Water-soluble cationic poly(p-phenyleneethynylene)s (PPEs): Effects of acidity and ionic strength on optical behavior

Article abstract of DOI:10.1021/ma048717k

Three cationic PPEs P1′-P3′ were obtained through quaternization of their neutral polymers, of which all side chains were respectively featured with a tertiary amino group (P1), an alternative tertiary amino and tri(ethylene glycol)methyl ether group (P2)

Full text of DOI:10.1021/ma048717k

Macromolecules 2005, 38, 2927-2936
2927
Water-Soluble Cationic Poly(p-phenyleneethynylene)s (PPEs): Effects of
Acidity and Ionic Strength on Optical Behavior
Qu-Li Fan, Ying Zhou, Xiao-Mei Lu, Xiao-Yuan Hou, and Wei Huang*
Institute of Advanced Materials (IAM), Fudan University, 220 Handan Road,
Shanghai 200433, The People’s Republic of China
Received June 26, 2004; Revised Manuscript Received January 12, 2005
ABSTRACT: Three cationic PPEs P1′-P3′ were obtained through quaternization of their neutral
polymers, of which all side chains were respectively featured with a tertiary amino group (P1), an
alternative tertiary amino and tri(ethylene glycol)methyl ether group (P2), and an alternative tertiary
amino and dodecyloxy group (P3). P1′ showed intrinsic water-solubility although its quaternization degree
was only 45%. Slightly blue-shifted UV-vis absorption maxima in an acidic environment (maximal ∆λmax
≈ 3 nm) and obvious red-shifted absorption (maximal ∆λmax ≈ 30 nm) and emission maxima (maximal
∆λmax ≈ 20 nm) in an alkaline environment at pH < 13 are explained in terms of pH-induced planarization
of the conjugated backbone. After the pH was increased to 14, a new absorption (λmax ≈ 478 nm) and
emission (λmax ≈ 495 nm) peak appeared in the redder region and the fluorescence intensity further
dramatically decreased, implying the occurrence of the conformational change from the planarization of
conjugated backbone to the interchain aggregation. The fluorescence enhancement of P1′ in an acidic
environment and the fluorescence decrease of P1′ in an alkaline environment are attributed to the pH-
influenced quenching ability of the nonbonding electron pair on the nitrogen atom, which belongs to the
unquaternized tertiary amino group on the side chain, and the interchain aggregation. Such pH-influenced
fluorescence intensity can be efficiently buffered by adding salts at pH < 10. Although adding salts in a
neutral environment only creates the planarization of PPE conjugated backbone, adding salts in an
alkaline environment significantly promotes the formation of interchain aggregation. Amplified quenching
of P1′ by anionic quencher Fe(CN)64- was observed and its Ksv increased significantly after adding salts
or increasing pH to 14, for example from Ksv ) 9.6 × 105 M-1 at pH ) 7 to 7.7 × 106 M-1 at pH ) 10 when
[NaCl] ) 0.1 M, due to the formation of elongated π-conjugation.
Introduction
Anionic FWSCPs are a versatile class of polyelectro-
lytes which can be easily fabricated through grafting
different types of anionic groups (sulfonated, carboxylic
and phosphonated group) to the side chains of conju-
In the past few years, fluorescent water-soluble
conjugated polymers (FWSCPs) containing charged
groups have showed great potential as novel chemo or
biosensors due to their amplified fluorescence quenching
ability upon electron or energy transfer by quenchers
with counterions.1,2 To develop FWSCPs as practical
sensors, the optophysical properties of FWSCPs in
different aqueous environments were studied inten-
sively and they showed that their optical properties and
quenching behavior varied in accordance with the
change of local environments, such as ionic strength,3
charge on quencher,4 concentration of surfactant5,6 or
polyelectrolyte7,8 with counterions, and type of sub-
strate.7 Most of the above investigations were performed
in a neutral environment. In practice, analysis in the
environments with different pH is nearly inevitable,
while the related pH-influenced analytical efficiency is
seldom reported.9 Although Pinto and Schanze recently
studied the optical properties of an anionic poly(p-
phenyleneethynylene) (PPE) in an alkaline environment
and reported its interchain aggregation with pH de-
crease,10 investigation of the influence of pH and its
combination with other factors, e.g., ionic strength) on
the conformational change and the related optical
behavior of FWSCPs in both acidic and alkaline envi-
ronments is more significant and exigent to develop
FWSCPs as good biosensors in a wider scope.
gated polymers with various backbones.11 Since a water-
-
soluble sulfonated poly(phenylenevinylene) (PPV-SO3
)
was first used by Chen et al. to detect proteins,1 anionic
FWSCPs have been the subject of intensive studies on
their application as biosensors.12 Their optophysical
properties in an aqueous solution were highly in-
vestigated.3-8,13,14 Compared with anionic FWSCPs,
cationic FWSCPs attracted little attention previously
because of no report on them as biosensors and their
relatively fewer types (cationic groups are mostly am-
monium groups).11 In 1999, Schanze and Reynolds first
reported the amplified fluorescence quenching in a poly-
(p-phenylene)-based cationic polyelectrolyte.15 Most re-
cently, cationic ammonium-functionalized polyfluorene
(PF-NEt3+) was successfully developed to identify
specific anionic DNA2,16 and a major effort is being
focused on determining the relationship between poly-
fluorene chemical structure and its corresponding
biosensitivity.17-19 The novel application of cationic
FWSPs encouraged us to develop additional cationic
FWSCP types and investigate their optophysical prop-
erties in an aqueous solution.
PPE is a kind of conjugated polymer which is suitable
to study optical property-structure relationship because
of its good optical response on environmental variation
through the relatively free rotation of alkyne-aryl
single bond and interchain aggregation.20 In addition,
because of the high delocalization of singlet exciton in
PPEs and the rapidness of the energy migration along
* To whom correspondence should be addressed. Telephone:
+86 (21) 5566 4188. Fax: +86 (21) 6565 5123. E-mail:
wei-huang@fudan.edu.cn/iamdirector@fudan.edu.cn.
10.1021/ma048717k CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/01/2005

2928 Fan et al.
Macromolecules, Vol. 38, No. 7, 2005
oxy)benzene, 1,4-diiodo-2,5-bis{2-[2-(2-methoxyetho-xy)ethoxy]-
ethoxy}benzene, and 1,4-diiodo-2,5-hydroquinone were syn-
thesized according to the literature procedures.20,21b,27
the conjugated backbone, PPEs used as chemosensors
in organic solvent and as films have been widely
reported by Swager et al.21 Currently, the preparation
of water-soluble PPEs and the study on their optical
properties in aqueous solution have attracted much
attention to fabricate novel chemo or biosensors. In
such, different types of anionic water-soluble PPEs were
widely investigated.10,22-25 However, for cationic PPEs,
as far as we know, only Swager et al. reported its
application in layer-by-layer self-assembling ultrathin
film through electrostatic attraction.26
Herein, we report the successful synthesis of a neutral
PPE with quaternizable tertiary amino groups on all
side chains. Quaternization after polymerization was
chosen to obtain a new cationic water-soluble conjugated
polyelectrolyte. In an attempt to develop intrinsically
water-soluble PPE polyelectrolytes, tertiary amino group
(the quaternizable group) was introduced into all side
chains of the neutral conjugated polymer instead of one
moiety of the side chains26 and expected to enhance the
charge density on the conjugated polyelectrolyte by
quaternization. The other two cationic PPE polymers
containing side chains with different hydrophilicity
respectively were also synthesized and the water solu-
bility of the three cationic PPEs was compared with each
other. The pH- and ionic-strength-influenced conforma-
tional change and the related optical behavior of the new
water-soluble PPE were intensively investigated in both
acidic and alkaline environments (pH ) 2∼14).
Synthesis. 1,4-Bis[3-(N,N-diethylamino)-1-oxapropyl]-
2,5-diiodobenzene (Monomer 1). A 250 mL round-bottom
flask with magnetic stirring bar was charged with anhydrous
potassium carbonate (24.84 g, 0.18 mol), 1,4-diiodo-2,5-hyd-
roquinone (10.86 g, 0.03 mol), and 150 mL of acetone. The
stirred mixture was sparged with nitrogen for 15 min and then
refluxed for about 30 min. After 30 min of refluxing, 2-chlo-
rotriethylamine hydrochloride (12.38 g, 0.072 mol) was added
into the round-bottom flask and the mixture was then refluxed
for 3 days. The precipitate mixture was filtered away, and the
filtrate was rotary evaporated. The residue was poured into
water and extracted with ether three times, and the combined
organics were washed with 10% aqueous sodium hydroxide
twice, water twice, and brine once. The solution was dried over
magnesium sulfate, filtered and stripped off solvent by rotary
evaporation to yield crude solid. The crude product was
recrystallized with hexane to afford colorless crystals (12 g,
1
yield 71%). Mp: 76-78 °C. MS: m/z 559.9. H NMR (CDCl3,
ppm): δ 7.23 (s, 2 H), 4.02 (t, 4 H, J ) 6.4 Hz), 2.93 (t, 4 H, J
) 6.4 Hz), 2.67 (q, 8 H, J ) 6.4 Hz), 1.10 (t, 12 H, J ) 6.8 Hz).
13C NMR (CDCl3, ppm): δ 153.4, 123.4, 86.5, 69.8, 52.0, 48.4,
12.6. Anal. Calcd for C18H30I2N2O2: C, 38.59; H, 5.40; N, 5.00;
I, 45.30. Found: C, 38.99; H, 5.32; N, 4.84.
1,4-Diethynyl-2,5-bis[3-(N,N-diethylamino)-1-oxapro-
pyl]benzene (Monomer 2). A 4.66 g (0.01 mol) sample of
monomer 1, 0.35 g (0.5 mmol) of PdCl2(PPh3)2 and 0.0952 g
(0.5 mmol) of CuI were dissolved in 40 mL of diisopropylamine.
2.156 g (0.022 mol) of (trimethylsilyl)acetylene was added into
the vigorously stirred solution at room temperature under
nitrogen protection. After the addition was finished, the
reaction mixture was stirred at reflux for 3 h. After the solvent
was evaporated under reduced pressure, the residue was
poured into 100 mL of water and extracted with chloroform
three times. The combined organic layer was washed with
water twice and brine once and dried over MgSO4. After the
mixture was filtered, 5 mL of hydrazine monohydrate dissolved
in10 mL of water was poured into the filtrate and stirred
vigorously for 12 h. The organic layer was separated and
washed with water three times and brine once and dried over
MgSO4. After the solvent was evaporated, the residue was
dissolved in a mixture of KOH (1 g in 2 mL of water) and 100
mL of methanol and stirred at room temperature for 1 h. After
evaporation of the solvent, the mixture was subjected to a
CHCl3/H2O workup. The organic phases were combined and
dried over MgSO4. The solvent was removed under reduced
pressure, and the deep black residue was added into 100 mL
of saturated HCl methanol solution and stirred for 30 min at
room temperature. The solvent was evaporated and the crude
product was recrystallized with ethanol to afford light yellow
crystals (3.1 g, yield 72%). Mp: 230-1 °C. 1H NMR (D2O,
ppm): δ 7.17 (s, 2 H), 4.34 (t, 4 H, J ) 5.2 Hz), 3.84 (s, 2 H),
3.58 (t, 4 H, J ) 4.8 Hz), 3.32 (q, 8 H, J ) 7.2 Hz), 1.28 (t, 12
H, J ) 7.2 Hz).
The abovementioned compound was dissolved in 50 mL of
water and K2CO3 aqueous solution (3 g in 50 mL water) was
added dropwise at room temperature and stirred for 30 min.
After the stirring, the obtained light yellow organic layer was
separated and the water layer was extracted with chloroform
three times. The combined organic layers were washed with
water twice and brine once and dried over MgSO4. The mixture
was filtered, and the filtrate was evaporated using rotary
evaporator to afford yellow crystals (2.49 g, yield 97%). Mp:
70-1 °C. MS: m/z 356.2.1H NMR (CDCl3, ppm): δ 6.99 (s, 2
H), 4.07 (t, 4 H, J ) 6.0 Hz), 3.34 (s, 2 H), 2.93 (t, 4 H, J ) 6.0
Hz), 2.66 (q, 8 H, J ) 7.2 Hz), 1.09 (t, 12 H, J ) 7.2 Hz). 13C
NMR (CDCl3, ppm) δ 154.4, 118.0, 113.6, 83.0, 80.1, 68.7, 51.9,
48.3, 12.4. Anal. Calcd for C22H32N2O2: C, 74.12; H, 9.05; N,
7.86. Found: C, 74.05; H, 8.99; N, 7.73.
Experimental Section
General Methods. The NMR spectra were collected on a
Varian Mercury Plus 400 spectrometer with tetramethylsilane
as the internal standard. FT-IR spectra were recorded on a
Shimadzu IRPrestige-21 FTIR-8400s spectrophotometer by
dispersing samples in KBr. Mass spectra (MS) were obtained
using a HP 5973MS mass spectrometer at an ionizing voltage
of 70 eV. UV-vis spectra were recorded on a Shimadzu 3150
PC spectrophotometer. The concentrations of copolymer solu-
tions were adjusted to about 0.01 mg/mL or less. Fluorescence
measurement was carried out on a Shimadzu RF-5301 PC
spectrofluorophotometer with a xenon lamp as a light source.
Thermogravimetric analysis (TGA) was performed on a Shi-
madzu thermogravimetry and differential thermal analysis
DTG-60H at a heating rate of 10 °C/min under N2. Elemental
microanalyses were carried out on a Vario EL III CHNOS
Elementar analyzer. Gel permeation chromatography (GPC)
analysis was conducted with a HP1100 HPLC system equipped
with 7911GP-502 and GP NXC columns using polystyrenes
as the standard and tetrahydrofuran (THF) as the eluant at
a flow rate of 1.0 mL/min and 35 °C. Time-correlated single
photon fluorescence studies were performed using an Edin-
burgh Instruments LifeSpec-PS spectrometer. The LifeSpec-
PS comprises a 371 nm picosecond laser (PicoQuant PDL
800B) operated at 2.5 MHz and a Peltier-cooled Hammamatsu
microchannel plate photomultiplier (R3809U-50). Lifetimes
were determined from the data using the Edinburgh Instru-
ments software package.
The pH-dependent electronic spectra were investigated in
polymer aqueous solution (5 µM) in which the pH was varied
from 2 to 12. The quenching behavior was studied by compar-
ing the fluorescence intensities of polymer aqueous solutions
in the presence of quenchers with different concentrations.
Milli-Q water used in preparing the aqueous solutions of the
polymers and quenchers was purged with nitrogen for 4 h
before using.
Materials. All chemical reagents used were purchased from
Aldrich Chemical Co. THF was purified by distillation from
sodium in the presence of benzophenone. Toluene was purified
by distillation from sodium. Other organic solvents were used
without any further purification. 2,5-Diiodo-1,4-bis(dodecyl-
1,4-Diphenylethynyl-2,5-bis[3-(N,N-diethylamino)-1-
oxapropyl]benzene (M1). First, 2.8 g (0.005 mol) sample of
monomer 1, 0.29 g (0.25 mmol) of Pd(PPh3)4, and 0.19 g (0.001
mol) CuI were dissolved in 15 mL of diisopropylamine. Then

Macromolecules, Vol. 38, No. 7, 2005
Water-Soluble Cationic PPEs 2929
1.12 g (0.011 mol) of phenylacetylene was added into the
vigorously stirred solution at room temperature under nitrogen
protection. After the addition was finished, the reaction
mixture was stirred at reflux for 12 h. The purification
procedure of monomer 2 was followed to obtain 1,4-Diphenyl-
ethynyl-2,5-bis[3-(N,N-diethylamino)-1-oxapropyl]benzene as
quaternization, a yellow precipitate was produced in the
solution. The resulting precipitate was collected on a frit at
reduced pressure and dried to get 0.41 g of the desired product
(yield 75%). 1H NMR (D2O, ppm): δ 7.26 (br, 2 H), 4.44 (br, 4
H), 3.73 (br, 1.8 H), 3.60 (br, 2.2 H), 3.40 (br, 4.3 H), 3.31 (br,
5.4 H), 1.20 (br, 12 H). 13C NMR (D2O, ppm): δ 153.3, 117.6,
114.1, 91.4, 64.8, 63.6, 58.5, 56.0, 54.3, 51.4, 49.5, 9.1, 7.6. FT-
IR (KBr pellet, cm-1): 3458 (br), 3056, 2970, 2947, 2870, 2659,
2484, 2202, 1510, 1467, 1422, 1397, 1275, 1216, 1059, 1020,
951, 864, 785, 715, 520. Anal. Calcd for C20H30N2O2‚0.9C2H5-
Br‚0.5H2O: C, 59.84; H, 8.18; N, 6.40; Br, 16.44. Found: C,
58.04; H, 7.35; N, 5.55; Br, 17.87. The water present in the
elemental analysis results was based on mass loss in the TGA
and the bromide determination was carried out by Scho¨niger
combustion method and then titration with HgNO3.
1
white crystals (1.81 g, yield 71%). H NMR (CDCl3, ppm): δ
7.51 (m, 4 H), 7.37 (m, 6 H), 7.09 (s, 2 H), 4.11 (t, 4 H, J ) 5.2
Hz), 2.94 (t, 4 H, J ) 5.2 Hz), 2.69 (q, 8 H, J ) 7.2 Hz), 1.05
(t, 2 H, J ) 7.2 Hz).13C NMR (CDCl3, ppm): δ 153.6, 131.3,
128.4, 123.5, 116.6, 114.0, 94.6, 85.7, 67.8, 51.2, 10.6.
1,4-Diphenylethynyl-2,5-bis[3-(N,N,N-triethylammo-
nium)-1-oxapropyl]benzene Dibromide (M1′). First 1.02
g (0.002 mol) of M1 was dissolved in 40 mL THF, and then 10
mL bromoethane was added into the solution under nitrogen
protection. The mixture was stirred with refluxing for 5 days
and then filtered to get the crude product. The crude product
was recrystallized with ethanol to afford pure light yellow
P2′: Most of the bromoethane and THF was evaporated and
then the quaternized polymer was precipitated in 100 mL of
acetone, collected by centrifugation and dried overnight in
1
1
crystals (0.95 g, yield 65%). H NMR (CD3OD, ppm): δ 7.53
vacuo at 50 °C (0.345 g, yield 71%). H NMR (CD3OD, ppm):
(m, 4 H), 7.42 (m, 6 H), 7.32 (s, 2 H), 4.50 (m, 4 H, J ) 4.2
Hz), 3.83 (t, 4 H, J ) 4.2 Hz), 3.52 (q, 12 H, J ) 7.2 Hz), 1.07
(t, 18 H, J ) 7.2 Hz). 13C NMR (CD3OD, ppm): δ 154.0, 132.6,
130.1, 129.8, 124.1, 118.4, 115.2, 96.3, 86.2, 64.1, 57.1, 55.0,
8.1.
δ 7.37 (br, 2 H), 7.27 (br, 2 H), 4.57 (br, 4 H), 4.29 (br, 4 H),
3.94 (br), 3.78 (br), 3.63 (br), 3.52 (br), 3.35 (br), 1.37 (br, 17.1
H). 13C NMR (CD3OD, ppm): δ 153.3, 152.7, 118.1, 117.9,
114.3, 92.3, 91.4, 72.5, 71.5, 71.3, 71.0, 69.7, 64.8, 63.6, 60.0,
58.6, 56.2, 54.2, 51.8, 49.5, 9.2, 7.4. FT-IR (KBr pellet, cm-1):
3430 (br), 3056, 2980, 2932, 2880, 2650, 2483, 2203, 1508,
1464, 1423, 1398, 1275, 1218, 1097, 1052, 1026, 949, 852, 790,
716, 527. Anal. Calcd for C42H62N2O10‚1.7C2H5Br‚H2O: C,
56.91; H, 7.63; N, 2.92; Br, 14.18. Found: C, 58.77; H, 7.75;
N, 2.67; Br, 15.50.
General Procedure for the Preparation of P1, P2, and
P3. Under argon protection, diisopropylamine/toluene (3:7, 35
mL) was added to a 50 mL round-bottom flask containing a
0.272 g (0.765 mmol) sample of monomer 2, 0.75 mmol of
diiodobenze monomer (0.420 g of monomer 1, 0.491 g of 1,4-
diiodo-2,5-bis{2-[2-(2-methoxyethoxy)ethoxy]ethoxy}benzene
or 0.524 g of 2,5-diiodo-1,4-bis(dodecyloxy)benzene), 51.9 mg
(0.045 mmol) of Pd(PPh3)4, and 42.8 mg (0.225 mmol) of CuI.
The mixture was heated at 70 °C for 24 h and then subjected
to a CHCl3/H2O workup. The combined organic phase was
washed with water NH4OH (50%) twice, water twice, and brine
once and dried over MgSO4. The solution was removed in
vacuo, and the residue was redissolved in 10 mL of CHCl3 and
reprecipitated in methanol twice. The mixture was filtered to
afford a yellow solid.
P3′: Most of the bromoethane and THF was evaporated and
then the quaternized polymer was precipitated in 100 mL of
acetone, collected by centrifugation and dried overnight in
1
vacuo at 50 °C (0.392 g, yield 77%). H NMR (CD3OD, ppm):
δ 7.34 (br, 2 H), 7.19 (br, 2 H), 4.54 (br, 4 H), 4.12 (br, 4 H),
3.74 (br, 4 H), 3.51 (br, 11.8 H), 1.85 (br, 4 H), 1.57 (br, 4 H),
1.37 (br), 1.27 (br), 0.89 (br, 6 H). 13C NMR (CD3OD, ppm): δ
154.2, 153.3, 117.8, 114.9, 91.2, 70.3, 64.8, 54.3, 51.3, 49.3, 32.1,
29.8, 29.6, 26.3, 22.8, 13.5, 8.7, 7.2. FT-IR (KBr pellet, cm-1):
3420 (br), 3056, 2968, 2924, 2853, 2627, 2475, 2203, 1510,
1467, 1422, 1392, 1271, 1214, 1057, 1020, 947, 863, 798, 718,
517. Anal. Calcd for C52H82N2O4‚1.9C2H5Br‚2.5H2O: C, 63.75;
H, 9.25; N, 2.66 Br, 14.44. Found: C, 61.68; H, 8.47; N, 2.78;
Br, 14.92.
1
P1: 0.42 g (yield 85%). H NMR (CDCl3, ppm): δ 7.04 (s, 2
H), 4.12 (t, 4 H), 2.95 (t, 4 H), 2.70 (q, 8 H), 1.08 (t, 12 H). 13
C
NMR (CDCl3, ppm): δ 153.8, 117.5, 114.6, 91.9, 68.8, 52.1,
48.4, 12.6. FT-IR (KBr pellet, cm-1): 3429 (br), 3055, 2967,
2930, 2872, 2816, 2199, 1512, 1464, 1426, 1379, 1275, 1211,
1042, 953, 860, 802, 740, 717, 510. Anal. Calcd for
C20H30N2O2: C, 72.69; H, 9.15; N, 8.48. Found: C, 71.95; H,
8.68; N, 8.04.
Results and Discussion
Synthesis of Monomers and Polymers. The prepa-
ration of monomers is shown in Scheme 1. 2,5-
Bis[3-(N,N-diethylamino)-1-oxapropyl)-1,4-diiodoben-
zene] (monomer 1) was prepared from 2,5-diiodohydro-
quinone by a reaction with 2-(diethylamino)ethyl chlo-
ride hydrochloride in refluxing acetone in the presence
of excess anhydrous potassium carbonate. Conse-
quently, treating monomer 1 with trimethylsilyl acety-
lene afforded di(trimethylsilyl)ethynyl compounds which
was further converted to the key monomer 2 under base
treatment. Syntheses of the polymers are outlined in
Scheme 2. The preparation of neutral polymers P1-
P3 was accomplished via Sonogashira reaction of cor-
responding monomers with key monomer 2 in the
mixture of toluene and diisopropylamine solution in the
presence of Pd(PPh3)4/CuI catalyst at 70 °C for 1 day.
Conversion of the neutral polymers to the final cationic
polymers was achieved by treating P1-P3 with bro-
moethane in the mixture of DMSO and THF (1:4) at 50
°C for 5 days. In contrast to the direct synthesis of
cationic PPE by Swager et al.,26 quaternization after
polymerization was adopted to obtain cationic PPEs in
our present work because those neutral analogues can
be purified and analyzed by a conventional method.28,29
Furthermore, the tunable quaternization degree of
cationic PPEs obtained by quaternization after polym-
1
P2: 0.46 g (yield 81%). H NMR (CDCl3, ppm): δ 7.07 (s, 2
H), 7.04 (s, 2 H), 4.24 (br, 4 H), 4.15 (br, 4 H), 3.93 (br, 4 H),
3.80 (br, 4 H), 3.65 (br, 8 H), 3.53 (br, 4 H), 3.36 (s, 6 H), 2.99
(br, 4 H), 2.71 (br, 8 H), 1.09 (t, 12 H). 13C NMR (CDCl3,
ppm): δ 153.8, 118.1, 117.5, 114.9, 114.6, 92.0, 72.1, 71.7, 71.2,
71.0, 70.5, 70.3, 68.8, 52.0, 48.4, 12.6. FT-IR (KBr pellet, cm-1):
3483 (br), 3056, 2967, 2929, 2873, 2816, 2200, 1510, 1456,
1425, 1372, 1277, 1218, 1107, 1042, 953, 858, 803, 741, 717,
512. Anal. Calcd for C42H62N2O10: C, 66.82; H, 8.28; N, 3.71.
Found: C, 64.15; H, 7.90; N, 3.28.
1
P3: 0.48 g (yield 80%). H NMR (CDCl3, ppm): δ 7.06 (s, 2
H), 7.03 (s, 2 H), 4.14 (br, 4 H), 4.05 (br, 4 H), 2.99 (br, 4 H),
2.70 (q, 8 H), 1.88 (br, 4 H), 1.52 (br, 4 H), 1.43-1.22 (br, 32
H), 1.09 (t, 12 H), 0.89 (t, 6 H).13C NMR (CDCl3, ppm): δ 153.7,
117.5, 114.6, 91.9, 70.0, 68.8, 51.9, 48.3, 32.0, 29.8, 29.7, 29.5,
29.4, 26.0, 22.7, 14.1, 12.2. FT-IR (KBr pellet, cm-1): 3446 (br),
3057, 2964, 2924, 2868, 2815, 2200, 1510, 1463, 1428, 1386,
1278, 1213, 1043, 955, 861, 804, 740, 721, 511. Anal. Calcd
for C52H82N2O4: C, 78.15; H, 10.34; N, 3.51. Found: C, 76.64;
H, 9.80; N, 3.08.
General Procedure for the Preparation of P1′, P2′, and
P3′ via Quaternization of the Neutral Polymers (P1, P2,
and P3, Respectively). A 50 mL round-bottom flask with a
magnetic spin bar was charged with 1 mmol (based on repeat
unit) of neutral polymer (0.330 g of P1, 0.378 g of P2, or 0.400
g of P3). The polymer was dissolved in 20 mL of THF. To this
was added bromoethane (1.09 g, 10 mmol) and 5 mL of DMSO.
The solution was stirred at 50 °C for 5 days. P1′: During the

2930 Fan et al.
Macromolecules, Vol. 38, No. 7, 2005
Scheme 1. Synthetic Route for Monomers and Model Compound M1′
erization makes them attractive for applications in
organic LEDs and as fluorescence biosensors through
layer-by-layer self-assembly.30 Gel permeation chroma-
tography reveals that all of the neutral polymers showed
reasonably high molecular weight (see Table 1). Ther-
mogravimetric analysis (see Figure 1) shows Td at
nearly 200 °C of those quaternized polymers made them
enough serve as chemo- or biosensor because all of
applications proceed at room temperature.
To compare the effect of hydrophilicity of the side
chains on water solubility of the polymers, dodecyloxy
or tri(ethylene glycol)methyl ether groups were also
employed, as our group previously adopted in the
cationic PPV system.31 All the neutral polymers P1-
P3 were readily soluble in common organic solvents
such as chloroform and THF but insoluble in methanol,
DMSO and DMF. P1′ and P2′ exhibited good solubility
in both methanol and water, while P3′ could be dis-
solved in methanol but not water due to the existence
of long hydrophobic side chains.31
The quaternization degrees (QDs) of P1′-P3′ could
be determined by 1H NMR spectra. As shown in Figure
2, the neutral polymer P1 exhibited two well-resolved
peaks at 2.95, and 2.70 ppm corresponding to the
methylene groups adjacent to the nitrogen (-CH2N-)
atoms. After the treatment with bromoethane, all
signals corresponding to -CH2N- split into two peaks,
which arise from the quaternized (low field) and un-
quaternized components.29,30 The relative integrals of
each pair of the split peaks can thus be used to estimate
the QD of P1′ which was 45%. However, for P2′ and
P3′, the QD of P2′ (85%) was determined by the relative
integrals of the methylene (-NCH2CH2O-) and methyl
(-NCH2CH3) peaks and that of P3′ (95%) was calcu-
lated from the relative integrals of the methylene (in
-OCH2CH2N-) and methylene (in -NCH2CH3) peaks.
The higher QDs of P2′ and P3′ than that of P1′,
combined with the fact that P1′ precipitated while P2′
and P3′ showed good solubility during their quaterniza-
tion process, indicated that, under the same quater-
nization conditions, the existence of longer side chains
may be more beneficial to the completion of the quat-
ernization. It is noteworthy that although the quater-
nization degree of P1′ is only 45%, much lower than
what we expected, 1H NMR measurement showed that
water is a better solvent for P1′ than for P2′ and other
reported cationic PPE derivatives with similar struc-
ture26 as P2′, indicating clearly that introducing more
percentage of tertiary amino groups into the side chains
of neutral PPE is very efficient to obtain cationic PPE
with intrinsical water solubility.
General Optical Properties. The UV-vis and pho-
toluminescence spectra of the neutral polymers in
CHCl3 are shown in Figure 3a. P1-P3 exhibited a
strong absorption peak occurring at 433, 430, and 444
nm respectively, while all the emission peaks of P1-
P3 appeared at about 472 nm with a vibronic band
shoulder at 506 nm. All absorption and emission spectra
were almost identical to what Wrighton et al. reported
previously,32 indicating that the electronic properties of
these conjugated polymers were predominantly gov-
erned by the rigid-rod and highly conjugated polymer
backbone and ancillarily influenced by the nature of the
attached side chains.
Figure 3b shows the UV-vis and photoluminescence
spectra of the quaternized polymers in CH3OH and
water. In comparison with that of the neutral polymers,
the absorption and emission peaks of all quaternized
polymers present obvious blue-shift (detailed data please
see Table 1). This is resulted from the mutual repulsion
among the positive charges leading to a more twisted
main chain conformation, and hence a decreased effec-
tive conjugation length.28-31 Meanwhile, the absorption
and emission maxima of those quaternized polymers are

Macromolecules, Vol. 38, No. 7, 2005
Water-Soluble Cationic PPEs 2931
Scheme 2. Synthetic Routes for Neutral and Quaternized PPEs
Table 1. Characterization of Neutral Polymers and Quaternized Polymersa
GPC
absorption λmax/nm
emission λmax/nm
polymer
Mn
Mw
PDI
CHCl3
CH3OH
H2O
CHCl3
CH3OH
H2O
P1
P2
P3
P1′
P2′
P3′
26 700
20 300
47 100
61 200
46 700
138 800
2.29
2.30
2.95
433
430
444
471
470
474
403
416
426, 480
378
404
457
463
466, 496
444
457
a Mn, Mw, and PDI of the polymers were determined by gel permeation chromatography using polystyrene standards.
remarkably dependent on the solvent, showing an
obvious bathochromic shift with the decrease of solvent
polarity. Interestingly, a new absorption maximum was
observed at about 480 nm for P3′. Note that P3′ has an
alternatively hydrophilic-hydrophobic side chain pat-
tern, the conjugated segments may have a strong
tendency to form interchain aggregates.22 An aggrega-
tion absorption at about 470 nm was previously ob-
served by Swager et al. in investigating the LB films of
a series of PPE derivatives, which is quite close to the
value in our system.33 In addition, the formation of
interchain aggregation can be further evidenced by the
fact that such a new absorption peak could disappear
thoroughly by adding THF (good solvent for those
hydrophobic groups) while showed no response to the
introduction of water (good solvent for those hydrophilic
groups). The abovementioned results strongly support
our previous conclusion that increasing the percentage
of hydrophilic groups is beneficial to improving the
water-solubility of conjugated polymers.
pH-Influenced UV)Vis Absorption and Photo-
luminescence of P1′. It was found that the yellow color
of P1′ aqueous solution changed into light color when
adding HCl aqueous solution while turned into deep
yellow color after adding NaOH aqueous solution. This
color tunability was similar to that of water-soluble
anionic polythiophene by changing the countercation,34
indicating the obvious pH-induced variation of UV-vis
absorption for P1′.
Figure 4a shows the UV-vis absorption of P1′ aque-
ous solution with different pH. As shown in Figure 4a,
the absorption maximum was slightly blue-shifted when
pH < 7.0, indicating the formation of a less effective
conjugated length in the backbone. Because of the
existence of unquaternized amino groups (55%) in P1′,
adding acid was considered to acidify those unquater-

2932 Fan et al.
Macromolecules, Vol. 38, No. 7, 2005
addition, the absorption maximum of P1′ red-shifted
obviously but no new absorption maximum appeared in
redder region in an alkaline environment at pH < 13.
However, when pH increased to 14, a new absorption
peak appeared (λmax ) 476 nm) after the initial absorp-
tion maximum ended its red shift at λmax ) 408 nm,
indicating that the interchain aggregation was formed.
This phenomenon suggests that the planarization of P1′
conjugated backbone exist before its formation of inter-
chain aggregation with pH increase, which is different
from the results reported earlier where the formation
-
of interchain aggregation for PPE-SO3 with pH de-
crease was the only origin of the spectra shift.10
The investigation of the pH-influence on emission
maxima of P1′ further demonstrated the above conclu-
sion. In Figure 4b, emission maximum little shifted in
an acidic environment while red-shifted gradually in an
alkaline environment. Combined with the gradual red-
shift without the formation of a new absorption and
emission peak, the maximal emission ∆λmax ≈ 20 nm
in an alkaline environment at pH < 13 which is similar
to the red shift of the PPE model compound by
planarization35 whereas lower than the reported red-
Figure 1. Thermalgravimetric analyses of the neutral and
quaternized PPEs.
-
shift (> 70 nm) of anionic PPE-PO3 from interchain
aggregation10,22 strongly evidenced the formation of
planarization of the conjugated backbone, not interchain
aggregation. Time-resolved photoluminescent measure-
ment excited at 380 nm in P1′ aqueous solution with
different pH (see Table 2) revealed a rapid monoexpo-
nential decay and no long-lived fluorescence (<1 ns, due
to interchain aggregation).10,22 This further confirmed
the nonexistence of interchain aggregation in an alka-
line environment at pH < 13. When pH increased to
14, a new emission peak (λmax ) 494 nm, ∆λmax ≈ 50
nm) and another long-lived fluorescence (≈6 ns) ap-
peared, indicating the formation of interchain aggrega-
tion. We believe that when pH increased, the positive
charge on the side chain was partially neutralized by
OH- (R4N+ + OH- T R4NOH). Consequently, the
decreased charge density on polymer chain reduced the
mutual repulsion among positive charges28-31 and re-
sulted in the planarization of the conjugated backbone.
As pH continuously increased, the planarization of the
conjugated backbone reached saturation and then the
interchain aggregation was formed to retain the water-
solubility of P1′.
The pH-influenced fluorescence intensity of P1′ aque-
ous solution is described in Figure 4c. When pH < 7,
the fluorescence intensity of P1′ enhanced gradually and
reached a plateau at about pH ) 4.5. A previous study
by Wrighton et al. showed that in comparison with
alkoxy-substituted PPEs, the PPEs functionalized with
tertiary amino group exhibited significantly lower PL
intensity due to the interaction of the nonbonding
electron pair on the nitrogen atom with the conjugated
π-system which results in quenching of the excited state
through intramolecular charge transfer.32 Hence, the
enhanced fluorescence intensity in an acidic environ-
ment can be attributed to the decreased interaction, by
protonation, of those unquaternized tertiary amino
group (55%) with the π-conjugated backbone. It is
consistent with the higher fluorescence intensity of
protonated amino-functionalized poly(p-phenylene)s than
neutral amino-functionalized poly(p-phenylene)s re-
ported by Reynolds et al.28
Figure 2. 1H NMR spectra of neutral polymers P1-P3 in
CDCl3 and quaternized polymers P1′ in D2O and P2′-P3′ in
CD3OD.
nized amino groups into cationic ammonium groups and
thus enhance the strong electrostatic repulsion among
the positive charges which resulted in the more torsional
angle of the conjugated backbone.28-31 The increased
torsional structure due to the free internal rotation of
alkyne-aryl single bond shortened the efficient π-π
conjugation system of PPE, and led to a blue-shift of
absorption maximum.
After dropping NaOH aqueous solution (0.1 M) into
P1′ solution at a high concentration (0.01 M), the P1′
solution color initially changed into deep yellow color,
and then an orange solid was precipitated from P1′
solution. To clarify if such a precipitate resulted from
the destruction of the chemical structure of P1′ under
an alkaline environment, the HCl solution was added
to neutralize NaOH solution. It was showed that after
being neutralized by HCl solution, the precipitate disap-
peared and was redissolved in aqueous solution. With
the nonchanged NMR spectra of P1′ after such a
treatment, it can be demonstrated that P1′ is not
damaged by NaOH. Thus, the precipitate with pH
increase can be attributed to the formation of aggrega-
tion.10 Furthermore, we found that upon dropping
NaOH aqueous solution into P1′ solution at a much
lower concentration (5 µM), a deeper color of P1′ solution
also became visible but no precipitate appeared. In
When pH > 7, the fluorescence intensity of P1′ started
to decrease swiftly at pH ) 9.0 and arrived at the first

Macromolecules, Vol. 38, No. 7, 2005
Water-Soluble Cationic PPEs 2933
Figure 3. UV-vis absorption and PL emission spectra of (a) neutral polymers P1-P3 in chloroform and (b) quaternized polymers
P1′-P3′ in water or methanol.
4-
Figure 4. pH dependence of UV-vis absorption maxima, emission maxima, PL intensity, and fluorescecne quenching by Fe(CN)6
of P1′ in aqueous solution with various salt concentrations. Key: (-9-) [NaCl] ) 0 M; (-b-) [NaCl] ) 0.1 M; (-2-) [NaCl] )
1 M. The new observable absorption and emission maxima: (-O-) [NaCl] ) 0.1 M; (-4-) [NaCl] ) 0.1 M. In Figure 4c, PL0
refers to the fluorescence intensity of P1′ aqueous solution at pH ) 7 and [NaCl] ) 0 M.
low point (0.4, compared with the pristine one at pH )
7) at about pH ) 10. As we discussed previously, the
planarization of the conjugated backbone, not interchain
aggregation, formed in an alkaline environment at pH
< 13. Thus, the general explanation that the reduced
fluorescence intensity of anionic PPE in aqueous solu-
tion resulted from interchain aggregation22 was not
forth increased the activity of the unquaternized tertiary
amino group, which led to its enhanced quenching
ability on the conjugated backbone. When pH increased
to 14, fluorescence intensity dramatically decreased to
a new nadir (PL/PL0 ) 0.12), arising from the formation
of interchain aggregation. Furthermore, the pH-influ-
enced fluorescence intensity of P1′ may be evident in
fluorescence lifetime data (see Table 2). P1′ showed a
longer fluorescence lifetime (τ ) 0.57 ns) at pH ) 4 than
that at pH ) 7 (τ ) 0.48 ns) while a shorter lifetime (τ
) 0.42 ns) at pH ) 10; all of this is in accordance with
the enhanced fluorescence intensity in an acidic envi-
ronment and decreased fluorescence intensity in an
alkaline environment and strongly indicated the exist-
ence of pH-dependent excited-state quenching.
1
appropriate for our system. H NMR spectra in Figure
2 show that the signal of unquaternized tertiary amino
group of P1′ shifted significantly to the lower field
compared with that of tertiary amino group on its
neutral analogue P1, and closed tightly to the signal of
quaternized ammonium group of P1′. This indicates the
reduced electron cloud density of the nonbonding elec-
tron pair on the nitrogen atom of unquaternized tertiary
amino group under the positive-charge-rich environ-
ment. Thus, it was suggested that the neutralization of
quaternized ammonium group by OH- in an alkaline
environment reduced positive charge density, and hence-
The possible influence on the spectral changes of
conjugated polymers by ionic strength has been studied
by several researchers.3,36 Herein, we studied the influ-
ence of ionic strength on the optical properties of P1′

2934 Fan et al.
Macromolecules, Vol. 38, No. 7, 2005
Table 2. Photophysical Properties and Fluorescence Quenching of P1′ in Aqueous Solution with Different pH and Ionic
Strengtha
absorption
emission
τ/ns at 450 nm
τ/ns at 500 nm
pH
4
[NaCl]/M
λmax/nm
λmax/nm
(amplititude/%)
(amplititude/%)
Ksv/M-1
0.0
0.1
1.0
0.0
0.1
1.0
0.0
0.1
1.0
0.0
0.1
1.0
375
376
390
378
385
405
390
405
405, 478
408, 476
407, 478
408, 478
444
445
454
444
445
454
462
464, 494
464, 494
464, 494
464, 495
465, 496
0.57 (100)
0.55 (100)
0.51 (100)
0.48 (100)
0.53 (100)
0.50 (100)
0.42 (100)
0.40 (90), 5.46 (10)
0.39 (81), 4.76 (19)
0.39 (79), 5.70 (21)
0.40 (75), 6.01 (25)
0.37 (73), 6.18 (27)
0.59 (100)
0.56 (100)
0.54 (100)
0.50 (100)
0.56 (100)
0.52 (100)
0.50 (100)
0.55 (55), 5.80 (45)
0.49 (53), 5.32 (47)
0.57 (48), 6.50 (52)
0.55 (45), 6.48 (55)
0.50 (45), 6.60 (55)
3.8 × 106
9.4 × 105
3.8 × 105
4.1 × 106
9.6 × 105
4.2 × 105
6.0 × 106
7.7 × 106
2.2 × 106
2.0 × 107
1.1 × 107
2.7 × 106
7
10
14
a pH was adjusted with HCl and NaOH aqueous solutions at different concentrations (1, 1 × 10-1 M, 1 × 10-2 M, 1 × 10-3 M, 1 × 10-4
M, and 1 × 10-5 M).
the polymer chains soluble in water.10 In addition, the
planarization of P1′ backbone induced by OH- and salts
further afforded a good precondition for the formation
of interchain π-aggregation.32
In conclusion, from Figure 4, it is visible that upon
increasing pH or adding salts or both, the absorption
and emission peak always initially red shifted and
stopped at one position (λmax ≈ 408 nm for UV and λmax
≈ 464 nm for PL), and then a new peak appeared on a
redder position (λmax ≈ 478 nm for UV and λmax ≈ 496
nm for PL). This clearly indicates that the pH- and salt-
induced conformational change occurred from the pla-
narization of the conjugated backbone to the interchain
aggregation.
Interestingly, when NaCl solution was used as a
fluorescence buffer solution for P1′, the H+-induced
Figure 5. UV-vis absorption and PL emission spectra of of
P1′ in aqueous solution with various pH and salt concentra-
fluorescence enhancement disappeared. The presented
tions.
stable fluorescence intensity at pH e 9 in buffer solution
(see Figure 4c) made it very attractive in practical
aqueous solution at different pH, which has never been
applications in a broader pH region. Besides, it was
investigated before. As shown in Figure 4, parts a and
found that the fluorescence intensity first increased
b, the absorption and emission maxima red shifted with
when [NaCl] ) 0.01 M and then decreased when [NaCl]
the increase of ionic strength. It is noteworthy that
increased to 0.1 M. At present, we cannot explain those
when adding salts at pH ) 7, the absorption maxima
phenomena related to ionic strength while the influence
only red shifted to 405 nm and no new absorption and
from ionic strength on the quenching of conjugated
emission peak appeared at redder region, showing that
backbone by nonbonding electron pair may exist. In
adding salts at a neutral environment only leads to the
contrast to the stable fluorescence intensity at pH e 9,
planarization of P1′ conjugated backbone. This result
the fluorescence intensity of P1′ decreased swiftly at pH
can be explained in terms of the effective screening of
) 10 after increasing ionic strength. Because of the
formation of nonemissive decay channels at interchain
aggregated state, we thus attribute the dramatic de-
crease of fluorescence intensity to the enhanced inter-
chain aggregation at pH ) 10.1 In addition, the more
extended π-conjugation obtained from the interchain
aggregation may also be conducive to enhance the
quenching ability of tertiary amino group on conjugated
backbone, giving rise to more fluorescence quenching.22
Stern-Volmer Study at Different pH. One prom-
ising application of FWSCPs as highly sensitive chemo-
or biosensors was found to be based on their rapid and
collective response to relatively small perturbations in
local environment.37 A low concentration of quencher
is sufficient to extinguish the fluorescence from the
conjugated segments through electron transfer or en-
ergy transfer due to facile energy migration along the
conjugated backbone, and relatively strong binding of
the quencher with FWSCPs by electrostatic attraction.
Therefore, to evaluate the amplified quenching effect
of our water-soluble cationic PPE, the fluorescence
electrostatic repulsion3 between positive ammonium
groups by adding ions which led to the planarization of
the conjugated backbone. However, when adding salts
at pH ≈ 10, new absorption (≈478 nm) and emission
(≈495 nm) maxima started to appear (see Figure 5). The
redder absorption and emission maxima, close to that
of P1′ at pH ) 14 belonging to interchain aggregation,
strongly implied the formation of salt-assisted inter-
chain aggregation for P1′. Time-resolved photolumines-
cent measurement showed that after adding salts at pH
) 10, the fluorescence decay changed into biexponential
and can be fitted to two components with τ ≈ 0.4 and 5
ns (see Table 2). This observed new long lifetime
emission decay component further confirmed the exist-
ence of interchain aggregation.10,22 We have discussed
previously that when pH increased to 10 the planariza-
tion of P1′ backbone was formed. Thus, adding salts at
pH ) 10, the retained charge of P1′ was screened and
the hydrophilicity of P1′ further decreased. Conse-
quently, the polymer backbones tended to aggregate in
order to optimize their hydrophobic properties and keep

Macromolecules, Vol. 38, No. 7, 2005
Water-Soluble Cationic PPEs 2935
The Ksv of P1′ vs pH was depicted in Figure 4d. When
pH < 7, although the fluorescence intensity was en-
hanced, Ksv was nearly unchanged, indicating that the
addition of H+ hardly influenced the quenching ef-
ficiency of P1′. However, the Ksv slightly increased in
an alkaline environment at pH < 13. Recent studies
have reported that conjugated polymers have more
amplified quenching than conjugated oligomer, because
their extended π-aggregation enhanced the delocaliza-
tion of singlet exciton and the rapidness of the energy
migration.1 Thus, we suggest that the amplified quench-
ing efficiency result from the OH--induced extended
conjugated length. After adding salts at pH ) 7, Ksv
decreased with the increase of ionic strength (see Table
2). This result is consistent with the notion that add
ions will screen the electrostatic attraction between
conjugated polyelectrolyte and quencher with counter-
ions and remove quencher from the vicinity of conju-
gated polyelectrolyte and finally decrease the Ksv.
Adding salts at pH ) 10 or increasing pH to 14 resulted
in a more enhanced quenching ability, which can be
explained by the formed interchain aggregation that
could lead to further amplification of fluorescence
quenching through the occurring of interchain exciton
migration.1,22
Conclusions
In summary, we report on the successful syntheses
of a series of cationic PPEs. P1′, prepared through
quaternization of the neutral PPE with tertiary amino
group in all side chains, exhibits intrinsical water-
solubility compared with other cationic PPEs P2′ and
P3′ containing side chains with different hydrophilicity.
Interchain aggregation was observed for P3′ in metha-
nol, which may result from the presence of long chain
hydrophobic groups. Study on the variation of the
absorption and emission spectra of P1′ in aqueous
solution with pH exhibits that with pH increase, the
planarization of the conjugated backbone is first formed
before the formation of interchain aggregation. Besides,
the pH-influenced fluorescence intensity of P1′ was also
found and explained by pH-influenced quenching of
π-conjugated backbone by unquaternized tertiary amino
group. Such a pH-influenced fluorescence intensity can
be efficiently buffered by adding salts at pH < 10.
Although adding salts in a neutral environment only
creates the planarization of P1′ conjugated backbone,
adding salts in an alkaline environment significantly
promotes the formation of interchain aggregation. The
investigation of fluorescence quenching of P1′ by
4-
Figure 6. (a) Stern-Volmer plot of P1′ quenched by Fe(CN)6
in aqueous solution at pH ) 7 and [NaCl] ) 0 M. Inset: the
4-
part of Stern-Volmer plot of P1′ quenched by Fe(CN)6 at
low concentration. (b) Stern-Volmer plot of model compound
4-
M1′ quenched by Fe(CN)6 in aqueous solution.
quenching of P1′ by anionic quencher Fe(CN)6 was
4-
compared with its model compound M1′. Figure 6
showed the Stern-Volmer plots of P1′ and M1′ quenched
4-
by Fe(CN)6 in aqueous solution. As shown in Figure
6, P1′ exhibited obviously amplified quenching that the
quenching of P1′ by anionic Fe(CN)64- (Ksv ) 4.1 × 106
M-1) is three orders more efficient than its model
compound M1′ (Ksv ) 1.3 × 103 M-1) due to the high
delocalization of singlet exciton and the rapidness of the
energy migration along the conjugated backbone of
P1′.1,21a,21b The upward Stern-Volmer curve which was
observed for the fluorescence quenching of P1′ is dif-
ferent from the straight line for M1′. This significant
difference can be explained by the existence of sphere-
of-action for polymer chain in aqueous solution,3 which
could be described by the modified Stern-Volmer equa-
tion:
4-
Fe(CN)6 exhibits that such a water-soluble cationic
PPE possesses amplified sensitivity on anionic quench-
er. In addition, its Ksv highly increased in an alkaline
environment due to the elongated π-conjugation from
the planarization of conjugated backbone and the in-
terchain aggregation.
F0
) (1 + KSsv[Q])
F
unmodified Stern-Volmer equation
(straight line)38
Acknowledgment. This work was financially sup-
ported by the National Natural Science Foundation of
China under Grants 60325412, 90406021, and 50428303
as well as the Shanghai Commission of Science and
Technology under Grants 022261042, 03DZ11016, and
04XD14002 and the Shanghai Commission of Education
under Grant 2003SG03.
F0
) (1 + KsSv[Q])eRV[Q]
F
modified Stern-Volmer equation (upward curve)
where F0 is the fluorescence intensity, F is the fluores-
cence intensity, [Q] is the quencher concentration, Ksv
is the Stern-Volmer constant, V is the volume constant,
and R is used to account for the charge-induced en-
hancement of the local quencher concentration.
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