Synthesis and photoproperties of Eu(III)-bearing star polymers as luminescent materials

Article abstract of DOI:10.1002/pola.26652

Water-soluble luminescent material was developed by introducing europium (Eu(III)) ions into the core of a star polymer. Living radical polymerization was used to obtain the star polymer. The strategy to introduce Eu(III) ions into the star polymer was st

Full text of DOI:10.1002/pola.26652

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Synthesis and Photoproperties of Eu(III)-Bearing Star Polymers as
Luminescent Materials
Ahmad Kusumaatmaja,1 Tsuyoshi Ando,1 Kayo Terada,1 Shiho Hirohara,2 Takuya Nakashima,1
Tsuyoshi Kawai,1 Takaya Terashima,3 and Masao Tanihara1
1Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0192, Japan
2Department of Chemical and Biological Engineering, Ube National College of Technology, Ube, Yamaguchi 755–8555, Japan
3Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyotodaigaku-Katsura, Nishikyo-Ku,
Kyoto 615–8510, Japan
Correspondence to: T. Ando (E-mail: tando@ms.naist.jp)
Received 18 January 2013; accepted 15 February 2013; published online 00 Month 2013
DOI: 10.1002/pola.26652
ABSTRACT: Water-soluble luminescent material was developed
by introducing europium (Eu(III)) ions into the core of a star
polymer. Living radical polymerization was used to obtain the
star polymer. The strategy to introduce Eu(III) ions into the
star polymer was studied using poly(methyl methacrylate) as
an arm. The best Eu(III) ion introduction was obtained by si-
multaneous introduction, resulting in about 30 mmol/g-poly-
mer, which needed only one step for synthesis. The
utilization of a hydrophilic polymer such as poly(ethylene ox-
ide) (PEO) as an arm produced a water-soluble star polymer.
The Eu(III)-bearing PEO star polymer obtained in this study
was water soluble and showed fluorescence. In addition, it
was stable in water after 1 month. The Eu(III)-bearing star
polymer exhibited luminescent properties under UV light irra-
diation with relatively high quantum yields of 60% in organic
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solution and 19% in aqueous solution.
2013 Wiley Periodi-
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KEYWORDS: living radical polymerization; luminescence; micro-
encapsulation; metal-polymer complexes; star polymers;
water-soluble polymers
applications, lanthanide(III)-based luminescent materials
should be water soluble. There have been some attempts to
obtain water-soluble lanthanide(III) luminescent materials
by complexation of lanthanide(III) (europium (Eu), terbium
(Tb)) with pyridine-2,4,6-tricarboxylic acid (H3PTA)13 or
incorporation into nanoparticles.16–19 Water-soluble Eu(III)
and Tb(III) complexes have been reported as binding tags
for studying protein interactions,20,21 or bacterial spore,22
or singlet oxygen23 detection, for signaling of carbonate
chelation,24 as labels for prolactin in human serum,25 and
as luminescent tags on magnetic nanoparticles for cell
imaging applications.26 However, it is known that Eu(III)
complexes are unstable in aqueous medium27 and thus
there are few water-soluble polymers that have lanthanide
ions in the side chain. Multidentate ligands are required to
stabilize lanthanide ions in water and there are some com-
plicated synthetic chemistries to obtain such lanthanide-sta-
bilizing monomers. It is equally important to note that the
multidentate group-containing monomer may cause side
reactions via interaction with polymerization catalysts or
propagating species.
INTRODUCTION The construction of various functional poly-
mers has been studied in recent years. Furthermore, control
of polymer size, monomer sequences, number and position
of functional groups, and polymer architecture is still state-
of-the-art material science issues.1–3 One of the targets of
functional polymers is an area of biorelated polymers in
which drug or gene delivery,4 materials for medical devi-
ces,5 and bioimaging probes6 are applied. Many polymers
have been reported as bioimaging probes, some of which
have organic luminescent dyes such as Rhodamine7–9 and
coumarin10,11 in their side chain or terminal. Organic lumi-
nescent dyes, however, have several drawbacks, being easily
photobleached and decomposed because of poor photo-
chemical stability.12 Lanthanide(III) complex-based lumines-
cent dyes have been attracting attention as alternatives to
organic luminescent dyes in recent years. Lanthanide(III)-
based luminescent materials generally have high photo-
chemical stability in both the ground and the excited states
and the long lifetime of the excited states allows the use of
time-resolved detection, a definite asset for bioassays and
luminescent microscopy.13–15 Considering the biological
Additional Supporting Information may be found in the online version of this article.
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Living radical polymerization (LRP) has been applied for the
syntheses of various functional polymeric materials in recent
years because the growing species (radicals) are stable for
many types of functional groups. Various metal-conjugating
polymers have been prepared by LRP.28,29 Copolymerization
of divinyl monomer and ligand monomer, which can coordi-
nate to a polymerization metal catalyst, from living polymer
(macroinitiator) leads to a star polymer bearing many metal
complexes in the core in a one-pot reaction, which is a very
simple method to introduce metal ions.30–34 Interestingly,
the stability of the metal complex in the core of the star
polymer was higher than that of the corresponding metal
complex, probably because of the chelating effect.34 These
results prompted us to investigate the introduction of lantha-
nide ions into a star polymer as an easy method to prepare
a water-soluble and water-stable luminescent polymer. We
employed poly(ethylene oxide) (PEO) as a water-solubilizing
by multiangle laser light scattering (MALLS) in DMF contain-
ing 10 mM LiBr at 40 ꢀC on a Dawn E instrument (Wyatt
Technology, Ga-As laser, k 5 690 nm). 1H, 13C, and 31P NMR
spectra were obtained on a JEOL. JNM-ECP 500. 1H and 13C
NMR chemical shifts were determined using tetramethylsi-
lane (TMS) as an internal standard, whereas 31P NMR chemi-
cal shifts were determined using 85% phosphoric acid as an
external standard. UV–vis spectra were recorded on a JASCO
V-570 spectrophotometer. Microwave-induced plasma–mass
spectrometry (MIP–MS) P-6000 (Hitachi) was used to deter-
mine the Eu(III) ion concentrations in the core of the star poly-
mer. Fluorescence spectra were recorded on an F-2500
spectrophotometer (Hitachi). Elemental analyses were per-
formed on a PerkinElmer 2400 series II CHNS/O elemental an-
alyzer (PerkinElmer). The emission quantum yields were
determined using an absolute PL quantum yields measurement
system (Hamamatsu Photonics, C9930-02). Emission lifetimes
were measured with the third harmonic (355 nm) of a Q-
switched Nd-YAG laser (Spectra Physics, INDI-50, fwhm5 5 ns,
k 5 1064 nm) and a photomultiplier (Hamamatsu Photonics,
R5108, response time ꢁ1.1 ns). The hydrodynamic diameter of
the Eu-bearing PEO star polymer was measured using a
dynamic light-scattering spectrometer equipped with a 633 nm
He–Ne laser (Zetasizer Nano-ZS, Malvern).
chain and
a
tris(hexafluoroacetylacetonato)europium(III)
bis(triphenylphosphine oxide) [Eu(III)-tppo] complex deriva-
tive, which is a typical luminescent complex having a high
quantum yield and is easy to prepare.35,36 We first explored
a suitable introduction method for Eu(III) ion using poly
(methyl methacrylate) (PMMA) as a model arm polymer for
star polymer formation and then prepared Eu(III)-bearing
PEO star polymer. The photoproperties (quantum yield and
emission lifetime) of the obtained polymers are also
discussed.
Synthesis of p-Styryldiphenyl phosphine oxide
p-Styryldiphenyl phosphine oxide (SDPO) was synthesized
according to the previously published literature.39 SDP (17.4
mmol, 5.00 g) was added to a round-bottom flask and then
dissolved in 1,2-dichloroethane (100 mL). Saturated aqueous
solutions of oxone (34.8 mmol, 21.4 g) and methanol (20
mL) were added to the reaction flask and the mixture was
left to stir for about 2 h. The reaction mixture and a large
excess of water were added in a separatory funnel, and the
two layers were separated. The organic layer was retained
and the solvent was removed in vacuum. The sticky solid
was washed with cyclohexane and then filtered to obtain
SDPO as a white powder (yield, 88%). The product was
EXPERIMENTAL
Materials
Methyl methacrylate (MMA, Tokyo Chemical Industry; purity,
>99%), ethylene glycol dimethacrylate (EGDMA, Sigma-
Aldrich; purity, >98%), and toluene (Sigma-Aldrich; purity,
>99%) were purified by distillation over calcium hydride
under reduced pressure before use. Poly(ethylene glycol)
monomethyl ether (Sigma-Aldrich, Mn 5 10,000 g/mol),
Ru(Ind)Cl(PPh3)2 (STREM; purity, >98%), 1,1,1,5,5,5-hexa-
fluoro-2,4-pentanedione (Sigma-Aldrich; purity, >98%), p-
styryl diphenyl phosphine (SDP, donated by Hokko Chemical
Industry), and europium acetate n-hydrate (Wako Pure
Chemical Industries; purity, >98%) were used as received.
Chlorine-terminated (PEO–Cl macroinitiator) was synthesized
according to the literature.37,38 Tris(hexafluoroacetylacetona-
to)europium(III) dihydrate [Eu(hfa-H)3(H2O)2] was prepared
according to the previously published literature.35
1
characterized by H, 13C, and 31P NMR.
1H NMR (500.17 MHz, CDCl3, TMS 5 0 ppm, d ppm): 7.44–
7.69 (14H, m, ArH), 6.74 (1H, dd, J 5 17.6, 10.8 Hz, CH), 5.86
(1H, dd, J 5 17.6, 0.8 Hz, CH2), 5.38 (1H, dd, J 5 11.2, 0.8 Hz,
CH2). 13C NMR (100.40 MHz, CDCl3 5 77.0 ppm,d ppm):
141.12 (Cipso), 135.94 (s, C2), 132.98 (CipsoP), 132.51 (s,
CipsoP), 132.10–132.40 (Car, meta, Cipso, Car, orthoP),
128.62 (d, Car, metaP, J 5 12.3 Hz), 126.31 (d, Car, ortho,
J 5 12.2 Hz), 116.73 (s, C2). 31P NMR (202.47 MHz, CDCl3, d
ppm): 30 ppm. Anal. calcd for C20H17PO: C, 78.6; H, 5.6%.
Found: C, 78.4; H, 5.6%.
Instruments
Gel permeation chromatography (GPC) on three linear-type
poly(2-hydroxyethyl methacrylate) gel columns (Shodex SB-
806M; exclusion limit 2 3 107; 0.8 cm i.d. 3 30 cm) that
were connected to a JASCO PU-2080 precision pump (Jasco),
a JASCO RI-2031 refractive index detector (Jasco), and a
JASCO UV-2074 UV/Vis detector (Jasco) set at 270 nm was
used to determine molecular weights (Mn) and molecular
weight distributions (Mw/Mn) of polymer samples with
respect to PMMA standards. The absolute weight-average
molecular weight (Mw) of the star polymers was determined
Synthesis of
tris(hexafluoroacetylacetonato)europium(III)
bis(styryldiphenylphosphine oxide) (Eu(hfa-H)3(SDPO)2)
Methanol (25 mL) containing Eu(hfa-H)3(H2O)2 (1.00 g, 1.25
mmol) and SDPO (0.76 g, 2.50 mmol) was stirred for 24 h in
a dry round-bottom flask. The reaction mixture was concen-
trated using a rotary evaporator and the product was
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SCHEME 1 Syntheses of Eu(III)-bearing star polymer by LRP: sequential (a) and simultaneous (b) introduction.
precipitated by addition of excess hexane (yield, 80%). The
mL, 1.130 M in toluene), and Ru(Ind)Cl(PPh3)2 (8.4 mmol,
7.3 mg) into the solution of PMMA (0.084 mmol, 1.0 g) in
toluene (4.52 mL). The final mole ratio was PMMA/EGDMA/
SDPO/Ru(Ind)Cl(PPh3)2/n-Bu3N 5 13.3/133/66.5/1.33/26.6.
The mixture was placed in an oil bath at 80 ꢀC. PMMA star
formation was observed by GPC analysis. The reaction was
terminated by cooling in an ice bath. The product was then
purified by silica gel column chromatography using toluene
as an eluent (for removing the remaining Ru catalyst) and
precipitated in hexane (yield: 94%, from the GPC curve
area). The Eu(III) ion was introduced into the core of the
PMMA star by the following technique. In a glass flask with
a three-way stopcock under argon gas, PMMA star (with
phosphine oxide amount 5 18.2 mmol, 100 mg) and Eu(hfa-
H)3(H2O)2 (9.1 mmol, 7.4 mg) was dissolved in mixed solvent
(2 mL, toluene:EtꢀOH 5 1:1 v/v). The solution was placed in
an oil bath at 70 C for 24 h. After cooling to room tempera-
ture, the mixture was precipitated in methanol to recover
Eu-PMMA star polymer.
1
product was characterized by H and 31P NMR.
1H NMR (500.17 MHz, CDCl3, TMS5 0 ppm, d ppm): 7.60–
9.00 (28H, m, ArH), 6.85 (2H, dd, J 5 17.6, 10.8 Hz, 2-CH),
5.91 (2H, dd, J 5 17.6, 0.8 Hz, CH2,), 5.65 (3H, s, CH3), 5.40
(2H, dd, J 5 11.2, 0.8 Hz, CH2,). 31P NMR (202.47 MHz,
CDCl3, d ppm): 29.00 ppm. Anal. calcd for C55H37O8F18P2Eu:
C, 47.81; H, 2.70%. Found: C, 47.58; H, 2.87%.
Synthesis of Eu(III)-Bearing PMMA Star Polymer
The polymerization was carried out using a syringe tech-
nique under argon in a flask equipped with a three-way
stopcock. The reaction solution was prepared by adding
sequentially in the order: Ru(Ind)Cl(PPh3)2 (0.20 mmol, 172
mg), toluene (26 mL), and MMA (200 mmol, 21.4 mL), n-
Bu3N (2.00 mmol, 2.00 mL, 1.0 M in toluene), and ECPA
(2.00 mmol, 0.34 mL). The total volume of the reaction mix-
ture was thus 50 mL. The flask was then placed in an oil
bath at 80 ꢀC. Samplings were done in a predetermined pe-
riod to estimate the conversion. The reaction was terminated
by cooling in an ice bath. The obtained PMMA was purified
by silica gel column chromatography (using toluene as an
eluent) and precipitated in hexane. The pure PMMA was
characterized by GPC (Mn 5 10,700; Mw/Mn 5 1.3).
For the simultaneous introduction method [Scheme 1(b)],
PMMA star polymer was synthesized by adding EGDMA
(0.40 mmol, 0.40 mL, 1.0 M in toluene), Eu(hfa-H)3(SDPO)2
(0.10 mmol, 0.14 g), n-Bu3N (0.080 mmol, 0.10 mL, 710 mM
in toluene), and Ru(Ind)Cl(PPh3)2 (4.0 mmol, 3.4 mg) to the
solution of PMMA in toluene (2.5 mL). The final mole ratio
was PMMA/EGDMA/Eu(hfa-H)3(SDPO)2/Ru(Ind)Cl(PPh3)2/n-
Bu3N 5 13.3/133/33.3/1.33/26.6. The solution was placed
in an oil bath at 80 ꢀC. Eu-PMMA star formation was
observed by GPC analysis. The reaction was terminated by
cooling in an ice bath. The product was purified by silica gel
column chromatography using toluene as an eluent and
Two methods were employed for introducing Eu(III) ions
into the PMMA star polymer: sequential and simultaneous
introduction. For the sequential introduction method
[Scheme 1(a)], PMMA star was synthesized by adding
EGDMA (0.84 mmol, 0.84 mL, 1.0 M in toluene), SDPO (0.42
mmol, 0.42 mL, 1.0 M in toluene), n-Bu3N (0.17 mmol, 0.15
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FIGURE 1 MWD curves of Eu-PMMA star obtained from PMMA arms by (a) sequential introduction with EGDMA and Eu(SDPO) in
toluene at 80 ꢀC: [PMMA]0 5 13.3 mM; [EGDMA]0 5 133 mM; [SDPO]0 5 66.5 mM; [Ru(Ind)Cl(PPh3)2]0 5 1.33 mM; [n-Bu3N]0 5 26.6
mM and (b) simultaneous introduction obtained from PMMA arms with EGDMA and Eu(SDPO) in toluene at 80 ꢀC: [PMMA]0 5 13.3
mM; [EGDMA]0 5 133 mM; [Eu(SDPO)]0 5 33.3 mM; [Ru(Ind)Cl(PPh3)2]0 5 1.33 mM; [n-Bu3N]0 5 26.6 mM.
precipitated in methanol (yield: 88%, from GPC curve area).
The introduction of core-bound Eu(III) ion was characterized
by UV–vis spectroscopy and MIP–MS.
RESULTS AND DISCUSSION
Synthesis of Eu-PMMA Star: Investigation of Eu(III) Ion
Introduction
Synthesis of Eu(III)-Bearing PEO Star Polymer
Introduction of Eu(III) ions into the core of the star polymer
was first investigated using chlorine-terminated PMMA as a
model arm polymer. Two introduction strategies were exam-
ined. First, ligand groups and Eu(III) ions were sequentially
introduced in this order (sequential introduction, Scheme
1(a)) because the Eu(III) center might affect polymerization
catalyst for star polymer synthesis. EGDMA as a linking
agent and SDPO as a ligand monomer for Eu(III) core were
copolymerized from a PMMA macroinitiator coupled with
Ru(Ind)Cl(PPh3)2 and n-Bu3N in toluene at 80 ꢀC to form a
ligand-bearing star polymer with final molar ratio of
used compounds PMMA/EGDMA/SDPO/Ru(Ind)Cl(PPh3)2/n-
Bu3N 5 1/10/5/0.1/2. After 48 h, the PMMA arm, EGDMA,
and SDPO were successfully consumed to give a ligand-bear-
ing PMMA star in high yield (94% calculated from the area
under the GPC curve) [Fig. 1(a)]. Unreacted PMMA arms and
Eu-PEO star polymer was synthesized by adding EGDMA
(0.075 mmol, 0.075 mL, 1.0 M in toluene), Eu(hfa-H)3(SDPO)2
(0.075 mmol, 104 mg), n-Bu3N (0.060 mmol, 0.085 mL, 710
mM in toluene), and Ru(Ind)Cl(PPh3)2 (1.3 mmol, 2.6 mg) to a
solution of PEO-Cl macroinitiator (Mn 5 10,000 g/mol, 0.030
mmol, 300 mg) in toluene (2.1 mL) using the simultaneous
introduction method. The final mole ratio was thus PEO-Cl/
EGDMA/Eu(hfa-H)3(SDPO)2/Ru(Ind)Cl(PPh3)2/n-Bu3N 5 13.3/
33.3/33.3/1.33/26.6. Immediately after mixing, the mixture
ꢀ
was placed in an oil bath at 80 C. Eu-PEO star formation was
observed by GPC analysis. The reaction was terminated by
cooling in an ice bath. The product was then purified by silica
gel column chromatography and precipitated in cold ether.
The Eu(III) ion concentration was determined by UV–vis
spectroscopy and MIP–MS.
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TABLE 1 Eu(III)-Bearing Star Polymers with Different Arm Polymers
a
a
a
b
Mw,arm
Mw,star
Mw/Mn,star
Mw,star
f
(No.
[Eu(III)]
(mmol/g-polymer)
Arm
(GPC)
(GPC)
(GPC)
(MALLS)
of Arms)
PMMA (sequential)
PMMA (simultaneous)
PEO
13,900
13,900
12,300
85,000
100,000
125,000
1.21
1.29
1.58
340,000
210,000
96,000
18
11
6
0.33c
26c
33c
1.78d
28d
39d
a
c
Determined by GPC calibrated with PMMA standards.
Determined by UV–vis spectroscopy.
Determined by MIP–MS.
b
d
Mole fraction of arm monomer 3 Mw,star(MALLS)/Mw,arm.
a Ru catalyst were removed by precipitation with toluene/
methanol and silica gel column chromatography with toluene
as an eluent. The ligand-bearing PMMA star polymer was
then mixed with Eu(hfa-H)3(H2O)2 in toluene/methanol to
introduce Eu(III) ions into the core. The Eu-PMMA star had
a high molecular weight (Mw 5 340,000 from GPC-MALLS)
and relatively narrow molecular weight distribution (MWD;
Mw/Mn 5 1.21 from GPC) (Table 1). By comparison with Mw
of the PMMA arm and using the literature method,29,30,37
the number of arms on the star was calculated as about 18
(Table 1).
was about 80 times larger than that using the sequential
introduction procedure. The difference may arise from the
phosphine oxide groups being located in a three-dimension-
ally random manner in the core in the star polymer forma-
tion for sequential introduction and the ratio of two
phosphine oxide groups that can coordinate to Eu(III) ion
was very low, whereas the phosphine oxide groups had al-
ready coordinated and they were located at the appropriate
position in the star polymer formation by simultaneous
introduction. The concentration of Eu(III) ions estimated
from UV absorption for the star polymer prepared by the si-
multaneous procedure was very close to that determined by
MIP–MS. However, the concentration of Eu(III) by the se-
quential method estimated from UV absorption was much
lower (0.33 mmol/g-polymer) than that measured by MIP–
MS (1.78 mmol/g-polymer). This result is probably owing to
coordination not only to the triphenylphosphine oxide ligand
but also to the carbonyl group in the PMMA side chain in
the case of sequential introduction. From these results, the
simultaneous introduction procedure is judged to be supe-
rior to the sequential introduction procedure.
As a second method, the Eu(III)-bound ligand monomer
[Eu(SDPO)] was introduced in one step (simultaneous intro-
duction, Scheme 1(b)). Eu(SDPO) was prepared separately
and used for the star polymer formation in conjunction with
EGDMA, to give the Eu-PMMA star in high yield (ca. 88%),
which was a little lower than that of the sequential introduc-
tion procedure [Fig. 1(b)], which suggested that the
Eu(SDPO) does not show crucial inhibition of the catalytic
activity of the Ru complex. The obtained PMMA star polymer
had about 11 arms and its MWD was relatively narrow (Mw/
Mn 5 1.29 from GPC). The number of arms was a little
smaller than that prepared by the sequential introduction
procedure. These results might be owing to some weak
interaction between the Eu(SDPO) and the Ru complex or
lower reactivity of Eu(SDPO) than that of SDPO because
Eu(SDPO) is much bulkier than SDPO.
Synthesis of Eu-PEO Star
Synthesis of water-soluble Eu(III)-bearing star polymer was
then examined. PEO was employed as an arm polymer and
the a-chloro-a-phenylacetyl group was introduced at the end
of the PEO chain (PEO–Cl macroinitiator).38,40,41 As a result
of the above investigation, the Eu-PEO star was prepared by
simultaneous introduction (Scheme 2). To obtain the water-
soluble Eu-PEO star, some adjustments in the concentration
ratio of the linking agent EGDMA to PEO–Cl macroinitiator
were required. The water-soluble Eu-PEO star was obtained
at the ratio of [EGDMA]0/[PEO-Cl]0 of 2.5. The star formation
proceeded more slowly than in the case of PMMA star for-
mation and the yield of the Eu-PEO star was lower, about
55% at 158 h from the GPC curve area (Fig. 2). The core of
the star polymer was hydrophobic; however, the water-solu-
ble PEO arms had eliminated the hydrophobic property of
the core and water-soluble Eu(III)-bearing star polymer was
obtained. The obtained Eu-PEO star was characterized using
GPC, resulting in high molecular weight (Mw 5 125,000) and
relatively narrow MWD (Mw/Mn 5 1.58).
Quantification of the Introduced Eu(III) Ions: Effect of
Introduction Procedures
The amount of introduced Eu(III) ion was quantified by
means of UV absorption30 and MIP–MS technique. The
Eu(SDPO) complex and the Eu(III)-bearing star polymers
have kmax at 305 nm in the UV–vis spectra while the ligand
monomer SDPO showed no absorbance (Supporting Informa-
tion Fig. S1). The concentration of Eu(SDPO) complex was
calibrated by the absorption at 305 nm (Supporting Informa-
tion Fig. S2). UV–vis spectra of the obtained Eu-PMMA star
using the sequential and simultaneous methods were very
close to that of the Eu(SDPO) complex and the concentra-
tions were estimated according to the calibration. The
amounts of Eu(III) in the core of the star polymers obtained
via the sequential and simultaneous methods were 0.33 and
26 mmol/g-polymer, respectively (Table 1). The amount of
Eu(III) ions using the simultaneous introduction procedure
The concentration of Eu(III) ion in the Eu-PEO star was also
evaluated to be 33 mmol/g-polymer from its UV absorption
spectrum and was in good agreement with that determined
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SCHEME 2 Synthesis of water-soluble Eu(III)-bearing star polymer.
from MIP–MS, 39 mmol/g-polymer. The diameter of the Eu-
PEO star was found using DLS to be 18 nm.
take place. Eu-PEO star polymer had an identical UV–vis
spectrum in water to that in CH2Cl2. The absorbance at 305
nm of the aqueous solution of Eu-PEO star was monitored
for 1 month (Fig. 3). The absorbance decreased in the initial
5 days but was almost constant after that, which implies sta-
bility of Eu-PEO star in water. In contrast, Eu(SDPO) showed
slow decomposition in water/MeOH (50/50 v/v), resulting
in about 25% decrease in absorbance in 30 days. Specifically,
stabilized coordination chemistry of star polymers in aque-
ous media has also been reported for Fe-bearing star
polymer.34
In contrast, water-insoluble star polymer was formed with
higher [EGDMA]0/[PEO-Cl]0 ratios (data not shown). If too
much hydrophobic EGDMA and Eu(SDPO) were employed,
the formed core would be larger and could not be covered
with hydrophilic PEO arms, resulting in the water-insoluble
polymer.
Stability of Eu-PEO Star in Water
The stability of Eu-PEO star in water was examined because
the Eu(III) ion has considerable hydrophilic character and
ligand exchange between phosphine oxide and water may
Luminescent Photoproperties of Eu(III)-Bearing
Star Polymers
Figure 4 shows a photograph of the fluorescence of Eu-
PMMA star (a) and Eu-PEO star (b) solution with 14 mM of
[Eu(III)] in CH2Cl2 under 365-nm UV irradiation. Both Eu-
bearing star polymers exhibited strong fluorescence and the
emission intensity of Eu-PEO star was stronger than that of
Eu-PMMA star.
Figure 5 shows fluorescence spectra of Eu-PEO star, Eu-
PMMA star, and Eu(SDPO) in different solvents. Fluorescence
spectra were normalized with respect to the intensity of the
magnetic dipole transition (5D0–7F1) at 595 nm, which is
known to be less sensitive to the surrounding environment
of the Eu(III) ion.42 In CH2Cl2 [Fig. 5(a)], a good solvent for
FIGURE 2 MWD curves of Eu-PEO star obtained from PEO
arms with EGDMA and Eu(SDPO) in toluene at 80 ꢀC:
[PEO]0 5 13.3 mM; [EGDMA]0 5 33.3 mM; [Eu(SDPO)]0 5 33.3
mM; [Ru(Ind)Cl(PPh3)2]0 5 1.33 mM; [n-Bu3N]0 5 26.6 mM.
FIGURE 3 The stability of Eu-PEO star and Eu(SDPO) in water
and mixed water–methanol, respectively.
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and the quantum yield. In MeOH, the quantum yield of
Eu-PEO star and Eu(SDPO) (entries 2 and 7, respectively)
dramatically decreased and the emission lifetime also
decreased to about half of that in CH2Cl2, which corresponds
to a much higher knr value in MeOH. Such a significant differ-
ence seems to originate from a modification in the coordina-
tion geometry in MeOH. As the core of the star polymer
consists of many ester groups, MeOH molecules can enter
the core and interact with the Eu complex to change the
coordination geometry, resulting in lower 5D0–7F2 emission
and a reduced quantum yield. The nonradiative transitions
of lanthanide complexes are affected by the high-vibrational
frequencies of CAH and OAH bonds of the solvent.42,43 Con-
sidering the solvation effects, together with increasing
FIGURE 4 Eu(III)-PMMA star (a) and Eu(III)-PEO star (b) under
365 nm of UV light irradiation in CH2Cl2, [Eu] 5 14 lM.
all of the samples (Eu-PEO star, Eu-PMMA star, and Eu(SDPO)),
the fluorescence spectra were almost identical with the three
5
emission bands at 581 nm (electric dipole transition, D0–7F0),
5
595 nm, and 615 nm (electric dipole transition, D0–7F2). The
emission intensity at 615 nm is known to depend markedly
on the coordination geometry42 and the similarities of the
spectra of the samples suggest that the coordination geome-
tries around the Eu(III) center of Eu(SDPO) are maintained
even in the star polymer with hydrophobic and hydrophilic
arms. In both water and deuterated water, the fluorescence
spectra of Eu-PEO star were close to that in CH2Cl2 with the
slight decrease in the relative intensity of the 615 nm band,
suggesting stability of the coordination geometry around the
Eu(III) center against environmental conditions [Fig. 5(b)]. In
contrast, the fluorescence spectra of Eu-PEO star and
Eu(SDPO) in MeOH were clearly different from those in CH2Cl2
and water. The relative intensities of the emission band of the
5D0–7F2 electronic dipole transition were much smaller than
those in CH2Cl2 and water and exhibited a slight red shift of
the emission peak to 618 nm. These results indicate that the
coordination geometries of Eu-PEO star and Eu(SDPO) in
MeOH were different from those in CH2Cl2 and water although
its origin is not clear at present.
The emission lifetimes and quantum yields of Eu-PEO star,
Eu-PMMA star, and Eu(SDPO) in different solvents were also
measured (Table 2). In CH2Cl2 (entries 1, 5, and 6), a rela-
tively long emission lifetime of about 0.8 ms was observed
for all Eu(III) compounds. The quantum yield of Eu-PEO star
was quite high (59%), which corresponds to a relatively low
nonradiative rate constant (knr), which was close to those of
Eu(SDPO) in various conditions. The quantum yield of Eu-
PMMA star was lower (44%) than that of Eu(SDPO) because
of a smaller radiative rate constant (kr). The reason for the
difference in quantum yields is not clear; however, the
nature of the surrounding arm polymer might affect kr, knr,
FIGURE 5 Fluorescence spectra of (a) Eu-PMMA star, Eu-PEO
star, and Eu(SDPO) in CH2Cl2 and (b) Eu-PEO star and Eu(SDPO)
in different solvents (excitation wavelength: 365 nm).
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TABLE 2 Emission Lifetimes and Quantum Yields of Eu(III)-Star Polymer and Eu(III)-Monomer in Different Solvents
Lifetime
(s, ms)a
Quantum
Entry
Sample
Solvent
yield (U, %)
kr (s21 b
)
knr (s21 c
)
1
2
3
4
5
6
7
Eu-PEO star
Eu-PEO star
Eu-PEO star
Eu-PEO star
Eu-PMMA star
Eu(SDPO)
CH2Cl2
MeOH
H2O
0.83
0.32
0.76
0.90
0.83
0.75
0.42
59
4
7.1 3 102
1.3 3 102
2.5 3 102
2.1 3 102
5.3 3 102
9.2 3 102
2.4 3 102
4.9 3 102
30.0 3 102
10.6 3 102
9.0 3 102
6.7 3 102
4.1 3 102
21.0 3 102
19
19
44
69
10
D2O
CH2Cl2
CH2Cl2
MeOH
Eu(SDPO)
a Emission lifetime of the Eu(III) complexes was measured by excitation
at 365 nm.
b Radiative rate constants kr 5 U/s observed.
c Nonradiative rate constants knr 5 1/s 2 kr.
abilities of direct coordination of solvent molecules to the
Eu(III) center, the solvent molecule motions consumed more
energy, and thus the energy transfer could not be carried
out perfectly.44
soluble luminescent polymer is a promising candidate for bi-
ological applications such as bioimaging.
ACKNOWLEDGMENTS
The quantum yield of Eu-bearing PEO star in water was
about 19% (entry 3) and is higher than some previous Eu-
based fluorescence dyes such as [40-(10-methyl-9-
anthryl)22,20:60,200-terpyridine-6,600-diyl]bis(methylenenitri-
lo)tetrakis(acetate)-Eu(III), which has been reported to show
a quantum yield of 0.9%.24 A water-soluble Eu(III) complex
with modified pybox ligand exhibited a high quantum yield
in H2O, 30%45; however, it required much more complicated
synthetic chemistry than that of the present Eu-PEO star.
This research was partially supported by a Grant-in-Aid for Sci-
entific Research (No. 2130064) from the Ministry of Education,
Culture, Sports, Science, and Technology, Japan (MEXT Grant).
The authors also thank Mitsuo Sawamoto for the use of the
GPC-MALLS instrument in his lab.
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