# Photon Conversion and Radiation Synergism in Eu/Tb Complexes Incorporated Poly Methyl Methacrylate

Authors: P. Xiao; J. J. Zhang; Z. Q. Wang; H. Lin
Journal: Advances in Materials Science and Engineering
 (2016)

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## Abstract

Green and red emissions in terbium and europium benzoic acids (TBA and EBA) incorporated poly methyl methacrylates (PMMA) are exhibited. Larger intensity parametersΩ
2 (11.13
×
10
-
20 cm2) and Ω
4 (13.15
×
10
-
20 cm2) of E
u
3
+ indicate a high inversion asymmetrical and strong covalent environment around lanthanide (Ln) ions and maximum emission cross section σ
e
m
-
m
a
x (2.45
×
10
-
21 cm2) for the dominant transition D
5
0
→
F
2
7 of E
u
3
+ reveals the effective photon conversion capacity in EBA incorporated PMMA. Color variation and fluorescence enhancement are validated to be originated from energy transfer and synergistic effect in TBA/EBA coincorporated PMMA system. The results indicate that Eu and Tb complexes incorporated PMMA with effective photon conversion hold great prospect in increment of lighting quality and enhancement of solar-cell efficiency employed in outer space.


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## Body

## 1. Introduction

Dye-sensitized solar cells (DSSCs) have attracted extensive attention as one of the promising alternatives for silicon solar cells [1–4]. However, most of the solar radiation energy in the ultraviolet (UV) and infrared (IR) regions is not utilized since DSSCs have a maximum absorption in visible region, which limits the power conversion efficiency of DSSCs [5]. Hence, developing low-cost and high-efficient technologies evokes a great deal of interest in the past decades [6]. Recently, investigations have been reported on the efficient fluorescence generated in lanthanide (Ln) ions incorporated TiO2 as photoelectrode, which are deemed to be a new route for high-quality conversion efficiency in DSSCs [7–9]. Ln ions can enhance light harvesting via converting UV and near infrared (NIR) radiation to visible emission, so that the conversion efficiency of solar cells will be improved effectively [10–21].The luminescence of Ln ions originates from the intra-4f transitions, which are forbidden in principle, resulting in relatively low emission efficiency [22]. An effective approach to increase the luminescent efficiency is that chelating Ln ions with organic ligands, which possess advantages such as long emission lifetimes, narrow bandwidths, and large Stokes shifts [23–34]. Since the Ln organic complexes are poor in thermal performances, mechanical stabilities, and processing properties, incorporating these complexes into organic polymers is an ideal method to make up these shortcomings [35–43]. Poly methyl methacrylate (PMMA) exhibits high transparency in the visible spectral region, excellent weather resistance, and high mechanical strength. Moreover, PMMA contains carbonyl groups along with its carbon chain that can be interacted with Ln complexes [44–50]. These characteristics make it suitable as a host material for lanthanide complex doping.Here in this work, terbium and europium benzoic acid (TBA and EBA) coincorporated PMMA have been prepared. Green and red fluorescence have been observed in TBA and EBA incorporated PMMA, respectively, and the variation of color coordinates originated from synergistic effect of TBA and EBA have been captured. Spontaneous emission probabilities, fluorescence branching ratios, and emission cross sections of intrinsic transitions of Eu3+ in PMMA have been evaluated. Thermogravimetric analysis (TGA) was carried out to give a detailed discussion of thermal property of PMMA after the addition of TBA. The analyses of lanthanide complexes incorporated PMMA reveal a promising application prospect for improving illumination quality of lighting devices and conversion efficiency of solar cells employed in outer space.


## 2. Experiments

TBA and EBA were synthesized according to the following procedures. Initially, LnCl3 was synthesized from the reaction between Ln oxides and hydrochloric acid. Then LnCl3 ethanol solution was added dropwise into the benzoic acid (BA) ethanol solution, and the molar ratio of Ln ions to BA was 1 : 3. Subsequently, sodium hydroxide dissolved in ethanol was used to adjust the pH of the mixture to remain between 6 and 7. Instantaneously the precipitate was formed and reaction mixture was stirred adequately for 2 hours at room temperature. The precipitate was filtered off by a Büchner funnel repeatedly, washed with ethanol and water, and dried in an oven at 80°C overnight.A certain amount of PMMA powder was dissolved in tetrahydrofuran (THF) solution followed by the addition of the lanthanide complexes. The well-mixed solutions were heated at 60°C for 1 hour using a thermostat water bath and the samples were prepared by evaporating the solvent slowly from the solutions.The infrared spectra of the samples were recorded at room temperature using a Spectrum One-B FTIR spectrophotometer in the range of 400–4000 cm−1. Thermogravimetric analysis scan was carried out by an American TA Company Q-50 thermogravimetric analysis system at a heating rate of 10°C/min under dynamic nitrogen atmosphere with a stream of 50 mL/s. Using the Metricon 2010 prism coupler, the refractive indices of the 0.2 wt% EBA incorporated PMMA were measured to be 1.4870 and 1.4777 at 635.96 and 1536.9 nm, respectively. The refractive indices at other wavelengths can be solved by Cauchy’s equation n
=
A
+
B
/
λ
2 with A
=
1.4758 and B
=
4526 nm2. The excitation and emission spectra were recorded by a Hitachi F-7000 fluorescence spectrophotometer and the relative spectral power distributions were measured by a Jobin Yvon Fluorolog-3 spectrophotometer with the calibration of a referenced halogen lamp. The luminescence pictures of the samples were taken using a Sony α200 digital camera.


## 3. Results and Discussion

Infrared spectroscopy is a useful technique to reflect the supramolecular interaction between the molecules. Taking an example of TBA incorporated PMMA, Fourier transform infrared (FTIR) spectra of pure PMMA and TBA incorporated PMMA in the spectral range 400–4000 cm−1 are shown in Figure 1. The peaks at 989 and 1384 cm−1 are attributed to C–O–C symmetric stretching and O–CH3 deformation, respectively, corresponding to fingerprint vibrations of the PMMA [51]. The intense and sharp band at 1729 cm−1 for the PMMA corresponds to the (–COOR), which is assigned to symmetrical stretching vibration of the carbonyl group [52]. The asymmetric stretching vibration modes of CH3 and CH2 show a strong broad band centered at about 2952 cm−1, indicating a high hydrogen content in the carbon backbone of PMMA [53]. Moreover, new peaks at 1603 and 1554 cm−1 appear, corresponding to the asymmetric stretching vibration of COO–, which demonstrates the coordination between the BA and the Ln ions [54].Figure 1
FTIR spectra of (a) PMMA and (b) 1 wt% TBA incorporated PMMA.
(a)
(b)The IR spectra show that the positions of carbonyl vibration in PMMA and TBA incorporated PMMA are located at 1729 and 1732 cm−1, respectively. The wavenumber difference reveals that the terbium complex is stabilized through chemical interactions with the oxygen atoms of the carbonyl group of PMMA [55], since the oxygen atoms of the carbonyl group in PMMA are a kind of hard base and lanthanide ions are regarded as a kind of hard acid according to HSAB theory [56]. When T
b
3
+ ion interacts with oxygen atom of the carbonyl group, the electron density changes, consequently leading to the position shift corresponding to its characteristic vibration [57].Thermal property is of great significance to PMMA for practical applications as optical materials and it is strongly influenced by the chain lengths of the polymer, the placements of the ester, and the methyl groups along the polymer backbone [58]. Thermogravimetric analyses (TGA) of PMMA and TBA incorporated PMMA have been carried out to explore their degradation behavior, as illustrated in Figure 2. TGA thermogram of PMMA appears with a maximum value at 143°C on the weight loss rate curve and shows a 10 wt% total weight loss at 260°C, which is attributed to the solvent removal from the TBA incorporated PMMA. The figure reveals that the major weight loss events that occurred begin at the temperature of 300°C, leaving essentially no residue at 420°C, indicating the thermal decomposition behavior of pure PMMA. In addition, the TGA data show that TBA incorporated PMMA has no weight loss event within 260–330°C due to the water molecules coordinated to the T
b
3
+ ion of the hydrate precursor complex after the doping reaction, and it can be deduced that the TBA framework decomposed and turned into terbium oxide at the temperature of 450°C.Figure 2
Thermogravimetric analysis of (a) PMMA and (b) 1 wt% TBA incorporated PMMA. Curves 1 and 3 are the weight loss curves, and curves 2 and 4 are the weight loss rate curves.
(a)
(b)TBA incorporated PMMA emits bright green fluorescence under the radiation of UV lamp as shown in inserted photo of Figure3. The excitation spectrum of PMMA polymer incorporated with TBA by monitoring the emission at 546 nm is shown as curve 1 in Figure 3. The excitation spectrum is dominated by an intense broad band in the 220–300 nm region, which can be assigned to the absorption of Ln organic complex. The intense band confirms that TBA incorporated PMMA can be efficiently excited under the short-wavelength UV radiation.Figure 3
Excitation (curve 1) and emission (curve 2) spectra for 0.2 wt% TBA incorporated PMMA. Inserted photo: fluorescence from TBA incorporated PMMA under 254 nm UV radiation.Curve 2 in Figure3 presents the emission spectrum of the PMMA polymer incorporated with TBA under 254 nm excitation. From the emission spectrum, four emission transitions have been identified at 488, 546, 583, and 619 nm, respectively, assigning to the characteristic D
4
5
→
F
J
7 (J
=
6,5
,
4,3) transitions within the 4f8 configuration of the T
b
3
+ ions [59–61]. The most intense emission peak corresponding to D
4
5
→
F
5
7 transition of T
b
3
+ ions is located at 546 nm with a full-width at half-maximum (FWHM) of only 10 nm, and its intensity is apparently stronger than other peaks, resulting in a bright green emission under 254 nm UV radiation. Furthermore, the PMMA matrix influences the coordination environment of T
b
3
+ ions, which causes the change of energy transfer probability from BA to T
b
3
+ and the increase of electric-dipole transition intensity [62].Luminescence color for the lanthanide complexes incorporated PMMA is a psychophysical property of human eye and it needs to be expressed mathematically for precision. Thus, marking color coordinates on standard chromaticity diagram is necessary. The Commission International del’Eclairage- (CIE-) 1931 color coordinates can be calculated using the following formula:(1)
x
=
X
X
+
Y
+
Z
,
y
=
Y
X
+
Y
+
Z
,
z
=
Z
X
+
Y
+
Z
,where X, Y, and Z are the three tristimulus values. The tristimulus values for a color can be derived through the relative spectral power distribution P
(
λ
) by (2)
X
=
∫
P
λ
x
-
λ
d
λ
,
Y
=
∫
P
λ
y
-
λ
d
λ
,
Z
=
∫
P
λ
z
-
λ
d
λ
,where λ is the wavelength of the equivalent monochromatic light and x
-
(
λ
), y
-
(
λ
), and z
-
(
λ
) are the three color-matching functions. The values of (
x
,
y
) coordinate for TBA incorporated PMMA are calculated to be (0.3327, 0.5877), as point 5 in Figure 4.Figure 4
CIE (x
,
y) chromaticity diagram indicating the color coordinates of the fluorescence in samples, and the coordinates corresponding to points from 1 to 5 are in 0.2 wt% EBA, 0.4 wt% TBA-0.2 wt% EBA, 0.8wt% TBA-0.2 wt% EBA, 1.4 wt% TBA-0.2 wt% EBA, and 0.2 wt% TBA incorporated PMMA.The emission spectrum of EBA incorporated PMMA under 254 nm excitation is presented in Figure5(a), which consists of five emission bands D
0
5
→
F
J
7 (J
=
0,1
,
2,3
,
4) located at 578, 591, 617, 650, and 698 nm, respectively. Intensity parameters Ω
t (t
=
2,4
,
6), which were calculated from the emissions of E
u
3
+, are important indicators to predict some radiative properties [63, 64]. The integrated fluorescence intensity ratio between the electric-dipole (A
e
d) and the magnetic-dipole (A
m
d) transitions has been widely used as a criterion of the coordination status and the site asymmetry for the lanthanide ions [65–67], which is calculated based on the emission photon ratio of D
0
5
→
F
J
7 (J
=
2,4
,
6) transition to D
0
5
→
F
1
7 transition, following the formula(3)
A
m
d
A
e
d
=
N
J
N
1
.Figure 5
Emission spectra of (a) 0.2 wt% EBA, (b) 0.4 wt% TBA-0.2 wt% EBA, (c) 0.8 wt% TBA-0.2 wt% EBA, and (d) 1.4 wt% TBA-0.2 wt% EBA incorporated PMMA under 254 nm UV radiation. Inserted photos: fluorescence from corresponding samples.
(a)
(b)
(c)
(d)The transitions ofE
u
3
+ from D
0
5 to F
J
7 (J
=
2,4
,
6) belong to electronic dipole transition and the spontaneous radiative transition probability A
e
d from ψ
J to ψ
′
J
′ is described by(4)
A
e
d
=
64
π
4
e
2
ν
3
3
h
2
J
+
1
n
n
2
+
2
2
9
∑
t
=
2,4
,
6
Ω
t
ψ
J
U
t
ψ
′
J
′
2
,where h is the Planck constant, e is the electron charge, n is the refractive index, J is the total angular momentum of the excited state, ν is the wavenumber of transition, Ω
t is the Judd-Ofelt parameters, and the term ψ
J
U
(
t
)
ψ
′
J
′
2 is the square of the reduced matrix elements. The transition D
0
5
→
F
1
7 is a magnetic-dipole transition and the spontaneous transition probability A
m
d is reduced by(5)
A
m
d
=
64
π
4
ν
3
3
h
2
J
+
1
n
3
S
m
d
,where S
m
d is the magnetic-dipole line strength, which is a constant and independent of the medium, so the value of A
m
d can be estimated by the value of fluoride glass (A
m
d
′
=
60.3 s−1). The relationship is (6)
A
m
d
=
n
n
′
3
A
m
d
′
,where n
′
(
=
1.522
) is the refractive index of fluoride glasses [68].Based on the relative spectral power distributionP
(
λ
) of the EBA incorporated PMMA, which is shown in Figure 6(a), the relative photon distribution N
(
v
) has been derived as(7)
N
v
=
λ
3
h
c
P
λ
,where λ is the wavelength, h is the Planck constant, and c is the velocity of light. The relative photon distribution was presented in Figure 6(b), and the emitted photon numbers were calculated using the Gaussian multipeaks fitting and listed in Table 1.Table 1
Relative photon numbers and number ratios in EBA incorporated PMMA.
Transition
Wave number (cm−1)
Relative photon number (1019 a.u.)
Relative photon number ratio
5
D
0
→
7
F
1
(
N
1
)
16920
1.002
—
5
D
0
→
7
F
2
(
N
2
)
16256
5.800
N
2
/
N
1
=
5.7
9
5
D
0
→
7
F
4
(
N
4
)
14327
3.326
N
4
/
N
1
=
3
.
32Figure 6
(a) Relative spectral power distribution and (b) relative photon distribution of 0.2 wt% EBA incorporated PMMA under 254 nm excitation.
(a)
(b)Due to the selection rules and the characteristics of transition intensities inE
u
3
+ ions, each of the ψ
J
U
t
ψ
′
J
′
2 parameters determines the intensities of the transitions since the remaining two are zero [69]. The intensity parameters Ω
t (t
=
2
,
4) of EBA incorporated PMMA were calculated to be 11.13
×
10
-
20 and 13.15
×
10
-
20 cm2, respectively. The value of Ω
6 was not obtained due to the absence of D
0
5
→
F
6
7 transition emission. The value of Ω
2 and Ω
4 reflects large chemical bond overlap polarizability to some degree, indicating a high inversion asymmetrical and strong covalent environment around E
u
3
+ in PMMA [67].The spectroscopic parameters including spontaneous emission probabilities of electric-dipole transitionA
e
d and magnetic-dipole transition A
m
d, total transition probability A
T, fluorescence branching ratios β, and radiative lifetime τ
r
a
d are calculated by Ω
t values and listed in Table 2. The fluorescence branching ratio β of the D
0
5
→
F
2
7 transition centered at 617 nm accounts for 57.23%, which is higher than those of other transitions, predicting that the D
0
5
→
F
2
7 transition emission is dominant among the transitions of E
u
3
+ in EBA incorporated PMMA.Table 2
Predicted spontaneous transition probabilities, branching ratios, and radiative lifetime of EBA incorporated PMMA.
Transition
U
(
2
)
2
U
(
4
)
2
U
(
6
)
2
A
e
d (s−1)
A
m
d (s−1)
A
T (s−1)
β (%)
τ
r
a
d (ms)
5
D
0
→
7
F
1
0
0
0
0
56.44
570.88
9.89
1.75
5
D
0
→
7
F
2
0.0032
0
0
326.71
0
57.23
5
D
0
→
7
F
4
0
0.0023
0
187.73
0
32.88To further evaluate the energy extraction efficiency for optical material, it is of great significance to discuss the emission cross sectionσ
e. The emission cross section for the transition emissions arising from D
0
5 level of E
u
3
+ reveals the radiative transition probabilities, which are determined through the Fuchtbauer-Ladenburg (FL) formula [69]:(8)
σ
e
=
A
r
a
d
8
π
c
n
2
λ
5
N
λ
∫
λ
N
λ
d
λ
,where n, A
r
a
d, and N
(
λ
) represent the refractive index, spontaneous emission probability, and emission photon distribution, respectively. The σ
e profiles are shown in Figure 7 and the maximum values of σ
e for D
0
5
→
F
J
7 (J
=
1,2
,
4) transitions are deduced to be 0.377
×
10
-
21, 2.451
×
10
-
21, and 1.799
×
10
-
21 cm2, respectively, and the large emission cross section enables the EBA incorporated PMMA to be a promising material for active optical devices.Figure 7
Emission cross section profiles of 0.2 wt% EBA incorporated PMMA.The visible emission spectra of TBA/EBA coincorporated PMMA under 254 nm excitation are presented in Figure5. With the introduction of TBA into EBA incorporated PMMA, seven strong and sharp emission bands are recorded under the same excitation condition. The 491 and 546 nm emission bands are attributed to the D
4
5
→
F
6
7 and D
4
5
→
F
5
7 characteristic transitions of T
b
3
+, and the emission bands at 582, 593, and 617 nm are ascribed to the transitions from both T
b
3
+ and E
u
3
+ ions. Other bands peaking at 653 and 702 nm are typical emission features for the D
0
5
→
F
J
7 (J
=
3,4) transitions of E
u
3
+ [60]. EBA single incorporated and TBA/EBA coincorporated PMMA samples exhibit various color fluorescence under the excitation of 254 nm UV radiation, as shown in the inserted photo of Figure 5. In order to exhibit the trace of color variation, the CIE color coordinates for EBA single incorporated and TBA/EBA coincorporated PMMA samples were calculated based on the relative spectral power distributions, as listed in Table 3. In the cases of a fixed concentration of EBA, the fluorescence color coordinates in CIE chromaticity diagram move along right boundary from the bottom to the top with the concentration of TBA varying from 0 to 1.4 wt%, as marked points 1, 2, 3, and 4 in Figure 4. The variation of color coordinates is originated from the superimposed effect of TBA and EBA, indicating that Ln complexes incorporated PMMA are a promising candidate for radiation conversion system and lighting device. Furthermore, intrinsic transition intensities of E
u
3
+ increase due to energy migration from T
b
3
+, and the energy transfer process can be further exposed through the excitation behaviors [70, 71].Table 3
CIE chromaticity coordinates of the lanthanide complexes coincorporated PMMA.
Points in CIE diagram
EBA (wt%)
TBA (wt%)
x-coordinate
y-coordinate
1
0.2
0
0.6554
0.3397
2
0.2
0.4
0.6015
0.3816
3
0.2
0.8
0.5863
0.3967
4
0.2
1.4
0.5569
0.4200
5
0
0.2
0.3327
0.5877In order to ascertain the occurrence of energy transfer betweenT
b
3
+ and E
u
3
+, the excitation spectra are recorded by monitoring at 610 nm, which corresponds to D
0
5
→
F
2
7 emission transition of E
u
3
+. It is comprised of a broad excitation band in the spectral region of 240–280 nm as presented in Figure 8, and the highest intensity is achieved in 0.8 wt% TBA-0.2 wt% EBA incorporated PMMA. The enhancement factor of 610 nm emissions between TBA/EBA coincorporated and EBA single incorporated PMMA under different excitation wavelengths has been derived from the excitation spectra, as shown in Figure 9. The enhancement factor is obviously larger than 1 when the excitation wavelength is located in 220–300 nm, which indicates that TBA/EBA coincorporated PMMA can be more efficiently excited by UV radiation compared with EBA single incorporated PMMA, especially in the short-wavelength UV region.Figure 8
Excitation spectra of (a) 0.2 wt% EBA, (b) 0.4 wt% TBA-0.2 wt% EBA, (c) 0.8 wt% TBA-0.2 wt% EBA, and (d) 1.4 wt% TBA-0.2 wt% EBA incorporated PMMA.
(a)
(b)
(c)
(d)Figure 9
Enhancement factor of 610 nm fluorescence between 0.8 wt% TBA-0.2 wt% EBA coincorporated and 0.2 wt% EBA incorporated PMMA.The luminescence mechanism of Ln complex incorporated PMMA is depicted in Figure10. In EBA/TBA single incorporated PMMA, the Ln complex is excited from the singlet ground state S0 to the singlet excited state S
1 by UV absorption first and then relaxes to the triplet state T
2 via intersystem crossing; after that an internal conversion T
2
→
T
1 takes place in triplet state. Subsequently, the energy is nonradiatively transferred from the triplet state of the ligands to the resonance state of the coordinated T
b
3
+ or E
u
3
+ [72–75]. Finally, the T
b
3
+ or E
u
3
+ emits visible characteristic fluorescence by the radiative transitions from the excited state to the ground states. Besides, in EBA/TBA coincorporated PMMA, a part of excitation energy in T
b
3
+ can be transferred to D
3
5 level of E
u
3
+ with the assistance of chemical bonds and then the excited E
u
3
+ transition radiatively to F
J
7 (J
=
0,1
,
2,3
,
4) emitting visible light, which consequently bring about the enhanced visible fluorescence emission and improved photon conversion. Effective intramolecular and intermolecular energy transfers make TBA/EBA coincorporated PMMA promising for enhancement of lighting quality and conversion efficiency of solar cells employed in outer space.Figure 10
Schematic diagram of luminescence mechanism in TBA/EBA incorporated PMMA.


## 4. Conclusions

Efficient green and red fluorescence is observed from terbium and europium benzoic acids (TBA and EBA) incorporated poly methyl methacrylate (PMMA) under ultraviolet (UV) radiation. Judd-Ofelt intensity parametersΩ
2 and Ω
4 of EBA incorporated PMMA have been derived to be 11.13
×
10
-
20 and 13.15
×
10
-
20 cm2, respectively, indicating a high inversion asymmetrical and strong covalent environment around Ln ions in PMMA. The maximum emission cross sections for D
0
5
→
F
2
7 transition in EBA incorporated PMMA are 2.451
×
10
-
21 cm2, revealing the potential UV→Visible photons conversion capacity of E
u
3
+. Color variation and fluorescence enhancement in TBA/EBA coincorporated PMMA are originated from superimposed effect of TBA and EBA. Furthermore, emission intensity of E
u
3
+ has been enhanced due to energy migration from T
b
3
+ with the assistant of chemical bonds. Effective photon conversion and superimposed effect in TBA/EBA coincorporated PMMA suggest that the Ln complexes incorporated PMMA is a potential candidate for lighting quality improvement and conversion efficiency increment of DSSCs.



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*Source: 2618253-2016-05-10.xml*