Spectroscopic study on the photophysical properties of novel lanthanide complexes with long chain mono-L phthalate (L = hexadecyl, octadecyl and eicosyl)

Article abstract of DOI:10.1016/j.saa.2004.12.036

Ortho-phthalic anhydride was modified with long chain alcohol (1-hexadecanol, 1-octadecanol and 1-eicosanol) to their corresponding mono-L phthalate (L = hexadecyl, octadecyl and eicosyl), i.e. monohexadecyl phthalate (16-Phth), monooctadecyl phthalate (18-Phth), and monoeicosyl phthalate (20-Phth), respectively. Nine novel lanthanide (Eu³⁺, Tb³⁺ and Dy³⁺) complexes with these three mono-L phthalate ligands were synthesized and characterized by elemental analysis and IR spectra. The photophysical properties of these complexes were studied in detail with various spectroscopes such as ultraviolet-visible absorption spectra, low temperature phosphorescence spectra and fluorescent spectra. The ultraviolet-visible absorption spectra show some band shifts with the different chain-length of phthalate monoester and homologous lanthanide complexes. From the low temperature phosphorescent emission, the triplet state energies for these three ligands were determined to be around 22,650 cm^-1 (16-Phth), 23,095 cm^-1 (18-Phth) and 22,400 cm^-1 (20-Phth), respectively, suggesting they are suitable for the sensitization of the luminescence of Eu³⁺, Tb³⁺ and Dy³⁺. The fluorescence excitation and emission spectra for these lanthanides complexes of the three ligands take agreement with the above predict from energy match.

Full text of DOI:10.1016/j.saa.2004.12.036

Spectrochimica Acta Part A 62 (2005) 269–275
Spectroscopic study on the photophysical properties of novel
lanthanide complexes with long chain mono-L phthalate
(L = hexadecyl, octadecyl and eicosyl)
Bing Yan∗, Bing Xu
Department of Chemistry, Tongji University, Siping Road 1239, Shanghai 200092, PR China
Received 20 October 2004; received in revised form 11 December 2004; accepted 17 December 2004
Abstract
Ortho-phthalic anhydride was modified with long chain alcohol (1-hexadecanol, 1-octadecanol and 1-eicosanol) to their corresponding
mono-L phthalate (L = hexadecyl, octadecyl and eicosyl), i.e. monohexadecyl phthalate (16-Phth), monooctadecyl phthalate (18-Phth), and
monoeicosyl phthalate (20-Phth), respectively. Nine novel lanthanide (Eu3+, Tb3+ and Dy3+) complexes with these three mono-L phthalate
ligands were synthesized and characterized by elemental analysis and IR spectra. The photophysical properties of these complexes were
studied in detail with various spectroscopes such as ultraviolet–visible absorption spectra, low temperature phosphorescence spectra and
fluorescent spectra. The ultraviolet–visible absorption spectra show some band shifts with the different chain-length of phthalate monoester
and homologous lanthanide complexes. From the low temperature phosphorescent emission, the triplet state energies for these three ligands
were determined to be around 22,650 cm−1 (16-Phth), 23,095 cm−1 (18-Phth) and 22,400 cm−1 (20-Phth), respectively, suggesting they are
suitable for the sensitization of the luminescence of Eu3+, Tb3+ and Dy3+. The fluorescence excitation and emission spectra for these lanthanides
complexes of the three ligands take agreement with the above predict from energy match.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Lanthanide complexes; Long chain phthalate monoester; Photophysical property; Energy transfer; Spectroscopy
tives are more suitable for luminescence of Eu3+ [11–15]
while most aromatic carboxylic acids are more suitable for
the luminescence of Tb3+ [16–20]. We also have studied the
energy match and intramolecular energy transfer mechanism
in ternary lanthanide complexes with aromatic carboxylic
acids and 1,10-phenanthroline in details [21,22], indicating
there still exist intramolecular energy transfer mechanism
between ligands (i.e. from aromatic carboxylic acids and
1,10-phenanthroline). On account of the energy match and
energy transfer mechanism, it can be realized to predict the
luminescent properties of lanthanide complexes. Phthalate or
aminobenzoate derivative have been found to exhibit favor-
able properties of luminescence and film formation [23,24].
In this context, using ortho-phthalic anhydride as staring
material, three kinds of long chain alcohol (1-hexadecanol,
1-octadecanol and 1-eicosanol) were grafted to achieve their
mono-L phthalates (L = hexadecyl, octadecyl and eicosyl),
i.e. monohexadecyl phthalate (16-Phth), monooctadecyl
1. Introduction
Lanthanide complexes have received much attention
because of their interesting photophysical properties which
have potential applications in the luminescent probes or
labels for chemical or biological macromolecules [1,2] and
the active center for luminescent materials [3,4] or electro-
luminescent devices [5,6]. Considerable studies have been
focused on the design and assembly of lanthanide complexes
with organic ligands such as aromatic carboxylic acids,
␤-diketones, cryptands, calixarenes and heterocyclic ligands,
etc. These organic molecules possess strong absorption of
ultraviolet light and then occur an effective intramolecular
energy transfer process to central lanthanide ions [7–10]. For
binary complexes, it was found that most ␤-diketone deriva-
∗
Corresponding author. Tel.: +86 21 65984663; fax: +86 21 65982287.
E-mail address: byan@tongji.edu.cn (B. Yan).
1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2004.12.036

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B. Yan, B. Xu / Spectrochimica Acta Part A 62 (2005) 269–275
3.47 (2H), 2.17 (2H), 1.77 (2H), 1.26 (28H), 0.88 (3H), 11.20
( COOH).
2.2. Synthesis of lanthanide complexes with mono-L
phthalate
Scheme 1. Synthesis of long chain ester by modification of ortho-phthalic
anhydride with alcohol (n = 15, 17, 19).
The lanthanide oxides (Eu2O3, Tb4O7 and Dy2O3) were
converted to their nitrates by treatment with concentrated ni-
tric acid. Mono-L phthalate (0.390 g for 16-Phth, 0.418 g for
18-Phth and 0.446 g for 20-Phth; 1.0 mmol) was dissolved
into the aqueous solution of Na2CO3 (0.1 mol L−1). Then
aqueous solution of lanthanide nitrate (Ln(NO3)3·6H2O,
L = Eu, Tb, Dy; 0.33 mmol) was added very slowly to the
above mixed solution with vigorous stirring, resulting in
the white solid products (Scheme 2). Continuing to stir for
1 h, the product was filtered, washed with ethanol and wa-
ter repeatedly and dried in desiccator. The compositions
of the complexes were confirmed by elemental analysis:
Ln(Lꢀ)2NO3, Ln = Eu, Tb, Dy; Lꢀ = 16-Phth, 18-Phth, 20-
Phth. Anal. Calcd. for C48H74O11NEu: C, 58.64; H, 7.46; N,
1.41; Found: C, 58.26; H, 7.10; N, 1.34. For C52H82O11NEu:
C, 59.54; H, 7.82; N, 1.34; Found: C, 59.90; H, 7.29; N,
1.29. For C56H90O11NEu: C, 60.87; H, 8.15; N, 1.27; Found:
C, 60.41; H, 7.47; N, 1.31. For C48H74O11NTb: C, 57.66;
H, 7.41; N, 1.40; Found: C, 58.16; H, 7.07; N, 1.32. For
C52H82O11NTb: C, 59.15; H, 7.77; N, 1.33; Found: C, 58.75;
H, 7.36; N, 1.28. For C56H90O11NTb: C, 60.49; H, 8.10; N,
1.26; Found: C, 60.61; H, 7.53; N, 1.27. For C48H74O11NDy:
C, 57.46; H, 7.38; N, 1.40; Found: C, 57.21; H, 7.11; N, 1.29.
For C52H82O11NDy: C, 58.95; H, 7.75; N, 1.32; Found: C,
59.10; H, 7.41; N, 1.26. For C56H90O11NDy: C, 60.30; H,
8.08; N, 1.26; Found: C, 60.61; H, 7.63; N, 1.23.
phthalate (18-Phth), and monoeicosyl phthalate (20-Phth),
respectively. The corresponding lanthanide (Eu3+, Tb3+
and Dy3+) complexes with these ligands were synthesized
and characterized. The photophysical properties for these
long esters and their lanthanide complexes were studied in
detail, which can be expected to have practical application
to fabricate the Langmuir–Blodgett films.
2. Experimental
2.1. Synthesis of mono-L phthalate by the modification
of ortho-phthalic anhydride
The synthesis of mono-L phthalates was adopted as
the similar procedure in ref. [25]. Ortho-phthalic anhy-
dride (2.96 g/mmol) was mixed with equimolar amount of
long chain alcohols (1-hexadecanol (4.85 g), 1-octadecanol
(5.41 g) and 1-eicosanol (5.97 g)) in a flask. Then the solid
mixtures were placed in an oil bath at the temperature
110–115 ◦C and react for 15 h. Finally the samples were re-
crystallized with n-hexane three times to afford the prod-
ucts (Scheme 1). The compositions of the aim products were
confirmed by elemental analysis: Calcd. for C24H37O4 (mp:
65 ◦C) C, 73.85; H, 9.69; Found: C, 74.21; H, 9.20; H NMR
δ 7.97 (1H), 7.75 (1H), 7.60 (2H), 4.33 (2H), 3.49 (2H),
2.24 (26H), 1.18 (3H), 10.50 ( COOH). For C26H41O4 (mp:
68 ◦C): C, 74.64; H, 10.05; Found: C, 74.95; H, 9.59; H NMR
δ 7.97 (1H), 7.76 (1H), 7.59 (2H), 4.34 (2H), 3.35 (2H), 2.24
(2H), 1.73 (2H), 1.25 (26H), 0.89 (3H), 10.90 ( COOH). For
C28H45O4 (mp: 70 ◦C): C, 75.34; H, 10.31; Found: C, 75.61;
H, 9.58; H NMR δ 7.97 (1H), 7.70 (1H), 7.60 (2H), 4.30 (2H),
2.3. Physical measurements
Elemental analyses (C, H, N) were carried out by the
Elementar Cario EL elemental analyzer. Infrared spec-
troscopy with KBr pellets was performed on a Nico-
let Nexus 912 AO446 model spectrophotometer in the
4000–400 cm−1. Ultraviolet absorption spectra were ob-
1
tained with an Agilent 8453 spectrophotometer. H NMR
spectra were recorded in chloroform on a Bruker AVANCE-
Scheme 2. Synthesis of lanthanide complexes with long chain ester (Ln = Eu, Tb, Dy; n = 15, 17, 19).

B. Yan, B. Xu / Spectrochimica Acta Part A 62 (2005) 269–275
271
500 spectrometer with tetramethylsilane (TMS) as internal
reference. Low temperature phosphorescence spectrum was
determined Perkin-Elmer LS-55 spectrophotometer at 77 K.
The fluorescence (excitation and emission) spectra were de-
termined with Perkin-Elmer LS-55 spectrophotometer: exci-
tation slit width = 10 nm, emission slit width = 5 nm.
3. Results and discussion
All the IR spectra of these lanthanide complexes show the
similar features. For example, in the IR spectra of Ln(16-
Phth)2NO3 system, the characteristic absorption peaks of
carboxylic group COO− appear (1600, 1600, 1587 cm−1 for
νas(COO ) and 1405, 1427, 1405 cm−1 for νs(COO ), re-
spectively) while there does not exist for free 16-Phth ligand,
suggesting that the oxygen atoms of mono 16-Phth carbonyl
group are coordinated with Ln3+. Both the free ligand and its
complexes show the characteristic absorption bands of car-
bonyl group (1743 cm−1 (strong) for 16-Phth and 1723 cm−1
for Ln(16-Phth)2NO3) except for the weaker absorption in-
tensity for complexes than that of 16-Phth, which indicates
that there still exist one carbonyl group of long chain es-
ter and these carbonyl groups coordinated with Ln ions for
the absorption band shift to low frequency. In the IR spec-
tra of Ln(18-Phth)2NO3 system, the characteristic absorp-
tion peaks of carboxylic group COO− appear (1509, 1503,
1503 cm−1 for νas(COO ) and 1386, 1386, 1386 cm−1 for
νs(COO ), respectively) while it is not observed for free 18-
Phth ligand, indicating that the oxygen atoms of mono 18-
Phth carbonyl group are coordinated with Ln3+. Both the free
ligand and its complexes show the characteristic absorption
bands of carbonyl group (1744 cm−1 (strong) for 18-Phth and
1724 cm−1 for Ln(18-Phth)2NO3) except for the weaker ab-
sorption intensity for complexes than that of 18-Phth, which
suggests that there still exist one carboxyl group of long chain
ester. In the IR spectra of Ln(20-Phth)2NO3 system, the char-
acteristic absorption peaks of carboxylic group COO− ap-
pear (1600, 1600, 1606 cm−1 for νas(COO ) and 1405, 1405,
1405 cm−1 for νs(COO ), respectively) while free 20-Phth
Fig. 1. Ultraviolet–visible absorption spectra of long chain esters.
ligand does not present, which illuminates that the oxygen
atoms of mono 20-Phth carbonyl group are coordinated with
Ln3+. Both the free ligand and its complexes show the char-
acteristic absorption bands of carbonyl group (1749 cm−1
(strong) for 20-Phth and 1724 cm−1 (1729 cm−1) for Ln(20-
Phth)2NO3) except for the lower frequency and weaker ab-
sorption intensity for complexes than that of 20-Phth, verify-
ing that there still exist one carboxyl group of long chain ester
which also take part in the coordination with Lanthanide ions.
Besides this, NO3− also participate the coordination to Ln3+,
showing two absorption bands for stretching vibrations of
N O (1472.70, 1470.28, 1463.28 cm−1) and NO2 (1301.62,
1288.65, 1282.16 cm−1), which suggest that the coordina-
tion between Ln3+ ions and NO3 ions belong to bidentate
−
chelation effect (Table 1).
Fig. 1 shows the ultraviolet–visible absorption spectra for
the three long chain esters (16-Phth, 18-Phth and 20-Phth).
They all exhibit domain absorption peaks in the ultraviolet
region in the range of 200–400 nm, and the maximum
absorption peaks are located at 249 nm (16-Phth), 247 nm
(18-Phth) and 243 nm (20-Phth), respectively. Besides, there
exist one shoulder peak at around 277 nm for the three long
Table 1
The IR spectra and band assignments of the lanthanide complexes
Complex
νs(CH3)
νas(CH3)
ν(C O)
νs(COO
)
νas(COO
)
νs(C
O
C)
νas(C
O
C)
ν(N O)
ν(NO2)
16-Phth
2845.41
2858.38
2851.89
2851.89
2923.24
2916.76
2923.24
2916.76
1742.70
1723.24
1723.24
1723.24
1074.00
1139.46
1139.46
1139.46
1262.76
1269.19
1275.68
1269.19
Dy-16-Phth
Eu-16-Phth
Tb-16-Phth
1405.41
1427.66
1405.41
1600.00
1600.00
1587.03
1472.70
1472.70
1472.70
1301.62
1301.62
1301.62
18-Phth
2851.98
2851.89
2845.41
2851.89
2923.24
2916.76
2923.24
2910.27
1743.78
1723.73
1723.73
1723.73
1126.49
1055.00
1074.00
1074.59
1282.16
1262.70
1262.70
1262.70
Dy-18-Phth
Eu-18-Phth
Tb-18-Phth
1385.95
1385.95
1385.95
1509.19
1502.70
1502.70
1470.28
1470.28
1470.28
1288.65
1288.65
1288.65
20-Phth
2851.89
2845.41
2845.41
2851.89
2923.24
2916.76
2923.24
2916.76
1749.19
1723.37
1729.37
1729.37
1074.59
1074.59
1132.97
1081.08
1249.37
1269.19
1269.29
1262.70
Dy-20-Phth
Eu-20-Phth
Tb-20-Phth
1405.41
1405.41
1405.41
1600.00
1600.00
1606.49
1463.28
1463.28
1463.28
1282.16
1282.16
1282.16

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B. Yan, B. Xu / Spectrochimica Acta Part A 62 (2005) 269–275
Table 2
The phosphorescence spectra data of aromatic carboxylic acids
Ligands Triplet state
energies
ꢀE(Tr-Eu3+
)
ꢀE(Tr-Tb3+
)
ꢀE(Tr-Dy3+
)
(cm−1
)
(cm−1
)
(cm−1
)
(cm−1
)
16-Phth 22650
18-Phth 23095
20-Phth 22400
5385
5830
5135
2150
2595
1900
1650
2095
1400
chain ligands (for 18-Phth, it is not apparent). Both of them
are ascribed to the characteristic absorption peaks of phenyl
cycle with long-chain substituted groups. The little distinc-
tion of band position and intensity is due to the different chain
length (16-Phth–18-Phth–20-Phth). It also can be observed
that the maximum absorption bands exhibit the slightly
blue shifts with the increase of chain length, suggesting the
increase of substituted alcohol chain has influence on the
electron distribution in along the phenyl cycle. Fig. 2(a–c)
presents the absorption spectra of Ln-16-Phth, Ln-18-Phth
and Ln-20-Phth (Ln = Eu, Tb), respectively. All of them
show the similar red shifts compared to the corresponding
free ligands: 252 nm for Ln-16-Phth, 250 nm for Ln-18-Phth
and 245 nm for Ln-20-Phth, separately, indicating that the
coordination effects between Ln3+ and ligands form more
extensive conjugated systems than free ligands.
We further measured the low temperature phosphores-
cence spectra of the three long chain ligands at 77 K. From
the maximum phosphorescent emission band (441.5 nm
for 16-Phth, 433 nm for 18-Phth and 446.5 nm for 20-
Phth, respectively), the triplet state energies can be deter-
mined as 22,650 cm−1 (16-Phth), 23,095 cm−1 (18-Phth) and
22,400 cm−1 (20-Phth), separately. The energy differences
between the triplet state of thee ligands (16-Phth, 18-Phth
and 20-Phth) and the resonance energy levels of Eu3+ (5D0,
17,265 cm−1), Tb3+ (5D4, 20,500 cm−1) and Dy3+ (4F9/2
,
21,000 cm−1) can be calculated and the detailed data have
been shown in Table 2. From these energy differences, it
can be see that all the three long chain esters can sensi-
tize the three lanthanide ions effectively. According to the
intramolecular energy mechanism [26–29], the intramolecu-
lar energy transfer efficiency depends chiefly on two energy
transfer processes: the first one leads from the triplet level
of ligands to the emissive energy level of the Ln3+ ion by
Dexter’s resonant exchange interaction [30]; the second one
is just an inverse energy transfer by a thermal deactivation
mechanism. Both energy transfer rate constants depend on
the energy differences between the triplet level of the ligands
and the resonant emissive energy of Ln3+. Intramolecular en-
ergy transfer efficiency in lanthanide complexes conforms to
Dexter’ exchange energy transfer theory [30]:
Fig. 2. Ultraviolet–visible absorption spectra of lanthanide complexes with
long chain esters: (a) Ln-16-Phth; (b) Ln-18-Phth; (c) Ln-20-Phth (Ln = Eu,
Tb; n = 15, 17, 19).
ꢀ
ꢁ
ꢄ
ꢂ ꢃ
2πZ2
r
KET
=
exp
Fd(E)Fa(E) dE
(1)
R
l
KET is the rate constant of energy transfer, r the intermolecu-
lar distance between the energy donor and acceptor atoms, l
the van der Waals’ radius, the integral represents the overlap

B. Yan, B. Xu / Spectrochimica Acta Part A 62 (2005) 269–275
273
between the luminescence spectrum of the ligands and the
absorption spectrum of Ln3+ (Fd(E): the luminescence spec-
trum of energy donor (ligand), (Fa(E): absorption spectrum
of energy acceptor (Ln3+), and 2πZ2/R the constant relating
to the mutual distance between Ln3+ and coordinated atom.
r and l are both considered to be constant for intramolecu-
lar energy transfer processes. From Eq. (1), KET increases
with decreasing energy difference ꢀE(Tr-Ln3+) between the
triplet state energy of ligands and the resonance emission en-
ergy of Eu3+ and Tb3+. Thus, ligands with a large energy
difference cannot sensitize rare earth ions effectively. On the
other hand, there exists an inverse energy transfer process
which affects luminescence intensity by temperature (K(T)
[31] is the rate constant of inverse energy transfer process
(thermal deactivation process), A the preexponential factor):
ꢀ
ꢁ
−ꢀE(Tr-Ln3+
)
K(T) = A exp
(2)
RT
It can be seen that the activation energy is approximately
equal to ꢀE(Tr-Ln3+) in the inverse energy transfer process;
therefore, a decreasing energy difference increases K(T).
Based on this evidence, the conclusion can be drawn that
ꢀE(Tr-Ln3+) is of opposite influence on the two energy trans-
fer processes and an optimal value can be assumed to exist.
The excitation spectra of these lanthanide complexes
show that they have no effective absorption in narrow wave-
length ultraviolet region of the range 200–300 nm. The ef-
fective energy absorption mainly takes place in the long
ultraviolet–visible region of 350–450 nm. Fig. 3(a–c) gives
the excitation spectra of Eu, Tb and Dy solid complexes
with the three ligands. The excitation bands for Eu com-
plexes under the red emission of 613 nm show broad exci-
tation bands and the maximum excitation peaks are located
around 392.5 nm for Eu-16-Phth, 394.5 nm for Eu-18-Phth
and 395.5 nm for Eu-20-Phth, respectively. The excitation
bands for Tb complexes under the green emission of 544 nm
exhibit a broad excitation bands and the maximum excitation
peaks are located around 350.5 nm for Tb-16-Phth, 351 nm
for Tb-18-Phth and 330.5 nm for Tb-20-Phth, respectively.
The excitation bands for Dy complexes under the blue emis-
sion of 484 nm present a broad excitation bands and the max-
imum excitation peaks are located around 331 nm for Dy-
16-Phth, 333.5 nm for Dy-18-Phth and 330.5 nm for Dy-20-
Phth, respectively. We further measured the corresponding
emission spectra by selective excitation with the four differ-
ent excitation wavelengths, they show the similar emission
position except for different luminescent intensities.
Fig. 3. Excitation spectra of lanthanide complexes with long chain esters:
(a) Eu-L; (b) Tb-L; (c) Dy-L (L = 16-Phth, 18-Phth, 20-Phth).
Figs. 4–6 show the selected emission spectra of Eu, Tb
and Dy complexes with these three ligands. For europium
complexes, the emission spectra show two apparent emis-
sion peaks under the excitation of 395 nm: 590, 615 nm for
Eu-16-Phth, 590, 616 nm for Eu-18-Phth and 590.5, 616 nm
for Eu-20-Phth, respectively, corresponding with the char-
supersensitive red 5D0 → F2 transition are the strongest. For
7
Tb complexes, the emission spectra show two apparent emis-
sion peaks under the excitation of 352 nm: 490.5, 543 nm for
Tb-16-Phth, 490, 543 nm for Tb-18-Phth and 488, 543.5 nm
for Tb-20-Phth, respectively, ascribed to be the character-
5
7
acteristic emissions originated from D0 → FJ transitions
5
7
(J = 1, 2) of Eu3+ ion. Among the luminescent intensities of
istic emission D4 → FJ (J = 6, 5) transition of Tb3+ ion.

274
B. Yan, B. Xu / Spectrochimica Acta Part A 62 (2005) 269–275
Among the 5D4 → F5 transition exhibits the strongest green
7
emission. For Dy complexes, the luminescence spectra show
two apparent emission peaks under the excitation of 356 nm:
one is at 482 nm for Dy-16-Phth, 483.5 nm for Dy-18-Phth
and 482.5 nm Dy-20-Phth; the other is at 572 nm for Dy-16-
Phth, 574.5 nm for Dy-18-Phth and 572.5 nm for Dy-20-Phth,
separately, which correspond to the characteristic emission
6
4F9/2 → HJ (J = 15/2, 13/2) transition of Dy3+ ion, respec-
tively.
4. Conclusions
Three long chain ester ligands have been synthesized by
the modification of ortho-phthalic anhydride with long chain
alcohols and their nine complexes with lanthanide ions (Eu3+,
Tb3+, Dy3+) have been prepared. The photophysical proper-
ties of them have been studied with ultraviolet spectra, low
temperature phosphorescent spectra, excitation and emission
spectra. The triplet state energy of the three long chain ligands
are determined to be 22,650 cm−1 (16-Phth), 23,095 cm−1
(18-Phth) and 22,400 cm−1 (20-Phth), respectively with the
phosphorescence spectra. The energy matchbetweenthe long
chainligandsandlanthanideions(Eu3+, Tb3+, Dy3+)hasbeen
studied to predict that the optimum energy transfer exists be-
tween the ligand and Ln3+.
Fig. 4. Emission spectra of Eu complexes with long chain esters.
Fig. 5. Emission spectra of Tb complexes with long chain esters.
Fig. 6. Emission spectra of Dy complexes with long chain esters.
Acknowledgement
This work was supported by the National Natural Science
Foundation of China (20301013).
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