Highly luminescent hydrogels synthesized by covalent grafting of lanthanide complexes onto PNIPAM: Via one-pot free radical polymerization

Article abstract of DOI:10.1039/c6tc00336b

Highly luminescent PNIPAM-Ln(DPA)₃ hydrogels have been synthesized by one-pot free radical polymerization in aqueous systems at room temperature. We firstly synthesized a bifunctional chelating ligand named as 4-vinylpyridine-2,6-dicarboxylic acid (4-VDPA), which contains two different moieties: (1) a pyridine-2,6-dicarboxylic acid (DPA) moiety that can sensitize the luminescence of Tb³⁺ and Eu³⁺ and (2) a reaction functional group that can be polymerized by free radical initiation. Therefore, the luminescent lanthanide complexes can be grafted onto the hydrogel matrix via one-pot free radical polymerization of [Ln(4-VDPA)₃]^3-, N-isopropylacrylamide (NIPAM) and N,N′-methylenebis(2-propenamide) (MBA). The results indicated that the final hydrogels possess bright luminescence, long luminescence lifetimes and high quantum efficiency. Moreover, the emission colors of the hydrogels can be tuned from red, reddish orange, orange-yellow, yellow, and green by varying the molar ratio of added Eu³⁺ to Tb³⁺.

Full text of DOI:10.1039/c6tc00336b

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Highly luminescent hydrogels synthesized by covalent grafting of
lanthanide complexes onto PNIPAM via one-pot free radical
polymerization
Received 00th January 20xx,
Qing-Feng Li,a Xiaodi Du,a Lin Jin,a Mengmeng Hou,a Zhenling Wang, *a and Jianhua Hao*b
Highly luminescent PNIPAM-Ln(DPA)3 hydrogels have been synthesized by one-pot free radical polymerization in aqueous
system at room temperature. We firstly synthesized a bifunctional chelating ligand named as 4-vinylpyridine-2,6-
dicarboxylic acid (4-VDPA), which contains two different moieties: (1) a pyridine-2,6-dicarboxylic acid (DPA) moiety that
can sensitize the luminescence of Tb3+ and Eu3+, (2) a reaction functional group that can be polymerized by free radical
initiation. Therefore, the luminescent lanthanide complexes can be grafted onto the hydrogel matrix via one-pot free
radical polymerization of [Ln(4-VDPA)3]3-, N-isopropylacrylamide (NIPAM) and N, N'-Methylenebis(2-propenamide) (MBA).
The results indicated that the final hydrogels possess bright luminescence, long luminescent lifetimes and high quantum
efficiency. Moreover, the emission colors of the hydrogels can be tuned from red, reddish orange, orange-yellow, yellow,
and green by varying the molar ratio of added Eu3+ to Tb3+.
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
smart
hydrogel
poly[(N-isopropylacrylamide)-co-(methacrylic
Introduction
acid)],15 and the as-prepared hybrid hydrogel can be used as a
luminescent probe for controlled drug release and cell imaging.
Hydrogels have gained considerable momentum due to their
unique three-dimensional polymeric networks structure, multiple-
stimulus responsive properties and applications in controlled
drug/protein release, sensing, adsorption, etc.1-5 Recently, great
efforts have been made toward modifying hydrogels with certain
functional groups to expand their potential applications. Therefore,
numerous multifunctional hydrogels have been obtained by self-
assembly technique or chemical synthesis methods.6-8 Among them,
luminescent hydrogels are widely investigated due to their extra
optical application in luminescent probes, optical imaging,
optoelectronic devices, and so on. A variety of luminophores such
as organic luminescent dyes, up-conversion luminescent particles
and carbon dots were incorporated into the hydrogel matrixes by
physical or chemical methods to prepare the luminescent
hydrogels.9-12 For instance, Wang et al. prepared an intelligent
luminescent hydrogels by physically incorporating a propeller-
shaped dye molecule, and these luminescent hydrogels possess
organic solvents/thermal dual-responsive properties.13 Jr and co-
works synthesized supramolecular lanthanide luminescent
hydrogels by self-assembled hydrothermal reactions of β-amino
acid and lanthanide ions.14 Lin et al developed a new pH-induced
thermally responsive luminescent hydrogel by grafting of up-
conversion luminescent microspheres of NaYF4:Yb3+/Er3+ onto the
Poly(N-isopropylacrylamide) (PNIPAM) is
a temperature-
responsive polymer, which shows a unique reversible phase
transition at the lower critical solution temperature (LCST) around
32 oC. Thus, luminescent PNIPAM hydrogels may simultaneously
exhibit luminous and temperature-sensitive characteristics.
Actually, there are some reports on the synthesis of luminescent
PNIPAM hydrogels, and the luminophores were successfully grafted
onto the polymer matrix by covalently attaching organic fluorescent
dyes like pyrene or 3-hydroxyflavones to PNIPAM chains.16,17
However, the broadband emission and aggregation-caused
quenching characteristics of dye molecules make them difficult to
obtain a luminescent PNIPAM hydrogel with high luminescent
efficiency and color purity.
Among all the luminophores, lanthanide complexes are
considered as one of the most promising optical materials due to
their outstanding emission properties, including high luminous
efficiency and color purity, long luminescent lifetime, wide range of
emission wavelengths from ultraviolet (UV) to near-infrared (NIR)
regions.18-21 In these lanthanide complexes, pyridine-2, 6-
dicarboxylic acid (DPA) and its derivatives are a well known chelate
group for sensitizing the luminescence of Tb3+ or Eu3+ ions.22-25 The
lowest energy level of the triplet state of DPA (26600 cm-1) is higher
than the resonant energy level of Tb3+ (5D4, 20430 cm-1) and Eu3+
(5D0, 17250 cm-1), indicating that the energy transfer can be
proceeded between the DPA ligand and the centers of Tb3+ or Eu3+
ions.26 Moreover, lanthanide ions are able to coordinated with
three nitrogen atoms and six carboxylate oxygen atoms from three
different DPA moieties, and water molecule is excluded from the
coordination sphere of Ln3+.22,23 Hence, it is reasonable to envisage
a The Key Laboratory of Rare Earth Functional Materials and Applications, Zhoukou
Normal University, Zhoukou 466001, Henan, P. R. China.
E-mail: liqingf335@163.com, zlwang2007@hotmail.com; Fax/Tel:+86-394-8178518
bDepartment of Applied Physics, The Hong Kong Polytechnic University, Hong Kong,
P. R. China.E-mail: jh.hao@polyu.edu.hk
†Electronic Supplementary Information (ESI) available: SEM, CIE, PL emission
spectra and luminescence decay curves. See DOI: 10.1039/x0xx00000x
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the combination of DPA complexes with PNIPAM hydrogels to
obtain new luminescent materials. Herein, we report the synthesis
of luminescent PNIPAM-Ln(DPA)3 hydrogels based on lanthanide
complexes by one-pot free radical polymerization. Firstly, we
synthesized
a bifunctional chelating ligand named as 4-
vinylpyridine-2, 6-dicarboxylic acid (4-VDPA), which contains two
functional groups: (1) chelating group for lanthanide ions; (2)
electron deficient double bond that can be polymerized by free
radical initiation. And then the corresponding luminescent
lanthanide complexes were introduced into temperature-sensitive
PNIPAM hydrogel by chemical copolymerization of [Ln(4-VDPA)3]3-
with N-isopropylacrylamide (NIPAM) and N, N'-Methylenebis(2-
propenamide) (MBA). Considering their unique luminescence
properties, we hope that this work could provide a new strategy for
fabricating lanthanide luminescent PNIPAM hydrogels through one-
pot free radical polymerization.
Fig. 1 Digital photos of the hydrogels: PNIPAM (a), PNIPAM-
Tb(DPA)3 (b) and PNIPAM-Eu(DPA)3 (c) under daylight, and PNIPAM-
Tb(DPA)3 (d) and PNIPAM-Eu(DPA)3 (e) under UV light (254 nm, 6
W)
Results and discussion
NMR analysis
The synthesis procedure of 4-VDPA and luminescent hydrogels
PNIPAM-Ln(DPA)3 is illustrated in Scheme 1. The bifunctional
organic ligand (4-VDPA) was synthesized starting from chelidamic
acid. Luminescent PNIPAM-Ln(DPA)3 hydrogels were prepared by
free radical polymerization of N-isopropyl acrylamide (NIPAM) and
N, N'-Methylenebis(2-propenamide) (MBA) with [Ln(4-VDPA)3]3-.
The obtained PNIPAM, PNIPAM-Tb(DPA)3 and PNIPAM-Eu(DPA)3
hydrogels show the transparent characteristic under daylight, while
PNIPAM-Tb(DPA)3 and PNIPAM-Eu(DPA)3 exhibit strong visible
green and red luminescence under irradiation with a UV lamp (Fig.
1). The microstructures of PNIPAM-Tb(DPA)3 and PNIPAM-Eu(DPA)3
are very similar to that of PNIPAM (Fig. S1).
The coordination modes of 4-VDPA to lanthanide ions were
investigated by monitoring changes in the 1H-NMR spectra of 4-
VDPA upon titration with paramagnetic lanthanide ions (Fig.
2).27,28 Paramagnetic Ln3+ ions exhibit very different magnitudes of
the χ tensor and their associated anisotropy,29 therefore, we select
moderately paramagnetic Yb3+ ion as the tested lanthanide ion to
facilitate the analysis by NMR. When small amount of Yb3+ ions was
added into 4-VDPA solution, the peak intensity of 4-VDPA signals
decreased and some new signals corresponding to coordination of
4-VDPA and Yb3+ with 3:1 ratio ([Yb(4-VDPA)3]3-) generated (peaks
labelled with star in Fig. 2b), showing slow exchange between 4-
VDPA and Yb3+ in NMR timescale. When the ratio of Yb3+ and 4-
VDPA is larger than 1:3, the peak intensity of [Yb(4-VDPA)3]3- signals
decreased and some new signals corresponding to coordination of
4-VDPA and Yb3+ with 2:1 ratio ([Yb(4-VDPA)2]-) generated (peaks
labeled with star in Fig. 2c). When the ratio of Yb3+ and 4-VDPA is
larger than 1:2, the peak intensity of [Yb(4-VDPA)2]1- signals
decreased and some new signals corresponding to coordination of
4-VDPA and Yb3+ with 1:1 ratio ([Yb(4-VDPA)]+) generated (peaks
labelled with star in Fig. 2d). The titration experiments showed that
there are three manners for Yb3+ coordinated with 4-VDPA, and the
unique complex of [Yb(4-VDPA)3]3- could be obtained by tuning the
ratio of 4-VDPA and Yb3+ larger than 3:1. Because the similar
chemical characteristic of lanthanides, the coordination modes of 4-
VDPA to Tb3+ or Eu3+ should be identical to that of 4-VDPA to Yb3+.
Scheme 1 Synthesis procedures of 4-vinylpyridine-2, 6-dicarboxylic
acid (4-VDPA) and the luminescent hydrogels PNIPAM-Ln(DPA)3.
The wavy lines represent the polymeric portion of the hydrogels
(P(NIPAM-co-MBA-co-[Ln(4-VDPA)3]3-))
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Fig. 2 NMR spectra of 0.5mL, 1mM 4-VDPA (a), upon titration with
10mM Yb3+, 13μL (b), 22μL (c), 58μL (d)
FT-IR spectra
Fig. 4 Thermogravimetry (TG) curves of PNIPAM, PNIPAM-Eu(DPA)3
and PNIPAM-Tb(DPA)3
The FT-IR spectra of Na3[Tb(4-VDPA)3], PNIPAM, PNIPAM-Tb(DPA)3
and PNIPAM-Eu(DPA)3 are displayed in Fig. 3. The peaks at 3431cm-1
are attributed to the stretching vibration bands of N-H,30,31 whereas
the peaks at 2972, 2926 and 2874 cm-1 can be attributed to
asymmetric and symmetric stretching vibration modes of saturated
C-H, respectively (Fig. 3b to d),30 and the characteristic peaks at
1637 and 1536 cm-1 are attributed to the C=O stretching vibration
of secondary amide.30 Meanwhile, three bands appeared at 1608,
1425 and 1395 cm-1 correspond to the skeleton vibration of
The thermal stabilities of PNIPAM, PNIPAM-Eu(DPA)3 and PNIPAM-
Tb(DPA)3 were evaluated by thermogravimetric analysis (TGA). The
TGA curve of PNIPAM-Eu(DPA)3 and PNIPAM-Tb(DPA)3 is quite
similar to that of PNIPAM (Fig. 4). From the TGA curve of PNIPAM,
PNIPAM-Eu(DPA)3 and PNIPAM-Tb(DPA)3, three main steps of
o
weight loss can be observed. The initial weight loss below 350 C, is
ascribed to the loss of physically adsorbed water and other small
molecules. While the large weight loss occurs from 350°C to 430 °C,
which is ascribed to the thermal decomposition of organic moieties.
Then it is followed by a slight weight loss in the temperature of 430-
900 oC. In addition, it is difficult to find the weight loss of lanthanide
complexes in the hydrogels from the TGA curves because the
lanthanide complexes are only accounted for about 1% (wt) of the
entire dry hydrogel. The weight fraction (ω) of lanthanide complex
in the dry lydrogel was calculated by the equation as follow:
pyridine ring (Fig. 3a,
c
and d),19 proving the successful
incorporation of DPA complexes onto PNIPAM.
ω = Weight of added lanthanide complex/Weight of dry hydrogel
Optical properties
The photoluminescence (PL) properties of PNIPAM-Tb(DPA)3 and
PNIPAM-Eu(DPA)3 were investigated as shown in Fig. 5. Both the PL
excitation spectra of PNIPAM-Tb(DPA)3 (Fig. 5a) and PNIPAM-
Eu(DPA)3 (Fig. 5c) exhibit a similar broad band with a maximum
value at 282 nm, which can be related to the π→π* electronic
transition of the lanthanide complexes. This result shows that both
of the hydrogels can be excited by the UV light with the same
wavelength. As expected, upon excitation at 282 nm, the emission
spectrum of PNIPAM-Tb(DPA)3 shows the characteristic emission of
Fig. 3 FT-IR spectra of Na3[Tb(4-VDPA)3] (a), PNIPAM (b), PNIPAM-
Tb(DPA)3 (c) and PNIPAM-Eu(DPA)3 (d)
5
Tb3+ peaked at 491, 542, 582, 621 nm, ascribed to the D4→7FJ (J=6,
5, 4, 3) transitions of Tb3+ ions,
respectively (Fig. 5b), while
32-37
PNIPAM-Eu(DPA)3 gives the four prominent peaks at 591, 615, 649
Thermal analysis
5
and 696 nm, assigned to the D0→7FJ (J= 1, 2, 3, 4) transitions of
Eu3+ ions,38-44 respectively (Fig. 5d). It is noteworthy that the
emission peaks of PNIPAM-Tb(DPA)3 appear obvious splitting at 542
and 582 nm (Fig. 5b, inside the red dashed box), proving that the
mole ratio of metal (Tb3+) to ligand (DPA) is 1:3 in PNIPAM-
Tb(DPA)3.45 This finding is consistent with the result of NMR
titration (Fig. 2).
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Fig. 5 Excitation and emission spectra of PNIPAM-Tb(DPA)3 (a, b)
and PNIPAM-Eu(DPA)3 (c, d)
As observed by digital photos, PNIPAM-Tb(DPA)3 and PNIPAM-
Eu(DPA)3 show bright green and red emission under UV light
excitation (Fig. 1). The Commission Internationale de I'Eclairage (CIE)
chromaticity coordinates also indicates that the emission of
PNIPAM-Tb(DPA)3 and PNIPAM-Eu(DPA)3 lie in green (x = 0.32, y =
0.58) and red (x = 0.66, y = 0.33) region, respectively (Fig. S2).
Moreover, the CIE chromaticity coordinate of the PNIPAM-Eu(DPA)3
is close to the National Television Standard Committee (NTSC)
standard values for red (x = 0.67, y = 0.33), which indicates that
PNIPAM-Eu(DPA)3 exhibits a high color purity.
Fig. 6 Luminescence decay curve of PNIPAM-Tb(DPA)3 (a) and
PNIPAM-Eu(DPA)3 (b)
Table 1 The PL data, including excitation wavelength (λex), emission
wavelength (λem), lifetimes (τ) and quantum efficiency (η) of [Ln(4-
VDPA)3]3-, [Ln(DPA)3]3-, PNIPAN-Ln(DPA)3 and other analogous
The lifetime value of Tb3+ or Eu3+ in the hydrogel is determined
by the corresponding decay curve that can be well fitted into a
single exponential function (Fig. 6), which implies that the average luminescent material Ln-dpa-Si-MSNs, Ln = Tb or Eu), a 0.1 mmol/L
coordination environment of Tb3+ or Eu3+ is uniform in PNIPAM-
aqueous solution.
Ln(DPA)3 (Ln = Tb or Eu) hydrogels. According to the fitting results,
the lifetime values of Tb3+ and Eu3+ are determined to be 1.43 and
λex
(nm)
τ
η
(%)
Sample
λem (nm)
(ms)
1.86 ms, respectively. The obtained PNIPAM-Ln(DPA)3 (Ln = Tb or
Eu) hydrogels have excellent luminescent properties and the
quantum efficiency of the PNIPAM-Tb(DPA)3 and PNIPAM-Eu(DPA)3
are about 63.8% and 71.5%, respectively. The optical data like
excitation wavelength (λex), emission wavelength (λem), lifetimes (τ)
and quantum efficiency (η) of our samples and other analogous
luminescent material are listed in Table 1. It appears that the
lifetimes of PNIPAM-Ln(DPA)3 are longer than those of other hybrid
materials containing DPA complexes (Ln-dpa-Si-MSNs).46 The
quantum efficiency of [Tb(DPA)3]3- or [Eu(DPA)3]3- in the aqueous
solution was lower than that of PNIPAM-Tb(DPA)3 or PNIPAM-
Tb(DPA)3, respectively (dry hydrogel) (Table 1). The reason is
probably that the luminescence of lanthanide complexes is easily
quenched in aqueous solution by non-radiative processes through
O-H vibrations of water molecular.
[Tb(4-VDPA)3]3- a
[Eu(4-VDPA)3]3- a
[Tb(DPA)3]3- a
[Eu(DPA)3]3- a
Tb-dpa-Si-MSNs ref45
─
─
─
─
315
286
286
281
594, 615, 649, 696
492, 543, 582, 622
592, 615, 649, 695
490, 546, 582, 622
0.86
2.01
1.58
0.86
1.8
39.9
34.6
─
Eu-dpa-Si-MSNs ref45
281
282
592, 614, 650, 694
491, 542, 581, 621
0.62
1.43
─
PNIPAM-Tb(DPA)3
63.8
PNIPAM-Eu(DPA)3
282
591, 615, 649, 696
1.86
71.5
However, the hydrogels (PNIPAM-Tb(DPA)3 or PNIPAM-
Eu(DPA)3) have no obvious Tb3+ or Eu3+ emission under the
irradiation with a UV lamp at the beginning of the polymerization,
and the luminescent intensities of PNIPAM-Tb(DPA)3 and PNIPAM-
Eu(DPA)3 were gradually enhanced as the polymerization reaction
was occured. Hence we measured the luminescent properties of
[Tb(4-VDPA)3]3- and [Eu(4-VDPA)3]3-. As expected, [Tb(4-VDPA)3]3-
has no characteristic emission of Tb3+ under the irradiation of UV
light (Fig. S3), and [Eu(4-VDPA)3]3- shows very weak emission of Eu3+
compared with that of [Eu(DPA)3]3- (Fig. S4), the luminescent
4 | J. Name., 2012, 00, 1-3
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intensity and quantum efficiency of [Ln(4-VDPA)3]3- are also much
lower and lifetimes are shorter (Fig S5) than those of [Ln(DPA)3]3-
(Ln = Tb or Eu) (Table 1). These results show that 4-VDPA is not an
efficient ligand for sensitizing the luminescence of Tb3+ and Eu3+
ions. However, when the polymerization reaction is completed, the
vinyl group (H2C=CH-) of 4-VDPA (Fig. 7a) was converted to a
saturated C-C bond (-CH2-CH2-) and then a [Ln(DPA)3]3- complex
moiety was generated (Fig. 7b). Therefore, the PNIPAM-Tb(DPA)3
and PNIPAM-Eu(DPA)3 show bright green and red emission,
respectively. Besides, this characteristics of [Ln(4-VDPA)]3- also
make it suitable for application in the luminescent detection of free
radicals. Moreover, the excitation spectra of PNIPAM-Tb(DPA)3 or
PNIPAM-Eu(DPA)3 (Fig. 5a or c) are very similar to those of
Na3[Tb(DPA)3] or Na3[Eu(DPA)3] (Fig. S6), respectively, which also
indicates that the lanthanide complexes are successfully grafted
onto the hydrogels via one-pot free radical polymerization.
Fig. 8 The emission spectra of the luminescent hydrogel upon
excitation at 282 nm: (a) PNIPAM-Tb(DPA)3; (b) PNIPAM-
Eu1Tb3(DPA)3; (c) PNIPAM-Eu1Tb1(DPA)3; (d) PNIPAM- Eu2Tb1(DPA)3;
(e) PNIPAM-Eu(DPA)3
Conclusions
In summary, we developed
a novel strategy for preparing
luminescent hydrogels through one-pot free radical polymerization.
Lanthanide DPA complexes were successfully grafted onto the
PNIPAM hydrogel through constructing the proper bifunctional
organic ligand. It is worthy to point out that the luminescent
intensities of the hydrogels were gradually enhanced as the
polymerization is conducted, implying that luminescent lanthanide
complexes were successfully grafted onto the hydrogel matrix.
Upon an excitation with UV light, the obtained PNIPAM-Tb(DPA)3
and PNIPAM-Eu(DPA)3 hydrogels exhibit the characteristic emission
of Tb3+ (green) and Eu3+ (red), long luminescent lifetimes and high
quantum efficiency. Moreover, the emission colors of the hydrogels
can be tuned by modulating the molar ratio of added Eu3+ to Tb3+.
Fig. 7 Structure diagram of [Ln(4-VDPA)3]3- complexes and PNIPAM-
Ln(DPA)3 (Ln = Tb or Eu)
As mentioned above, PNIPAM-Tb(DPA)3 and PNIPAM-Eu(DPA)3
have prominent luminescent properties and multiple color emission
under the excitation of UV light with the same wavelength, hence
the luminescent color of hydrogels should be tuned by changing the
relative content of Tb3+ and Eu3+.47 As shown in Fig. 8, the emission
spectra of PNIPAM-EuxTby(DPA)3 show characteristic emission peaks
of Eu3+ ions at 591, 615, 649 and 696 nm and Tb3+ ions at 491, 542,
582 and 622 nm, respectively. Moreover, the intensity ratio of the
red to green emission decreases drastically upon increasing the Tb3+
concentration. Therefore, the luminescent colors of PNIPAM-
EuxTby(DPA)3 varied from red, reddish orange, orange-yellow,
yellow to green correspondingly (Fig. S7). However, the luminescent
lifetime values of the excited state 5D4 (Tb3+) in PNIPAM-
EuxTby(DPA)3 are almost unchanged, which indicated that the
energy transfer from Tb3+ to Eu3+ is not obvious in these hydrogels
(Fig. S8). 48, 49 Taking into account the transparent, long luminescent
lifetimes, high quantum efficiency, multi-color emission and
temperature-responsive characteristics of PNIPAM-Ln(DPA)3, these
hydrogels are promising in the application of optical systems,
controlled drug release, ion detection, bio-imaging and so on.
Experimental section
Diethyl 4-bromopyridine-2, 6-dicarboxylate (Compound 2)50
Diethyl 4-bromopyridine-2, 6-dicarboxylate was synthesized from
1
chelidamic acid, as previously reported. H-NMR (400MHz, CDCl3) δ
ppm: 8.45 (2H, s), 4.52 (4H, q, J=7.14Hz), 1.48 (6H, t, J=7.14Hz). 13C-
NMR (100MHz, CDCl3) δ ppm: 163.5, 149.5, 134.9, 131.1, 62.7, 14.1.
Diethyl 4-vinylpyridine-2, 6-dicarboxylate (Compound 3) 51
Diethyl 4-bromopyridine-2, 6-dicarboxylate (1.2g, 4 mmol), PdBr2
(53 mg, 0.2 mmol), 2-(di-tert-butylphosphino)biphenyl (120 mg, 0.4
mmol), tetrabutylammonium fluoride (TBAF; 8 mL, 8 mmol) in
tetrahydrofuran
stock,
and
1,3,5,7-tetravinyl-1,3,5,7-
tetramethylcyclotetrasiloxane (1.4 g, 4 mmol) were added to
tetrahydrofuran (20 mL). The resulting solution was stirred at 60 °C
for about 3 h under an atmosphere of argon, and then cooled down
to room temperature. Solvent was removed by distillation using a
rotary evaporator, and then ethyl acetate (20 mL) was added to the
residue. The organic phase was washed with brine, dried with
anhydrous Na2SO4. After filtration and distillation, the resulting
yellowish oil was purified by column chromatography on silica gel
using ethyl acetate:petroleum ether (1:3 by volume) as eluent to
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yield a pale yellow oil (0.75g, 76%). 1H-NMR (400MHz, CDCl3) δ
ppm: 8.25 (2H, s), 6.80 (1H, dd, J=17.58, 10.81), 6.18 (1H, d,
J=17.58), 5.67 (1H, d, J=10.81), 4.50 (4H, q, J=7.14), 1.47 (6H, t,
J=7.14). 13C-NMR (100MHz, CDCl3) δ ppm: 164.5, 148.9, 147.3,
133.3, 124.5, 121.0, 62.1, 14.0.
Acknowledgements
The work was financially supported by the National Natural Science
Foundation of China (Nos. 21401218, 21171179, 51572303),
Excellent Youth Foundation of He'nan Scientific Committee (no.
134100510018), Program for Innovative Research Team (in Science
and Technology) in University of Henan Province (14IRTSTHN009),
Innovation Scientists and Technicians Troop Construction Projects
of Henan Province (2013259), Henan Province Key Discipline of
Applied Chemistry (201218692). We thank Prof. Xun-Cheng Su and
Dr Jia-Liang Chen at Nankai University for their kind help on NMR
analysis and measurement.
4-vinylpyridine-2, 6-dicarboxylic acid (4-VDPA)
Diethyl 4-vinylpyridine-2, 6-dicarboxylate (0.7 g) was added to 8 mL
of water and alcohol mixed solution (1:1 by volume), and then
NaOH solution (2 mol/L, 3.5 mL) was added. This solution was
stirred at room temperature for about 5 h, and the pH of the above
mixture was adjusted to about 2 by hydrochloric acid. The resulting
white suspension was filtered, washed with deionized water and
1
Notes and references
dried under vacuum. H-NMR (400MHz, D2O, pD ≥ 12) δ ppm: 7.91
(2H, s), 6.77 (1H, dd), 6.08 (1H, d), 5.52 (1H, d). 13C-NMR (100MHz,
D2O, pD ≥ 12) δ ppm: 172.8, 153.3, 147.4, 134.2, 122.0, 120.0. MS:
192.0296 (M-1)-.
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H2O, the mixture was degassed by bubbling with N2 gas for about
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immersed into deionized water to remove the residual monomers.
PNIPAM was prepared by this method without using [Ln(4-VDPA)3]3-
. The corresponding dry hydrogels were obtained after drying under
vacuum at 60 oC for 24h.
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DOI: 10.1039/C6TC00336B
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Journal of Materials Chemistry C
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DOI: 10.1039/C6TC00336B
Highly luminescent hydrogels synthesized by covalent grafting of
lanthanide complexes onto PNIPAM via one-pot free radical
polymerization
Qing-Feng Li, Xiaodi Du, Lin Jin, Mengmeng Hou, Zhenling Wang * and Jianhua Hao*
Transparent and highly luminescent hydrogels synthesized by covalent grafting of
lanthanide complexes onto PNIPAM exhibit tunable multi-color luminescence
property.
TOC Figure