Design and synthesis of fluorescence "turn-on" chemosensors based on photoinduced electron transfer in conjugated polymers

Article abstract of DOI:10.1021/ma047983v

A new approach to fluorescence "turn-on" chemosensors based on a photoinduced electron transfer (PET) strategy involving conjugated polymers has been developed. Two new conjugated polymers dea-PPETE and tmeda-PPETE were synthesized and characterized. These two polymers use diethylamino and N,N,N'-trimethylethylenediamino as receptors, respectively, on a poly[p-(phenyleneethylene)-alt-(thienyleneethynylene)] (PPETE) fluorescent conjugated polymer backbone. The polymers were found to be relatively weakly emissive at λ_max ～ 488 nm with quantum yields of 0.11 and 0.09, respectively, at room temperature in THF solution. Initial investigations show that the tmeda-PPETE selectively detects some metal cations by an observed increase in fluoresence. In particular, Hg²⁺ in aqueous solution causes the fluorescence of tmeda-PPETE to increase by a factor of 2.7 at less than micromolar concentrations. The photophysical results are consistent with a PET mechanism for fluorescence quenching, which is removed upon binding of the analyte.

Full text of DOI:10.1021/ma047983v

2844
Macromolecules 2005, 38, 2844-2849
Design and Synthesis of Fluorescence “Turn-on” Chemosensors Based
on Photoinduced Electron Transfer in Conjugated Polymers
Li-Juan Fan, Yan Zhang, and Wayne E. Jones, Jr.*
Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13902
Received September 30, 2004; Revised Manuscript Received January 26, 2005
ABSTRACT: A new approach to fluorescence “turn-on” chemosensors based on a photoinduced electron
transfer (PET) strategy involving conjugated polymers has been developed. Two new conjugated polymers
dea-PPETE and tmeda-PPETE were synthesized and characterized. These two polymers use diethylamino
and N,N,N′-trimethylethylenediamino as receptors, respectively, on a poly[p-(phenyleneethylene)-alt-
(thienyleneethynylene)] (PPETE) fluorescent conjugated polymer backbone. The polymers were found to
be relatively weakly emissive at λmax ∼ 488 nm with quantum yields of 0.11 and 0.09, respectively, at
room temperature in THF solution. Initial investigations show that the tmeda-PPETE selectively detects
some metal cations by an observed increase in fluoresence. In particular, Hg2+ in aqueous solution causes
the fluorescence of tmeda-PPETE to increase by a factor of 2.7 at less than micromolar concentrations.
The photophysical results are consistent with a PET mechanism for fluorescence quenching, which is
removed upon binding of the analyte.
Introduction
process. However, this report did not extend this
methodology to sensors.
Fluorescent chemosensors are gaining increased at-
tention due to their high sensitivity and ease of mea-
surement.1 In particular, conjugated polymer chemosen-
sors have recently been used with great success for the
detection of a range of analytes from biomolecules to
explosives.2 Fluorescent conjugated polymers have sev-
eral advantages over small molecule sensors due to
enhancements associated with electronic communication
between receptors along the polymer backbone,2 pro-
cessability, and ease of structural modification. The vast
majority of the detection methods employed involve a
“turn-off” effect in the presence of analytes.2-4 A more
sensitive detection method would involve design of a
“turn-on” fluorescent sensor. While the “turn-on” ap-
proach has been demonstrated for a number of small
molecule sensors, we find only one literature example
reporting conjugated polymers as fluorescence “turn-
on”chemsensor.5
Photoinduced electron transfer (PET) sensors are an
important family of chemosensors,6 and small molecule
fluorescence turn-on sensors have been developed and
studied on the basis of the PET mechanism.7 In such
systems, the receptors usually contain a relatively high-
energy nonbonding electron pair. In the absence of
analytes, this electron pair quenches the fluorescence
of the fluorophore by rapid intramolecular electron
transfer from the receptor to the excited fluorophore,
as shown in Figure 1. When this electron pair is
coordinated to Lewis acid cations in solution, the HOMO
of the receptor is lowered. This decreases the driving
force for the PET process and can turn on the fluores-
cence of the chromophore. One literature example8 of
PET in polymer chemosensors involves incorporation of
dyes into the polymer backbones via copolymerization
with other monomers. The polymer backbones were
used simply as inert scaffolds to position the fluoro-
phores and receptors. To our knowledge, there is only
one literature report9 using the conjugated polymer
backbone directly as the fluorophore for the PET
By combining the transducer and receptor compo-
nents on the molecular wire polymer backbone, a more
versatile sensory system can be prepared. Previous
conjugated polymer-based fluorescent sensors have
taken advantage of the rapid mobility of the exciton
along the polymer backbone to yield enhanced sensitiv-
ity. This mechanism will not provide a sensitivity
enhancement in a PET-based fluorescence turn-on sen-
sor. However, it has previously been proposed that the
support of small molecule sensors on polymer substrates
will yield more processable and useable systems.8,10
Further, we have recently shown that the changes in
the conjugated polymer backbone can be used to en-
hance the selectivity of the system.4b Thus, we anticipate
both processing- and selectivity-based enhancements as
a result of combining the PET receptors with the
conjugated polymer. Further, loading variations on the
conjugated polymer can also provide for a variable
fluorescence through changes in conjugation.
On the basis of our previous work using poly[p-
(phenyleneethylene)-alt-(thienyleneethynylene)] (PPETE)
as a conjugated polymer backone and functionalizing it
with different receptors,4 we have designed a strategy
to synthesize a series of fluorescence “turn-on” polymer
sensors with the PPETE polymer backbone as the
fluorophore and different amino groups as the receptors.
With this synthetic strategy we seek to develop an
addressable family of conjugated polymer PET sensors
that respond to different analytes by the “turn-on”
mechanism. Here we report the successful preparation
and characterization of the first two polymers synthe-
sized by this method, dea-PPETE and tmeda-PPETE.
These polymers use N,N-diethylamino and N,N,N′-
trimethylethylenediamino groups as receptors, respec-
tively (Scheme 1). The sensing responses of each poly-
mer toward several different cations were examined in
order to evaluate the potential applications of these
materials as PET sensors.
Experimental Section
Materials. All materials were purchased from Aldrich and
used as received unless otherwise noted. The compounds 1,4-
* Corresponding author: Tel 607-777-2421; Fax 607-777-4478;
e-mail wjones@binghamton.edu.
10.1021/ma047983v CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/09/2005

Macromolecules, Vol. 38, No. 7, 2005
Fluorescence “Turn-on” Chemosensors 2845
Figure 1. Orbital energy diagram for fluorescent PET sensors before and after binding cation.
Scheme 1. Synthesis of Conjugated Polymers Containing Amino Receptors
diethyl-2,5-didodecyloxybenzene11 and 3-bromomethylthio-
phene12 were synthesized according to the literature methods.
Satisfactory NMR characterization of all stable intermediates
was observed in each case.
mixture was acidified with 6 M HCl and the aqueous layer
washed with ether. The aqueous layer was made basic by
adding 20% Na2CO3 solution, and the mixture was extracted
with ether (2 × 50 mL). The combined organic layer was dried
over MgSO4. Removal of the solvent in vacuo gave a light
General Methods. 1H and 13C NMR spectra were recorded
on a Bruker AM-360 spectrometer. Elemental analyses were
performed by QTI Inc. in New Jersey. IR spectra were obtained
on a FT-IR Bruker Equinox55 spectrometer at a nominal
resolution of 2 cm-1. The samples were prepared by adding
monomers or polymers into KBr, and the mixture was ground
to a fine power and pressed to form disks. The molecular
weights and distribution were determined by gel permeation
chromatography (GPC) using a solution of 0.1 vol % triethy-
lamine in toluene as the mobile phase at 40 °C, relative to
polystyrene standards. The GPC instrument is equipped with
a Waters 510 pump, 410 differential refractive index detectors,
and Waters Styragel HT6, HT4, and HT2 (7.8 mm × 300 mm)
columns. UV-vis spectra were obtained on a Perkin-Elmer
Lambda 2S spectrophotometer in tetrahydrofuran (THF) solu-
tion using 1 cm quartz cuvette cells. Fluorescence spectra were
measured on a SLM 48000s spectrofluorometer using excita-
tion at 410 nm with 4 nm slits. The fluorescence solutions were
prepared as described previously.4b All the cationic solutions
(Hg2+, Hg2+, Zn2+, Ca2+) were prepared by dissolving their
chlorides in water. The quantum yield of fluorescence was
determined relative to quinine sulfate in 0.5 M H2SO4 solutions
with a quantum yield of 0.546, excited at 365 nm.13 Lifetime
experiments were carried out as described previously.4b
Synthesis. 2,5-Dibromo-3-bromomethylthiophene was pre-
pared by a modification of a previous literature report.14
Anhydrous sodium bicarbonate (4.20 g, 0.047 mol) was added
to a solution of 3-bromomethylthiophene (3.54 g, 0.020 mol)
in 50 mL of chloroform, followed by the dropwise addition of
bromine solution (8.16 g in 50 mL of CHCl3) over a period of
1 h. The reaction mixture was stirred overnight at room
temperature and then filtered. A highly lachrymatory brown
oil remained following vacumm removal of solvent and residual
bromine. The compound was previously reported to be unstable
due to the bromomethyl group on the thiophene ring.15
However, satisfactory NMR relative to previous literature
reports was obtained, and this material was used for the next
step immediately (5.29 g, 0.016 mol, yield 79%). 1H NMR (360
MHz, CDCl3): δH, 6.98 (s, 1H), 4.35 (s, 2H).
1
yellow oil (2.87 g, 8.8 mmol, yield 88%). H NMR (360 MHz,
CDCl3): δH, 6.98 (s, 1H), 3.44 (s, 2H), 2.51 (q, 4H), 1.03 (t, 6H)
ppm. 13C (360 MHz, CDCl3): 140.91, 131.66, 110.47, 108.92,
51.12, 46.79, 11.73 ppm. Elemental analysis: Calcd for C9H13-
Br2NS: C, 33.04%; H, 3.98%; N, 4.28%. Found: C, 32.62%; H,
3.69%; N, 4.15%. FTIR wavenumber (cm-1): 3061, 2969, 2934,
2873, 2808, 1576, 1556, 1544, 1383, 1205, 1079, 1022, 857, 806,
436.
N-(2,5-Dibromothiophen-3-ylmethyl)-N,N,N′-trimethylethane-
1,2-diamine. A mixture of 3.35 g (0.010 mol) of 2,5-dibromo-
3-bromomethylthiophene in 63 mL of ether, 3.88 mL (0.030
mol) of N,N,N′-trimethylethylenediamine, 38 mL of water, and
6.3 g of Na2CO3 was stirred for 19 h. The reaction mixture
was acidified with 6 M HCl, and the aqueous layer was washed
with ether. The aqueous layer was made basic by adding 20%
Na2CO3 solution, and the mixture was then extracted with
ether (2 × 50 mL). The combined organic layer was dried over
MgSO4. Removal of the solvent in vacuo gave a light yellow
1
oil (2.53 g, 7.1 mmol, yield 71%). H NMR (360 MHz CDCl3):
δH, 6.94 (s, 1H), 3.39 (s, 2H), 2.42-2.48 (t, 2H), 2.34-2.40 (t,
2H), 2.21 (s, 3H), 2.19 (s, 6H) ppm. 13C (360 MHz, CDCl3):
139.68, 131.64, 110.70, 109.87, 57.35, 55.83, 55.08, 45.78, 42.34
ppm. Calcd for C10H16N2Br2S: C, 33.71%; H, 4.49%; N, 7.87%.
Found: C, 33.54%; H, 4.36%; N, 7.62%. FTIR: wavenumber
(cm-1): 3095, 2917, 2943, 2765, 1544, 1463, 1428, 1356, 1168,
1127, 1034, 1004, 914, 839, 477.
dea-PPETE. Diisopropylamine (2 mL) was added to a
mixture of 2,5-dibromothiophen-3-ylmethyldiethylamine (0.327
g, 1 mmol), 1,4-diethyl-2,5-didodecyloxybenzene (0.494 g, 1
mmol), Pd(PPh3)4 (58 mg, 0.05 mmol), and CuI (20 mg, 0.10
mmol) in 10 mL of anhydrous THF under an argon atmo-
sphere. The mixture was refluxed for 24 h, and then chloroform
(20 mL) was added. The organic phase was washed twice with
dilute NaHCO3 solution. The solvent was removed under
reduced pressure, and the residue was washed with hot water
and hot methanol. The crude product was dissolved in chlo-
roform and then precipitated in methanol twice to give a brown
solid (0.579 g, yield 88%). 1H NMR (360 MHz, CDCl3): δH,
6.98-7.46 (3H), 3.95-3.99 (4H), 3.47-3.72 (2H), 2.49-2.58
(4H), 1.03-1.86 (52H). UV-vis λmax: 332, 436 nm. Emission
λmax: 486 nm. FTIR: the formation of internal ethynyl link
was confirmed by the presence of the 2193 cm-1 stretch.
2,5-Dibromothiophen-3-ylmethyldiethylamine. A mixture of
3.35 g (0.010 mol) of 2,5-dibromo-3-bromomethylthiophene in
63 mL of ether, 3 mL (0.030 mol) of diethylamine, 38 mL of
water, and 6.3 g of Na2CO3 was stirred for 19 h. The reaction

2846 Fan et al.
Scheme 2. Synthesis of Monomer
Macromolecules, Vol. 38, No. 7, 2005
the amino receptor connected to the thiophene ring by
a methylene spacer (monomer a). The 3-bromometh-
ylthiophene was synthesized from commercially avail-
able 3-methylthiophene with a free radical reaction
under nitrogen.12 Bromination of this compound with
Br2 in the presence of NaHCO3 gave 2,5-dibromo-3-
bromomethylthiophene, which reacted with a secondary
amine to give the desired monomer a.
The polymers were thoroughly characterized includ-
ing FTIR, NMR, GPC, and photophysical characteriza-
tion. The typical infrared stretching absorptions of the
terminal alkyne, including a strong, sharp absorption
at 3286 cm-1 for acetylenic C-H stretching vibration
and a weak, but sharp, absorption at 2106 cm-1 for the
CtC stretching vibration in the monomer, were absent
in both polymers. Instead, a broad, weak absorption
around 2193 cm-1 appeared, consistent with the forma-
tion of internal ethynyl link.17 The end groups C-H and
C-Br of these two polymers were below the signal-to-
tmeda-PPETE. Diisopropylamine (2 mL) was added to a
mixture of N-(2,5-dibromothiophen-3-ylmethyl)-N,N,N′-tri-
methylethane-1,2-diamine (0.356 g, 1 mmol), 1,4-diethyl-2,5-
didodecyloxybenzene (0.494 g, 1 mmol), Pd(PPh3)4 (58 mg, 0.05
mmol), and CuI (20 mg, 0.10 mmol) in 10 mL of anhydrous
THF under an argon atmosphere. The mixture was refluxed for
24 h, and then chloroform (20 mL) was added. The organic
phase was washed twice with dilute NaHCO3 solution. The sol-
vent was removed under reduced pressure, and the residue was
washed with hot water and hot methanol. The crude product
was dissolved in chloroform and then precipitated in methanol
twice to give a brown solid (0.564 g, yield 82%). 1H NMR (360
MHz, CDCl3): δH, 6.9-7.2 (3H), 3.4-3.7 (2H), 2.1-2.3 (9H),
2.3-2.6 (4H), 1.8-1.9 (46H). UV-vis λmax: 332, 446 nm. Emis-
sion λmax: 488 nm. FTIR: the formation of internal ethynyl
link was confirmed by the presence of the 2194 cm-1 stretch.
1
noise ratio in H NMR and 13C NMR, also indicating
the formation of polymer. There were also several weak
peaks in the aromatic region that can be assigned to
catalyst residue. For this polymerization methodology,
there has been no effective way to totally remove the
catalyst residue.18 To our surprise, there were three
peaks between 3.4 and 3.8 ppm where only one peak
was expected for the proton in the methylene spacer
between the thiophene ring and the amino receptor
(Figure 2). 13C DEPT (135°) and 2D heteronuclear (C,
H)-correlated NMR experiments show that these peaks
are related to a single secondary carbon. One explana-
tion could be that because the substituents on both
monomers are not very bulky and relatively flexible, the
polymerization process can take place via head-to-tail
or head-to-head-tail-to-tail. As the result of this regioi-
somerism, the methylene protons are in different elec-
tronic environments, which result in multiple peaks in
Results and Discussion
Synthesis. The polymers dea-PPETE and tmeda-
PPETE were prepared by a step growth polymerization
employing the palladium-catalyzed cross-coupling16 of
the receptor loaded monomer a and 1,4-diethynyl-2,
5-didodecyloxybenzene (monomer b), as shown in Scheme
1. Monomer b was synthesized from 1,4-hydroquinone
in four steps as described in the literature.11 dea-PPETE
and tmeda-PPETE are brown solids and very soluble
in common organic solvents such as THF and chloro-
form. Scheme 2 shows the three-step reaction used to
prepare the 2,5-dibrominated thiophene monomers with
1
the H NMR.
Figure 2. 1H NMR spectra of tmeda-PPETE and dea-PPETE.

Macromolecules, Vol. 38, No. 7, 2005
Fluorescence “Turn-on” Chemosensors 2847
Figure 3. UV-vis and emission spectra of model PPETE, dea-PPETE, and tmeda-PPETE in THF solution at room temperature.
Figure 4. (a) Fluorescence enhancement of tmeda-PPETE in THF upon addition of metal cations. (b) Emission spectra of tmeda-
PPETE upon addition of different concentration of Hg2+. Polymer concentration was held at constant 5.0 × 10-6 M in receptor
unit.
Table 1. Molecular Weight and Polydispersity of
Because of the strong interaction between the amino
group and the GPC column, the GPC experiments were
carried out using a solution of 0.1% triethylamine (TEA)
in toluene as the mobile phase. Reproducible negative
peaks were observed with a refractive index detector
for both polymers. The molecular weights were calcu-
lated relative to polystyrene standards, and the results
are summarized in Table 1. Considering the rigidity of
the PPETE backbone compared to the relatively flexible
polystyrene standard, the degree of polymerization
shown here might be overestimated by 2-3 times.4a,18
Photophysical Properties. The absorption and
emission spectra of the model polymer PPETE,4 dea-
PPETE, and tmeda-PPETE are shown in Figure 3. The
absorption and emission spectra of dea-PPETE and
tmeda-PPETE are similar to that of the model PPETE.
On the basis of previous studies,4 the absorption peaks
around 440 nm can be assigned to π-π* transitions from
the conjugated polymer backbone. All the emission
spectra show a maximum and a shoulder, which can be
attributed to a single transition with vibronic structure.4
From the similarity in absorption and emission spectra
between model PPETE and the new polymers, we
conclude that introduction of the amino receptors does
not lead to significant electronic or structural distortion
of the new polymers compared to the model polymer.
The quantum yields of fluorescence for dea-PPETE and
for tmeda-PPETE were found to be 0.11 and 0.09,
respectively. For comparison, the quantum yield for
model PPETE is 0.54.4 The lower values of the quantum
yields of these two polymers compared to the model
polymer are consistent with a PET reduction of the
dea-PPETE and tmeda-PPETEa
deg of
PDI polymerization
Mw
Mn
dea-PPETE
1.20 × 105 6.18 × 104 1.76
206
106
tmeda-PPETE 1.00 × 105 3.67 × 104 2.73
a Determined by GPC at 40 °C relative to polystyrene in
trimethylamine (0.1 vol %) toluene solution.
excited state as expected in the absence of coordinating
analytes (Figure 1). The incomplete PET quenching of
these two polymers may be ascribed to a kinetic
competition between the regular fluorescence process
and the PET process or the presence of localized
fluorescent domains that are not coupled to a receptor
site. This has been suggested previously for PET in
fluorescent polymers.19
Cation Sensitivity. The influence of various metal
chlorides and HCl on the fluorescence behavior of the
tmeda-PPETE polymer was investigated (Figure 4a).
The fluorescence was observed to increase upon titrating
aqueous solution of Hg2+, Zn2+, Ca2+, and H+ into the
polymer THF solution.4b We selected the carrier solvent
based on its miscibility with H2O, which was the target
medium for cation detection in the environment. Addi-
tion of Hg2+ gave the maximum response with a 2.7-
fold intensity enhancement at saturation (micromolar
concentrations). Similar titrations were also carried out
on the dea-PPETE, but the enhancement effects were
very small, usually less than 1.2-fold. Thus, only the
tmeda-PPETE polymer successfully demonstrated a

2848 Fan et al.
Macromolecules, Vol. 38, No. 7, 2005
fluorescence “turn-on” response to metallic cations as
designed.
receptors onto fluorescent conjugated polymers as PET
sensors. A new class of sensors for a variety of cations
can be envisioned with this easily modified system. On
the basis of the design concept, future work is being
directed to increase the sensitivity and selectivity of this
class of chemosensors.
Acknowledgment. The authors thank Dr. Clifford
B. Murphy for the lifetime measurement and useful
discussion on the photophysical studies, Dr. Brendan
R. Flynn and Dr. Stanley K. Madan for the careful
review of this manuscript, and Dr. Elizabeth Brown for
her technical assistance. This research was funded by
a grant from the National Institute of Health (NIH
Grant 1R15ES10106-01).
The different responses of dea-PPETE and tmeda-
PPETE to the same cations may be explained by the
relative energy levels of the two amino receptors com-
pared to the HOMO and LUMO of the polymer back-
bone. On the basis of the oxidation potentials of related
small molecules,20 we propose that the lone electron pair
on diethylamino group has a lower energy level than
that on the N,N,N′-trimethylethylenediamino group. In
the absence of the cations, the driving force of the PET
process is smaller for the tmeda-PPETE than for dea-
PPETE (Figure 1). This is consistent with the observa-
tion that dea-PPETE has a higher quantum yield of
fluorescence than tmeda-PPETE. When the electron
pair on the receptor is coordinated to the cations, the
driving force of the PET process for both polymers is
decreased based on literature oxidation potentials vs
SCE.20 On the basis of comparisons to model com-
pounds,21 the monodentate dea-PPETE has a smaller
binding constant to metal cations as compared to tmeda-
PPETE. The higher equilibrium constant could also
account for the enhanced fluorescence “turn-on” behav-
ior for tmeda-PPETE.
As shown in Figure 4a, when tmeda-PPETE was
titrated by Ca2+, H+, and Zn2+, the fluorescence “turn-
on” response is more rapid than by Hg2+, though these
cations did not yield maximum fluorescence enhance-
ment. For Ca2+, H+, and Zn2+, the fluorescence enhance-
ment saturation was reached at a very early stage (∼5
ppm), when the cation concentration was close to the
concentration of the receptor unit in the solution. When
the addition of these cations reached very high concen-
tration, the fluorescence intensity actually decreased
slightly. This is likely due to the dilution of the polymer
solution with the aqueous cation solution, which is more
easily corrected at low concentration. The polymer
response toward different cations is expected, given the
different association constants between cations and the
amino receptor.
Supporting Information Available: FTIR, 1H NMR, and
13C NMR spectra of the monomers and polymers; 13C DEPT
(135°) and 2D heteronuclear (C, H)-correlated NMR spectra
of dea-PPETE. This material is free of charge via the Internet
at http://pubs.acs.org.
References and Notes
(1) (a) Nolan, E. M.; Lippard, S. J. J. Am. Chem. Soc. 2003, 125,
14270. (b) Wolf, C.; Mei, X. F. J. Am. Chem. Soc. 2003, 125,
10651. (c) He, H. R.; Mortellaro, M. A.; Leiner, M. J. P.;
Fraatz, R. J.; Tusa, J. K. J. Am. Chem. Soc. 2003, 125, 1468.
(d) Hayashita, T.; Qing, D.; Minagawa, M.; Lee, J. C.; Ku, C.
H.; Teramae, N. Chem. Commun. 2003, 2160. (e) Yang, W.
Q.; Yan, J.; Fang, H.; Wang, B. H. Chem. Commun. 2003,
792.
(2) (a) Swager, T. M. Acc. Chem. Res. 1998, 31, 201. (b) Zhou,
Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 12593. (c)
McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev.
2000, 100, 2537.
(3) (a) Fan, C.; Plaxco, K. W.; Heeger, A. J. J. Am. Chem. Soc.
2002, 124, 5642. (b) Wang, D.; Wang, J.; Moses, D.; Bazan,
G. C.; Heeger, A. J. Langmuir 2001, 17, 1262. (c) Wang, J.;
Wang, D.; Miller, E. K.; Moses, D.; Bazan, G. C.; Heeger, A.
J. Macromolecules 2000, 33, 5153. (d) McQuade, D. T.;
Hegedus, A. H.; Swager, T. M. J. Am. Chem. Soc. 2000, 122,
12389.
(4) (a) Zhang, Y.; Murphy, C. B.; Jones, W. E., Jr. Macromolecules
2002, 35, 630. (b) Murphy, C. B.; Zhang, Y.; Troxler, T.; Ferry,
V.; Martin, J. J.; Jones, W. E., Jr. J. Phys. Chem. B 2004,
108, 1537.
(5) Tong, H.; Wang, L.; Jing, X.; Wang, F. Macromolecules 2003,
36, 2584.
(6) (a) de Silva, A. P.; Gunaratne, H. O. N.; Gunnlaugsson, T.;
Huxley, A. J.; McCoy, C. P.; Rademacher, J. T.; Rice, T. E.
Chem. Rev. 1997, 97, 1515. (b) Valeur, B.; Leray, I. Coord.
Chem. Rev. 2000, 205, 3. (c) Zang, L.; Liu, R.; Holman, M.
W.; Nguyen, K. T.; Adams, D. M. J. Am. Chem. Soc. 2002,
124, 10640.
Upon binding these cations, there is negligible shift
in both the UV-vis absorption spectra and the excita-
tion spectra and no change in lifetime for both polymers.
In some emission spectra of tmeda-PPETE, a 2-4 nm
red shift of the emission maximum was observed with
the increase in fluorescence intensity, but the emission
profile shift is within experimental noise (Figure 4b).
On the basis of these photophysical studies, we can
conclude that the ion complexation does not alter the
conformation of the polymers in either ground state or
excited state. Given the favorable driving force for
electron transfer, these results are consistent with the
intensity enhancement resulting from a “switching-off”
of the PET process as shown in Figure 1.
(7) (a) Huston, M. E.; Haider, K. W.; Czarnik, A. W. J. Am. Chem.
Soc. 1988, 110, 4460. (b) Huston, M. E.; Akkaya, E. U.;
Czarnik, A. W. J. Am. Chem. Soc. 1989, 111, 8735. (c) Van
Arman, S. A.; Czarnik, A. W. J. Am. Chem. Soc. 1990, 112,
5376.
(8) Grabchev, I.; Qian, X. H.; Xiao, Y.; Zhang, R. New J. Chem.
2002, 26, 920.
(9) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science
1992, 258, 1474.
(10) Daffy, L. D.; de Silva, A. P.; Nimal Gunaratne, H. Q.; Hunder,
C.; Mark Lunch, P. L.; Werner, T.; Wolfbeis, O. Chem.sEur.
J. 1998, 4, 1810.
Conclusion
We present here for the first time the application of
a PET strategy for the design of conjugated polymer
fluorescence “turn-on” chemsensors. Two new polymers
dea-PPETE and tmeda-PPETE were synthesized and
characterized in this initial study. These two polymers
have relatively low quantum yields in room temperature
solution compared to the model PPETE, due to fluores-
cence quenching. The tmeda-PPETE polymer shows
fluorescence enhancement upon binding Hg2+, protons,
and other divalent cations. The synthetic strategy
provides a platform to introduce a broad series of amino
(11) Swager, T. M.; Gil, C. J.; Wrighton, M. S. J. Phys. Chem.
1995, 99, 4886.
(12) Mandal, S. S.; De Chakraborty, J. J. Chem. Soc., Perkin
Trans. 1 1999, 2639.
(13) Pang, Y.; Li, J.; Barton, T. J. J. Mater. Chem. 1998, 8, 1687.
(14) Lemaire, M. J. Chim. Phys. Phys.-Chim. Biol. 1989, 86, 93.
(15) Campaigne, E.; Lesuer, W. M. J. Am. Chem. Soc. 1948, 70,
1555.
(16) Yamamoto, T.; Honda, K.; Ooba, N.; Tomaru, S. Macromol-
ecules 1998, 31, 7.
(17) Hesse, M.; Meier, H.; Zeeh, B. Spectroscopic Methods in
Organic Chemistry; Georg Thieme Verlag: Stuttgart, 1997.
(18) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605.

Macromolecules, Vol. 38, No. 7, 2005
Fluorescence “Turn-on” Chemosensors 2849
(19) Guillet, J. Polymer Photophysics and Photochemistry; Cam-
bridge University Press: Cambridge, UK, 1985.
(20) Meites, L.; Zuman, P. Electrochemical Data; John Wiley &
Sons: New York, 1974.
(21) Harris, D. C. Quntitative Chemical Analysis, 5th ed.; W.H.
Freeman and Co.: New York, 1999.
MA047983V