Copoly(p-phenylene) containing azacrown ether: Synthesis, optical properties, and application for chemical sensor

Article abstract of DOI:10.1002/pola.26811

Copoly(p-phenylene) (P1) containing pendent azacrown ether and ethylene glycol ether was prepared by the Suzuki coupling reaction and applied for chemical sensor. Poly(p-phenylene) derivative P0 without the azacrown ether groups were also synthesized for comparative study. The photophysical and sensing properties were investigated by absorption and photoluminescence (PL) spectroscopy to elucidate the influence of the azacrown ether groups. The P1 exhibited specific selectivity and high sensitivity toward Zn²⁺, with the Stern-Volmer coefficient (K_sv) being 3.66 × 10⁶ M^-1 at low concentration in mixture solvent of tetrahydrofuran and water (THF/H₂O = 9/1, v/v). The P1 maintained high selectivity toward Zn²⁺ in the presence of fifteen interfering metal cations. This results indicate that copoly(p-phenylene) (P1) is a promising functional material for chemical sensors. Copyright

Full text of DOI:10.1002/pola.26811

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Copoly(p-phenylene) Containing Azacrown Ether: Synthesis, Optical
Properties, and Application for Chemical Sensor
Chia-Shing Wu, Chiao-Pei Chen, Yun Chen
Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan
Correspondence to: Y. Chen (E-mail: yunchen@mail.ncku.edu.tw)
Received 7 April 2013; accepted 1 June 2013; published online
DOI: 10.1002/pola.26811
ABSTRACT: Copoly(p-phenylene) (P1) containing pendent aza-
crown ether and ethylene glycol ether was prepared by the Suzuki
coupling reaction and applied for chemical sensor. Poly(p-phenyl-
ene) derivative P0 without the azacrown ether groups were also
synthesized for comparative study. The photophysical and sens-
ing properties were investigated by absorption and photolumi-
nescence (PL) spectroscopy to elucidate the influence of the
azacrown ether groups. The P1 exhibited specific selectivity and
high sensitivity toward Zn21, with the Stern-Volmer coefficient
(Ksv) being 3.66 3 106 M21 at low concentration in mixture sol-
vent of tetrahydrofuran and water (THF/H2O 5 9/1, v/v). The P1
maintained high selectivity toward Zn21 in the presence of fifteen
interfering metal cations. This results indicate that copoly(p-phen-
ylene) (P1) is a promising functional material for chemical sen-
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sors. 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym.
Chem. 2013, 00, 000–000
KEYWORDS: conjugated polymers; optics; sensors
thus called a fluoroionophore.20 Among the typical conju-
gated fluorescent materials,21 poly(p-phenylene)s (PPPs)
derivatives show good thermal stability and exhibit good
photoluminescence (PL) efficiency.22 Moreover, ethylene
glycol groups extremely shows good solubility in highly polar
organic solvents.
INTRODUCTION Conjugated polymers have gathered a lot of
attention due to their potential applications in light-emitting
diodes,1,2 chemical sensors,3,4 organic solar cells,5,6 and field-
effect transistors.7,8 Conjugated polymers-based fluorescent sen-
sors have emerged as an important chemical sensory material
attributed to its high sensitivity in transforming chemical signal
into electrical or optical signals upon binding with analyte.
Recently, there has been a growing need for developing highly
sensitive and selective probes for the detection of metal ions in
biological and environmental samples. A variety of divalent
metal ions are known to be involved in the structural, catalytic,
and regulatory aspects of the biological system, and some such
metal ions serve as prognostics of certain human diseases.9,10
In this work, we report the synthesis and characterization of
a
novel copoly(p-phenylene) P1 containing pendant
azacrown ether and ethylene glycol ether groups. The PL of
P1 is significantly quenched only by Zn21, even in the pres-
ence of 15 interfering cations. Moreover, the color changes
after recognition are observable by naked eye. Accordingly, it
is a promising chemical sensor for Zn21
.
Zinc is an essential nutrient required for normal growth and
development11 and for key cellular processes such as DNA
repair12,13 and apoptosis.14 Zinc concentrations are tightly regu-
lated under normal physiological conditions.11,15 Failure to
maintain zinc homeostasis has been implicated in a number of
pathologies including diabetes16 and Alzheimer’s disease.17
Zinc metal is a soil pollutant too, significant concentrations of
which cause phytotoxic effects on soil microbes. Therefore,
selective discrimination of zinc ions is still considered an
appealing area of research.18,19 Although, monoaza-15-crown-5
is known to bind zinc ion, and surprisingly Zn21 sensors incor-
porating this receptor are scarce in the literature.19
EXPERIMENTAL
Materials
Acetyl chloride, triethylene glycol, ferrocene, n-bromosuccini-
mide, 1,10-azobis (cyclohexane carbonitrile), 2-methoxyethanol,
bromobenzene, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxab
orolane, and NaH (60 wt % dispersed in mineral oil) were pro-
cured from Acros Co. N-phenyldiethanolamine, sodium ethox-
ide, p-toluenesulfonyl chloride, triphenylphosphine, 2,5-
dibromo-p-xylene, tert-butyllithium, and tetrakis (triphenyl-
phosphine) palladium [Pd(PPh3)4] were acquired from Alfa
Aesar. Triethylamine, potassium tert-butoxide, and phosphoryl
trichloride was purchased from Merck, Lancaster, and Aldrich,
respectively, and used without further purification.
Fluorescent chemosensor consists of a fluorophore (signaling
moiety) linked to an ionophore (recognition moiety) and is
Additional Supporting Information may be found in the online version of this article.
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Tetrahydrofuran (THF) was dried with potassium hydroxide
before usage. All the reagents were used without further purifi-
cation expect specifically notified.
compound 1. Chloroform and sodium acetate solution
(24.7 g, 0.3 mol) was added slowly under ice bath and
stirred at room temperature overnight. The mixture was
extracted with chloroform and the organic layer was
dried (MgSO4) and concentrated under reduced pressure
to give 2 (12.2 g, 92.8%). 1H NMR (400 MHz, CDCl3,
TMS, 25ꢀC): d 9.74 (s, H, ACHO) 6.81–6.83.(d, 2H, J 5 8.6
Hz, ArAH), 7.73–7.75 (d, 2H, J 5 8.7 Hz, ArAH), 4.26–4.29
(t, 4H, J 5 12.2 Hz, ACH2A), 3.71–3.74 (t, 4H, J 5 12.3 Hz,
ACH2A), 2.04 (s, 6H, ACH3). Anal. Calcd. for C15H19NO5
(%): C, 61.42; H, 6.53; N, 4.78. Found: C, 61.09; H, 6.56; N,
4.65. FT-IR (KBr, cm21): m 1676 (ACHO stretch), 1739
(C@O stretch), 2815 (ACH aldehyde), and 2963 (ACH
stretch).
Instruments
All synthesized compounds were identified by 1H NMR,
FT-IR, and elemental analysis (EA). The 1H NMR spectra
were recorded on a Bruker AMX-400 MHz FT-NMR, and the
chemical shifts were reported in ppm using tetramethylsi-
lane (TMS) as an internal standard. The FT-IR spectra were
measured as KBr disk on a Fourier transform infrared
spectrometer, model 7850 from Jasco. The EA was carried
out on a Heraus CHN-Rapid elemental analyzer. The ther-
mogravimetric analysis (TGA) was performed under
a
nitrogen atmosphere using a PerkinElmer TGA-7 thermal
analyzer at a heating rate of 10ꢀC/min. The thermal behav-
iors and thermal transitional properties of the polymers
were investigated using a differential scanning calorimeter
(DSC), Mettler DSC 1, with a heating rate of 10ꢀC/min.
Absorption and PL spectra were measured on a Jasco V-550
spectrophotometer and a Hitachi F-4500 spectrofluorometer,
respectively.
Synthesis of 4-Bis(2-hydroxyethyl)amino)benzaldehyde (3)
A mixture of 2 (10 g, 34 mmol), methanol (150 mL) and
sodium carbonate solution (10.9 g, 102 mmol) was stirred at
room temperature overnight. Few drops of 1N HCl were
added to the mixture; and then the methanol was removed
by rotary evaporation. The mixture was extracted with chlo-
roform and the organic layer was dried (MgSO4) and concen-
trated under reduced pressure. The residue was purified by
column chromatography eluted with ethyl acetate to give 3
Preparation of Metal Ion Solutions and Titration
1
The chloride salts of Li1, Na1, K1, Ag1, Cu21, Ca21, Zn21
,
(4.9 g, 69.5 %). H NMR (400 MHz, CDCl3, TMS, 25ꢀC): d
Co21, Ba21, Pb21, Cd21, Ni21, Fe21, Hg21, Fe31, Al31, and
Ru31 were dissolved in a mixture solvent of THF/H2O (9/1,
v/v) or ethanol/H2O (9/1, v/v) to prepare their correspond-
ing stock solutions (1024 M). All of the optical measure-
ments were operated right after the test solutions were
prepared and thoroughly mixed. The Stern-Volmer constant
(Ksv) was estimated from Stern-Volmer plot, which correlates
9.62 (s, H, ACHO) 7.63–7.66 (d, 2H, J 5 8.7 Hz, ArAH),
6.68–6.71 (d, 2H, J 5 8.7 Hz, ArAH), 3.86–3.89 (t, 4H,
ACH2A), 3.65–3.68 (t, 4H, ACH2A), 3.40–3.45 (d, 2H, J 5 16.4
Hz, AOH). Anal. Calcd. for C11H15NO3 (%): C, 63.14; H, 7.23; N,
6.69. Found: C, 63.07; H, 7.28; N, 6.59. FT-IR (KBr, cm21):
m 1667 (AC@O stretch), 2753 (ACH of aldehyde), and 3352
(AOH stretch).
fluorescence quenching (Fo/F)
2 1 versus quencher’s
Synthesis of Triethyleneglycol Ditosylate (4)
concentration.
A mixture of p-toluenesulfonyl chloride (5.75 g, 0.03 mol),
triethylene glycol (1.5 g, 0.01 mol) and THF (15 mL) was
mechanically stirred at 0ꢀC. Aqueous solution of KOH (16 M,
4 mL) was added portionwise to the mixture within 1 h and
then stirred at room temperature for 7 h. The mixture was
poured into ice water, the appearing white precipitate was
collected by filtration and recrystallized from methanol/
water to afford white crystal of 4 (4.05 g, 88.6%, m.p.
83.5–84ꢀC). 1H NMR (400 MHz, CDCl3, TMS, 25ꢀC):
7.25–7.79 (m, 8H, aromatic region), 4.1 (m, 4H), 3.6 (m, 4H),
3.5 (s, 4H), and 2.4 (s, 6H). FT-IR (KBr, cm21): 1600
(aromatic ring), 1129 (CAOAC), 1175 (sulfonate). Anal. Calc.
for C20H26O8S2 (%): C, 52.39; H, 5.72. Found: C, 52.37;
H, 5.72.
Synthesis of N,N-Bis(acetoxyethyl)phenylamine (1)
A mixture of N-phenyldiethanolamine (8.0 g, 44 mmol),
triethylamine (13.3 g, 132 mmol), and THF (40 mL) was
added to a solution of acetyl chloride (9.3 mL, 132 mmol) in
THF (40 mL). The mixture was stirred at 35ꢀC for 20 h.
Then, 40 mL water was added dropwise to the mixture
slowly. After removal of THF by rotary evaporation, the mix-
ture was extracted with chloroform; the organic layer was
dried (MgSO4) and concentrated under reduced pressure.
The residue was distillated in vacuo (210ꢀC, 7 torr) to obtain
1
1 (9.7 g, 82.9 %). H NMR (400 MHz, CDCl3, TMS, 25ꢀC):
d 7.20–7.24 (t, 2H, J 5 16 Hz, ArAH), 6.70–6.76 (m, 3H,
ArAH), 4.21–4.24 (t, 4H, J 5 12.5 Hz, ACH2A), 3.59-3.63
(t, 4H, J 5 12.6 Hz, ACH2A), 2.04 (s, 6H, ACH3). Anal. Calcd.
for C14H19NO4 (%): C, 63.38; H, 7.22; N, 5.28. Found: C,
62.98 7.18; N, 5.22. FT-IR (KBr, cm21): m 1735 (C@O stretch),
2963 (ACH stretch).
Synthesis of 4-(Monoaza-15-crown-5)benzaldehyde (5)
A mixture of compound 3 (0.4 g, 2.09 mmol) and NaH (0.8 g,
60 % in mineral oil, 20 mmol) in dried THF (300 mL) was
refluxed for 30 min under nitrogen atmosphere. The mixture
was added dropwise with a solution of compound 4 (1.0 g, 2.1
mmol) in THF (200 mL) and then stirred at 70ꢀC for 2 days.
After adding an aqueous solution of H2SO4 (2 M, 50 mL), the
THF was removed by rotary evaporation. The mixture was then
extracted with chloroform and the organic layer was dried
Synthesis of 4-(Bis(2-acetoxylethyl)amino)benzaldehyde (2)
Phosphoryl trichloride (6.9 g, 45 mmol) was added
slowly to N,N-dimethylformamide (13.8 g, 188 mmol)
under ice bath and stirred at room temperature for 1 h,
and then reacted at 75ꢀC for additional 12 h after adding
2
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SCHEME 1 Synthetic procedures of diborate monomer (9) and dibromo monomer (DC).
(MgSO4) and concentrated under reduced pressure. The crude
product was purified by column chromatography (eluent: ace-
7.77; N, 4.14. FT-IR (KBr, cm21): m 2798 (ACH aldehyde), 1667
(AC@O stretch), and 1124 (CAO stretch).
1
tone/n-hexane 5 3/2) to give 5 (0.3 g, 44.5 %). H NMR (400
MHz, CDCl3, TMS, 25ꢀC): d 9.73 (s, H, ACHO) 7.70–7.73 (d, 2H, J
5 8.8 Hz, ArAH), 6.71–6.74 (d, 2H, J 5 8.7 Hz), 3.52 (m, 4H,
crown ether), 3.77–3.80 (m, 16H, crown ether). Anal. Calcd. for
Synthesis of 1,4-Dibromo-2,5-bis(bromomethyl)benzene
(6)
A mixture of 2,5-dibromo-p-xylene (6 g, 22.73 mmol) and
N-bromosuccinimide (8.9 g, 50 mmol) in benzene (60 mL) was
C17H25NO5 (%): C, 63.14; H, 7.79; N, 4.33. Found: C, 62.68; H,
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cooled to 278ꢀC, added dropwise with tert-BuLi (10 mL,
TABLE 1 Characterization and Thermal Properties of the
Polymers
1.9 M in pentane, 19 mmol) and stirred for 4 h. And then
2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane
(3.3
Polymer Mn (3103) Mw (3103) PDIa Tg (ꢀC)b Td (ꢀC)c
a
a
mL, 16 mmol) was added to the reaction mixture and
stirred for 1 h. The mixture was extracted using ether and
water, dried over MgSO4, and dried by rotary evaporation.
The residue was recrystallized from n-hexane to afford
light yellow crystal of 9 (0.9 g, 33.5%, m.p. 74–75ꢀC). 1H
NMR (400 MHz, CDCl3, TMS, 25ꢀC): d 7.60 (s, 2H, ArAH),
4.77 (s, 4H, ACH2A), 3.59–3.61 (m, 4H), 3.53–3.56 (m,
4H), 3.37 (s, 6H, ACH3), 1.34 (s, 24H, ACH3). Anal. Calcd.
for C26H44B2O8 (%): C, 40.80; H, 4.89. Found: C, 40.93;
H, 5.03. FT-IR (KBr, cm21): m 1105 (CAO stretch), 2877
(OACH2).
P0
P1
2.28
6.79
3.47
1.52 195
2.0
346
362
13.58
–
a
Mn, Mw, and PDI of the polymers were determined by gel permeation
chromatography using polystyrene standards in CHCl3.
The glass transition temperature was measured by DSC at a scan rate
b
of 10 ꢀC/min.
c
Thermal decomposition temperature at 5 wt % loss: measured by TGA
at a heating rate of 10 ꢀC/min under nitrogen.
refluxed under nitrogen atmosphere. And then 1,10-azo-
bis(cyclohexane carbonitrile) was added to the mixture.
The mixture was stirred at 80ꢀC for 1 day under nitrogen
atmosphere. The mixture was extracted with chloroform
and the organic layer was dried (MgSO4) and concen-
trated under reduced pressure. After removal of CHCl3 by
a rotary evaporator, it was re-crystallized from ethanol to
Synthesis of 2,5-Dibromo-1,4-bis[ethene-bis(monoaza-
15-crown-5)styryl] (DC)
A mixture of compound 5 (3.76 g, 11.64 mmol) and
compound 7 (2.21 g, 2.33 mmol) in dried ethanol (5 mL)
was added dropwise with sodium ethoxide (3.77 mL, 11.64
mmol) and stirred at room temperature overnight under
nitrogen atmosphere. The mixture was extracted with chloro-
form, the organic layer was dried (MgSO4) and concentrated
under reduced pressure. The crude product was purified by
column chromatography (eluent: acetone/n-hexane 5 1/1,
EA) to give monomer DC (0.8 g, 39%). 1H NMR (400 MHz,
CDCl3, TMS, 25ꢀC): d 7.82 (s, H, ArAH), 7.41–7.43 (d, 2H,
J 58.8 Hz, ArAH), 7.11–7.15 (d, 2H, AC@CHA), 6.94–6.98
(d, 2H, AHC@CA), 6.66–6.68 (d, 2H, J 5 8.7 Hz, ArAH),
3.76–3.79 (m, 8H, crown ether), 3.64–3.67 (m, 32H, crown
ether). Anal. Calcd. for C42H54Br2N2O8 (%): C, 57.67; H, 6.22;
N, 3.20. Found: C, 57.50; H, 6.22; N, 3.19. FT-IR (KBr, cm21):
m 1138 (CAO stretch), 2883 (OACH2).
afford white crystal of
6
(4.32 g, 43.7%, m.p.
1
157.5–158ꢀC). H NMR (400 MHz, CDCl3, TMS, 25ꢀC): d
7.66 (s, 2H, ArAH), 4.51 (s, 4H, ACH2A). Anal. Calcd. for
C8H6Br4 (%): C, 22.78; H, 1.43. Found: C, 22.84; H, 1.52.
Synthesis of 2,5-Dibromo-1,4-
bis[methylene(triphenylphosphonium bromide)] (7)
To a mixture of compound 6 (5 g, 11.8 mmol) and triphenyl-
phosphine (9.32 g, 35.4 mmol) and DMF (100 mL) was
stirred at 100ꢀC for 18 h under nitrogen atmosphere. The
mixture was poured into an excess of ethyl ether, the appear-
ing solid was collected by filtration and re-crystallized from
methanol to afford white crystal of 7 (7.1 g, 63.4%). 1H
NMR (400 MHz, CDCl3, TMS, 25ꢀC): d 7.64–7.81 (m, 30 H,
ArAH), 7.37 (s, 2H, J 5 8.7 Hz, ArAH), 5.67–5.69 (m, 4H),
3.52 (m, 4H, crown ether), and 3.77–3.80 (m, 16H, crown
ether). Anal. Calcd. for C44H36Br4P2 (%): C, 54.8; H, 3.97.
Found: C, 54.78; H, 4.08.
Synthesis of Polymers P0 and P1
To a 25-mL glass reactor was added with compound 9
(0.116 g, 0.23 mmol), DC (0.2 g, 0.23 mmol), aqueous solu-
tion of 2 M K2CO3 (5 mL), Pd(PPh3)4, and toluene (10 mL)
under nitrogen. The mixture was stirred at 80ꢀC for 2 days
after adding Aliquat 336. The polymerization proceeded for
additional 12 h after adding bromobenzene (0.018 g, 0.115
mmol) as end-capping agent. The mixture was extracted
with distilled water three times after adding chloroform. It
was then poured into a lot of amount of n-hexane and the
appearing precipitate was collected by filtration. The precipi-
tate was purified by extracting with n-hexane for 2 days
using a Soxhlet extractor to provide P1 (0.1 g, 45.5 %). The
synthesis procedures of homopolymer (P0) are similar to
those used in the preparation of P1 except that compound 8
was used instead of DC. The yield was 60% (0.12 g).
Synthesis of 1,4-Dibromo-2,5-bis[(2-
methoxyethoxy)methyl]benzene (8)
A mixture of compound 6 (3.7 g, 8.8 mmol), 2-methoxyethanol
(4 g, 52.6 mmol), and NaH (2.11 g, 60 % in mineral oil, 52.6
mmol) in dried THF (50 mL) was refluxed for 1 day under nitro-
gen atmosphere. The mixture was then extracted with chloro-
form and the organic layer was dried (MgSO4) and
concentrated under reduced pressure. It was recrystallized
from ethanol to afford light yellow crystal of 8 (2.2 g, 61.1%,
m.p. 73.5–74ꢀC). 1H NMR (400 MHz, CDCl3, TMS, 25ꢀC): d 7.68
(s, 2H, ArAH), 4.58 (s, 4H, ACH2A), 3.71–3.73 (m, 4H),
3.61–3.63 (m, 4H), 3.42 (s, 6H, ACH3). Anal. Calcd. for
C
14H20Br2O4 (%): C, 54.8; H, 3.97. Found: C, 54.88; H, 4.08.
P1
FT-IR (KBr, cm21): m 1105 (CAO stretch), 2877 (OACH2).
1H NMR (400 MHz, CDCl3, TMS, 25ꢀC): d 7.74, 7.35–7.59,
7.15, 6.55–6.60, 6.34 (m, 16H, ArAH and AHC@CHA),
4.42–4.45 (m, 4H, ArACH2AO), 3.63, 3.49, 3,37 (m, 48H,
crown ether and OACH2A), 3.15–3.17 (m, 6H, ACH3). Anal.
Calcd. for C56H74N2O12 (%): C, 69.54; H, 7.71; N, 2.90. Found:
C, 68.55; H, 7.61; N, 2.74.
Synthesis of 2,20-[2,5-Bis(2-methoxyethoxy)methyl]-1,4-
phenylene-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane) (9)
To a 50-mL glass reactor was added with compound 8
(2.2 g, 5.3 mmol) and dried THF (35 ml). The mixture was
4
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FIGURE 1 1H NMR spectra of (a) compound DC, (b) P0, and (c) P1.
P0
ACH2A), 3.32–3.36 (m, 6H, ACH3). Anal. Calcd. for
14H20O4 (%): C, 66.65; H, 7.99. Found: C, 65.54; H,
7.86.
1H NMR (400 MHz, CDCl3, TMS, 25ꢀC): d 7.47 (s, 2H,
ArAH), 4.36–4.39 (m, 4H, ArACH2AO), 3.64–3.68 (m, 8H,
C
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FIGURE 2 Normalized absorption spectra and PL emission
spectra of P1 (a) in CHCl3 solution (1.0 3 1025 M) (b) in the
film state.
FIGURE 3 Absorption spectra of P1 with various metal ions in
solution (THF/H2O 5 9/1, v/v).The concentration of P1 and
metal ions were fixed at 1025 M and 1024 M, respectively.
RESULT AND DISCUSSION
The chemical shifts at 6.34–7.74 ppm (a) are assigned to
aromatic and vinylene protons, whereas those at 3.37–3.63
ppm (c) are attributed to the protons in crown ether and
ethylene glycol ether groups.
Synthesis and Characterization of Monomer DC and
Polymers
Novel dibromo-monomer (DC) containing azacrown ether
moiety was synthesized from compound 5 and compound 7
via several steps as shown in Scheme 1. The structure of DC
was satisfactorily identified by 1H NMR spectra and EA.
Copoly(p-phenylene) containing pendent azacrown ether
groups (P1) was successfully synthesized from DC and com-
pound 9 by the Suzuki coupling reaction using Pd(PPh3)4 as
catalyst. The polymerization results and thermal properties
of P0 and P1 are summarized in Table 1. Figure 1(a) shows
The weight-average molecular weights (Mw) of P0 and P1,
determined by gel permeation chromatography using mono-
disperse polystyrenes as standard, were 3.47 3 103 and
13.58 3 103 with the polydispersity indexes (PDI) being
1.52 and 2.0, respectively. The P1 is soluble not only in
common organic solvent but also in high polar solvents such
as ethanol due to polar nature of the crown ether and ethyl-
ene glycol ether moieties. Moreover, incorporation of crown
ether moiety provides the ability to capture metal ion.
Accordingly, the resulting conjugated polymers might be
applied as polymer-based chemosensor. Thermal stability
and glass-transition temperature (Tg) were evaluated by TGA
1
the H NMR spectra of DC, in which chemical shifts at 7.11–
7.15 (c) and 6.94–6.98 ppm (d) have been assigned to pro-
tons of vinylene groups. The chemical shifts at 7.82 (a),
7.41–7.43 (b) and 6.66–6.68 ppm (e) are attributed to aro-
matic protons of 1,4-distyrylphenylene. The chemical shifts
at 3.64-3.79 ppm are readily assigned to the protons of
crown ether moiety, among which the ones at 3.76–3.79
ppm (f) are assigned to the protons symmetrically adjacent
1
to the nitrogen atom. Figure 1(b) shows the H NMR spectra
of P0. The peaks at 7.47 ppm (a) and 3.64–3.68 ppm (c) are
assigned to the protons of phenylene and pendant ethylene
glycol ether. And the chemical shifts at 4.36–4.39 ppm
(b) and 3.32–3.36 ppm (d) are ascribed to benzyl and
methyl protons, respectively. Using aforementioned results,
the 1H NMR peaks of P1 can be assigned readily [Fig. 1(c)].
TABLE 2 Optical Properties of Polymers
Solutiona
Film
Absorption
(nm)
Absorption PL
Polymer (nm)
(nm)b UPL
PL (nm)b
P0
P1
250
410
385
497
0.17 258
0.43 417
400
504
FIGURE 4 Fluorescence spectra of P1 with various metal ions
in solution (THF/H2O 5 9/1, v/v).The concentration of P1 and
metal ions were fixed at 1025 M and 1024 M, respectively. Exci-
tation wavelength was 411 nm.
a
In CHCl3 (1 3 1025 M).
b
The excitation wavelength was 350 nm.
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497 nm. In film state, the absorption peaks of P0 and P1 are
shifted slightly to 258 nm and 417 nm respectively, with the
emission maxima being moved bathochromically to 400 and
504 nm. The spectral red-shift in going from solution to film
state is attributable to aggregate formation, which is caused
by strong intermolecular interaction between pendent polar
ether groups. Figures S4 and S5 (Supporting Information)
show the absorption and fluorescence spectral variations of
P1 in various solvents. They are red-shifted slightly with an
increase in solvent polarity, indicating that the dipole
moment in the excited state (lE) is larger than in the ground
state (lG). Solvent dipoles reorient or relax around excited
chromophore to lower its energy that leads to gradual red-
shift in emission with increasing solvent polarity.
FIGURE 5 Fluorescence emission response profile of P1 upon
addition of various metal ions in solution (THF/H2O 5 9/1, v/v).
Chemosensory Properties
The concentration of P1 and metal ions were fixed at 1025
and 1024 M, respectively. Excitation wavelength was 411 nm.
M
Crown ether has been commonly employed as molecular
host to complex with alkali and alkaline earth metal cations,
transition metal cations and ammonium cations. Combination
of crown ether receptor with conjugated polymer backbone
renders amplified fluorescence and absorption spectral varia-
tions in the presence of different metal ions.23 The P1 is a
functional material consisting of conjugated backbone and
pendent crown ether as recognition moiety. Therefore, its
applications as chemical sensor for cations can be expected.
Fluorescence has been a widely used sensing signal in chem-
ical sensors; it offers diverse transduction schemes based on
change in intensity, energy transfer, or wavelength shift. To
investigate its applicability in contaminated aqueous solu-
tion, the sensory characteristics of P1 were studied in two
aqueous mixture solvent systems (THF/H2O 5 9/1 and etha-
nol/H2O 5 9/1). Figures 3 and 4 and Figures S6 and S7
(Supporting Information) show the absorption and PL spec-
tral variations of P1 in the presence of various cations. In
the presence of Zn21, the absorption spectra demonstrate
obviously blue-shift with simultaneously diminished inten-
sity. However, the absorption spectral intensity and wave-
length remain almost unchanged in the presence of other
and DSC, respectively. As shown in Figure S1 (Supporting
Information), thermal decomposition temperature (at 5%
weight loss) of P0 (346ꢀC) is lower than that of P1 (362ꢀC).
Incorporation of conjugated 1,4-distyrylphenylene structure
raises thermal resistance slightly. The glass transition tem-
perature (Tg) of P0 was 195ꢀC, while no clear Tg was
observed for P1 (Figure S2 Supporting Information).
Optical Properties
Figure S3 (Supporting Information) and Figure 2 show the
absorption and fluorescence (PL) spectra of P0 and P1 in
CHCl3 and as thin films spin-coated from CHCl3 solution
(10 mg/mL), with the corresponding optical data summar-
ized in Table 2. In CHCl3 solution, P0 and P1 show absorp-
tions peaked at 250 and 410 nm, respectively, which can be
attributed to the p-p* electronic transitions of PPP main
chain and 1,4-distyrylphenylene chromophore, respectively.
And their fluorescence spectra show peaks at 385 nm and
FIGURE 6 Absorption spectrum of various metal ions and fluorescence spectrum of P1 in solution (THF/H2O 5 9/1, v/v), excitation
wavelength was 411 nm.
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FIGURE 7 (a) Fluorescence spectral variations and (b) Stern-Volmer plot of P1 with varying concentration of Zn21 in solution
(THF/H2O 5 9/1, v/v). The concentration of P1 was fixed at 1025 M. Excitation wavelength was 411 nm.
cations tested. The PL intensity is also significantly quenched
only in the presence of Zn21, as demonstrated in its fluores-
cence emission response profiles toward various metal ions
in the two solvent mixtures (Fig. 5 and Supporting Informa-
tion Fig. S8). In addition, Swager and coworkers reported a
highly specific and sensitive transduction mechanism that
uses ion-mediated interpolymer aggregation.3(b) In our case,
slight absorption spectral blue shift and obvious quenching
in the presence of Zn21 imply the quenched PL is not attrib-
uted to inter-polymer aggregation.
energy is readily transferred to the bound metal cation which is
in the proximity of the excited chromophore. The obvious
absorption spectral blue-shift is caused by the interaction of
Zn21 with azacrown ether which destabilizes the excited state
more than the ground state. Fluorescence quenching could be
induced from the transfer of an electron from the excited state
directly to the metal center.24 Furthermore, quenching effi-
ciency depends on the degree of overlap between absorption
spectrum of metal ion and fluorescence spectrum of fluoro-
phore.25,26 As shown in Figures 6 and S9 (Supporting Informa-
tion), the larger overlap of Zn21 absorption with emission of P1
results in significant quenching (energy transfer) in solutions.
The PL intensity decreases with an increase in concentration of
Zn21 (Figs. 7 and S10 Supporting Information).
Although the monovalent cations would be a good cavity fit for
the present macrocycle (1.77–2.2 Å), such as Li1 (1.36 Å) and
Na1 (1.9 Å), but their absorption spectral intensities and wave-
lengths remain almost unchanged. However, high selectivity of
P1 suggests its potential application as unique chemical sensor
for Zn21. The selectivity is probably not only ascribed to the
size match of Zn21 (1.48 Å) and cavity (1.77–2.2 Å) of azacrown
ether. In addition, binding of divalent ions with the crown ether
leads to more stable complex than univalent ions. Photo-excited
The Stern-Volmer plot for Zn21 shows upward curvature,
suggesting that the quenching is attributed to the sphere of
action mechanism.27 At low Zn21 concentration the PL emis-
sion diminish is attributed to static quenching, while at
higher concentration dynamic quenching dominates. As the
Zn21 concentration is increased, the possibility of collision
probability is raised and the distance between Zn21 and flu-
orophore is shortened that leads to efficient energy transfer.
The Stern-Volmer constants (Ksv) of P1 toward Zn21 in THF/
H2O (v/v 5 9/1) were 1.07 3 105 M21 and 3.66 3 106 M21
for the static and dynamic quenching regions. However, in
ethanol/H2O mixture the constants were decreased to 3.61
FIGURE 8 Fluorescence spectral quenching of P1 by Zn21 inter-
fered by various metal ions (Li11 Na11 K11 Ag11 Ca211
Ba211 Cu211 Cd211 Ni211 Pb211 Co211 Fe211 Fe311
Al311 Ru31). THF/H2O (v/v 5 9/1). The concentration of P1 and
metal ions were fixed at 1025 M and 1024 M, respectively. Exci-
tation wavelength was 411 nm.
FIGURE 9 Color changes of P1 in the presence of various
metal ions and the target metal ions Zn21. (Left image: only
P1, middle image: P1 1 other cations, right image: P1 1 other
cations 1 Zn21). (a) THF/H2O 5 9/1, v/v and (b) ethanol/H2O 5
9/1, v/v.
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quenched by Zn21 cation, with static (dynamic) Stern-Volmer
constants (Ksv) being 1.07 3 105 M21 (3.66 3 106 M21) in
THF/H2O and 3.61 3 104 M21 (1.62 3 105 M21) in etha-
nol/H2O mixtures. The selective recognition of P1 toward
Zn21 was still maintained in the presence of fifteen interfer-
ing metal cations. This results indicate that P1 is a promising
3 104 M21 and 1.62 3 105 M21, respectively. The Ksv values
in THF/H2O mixture are almost one order larger than those
in ethanol/H2O mixture. This is attributable to the viscosity
difference between the two solvent mixtures. However, for
the THF/H2O 5 9/1 and ethanol/H2O 5 9/1 solvent
systems, the PL intensity is significantly quenched as the
concentration ratio of [Zn21]/[azacrown ether] is about five
(Figure S11 Supporting Information).
chemical sensor for the detection of Zn21
.
The Ksv value can be estimated by eq 1, where kq is bimolec-
ular quenching constant, s0 was the lifetime of fluorophore
in the absence of quencher, fQ is the quenching efficiency,
k0 is diffusion controlled bimolecular rate constant, R is the
collision radius which is generally assumed to be the sum of
the molecular radii of the fluorophore (Rf) and quencher
(Rq), and D is the sum of the diffusion coefficients of the
fluorophore (Df) and quencher (Dq), and N was Avogadro’s
number. And eq 2 is used to calculate D (diffusion coeffi-
cient), where j is Boltzmann’s constant, T is temperature,
g is the solvent viscosity, and R is the molecular radius.21
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CONCLUSIONS
We have successfully synthesized a novel copoly(p-phenyl-
ene) P1 containing pendent azacrown ether groups and a
homopolymer P0 by the palladium-catalyzed Suzuki coupling
reaction. Fluorescence of P1 solution was selectively
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