Poly[p-(phenyleneethynylene)-alt-(thienyleneethynylene)] polymers with oligopyridine pendant groups: Highly sensitive chemosensors for transition metal ions

Article abstract of DOI:10.1021/ma011479y

We report the synthesis and characterization of a series of luminescent chemosensory materials that have the conjugated polymer poly[p-(phenyleneethynylene)-alt-(thienyleneethynylene)] (PPETE) as a backbone and oligopyridine pendant sites as receptors. Th

Full text of DOI:10.1021/ma011479y

630
Macromolecules 2002, 35, 630-636
Poly[p-(phenyleneethynylene)-alt-(thienyleneethynylene)] Polymers with
Oligopyridine Pendant Groups: Highly Sensitive Chemosensors for
Transition Metal Ions
Ya n Zh a n g, Cliffor d B. Mu r p h y, a n d Wa yn e E. J on es, J r .*
Chemistry Department and Institute for Materials Research, State University of New York at
Binghamton, Binghamton, New York 13902-6016
Received August 16, 2001; Revised Manuscript Received October 23, 2001
ABSTRACT: We report the synthesis and characterization of a series of luminescent chemosensory
materials that have the conjugated polymer poly[p-(phenyleneethynylene)-alt-(thienyleneethynylene)]
(PPETE) as a backbone and oligopyridine pendant sites as receptors. These polymers are soluble in
common organic solvents and highly emissive. Investigations reveal that these polymers are highly
sensitive to transition metal ions such as Ni2+. For the terpyridine-receptor polymer ttp-PPETE, we
observed ∼5% initial emission quenching by Ni2+ concentration as low as 4 × 10-9 M. The polymers also
show selectivity to different transition metal ions. The ether-linked polymer ttp-O-PPETE is also
prepared which successfully demonstrates that complete conjugation is not required for emission
quenching. However, the vinylene-linked ttp-PPETE was found to have a higher quenching efficiency.
In tr od u ction
The toxicity of certain metal ions has been a constant
cause of environmental concern. Thirteen heavy metal
ions are listed as “priority pollutants” by the Environ-
mental Protection Agency (EPA).1 These metals, in
different oxidation states, include chromium, manga-
nese, cobalt, copper, zinc, molybdenum, silver, mercury,
cadmium, lead, and nickel. Although a number of
laboratory-based methods are available for determining
the presence of trace metals/radionuclides in the envi-
ronment,2 there is an increasing need for the develop-
ment of field-based sensors and remediation devices,
demanding new chemosensory materials and novel, low-
cost synthetic designs.
Because of their high sensitivity and ease of measure-
ment, fluorescent sensors have received significant
attention for a variety of environmental applications.3
Conjugated polymers as fluorescent sensors are par-
ticularly attractive due to their enhanced electronic
communication properties. Several groups including the
seminal work by Swager et al.4 have demonstrated that
polyreceptor assemblies, electronically connected by a
conjugated “molecular wire” polymer, exhibit large
sensitivity enhancement over conventional molecule-
based fluorescent chemosensors. The energy-transfer
process can be illustrated using the state diagram
shown in Figure 1. The initial excitation leads to
formation of an exciton which can rapidly migrate
between isoenergetic sites along the conjugated polymer
backbone to a low-energy acceptor site. The result is an
efficient fluorescence quenching mechanism even at
very low quencher concentrations since the binding of
one receptor site results in efficient quenching of several
emitting units along the polymer backbone.
F igu r e 1. Illustration of energy-transfer quenching through
a conjugated polymer.
polymers.5-7 Wang and Wasielewski have demonstrated
that when 2,2′-bipyridine was introduced into a conju-
gated polymer backbone, the absorption and emission
profiles changed significantly upon binding different
transition metals to the polymer.5 More recently,
Kimura et al. prepared PPV-based conjugated polymers
with terpyridine units bound directly to the conjugated
polymer framework. In the presence of transition met-
als, both the absorption and emission spectrum were
found to shift in a quantifiable manner.6
Here we report the synthesis and characterization of
a series of new chemosensory polymers, poly[p-(phe-
nyleneethynylene)-alt-(thienyleneethynylene)] (PPETE)
with oligopyridine pendant groups as receptors for
transition metals. This new system takes advantage of
the strong conjugation and luminescence properties of
the polyarylene ethynylene backbone and multidentate
Lewis base coordinating ability of oligopyridines to yield
a highly effective transition-metal chemosensor. Given
the conformational flexibility of the linking group in our
PPETE systems, a fundamental question, which re-
sulted from preliminary investigations, involved the role
of pendant receptor conjugation with the backbone in
the quenching mechanism. In previous literature ex-
amples, the receptor site was bound within the conjuga-
tion of the backbone.5,6 To address this question, we
have prepared two different PPETEs (ttp-PPETE and
ttp-O-PPETE) with different linker groups which vary
the extent of conjugation between the backbone and the
receptor site (Figure 2). Fluorescence quenching experi-
Lewis bases such as oligopyridyl ligands are known
to coordinate a large number of transition metal ions.
There are only a limited number of reports involving
incorporating oligopyridyl ligands into conjugated
* Corresponding author. Tel 607-777-2421; FAX 607-777-4478;
e-mail wjones@binghamton.edu.
10.1021/ma011479y CCC: $22.00 © 2002 American Chemical Society
Published on Web 12/20/2001

Macromolecules, Vol. 35, No. 3, 2002
PPETE Polymers with Oligopyridine Groups 631
F igu r e 2. Structure of PPETE polymers.
Sch em e 1. Syn th esis of P P ETE P olym er s by P a lla d iu m -Ca ta lyzed Cou p lin g
was stirred at room temperature for 1 h. The yellow suspension
gradually turned light yellow in solution and was subsequently
concentrated by evaporation. Ether (80 mL) was added, and
the organic layer was washed with water (3 × 20 mL), dried
with MgSO4, and concentrated in vacuo. Chromatography of
the residue (silica, hexane:ethyl acetate 6:1) yielded compound
ments enable us to determine whether complete conju-
gation is a requirement for efficient chemosensory
activity.
Exp er im en ta l Section
1
Ma ter ia ls. All materials were purchased from Aldrich and
used as received unless otherwise noted. The compounds 1,4-
diethynyl-2,5-diiododecyloxybenzene (4),8 5-bromomethyl-2,2′-
bipyridine (7),9 4′-(4-bromomethyl-phenyl)-[2,2′:6′,2′′]-terpyri-
dine (8),10 and 2,5-diiodo-3-dodecylthiophene11 were synthesized
as described previously. Satisfactory NMR characterization of
all stable intermediates was observed in each case.
7 as off-white crystals (0.70 g, 86%). H NMR (CDCl3): δ 1.59
(br, 1H), 4.56 (s,2H), 7.01 (s, 1H). 13C NMR (CDCl3): 59.26,
109.19, 111.40, 130.47, 141.43. Elemental analysis: Calcd for
C5H4Br2SO: C, 22.06%; H, 1.47%. Found: C, 22.21%; H,
1.45%.
5-2[(2,5-Dibromo-thiophen-3-yl)-vinyl]-2,2′-bipyridine (1)
(Scheme 2, eq 2). A mixture of 5-bromomethyl-2,2′-bipyridine
(0.12 g, 0.5 mmol) and triethyl phosphite (2 g) was heated
slowly to 120 °C for 1 h. Excess triethyl phosphite was removed
from the reaction mixture by vacuum distillation to give a faint
yellow oil. The residue was dissolved in THF; 5 (0.14 g, 0.5
mmol) was added. When dissolution was complete, KOBut
(0.55 mL, 1 M in THF) was added. The mixture was stirred at
room temperature for 2 h. The product mixture was poured
into ethanol. The solid was filtered and subsequently recrys-
tallized from THF and ethanol to give an off-white solid (yield
Gen er a l Meth od s. NMR (1H and 13C) spectra were re-
corded on an AM-360 spectrometer. Elemental analyses were
performed by QTI, Inc. Gel permeation chromatography was
used to measure the molecular weight of all polymers in
toluene relative to polystyrene standards. UV-vis spectra of
the polymers were obtained on a Perkin-Elmer Lambda 2S
spectrophotometer in tetrahydrofuran (THF) solution unless
otherwise noted. Fluorescence spectra were measured on an
SLM 48000s fluorimeter with variable excitation between 300
and 460 nm with 4 nm slits. Fluorescence solutions were
prepared with absorption at the excitation maximum of 0.1-
0.2 o.d. Quantum yields were determined relative to an-
thracene in ethanol with a quantum yield of 0.27 ( 0.03.12,13
Lifetimes were measured using single-photon counting at the
Regional Laser and Biotechnology Laboratory at University
of Pennsylvania using a system that has been described
elsewhere.14
Syn th esis. 2,5-Dibromothiophene-3-carbaldehyde (5) (Scheme
2, eq 1). To a solution of 3-thiophenecarboxaldehyde (2.5 g,
0.020 mol) in 50 mL of chloroform was added anhydrous
sodium bicarbonate (4.2 g), followed by the dropwise addition
of a solution of bromine (8.2 g in 50 mL of chloroform) over a
period of 1 h. The reaction mixture was stirred overnight at
room temperature and then filtered. The filtrate was washed
with water (2 × 100 mL) and dried over MgSO4. The solvent
was evaporated, and the solid residue was chromatographed
(silica, hexane:ethyl acetate 20:1) to give a light yellow solid
6 (yield: 75%).1H NMR (360 MHz, CDCl3): δ 9.78 (s, 1H,
CHO), 7.33 (s, 1H). 13C NMR (CDCl3): 189.14, 139.33, 128.68,
124.18, 113.36. Elemental analysis: Calcd for C5H2Br2SO: C,
22.24%; H, 0.74%. Found: C, 23.03%; H, 0.87%.
1
76%). H NMR (360 MHz, CDCl3): 8.72 (m, 2H), 8.43 (d, 2H),
7.97 (dd, 1H), 7.83 (t, 1H), 7.32 (m, 1H), 7.24 (s, 1H), 7.01 (dd,
2H, trans-vinyl-H). Elemental analysis: Calcd for C16H10
-
Br2N2S: C, 45.50%; H, 2.37%; O, 6.64%. Found: C, 46.11%;
H, 2.55%; O, 6.58%.
4′-{4-[2-(2,5-Dibromothiophen-3-yl)-vinyl]phenyl}-2,2′:6′,2′′-
terpyridine (2) was synthesized from 4′-(4-bromomethylphe-
nyl)-2,2′:6′,2′′-terpyridine by the same method described for 1
above (Scheme 2, eq 3), producing a yellow solid 2 (yield 80%).
1H NMR (360 MHz, CDCl3): δ 8.75 (s, 2H), 8.73 (d, 2H), 8.67
(d, 2H), 7.92 (d, 2H), 7.86 (td, 2H), 7.62 (d, 2H), 7.35 (td, 2H),
7.24 (s, 1H, 4-pyrrole-H), 7.02 (dd, 2H, trans-vinyl-H). Elemen-
tal analysis: Calcd for C27H17Br2N3S: C, 56.35%; H, 2.96%;
N, 7.30%. Found: C, 55.19%; H, 2.81%; N, 7.07%.
4′-[4-(2,5-Dibromothiophen-3-yl-methoxymethyl)phenyl]-2,2′:
6′,2′′-terpyridine (3) (Scheme 2, eq 4). NaH (98 mg, 2.45 mmol)
was added to a solution of (2,5-dibromothiophen-3-yl)methanol
(6) (0.67 g, 2.45 mmol) in THF (30 mL). After initial gas
evolution ceased, 8 (2.0 g, 3.48 mmol) was added. The reaction
mixture was refluxed for 5 h under nitrogen, cooled to room
temperature, and diluted with ether (100 mL). The organic
layer was washed with water (3 × 30 mL), dried over MgSO4,
and concentrated in vacuo. Chromatography of the residue
(silica, hexane:ethyl acetate 10:1) yielded 3 as a white crystal
(1.3 g, 60%). 1H NMR (CDCl3): δ 8.75 (s, 2H), 8.73 (d, 2H),
(2,5-Dibromothiophen-3-yl)methanol (6) (Scheme 2, eq 1). To
a suspension of 5 (0.80 g, 3.0 mmol) in 85% ethanol (30 mL)
was added sodium borohydride (0.34 g, 8.9 mmol). The mixture

632 Zhang et al.
Macromolecules, Vol. 35, No. 3, 2002
Sch em e 2. Syn th esis Sequ en ce for th e 2,5-Dibr om oth iop h en e Un its (1-3)
8.67 (d, 2H), 7.92 (d, 2H), 7.89 (td, 2H), 7.48 (d, 2H), 7.37 (td,
2H), 7.02 (s, 1H), 4.60 (s, 2H) 4.46 (s,2H). 13C NMR (CDCl3):
156.17, 155.87, 149.93, 149.06, 139.11, 138.71, 137.95, 136.95,
130.91, 129.63, 128.30, 127.76, 127.43, 123.85, 121.41, 118.86,
71.94, 65.94. Elemental analysis: Calcd for C27H19OBr2N3S:
C, 54.64%; H, 3.20%; N, 7.08%. Found: C, 56.57%; H, 3.29%;
N, 7.53%.
(yield 80%).15 Elemental analysis: Calcd for C61H67O2N3S: C,
76.90%; H, 7.40%; N, 4.64%. Found: C, 74.25%; H, 7.50%; N,
4.46%.
ttp -O-P P ETE. This material was prepared by reaction
of compounds 3 and 4 using a procedure identical to bp-
PPETE (yield 78%). Elemental analysis: Calcd for
C
61H71O3N3S: C, 79.14%; H, 7.68%; N, 4.54%. Found: C,
bp -P P ETE. Diisopropylamine (2 mL) was added to a
mixture of compound 1 (0.50 g, 0.82 mmol), 1,4-diethyl-2,5-
dihexadecyloxybenzene (4) (0.48 g, 0.82 mmol), Pd(PPh3)4 (50
mg, 0.043 mmol), and CuI (40 mg, 0.21 mmol) in 10 mL of
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
organic phase was collected and dried over MgSO4. After the
solvent was removed under reduced pressure the residue was
washed with hot acetone, hot water, and hot MeOH to afford
a yellow solid (yield 88%). Elemental analysis: Calcd for
74.36%; H, 7.70%; N, 3.71%.
Mod el P P ETE. This material was prepared by reaction of
2,5-diiodo-3-dodecylthiophene and 4 using a procedure identi-
cal to bp-PPETE (yield 85%). Elemental analysis: Calcd for
C
50H78O2S: C, 80.86%; H, 10.51%. Found: C, 79.19%; H,
10.43%.
Resu lts a n d Discu ssion
Syn th esis. Following the seminal work of Heck and
others,16 the palladium-catalyzed cross-coupling reac-
tion has become a powerful tool in preparing a wide
range of PAE type conjugated polymers with functional
or multifunctional groups.17 To synthesize the PPETE
C
50H62O2N2S: C, 79.58%; H, 8.22%; N, 3.71%. Found: C,
75.66%; H, 8.16%; N. 3.25%.
ttp -P P ETE. This material was prepared by reaction of
compounds 2 and 4 using a procedure identical to bp-PPETE

Macromolecules, Vol. 35, No. 3, 2002
PPETE Polymers with Oligopyridine Groups 633
Sch em e 3. Syn th esis of Mod el P olym er P P ETE
Ta ble 1. Molecu la r Weigh ts a n d P olyd isp er sities of
P P ETE P olym er s Deter m in ed by GP C Rela tive to
P olystyr en e Sta n d a r d s in THF Solu tion a t Room
Tem p er a tu r e
The number-average molecular weight (Mn) and poly-
dispersity of these polymers were determined by GPC
relative to polystyrene standards. The results of the
polycondensations are summarized in Table 1. Given
the rigidity of the PPETE polymers, the Mn might be
overestimated by a factor of 2-3 since the standard
polystyrene is a flexible coil polymer19 while the PPETE
polymer was expected to be more rigid.
polymers
PPETE
bp-PPETE ttp-PPETE ttp-O-PPETE
Mn
3.2 × 105 1.8 × 104
1.7 × 105
1.9 × 105
polydispers- 2.8
2.5
1.4
1.5
ity (Mw/Mn)
P h otop h ysica l P r op er ties. Absorption and emis-
sion spectra were collected for all polymer systems in
THF solution. The photophysical data are summarized
in Table 2. For the model polymer PPETE, only one
peak is observed in the absorption spectrum (λmax ) 448
nm). This can be assigned to the π-π* transition of the
conjugated polymer backbone. All of the receptor-
substituted polymers have two absorption bands. The
lower energy bands (λmax ) 444-462 nm) can be
assigned to π-π* transitions on the conjugated polymer
backbone. The higher energy bands (around 300 nm)
can be assigned to a π-π* and n-π* transitions in the
pendant bipyridyl or terpyridyl group by comparison to
the absorption spectra of the monomer (1-3). From the
absorption data shown (Figure 3a), it is clear that both
the lower energy bands of the bp-PPETE and ttp-
PPETE are red-shifted and broadened relative to that
of the model polymer PPETE. This can be rationalized
on the basis of the increased conjugation of the polymer
backbone since the pendant groups were connected to
the backbone by conjugated double bonds. On the other
hand, the lower energy peak in absorption of ttp-O-
PPETE shows no red shift compared to the model
PPETE polymer. This is consistent with a lack of
conjugation with the pendant group of the polymer
backbone.
The polyarylene ethynylene class of polymers is
known to be highly emissive with quantum yields of 0.7
and higher.19 All of the polymers prepared here were
found to be highly emissive with quantum yields of
0.38-0.54 (Table 2). A representative emission spectrum
is shown in Figure 3b. The emission spectra of bp-
PPETE, ttp-PPETE, and ttp-O-PPETE were nearly
identical. This is consistent with limited perturbation
of the lowest energy π-π* state of the polymer by the
receptor. All three polymers were broadened relative to
that of the model polymer PPETE. The emissions of bp-
PPETE and ttp-PPETE polymers also showed a red
shift while the ttp-O-PPETE has no red shift compar-
ing to the model PPETE polymer. This result agrees
with the trend in absorption spectra.
polymers using this reaction (Scheme 1), the corre-
sponding precursors 1,4-diethynyl-2,5-didodecyloxyben-
zene (4) and 2,5-dibromo-thiophene substituted with
different binding sites (1-3) were first synthesized. 1,4-
Diethynyl-2,5-didodecyloxybenzene (4) was synthesized
from 1,4-hydroquinone in four steps as described previ-
ously.8
The synthetic sequence for the 2,5-dibromo-thiophene
units (1-3) is outlined in Scheme 2. The compound 2,5-
dibromothiophene-3-carbaldehyde (5) was readily syn-
thesized from the bromination reaction of 3-thiophen-
ecarboxaldehyde with Br2 in the presence of NaHCO3
(Scheme 2, eq 1). Initial attempts to synthesize 2,5-
diiodo-3-thiophenecarboxaldehyde with I2 (catalyzed by
HgO or Hg(OAc)2) according to literature methods were
not successful. This is likely due to the presence of the
aldehyde group in the reactant thiophene monomer.
Bipyridine phosphite or terpyridine phosphite was
prepared by reacting 5-bromomethyl-2,2′-bipyridine (7)
or 4′-(p-bromomethylphenyl)-2,2′:6′2′′-terpyridine (8)
with triethyl phosphite and subsequently treated in situ
with 2,5-dibromo-3-thiophenecarboxaldehyde (5) under
Horner-Wittig-Emmons conditions.18 This gives the
polymer precursors 1 and 2 with a yield of 76% and 80%,
respectively (Scheme 2, eqs 2 and 3).
The precursor compound (2,5-dibromothiophen-3-yl)-
methanol (6) can be easily prepared by reduction of 2,5-
dibromo-3-thiophenecarboxaldehyde (5) using sodium
borohydride (yield 86%) (Scheme 2, eq 1). Thus, the
ether-linked monomer 3 was obtained by adding the
bromide 8 to a suspension of the sodium salt of alcohol
6, which was generated from the reaction of 6 with
sodium hydride (Scheme 2, eq 4).
The polymerizations in all cases were performed
under Heck coupling conditions.16 Substituted dibro-
mothiophene was reacted with diethynylbenzene in
THF catalyzed by 5 mol % Pd(PPh3)4 and 5 mol % CuI
(Scheme 1). For comparison purposes, the model poly-
mer PPETE was synthesized from 2,5-diiodo-3-dode-
cylthiophene and 4 using the same conditions (Scheme
3). This polymer has the same backbone as the other
PPETE polymers but has long alkyl chains in place of
the receptor connected to the backbone. IR and NMR
spectra of the polymers indicated that the polymeriza-
tion was complete as evidenced by the disappearance
of the acetylenic groups. Further, the formation of the
ethynyl link was confirmed by the presence of 2184 cm-1
stretch (for ttp-PPETE) in the FTIR. Each of the
polymers was notably fluorescent and very soluble in
common organic solvents such as THF, chloroform, and
toluene.
The emission lifetimes measured by single photon
counting were found to be less than 1 ns. This is
consistent with the π-π* nature of the excited states.
Emission decays could not be satisfactorily fit as single-
exponential, unimolecular process. In each case, life-
times were best fit by a biexponential decay model (eq
1). Multiexponential emission decays have been ob-
served previously for this class of PAE polymers.19
y(t) ) A1 exp(-t/τ1) + A2 exp(-t/τ2)
(1)

634 Zhang et al.
Macromolecules, Vol. 35, No. 3, 2002
F igu r e 4. Emission spectra recorded in THF at room tem-
perature for Ni2+ complexes with ttp-PPETE. The polymer
concentration is 3.08 × 10-6 M corresponding to receptor unit.
F igu r e 3. Absorption (a) and emission (b) spectra of the
PPETE polymers in THF solution.
Ch em osen sor Qu en ch in g Resp on se. The addition
of a Lewis base receptor to the conjugated polymer
backbone provides the opportunity to observe modulated
fluorescence and absorption response characteristics in
the presence of Lewis acids such as toxic transition
metals. The emission of the polymers was monitored in
THF using steady-state fluorescence spectroscopy. The
emission intensity of polymer ttp-PPETE as a function
of concentration of Ni2+ in THF is shown in Figure 4.
We observed 5% emission quenching at Ni2+ concentra-
tions as low as 4 × 10-9 M20 (not shown in this figure).
The emission intensity decreases to about 60% of the
initial intensity at concentrations as low as 2.0 × 10-7
M. In these experiments, the terpyridyl receptor con-
centration was held fixed at 3.08 × 10-6 M. Control
experiments using the model polymer PPETE show no
emission quenching in the presence of any of the
transition-metal ions in THF solution. This suggests
that the emission quenching response we observed
F igu r e 5. Emission quenching of ttp-PPETE polymer by
different transition metal ions. The polymer concentrations are
held fixed at 3.08 × 10-6 M corresponding to receptor unit.
Transition metal ions are 1.54 × 10-6 M.
involves the interaction of the transition metal quench-
ers with the bipyridyl or terpyridyl receptors.
The polymer ttp-PPETE also shows sensitivity to
several other transition-metal ions such as Cr6+, Co2+
,
Cu2+, and Mn2+. Because of the varying chelating ability
of terpyridine with different transition metals, this
polymer shows some selectivity toward the different
transition metals (Figure 5). In these experiments, both
the ttp-PAE polymer and transition-metal quencher
concentration were held fixed at 3.0 × 10-6 and 1.5 ×
10-7 M, respectively. The polymer was quenched to
29.0% of its initial intensity in the presence of Ni2+
under these conditions, while 79.6% of the intensity
Ta ble 2. P h otop h ysica l P r op er ties of P P ETE P olym er s in THF Solu tion a t Room Tem p er a tu r e
lifetime (component)
absorbance (λmax, nm)
emission (λmax, nm)
τ1(ns) (A1)
τ2 (ns) (A2)
quantum yield
model PPETE
bp-PPETE
448
496
508
508
488
0.547 (0.854)
0.582 (0.758)
0.567 (0.617)
0.599 (0.586)
0.081 (0.146)
0.122 (0.242)
0.208 (0.383)
0.233 (0.414)
0.54
0.38
0.40
0.45
344, 462
338, 454
276, 444
ttp-PPETE
ttp-O-PPETE

Macromolecules, Vol. 35, No. 3, 2002
PPETE Polymers with Oligopyridine Groups 635
sion intensity with ttp-O-PPETE. Thus, the conjuga-
tion of the receptors to the polymer backbone clearly
enhances the electronic communication and the sensi-
tivity of polymer chemosensory behavior. However, we
conclude that the conjugation of receptors to the polymer
backbone is not required for the fluorescence response.
The successful demonstration of quenching in the
absence of conjugation to the receptors provides more
design flexibility in the synthesis of new conjugated
polymer chemosensors.
Con clu sion
We have successfully designed and synthesized a
series of conjugated PPETE polymers with different
oligopyridyl receptors using a palladium-catalyzed cou-
pling reaction. Each of the polymers is notably fluores-
cent and very soluble in common organic solvents,
making them suitable for application to large-scale spin-
coating production. These polymers show significant
sensitivity to transition-metal ions due to the enhanced
electronic communication properties of the conjugated
polymers. Further, our result shows that complete
conjugation of the pendant receptors to the polymer
backbone is not required for enhanced emission quench-
ing. However, the fully conjugated, vinylene-linked, ttp-
PPETE was found to have the highest overall quenching
efficiency.
F igu r e 6. Stern-Volmer plots of ttp-PPETE (circle) and bp-
PPETE (square) emission quenched by Ni2+ in THF at room
temperature. The polymer concentrations are both 3.08 × 10-6
M corresponding to receptor unit.
remained when Cd2+ was used as the quencher. No
quenching was observed in the presence of common
cations such as Na+ or Ca2+. The polymer was also not
responsive to Pb2+ and Hg2+
.
The bp-PPETE and ttp-PPETE polymers are
quenched to varying degrees by the different transition
metals tested. Figure 6 shows a Stern-Volmer quench-
ing analysis of bp-PPETE and ttp-PPETE polymer
emission quenched by [Ni2+]. In these experiments, the
concentrations of these two polymers were 3.08 × 10-6
M in receptor sites. The ttp-PPETE was more sensitive
Ack n ow led gm en t. The authors thank Dr. Biwang
J iang for useful discussion and Dr. Thomas Troxler from
University of Pennsylvania for assistance with the
lifetime measurements. This research was funded by a
grant from the National Institute of Health (NIH) under
Grant 1R15ES10106-01 and the Integrated Electronics
and Engineering Center (IEEC) at the State University
of New York at Binghamton. The IEEC receives funding
from the NY State Science and Technology Foundation,
the National Science Foundation, and a consortium of
industrial members.
to Ni2+ ion, where a Ni2+ concentration of 1.0 × 10-6
M
quenched ttp-PPETE to 10.9% of its original emission
intensity, while only to 38.2% for emission intensity of
bp-PPETE. This is consistent with the tridentate
nature of the terpyridyl pendant, while the bipyridyl
group is bidentate. It is interesting to note the nonlinear
nature of the Stern-Volmer plot in Figure 6. Dynamic
quenching processes would be expected to give a straight
line with a Y-intercept of 1. Preliminary data in our lab
show that there is no change in the lifetime as a function
of metal concentration,21 suggesting that in this case
the mechanism of the quenching process involves com-
plexation rather than collisional deactivation. Nonlinear
behavior for polymer-based fluorescence quenching has
been observed previously22 and is the basis of continuing
analysis in our lab.23
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra of
polymers. This material is available free of charge via the
Internet at http://pubs.acs.org.
Refer en ces a n d Notes
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To understand whether the conjugation between the
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Manuscript in preparation.
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