Enhanced conjugated polymer fluorescence quenching by dipyridinium-based quenchers in the presence of surfactant

Article abstract of DOI:10.1021/jp0455071

Poly[(2-methoxy-5-propyloxysulfonate)phenylene vinylene] (MPS-PPV) was synthesized directly from its bischloromethylated monomer, considerably reducing the total number of steps involved in the polymer preparation. For the first time, a simple technique o

Full text of DOI:10.1021/jp0455071

J. Phys. Chem. B 2005, 109, 3873-3878
3873
Enhanced Conjugated Polymer Fluorescence Quenching by Dipyridinium-Based Quenchers
in the Presence of Surfactant
Jyoti Dalvi-Malhotra*,† and Liaohai Chen‡
Customization DiVision, Brewer Science, Inc., Rolla, Missouri 65401, and Biosciences DiVision,
Argonne National Laboratory, Argonne, Illinois 60439
ReceiVed: October 3, 2004; In Final Form: January 5, 2005
Poly[(2-methoxy-5-propyloxysulfonate)phenylene vinylene] (MPS-PPV) was synthesized directly from its
bischloromethylated monomer, considerably reducing the total number of steps involved in the polymer
preparation. For the first time, a simple technique of ultracentrifugation was employed for final purification
of the polymer. The interactions among the polymer, surfactant, and quencher molecules, as well as amplified
fluorescence quenching and fluorescence enhancement associated with the interactions, were investigated
and discussed. When compared with methyl viologen [MV]2+, higher values of Stern-Volmer constant KSV
values on the order of g107 M-1 were observed for the newly synthesized N-(2-carboxyhexadecanoyl)-N′-
methyl-4,4′-bipyridinium iodide bromide ([CHMB]2+) quencher in the presence of 1,2-dioleoyl-3- trimethyl-
ammonium propane (DOTAP) surfactant. Comparisons of surfactants demonstrated that the KSV of [CHMB]2+
was 10-fold higher in the presence of dodecyltrimethylammonium bromide (DTAB) surfactant than with
DOTAP. Polymer fluorescence was totally recovered upon addition of DOTAP surfactant to a MV-quenched
polymer system, whereas only 50% of fluorescence was recovered upon addition of DOTAP surfactant to the
CHMB-quenched polymer solution. In contrast, no fluorescence was recovered when DTAB was added to
either the MV- or CHMB-quenched polymer systems. Thus, fluorescence enhancement was observed for the
polymer complex with DOTAP, whereas fluorescence quenching was predominant in the polymer complex
with DTAB. Such studies will not only help to better understand the intrinsic properties of the ionic conjugated
polymer and amplified fluorescence quenching and enhancement but also provide guidelines to develop the
next generation of ionic conjugated-polymer-based biosensors.
Introduction
electrons and holes. The fluorescence of a conjugated polymer
comes from the radiative decay that results from recombination
of the electrons and holes. Due to the conjugated nature of the
polymer, the electrons can migrate along the polymer chains.
Consequently, one quenching site can affect multiple excitation
sites of the polymer, which leads to amplified fluorescence
quenching.
Fluorescence quenching involves deactivation of an excited
molecule at an excited singlet state by long- or short-range
interaction of the molecule with a quencher molecule.1 The
quenching efficiency is determined by a variety of factors
including, for example, orientational and interactive (electron
transfer, dipole-dipole, etc.) parameters. A quantitative measure
of the fluorescence quenching can be achieved by determining
Recently, Whitten and co-workers3, Heeger and co-workers,4
and several other groups5 including our own6 extended amplified
fluorescence quenching to ionic conjugated polymers. By using
oppositely charged quencher molecules, a Stern-Volmer con-
stant (KSV) as high as 108-1010 M-1 for quenching the
fluorescence of an ionic conjugated polymer can be achieved.
Virtually one quencher molecule was able to quench the
fluorescence of an entire polymer chain.6b Although the mech-
anism remains under investigation, such an aggressively ampli-
fied fluorescence quenching (AAFQ) is believed to arise due
to (i) quench-induced conformational change of the ionic
conjugated polymer and (ii) extremely rapid exciton diffusion
along the conjugated polymer chain to the quencher “trap site”.
Removing the quencher molecule from a quenched ionic
conjugated polymer chain will result in resuming the fluores-
cence of the entire polymer chain. Such an amplification process
has led to the development of a novel biosensor platform. Such
sensors have the potential to detect targeted biomolecules (such
as proteins, viruses, bacteria, spores, cells, microorganisms,
antibodies, antibody fragments, nucleic acids and toxins) at
subpicomolar range.7
the Stern-Volmer constant, KSV
:
I0/I ) 1 + KSV[quencher]
where I0 is the intensity of fluorescence in the absence of the
quencher and I is the intensity of fluorescence in the presence
of the quencher. The equation reveals that I0/I increases in direct
proportion to the concentration of the quenching moiety, and
the constant KSV defines the efficiency of quenching. When all
other variables are held constant, the higher the KSV, the lower
the concentration of quencher required to quench the fluores-
cence.
Amplified fluorescence quenching is a phenomenon associ-
ated with conjugated polymers whose mechanism was first
investigated by Zhou and Swager.2 Upon photoexcitation of a
conjugated polymer, the electrons from the valence band are
excited to the conducting band, thus generating charge-separated
* Corresponding author: e-mail jmalhotra@brewerscience.com.
† Brewer Science, Inc.
‡ Argonne National Laboratory.
10.1021/jp0455071 CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/09/2005

3874 J. Phys. Chem. B, Vol. 109, No. 9, 2005
Dalvi-Malhotra and Chen
SCHEME 1: Synthesis of Polymer from the
Bischloromethylated Monomer
Figure 1. Fluorescence intensity of the polymer [MPS-PPV] ) 10-5
M (in monomer repeat units) (excited at 450 nm) as a function of
DOTAP concentration [2 µM, 4 µM, 6 µM, 8 µM, and 1 × 10-5 M].
Inset: Emission spectra of MPS-PPV in water and as its DOTAP
complex.
SCHEME 2: Synthesis of Dipyridine-Based Quencher
Molecule CHMB 7
On the other hand, it also has been demonstrated that the
fluorescence of ionic conjugated polymers can be enhanced by
interaction with certain surfactants6a and polyelectrolytes.8 For
example, the addition of small amounts of certain surfactant
molecules (∼1-10 surfactant molecules/polymer chain) can
enhance the fluorescence of an ionic conjugated polymer up to
10-fold.6a It is believed that the fluorescence enhancement is
due to the formation of polymer-surfactant complexes, in which
the polymer adapts an ordered conformation in the presence of
surfactant, resulting in enhancement in the fluorescence intensity.6a
In this work, we explore the simultaneous interaction of ionic
conjugated polymers with both quencher molecules and sur-
factant molecules and study the two competing processes of
the polymer associated with fluorescence quenching (by quench-
er molecules) and fluorescence enhancement (by surfactant
molecules). The ionic conjugated polymer used is MPS-PPV.
For surfactants, we used DOTAP, which is well-known to retain
its tendency to assemble in bilayers in solution, and DTAB,
which has a tendency to aggregate in the micelle format. For
quenching molecules, we used both a conventional quencher
molecule, [MV]2+, and a newly synthesized quencher, [CHMB]2+
(Scheme 2). Key results include our demonstration that fluo-
rescence quenching is predominant in the mixture of polymer,
quencher, and DTAB, whereas the fluorescence enhancement
is dominant in the mixture of polymer, quencher, and DOTAP.
A possible mechanism associated with the observations has been
discussed. Since amplified fluorescence quenching and enhance-
ment is the key component for signal transduction, this study
opens new avenues for designing the next generation of ionic
conjugated-polymer-based biosensors.
lization (methanol). The purification was also carried out by
using an ultracentrifuge operated at 100 000 rpm (435000g) for
2 h. The purified polymer was characterized by dynamic light
scattering, capillary electrophoresis, and different spectroscopic
techniques such as NMR, UV-vis, and fluorescence. Light
scattering experiments revealed an average polymer mass of
21 kDa (Scheme 1).
Fluorescence of MPS-PPV in the Presence of Surfactant
Molecules. MPS-PPV synthesized as described above is a
reddish powder and is totally soluble in water. The UV-visible
spectrum of the polymer in aqueous solution showed one major
broad peak with maximum absorption around 470 nm. The
Results
Synthesis of MPS-PPV. As indicated in Scheme 1, the MPS-
PPV derivative was prepared in an anhydrous mixture of
solvents N,N-dimethylformamide (DMF) and tetrahydrofuran
(THF) (1:1) via dehydrohalogenation of the corresponding
bischloromethylated monomer 3 at 45 °C. Our initial attempts
at using t-BuLi as a base led to the formation of oligomers.
Polymerization was successfully carried out in the presence of
an excess of t-BuOK for 2 days. The reaction was carried out
in the absence of light to avoid any oxidation. After removal
of the solvents, the crude product was first dialyzed against water
by use of a Slide-A-Lyzer dialysis membrane with 10K
molecular weight cutoff (MWCO) and then purified by crystal-
emission spectra of an aqueous solution of MPS-PPV (λem
)
569 nM) is shown in Figure 1 (inset). In contrast to previous
reports,6b the fluorescence of MPS-PPV was independent of the
excitation wavelength.
The addition of DOTAP to the MPS-PPV solution caused
an enhancement in MPS-PPV fluorescence. Figure 1 demon-
strates the increase in fluorescence intensity of MPS-PPV as
the DOTAP concentration was increased. The enhancement
effect was saturated when the polymer-DOTAP ratio reached
1:0.6, at which point the emission intensity of MPS-PPV was
enhanced 5-fold. The inset of Figure 1 shows the emission

Enhanced Conjugated Polymer Fluorescence Quenching
J. Phys. Chem. B, Vol. 109, No. 9, 2005 3875
M-1, while the average KSV value for [MV]2+ was 5 × 106
M-1 (Figure 3A). This implies that [CHMB]2+ quenches the
fluorescence of MPS-PPV-surfactant complex more efficiently
compared to [MV]2+. Similar experiments were also conducted
with DTAB surfactant. As showed in Figure 3B, both [MV]2+
and [CHMB]2+ quenched the fluorescence of the MPS-PPV-
DTAB complex with a similar KSV value of 2 × 108 M-1. It
should be noted that the quenching constants obtained in the
DTAB system were significantly higher (10-fold) than those
for the DOTAP system, demonstrating a greater quenching
efficiency of [CHMB]2+ with DTAB than DOTAP.
ii. Fluorescence Enhancement for Quenched MPS-PPV. To
further explore the interactions among the polymer, surfactant,
and quencher molecules, a second set of complementary
experiments were conducted. In this study, the fluorescence of
MPS-PPV was first quenched by quencher molecules, and then
the fluorescence enhancement of quenched MPS-PPV with
different surfactants was determined. As shown in Figure 4B,
no fluorescence enhancement was observed when DTAB was
added to the CHMB-quenched polymer solution. In contrast,
fluorescence of the CHMB-quenched polymer was enhanced
about 50% when only 10-8 M DOTAP was added to the
quenched polymer solution (Figure 4A). In the case where
[MV]2+ was used as the quencher molecule, no fluorescence
enhancement was observed when DTAB was added to the MV-
quenched polymer solution (Figure 4D). However, fluorescence
was fully recovered when the same concentration of DOTAP
(10-8 M) was added to the MV-quenched polymer solution
(Figure 4C).
Figure 2. Stern-Volmer plots for quenching of the fluorescence (λex
) 450 nm) of MPS-PPV in aqueous solution without surfactant ([MPS-
PPV] ) 10-5 M). These measurements were used to calculate KSV
values.
spectra from a polymer solution with and without added
surfactant (1:0.8 polymer surfactant ratio). A sharper peak with
increased intensity and a slight blue shift was observed upon
addition of surfactant. In addition, the presence of a shoulder
at ∼604 nm was noted when DOTAP was employed.
Fluorescence of MPS-PPV in the Presence of Quencher
Molecules. The fluorescence of MPS-PPV can be quenched
efficiently by both [MV]2+ quencher and [CHMB]2+quencher.
As shown in Figure 2, the ratio of fluorescence intensities
without quencher to the intensity with quencher [I0/I] increased
linearly with quencher concentration. The linear (static quench-
ing) Stern-Volmer constant (KSV) values obtained from the
slopes of the fitted lines are 4.5 × 1010 for [CHMB]2+ and 1.5
× 1010 for [MV]2+. Results demonstrate that, in the absence of
surfactant, [CHMB]2+ is a slightly stronger quenching molecule
than [MV]2+, and both of them have similar KSV within the
same order of magnitude.
Discussion
Various substituted PPV derivatives have been produced
either via a processible precursor polymer9 or directly from the
respective monomers.10 In this work, we reduced the total
number of steps involved in the synthesis of PPV derivatives
to three by using sultones as a source of ionic side chain
(Scheme 1). For polymer isolation and purification, we applied
several protein purification methods, including dialysis and
ultracentrifugation.
Although MPS-PPV has been used by many groups, to the
best our knowledge, there is no detailed discussion about its
synthesis directly from the monomer. Polymers obtained from
polymer precursor routes may not be ideal, owing to the
difficulties in achieving complete elimination of the labile
groups, and also to the larger number of parameters affecting
the conversion of the precursor to the conjugated material
(pressure, duration, rate of temperature increase/decrease). Here
the Gilch route11 was adapted for synthesis of the polymer
directly from its monomer. Results provided a simplified
procedure for producing polymer with distinguished spectral
characteristics.
So far, ultracentrifugation has been used mainly for the
separation of proteins of different molecular mass, but for the
first time we report the use of ultracentrifugation for the final
purification of conjugated polymers, in place of crystallization.
Based on the principle of molecular size, the ultracentrifugation
technique offers separation for short times at very high speeds.
At the end of 2 h of ultracentrifugation at 100K rpm, a purified
polymer was conveniently obtained.
The interaction of MPS-PPV with counterionic surfactant
DTAB has been studied previously. It was demonstrated that
the interaction can make the polymer backbone ordered,
eliminate the interchain interactions, and enhance the polymer
fluorescence. As expected, fluorescence enhancement was also
Fluorescence of MPS-PPV in the Presence of Surfactant
and Quencher Molecules. To investigate the interactions
among the polymer, quencher, and surfactant molecules, two
sets of experiments were carried out. In the first set, the
surfactant was added to the polymer solution to form the
polymer-surfactant complex first, and then the behavior of the
quencher against the complex was examined. In the second set,
the fluorescence of the polymer was first quenched by quencher
molecules, and then the surfactant was added to the quenched
polymer solution. The behavior of surfactant against quenched
polymer was examined.
i. Fluorescence Quenching for MPS-PPV-Surfactant Com-
plex. Experiments were conducted where fluorescence quench-
ing constants of quencher molecule against MPS-PPV-
surfactant complex were measured as a function of surfactant
concentration. In brief, DOTAP was added to the MPS-PPV
solution (1 × 10-5 M). The resulting solution exhibited
enhanced fluorescence compared to MPS-PPV itself due to the
formation of the MPS-PPV-DOTAP complex (Figure 2). To
this complex solution were added quencher molecules (2 × 10-9
to 1 × 10-8 M), and the Stern-Volmer quenching constant was
determined. By varying the surfactant concentration, a quenching
constant at each surfactant concentration was calculated. As
demonstrated in Figure 3A, the quenching constants of both
quencher molecules (CHMB and MV) against the polymer-
DOTAP complex were plotted vs the concentration of DOTAP.
Results show that [CHMB]2+ quenched the fluorescence of the
MPS-PPV-DOTAP complex with an average KSV value of 2.5
× 107 M-1 with variations between 1.5 × 107 and 3.1 × 107

3876 J. Phys. Chem. B, Vol. 109, No. 9, 2005
Dalvi-Malhotra and Chen
Figure 3. Quenching constant (KSV) of CHMB2+ and of MV2+ to MPS-PPV as a function of surfactant concentration (λex ) 450 nm): [MPS-PPV]
) 10-5 M; [quencher] ) (2-10) × 10-9 M; (A) with DOTAP; (B) with DTAB.
Figure 4. (A, B) Emission spectra of quenched MPS-PPV at different sequence of addition of surfactant (excited at 450 nm): [MPS-PPV) ) 10-5
M; [CHMB] ) 10-9 M; (A) [DOTAP] ) 10-8 M; (B) [DTAB] ) 10-8 M). (C, D) Emission spectra of quenched MPS-PPV at different sequences
of addition of surfactant: [MPS-PPV] ) 10-5 M; [MV] ) 10-9 M; (C) [DOTAP] ) 10-8 M; (D) [DTAB] ) 10-8 M.
observed when DOTAP surfactant interacts with MPS-PPV
(Figure 1). The formation of MPS-PPV-DOTAP complex
serves to extend the MPS-PPV chains and inhibit the folding
of the polymer chains, reducing the conformational disorder and
increasing the fluorescence efficiency. This was confirmed by
several spectral features, including a narrowed and slightly blue-
shifted fluorescence spectrum, together with the emergence
of vibronic structure (presence of a shoulder peak) in emis-
sion.
The amplified fluorescence quenching of ionic conjugated
polymers is believed to occur through two major mechanisms:
(1) efficient energy or electron transfer between the polymer
and quencher and within the polymer chain and (2) quencher-
induced polymer conformational change, inducing interchain
interactions and thus quenching of the polymer fluorescence.
In both cases, the attachment of the quencher molecule to the
polymer backbone is critical for polymer fluorescence quench-
ing. Since [CHMB]2+ shares the same electron donation core
as [MV]2+, comparable quenching efficiencies were observed
in the absence of surfactant (Figure 2). In this case, both
quenchers can attach to the polymer backbone via attractive
electrostatic interactions. The trend for enhanced quenching
efficiency of [CHMB]2+ over [MV]2+ in the presence of
DOTAP (Figure 3) supports the hypothesis that DOTAP forms
self-assembling structures that facilitate the interaction of
[CHMB]2+ with the polymer backbone due to hydrophobic
interactions between the long aliphatic carbon tail of [CHMB]2+
and the surfactant assemblies. This hydrophobic interaction
would help [CHMB]2+ to penetrate well through surfactant
chains to reach the polymer chains. However, the situation for
DTAB was quite different. Due to its high cmc, no obvious
self-assembling of DTAB was observed. DTAB may spread
more sparsely along the MPS-PPV polymer chain, explaining
why both [MV]2+ and [CHMB]2+ showed similar quenching
behavior against the MPS-PPV-DTAB complex. (Figure 3).
This interpretation would also explain why the average quench-
Although DOTAP and DTAB share the same ionic head-
group, the major difference between the two surfactants is that
the critical micelle concentration (cmc) of DOTAP is signifi-
cantly lower (7 × 10-5 M)12 than the cmc of DTAB (3.8 ×
10-3 M).13 Thus, DOTAP has a much stronger tendency for
self-assembling. DOTAP has been a popular transinfectant
reagent for cross-membrane delivery of DNA molecules due to
its ability to complex with polyanionic DNA and form self-
assemblies around the DNA chain. Although the bulk surfactant
concentrations were below the critical micelle concentration
under the experimental conditions, due to the coulombic
interaction between the polymer and surfactant, the local
surfactant concentration near the polymer might be higher than
the cmc, resulting in self-assembling of surfactant around the
polymer chains. This would be especially true for the DOTAP
molecules, and it may play an important role in the rationale
for the interactions of polymer, surfactant, and quencher
molecules.

Enhanced Conjugated Polymer Fluorescence Quenching
J. Phys. Chem. B, Vol. 109, No. 9, 2005 3877
on the polymer-surfactant complex indicates that the electro-
static interaction between polymer and quencher, with surfactant
present, remains unchanged over a wide range of temperature.
This behavior clearly shows that no temperature-induced
structural changes occur in the quenched polymer-surfactant
complex and that the interaction of quencher with the polymer-
surfactant complex is stable even at elevated temperatures.
Conclusion
MPS-PPV was successfully synthesized directly from its
bischloromethylated monomer, considerably reducing the total
number of steps involved in the polymer preparation. For the
first time, a simple technique of ultracentrifugation was
employed for final purification of the polymer. The interactions
among the polymer, surfactant, and quencher molecules, as well
as amplified fluorescence quenching and fluorescence enhance-
ment associated with the interactions, were investigated and
discussed. When compared with [MV]2+, higher KSV values
(g107 M-1) were observed for the carboxylate ester [CHMB]2+
7 in the presence of DOTAP surfactant. Comparisons of
surfactants demonstrated that the quenching constant of
[CHMB]2+ was 10-fold higher in the presence of DTAB
surfactant than with DOTAP. Polymer fluorescence was totally
recovered upon addition of DOTAP surfactant to a MV-
quenched polymer system, whereas only 50% of fluorescence
was recovered upon addition of DOTAP surfactant to the
CHMB-quenched polymer solution. In contrast, no fluorescence
was recovered when DTAB was added to either the MV- or
CHMB-quenched polymer systems. Thus, in the competition
between fluorescence enhancement (surfactant-induced confor-
mational change associated with MPS-PPV) and fluorescence
quenching (electrostatic interaction between polymer-quencher
complex), fluorescence enhancement played a major role for
the polymer complex with DOTAP, whereas fluorescence
quenching was predominant in the polymer complex with
DTAB.
Figure 5. (A) Surfactant-induced conformational change (random
coiled to an ordered form) associated with the polymer: Polymer
partially covered by quencher in the presence of surfactant. (B) Polymer-
bound DOTAP bilayers. (C) Randomly spread DTAB molecules along
polymer chain.
ing constant values for both quenchers were 10-fold higher when
DTAB surfactant was used compared to DOTAP surfactant. The
self-assembling of DOTAP apparently shielded the interaction
of quencher molecules with polymer.
Second, supportive data to indicate the self-assembly of
DOTAP around the polymer chain came from the experiment
in which surfactant was added to a quenched polymer solution.
When the polymer fluorescence was quenched by [MV]2+ or
[CHMB]2+, the addition of DTAB did not enhance the polymer
fluorescence at all. On the other hand, DOTAP significantly
enhanced the quenched polymer fluorescence in both cases
(Figure 4). Since the DOTAP concentration used was very low
(10-8 M), it is unlikely that the quenched molecules were
detached from the polymer backbone. Thus, fluorescence
quenching was predominant in the mixture of polymer, quencher
molecule, and DTAB, while fluorescence enhancement played
an important role in the mixture of polymer, quencher molecule,
and DOTAP. As mentioned above, charge transfer between the
polymer and quencher and quencher-induced conformational
change are the two main fluorescence quenching pathways, and
the surfactant-induced conformational change is the major
fluorescence enhancement pathway. We propose that a change
in polymer conformation due to the self-assembly of DOTAP,
which leads to fluorescence enhancement, overwhelms the
fluorescence quenching in the polymer-quencher-DOTAP
system; whereas more efficient interactions of polymer and
quencher molecules enable the dominates of polymer fluores-
cence quenching due to the lack of self-assembly of DTAB in
low surfactant concentration conditions. A diagram depicting
the proposed model of polymer-quencher-surfactant interac-
tions is illustrated in Figure 5.
Experimental Section
Materials. Chemicals were purchased from Aldrich and used
as received. Fluorescence spectra were measured on a Photon
1
Technology International fluorescence PTI system. H NMR
spectra were recorded on a Bruker (300 MHz) spectrometer.
Electropherograms were run on a Beckman P/ACE system 5000.
Procedures: Sodium 4-Methoxyphenoxypropane Sulfonate
(1). To a solution of methoxyphenol [2 g, 0.016 mol] in dry
methanol [10 mL] was added NaOMe [0.87 g, 0.016 mol]. The
mixture was allowed to stir for 2 h at 45 °C under N2. Propane
sultone [2 g, 0.0164 mol] was added to the mixture, which was
then refluxed under an inert atmosphere for 2-3 h. After
cooling, methanol was removed under reduced pressure. The
product was recrystallized in methanol. Yield ) 85%. 1H NMR
(D2O): 1.9-2.00 (q, 2H), 2.4-2.6 (t, 2H), 3.6 (s, 3H), 3.9 (t,
2H), 6.78 (s, 4H).
Sodium 4-Methoxyphenoxybutane Sulfonate (2). A mixture
of methoxyphenol (2 g, 0.016 mol) and NaOMe (0.87 g, 0.016
mol) was taken in 10 mL of dry methanol. After in situ
generation of phenoxide ion as mentioned for 1, butane sultone
(1.5 mL, 0.015 mol) was added dropwise over 20 min. The
reaction mixture was then refluxed for 2-3 h. After removal
of solvent under reduced pressure, the product was then purified
by recrystallization in methanol. Yield ) 81%. 1H NMR
(D2O): 1.87-1.89 (m, 4H), 2.97 (t, 2H), 3.8 (s, 3H), 4.1 (t,
2H), 7.00 (bifurcated s, 4H).
It should be noted that the MPS-PPV-DOTAP complex was
very stable up to 70 °C. This property is supported by an
experiment in which the quenching constant (KSV) of [CHMB]2+
to MPS-PPV (10-5 M) was determined as a function of
temperature in the presence of DOTAP (10-7 M). KSV values
remained constant as the temperature was increased from 35 to
70 °C. The trend for temperature insensitivity of quencher action

3878 J. Phys. Chem. B, Vol. 109, No. 9, 2005
Dalvi-Malhotra and Chen
General Procedure Followed for Chloromethylation.14 To
dry 1,4-dioxane (5 mL) was added 1 ( 1 g, 3.73 mmol) or 2 (1
g, 3.54 mmol), followed by addition of concentrated HCl (0.5
mL). A slow stream of HCl(g) was allowed to pass through the
reaction mixture. An aqueous formalin solution (40%) was
added at 30 min intervals (3 × 0.3 mL), during which HCl(g)
was allowed to pass. After completion of formalin addition,
reaction mixture was stirred for another 3 h under HCl(g)
atmosphere. Concentrated HCl (2 mL) was added into the ice-
cooled reaction mixture. It was then allowed to stand overnight
at room temperature, and the precipitated product was dried over
KOH pellets in a vacuum desiccator. Chloromethylated product
was used for polymerization without any further purification.
Sodium 2,5-Bis(chloromethyl)-4-methoxyphenoxypropane Sul-
fonate (3). Yield ) 78%. 1H NMR (D2O): 2.15-2.25 (q, 2H),
3.1 (t, 2H), 3.82 (s, 3H), 4.1-4.2 (t, 2H), 4.62 (s, 4H), 7.3
(bifurcated s, 2H).
pressure. The obtained colorless oil was dried under vacuum
to give a solid, which was used without any further purification.
NMR showed the absence of an acid peak. Yield ) 81%.
N-(2-Carboxyhexadecanoyl)-N′-methyl-4,4′-bipyridinium Io-
dide Bromide (7). The mixture of N-methyl-4-(4′-pyridyl)-
pyridinium iodide (0.46 g, 1.55 mmol) and 16-bromohexade-
canoic acid methyl ester (0.5 g, 1.86 mmol) was taken in dry
DMF (10 mL). It was heated under an inert atmosphere for 2
days at 95 °C. The obtained carboxylate ester was crystallized
1
in methanol. Yield ) 69%. H NMR (D2O): 1.25 (s, 3H), 2.3
(t, 2H), 2.75 (t, 7H), 2.92 (d, 6H), 3.45 (s, 3H), 3.62 (s, 3H),
4.85 (s, 3H), 9.00-9.1 (m, 4H), 9.6 (d, 2H), 9.7 (d, 2H).
Acknowledgment. This research was supported by LDRD,
Argonne National Laboratory (ANL). We thank Dr. P Rickert,
Dr. Gerald Rex, III, and Dr. M Chen for generously allowing
us to use their NMR facility at ANL. J.D.M. thanks Dr. M.
Bhattacharyya (ANL) for her helpful discussions and for
reviewing the manuscript. J.D.M. thanks Dr. Marion Thurnauer
(ANL) for her insightful comments to the manuscript. We extend
our thanks to Funing Yan for his input to the text.
Sodium 2,5-Bis(chloromethyl)-4-methoxyphenoxybutane Sul-
1
fonate (4). Yield ) 72%. H NMR (D2O): 1.8-2.0 (m, 4H),
3.00-3.1 (t, 2H), 3.88 (s, 3H), 4.1 (t, 2H), 7.15 (s, 2H).
General Procedure for Polymerization. The corresponding
bischloromethylated monomer (3 or 4, 5 mmol) was taken into
a 10 mL mixture of anhydrous DMF and THF (1:1). After
vigorous shaking, THF solution of t-BuOK (0.05 mol) was
added dropwise into the reaction mixture. It was allowed to
stir for 2 days at 45 °C under an inert atmosphere. Precipitated
polymer was purified by dialysis (10K MWCO). Average yield
of purified polymers was found be 55%.
References and Notes
(1) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 7017.
(2) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 12593.
(3) Jones, R. M.; Bergstedt, T. S.; McBranch, D. W.; Whitten, D. G.
J. Am. Chem. Soc. 2001, 123, 6726.
(4) (a) Wang, J.; Wang, D.; Miller, E. K.; Moses, D.; Bazan, G.;
Heeger, A. J. Macromolecules 2000, 33, 5153. (b) Wang, D.; Wang, J.;
Moses, D.; Bazan, G. C.; Heeger, A. J. Langmuir 2001, 17, 1262.
(5) Tan, C.; Pinto, M. R.; Schanze, K. S. Chem. Commun. 2002, 446.
(6) (a) Chen, L.; McBranch, D.; Wang, R., Whitten, D. Chem. Phys.
Lett. 2000, 330, 27. (b) Chen, L.; McBranch, D. W.; Wang, H.-L.; Helgeson,
R.; Wudl, F.; Whitten, D. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 12287.
(c) Chen, L.; Xu, S.; McBranch, D.; Whitten, D. J. Am. Chem. Soc. 2000,
122, 9302.
(7) Heeger, P.; Heeger, A. Proc. Natl. Acad. Sci. U.S.A. 1999, 96,
12219.
(8) Abe, S.; Chen, L. J. Polym. Sci. 2003, B 41, 1676.
(9) (a) Wessling, R. A.; Midland; Zimmerman, R. G.; Shepherd, M.
U.S. Patent 3,401,152, 1968. (b) Hsieh, B. R.; Johnson, G. E. U.S. Patent
5,558,904, 1996. (c) Son, S.; Dodabalapur, A.; Lovinger, A. J.; Galvin, M.
E. Science 1995, 269, 376.
(10) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Angew. Chem., Int.
Ed. Engl. 1988, 37, 402.
(11) Gilch, H. G.; Wheelwright, W. L. J. Polym. Sci., Part A-1 1966,
4, 1337.
(12) Zuidam, N. J.; Barenholz, Y. Int. J. Pharm. 1999, 183, 43.
(13) Cationic surfactants: Physical hemistry; Surfactant Science Series
37; Rubingh, D. N., Holland, P. M., Eds.; New York, 1991.
(14) Wood, J. H.; Gibson, R. E. J. Am. Chem. Soc. 1949, 71, 393.
(15) Willner, I.; Eichen, Y.; Frank, A. J.; Fox, M. A. J. Phys.Chem.
1993, 97, 7264.
Poly([2-methoxy-5-propyloxy sulfonate]phenylene Vinylene),
MPS-PPV, and Poly([2-methoxy-5-butyloxy sulfonate]phenylene
Vinylene), MBS-PPV: λem ) 565 nm when excited at 450 and
500 nm; 8.4 min retention time on capillary electrophoresis
(borate buffer, pH 9.2, pressure injection 5 s, 20 kV] when
compared with reference polymer, poly(sodium 4-styrene sul-
fonate) (∼70 K), with 9.1 min retention time, 22K molecular
weight shown from light scattering experiment.
N-Methyl-4-(4′-pyridyl)pyridinium iodide (5) was synthesized
according to the reported procedure.15 Yield ) 79%.1H NMR
(D2O): 4.4 (s, 3H), 7.9 (d, 2H), 8.4 (d, 2H), 8.6-8.9 (m, 4H).
16-Bromohexadecanoic Acid Methyl Ester (6). To a solution
of bromohexadecanoic acid (1 g) in methanol (22 mL) was
added concentrated HCl (0.25 mL). The reaction mixture was
refluxed for 4 h. After cooling, methanol was evaporated under
reduced pressure. The DCM (20 mL) solution of obtained oil
was three times washed with saturated Na2CO3solution. The
organic layer was dried over MgSO4 and then evaporated under