Novel multi-color photoluminescence emission phosphors developed by layered gadolinium hydroxide via in situ intercalation with positively charged rare-earth complexes

Article abstract of DOI:10.1039/c4tc02583k

The design of multi-color phosphors has aroused great interest for practical optical applications. Herein, for the first time, the stable positively charged Eu(iii) and Tb(iii) complexes EuL and TbL (L = 2,2′-(4-(2-ethoxyethoxy)pyridine-2,6-diyl)bis(4,5-dihydrooxazole)) have been successfully intercalated in situ into the gallery of layered gadolinium hydroxide (LGdH) utilizing chelation of picolinic acid (pa) anions via ligand exchange reaction. In particular, the resulting hybrid phosphors (LGdH-pa-Eu_1-xTb_xL) exhibit multi-color emissions by simply fine-tuning the molar ratio of Eu(iii)/Tb(iii), which show red and green primary colors, as well as intermediate colors excited at 310 nm. The luminescence spectra, decay time, and low-temperature phosphorescence spectra analysis indicate that other than the intramolecular energy transfer (ET) process from ligand to RE(iii) and intermolecular ET process from Tb(iii) to Eu(iii), the expected and interesting interaction between host (LGdH) and guest pa anions was also observed, which dramatically enhanced the absorption cross section of pa anions. Finally, a blue emission component 4,4′-distyrylbiphenyl sodium sulfonate (Tinopal CBS) was further introduced into the hybrid material LGdH-pa-Eu_0.2Tb_0.8L, to provide white light emission under the 330 nm excitation. In addition, to determine the potential of these hybrid phosphors in various applications, transparent and multi-color emission composite film devices have been fabricated with poly(methyl methacrylate) (PMMA) using the solvent-casting method.

Full text of DOI:10.1039/c4tc02583k

Journal of
Materials Chemistry C
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PAPER
Novel multi-color photoluminescence emission
phosphors developed by layered gadolinium
hydroxide via in situ intercalation with positively
charged rare-earth complexes†
Cite this: J. Mater. Chem. C, 2015, 3,
807
1
Tingting Shen, Yu Zhang, Weisheng Liu and Yu Tang*
The design of multi-color phosphors has aroused great interest for practical optical applications. Herein, for
0
the first time, the stable positively charged Eu(III) and Tb(III) complexes EuL and TbL (L ¼ 2,2 -(4-(2-
ethoxyethoxy)pyridine-2,6-diyl)bis(4,5-dihydrooxazole)) have been successfully intercalated in situ into
the gallery of layered gadolinium hydroxide (LGdH) utilizing chelation of picolinic acid (pa) anions via
x
ligand exchange reaction. In particular, the resulting hybrid phosphors (LGdH-pa-Eu1ꢀxTb L) exhibit
multi-color emissions by simply fine-tuning the molar ratio of Eu(III)/Tb(III), which show red and green
primary colors, as well as intermediate colors excited at 310 nm. The luminescence spectra, decay time,
and low-temperature phosphorescence spectra analysis indicate that other than the intramolecular
energy transfer (ET) process from ligand to RE(III) and intermolecular ET process from Tb(III) to Eu(III), the
expected and interesting interaction between host (LGdH) and guest pa anions was also observed, which
dramatically enhanced the absorption cross section of pa anions. Finally, a blue emission component
0
4
,4 -distyrylbiphenyl sodium sulfonate (Tinopal CBS) was further introduced into the hybrid material
Received 12th November 2014
Accepted 19th December 2014
LGdH-pa-Eu0.2Tb0.8L, to provide white light emission under the 330 nm excitation. In addition, to
determine the potential of these hybrid phosphors in various applications, transparent and multi-color
emission composite film devices have been fabricated with poly(methyl methacrylate) (PMMA) using the
solvent-casting method.
DOI: 10.1039/c4tc02583k
www.rsc.org/MaterialsC
exhibit multi-color PL emissions by simply tuning the relative
1
. Introduction
12,13
compositions of rare-earth ions,
which determines whether
Layered rare-earth hydroxides (LRHs) with general formula this kind of layer material is an excellent candidate for fabrication
mꢀ
RE
2
(OH)
5
(A
)
1/m$nH
2
O (RE ¼ rare-earth ions; A ¼ interlayer of multi-color phosphors. However, abundant water molecules
anions) have received considerable attention in recent years; they and hydroxyls contained in LRHs, which directly coordinate to
are composed of positively charged hydroxide layers of trivalent the rare-earth metal centres, may lead to drastic luminescence
14
rare-earth ions and negatively charged anions to maintain electric quenching. Consequently, studies on how to improve the
1
–4
neutrality. The combination of the rare-earth elements and emission intensity of LRHs phosphors have become more chal-
interlayer chemistry endows integrated materials with wide lenging. Until now, doping inert ions Y(III) and Gd(III) into the host
5
6
7
8
15–17
applications in medicalscience, catalysis, separations, sensors,
and luminescence devices. Following studies of synthesis, anion sensitizers have been the most common methods. To the best of
exchanges, and exfoliation properties of LRHs,
layer,
calcinations of LRHs crystals, and intercalation of
9
8
6
10,11
J
and due to the our knowledge, the energy level ( S7/2– I ) of Gd(III) excited states
unique electronic structure and outstanding color purity of rare- is very high, which cannot be sensitized by common organic
earth ions, the photoluminescence (PL) behavior of LRHs has sensitizers, but can transfer energy to photoluminesce active ones
been considered a hot spot. Furthermore, the synthesis of LRHs such as Eu(III), Tb(III), or Ce(III) to improve their optical proper-
18
with two or more active rare-earth ions in the host layer can ties. By annealing, the hydration of the rare-earth ions becomes
lower and changed coordination environments lead to strong PL
19
emissions. In contrast, the interlayer chemistry is emphasized
to assemble inorganic or organic sensitizers with the host layers
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu
Province, State Key Laboratory of Applied Organic Chemistry, College of Chemistry
and Chemical Engineering, Lanzhou University, Lanzhou 730000, P. R. China.
E-mail: tangyu@lzu.edu.cn; Fax: +86 931 8912582; Tel: +86 931 8912552
20,21
to form composite hybrids.
In view of the abovementioned
results, to maintain the integrity properties of the layered mate-
†
Electronic supplementary information (ESI) available: Powder X-ray diffraction, rial, our group utilized the synergistic effect of Gd(III) and organic
FT-IR spectrum, luminescence decays, photoluminescence spectra, additional
22
sensitizers to highly enhance the PL intensity of LTbH.

gures and tables. See DOI: 10.1039/c4tc02583k
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Paper
It was noted that these previous studies mainly focused on 2.2. Measurements
emission centers from the host layer and rarely work was
Powder X-ray diffraction (PXRD) patterns were recorded over the
focused on emitting different colors entirely from galleries of
LRHs. To the present, layered gadolinium hydroxide (LGdH)
has been studied widely due to the low cost of inert ions. High
effective magnetic moment and slow electronic relaxation rate
determine that LGdH is a good contrast agent for magnetic
ꢁ
2q range of 3–65 using a Rigaku-Dmax 2400 diffractometer with
Cu-Ka radiation. Elemental analysis was determined on an Ele-
1
mentar Vario EL analyzer. The H NMR spectrum was measured
using a Bruker 400 MHz spectrometer in D-trichloromethane
with tetramethylsilane [Si(CH ) ] as an internal standard. Fourier
3 4
23
resonance imaging. In this paper, LGdH is chosen as the host
matrix and the picolinic acid (pa, Fig. S1†) anions are rst
introduced into the galleries, and then positively charged rare-
transform infrared (FTIR) spectra of the materials were con-
ꢀ1
ducted within the 4000–400 cm wavenumber range by using a
Nicolet 360 FTIR spectrometer with the KBr pellet technique.
UV-vis absorption spectra were determined on a Varian
UV-Cary100 spectrophotometer. The phosphorescence spectrum
of Gd-pa was measured in a methanol–DMF mixture on a Hitachi
F-4500 spectrophotometer. The steady-state luminescence
spectra, lifetime measurements, and overall quantum yields of
the samples were measured using an Edinburgh Instrument
FSL920. A 450 W Xe arc lamp as the excitation source was used
for the steady-state measurements, and an Nd-pumped OPOlette
laser as the excitation source was used for lifetime measure-
ments. Overall quantum yields were determined by an absolute
0
earth complexes EuL and TbL (L ¼ 2,2 -(4-(2-ethoxyethoxy)-
pyridine-2,6-diyl)bis(4,5-dihydrooxazole), Fig. S1†) are in situ
intercalated into the galleries utilizing the chelation of pa
24
anions by ligand exchange reaction. Referring to the literature,
the Eu(III) and Tb(III) central ions can be sensitized efficiently by
25
the ligand L. Above all, the importance of resulting hybrid
phosphors can be summarized as follows. First, this work paves
the way for xing positively charged complexes between the
same charged layers of lamellar compound. Second, the non-
active host acts as not only a support matrix, but also an indirect
energy donor to activate rare-earth ions via guest pa to enhance
the PL emissions, which highly enhance the absorption cross
section of pa depending on the strong host–guest interaction,
such as coordination and electrostatic interaction. Third, the
host matrix keeps the rare-earth ions close to each other in
order for the energy transfer (ET) process from Tb(III) to Eu(III) to
take place, and then also acts as a protective shield to enhance
the photostability and thermal stability of intercalated organic
complexes. Fourth, color tuning can be realized by simple
adjustment of the relative molar ratios of Eu(III)/Tb(III). As an
integral part of this work, to expand the potential application of
method with an integrating sphere (150 mm diameter, BaSO
4
coating). Three parallel measurements were carried out for each
sample, so that the presented value corresponded to the arith-
metic mean value. The errors in quantum yield values associated
with this technique were estimated to be within 10%. All
measurements were carried out at room temperature. For the
molar extinction coefficient assay of pa, rst, pa was dissolved
into the deionized water to form a series of concentrations (0.005,
ꢀ1
0
.00625, 0.01, 0.0125, 0.025 g L ) and LGdH-pa was dispersed
into the deionized water to form the different concentrations of
ꢀ1
pa (0.0038, 0.0095, 0.019, 0.0285, 0.038 g L ). Then, all absor-
hybrid phosphors, the exchange of a blue emission center
bance values of the solutions at the 265 nm excitation wavelength
and solid diffuse reectance were measured on the Agilent Cary
0
4
,4 -distyrylbiphenyl sodium sulfonate (Tinopal CBS, Fig. S1†)
into the LGdH hybrid to acquire white PL emissions and toward
fabrication of LGdH-hybrid phosphors-doped poly(methyl
methacrylate) (PMMA) luminescent composite lm devices
5000 UV-Vis-NIR spectrophotometer.
26
2.3. Synthesis of LGdH precursor
using the solvent-casting method are also reported. A sche-
matic diagram showing the design of the multi-color PL emis-
sion hybrid phosphors is exhibited in Scheme 1. These ndings
may open up new avenues for the exploration of multifunctional
hybrids based on LRHs and fabrication of color-tunable emit-
ting composite lms, which can serve as promising materials
for use in various bioprobes and optical devices.
2
LGdH was obtained following a previously reported procedure.
Gd(NO $6H O (1.00 g, 2 mmol) and KNO (2.00 g, 19 mmol)
)
3 3
2
3
were dissolved in 20 mL of deionized water. KOH (2.00 M) was
added dropwise to the above mixture until pH ¼ 6.94, and then
the suspension was transferred into a Teon-lined stainless-steel
ꢁ
autoclave of 50 mL capacity reacting at 150 C for 48 h aer
stirring 20 min. The resulting solid was centrifuged, washed
ꢁ
with ethanol and water, and then dried at 70 C overnight. The
CHN and inductively coupled plasma (ICP) element analyses
showed the following chemical formula of the product:
Gd (OH)5.63(NO )0.79(CO )0.23$0.58H O (found: Gd, 65.61; C,
2 3 3 2
2
. Experimental section
2
.1. Materials
Gadolinium nitrates (Gd(NO
Tb(NO $6H O), and europium nitrates (Eu(NO
prepared by reacting Gd , Tb , and Eu
Shanghai Yuelong) with nitric acid, and then superuous nitric
3
)
3
$6H
2
O), terbium nitrates
$6H O) were
(99.99%,
0.58; H, 1.46; N, 2.92. Calc.: Gd, 65.05; C, 0.57; H, 1.42; N, 2.29%).
(
3
)
3
2
3
)
3
2
2
O
3
4
O
7
2 3
O
2.4. Synthesis of Gd(III) complex
acid was removed. PMMA and Tinopal CBS were purchased The ligand pa (3.0 mmol) was dissolved in ethanol at room
from Alfa Aesar. Chelidamic acid, 2-chloroethylamine hydro- temperature. The solution was added with triethylamine until
chloride, and 2-ethoxyethanol were obtained from Lanzhou pH ¼ 7–8. Gd(NO ) $6H O (1.0 mmol) was then dissolved in
3
3
2
Aihua Corporation. The reagents referring to the experiment ethanol solution and ltered into the pa solution with the
were used directly without further treatment.
observed sediment. Aer the mixture was stirred for 5 h, the
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Scheme 1 Schematic illustration for fabrication of LGdH-pa-Eu1ꢀxTb
and mechanism of color-tuning process.
x
L (x ¼ 0, 0.3, 0.5, 0.8, 1.0), LGdH-pa-Eu0.2Tb0.8L-CBS hybrid phosphors,
precipitate was centrifuged, washed with ethanol, and dried 5 : 1). Finally, a solution of a (2.00 g, 5 mmol) in 2-ethoxyethanol
in vacuum. CHN element analyses were carried out for (150 mL) by the addition of KOH (1.46 g, 26 mmol) was heated at
ꢁ
Gd(pa)
3
$H
2
O (found (%): C, 39.73; H, 2.17; N, 8.52. Calc. (%): C, 75 C for 2 days. Aer the solvent was removed, the crude
39.92; H, 2.61; N, 7.76).
product was puried via silica column chromatography
(
V
dichloromethane : Vmethanol ¼ 10 : 1). d
H 3 4
(400 MHz; CDCl ; Me Si):
2.5. Synthesis of RE(III) complexes
7.70 (s, 2H), 4.52 (t, 4H), 4.26 (m, 2H), 4.10 (t, 4H), 3.82 (m, 2H),
3
1
.59 (q, 2H), 1.24 (t, 3H). d
63.51, 148.10, 112.07, 68.32, 68.29, 68.16, 66.98, 54.95, 15.10.
C 3 4
(400 MHz; CDCl ; Me Si): 165.87,
The Eu(III) and Tb(III) complexes were prepared in a similar
manner by mixing ligand L (3.0 mmol) and RE(NO ) $6H O
3
3
2
ESI-MS: [M + H] m/z 306.12.
.7. Exchange of pa into LGdH (LGdH-pa)
(1.0 mmol) in acetonitrile. Aer the mixture was heated for 4 h,
the solvent was partially removed under reduced pressure and
the resulting material was dried in a vacuum oven overnight to
2
give the solid complexes. Elemental analysis was carried out for LGdH (172 mg) was treated in deionized water (2 mL) for 15 min
EuL (NO $3H O (found (%): C, 41.12; H, 4.79; N, 12.83. Calc. by ultrasonication, and subsequently dispersed into the mixed
3
3
)
3
2
(
(
4
%): C, 41.32; H, 4.85; N, 12.85), and for TbL (NO ) $3H O aqueous solution of pa (128.2 mg, 1.04 mmol) and NaOH (1 M)
3
3 3
2
ꢁ
found (%): C, 40.89; H, 4.80; N, 12.65. Calc. (%): C, 41.10; H, (pH ¼ 7.35). The mixture was treated hydrothermally at 100 C
.83; N, 12.78).
for 12 h. Then the resulting solid was centrifuged, washed with
deionized water, and dried at 50 C overnight.
ꢁ
0
.6. Synthesis of ligand 2,2 -(4-(2-ethoxyethoxy)pyridine-2,6-
2
diyl)bis(4,5-dihydrooxazole) (L)
2.8. In situ intercalation assembly of rare-earth complexes
into LGdH-pa (LGdH-pa-Eu1ꢀxTb
x
L, x ¼ 0, 0.3, 0.5, 0.8, 1.0)
The ligand L was synthesized using method reported in the
25
literature with some modications. Chelidamic acid (5.00 g, The hybrid phosphors LGdH-pa-Eu1 Tb L (x ¼ 0, 0.3, 0.5, 0.8,
ꢀx
x
2
7 mmol) and DMF (20 mL) were added into SOCl
then reuxed for 48 h. Aer removing the excess SOCl
-chloropyridine-2,6-dicarbonyl dichloride was dissolved again ultrasonication for 30 min. Next, the mixture of the ligand L and
in chloroform (100 mL), and then added dropwise into a solu- rare-earth nitrates with the molar ratio of L/RE(NO $6H
2
(20 mL), and 1.0) were prepared via in situ intercalation reaction. First, LGdH-
2
, the pa (41.6 mg) powder was dispersed in acetonitrile (5 mL) by
4
3
)
3
2
O ¼
tion of 2-chloroethylamine hydrochloride (6.95 g, 60 mmol) and 2 : 1 (the molar ratio of Eu(III)/Tb(III) ¼ 1 : 0, 0.7 : 0.3, 0.5 : 0.5,
ꢁ
KOH (6.40 g, 114 mmol) at 0 C and stirred vigorously for 2 h. 0.2 : 0.8, 0 : 1, respectively) in acetonitrile (5 mL) was added into
ꢁ
The organic phase was separated, washed with deionized water LGdH-pa, and treated hydrothermally at 130 C for 24 h. The
and saturated NaHCO , and dried with MgSO overnight. The resulting hybrid materials (LGdH-pa-Eu Tb L, x ¼ 0, 0.3, 0.5,
3
4
1ꢀx
x
solvent was removed to give a bright white solid (a) with puried 0.8, 1.0) were centrifuged, washed with acetonitrile, and dried at
ꢁ
by silica column chromatography (Vpetroleumether : Vethyl acetate
¼
50 C.
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.9. Preparation of white-light emission hybrids (LGdH-pa-
Paper
2
Eu0.2Tb0.8L-CBS)
Tinopal CBS (0.5 mg, 0.0009 mmol) and pa (32.0 mg,
0.25 mmol) were dissolved in deionized water (12 mL); NaOH
(
(
1 M) solution was added until pH ¼ 7.35; and then LGdH
ꢁ
43.0 mg) was dispersed in the mixture hydrothermally at 120 C
for 12 h. The resulting product LGdH-pa-CBS and the mixture of
the ligand L and rare-earth nitrates with the molar ratio of
ꢁ
Eu/Tb/L ¼ 0.2 : 0.8 : 2 were dispersed in acetonitrile at 130 C
for 24 h hydrothermally to get the nal hybrids LGdH-pa-
Eu0.2Tb0.8L-CBS.
2
.10. Preparation of multi-color PL emission composite lm
devices doped with hybrid phosphors (PMMA/LGdH-pa-
Eu1ꢀxTb L, PMMA/LGdH-pa-Eu0.2Tb0.8L-CBS)
x
ꢀ
1
The PMMA powder dissolved in DMF (100 mg mL ) was heated
ꢁ
ꢁ
Fig. 1 PXRD patterns (2q ¼ 3–60 ) of (a) LGdH, (b) LGdH-pa,
at 60 C for 30 min, and the hybrid LGdH-pa-Eu1ꢀxTb
LGdH-pa-Eu0.2Tb0.8L-CBS solid dispersed in DMF (20 mg mL
was treated with ultrasound for 1 h. Then, the prepared LGdH-
x
L or
(
c) LGdH-pa-EuL, (d) LGdH-pa-Eu0.7Tb0.3L, (e) LGdH-pa-Eu0.5Tb0.5L,
f) LGdH-pa-Eu0.2Tb0.8L, and (g) LGdH-pa-TbL.
ꢀ
1
)
(
pa-Eu1ꢀxTb
x
L or LGdH-pa-Eu0.2Tb0.8L-CBS solution was added
ꢁ
into the dissolved PMMA, and heating of the mixture at 60 C seriously aer intercalation of pa against the (400) and (220)
was continued for 2 h. Subsequently, the hybrid material was reections, which indicated that the regularity degrees became
cast onto clean quartz glasses and the multi-color composite less ordered or the thickness of LGdH platelets decreased along
15

lm device was obtained aer total evaporation of the solvent at the c-axis. Aer complexes went into the interlayer of
room temperature.
LGdH-pa, the relative intensity of (002) and (004) reections
became weaker, for the abovementioned reasons as well as
increased electron density at the midpoint of the interlayers due
3. Results and discussion
27
to the presence of the rare-earth ions at high loading (10%).
3.1. Structural analysis of LGdH precursor and hybrid
The HRTEM images of LGdH and LGdH-pa-Eu1ꢀxTb L clearly
x
phosphors
show the lattice fringes (see Fig. S2b and e†). All the as-prepared
samples feature the interplanar spacing of approximately
x
PXRD patterns of LGdH, LGdH-pa, and LGdH-pa-Eu1ꢀxTb L
0.31 nm, corresponding to the (220) plane of LRHs. These
(
x ¼ 0, 0.3, 0.5, 0.8, 1.0) are shown in Fig. 1, indicating that all
results further illustrate the integrity of the host layer before
and aer intercalation. From PXRD (Fig. 1g) and SAED
samples have a typical ordered, two-dimensional (2D) lamellar
stacking structure. Fig. 1a displays a series of strong (00l)
diffraction peaks, which are indexed as a layered gadolinium
(
x
Fig. S2f†) patterns, the cell parameters of LGdH-pa-Eu1ꢀxTb L
5
˚
˚
˚
were calculated (a z 12.6(2) A, b z 7.2(2) A, c z 29.8(3) A).
Results illustrate that the stacking direction was parallel to the
c-axis. The PXRD patterns of LGdH-pa-CBS and LGdH-pa-
Eu0.2Tb0.8L-CBS and detailed description are given in Fig. S3.†
The elemental analysis (Table S1†) of the solid LGdH-pa
shows the C/N molar ratio, 6.24, is very close to that in the pa
molecule (6.00). This value decreases to about 4 accompanied
hydroxide phase with a basal spacing of 0.83 nm. Meanwhile, a
number of very weak non-(00l) reections were also observed,
indicating ordered layers. Size and morphology of the products
were examined by TEM as shown in Fig. S2.† Well-arranged
spots in the selected area electron diffraction (SAED) patterns
shown in Fig. S2c† conrm this ordered structure within the ab
plane. The lattice parameters were estimated from PXRD and
˚
2
˚
˚
by incorporation of the rare-earth complexes, which may be due
SAED patterns (a z 12.6(2) A, b z 7.4(1) A, c z 16.6(4) A, and b
ꢀ
ꢁ
to the introduction of NO
3
anions in the rare-earth salts via the
z 92 of a monoclinic cell). As revealed in Fig. S2a and d,† the
prepared materials displayed a quasi-hexagonal plate-like
morphology with side length of about 200 nm. The inset in
Fig. S2d† clearly shows the layered structure. In the rst step of
the intercalation of pa anions into the LGdH gallery, the basal
in situ coordination reaction. Taking into account the steric
hindrance in the restricted interlayer region, the coordination
environment around each Tb(III) or Eu(III) ion contained only
ꢀ
one pa group, one L group and one or two NO3 anions, which
28
can satisfy the rare-earth preferred coordination number 7–9.
spacing expanded from 0.83 nm to 1.48 nm (Fig. 1b), indicating
ꢀ
As seen in Table S1,† the Eu(III)/Tb(III) molar ratio in the prod-
ucts is close to the feeding capacity. The amount of rare-earth
ions in the solid indicated that rare-earth complex intercalation
content reached up to 31.3%. Thanks to this well-ordered
disposition of pa anions in the interlayer space, the hybrid
LGdH-pa may be regarded as a porous structure, with channels
that the NO
3
was exchanged by the pa anions. As seen in
Fig. 1c–g, when different compositional rare-earth complexes
were xed in the LGdH-pa gallery to form the hybrids LGdH-pa-
Eu1 Tb L, compared with LGdH-pa, the systematic (00l)
ꢀx
x
reections shied slightly toward lower diffraction angles, and
the basal spacing expanded continually to 1.50 nm. Obviously,
the relative intensity of (002) and (004) reections decreased
6
running along the c direction. For purposes of further proving
1810 | J. Mater. Chem. C, 2015, 3, 1807–1816
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ꢀ
1
the fact that the complexes entered into the LGdH-pa gallery, N
2
group of rare-earth complexes are observed at 3095 cm ,
ꢀ1
ꢀ1
ꢀ1
adsorption–desorption isotherm curve experiments were 2932 cm , 2975 cm , and 2879 cm , further illustrating that
carried out. Results revealed that the signicant decrease in the rare-earth complexes entered the interlayer. From the FTIR
2
ꢀ1
2
ꢀ1
BET surface area from 22.20 m g (LGdH-pa) to 4.138 m g
LGdH-pa-Eu1ꢀxTb
the LGdH-pa was lled with rare-earth complexes.
spectra, the binding modes of ligands in LGdH may be the
L), indicating that the interlayer region of COO group and N atom of the pa anion to coordinate with
ꢀ
(
x
29
RE(III) ions; in addition, the ligand L supplies its three N atoms
The FTIR spectra of LGdH, LGdH-pa and LGdH-pa-EuL to simultaneously coordinate with the rare-earth ions. To satisfy
samples are shown in Fig. 2. The spectra of other LGdH-pa- the high coordination numbers of rare-earth ions, and consider
ꢀ
Eu1ꢀxTb
x
L samples are displayed in Fig. S4,† and the spectra of the two kinds of NO
3
in the LGdH gallery, we nally deduced
ꢀ
LGdH-pa-CBS and LGdH-pa-Eu0.2Tb0.8L-CBS are shown in that there were two NO3 around one rare-earth ion, where one
Fig. S5 in the ESI.† In Fig. 2a intense broad bands centred at takes part in coordination and the other is free in the interlayer
ꢀ
1
ꢀ1
3
569 cm and 3387 cm were observed, which are attributed space, which is also consistent with the elemental analysis
to the stretching vibration of the hydroxyl group from both the results. In view of FTIR and the elemental analysis results, the
interlayer water and hydroxide layers. Meanwhile, the absorp- complex stoichiometry was deduced to be 1 : 1 with L to rare-
tion peak attributed to the bending mode of water molecules earth ions in the layered-material LGdH gallery.
ꢀ
1
was observed at around 1634 cm . The intense band at
The UV-vis absorbance spectra of pa, LGdH, LGdH-pa, and
LGdH-pa-Eu0.5Tb0.5L were shown in Fig. S6.† Obviously, the
ꢀ
1
30
1384 cm corresponds to the n
3
mode of the nitrate species.
Aer exchanging with pa (Fig. 2b), the absorption peak at absorbance spectra of the intercalation material were not only a
ꢀ
1
ꢀ1
3
387 cm was red-shied to 3444 cm , conrming the inter- simple superposition of each component's spectra. The absor-
action between the host LGdH and the pa anions. The retained bance spectra of pa had an obvious red-shi (264–271 nm) when
ꢀ
1
ꢀ
3
coexisting with the pa was exchanged into the LGdH interlayer. It is commonly
band at 1384 cm , conrm that there are NO
pa anions and so with introducing the rare-earth complexes. accepted that absorbance spectrum red shi suggests that the
34
The asymmetric and symmetric stretching vibrations of the organic ligand should have been coordinated with metal ions.
COO group appear at 1639 cm , 1571 cm , 1449 cm , and The coordination of pa anions with Gd(III) ions of the LGdH
384 cm (overlapping with the nitrate band), which illustrate layer could have evolved due to the exible and high coordi-
ꢀ
ꢀ1
ꢀ1
ꢀ1
–
ꢀ
1
1
that the pa molecules entered into the gallery in the form of nation number characteristics of rare-earth ions. In addition,
31
anions. Aer in situ incorporation of rare-earth complexes the unique structure of corrugated charged slabs in hybrid
ꢀ
1
ꢀ1
ꢀ1
ꢀ1
(
Fig. 2c), these peaks shi to 1631 cm , 1567 cm , 1441 cm
,
LGdH indicates that depending on electrostatic attraction,
organic molecules in the gallery can directly coordinate to the
ꢀ1
and 1412 cm , respectively. Characteristic bands at 1593 cm
and 1478 cm can be attributed to stretching vibrations of the rare-earth host and not just interact. It is especially important
pyridine ring, whose intensities varied sharply when the that the absorbance spectra of LGdH-pa have an increased
complexes went into the gallery. Peak shis and intensity absorbance intensity and broad absorbance area aer interca-
variations suggest a strong interaction between the rare-earth lation of mixed rare-earth complexes. The continued red shi of
complexes and pa anions. Meanwhile, at 1290 cm (n ) the C
ꢀ1
32
ꢀ
1
absorbance spectra led us to conclude that there was a
4
2n
ꢀ
ꢀ
mode of NO3 is observed, illustrating that a part of NO had continued coordination interaction between pa anions with the
taken part in coordination as a bidentate agent. In Fig. 2c Eu(III) and Tb(III) complexes.
3
3
3
typical C–H stretching vibrations of pyridine ring and alkyl
3
.2. Photoluminescence behavior
The PL spectra of the powder samples LGdH-pa-Eu1ꢀxTb
LGdH-pa-Eu0.2Tb0.8L-CBS at room temperature are shown in
Fig. 3. The excitation spectra of LGdH-pa-Eu1ꢀxTb L were
basically similar (see Fig. S7†), and the maximum excitation
peak of the sample LGdH-pa-Eu1ꢀxTb L was detected at
approximately 310 nm, which were different from the pure-
x
L and
x
x
25
complexes solid at 290 nm (see Fig. S8†), indicating the
complicated coordination environments of the rare-earth ions
in the LGdH gallery. Upon being excited at 310 nm, the hybrid
phosphors retained their optical behavior of corresponding
rare-earth ions. At 489, 543, 582, 620, and 646 nm, the emission
5
7
peaks of Tb(III) were collected, corresponding to its D
J ¼ 6, 5, 4, 3, 2) transitions. Eu(III) also displayed its charac-
teristic emissions at 579, 592, 615, 649, and 693 nm, corre-
4 J
/ F
(
5
7
35
sponding to the D / F (J ¼ 0, 1, 2, 3, 4) transitions. In the
0
J
mixed component LGdH-pa-Eu Tb L, the color output
1
ꢀx
x
changed from red to green with relative intensities varying
Fig. 2 FTIR spectra of (a) LGdH, (b) LGdH-pa, and (c) LGdH-pa-EuL.
between the Eu(III) and Tb(III) bands. Aer intercalating the blue
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Fig. 3 (A) PL spectra of powder samples (a) LGdH-pa-EuL, (b) LGdH-pa-Eu0.7Tb0.3L, (c) LGdH-pa-Eu0.5Tb0.5L, (d) LGdH-pa-Eu0.2Tb0.8L,
(
e) LGdH-pa-TbL, lex ¼ 310 nm, and (f) white light emission LGdH-pa-Eu0.2Tb0.8L-CBS, lex ¼ 330 nm. (B) CIE chromaticity diagram showing (x, y)
emission color coordinates.
light emitter Tinopal CBS into the component LGdH-pa- Tb(III) and Eu(III) obviously changed, indicating that the ET
Eu0.2Tb0.8L gallery, the hybrid phosphors evidently emitted process occurred between Tb(III) and Eu(III). Considering the
white light excited at 330 nm. All colors are depicted in the above results, we deduced that there is a cascade energy
Commission Internationale de L'Eclairage (CIE) x–y chroma- transfer from pa to Tb(III), nally relaying to Eu(III) in the
ticity diagram shown in Fig. 3B. The CIE color coordinates of mixed-component hybrid phosphors.
each sample are listed in Table S2.† It can be seen that the
It is well known that the singlet and triplet states levels of the
hybrid phosphors exhibited an excellent property in tuning ligand play an important role in determining efficiencies of
37
multi-color PL emission.
.3. Energy transfer
intersystem crossing and ET from ligand to rare-earth ion. The
rst excited singlet energy (S ) levels and rst excited triplet
energy (T ) levels of ligand were determined by reference to the

1
3
1
upper absorption edge of the UV-vis spectrum and lower-wave-
length emission edge of the low-temperature phosphorescence
In order to conrm the fact that the host LGdH acted as not
only a support matrix, but also an important participant in
the assembly's entire ET process, the powders LGdH, pa,
LGdH-pa, LGdH-pa-EuL, LGdH-pa-TbL, and LGdH-pa-
Eu0.5Tb0.5L were chosen as samples. The low-temperature (77
K) phosphorescence emission spectra (Fig. 4) and excitation
spectra (Fig. S9†) were collected with the addition of other
step-by-step characterizations, with the aim of tracing the
energy transfer (ET) path between them. As seen in Fig. 4a,
pa has a very weak emission excited at the optimal 292 nm
wavelength, but its phosphorescence was signicantly
enhanced more than eight times when pa anions were inter-
calated into the layered-matrix LGdH gallery. Meanwhile, the
pa phosphorescence emission peak shied from 439 nm to
38
spectrum of the corresponding Gd(III) complex. The triplet and
ꢀ1
singlet state energy of the ligand L are 26 200 cm and 33 200
ꢀ
1
25
cm in acetonitrile solution, respectively. With regard to pa,
ꢀ
1
ꢀ1
the values were obtained as 27 322 cm and 36 496 cm
determined in the mixed solution of methanol and DMF
Fig. S10†). Latva's empirical rule states that an optimal
,
39
(
5
ligand-to-metal energy transfer process needs (DE ¼ T
1
ꢀ D
J
)
3
6
ꢀ
1
ꢀ1
2
500–4000 cm
for Eu(III) and 2500–4500 cm
for Tb(III).
While the truth is that the T
1
level of the ligand L has a distance
ꢀ
1
ꢀ1
of 8900 cm for Eu(III) and 5700 cm for Tb(III). As related to
ꢀ1
ꢀ1
pa, the gaps are 10 022 cm for Eu(III) and 6822 cm for Tb(III),
indicating that pa is not an excellent candidate compared with L
alone. But the strong host–guest interaction indicated that the
sensitized efficiency of pa is enhanced by the dramatically
increased absorption cross section in the LGdH gallery, which
was demonstrated by the molar extinction coefficient data of the
species pa at 265 nm investigated in deionized water before
4
52 nm in LGdH-pa, and the strong increased intensity and
red shi indicated that the LGdH host layer played a very
important role in helping guest pa to absorb energy even
though the host itself could not be easily excited at 281 nm.
When the TbL complexes were further introduced into the
LGdH-pa gallery, the emission intensity of pa decreased
acutely instead of the remarkable appearance of Tb(III) char-
acteristic emissions (Fig. 4b); this was attributed to ET from
pa to Tb(III). This phenomenon was consistent in Fig. 4c, weak
Eu(III) emissions can be observed and pa emissions still
existed albeit in decline to a certain extent, which directly
illustrated that pa is a better sensitizer to Tb(III) than Eu(III).
In the hybrid phosphors of mixed composition, LGdH-pa-
Eu0.5Tb0.5L (Fig. 4d), the relative emission intensities of
ꢀ1
ꢀ1
ꢀ1
ꢀ1
(3017.42 L mol
cm ) and aer (4299.01 L mol
cm )
intercalation. Moreover, the solid absorption intensity of pa was
increased 5 times aer intercalation. In general, the ligands L
and pa sensitized RE(III) simultaneously, and the conjunct
power impelled multi-color emissions.
The values of luminescence decays aimed at the LGdH-pa-
x
Eu1ꢀxTb L solid samples (Fig. S11–S15†) nearly match the
ꢀ
(x|t
1
)
ꢀ(x|t
2
)
double exponential equation: y ¼ A
1
e
+ A
2
e
0
+ y ,
verifying that there are two coordination environments around
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Fig. 4 Low-temperature (77 K) phosphorescence spectra of (a) LGdH-pa, pa, LGdH, (b) LGdH-pa and LGdH-pa-TbL, (c) LGdH-pa and
LGdH-pa-EuL, and (d) LGdH-pa and LGdH-pa-Eu0.5Tb0.5L. The spectra were obtained under the following optimal excitation wavelengths:
3
281 nm for LGdH-pa and LGdH-NO , 292 nm for pa, 290 nm for LGdH-pa-EuL, 310 nm for LGdH-pa-TbL, and 296 nm for LGdH-pa-Eu0.5Tb0.5L.
40
the rare-earth ions inside the solid products. Average lifetime mixed-component hybrids, and the value of E was high to 28%
4
1
hsi is expressed using the following method:
in LGdH-Pa-Eu0.7Tb0.3L (Table S3†). Above all, there were three
ET processes observed in the hybrid phosphors LGdH-pa-
Eu1ꢀxTb L: (1) the important one derives from ligand L to rare-
x
earth ions Eu(III) and Tb(III); (2) the ligand pa sensitizes Eu(III)
and Tb(III), that is, has an antenna effect; and (3) the intermo-
lecular ET process occurs from Tb(III) to Eu(III). All mechanisms
of the most probable energy transfer processes in LGdH-pa-
P
P
2
i
hsi ¼
A
i
s
/
A
s
i i
(1)
The A in the equation is the pre-exponential factor related to
statistical weights of each exponential, and s represents the
lifetime of each exponential decay. Average Eu(III) lifetime
increased with higher concentrations of Tb(III) (Table 1), indi-
cating signicant energy interactions in the mixed phase. The
ligand-sensitized luminescence quantum efficiency Foverall can
be measured using an integrating sphere apparatus. The
values for LGdH-pa-EuL and LGdH-pa-TbL were 7.4% and
6.15%, respectively; that of the mixed component hybrids were
among the two values, also enhanced following the concomi-
tantly increased concentration of Tb(III) (Table S3†). This
behavior may be directly related to the ET process from Tb(III) to
i
i
Eu1 Tb L are demonstrated in Fig. 5.
ꢀx
x
4
2
In order to better understand the ET mechanism in the
assembly LGdH-pa-Eu1ꢀxTb L, we prepared a series of pure
x
43
complexes Gd-pa-EuL, Gd-pa-Eu0.5Tb0.5L, Gd-pa-TbL to
perform contrast experiments. The synthesis, characteriza-
tion, and all properties of the complexes are shown in ESI.†
The PL spectra of solid complexes Gd-pa-EuL, Gd-pa-TbL, and
Gd-pa-Eu0.5Tb0.5L are shown in Fig. S16.† The characteristic
emission peaks of Eu(III)and Tb(III) were observed at the same
excitation wavelength of about 273 nm, which is close to the
excitation band of the complex Gd-pa (1 : 1) (Fig. S17†),
distinct from the assembly at 310 nm. The FTIR (Fig. S18†),
UV-vis absorption (Fig. S19†) and low-temperature (77 K)
phosphorescence spectra (Fig. S20†) of the contrast complexes
1
44
Eu(III) in the solid materials.
45
ET efficiency (E) is usually calculated as follows:
E ¼ 1 ꢀ s /s
da
(2)
a
a
where sda and s are the luminescence lifetimes of the donor in
x
were similar to the assembly LGdH-pa-Eu1ꢀxTb L. In addition,
the excited-state lifetime and quantum efficiency show that
there is no ET process from Tb(III) to Eu(III) in the complexes
the presence and absence of the acceptor. In the hybrid mate-
rials, the average lifetime of the donor Tb(III) was 1.011 ms
without the acceptor Eu(III) (Table 1), which decreased in the
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Table 1 Lifetime values for solid hybrid phosphors with different composition
Eu(III)
Tb(III)
Samples
s
1
(ms)
s
2
(ms)
hsi (ms)
s
1
(ms)
s
2
(ms)
hsi (ms)
LGdH-pa-EuL
0.494 (42.73%)
0.517 (52.83%)
0.615 (56.36%)
0.642 (40.30%)
0.883 (52.27%)
1.004 (47.17%)
1.226 (43.64%)
1.479 (52.70%)
0.717
0.747
0.883
1.083
LGdH-pa-Eu0.7Tb0.3
LGdH-pa-Eu0.5Tb0.5
LGdH-pa-Eu0.2Tb0.8
LGdH-pa-TbL
L
L
L
0.198 (22.16%)
0.221 (17.43%)
0.422 (23.36%)
0.456 (18.81%)
0.878 (77.84%)
0.950 (82.57%)
1.113 (76.64%)
1.137 (81.19%)
0.727
0.822
0.951
1.011
three steps. The removal of water molecules occurring between
ꢁ
ꢁ
6
0 and 160 C and the platform (160 to 250 C) suggested that
the low-hydrated phase was stabilized by the initial water loss.
The subsequent sharp weight loss in the temperature range
ꢁ
2
50–310 C can be attributed to the dehydroxylation of the host
ꢁ
hydroxide layers. The nal step starting from 310 C was
assigned to further decomposition of nitrate anions. Evidently,
it was shown that the maximum decomposition temperature of
the hybrid phosphor LGdH-pa-Eu0.5Tb0.5
L was enhanced
ꢁ
(
centred at 410.5 C) (Fig. S22b†) compared to the contrast
ꢁ
complex Gd-pa-Eu0.5Tb0.5L alone (334.1 C) (Fig. S22c†). We
concluded that the LGdH layer can enhance thermal stability of
the loading complex molecules.
x
Fig. 5 Energy transfer mechanisms for mixed LGdH-pa-Eu1ꢀxTb L
hybrid. Solid arrows represent three most probable ET processes.
3
.5. Application of hybrid phosphors
To suggest practical applications, we attempted to intercalate
the Tinopal CBS molecules to the hybrid LGdH-pa-Eu0.2Tb0.8
(
see Table S4†). In summary, all contrast experiments illus-
L
trated that the host LGdH acted as not only a supporting
matrix, but also a positive ET process participant to helping pa
absorb energy in the UV region by coordinating with guest pa
anions as well as keeping the rare-earth complexes close to
each other. Finally, the ET process from Tb(III) to Eu(III) was
by tuning the excited wavelength to get a white light emission
at 330 nm excitation as shown in Fig. 3. Next, all solid-
46
powder hybrid phosphors were doped into PMMA to fabricate
the composite lm devices. The PXRD patterns showed that
the diffraction peaks of LGdH were observed in all lm
samples (Fig. S23†) and the FTIR spectrum of the lm PMMA/
observed in the phosphors LGdH-pa-Eu1ꢀxTb
the UV-vis absorption spectra of pure Eu-L and Tb-L complexes
Fig. S19†), pa is not only a structural bridge to link host LGdH
x
L. Considering
ꢀ1
ꢀ1
LGdH-pa-EuL show bands at 1591 cm and 1565 cm , which
can be ascribed to the hybrid phosphors sample (Fig. S24†).
This result is mainly due to the fact that the lm formation
process using organic polymer seems highly compatible to the
hybrid phosphors because of intercalated complexes; thus, it
did not lead to formation of an exfoliated structure where
intercalated LGdH layers were completely separated. More
(
with positively charged rare-earth complexes, but also
contributes a portion of the energy to co-sensitize the char-
acteristic emissions of RE(III) with ligand L.
3.4. Photoluminescent stability and thermal stability
To compare photoluminescent stabilities, kinetic scans of interestingly, the excitation peak shis from 310 nm to 288
luminescence induced by UV light (310 nm) for the example nm when the hybrid phosphors were doped into PMMA
samples were carried out (Fig. S21†). Evidently, the decrease in (LGdH-pa-EuL as an example, Fig. S25†), and the optimal
emission intensities of Eu(III) and Tb(III) complexes was similar, excitation wavelength of the sample LGdH-pa-Eu0.2Tb0.8L-CBS
indicating similar coordination environments in the two shied from 330 nm to 320 nm when dispersed in PMMA,
complex samples. As expected, the decay curve for assembly conforming to interactions between hybrids and PMMA
4
7
LGdH-pa-EuL, LGdH-pa-Eu0.5Tb0.5L, and LGdH-pa-TbL support matrices. Optical and physical properties of the
presented higher photostability compared to the pure lms are shown in Fig. 6, and the CIE color coordinates of
complexes, which may be due to the shielding effect of the each lm sample are listed in Table S2.† Under excitation at
matrix LGdH layers.
288 nm, the lm exhibited multi-color emissions from red,
Thermal stabilities of example samples were further studied pink, orange, and yellow to green. The clear pictures and
by TG and DTG measurements (see Fig. S22 in ESI†). Fig. S22a† curled shape indicate that the lms possessed outstanding
shows that the host matrix LGdH underwent weight loss in transparency and mechanical exibility.
1814 | J. Mater. Chem. C, 2015, 3, 1807–1816
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Fig. 6 (A) Emission spectra of composite films: (a) PMMA/LGdH-pa-EuL, (b) PMMA/LGdH-pa-Eu0.7Tb0.3L, (c) PMMA/LGdH-pa-Eu0.5Tb0.5L,
(
3
d) PMMA/LGdH-pa-Eu0.2Tb0.8L, (e) PMMA/LGdH-pa-TbL, lex ¼ 288 nm, and (f) white light emission PMMA/LGdH-pa-Eu0.2Tb0.8L-CBS, lex
¼
20 nm. (B) CIE chromaticity diagram showing (x, y) emission color coordinates. (C) Photographs of all composite films were exposed to UV lamp
at 330 nm and day light.
emission. By using PMMA as the support matrix, uniform
composite lms can be obtained by the solvent-casting method
4
. Conclusions
We have proposed a simple idea to prepare multi-color emis- that exhibit homogeneous optical properties, high transparency
sion inorganic–organic hybrid phosphors by intercalating in and mechanical exibility. Therefore, this work enriches the
situ positively charged rare-earth complexes in LGdH gallery eld of multi-color emission phosphors based on LRHs, which
space using pa anion chelation. Furthermore, the PXRD, FTIR, are promising for use in various bioprobes and optical devices.
BET surface area, UV-vis absorption spectra, and luminescence
decays had been employed to prove that the rare-earth
complexes have been incorporated into the interlayer region.
Acknowledgements
The resulting hybrid phosphors exhibited tunable color outputs
depending on different concentrations of rare-earth ions. As
expected, three energy transfer channels were observed
This work was nancially supported by the National Natural
Science Foundation of China (21471071, 21431002).
including the two most important channels from ligands L and
pa to activate RE(III) ions with the third intermolecular energy Notes and references
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