from Alfa Aesar Chemical and
used as received. Other commercially available reagents were
from Sinopharm Chemical Reagent and were used without fur-
ther purification unless otherwise noted. Tetrahydrofuran was
dried by distillation from sodium metal and kept under argon.
Diisopropylamine was distilled over potassium hydroxide
before use. 3,6-Bis(2-(5-bromo-3-methylthienyl))pyridazine
1
afford a white solid (8.60 g, yield 60%). H NMR (400 MHz,
CDCl , d): 2.31 (s, 3H), 3.37 (s, 3H), 3.54–4.02 (m, 176 H),
3
ꢁ
1
7
.35 (d, ArAH), 7.80 (d, 2H). FTIR (KBr, cm ): 2869, 1963,
1721, 1638, 1459, 1351, 1299, 1250, 1198, 1102, 944, 846,
89. Anal. Calcd for C96 47S (%): C, 54.29; H, 8.77;
found: C, 53.32; H, 9.14.
2
5
40
(
1) and 1,4-diethynyl-2,5-didodecyloxybenzene (2) were
6
186
H O
synthesized according to the published procedures.
Measurements
1
H NMR spectra were recorded on an Inova 400 MHz NMR
PEG methyl-p-iodophenyl ether (PEG-I)
spectrometer with tetramethylsilane as the internal standard.
The FTIR spectra of the monomers and polymers were
obtained on Nicolet 6700 FTIR spectrophotometer (thermo)
with KBr pellets. Elemental microanalyses were carried out
on a Carlo-Erba Elemental Analyzer EA 1110. Gel permeation
chromatography (GPC) measurements were performed on a
Waters 1515 system with THF as the mobile phase and poly-
styrene as the standard. The UV–vis absorption spectra were
collected on a Hitachi U-3900/3900H spectrophotometer,
and photoluminescence (PL) emission spectra were meas-
ured on a Horiba FluoroMax-4 spectrofluorometer using exci-
tation at 440 nm with 1 nm slits. The concentration of the
polymers was held at 1 mM with respect to the repeating
unit of the conjugated polymer backbone. The fluorescence
quantum yields in solution of the polymers were determined
relative to quinine sulfate in 0.5 M H SO solutions with a
Potassium carbonate (2.40 g, 17.4 mmol) and 4-iodophenol
(1.50 g, 6.80 mmol) was slowly added into a solution of 3
(4.00 g, 3.45 mmol) in ethanol (100 mL) at room tempera-
ture. The reaction system was refluxed for 24 h and cooled
to room temperature. The resulting mixture was poured into
a large amount of water and extracted with methylene chlo-
ride. Then the organic layer was washed with water, and
dried over anhydrous magnesium sulfate. After filtration, the
solvent in the filtrate was removed in a rotary evaporator.
The obtained crude product was further purified by column
chromatography (silica gel, ethyl acetate eluent) to afford a
1
white solid (2.00 g, yield 50%). H NMR (400 MHz, CDCl
,
3
d): 3.37 (s, 3H), 3.54–4.02 (m, 176H), 6.70 (d, ArAH), 7.56
ꢁ
1
(d, 2H). FTIR (KBr, cm ): 2861, 1962, 1638, 1578, 1485,
1467, 1345, 1278, 1243, 1101, 999, 962, 850, 583, 529.
Anal. Calcd for C95H O45I (%): C, 52.53; H, 8.43; found: C,
183
2
4
quantum yield of 0.546, excited at 365 nm.
49.09; H, 8.24.
Synthesis
Poly[p-(ethynylenephenyleneethynylene)-alt-
thienylenepyridazinylenethienylene)] (PEPETPT)
3
,6-Bis(2-(5-bromo-3-methylthienyl))pyridazine (1)
(
Compound 1 was synthesized according to literature meth-
3
1
,6-Bis(2-(5-bromo-3-methylthienyl))pyridazine (1) (0.430 g,
.00 mmol), 1,4-diethynyl-2,5-didodecyloxybenzene (2)
2
5,40
ods.
Satisfactory NMR characterization of all stable inter-
mediates was observed. The final product was obtained as a
light-yellow solid (yield 75%). H NMR (400 MHz, CDCl , d):
(
0.494 g, 1.00 mmol), (PPh ) Pd (0.0580 g, 0.050 mmol),
3 4
1
3
and CuI (0.0200 g, 0.10 mmol) were placed in a pre-dried
Schlenk flask and deoxygenated by several cycles of vacuum-
argon cycling. 4 mL of dry diisopropylamine and then 20 mL
of dry THF were quickly added into the flask under an argon
atmosphere. The mixture was refluxed for 24 h under argon.
After cooling to room temperature, the reaction mixture was
precipitated in 400 mL of methanol. The precipitate was cen-
trifuged and redissolved in a minimal amount of THF. Then
the resulting polymer solution was precipitated in 400 mL of
methanol, centrifuged, and dried in vacuum for 24 h to
afford a brown solid (0.61 g, yield 80%). GPC (in THF, with
ꢁ
1
7
2
7
.66 (s, 2H), 6.95 (s, 2H), 2.52 (s, 6H). FTIR (KBr, cm ):
951, 2928, 2852, 1554, 1433, 1384, 1141, 1054, 993, 823,
48, 729. Anal. Calcd for C H Br N S (%): C, 39.07; H,
1
4
10
2 2 2
2.33; N, 6.51; found: C, 39.29; H, 2.49; N, 6.51.
1
,4-Diethynyl-2,5-didodecyloxybenzene (2)
Compound 2 was synthesized according to published proce-
4
1
dures. The product was a light-yellow crystalline solid (yield
1
8
3
0%). H NMR (400 MHz, CDCl , d): 6.95 (s, 2H), 3.95 (t, 4H),
3
.33 (s, 2H), 1.78 (m, 4H), 1.50–1.20 (m, 36H), 0.88 (t, 6H).
ꢁ
1
FTIR (KBr, cm ): 3285, 2920, 2848, 1500, 1465, 1384, 1273,
218, 1199, 1029, 864, 781, 670, 645. Anal. Calcd for C H O
4 54 2
4
polystyrene as the stand) Mn ¼ 2.20 ꢂ 10 g/mol; PDI ¼
1
3
1
2
.05. H NMR (400 MHz, CDCl , d): 7.6–7.8 (2H), 7.1–7.2
3
(
%): C, 82.59; H, 10.93; found: C, 82.85; H, 10.98.
(
2H), 6.85–7.05 (2H), 3.9–4.15(4H), 2.48–2.7(6H), 1.78–
ꢁ
1
PEG methyl tosylate (3)
Compound 3 was synthesized under argon atmosphere. PEG
monomethyl ether (13.2 g, 6.90 mmol) was dissolved in 15
2.0(4H), 1.1–1.48(36H), 0.7–0.95(6H). FTIR (KBr, cm ):
2920, 2849, 2360, 2190, 1655, 1639, 1551, 1495, 1446,
1410, 1379, 1273, 1210, 1094, 1025, 828, 718, 624, 562,
1
638
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4
3
71. Anal. Calcd for C H O N S (%): C, 75.59; H, 8.14; N,
48 62 2 2 2
.67; found: C, 72.93; H, 7.91; N, 3.36.
Poly(ethylene glycol)-b-oligo[p-(ethynylenephenyleneethy-
nylene)-alt-(thienylenepyridazinylenethienylene)]-b-poly
(
ethylene glycol) (PEG-OEPETPT-PEG)
A pre-dried Schlenk flask was charged with monomer 1
0.2139 g, 0.50 mmol), monomer 2 (0.3037 g, 0.61 mmol),
PEG-I (0.5165 g, 0.24 mmol), (PPh ) Pd (0.0481 g, 0.040
(
3
4
mmol), and CuI (0.0066 g, 0.035 mmol). The flask was
deoxygenated by several cycles of vacuum-argon cycling. 4
mL of dry diisopropylamine and 20 mL of dry THF were
quickly added into the flask under the protection of argon.
The final mixture was refluxed for 36 h under argon. After
cooling to room temperature, the reaction mixture was pre-
cipitated in 400 mL of methanol. The precipitate was centri-
fuged and redissolved in a small amount of THF. The poly-
mer was precipitated again into a large volume of methanol,
centrifuged and dried in vacuum for 24 h to afford a yellow
solid (0.561 g, yield 60%). GPC (in THF, with polystyrene as
1
3
FIGURE 1 H NMR spectrum of PEPETPT in CDCl .
unique properties, by combining the properties of the conju-
gated block, such as the charge transfer characteristic and
the strong fluorescence emission, with typical properties of
PEG, such as biocompatibility and good solubility in most
solvents.
3
1
the stand) Mn ¼ 7.1 ꢂ 10 g/mol; PDI ¼ 1.5. H NMR (400
MHz, CDCl , d): 7.6–7.8 (2H), 7.1–7.2 (2H), 6.85–7.05 (2H),
3
3
.9–4.15 (4H), 3.37–4.02 (179H), 2.48–2.7 (6H), 1.78–2.0
ꢁ
1
(
4H), 1.1–1.48 (36H), 0.7–0.95(6H). FTIR (KBr, cm ): 2923,
Before employing the strategy described in Scheme 2, several
other methods were explored when preparing such coil-rod-
2
1
Anal.
852, 2363, 2196, 1721, 1601, 1555, 1499, 1464, 1410,
386, 1276, 1210, 1115, 1026, 860, 836, 723, 564, 469.
32,43–45
coil polymers based on the literature reports.
End
Calcd
for
2 4 62 2 2 2 4 66 4
(C H O)44(C48H O N S ) (C48H O )
group modification strategy (Supporting Information Scheme
S1) was attempted. By using a slight excess of monomer 2,
the OEPETPT with terminal BCAH at both ends should be
synthesized. After purification, this polymer was supposed to
react with PEG-I under the Sonogashira coupling protocol to
yield the desired PEG-OEPETPT-PEG. However, the synthesis
was unsuccessful. An alternative method (Supporting Infor-
mation Scheme S2) was to modify the terminal BCAH with
(
8
C H O) (%): C, 65.46; H, 8.73; N, 1.47; found: C, 68.97; H,
.80; N, 1.91.
2
4
44
RESULTS AND DISCUSSION
Synthesis and Characterizations
The monomers and polymers were synthesized according to
the strategies in Scheme 2. Basically, compound 1 and com-
pound 2 were synthesized according to the published litera-
4-iodophenol right after the polymerization, directly in the
2
5,40,41
tures.
Compound 3 was prepared by reacting the com-
polymerization reaction mixture. After the purification, the
conjugated polymer with phenol end group was expected to
react with PEG-O-Ts (compound 3) through the nucleophilic
displacement of tosylate group to obtain the coil-rod-coil,
but this strategy did not work either. The merit for these
two strategies was that both rod and coil blocks can be well
characterized before being assembled together. The possible
reasons for these unsuccessful attempts could be the termi-
nal BCAH and phenol group did not survive through the pu-
rification process, or the poor solubility of the conjugated
polymer and the p-p induced aggregation in the solution
made end group modification extremely difficult.
mercial PEG monomethyl ether with toluene-p-sulfonyl
chloride, followed by a procedure similar to literature
4
2
description.
obtained by nucleophilic displacement of tosylate group with
-iodophenol. The structures of all these compounds were
Then, PEG-p-iodophenyl ether (PEG-I) was
4
confirmed by routine characterizations, as shown in the
Experimental section.
PEPETPT was synthesized by a conventional Pd-catalyzed
Sonogashira coupling polymerization, using (PPh ) Pd and
3
4
CuI as catalysts. The polymerization was performed in THF/
diisopropylamine mixed solvent under argon atmosphere.
The number average molecular weight of the resulting poly-
Thus, the one-step strategy in Scheme 2 was used in this
study. The merit of this strategy had been advanced in the
introduction. The coil-rod-coil polymer was successfully pre-
pared under the Palladium catalyzed Sonogashira coupling
protocol, by adding two bifunctional monomers (compound
4
mer was found to be 2.20 ꢂ 10 , with a polydispersity index
(
PDI) of 2.05. This molecular weight is a typical value for
conjugated polymers, and the PDI is very close to the ideal
molecular distribution in the polycondensation system. All
1
other characterizations, including H NMR (spectrum shown
1
and compound 2) and one monofunctional monomer
in Fig. 1), elemental analysis and FTIR further demonstrated
the successful synthesis of PEPETPT.
(
compound PEG-I) in a predetermined ratio. According to
the mechanism of condensation polymerization, the degree
of polymerization of the conjugated block can be tuned by
adjusting the ratio between the monomers or ratio between
As mentioned previously, introducing the PEG blocks to both
ends of the PEPETPT conjugated polymer may bring certain
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the two types of functional groups. Reducing the length of
conjugated block to a certain degree would not have great
influence on the photophysics, since the delocalized exciton
1
migration actually has limited distance. However, the solu-
bility can be improved, and the self-assembling or aggrega-
tion behavior in the solution will be affected. The molar ratio
of about 4:5:2 for monomer 1, monomer 2 and PEG-I was
used in this study, which was based on the comprehensive
46
consideration of the principle of polycondesation, the prin-
4
6
ciple of polymer reactivity, and the mechanism for palla-
4
7,48
dium catalysis.
Monomer 1 and monomer 2 are the typi-
cal bifunctional small molecular monomers for the
Sonogashira coupling, as in the synthesis of PEPETPT. PEG-I
is a monofunctional macromolecular monomer. Simply con-
sidering the Sonogashira coupling mechanism, the dibromo-
pyrene (monomer 1) and the iodoarene (PEG-I) should com-
pete for the coupling reaction with the bisalkyne. It is well
known that the reaction rate is affected by the chemical
structure of haloarene, such as the iodides react faster than
the bromides and the reactants with electron-withdrawing
groups react faster than those with electron-donating
1
3
FIGURE 2 H NMR spectrum of PEG-OEPETPT-PEG in CDCl .
1
ods is the end group analysis in H NMR spectrum. Figure 2
showed the spectrum of PEG-OEPETPT-PEG. Most proton
peaks from the conjugated block in the coil-rod-coil (Fig. 2)
were very similar to those for PEPETPT (Fig. 1), which had
been marked out in each figure. As expected, two additional
4
7
groups. In this study, the bromide has electron-withdraw-
ing group and the iodide has electron-donating group, which
makes the above two influencing factors cancel out each
other. However, from the viewpoint of conventional polymer
chemistry, it is also well known that the reaction rate of
functional groups in a macromolecule is much lower than
that of the corresponding low-molecular-weight analogs,
peaks, 3.64 ppm for methylene protons (CH ) and 3.37 ppm
for the methyl protons (CH ) in the end group apparently from
2
3
the PEG chain, appeared in Figure 2. Theoretically, the ratio of
area among different peaks is the same as the ratio of number
among the corresponding protons. In Figure 2, the integration
area of peak central around 2.55 ppm (marked as F, from the
methyl group on the thiophene ring) is about four times that of
the peak around 3.37 ppm (marked as P, from the methyl end
group). That means, each polymer chain with two methyl end
groups, has eight methyl groups on the thiopene, for average.
In addition, the ratios of other integration areas are also in pro-
portion with the corresponding number of protons in the coil-
rod-coil. The more detailed integration and calculation are
shown in Supporting Information as Figure S1 and related dis-
cussion. Ultimately, the NMR peak area analysis is consistent
with the proposed degree of polymerization according to the
feeding ratio.
4
6
such as small molecules or oligomers. Thus it can be envi-
sioned that PEG-I will be much less reactive than the mono-
mer 1 and the conjugated oligomers with the bromide end
group. Several factors could account for this slow rate. First,
the iodide end group will be embedded in the PEG coil most
of the time, which makes it less available for the reaction.
Second, the Pd catalytic process requires the reactants to fit
47,48
into one of the Pd empty orbital in a specific direction.
Consequently, having a macromolecular tail will make the
monomer more difficult to diffuse to where the catalyst is
and enter the catalytic circle. In addition, the bulky coiled
tail might also retard the monomer of another type to
approach the neighboring orbital in attempt to couple with
each other.
The GPC measurements have shown that PEG-OEPETPT-PEG
had a number average molecular weight of 7.10 ꢂ 10 with a
3
Thus, we can assume the PEG-I participates in the cross-cou-
pling reaction after the small molecular monomer analogs
has been consumed. The initial molar ratio between the
monomer dihaloarene and bisalkyne was about 4:5. That is,
the bifunctional monomer ratio (r) is about 0.8. In the case
of having no PEG-I in the system, the degree of polymeriza-
tion (Xn) will be 9, calculated by Xn ¼ (1 þ r)/(1 ꢁ r)
PDI of 1.5. Considering the PEG chain has a number average
molecular weight of 1900 with a PDI of 1.05, the molecular
weight of the middle rod segment can be obtained by subtract-
ing the molecular weight of two PEG segments from that of the
total polymer. The results are consistent with the situation
discussed above. That is, the degree of polymerization of the
conjugated block is about 9 (calculation shown in Supporting
Information). It is evident that the conjugated block in the coil-
rod-coil triblock polymer had been greatly shortened, compar-
ing with the PEPETPT. The PDI was also reduced, which was
consistent with the situation in the condensation polymeriza-
tion with nonstoichiometry between the two types of the
monomers. In addition, the elemental analysis results shown
in the experimental section are also consistent with the conju-
gated block having a degree of polymerization of 9.
46
according to the principle of condensation polymerization.
Thus, the average constituent of the conjugated oligomer
should be 4 monomeric units from dihaloarene and 5 mono-
meric units from bisalkyne, with terminal alkynes at both
ends, which will react with the PEG-I if extending the reac-
tion time.
Experimental determination of the conjugated block length
would be very useful but challenging. One of the useful meth-
1
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FIGURE 3 Normalized UV–vis absorption and emission spectra
of polymers in dilute THF solution at room temperature
(
excited at 440 nm).
The solubility of polymers was examined. Both of PEG-
OEPETPT-PEG and PEPETPT were soluble in many common
organic solvents, such as THF, DMF, DMSO, chloroform, and
1,4-dioxane, while PEG-OEPETPT-PEG turned out to be more
soluble compared with PEPETPT. THF was found to be the
best solvent for both polymers, among all the solvents
explored. PEG-OEPETPT-PEG can be easily redissolved in
THF with a concentration more than 5 mg/mL, while only
<
0.5 mg/mL can be achieved for PEPETPT. In addition, PEG-
FIGURE 4 The emission spectra of (a) PEPETPT and (b) PEG-
OEPETPT-PEG was slightly soluble in water, methanol, and
ethanol, while PEPETPT could not dissolve in these three sol-
vents. The improved solubility in most solvents for PEG-
OEPETPT-PEG can be mainly attributed to the introduction
of PEG coil block as well as the reduction of length of the
conjugated segment.
OEPETPT-PEG, respectively, in H
2
O/THF mixed solvent with dif-
ferent volume percentage of H O. The polymer concentrations
2
were fixed at 1 mM with respect to the repeating unit of the
conjugated backbone. The excitation was at 440 nm.
shoulder can be attributed to the existence of the vibrational
49,50
structure of the polymers.
Photophysical Properties
In both absorption and emission spectra, slight blue-shifting
was found for PEG-OEPETPT-PEG compared to PEPETPT,
which is consistent with the fact that PEG-OEPETPT-PEG has
a shorter conjugated segment. In addition, the tri-block co-
polymer was found to have a higher quantum yield of fluo-
rescence than PEPETPT. The self-quenching from the p-p
interaction among the conjugated blocks was greatly weak-
ened due to the protection of two PEG chains at the two
ends, and as a result an increase in emission was observed.
Thus, introduction of the PEG coil segments not only
increased the solubility, but also improved the photolumi-
nance properties.
The normalized UV–vis absorption and PL spectra of
PEPETPT and PEG-OEPETPT-PEG in dilute THF solution are
shown in Figure 3. The corresponding photophysical data is
shown in Table 1. The two polymers displayed very similar
spectral profiles, which is very typical for conjugated poly-
mers. The absorption maximum was around 442 nm for
PEPETPT and 439 nm for PEG-OEPETPT-PEG. The emission
peak was around 490 nm for PEPETPT and 478 nm for PEG-
OEPETPT-PEG. Both polymers have an emission shoulder,
which was about 35 nm away from the peak at a longer
wavelength. The absorption and emission should come from
the p-p* electronic transition along the conjugated polymer
backbone and the coexistence of the emission peak and
Fluorescent Emission and Aggregation Studies
TABLE 1 Photophysical Data for PEPETPT and PEG-OEPETPT-
Many applications of conjugated polymers require the pres-
ence of water, such as detection of pollutants directly from
the river or lake, bioimaging, biolabling, and biosensing.
However, most conjugated polymers are hydrophobic and
become aggregated, and more or less, quench the fluores-
cence, when introducing water into the system. THF was
selected as the organic solvent to dissolve the polymers
a
PEG
Polymers
k
max,abs (nm)
k
max,em (nm)
F
U (%)
PEPETPT
442
439
490
478
26
36
PEG-OEPETPT-PEG
a
Determined in dilute THF solution.
based on the fact that it was miscible with H O, and
2
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PEPETPT and PEG-OEPETPT-PEG exhibited strong fluores-
cence in THF, as described previously. The emission spectra
with the emission maximum around 517 nm. Slightly red-
shifting in the emission maximum (530 nm) with similar
emission intensity and broad spectra were also founded,
when 60–80% of water was present in the solvent. These
emission phenomena indicated that certain different emis-
sion species, such as the nonaggregated form, associated
forms with various degrees of aggregation, must coexisted in
the solution with 50–80% of water in the mixed solvent.
Increasing the water content to 90–99% led to an even
larger red-shift, with an emission maximum around 560 nm
and the intensity decreased, suggesting the tightly aggre-
gated form existed as the dominating emission species in the
solution and the self-quenching effect was very strong. How-
ever, the residual fluorescence was stronger compared to
of these polymers in H O/THF mixed solvent with various
2
compositions were recorded (Fig. 4). Generally, both poly-
mers exhibited a red-shifting in the wavelength of emission
peak and a decrease of emission intensity by increasing
water content in the mixed solvent. This phenomenon can be
attributed to the interactions or aggregation among the con-
jugated segments, which resulted in the enlargement in the
p system as well as the intermolecular fluorescent self-
quenching. However, the aggregation and self-quenching
behaviors of these polymers differed greatly from each other.
As shown in Figure 4(a), the emission of PEPETPT decreased
dramatically by increasing the percentage of H O in the
2
2
that of PEPETPT, when having 99% H O in the solvent.
mixed solvent. The intensity at the emission maximum was
quenched by 63% with the presence of 10% of H
2
O, com-
To evaluate further the fluorescence stability of the two poly-
mers toward water, small amounts of water were titrated
into 10 mL of polymer THF solutions. The emission spectra,
the relationship between the change in intensity and the
water volume are shown in Supporting Information Figure
S2. Generally, PEG-OEPETPT-PEG displayed a smooth decline
in emission intensity, with increasing the amount of H O
2
added directly to the polymer solution, whereas PEPETPT
showed a remarkable drop in the intensity.
pared with that in pure THF. The profile of spectrum
remained almost unchanged, at this point, with a peak
around 490 nm and a shoulder around 524 nm. When the
percentage of H O reached 20%, the emission intensity at
2
5
24 nm became almost as strong as the intensity at 490 nm.
Increasing the water content to 30%, the emission around
24 nm became stronger than that around 490 nm, and
accompanied with another emission peak emerged around
70 nm. Increasing the water percentage to 60% and further,
5
5
Absorption and Excitation Studies
the emission around 490 nm completely disappeared and
the emission around 570 nm became the dominating emis-
sion peak with the coexistence of the emission shoulder
around 524 nm. The residual fluorescence was very low
To investigate the origin of the fluorescence, the absorption
and excitation studies of polymers in certain typical mixed
solvent systems were also conducted. As shown in Support-
ing Information Figure S3, all the spectra for PEPETPT sys-
tems had water in the solvent, displaying an absorption pro-
file with two peaks (or one peak and one shoulder, around
with 99% H O in the solvent. The emission behavior of
2
PEPETPT in the mixed solvent is very typical for the com-
mon hydrophobic conjugated polymers. Addition of water
induced the aggregation among the conjugated polymer
backbones, and therefore the broadening and red-shifting of
emission spectra accompanied with self-quenching of the
fluorescence.
4
74 and 510 nm), which is significantly different from
PEPETPT in pure THF, with only one absorption peak around
42 nm. However, no significant change in the absorption
4
spectra (peaked around 439 nm) were found by adding
water into the system in the case of PEG-OEPETPT-PEG, with
the exception of a small shoulder emerging around 481 nm
in certain systems. The UV–vis absorption spectra further
confirmed that introduction of PEG to the ends of the conju-
gated block reduced the influence of water on the photophy-
sics of polymer solution.
PEG-OEPETPT-PEG showed a superior fluorescence stability
toward water, due to the presence of two PEG tails at both
ends of the conjugated block, which somewhat prevented
the interactions or aggregation among the OEPETPT seg-
ments. Initially, the polymer displays a strong emission with
the peak centered around 478 nm and a shoulder around
Excitation spectra were recorded to examine if different
emission peaks or shoulders come from the same electronic
transition for different emission species. The systems, with
solvent ratios between water and THF as 10:90, 30:70,
512 nm in THF dilute solution [Fig. 4(b)]. The polymer solu-
tion could tolerate a certain quantity of H O in the system
2
(
up to 20% of H O in the mixed solvent), without significant
2
changes in the emission wavelength and intensity, as well as
the profile of the spectrum. Interestingly, the introduction of
70:30, and 90:10, were selected for both polymers. For each
system, the emission wavelength was set at the emission
peak or shoulder wavelength in the steady-state fluorescence
spectra, as shown in Supporting Information Figures S4 and
S5, and excitation wavelength was scanned from 350 nm to
a wavelength that was 20 nm shorter than the emission
wavelength. Table 2 showed the excitation peaks or should-
ers existed in the polymer system with different solvent
composition.
1
0–20% water into the solvent resulted in a slightly
increased emission intensity at the peak by 7%, with respect
to the initial intensity in pure THF. This could possibly be
explained by the hydrophilic PEGs, which have a better solu-
bility in H O than THF. However, when the amount of H O
increased to 30%, the intensity decreased to 46% of the ini-
tial value. At this stage, loose aggregation might form but the
nonaggregated should be still the dominating emission
species. The spectra became broad and no fine vibronic
structure was found with 50% water in the mixed solvent
2
2
For PEPETPT, only the 10:90 system displayed a single exci-
tation peak around 437 nm for all emission, indicating that
1
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TABLE 2 The Wavelength (nm) of Excitation Peaks/Shoulders
wavelength, when the solvent was gradually changed from
in Different Polymer Systems
2
THF to H O, while the coil-rod-coil has enhanced fluorescence
stability toward its surroundings. This work demonstrated a
facile approach for synthesizing such coil-rod-coil copolymer,
which may applicable for other conjugated polymer system.
In addition, the unique combination the TPT-based conju-
gated segment with the PEG coils may result in some other
interesting properties and useful applications. The related
research is currently underway in our laboratory.
Solvent composition
(
2
H O:THF)
PEPETPT
PEG-OEPETPT-PEG
1
3
7
9
0:90
0:70
0:30
0:10
437
438
437, 475, 507
437, 475, 507
437, 475, 507
438
438
438, 480
ACKNOWLEDGMENT
This work was supported by the National Natural Science Foun-
dation of China (50773049, 20974075, and 21174099) and A
Priority Academic Program Development of Jiangsu Higher
Education Institution (PAPD).
only one electronic transition occurred in this system. How-
ever, for all other systems, there were three excitation
peaks/shoulders, which is consistent with the information
obtained previously that more than one emission species
existed in the solution. It is to be noted that sometimes
some ‘‘electronic communication’’ among different emission
species might exist. Thus, the excitation of a smaller p sys-
tem may result in the emission of a larger system, due to
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