Triphenylamine-based fluorescent conjugated copolymers with pendant terpyridyl ligands as chemosensors for metal ions

Article abstract of DOI:10.1002/pola.23890

Two well-defined triphenylamine-based fluorescent conjugated copolymers with pendant terpyridyl ligands were synthesized through Suzuki coupling polymerization and were further characterized by ¹H-NMR, ¹³C-NMR, gel permeation chromat

Full text of DOI:10.1002/pola.23890

Triphenylamine-Based Fluorescent Conjugated Copolymers with Pendant
Terpyridyl Ligands as Chemosensors for Metal Ions
YI CUI,1,2 QI CHEN,1 DAN-DAN ZHANG,1 JIE CAO,2y BAO-HANG HAN1
1National Center for Nanoscience and Technology, Beijing 100190, People’s Republic of China
2College of Chemistry and Chemical Engineering, Graduate University of Chinese Academy of Sciences, Beijing 100049,
People’s Republic of China
Received 6 October 2009; accepted 15 December 2009
DOI: 10.1002/pola.23890
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: Two well-defined triphenylamine-based fluores-
cent conjugated copolymers with pendant terpyridyl ligands
were synthesized through Suzuki coupling polymerization
and were further characterized by 1H-NMR, 13C-NMR, gel per-
meation chromatography, Infrared, and UV-vis spectra. Poly-
mer P-1, terpyridine-bearing poly(triphenylamine-alt-fluorene)
with a high fluorescence quantum yield (62%) shows much
higher sensitivities toward Fe3þ, Ni2þ, and Cu2þ as compared
with the other metal ions investigated. Especially, Fe3þ can
lead to an almost complete fluorescence quenching of poly-
mer P-1. Whereas, the analogous polymer P-2, in which N-
ethylcarbazole repeat units replace the fluorene units in P-1,
shows a very poor selectivity. It demonstrates that polymers
with a same receptor may show different sensitivity to analy-
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2010 Wiley
Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 1310–
1316, 2010
KEYWORDS: chemosensor; conjugated polymers; fluorescence;
metal–polymer complexes; terpyridine; triphenylamine
various disorders of most organisms.14,15 Therefore, high
sensitivity chemosensor for Fe3þ should be developed.
INTRODUCTION With high efficiencies in both photolumines-
cence and electroluminescence, fluorescent conjugated poly-
mers (FCPs) have attracted much attention as versatile sen-
sory materials during the past decade owing to their high
sensitivity, flexibility in synthesis, and ease in measure-
ment.1–4 FCPs are referred to as amplifying fluorescent poly-
mers associated with an efficient electronic communication
along the main backbone, and as the concept of molecular
wire, which was first established by Zhou and Swager.5,6
Analyte-binding with the polymer can happen in a very short
time, so a real-time detection as well as a trace detection
will be achieved due to their high sensitivity. Therefore, FCPs
provide a unique platform for exploiting novel chemo- and
biosensors.7–12
By introducing specific ligands such as bipyridine,16–18 ter-
pyridine,19–23 quinoline,24 and dipyrrolylquinoxaline25 to the
main backbones or side chains, conjugated polymers have
been widely employed for the metal ion detection owing to
the characteristic metal-to-ligand charge transfer. In addition,
terpyridine possesses a superb ability to coordinate a large
number of transition-metal ions and forms stable terpyri-
dine–metal ion complexes, so it is an important building
block for designation of sensory materials to detect some
transition-metal ions. Pang’s group reported that the poly-
mer of poly(phenylenevinylene) with terpyridyl ligand shows
the potential for Cu2þ and Zn2þ detection.26
The toxicity of certain metal ions has been a constant con-
cern of environmental problems. Thirteen transition-metal
ions, such as beryllium, chromium, manganese, cobalt, cop-
per, zinc, molybdenum, thallium, silver, mercury, cadmium,
lead, and nickel have been listed as ‘‘priority pollutants’’ by
the Environmental Protection Agency.13 Fe3þ is an essential
trace element, which plays a significant role in chemical and
biological processes. Fe3þ deficiency in the body leads to
anemia, and Fe3þ excess causes liver and kidney damage.
Furthermore, deficiency and overload of Fe3þ may induce
We have reported the synthesis of various FCPs with pend-
ant carbohydrates and their interactions with lectins or
DNA.27–29 To extend the potential application of FCPs in
chemo- and biosensing, herein, two well-defined triphenyl-
amine-based conjugated polymers containing terpyridyl
ligands were synthesized (Scheme 1) and well characterized
using 1H-NMR, 13C-NMR, Infrared (IR), and UV-vis spectra.
The polymers take advantage of the effective p-conjugation
and strong luminescence properties of the triphenylamine
†Present address: Department of Chemistry, Beijing Institute of Technology, Beijing 100081, People’s Republic of China.
Correspondence to: B.-H. Han (E-mail: hanbh@nanoctr.cn)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 1310–1316 (2010) 2010 Wiley Periodicals, Inc.
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SCHEME 1 Synthetic routes to monomers and copolymers.
and metal ion-coordinating ability of terpyridine to develop
a highly effective chemosensor for transition-metal ions. It is
found that one of the synthesized polymers, polymer P-1,
tem in tetrahydrofuran (THF). The number-average and
weight-average molecular weights (Mn and Mw) were esti-
mated by using a calibration curve of polystyrene standards.
Infrared (IR) spectra were recorded using a Perkin-Elmer
Paragon 1000 FT-IR spectrometer. Ultraviolet-visible (UV-vis)
spectra were measured using a Perkin-Elmer Lamda 900 UV-
vis-NIR spectrophotometer and quartz cells with 1 cm path
length. The fluorescence spectra were measured using a Per-
kin-Elmer LS55 luminescence spectrometer in a conventional
quartz cell with 1 cm path length. Thermogravimetric analy-
ses (TGA) were performed on a Pyris Diamond thermogravi-
metric/differential thermal analysis instrument (10 ꢀC
minꢁ1, nitrogen).
shows higher sensitivities and selectivities for Fe3þ, Ni2þ
,
and Cu2þ compared with the other metal ions investigated,
such as Al3þ, Mn2þ, Cr3þ, Pb2þ, Zn2þ, Co2þ, and Fe2þ. We
believe it will provide some clues for elucidation of the influ-
ence on the selectivity to metal ions owing to the structure
of the main backbone of the polymers.
EXPERIMENTAL
Materials and Measurements
All chemical reagents were commercially available and used
as received unless otherwise stated. Aldehyde 1 and terpyr-
idyl-benzyl phosphonate ester 3 (Scheme 1) were prepared
according to reported methods.19,30
Synthesis of Monomer and Polymers
Monomer 4
To the mixture of terpyridyl-benzyl phosphonate ester 3
(0.930 g, 2.15 mmol) and aldehyde 1 (1.13 g, 2.15 mmol) in
dry THF (20 mL) under argon, t-BuOK (1.12 g, excess) was
added slowly in portions. A yellow colored mixture was
obtained and became warm. The mixture was stirred under
argon for 24 h. After completion of the reaction (monitored
by TLC), the mixture was dispersed in ethanol (100 mL).
The residual solid was filtered and recrystallized from
The 1H- and 13C-NMR spectra were recorded on a Bruker
DMX 300 NMR spectrometer. The optical rotations were
measured with a JASCO DIP-1000 digital polarimeter. Mass
spectra were recorded with a VG PLATFORM mass spectrom-
eter using the ESI technique. The molecular weight and poly-
dispersity index (PDI) of the polymers were determined by
an Agilent 1100 gel permeation chromatography (GPC) sys-
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RESULTS AND DISCUSSION
anhydrous dichloromethane–ethanol, to give yellow crystals
4 (1.3 g, yield 75%). 1H-NMR (300 MHz, CDCl3): d ¼ 8.75
(d, J ¼ 12.0 Hz, 4H), 8.68 (d, J ¼ 7.2 Hz, 2H), 7.94–7.80 (m,
4H), 7.63 (d, J ¼ 8.4 Hz, 2H), 7.57–7.52 (m, 4H), 7.44 (d, J ¼
8.7Hz, 2H), 7.38–7.34 (m, 2H), 7.18 (d, J ¼ 8.4 Hz, 1H), 7.15
(d, J ¼ 8.7 Hz, 1H), 7.12 (d, J ¼ 8.7 Hz, 1H), 6.94–6.84(m,
5H). 13C-NMR (75 MHz, CDCl3): d ¼ 155.3, 155.0, 154.9,
148.1, 145.8, 137.3, 135.8, 129.1, 128.3, 126.7, 126.6, 126.2,
126.1, 125.9, 125.2, 125.0, 123.3, 122.8, 122.6, 120.3, 117.6,
117.5, 85.2. ESI(þ)-MS: calcd. for C41H28I2N4: 830.5 (M);
found 831.4 (Mþ1)þ.
Polymer Synthesis and Characterization
The synthetic routes to the monomers and copolymers are
outlined in Scheme 1. Through the Horner–Wadsworth–
Emmons (HWE) reaction (also called the Horner–Wittig reac-
tion),31 the known aldehyde 1 was coupled with terpyridyl-
benzyl phosphonate ester 3 (prepared from methylphenyl
terpyridine 2) to afford monomer 4. The vinylene group in
the side chain formed from the HWE reaction of 1 and 3 can
facilitate the electronic communication between triphenyl-
amine backbone and terpyridine side chain. A palladium-cat-
alyzed Suzuki coupling polymerization of the monomer 4
with corresponding diboronate (5, 6)32 furnished two well-
defined copolymers, poly(triphenylamine-alt-fluorene) and
poly(triphenylamine-alt-N-ethylcarbazole) bearing terpyridyl
ligands (P-1, P-2), respectively. Figure 1 shows the 1H-NMR
spectra of monomer 4, polymers P-1, and P-2 in CDCl3 solu-
tion. We can find that the signal peaks of protons in pyridyl
(d ¼ 8.8–8.5 ppm), which derived from both P-1 and P-2.
Ethyl (d ¼ 4.41–4.30 and 1.45 ppm) and hexyl (d ¼ 2.06–
1.85, 1.06, and 0.73 ppm) groups derived from P-1 and P-2,
respectively. The successful polymerization is also confirmed
by FT-IR spectra of the monomer and polymers as shown in
Figure 2. Compared with monomer 4, the absorption bands
at 2850–2950 cmꢁ1 on the FT-IR spectra of polymers P-1
and P-2 indicate the CAH stretching of alkyl chains from
diboronate monomers (5, 6). Both P-1 and P-2 show the
same characteristic absorption profiles in the range of 1425–
1570 cmꢁ1 for terpyridine and benzene rings.
General Procedure for Suzuki Coupling Polymerization
Under a nitrogen atmosphere, monomer 4 (0.3 mmol), dibo-
rate 5 or 6 (0.33 mmol), Pd(PPh3)4 (80 mg), and potassium
carbonate (0.5 g, 3.6 mmol) were placed in a 50 mL round-
bottom flask, and then THF-H2O (20 mL, 2/1, v/v) was
added. The mixture was stirred at 70 ꢀC for 48 h under a
nitrogen atmosphere. The resulting product was purified by
precipitation with methanol and washed with methanol–ace-
tone in a Soxhlet apparatus for 48 h.
1
P-1 was obtained as a gray powder with a yield of 85%. H-
NMR (300 MHz, CDCl3): d ¼ 8.74–8.68 (m, 6H), 7.87–7.80
(m, 6H), 7.78–7.40 (m, 10H), 7.39–6.98 (m, 12H), 2.06–1.85
(m, 4H), 1.06 (br, 12H), 0.73 (br, 10H). 13C-NMR (75 MHz,
CDCl3): d ¼ 156.3, 156.0, 149.1, 136.9, 136.8, 129.3, 128.0,
127.9, 127.6, 127.5, 127.2, 127.0, 126.8, 124.8, 123.9, 123.8,
121.4, 121.0, 118.6, 118.5, 118.4, 117.6, 31.5, 29.7, 23.8,
22.6, 22.5, 14.0.
P-2 was obtained as a yellow powder with a yield of 78%.
1H-NMR (300 MHz, CDCl3): d ¼ 8.75–8.69 (m, 6H), 8.26–
8.21(m, 2H), 7.90–7.81 (m, 4H), 7.78–7.63 (m, 6H), 7.61–
7.46(m, 6H), 7.42–7.01 (m, 10H), 4.41–4.30(br, 2H), 1.45 (br,
3H). 13C-NMR (75 MHz, CDCl3): d ¼ 156.4, 155.9, 149.5,
136.9, 133.2, 129.5, 128.2, 127.9, 127.6, 126.7, 124.8, 123.7,
122.4, 121.6, 120.3, 118.6, 117.7, 115.6, 111.5, 51.2, 13.5.
The number-average molecular weight and distribution were
obtained by GPC with polystyrene standards in THF to be
Mn ¼ 20950 with PDI ¼ 1.25 for P-1, and Mn ¼ 18700 with
PDI ¼ 1.06 for P-2. Both polymers P-1 and P-2 are easily
soluble in THF, DMF, and chloroform. The thermal stability of
the polymer P-1 was characterized by TGA. A slight mass
loss of 5% was observed from 300 to 340 ꢀC, and a main
mass loss of P-1 occurred at the temperature of around 425
ꢀC. The other data for physical properties of the polymers
are shown in Table 1.
Fluorescence Spectra of Polymer P-1 in the Presence of
Different Metal Ions
A solution of P-1 (1.0 lM) was prepared in THF. The solu-
tions of various metal ions (5.0 mM) were prepared in dis-
tilled water. A solution of P-1 (3.0 mL) was placed in a
quartz cell (1 cm width), and the fluorescence spectrum was
recorded. Different metal ion solutions were introduced (3.0
lL) into the polymer solution and the fluorescence intensity
was recorded at room temperature (excitation wavelength:
384 nm).
Optical Properties of the Copolymers
The UV-vis absorption and photoluminescence spectra of
polymers P-1 and P-2 in the THF solution (1.5 lM) are
shown in Figure 3. The polymer P-2 exhibits an absorption
maximum peak at 355 nm and an emission maximum peak
at 456 nm in THF solution, which are partly assigned to the
p–p* transition of the conjugated polymer backbone. The
stilbene-terpyridine side groups, which were reported to
possess a main absorption around 380 nm,33 definitely con-
tribute to the aforementioned absorption peak. Compared
with polymer P-2, red shifts in absorption and emission are
observed for polymer P-1, which shows an absorption maxi-
mum peak at 384 nm and an emission maximum peak at
510 nm in THF solution. The red shifts are ascribed to the
enhanced extent of p-orbital overlap and electron-donating
effect due to the dihexylfluorene residues in the backbone of
polymer P-1. Fluorescence quantum yields of the polymers
Spectrofluorometric Titration of Polymer P-1
with Fe31 Ion
Aliquots of Fe3þ ion in aqueous solution were added to a so-
lution of polymer P-1 in THF. The final concentration of
polymer P-1 is 0.5 lM, corresponding to the repeating unit.
After each addition, the sample was allowed to equilibrate
for 30 min before recording a spectrum. Addition of the
Fe3þ ion was continued until no change in the fluorescence
signal was observed. The excitation wavelength was 384 nm,
and the emission scan ranged from 400 to 750 nm.
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FIGURE 1 1H-NMR spectra of monomer 4, polymers P-1, and P-2.
were measured by a standard method using quinine bisulfate
Fluorescence Quenching by Various
Transition-Metal Ions
in a 0.1 N sulfuric acid as the reference absolute quantum ef-
ficiency (0.55). Polymer P-1 possesses a high fluorescence
quantum yield (0.62), however, polymer P-2 shows a much
lower fluorescence quantum yield (0.13), probably arising
from the lower extent of p-orbital overlap effect of the carba-
zole residues in the backbone of polymer P-2.34
The influence of various metal ions (Agþ, Cd2þ, Al3þ, Mn2þ
,
Cr3þ, Pb2þ, Zn2þ, Ni2þ, Fe3þ, Cu2þ, Co2þ, and Fe2þ) on the
fluorescence emission of polymer P-1 is shown in Figure 4.
The concentration of P-1 solution was fixed at 1.0 lM. The
metal ion solutions were used at a concentration of 5.0 lM.
As shown in Figure 4, Fe3þ, Cu2þ, and Ni2þ lead to an effi-
cient fluorescence quenching of the conjugated polymer.
Especially, nearly complete fluorescence quenching was
observed when Fe3þ was added. Whereas, the other metal
ions (Al3þ, Mn2þ, Cr3þ, and Pb2þ) result in a slight decrease
TABLE 1 Physical and Optical Properties of Polymers P-1 and
P-2
b
Absorbance Emission Quantum Td
a
Polymer Mn
PDI (kmax, nm)
(kmax, nm) yield
(ꢀC)
P-1
P-2
a
20,950 1.25 384
18,700 1.06 355
510
456
0.62
0.13
340
308
The number-average molecular weight (Mn), and polydispersity index
(PDI) were determined for P-1 and P-2 by GPC with polystyrene stand-
ards in THF.
Td was determined by TGA with heating rates of 10 ꢀC minꢁ1 under
b
FIGURE 2 FT-IR spectra of monomer 4, polymers P-1, and P-2.
nitrogen atmosphere.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 3 UV-vis absorption and fluorescence spectra of poly-
mers P-1 and P-2 in THF.
FIGURE 5 Emission quenching response profiles for P-1 by dif-
ferent metal ions in THF.
in the fluorescence emission of the polymer. This is consist-
ent with the poor coordination of these cations with terpyr-
idyl groups. The metal ions such as Zn2þand Fe2þ cause a lit-
tle heavier fluorescence quenching of P-1. The complexation
of metal ions do not cause any shift of the emission peak. On
the other hand, the addition of Zn2þ ions results in another
broad emission peak from 575 to 700 nm, as shown in Fig-
ure 4. This phenomenon is consistent with the coordination
of Zn2þ and terpyridine, which alters the electron density of
the polymer backbone.21 The distinct cation-responsive
behaviors reveal the different coordination abilities of transi-
tion-metal ions with terpyridyl receptors in the backbone of
the polymer. A possible mechanism for the fluorescence
quenching of P-1 is attributed to its aggregation derived
from P-1—metal ion complex formation.
ever, the addition of Fe3þ, Cu2þ, and Ni2þ ions into the solu-
tion of P-1 leads to a significant fluorescence quenching.
Especially, nearly complete fluorescence quenching was
observed when Fe3þ is added. This phenomenon shows that
the varying chelating ability of terpyridine with different
metal ions makes P-1 show a high sensitivity toward Fe3þ
,
which can be attributable to the unique properties of Fe3þ
and possible size-fit concept. For a d5 ion such as Fe3þ, the
five electrons may be present as two orbitals occupied by
pairs of electrons and one orbital having single occupancy.
So it could form an inner-orbital complex, which is more sta-
ble than the other terpyridine–metal complexes.35 In addi-
tion, the higher charge and smaller size of Fe3þ result in
high charge density, which make its electron-accepting easier,
thus lead to form stable terpyridine—Fe3þ complex.36 On
the basis of these results, it is very likely that polymer P-1 is
a highly sensitive and relatively selective chemosensor for
Fe3þ in the THF solution.
The distinctly different response profiles of P-1 upon the
addition of various metal ions could be easily seen in Figure
5. The fluorescence quenching of P-1 varied with different
metal ions. Most cations such as Al3þ, Mn2þ, and Cr3þ result
in a very limited influence on the polymer emission. How-
Figure 6 shows the fluorescence emission spectra of the P-1
in the absence and presence of Fe3þ ions at various concen-
trations. In these spectrofluorometric titration experiments,
the concentration of P-1 is 0.5 lM. The fluorescent emission
of the polymer gradually reduces with the increase in metal
ion concentrations stepwise. At the very beginning, the Fe3þ
concentration is only 0.1 lM, which is only 20% of the poly-
mer concentration, the fluorescence emission of P-1 is
quenched to 78% of its original emission intensity. And then
when the concentration of Fe3þ is 0.4 lM, almost 55.0% of
the emission of the polymer is quenched. At last, the emis-
sion was nearly completely quenched (98%) when the Fe3þ
concentration increases to 0.9 lM. In addition, there was a
small blue shift in the emission peak, which suggested a
shortening of the excitation migration distance in the back-
bone with increasing loading of quencher. Terpyridine pos-
sesses a superb ability to coordinate a large number of tran-
sition-metal ions and forms stable terpyridineꢁmetal ion
complexes of 2:1 stoichiometry.22,37 This makes terpyri-
dine—Fe3þ complex as the bridge of the photoelectron
energy transferring between the polymer backbones. Besides,
FIGURE 4 Fluorescence response profiles of copolymer P-1 in
THF upon addition of different metal ions.
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ARTICLE
ent electron-donating groups in the main backbones make the
polymers possess different selectivity to metal ions. Based on
the results reported above and by us, it is believed that by
selecting a proper type of the main backbone, the conjugated
polymers with pendant terpyridyl ligands may show distinct
selectivity to a certain transition-metal ion.
CONCLUSIONS
We have successfully designed and synthesized two well-
defined triphenylamine-based FCPs with pendant terpyridyl
ligands by Suzuki coupling polymerization. The polymers can
be a chemosensor for transition-metal ions. Polymer P-1
shows a significant sensitivity to transition-metal ions due to
the enhanced electronic communication properties of the con-
jugated polymers. Especially, Fe3þ ion can lead to complete flu-
orescence quenching of P-1. It demonstrates that the polymers
with a same receptor may show different sensitivity to analy-
tes owing to their different structure in the main backbone.
FIGURE 6 Fluorescence spectra and Stern—Volmer plot (inset)
of P-1 (0.5 lM) in the absence and presence of different con-
centrations of Fe3þ in solution: Fe3þ concentrations from top to
down are 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 lM.
The financial supports of the Ministry of Science and Technol-
ogy of China (National Basic Research Program, Grant No.
2007CB808000) and National Science Foundation of China
(Grant Nos. 20672025 and 20972035) is acknowledged.
Fe3þ may also bind to only one terpyridine ligand, where the
photoelectron energy transferring is from the backbone to
terpyridine—Fe3þ complex. Both of the two processes can
induce the fluorescence quenching of the polymer. These
results suggested that P-1 is a highly sensitive chemosensor
for Fe3þ in the THF solution. No other metal ions showed
such a significant response. It is interesting to note the non-
linear nature of the SternꢁVolmer plot in Figure 6 (inset),
suggesting that in this case the mechanism of the quenching
process involves complexation rather than collisional deacti-
vation. However, P-2 showed a poor selectivity to the metal
ions, while Co2þ, Ni2þ, Zn2þ, and Fe2þ could cause nearly
complete quenching of the polymer. The difference between
P-1 and P-2 is the structure in the main backbones, which
makes the two polymers possess completely different selec-
tivity to the metal ions. The only difference between the mo-
lecular structures of polymers P-1 and P-2 is the second
repeat unit (triphenylamine is defined as the first repeat
unit), which is fluorene and carbazole, respectively. The side
chain and the terminal terpyridine moiety are the same. We
believe that the fluorene units play a dominant role for co-
polymer P-1 in its high selectivity toward Fe3þ. The fluorene
moiety in P-1, poly(triphenylamine-alt-fluorene), is a better
electron-donating group than the carbazole in P-2. Therefore,
the photoelectron energy transferring from the backbone to
the terpyridineꢁmetal complex or photoelectron energy loss
in P-1 between backbones would be easy. Furthermore, the
p-orbital overlap extent in P-1 is enhanced by the 2,7-linked
fluorene unit as compared with P-2, poly(triphenylamine-alt-
N-ethylcarbazole), in which carbazole units are 3,6-linked.
Due to these reasons mentioned earlier, P-1 possesses a
higher fluorescence quantum yield than P-2 and shows a
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