Aggregation-mediated optical properties of pH-responsive anionic conjugated polyelectrolytes

Article abstract of DOI:10.1021/ja065061x

Conjugated polyelectrolyte copolymers containing 2,1,3-benzothiadiazole- (BT) and oligo(ethylene oxide)-substituted fluorene and phenylene units have been designed and synthesized. The phenylene pendent groups also have carboxylic acid functionalities, which allow probing the effect of pH on optical properties. The BT content in the backbone can be regulated at the synthesis stage. Dynamic light scattering studies show that polymers aggregate in water at low pH. Increased interchain contacts give rise to a lowering of the photoluminescence (PL) efficiency via self-quenching when the BT units are absent and increased levels of FRET from the phenylene-fluorene segments to BT. Furthermore, the PL efficiency of BT increases in the aggregated structures. Examination of solvent effects indicates that the increased BT efficiencies are likely due to decreased contact with water. The changes in PL efficiencies are reversible, showing that the aggregates are dynamic and not kinetically constrained.

Full text of DOI:10.1021/ja065061x

Published on Web 11/18/2006
Aggregation-Mediated Optical Properties of pH-Responsive
Anionic Conjugated Polyelectrolytes
Fuke Wang and Guillermo C. Bazan*
Department of Materials and Chemistry & Biochemistry, Institute for Polymers and Organic
Solids, UniVersity of California, Santa Barbara, California 93106
Received July 16, 2006; E-mail: bazan@chem.ucsb.edu
Abstract: Conjugated polyelectrolyte copolymers containing 2,1,3-benzothiadiazole- (BT) and oligo(ethylene
oxide)-substituted fluorene and phenylene units have been designed and synthesized. The phenylene
pendent groups also have carboxylic acid functionalities, which allow probing the effect of pH on optical
properties. The BT content in the backbone can be regulated at the synthesis stage. Dynamic light scattering
studies show that polymers aggregate in water at low pH. Increased interchain contacts give rise to a
lowering of the photoluminescence (PL) efficiency via self-quenching when the BT units are absent and
increased levels of FRET from the phenylene-fluorene segments to BT. Furthermore, the PL efficiency of
BT increases in the aggregated structures. Examination of solvent effects indicates that the increased BT
efficiencies are likely due to decreased contact with water. The changes in PL efficiencies are reversible,
showing that the aggregates are dynamic and not kinetically constrained.
Introduction
signaling fluorophores attached to biomolecular probes, thereby
providing signal intensities above those of single-molecule
The delocalized electronic structure of π-conjugated polymers
allows for effective coupling between optoelectronic segments.1
Properties such as charge transport,2 conductivity,3 emission
efficiency,4 and exciton migration2 are easily perturbed by
external agents, leading to large changes in measurable signals.5
Trace detection of analytes has been successfully accomplished
by making use of these amplification mechanisms.4,6,7 Conju-
gated polyelectrolytes (CPs) are conjugated polymers with
pendent functionalities capable of ionizing in high dielectric
media. Their solubility in water makes them appropriate for
interrogation of biological targets in aqueous media.4,7,8 These
water-soluble materials have been used as light-harvesting
molecules that deliver excitations upon recognition events to
reporters.5,9 Nonspecific contacts between nontarget species and
the hydrophobic CP backbone are complex and need special
attention for attaining selectivity as these interactions can easily
lead to misinterpretation of results.10
Differences in the efficiencies of intra- and interchain Fo¨rster
resonance energy transfer (FRET) have been used to design CPs
with optical signatures that are sensitive to DNA concentration.11
Interchain FRET is more effective as a result of more effective
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J. AM. CHEM. SOC. 2006, 128, 15786-15792
10.1021/ja065061x CCC: $33.50 © 2006 American Chemical Society

pH-Responsive Anionic Conjugated Polyelectrolytes
A R T I C L E S
a
Scheme 1. Preparation of P1-BTx
a Conditions: (i) 2 M Na2CO3, toluene, Pd(PPh3)4, reflux 48 h; (ii) TFA/CH2Cl2 (v/v ) 1/1); (iii) Na2CO3 (aq).
electronic coupling12 and the increased dimensionality of the
multichromophore system.13,14 For example, under dilute condi-
tions where multichain aggregation is negligible, emission of
poly((9,9-bis(6′-N,N,N,-timethylammoniumbromide)hexyl)-
fluorene-co-alt-1,4-phe nylene-co-alt-4,7-2,1,3,-benzothiadia-
zole (PFPB) containing 7% 2,1,3,-benzothiadiazole (BT) units
(PFPB7) occurs in the blue region of the spectrum (390-500
nm). Concentrated solutions emit with a green color character-
istic of the BT sites. The working hypothesis is that under these
increase of the local PFPB7 concentration, and a shift from the
blue-emitting fluorene-phenylene units to the green-emitting
BT is observed. It is possible to treat these spectral changes to
obtain linear calibration curves from which [DNA] can be
determined.11
The collective optical response of conjugated polymers can
thus be used to develop chemical and biological detection
technologies. Conversely, changes in spectral properties are also
useful for interrogating how the environment influences el-
ementary self-assembly processes.16 These organized structures
have clear implications on emerging technologies based on
organic semiconductors.17 In this contribution we disclose the
design and synthesis of structurally related water-soluble,
negatively charged CPs for which electrostatic interactions
typical of polyelectrolytes control FRET and fluorescence-
quenching processes. The structural design includes carboxylic
acid functionalities pendent to the delocalized backbone that
can be charged or neutral, depending on the solution pH. The
polymer charge was anticipated to control electrostatic repulsion
between chains and thus would lead to different levels of
aggregation and average interchain distances.18 We find the
unanticipated result that aggregation also controls the emission
efficiency of low-energy sites and propose that this is the result
of shielding from water within the aggregate structures.
conditions there is an increase of interchain contacts and
FRET from the fluorene-phenylene repeat units to the BT sites
is more efficient, relative to the situation where chains are more
isolated. Addition of a negatively charged polyelectrolyte (such
as DNA) induces interpolymer complexation,15 leading to an
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9
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A R T I C L E S
Wang and Bazan
Results and Discussion
Structural Design and Synthesis. Scheme 1 shows the
synthesis and structures of the polymers used in these studies.
Copolymerization of 1 with a stoichiometric quantity determined
by the sum of 2 and 4,7-dibromo-2,1,3-benzothiadiazole (3)
under Suzuki cross-coupling conditions using Pd(PPh3)4 and
Na2CO3 provides the neutral precursor polymers P1N-BTx in
which the BT content (x% relative to phenylene units) is
regulated by the ratio of 2 to 3. 2,7-Bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-9,9-di(1-(2-(2-methoxy-ethoxy)ethoxy-
)ethyl)fluorene (1) was obtained by alkylation of 2,7-dibromof-
luorene with 1-(2-(2-methoxy-ethoxy)ethoxy)-2-bromoethane,
followed by esterification with 2-isopropoxy-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane.19,20 Compound 2 was synthesized from 1,4-
dibromohydroquinone by modification of published proce-
dures.21 The P1N-BTx polymers are soluble in organic solvents
such as chloroform, toluene, and THF and can be purified by
repeated precipitation from toluene solutions into methanol.
Characterization by NMR spectroscopy and elemental analysis
is consistent with the proposed structures. Molecular weight
determination by GPC analysis against polystyrene standards
shows number-average molecular weights (Mn) for the neutral
precursor polymers P1N-BT0 (x ) 0, i.e., no BT units) and P1N-
BT15 (15% BT units) of 56K (PDI ) 1.7) and 26K (PDI )
1.6), respectively. In general, as the BT content increases, one
observes a decrease in average polymer molecular weight.
Hydrolysis of the ester groups occurs in a CH2Cl2 and
trifluoroacetic acid (TFA) mixture (v/v ) 1/1) at room tem-
perature. After solvent evaporation, the residue is treated with
saturated aqueous Na2CO3. The target materials are then purified
by dialysis against deionized (DI) water using 1 KD molecular
weight cutoff dialysis membranes. Final isolation requires
removal of water and drying in a vacuum oven at 50 °C for at
Figure 1. Effective diameter (ED) determined by dynamic light scattering
of P1-BT0 and P1-BT15 ([RU] ) 3.8 × 10-5 M) in water as a function of
pH.
Figure 2. Absorption spectra of P1-BT0 ([RU] ) 3.8 × 10-5 M) in water
as the pH is incrementally decreased from 7 to 1.
degree of aggregation by dynamic light scattering (DLS)
techniques.23
DLS results in Figure 1 show the effect of pH on the effective
diameter (ED) measured from solutions of P1-BT0 and P1-BT15
in water ([RU] ) 3.8 × 10-5 M, RU ) polymer repeat unit).
At pH > 5 the EDs for the two polymers are in the range of
300-400 nm. As the pH decreases, both polymers show a
sudden increase in particle size at pH ≈ 3.5. At pH ) 1, P1-
BT15 is slightly larger (3000 nm) than P1-BT0 (2700 nm). These
data are in agreement with substantial aggregation of chains
occurring upon protonation of pendent groups.
Absorption and Photoluminescence Spectra of P1-BT0 and
P1-BT15: Effect of pH. Figure 2 shows the absorption spectra
of P1-BT0 in water as a function of pH. For these measurements,
the pH of a 20 mL solution ([RU] ) 3.8 × 10-5 M) was adjusted
with HCl and/or NaOH using a pH meter. Once a stable reading
was obtained, 2 mL of the solution was transferred to a cuvette
for optical measurements. As shown in Figure 2, the absorption
spectra of P1-BT0 are dominated by the π-π* transition. The
absorption band becomes broader and the absorption maximum
(λmax) red shifts from 370 to 382 nm as the medium becomes
more acidic and chains aggregate. For comparison, films of P1-
BT0, where interchain contacts are maximized, show a λmax of
379 nm. A similar red shift in absorption with aggregation has
been observed for other conjugated polyelectrolytes.18,24 Integra-
tion of the absorption bands in Figure 2 allows for a relative
comparison of the oscillator strength; only a 5% decrease is
observed as the pH decreases from 7 to 1.25 Thus, neither
precipitation nor adsorption of P1-BT0 to the cuvette walls is a
significant process.
1
least 24 h. H NMR spectroscopy in CD3OD of the product
shows no signals in the 1.4-1.5 ppm range, indicating greater
than 95% removal of tert-butyl groups.
P1-BT0 and P1-BT15 are soluble in methanol or water. In
DI water, polymer concentrations of up to 2 mg/mL can be
obtained in the pH range from 5 to 12. Aqueous solutions of
P1-BT0 are visibly viscous under these conditions. Similar to
previously reported nonionic water-soluble conjugated polymers
with ethylene oxide side chains,22 the neutral form of both
polymers (i.e., at pH < 3) is also soluble in water, with
concentrations as high as 0.05 mg/mL.
pH-Dependent Aggregation of P1-BT0 and P1-BT15. One
of the goals of this study is to probe how the pH of the solution
influences the optical properties of P1-BT0 and P1-BT15. We
anticipated that protonation of the carboxylic groups at low pH
would render the polymers charge neutral and lead to a decrease
in interchain electrostatic repulsion. These conditions would
encourage interchain aggregation18 and thereby perturb elemen-
tary photophysical processes, such as FRET and fluorescence
quenching. In anticipation of these changes, we probed the
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pH-Responsive Anionic Conjugated Polyelectrolytes
A R T I C L E S
Figure 3. Photoluminescence (PL) spectra of P1-BT0 ([RU] ) 3.8 × 10-5
M) in water as a function of pH (λexc ) 370 nm).
Figure 5. (a) PL spectra of P1-BT15 ([RU] ) 4.3 × 10-5 M) in water as
a function of pH (λexc ) 370 nm). (b) PL spectra of P1-BT15 ([RU] ) 4.3
× 10-5 M) at pH 7 and 3 (λexc ) 440 nm).
Figure 4. Absorption spectra of P1-BT15 ([RU] ) 4.3 × 10-5 M) in water
as the pH is incrementally decreased from 7 to 1. Note that the spectra at
pH 7.0 and 5.0 overlap each other perfectly.
The photoluminescence (PL) spectra of P1-BT0 are shown
in Figure 3. There is little change in the intensity or shape of
the emission band as the pH decreases from 7 to 4. A reduction
of 64% in the emission intensity and a slight red shift of the
emission maximum (λPL) from 411 to 416 nm are observed when
the pH is lowered from 4 to 3. The PL characteristics then
remain essentially unchanged down to pH ) 1. The data are
consistent with self-quenching upon aggregation at low pH and
a red shift in emission as a result of additional interchain
delocalization.26
Changes induced by the pH on the absorption spectra of P1-
BT15 are shown in Figure 4. The absorption of P1-BT15 displays
two bands at 364 and 440 nm, which are assigned to the
fluorene-phenylene segments and the BT units, respectively.27
It is interesting to note that relative to P1-BT0, the absorption
red shift with decreasing pH appears to be less pronounced,
although this shift is difficult to quantify because of the overlap
between the two transitions. That the absorption spectra of P1-
BT15 in Figure 4 are not perturbed by the degree of aggregation
suggests that there are no measurable changes in the degree of
electronic delocalization in single chains.
Figure 6. PL response of P1-BT0 ([RU] ) 3.8 × 10-5 M) and P1-BT15
([RU] ) 4.3 × 10-5 M) as a function of pH. Data were normalized relative
to the maximum PL intensity of each polymer.
place, yields more intense emission at pH 3 than at pH 7 (Figure
5b). Examination of the excitation spectra by monitoring at 556
nm (Supporting Information) shows that a larger fraction of
initial excitations located at fluorene-phenylene sites is capable
of sensitizing BT emission at pH ) 3 than at pH ) 7. Thus,
aggregation of P1-BT15 chains results in more effective FRET
from the fluorene-phenylene segments to the BT units in an
increase in the PL quantum yield of BT.
The pKa of the pendent carboxylic functionalities can be
estimated by looking at 3-[2-(2-(2-hydroxy-ethyoxy)-ethoxy)-
ethoxy]propionic acid (HEPA), an analogue of the polymer side
chain. The pKa of HEPA was determined to be 3.7 ( 0.1 by
potentiometric titration with 0.1 N NaOH aliquots under
nitrogen. Thus, at pH > 3.7, the majority of the pendent groups
in P1-BT0 and P1-BT15 are expected to be deprotonated,
yielding the negatively charged chain. Figure 6 shows that the
plots of PL emission intensity for P1-BT0 (λem ) 411 nm) and
P1-BT15 (λem ) 556 nm) as a function of pH show a sharp
transition in the pH range from 3 to 4. There is an excellent
correlation between these data and those in Figure 1, where
interchain aggregation occurs within a similar pH range. It is
also informative to note that assuming that the inflection point
in Figure 1 occurs at the pKa of the polymer carboxylic groups,
The PL spectra of P1-BT15, with two prominent bands at
410 and 556 nm, are nearly indistinguishable in the pH range
from 7 to 4 (Figure 5a). However, decreasing the pH from 4 to
3 results in the disappearance of the high-energy band and in a
7-fold increase of the BT emission intensity at 556 nm. Little
change is observed when the pH further decreases to 1.
Excitation at 440 nm, where preferential absorption by BT takes
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A R T I C L E S
Wang and Bazan
Figure 7. PL intensities at 560 nm of P1-BT15 ([RU] ) 4.3 × 10-5 M)
upon cycling the pH between 3 and 10.
most of the changes occur within a single pH unit. From eq 1,
we see that the aggregation onset begins when less than
approximately 75% of the carboxylic groups are deprotonated.
Figure 8. PL spectra of P1N-BT15 ([RU] ) 4.0 × 10-5M) in THF with
water content from 0% to 10% in 1% increments: (a) λexc ) 370 nm and
(b) λexc ) 440 nm.
[HA]
pH - pKa ) -log
(1)
-
[
]
[A ]
Changes in the PL spectra as a function of pH are reversible.
Figure 7 shows that it is possible to modulate the PL intensity
of P1-BT15 ([RU] ) 4.3 × 10-5 M) by cycling the pH between
3 and 10. These data indicate that “activating” and “neutralizing”
the polymer charge by changing the pH results in reversible
aggregation. Furthermore, the time interval between the pH
change and PL measurements was on the order of less than 1
min. Thus, aggregate breakup is not kinetically constrained.
Aggregation and Optical Properties of Neutral Precursor
Polymers. To summarize the studies on charged polymers in
the preceding section, we find that PL self-quenching of P1-
BT0 and the more efficient FRET in the case of P1-BT15 at pH
< 4 are well accommodated within the framework provided by
more intimate interchain contacts within supramolecular ag-
gregates. A remaining issue is the lower quantum efficiency
for BT under basic conditions. To probe the possibility that the
increased efficiency is due to the more hydrophobic environment
in the aggregated structures we examined the optical properties
of the precursor polymers in mixed solvents. The changes in
the PL spectra of P1N-BT15 in THF upon addition of water are
shown in Figure 8. In contrast to P1-BT15 in basic aqueous
media, excitation at 370 or 440 nm in pure THF results in
emission that is almost completely BT-based. Thus, excited state
initially generated in the fluorene-phenylene segments decay
thermally or are transferred to BT sites by FRET. Surprisingly,
DLS shows that the ED in these solutions ([RU] ) 4.0 × 10-5
M) is ∼2500 nm, much larger than that observed for P1-BT15
at pH ) 7. Extensive aggregation of polymer chains in THF
therefore occurs at this concentration and is probably driven
by stacking of the polarizable π-delocalized backbone. The PL
intensity decreases 50% as the water content of in the THF
solution increases from 0% to 10%. DLS shows that with 10%
water content the effective diameter does not change substan-
tially (∼2400 nm) relative to pure THF. This information shows
that there is essentially no difference in the particle size as water
is included in the solvent, although changes within the internal
structure or the aspect ratio of these aggregates cannot be ruled
out. Altogether, these data suggest that the emission efficiency
of BT decreases as the water concentration increases, consistent
with a charge-transfer excited state that increases its rate of
Figure 9. PL spectra (λexc ) 370 nm) change of neutral precursor polymer
P1N-BT0 ([RU] ) 4.0 × 10-5 M) in THF with water content from 0% to
10% in 1% increments. The spectra have nearly perfect overlap with each
other.
nonradiative deactivation in more polar solvents.28 The charge-
transfer character is also confirmed by the red shift of the PL
maximum with increasing water content from 545 nm in pure
THF to 554 nm in THF with 10% water. Additionally, from
these data we infer that in solutions of P1-BT15 at pH < 4,
aggregation introduces the BT units within the interior of the
aggregates where water contact is not as pronounced, thereby
increasing the emission efficiency.29
A similar set of experiments to those illustrated in Figure 8
was done using P1N-BT0 instead of P1N-BT15. As shown in
Figure 9, there is no change in the PL spectra of P1N-BT0 in
THF upon addition of water up to a total concentration of 10%
by volume. DLS measurements show that the aggregate size of
P1N-BT0 in THF (EF ) 630 ( 30 nm) is not as large as that of
P1N-BT15 probably as a result of the higher fraction of ether
linkages along the polymer chain and the ensuing better
miscibility with the solvent.
Conclusions
Scheme 1 provides a method of synthesizing anionic CPs
bearing carboxylate functionalities in good yield using estab-
lished polymerization methods. The modular introduction of
(28) (a) Henry, B. R.; Morrison, J. D. J. Mol. Spectrosc. 1975, 55, 311. (b) Lin,
T.-S.; Braun, J. R. Chem. Phys. 1978, 28, 379. (c) Lim, E. C. J. Phys.
Chem. 1986, 90, 6770.
(29) Woo, H. Y.; Korystov, D.; Mikhailovsky, A.; Nguyen, T. Q.; Bazan, G.
C. J. Am. Chem. Soc. 2005, 127, 13794.
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pH-Responsive Anionic Conjugated Polyelectrolytes
A R T I C L E S
fluorene units with either BT or substituted phenylene co-
monomers provides control over the content of lower energy
sites along the polymer chain. The polymerization reaction yields
precursor polymers P1N-BTx that are soluble in organic solvents
and straightforward to characterize. Treatment of P1N-BTx with
acid, followed by deprotonation with base, yields P1-BTx
polymers, which are negatively charged in basic media.
DLS results provide concrete evidence that changing the pH
of P1-BT0 and P1-BT15 in water controls the degree of
aggregation of these polymers. Large aggregates are observed
when the pH is lower than ∼3.7, the pKa of the model compound
HEPA. Under these conditions it is appropriate to consider these
systems as colloidal suspensions. Increasing the pH results in
aggregate breakup, most likely as a result of electrostatic
repulsion between the negatively charge polymer chains;
however, the resulting EDs are considerably larger than what
would be expected on the basis of estimates for the molecular
size of independent polymer chains (30-60 nm). Discrepancies
of this type may arise from nonspherical aggregate and backbone
shapes and the assumptions made in modeling the DLS data.
From Figure 1 we observe that the general aggregation proper-
ties of P1-BT15 and P1-BT0 are nearly identical, consistent with
the similarities in polymer structure ones and identical pendent
functionalities.
Similarities in the absorption coefficients of P1-BT0 or P1-
BT15 as a function of pH indicate that there is little or no
precipitation, despite the large aggregate sizes. Comparison
against the much lower solubility of related alkyl-containing
or unsubstituted cationic fluorene-phenylene CPs highlights the
importance of the oligo(ethylene oxide) groups in improving
water solubility.19 Figure 4 shows that in the case of P1-BT15
the absorption spectra are not perturbed by the state of
aggregation, suggesting that there are no measurable changes
in the degree of electronic delocalization in individual chains.
That aggregation induces PL self-quenching in P1-BT0 and
improved FRET to the BT units in P1-BT15 could perhaps have
been predicted on the basis of enhanced interchain contacts, as
shown by previous studies.18,30,31 An important new observation
is that the PL efficiency of the BT units in P1-BT15 solutions
at low pH is considerably higher relative to conditions at high
pH. Water induces quenching in the BT emission of P1N-BT15
in THF. Since a similar quenching is not observed for P1N-
BT0 in THF, we conclude that this quenching is not due to
interchain contacts. On the basis of these data we propose that
aggregation of P1-BT15 at low pH reduces BT/water contacts
and decreases energy-wasting nonradiative relaxation processes.
From the pH dependence of PL in Figure 6 we see that the
responses of P1-BT0 and P1-BT15 occur over a narrow pH
range, even though the mechanisms by which the emissions are
modified are significantly different (PL self-quenching for P1-
BT0 vs FRET and PL emission enhancement for P1-BT15).
Changes in PL appear to be sharper than the aggregation
dependence on pH and suggest a higher level of cooperative
response by the optical units than the aggregation of chains. It
is also interesting that the inflection points in Figure 6 occur at
a lower pH (∼3.3) than the pKa of HEPA. Such an effect would
be expected when electrostatic repulsion between chains and
aggregate breakup take place when a fraction of pendent groups
are ionized.
In summary, we obtained new insight into how charge-
mediated aggregation of conjugated polyelectrolytes influences
the optical performance of the constituent units. Of particular
significance is that the optical output can be increased within
aggregates when the emitters are shielded from water contacts.
The response of P1-BT15 has been shown to be reversible and,
in contrast to standard single-molecule pH indicators, is a result
of electrostatic interactions typical of polyelectrolytes in com-
bination with the optical properties of conjugated polymers.
Experimental Section
General Details. 1H and 13C NMR spectra were collected on a Varian
ASM-100 400 MHz spectrometer. UV-vis absorption spectra were
recorded on a Shimadzu UV-2401 PC diode array spectrometer.
Fluorescence was measured using a PT1 (Lawrenceville, NJ) Quantum
Master fluorometer equipped with a Xenon lamp excitation source and
a Hamamatsu (Japan) 928 PMT using 90° angle detection for solution
samples. Mass spectrometry and elemental analysis were performed
by the UC Santa Barbara Mass Spectrometry Lab and elemental analysis
center. Reagents and chemicals were obtained from Aldrich Co. and
used as received. Monomers 221 and 332 were synthesized according to
reported procedures. Potentiometric titrations were performed with a
glass/reference electrode, calibrated with buffer solutions of pH 4, 8,
and 10. Titrations were performed using 0.1 N NaOH solution and a
microburet to 10 mL polymer solutions under nitrogen at room
temperature. Dynamic light scattering (DLS) was recorded using 10
mW HeNe laser (wavelength 633 nm) with photodiode detector BI-
APD (Brookhaven Instruments Co.).
2,7-Dibromo-9,9-di(1-(2-(2-methoxy ethoxy)ethoxy)ethyl)fluo-
rene. 1-(2-(2-Methoxy ethoxy)ethoxy)-2-bromoethane (55 g, 244 mmol)
was added to a mixture of 2,7-dibromofluorene (7.9 g, 24.4 mol) and
tetrabutylammonium bromide (TBAB) (0.78 g, 2.4 mmol) in 150 mL
of aqueous KOH (45%) and heated at 60 °C for 4 h. After the reaction
mixture was cooled to ambient temperature, the resulting solution was
extracted with CH2Cl2 (3 × 100 mL). The combined organic fractions
were washed with water, dilute HCl, and brine and dried over MgSO4.
The crude oily product was purified by chromatography on silica gel
using a mixture of ethyl acetate and n-hexane (2/5) as the eluent to
give 11.4 g (76% yield) of the product as a white solid: 1H NMR (400
MHz, CDCl3) δ (ppm): 7.53 (dd, 2H, J ) 3.6 Hz, J ) 1.6 z), 7.50 (s,
1H), 7.47 (dd, 2H, J ) 8.0 Hz, J ) 1.6 z), 3.53 (m, 4H), 3.49 (m, 4H),
3.39 (m, 4H), 3.35 (s, 6H), 3.21 (m, 4H), 2.76 (t, 4H, J ) 7.6 Hz),
2.34 (t, 4H, J ) 7.6 Hz). 13C NMR (100 MHz, CDCl3) δ (ppm): 151.0,
138.6, 130.8, 126.8, 121.8 121.4, 72.0, 70.6, 70.6, 70.2, 66.9, 59.2,
51.9, 39.6. MS (EI-MS) 616 (M). HRMS Calcd for C27H36Br2O6:
614.0879. Found: 614.0856.
2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-di(1-(2-
(2-methoxy ethoxy)ethoxy)ethyl)fluorene (1). To a stirred solution
of 2,7-dibromo-9,9-di(1-(2-(2-methoxy ethoxy)ethoxy)ethyl)fluorene
(5.6 g, 9.1 mmol) in anhydrous THF (80 mL) at -78 °C was added
dropwise n-butyllithium in hexane (7.6 mL, 2.5 M, 19 mmol) at -78
°C. The mixture was stirred for another 30 min, and 2-isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (4.0 g, 21.5 mmol) was added
rapidly at -78 °C. The reaction mixture was warmed to room
temperature and stirred for another 24 h. The reaction mixture was
poured into water and extracted with diethyl ether. The combined
organic layers were washed with brine and dried over magnesium
sulfate. The solvent was removed under reduced pressure, and the crude
produce was purified by chromatography with 2/5 ethyl acetate and
n-hexane to give 3.2 g of the product as colorless crystals (45% yield).
1H NMR (400 MHz, CDCl3) δ (ppm): 7.84 (s, 2H) 7.75 (dd, 4H, J )
7.6 Hz), 3.51 (m, 4H), 3.47 (m, 4H), 3.39 (m, 4H), 3.33 (s, 6H), 3.18
(30) (a) Fan, Q. L.; Zhou, Y.; Lu, X.-M.; Hous, Z.-Y.; Huang, W. Macromol-
ecules 2005, 38, 2927. (b) Kim, B.-S.; Chen, L.; Gong, J.; Osada, Y.
Macromolecules 1999, 32, 3964. (c) Kim, B.-S.; Fukuoka, H.; Gong, J. P.;
Osada, Y. Eur. Polym. J. 2001, 37, 2499.
(31) Nguyen, T.-Q.; Schwartz, B. J. J. Chem. Phys. 2002, 116, 8198.
9
J. AM. CHEM. SOC. VOL. 128, NO. 49, 2006 15791

A R T I C L E S
Wang and Bazan
(m, 4H), 2.65 (t, 4H, J ) 7.6 Hz), 2.44 (t, 4H, J ) 7.6 Hz), 1.39 (s,
12H). 13C NMR (100 MHz, CDCl3) δ (ppm): 148.9, 143.6, 134.5,
129.7, 120.0, 84.4, 72.3, 70.9, 70.9, 70.4, 67.4, 59.5, 51.4, 40.0, 25.4.
MS (EI-MS) 710 (M). HRMS Calcd for C39H60B2O10: 708.4445.
Found: 708.4467.
1.4 (18H). 13C NMR (100 MHz, CDCl3) δ (ppm): 168.5, 148.1, 146.5,
136.8, 134.7, 128.7, 126.5, 121.9, 117.1, 114.4, 78.1, 69.4, 68.3, 68.2,
68.1, 68.1, 68.0, 67.9, 67.6, 67.4, 66.8, 64.9, 64.5, 56.6, 48.8, 33.8,
25.7. Anal. Calcd for C59H88O18: C, 65.31; H, 8.11. Found: C, 64.86;
H 7.98.
General Polymerization Procedures. Polymerization reactions were
carried out under palladium-catalyzed Suzuki cross-coupling condi-
tions.32 Carefully purified 1, 2, and 3 were mixed in the appropriate
ratios with (Ph3P)4Pd (1 mol %) in a mixture of 2 M Na2CO3 (aq) and
toluene (1/1 v/v) under argon. After degassing, the reaction mixture
was refluxed with vigorous stirring for 24 h. After cooling to room
temperature, the organic layer was poured into methanol. The precipitate
was separated by filtration and washed with methanol and acetone.
Purification was accomplished by precipitation of toluene solutions into
methanol. The neutral polymers were obtained by filtration and drying
under vacuum. The obtained organic-soluble polymers are then
dissolved in CH2Cl2, and an equal amount of trifluoroacetic acid was
added. The mixture was then allowed to stir under argon at room
temperature overnight. The solvent was removed under reduced
pressured, and the residues were treated with saturated Na2CO3 (aq)
solution for 4 h at room temperature. The reaction mixture was dialyzed
in DI water to remove the salts. Solvent was removed, and the obtained
solids were dried under reduced pressure overnight. The resulting
polymers were soluble in water and methanol.
P1-BT0. 1H NMR (400 MHz, CD3OD) δ (ppm): 7.9 (4H), 7.7 (2H),
7.2 (2H), 4.2 (4H), 3.8 (4H), 3.7-3.6 (24H), 3.5-3.4 (12H), 3.3 (14H),
2.4 (4H).
P1N-BT15. Mn ) 25 702; Mw ) 41 809; PDI ) 1.62. Yield: 45%.
1H NMR (400 MHz, CDCl3) δ (ppm): 8.1 (1H), 7.9-7.7 (6H), 7.2
(1H), 4.2 (2H), 3.8 (4H), 3.7-3.6 (28H), 3.5-3.4 (16H), 3.3 (6H), 2.5
(4H), 1.4 (18H). 13C NMR (100 MHz, CDCl3) δ (ppm): 168.5, 151.7,
148.1, 146.5, 135.2, 134.7, 130.8, 128.7, 126.5, 125.7, 121.9, 117.1,
114.4, 78.1, 69.5, 68.3 68.3, 68.1, 68.1, 68.0, 67.9, 67.6, 67.4, 64.5,
49.0, 33.8, 25.7. Anal. Calcd for C55.1H80.5N0.3O16.2S0.15: C, 65.46; H,
7.97. Found: C, 65.05; H 7.71.
P1-BT15. 1H NMR (400 MHz, CD3OD) δ (ppm): 8.4 (0.5H), 8.0-
7.7 (5H), 7.3 (1H), 4.3 (2H), 3.9 (4H), 3.8-3.6 (28H), 3.5-3.4 (16H),
3.3 (10H), 2.5 (4H).
Acknowledgment. This work was supported by the NIH (GM
62958-01) and NSF (DMR-0606414).
Supporting Information Available: 1H NMR spectra of
polymers P1N-BT0, P1-BT0, P1N-BT15, and P1-BT15, titration
curve for HEPA, UV-vis spectrum of P1-BT0 film, and
excitation spectra of P1-BT15. This material is available free
of charge via the Internet at http://pubs.acs.org.
P1N-BT0. Mn ) 56 054; Mw ) 97 543, PDI ) 1.74. Yield: 57%.
1H NMR (400 MHz, CDCl3) δ (ppm): 7.7 (4H), 7.6 (2H), 7.1 (2H),
4.2 (4H), 3.8 (4H), 3.7-3.6 (28H), 3.5-3.4 (16H), 3.3 (6H), 2.5 (4H),
(32) (a) Kitamura, C.; Tanaka, S.; Yamashita, Y. Chem. Mater. 1996, 8, 570.
(b) Hou, Q.; Zhou, Q.; Zhang, Y.; Yang, W.; Yang, R.; Cao, Y.
Macromolecules 2004, 37, 6299. (c) Huang, J.; Niu, Y.; Yang, W.; Mo,
W.; Yuan, M.; Cao, Y. Macromolecules 2002, 35, 6080.
JA065061X
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15792 J. AM. CHEM. SOC. VOL. 128, NO. 49, 2006