Fluorescence enhancement of rare earth Tb(III) by Tm(III) in benzyl benzoylmethyl sulphoxide complexes

Article abstract of DOI:10.1002/bio.1368

A series of rare earth complexes [(Tb_xTm_y)L ₅(ClO₄)₂](ClO₄) A·3H₂O (x:y = 1.000:0.000, 0.999:0.001, 0.995:0.005, 0.990:0.010, 0.950:0.050, 0.900:0.100, 0.800:0.200, 0.700:0.300; L = C ₆H₅CH₂SOCH₂COC₆H ₅) (Tb(III) luminescence ion; Tm(III) doped inert ion) were synthesized and characterized by elemental analysis, infrared spectra (IR) and ¹H-NMR. The photophysical properties of these complexes were studied in detail using ultraviolet absorption spectra, fluorescent spectra and lifetimes. The fluorescence spectra of complexes indicated that the fluorescence emission intensity was significantly enhanced by Tm(III). The complexes showed the best luminescence properties when the mole ratio Tb(III):Tm(III) was 0.990:0.010. The fluorescence intensity could be increased to 390%. Additionally, phosphorescence spectra and the luminescence mechanisms are discussed. Copyright

Full text of DOI:10.1002/bio.1368

Research article
Received: 16 May 2011,
Revised: 28 September 2011,
Accepted: 7 October 2011,
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/bio.1368
Fluorescence enhancement of rare earth Tb(III)
by Tm(III) in benzyl benzoylmethyl sulphoxide
complexes
Wen-Xian Li,* Wen-Juan Chai, Yu Liu, Ying-Jie Li, Tie Ren, Jing Zhang and
Bo-Yang Ao
ABSTRACT: A series of rare earth complexes [(TbxTmy)L5(ClO4)2](ClO4)ꢀ3H2O (x:y = 1.000:0.000, 0.999:0.001, 0.995:0.005,
0.990:0.010, 0.950:0.050, 0.900:0.100, 0.800:0.200, 0.700:0.300; L = C6H5CH2SOCH2COC6H5) (Tb(III) luminescence ion; Tm(III)
doped inert ion) were synthesized and characterized by elemental analysis, infrared spectra (IR) and 1H-NMR. The photophysical
properties of these complexes were studied in detail using ultraviolet absorption spectra, fluorescent spectra and lifetimes.
The fluorescence spectra of complexes indicated that the fluorescence emission intensity was significantly enhanced by
Tm(III). The complexes showed the best luminescence properties when the mole ratio Tb(III):Tm(III) was 0.990:0.010.
The fluorescence intensity could be increased to 390%. Additionally, phosphorescence spectra and the luminescence
mechanisms are discussed. Copyright © 2011 John Wiley & Sons, Ltd.
Keywords: rare earth complex; benzyl benzoylmethyl sulphoxide; rare earth doped inert ion; fluorescence enhancement;
luminescence mechanisms
Tb(III). Tb(III) was the active ion and Tm(III) was the doped ion.
Introduction
Furthermore, the photophysical properties of these complexes,
Rare earth complexes of organic ligands show characteristic
such as the photoluminescence (PL) spectra, PL intensities, life-
luminescence properties of the rare earth ions [RE(III) ions].
Because of their inner 4f–4f transition characteristics, rare earth
organic complexes exhibit rich, narrow-band, long-lifetime
time decays and phosphorescence spectra, were also investi-
gated in detail. In this way, if the right amounts of doped rare
earth ions are introduced in complexes, superior rare earth
emissions in all the wavelength ranges from ultraviolet to the
luminescent materials may be obtained.
far infrared regions. The luminescence mechanism of the
organic rare earth complexes is that the ligand can absorb and
transfer energy to rare earth ions, then the rare earth ions emit
characteristic fluorescence; the property of the ligand is called
Experimental
Materials
an ‘antenna effect’ by Lehn (1). Because rare earth organic com-
plexes have superior luminescence properties, they have poten-
tial applications in areas such as fluorescence materials (2–4),
electroluminescence devices (5–8) and fluorescence probes
and labels in a variety of biological systems (9–12).
The purity of lanthanide oxide exceeds 99.99%. Rare earth perchlorates
were prepared by dissolving the oxide (99.99%) in HClO4 (2 mol/L). The
purity of benzyl mercaptan exceeds 99%. All other chemicals were of
analytical reagent grade.
Organic ligands of the rare earth complexes are abundant.
The main organic ligands include aromatic carboxylic acid,
1,10-phenanthroline (phen), pyridine and b-diketone (13–15).
Our recent work has been concerned with rare earth sulphoxide
complexes (16,17). Because sulphoxide ligands have good coor-
dination abilities and solubility, they have become a group of
important ligands for sensitizing the luminescence of RE(III) ions.
On the basis of previous research, some studies also indicate
that some of the doped inert rare earth ions, such as Tm(III),
Gd(III) and La(III), can also change the fluorescence properties
of the rare earth complexes (18–21). Hence, studying the effect
of different rare earth ions on the luminescence of rare earth
complexes is beneficial to finding potential applications of rare
earth luminous complexes.
Physical measurements
Elemental analysis was carried out on a Hanau analyser. Conductivity
measurements were made using a 10–3 mol/L solution in acetone on a
DDS–11D conductivity meter at room temperature. Rare earth contents
of the complexes were determined by EDTA titration, using xylenol-
orange as the indicator. The thermal behaviour was monitored on a
SDTQ600 differential scanning calorimeter and thermal gravimetric
analyser. The infrared spectra (IR; n = 4000–400 cm–1) were determined
* Correspondence to: W.-X. Li, College of Chemistry and Chemical Engineer-
ing, Inner Mongolia University, Hohhot 010021, People’s Republic of China.
E-mail: nmglwx@163.com
In this study, a series of rare earth sulphoxide complexes
[(TbxTmy)L5(ClO4)2](ClO4)ꢀ3H2O (L = C6H5CH2SOCH2COC6H5) had
been synthesized to improve the fluorescence emission of
College of Chemistry and Chemical Engineering, Inner Mongolia University,
Hohhot 010021, People’s Republic of China
Luminescence 2011
Copyright © 2011 John Wiley & Sons, Ltd.

W.-X. Li et al.
by the KBr pressed disc method on a Nicolet NEXUS-670 FT-IR spectro-
photometer. The ultraviolet spectra (190–400 nm) of the ligands and
the complexes were recorded on a Shimadzu TU-1901 double-beam
spectrophotometer and acetone was used as reference and solvent.
1H-NMR spectra were measured on a Bruker AC-300 spectrometer in
CDCl3. Fluorescence excitation and emission spectra were determined
on a FLS920 fluorescence photometer and the slit width was 1 nm. The
phosphorescence spectra were monitored using a SPEX1934D spectropho-
tometer at room temperature. Fluorescent decay curves were recorded
using a FLS920 combined steady state and lifetime spectrometer.
Five millimoles of the ligands (5 mmol) were weighed and dissolved in
ether. Then the mixed perchlorates were added in the solution of the
ligand and a white solid was precipitated. The mixture was stirred for
0.5 h. The products were washed with ether several times and then dried
in an oven at 50 ꢁC (yield > 90%).
Results and discussion
Properties of the complexes
Analytical data for the complexes, presented in Table 1, conformed
to [(TbxTmy)L5(ClO4)2](ClO4)ꢀ3H2O (x:y = 1.000:0.000, 0.999:0.001,
0.995:0.005, 0.990:0.010, 0.950:0.050, 0.900:0.100, 0.800:0.200,
0.700:0.300; L = C6H5CH2SOCH2COC6H5). All the complexes were
white powders, stable under atmospheric conditions and soluble
in acetone, dimethylformamide (DMF) and DMSO. The molar
conductivity values in acetone indicated that the complexes were
a type of 1:1 electrolyte (23).
Synthesis of the ligand
The synthesis route of the ligand is shown in Fig. 1.
Synthesis of benzyl benzoylmethyl sulphide(22). Sodium was dis-
solved in alcohol. Then benzyl mercaptan and a-bromoacetophenone
were added to the solution. The mixture was stirred continuously at
room temperature for 1 h. The solution was poured directly into water
and then extracted with ether. After the ether phase had volatilized com-
pletely, a pale pink solid was precipitated. The sulphide was a white crys-
talline substance which was purified by recrystallization from alcohol.
Yield, 60–70%; mp, 83–84 ꢁC. 1H-NMR (CDCl3, 300 MHz) d (ppm): 7.315–
7.948 (10H, m, ArH), 3.667 (2H, s, CH2 of C6H5COCH–2), 3.755 (2H, s, CH2
of C6H5CH–2). Anal. calcd. for C6H5CH2SCH2COC6H5: C, 74.38%; H, 5.79%;
S, 13.22%; found, C, 74.33%; H, 5.65%; S, 13.10%.
TG–DSC studies
TG–DSC analyses were carried out up to 400 ꢁC in N2 at a heating
rate of 10 ꢁC/min. The TG–DSC curves of rare earth(III) complexes
were similar; the curves of the Tb(III) complex are shown in Fig. 2.
The TG curve of Tb(III) complex showed that the first mass loss
occurred in the range 38.44–90.33 ꢁC and mass loss was 3.14%.
The result was coincided well with the release of the three water
content (3.00%). In addition, there were two exothermic peaks in
the DSC curve, which were attributed to the decomposition of
five ligands. At the same time, the obvious weight losses oc-
curred on the TG curve and mass loss percentage was 68.70%.
The result also coincided with the release of five ligands content
(71.61%). The final products were proved to be Tb(ClO4)3 (or Tb
(ClO4)3 and Tm(ClO4)3) when the temperature reached near
400 ꢁC. The total weight loss of the complexes was found to be
close to the calculated values. The results were consistent with
element analyses.
Synthesis of benzyl benzoylmethyl sulphoxide. Benzyl benzoyl-
methyl sulphide was dissolved in acetic acid and 30% hydrogen perox-
ide was added to it at once. The mixture was stirred continuously at
room temperature for 24 h. After the reaction, the mixture was extracted
with ether until the pH of the mixture was 7. Then a white solid was pre-
cipitated, filtered and dried in vacuum. Yield, 83–88%; mp, 126–129 ꢁC.
1H-NMR (CDCl3, 300 MHz) d (ppm): 7.920–7.264 (10H, m, ArH), 4.148
(2H, s, CH2 of C6H5COCH2–), 4.220 (2H, s, CH2 of C6H5CH–2). Anal. calcd.
for C6H5CH2SOCH2COC6H5: C, 69.77%; H, 5.43%; S, 12.40%; found, C,
69.89%; H, 5.20%; S, 12.18%.
Preparation of the complexes
Rare earth perchlorate (1 mmol) was weighed in the ratios of Tb(III) to
Tm(III) [Tb(III): Tm(III) = 1.000:0.000; 0.999: 0.001; 0.995:0.005; 0.990:0.010;
0.950:0.050; 0.900:0.100; 0.800: 0.200; 0.700:0.300) and dissolved in ether.
Figure 1. Synthesis scheme of benzyl benzoylmethyl sulphoxide.
Table 1. Composition analysis (%) and molar conductivities (Sꢀcm2ꢀmolꢂ1) of the rare earth complexes [25 C]
ꢁ
Complexes
C
H
S
RE
lm
TbL5(ClO4)3ꢀ3H2O
50.23 (49.96)
50.39 (49.96)
50.07 (49.96)
50.20 (49.96)
49.88 (49.93)
49.97 (49.93)
49.78 (49.90)
50.21 (49.88)
4.32 (4.22)
3.88 (4.22)
4.55 (4.22)
4.60 (4.22)
3.87 (4.22)
4.32 (4.22)
4.40 (4.21)
4.13 (4.21)
8.97 (8.88)
8.64 (8.88)
9.05 (8.88)
8.72 (8.88)
9.21 (8.88)
8.50 (8.88)
8.62 (8.87)
8.49 (8.87)
8.23 (8.83)
8.89 (8.83)
8.76 (8.83)
9.17 (8.83)
9.09 (8.88)
8.68 (8.88)
8.97 (8.93)
9.35 (8.98)
164.7
149.8
160.4
152.5
165.5
159.1
155.8
154.7
Tb0.999Tm0.001 L5(ClO4)3ꢀ3H2O
Tb0.995Tm0.005 L5(ClO4)3ꢀ3H2O
Tb0.990Tm0.010 L5(ClO4)3ꢀ3H2O
Tb0.950Tm0.050 L5(ClO4)3ꢀ3H2O
Tb0.900Tm0.100 L5(ClO4)3ꢀ3H2O
Tb0.800Tm0.200 L5(ClO4)3ꢀ3H2O
Tb0.700Tm0.300 L5(ClO4)3ꢀ3H2O
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Fluorescence enhancement in benzyl benzoylmethyl sulphoxide complexes
of the complexes shifted to a lower wave number by 56 cm–1
(Fig. 4), indicating that rare earth ions were bonded with the oxy-
gen atom in the sulphonyl group. The C = O stretching frequency
of the ligand appeared at 1688 cm–1 as the strongest absorption
in the IR spectra. After coordination, the C = O stretching frequency
of the complexes shifted to 1673–1676 cm–1, it was shown that the
oxygen atoms in the carbonyl group were also coordinated to rare
earth ions. The absorptions of phenyl group appeared at 3057 cm–1
(nC–H), 1597 cm–1 (nC=C), 753 cm–1 (dC–H) and 692 cm–1 (dC–H). In
addition, there were four additional bands at about 1139–1145,
1114, 1084–1185 and 625–628 cm–1; all of them were attributed
–
–
to the ClO4 group. When ClO4 was not coordinated there was
Td symmetry and there were two absorptions. When ClO4– was co-
ordinated, there was C3v symmetry and there were five absorptions
(24,25). In the IR spectra of the complexes, four absorptions could
be seen clearly. Therefore, ClO4– was not just with the Td symmetry
and some should have C3v symmetry. Combined with the molar
Figure 2. TG–DSC curves of TbL5(ClO4)3ꢀ3H2O.
–
conductivities, it could be deduced that two ClO4 s were bonded
with RE(III) through the oxygen atom.
Infrared spectra
The most important IR assignments in the spectra of the ligand
and [Tb990Tm010L5(ClO4)2](ClO4)ꢀ3H2O can be seen in Figs 3 and
4. Fig. 3 shows that the S =O group stretching modes of ligand L
appeared at 1037 cm–1. However, the S = O stretching frequency
UV spectra
Figure 5 and 6 show UV spectra of the free ligand and Tb(III) com-
plex in acetone solution. Both spectra showed a broad absorption
band in the near-UV range (200–350 nm), which was attributed to
the p–p* transition, based on the conjugated double bonds of the
ligand. An obvious blue shift was observed in the complex, owing
to the coordination of the ligand to Tb(III) (26).
Figure 3. IR absorption spectrum of the ligand.
Figure 5. UV spectrum of C6H5CH2SOCH2COC6H5.
Figure 4. IR absorption spectrum of [Tb0.990Tm0.010 L5(ClO4)2](ClO4)ꢀ3H2O.
Figure 6. UV spectrum of TbL5 (ClO4)3ꢀ3H2O
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W.-X. Li et al.
Fluorescence spectra
Table 2. Comparison of fluorescent data of TbL5(ClO4)
3ꢀ3H2O and Tb0.990Tm0.010 L5(ClO4)3ꢀ3H2O in the solid state
at room temperature
The fluorescence excitation and emission spectra of [(TbxTmy)L5
(ClO4)2](ClO4)ꢀ3H2O (x:y = 1.000:0.000, 0.999:0.001, 0.995:0.005,
0.990:0.010, 0.950:0.050, 0.900:0.100, 0.800:0.200, 0.700:0.300; L =
C6H5CH2SOCH2COC6H5) complexes were measured in solid state
at room temperature. Fluorescence spectra of the Tb(III) complex
and doped complex are shown in Fig. 7. The excitation spectra
were obtained by monitoring the emission of the Tb(III) at
544 nm, and all the complexes had similar excitation spectra.
There was an intensive and broad excitation band in the range
270–370 nm, whose maximum peak was at about 324–326 nm.
In the fluorescence emission spectra of the complexes, strong
emission intensities indicated that the ligand was a good or-
ganic ligand. It could absorb energy and transfer it to the RE
(III) ion. The rare earth complexes could emit the characteristic
fluorescence of the RE(III) ion. Data of the emission spectra for
the doped complexes are shown in Table 2. The emission peaks
of complexes were assigned to the characteristic 5D4–7FJ (J = 6, 5,
4 and 3) transitions of Tb(III) about at 489, 544, 583 and 621 nm,
respectively, and the fluorescence intensity at 544 nm was the
strongest intensity for the 5D4–7 F5 emission. From the emission
spectrum, when the ratio Tb(III):Tm(III) was 0.990:0.010, the fluo-
rescence intensity of the complex was the strongest intensity in
our experiment. The fluorescence intensity of [Tb990Tm010L5
(ClO4)2](ClO4)ꢀ3H2O could be increased to 390% compared with
[TbL5(ClO4)2](ClO4)ꢀ3H2O. With the increase of Tm(III), the change
of Tb(III) fluorescent emission intensity of lem = 544 nm can be
seen in Fig. 8. In the range of our records, the fluorescence inten-
sity of all the complexes were sensitized by Tm(III).
We also measured the fluorescence decay curves of Tb(III)
complex and [Tb0.990Tm0.010 L5(ClO4)2](ClO4)ꢀ3H2O-doped com-
plex; figs 9 and 10 show their fitted curves. The lifetime values
of [TbL5(ClO4)2](ClO4)ꢀ3H2O and [Tb0.990Tm0.010 L5(ClO4)2](ClO4)ꢀ
3H2O complexes were calculated by the single exponential
mode. The results indicated that the fluorescence lifetime of
the [Tb0.990Tm0.010 L5(ClO4)2](ClO4)ꢀ3H2O complex (579.96 ms)
was shorter than that of the Tb(III) complex (625.21 ms). Some
studies had shown that the stronger radiative transition may
correspond to a shorter radiative lifetime (27). The fluorescence
lifetime of [Tb0.990Tm0.010 L5(ClO4)2](ClO4)ꢀ3H2O was shorter than
that of the Tb(III) complex, so the emission efficiency was higher
and the fluorescence intensity was stronger than that of the Tb
(III) complex. In those systems we had studied previously (20),
Complexes
Slit width lEX lEM I (a. Energy
(nm) (nm) (nm) u.) transition
7
TbL5(ClO4)3ꢀ3H2O
1
324 489 1543 5D4 ! F6
7
544 4711 5D4 ! F5
7
583 699 5D4 ! F4
7
621
27 5D4 ! F3
7
Tb0.990Tm0.010 L5
(ClO4)3ꢀ3H2O
1
326 489 6853 5D4 ! F6
7
54418350 5D4 ! F5
7
582 2578 5D4 ! F4
7
621 1824 5D4 ! F3
Figure 8. Changes of Tb(III) fluorescent emission intensity with the increase of
Tm(III). Wavelength of Tb ion emission is lem = 544 nm
the two results showed that there may not be a good relation-
ship between the fluorescence lifetime and the fluorescence
intensity. There are many factors which affect the fluorescence
intensity, so more thorough research is still needed.
Phosphorescence spectra and luminescence mechanism
The phosphorescence spectrum of the ligand L was measured
using a SPEX1934D phosphorescence photometer in the solid
state at room temperature (Fig. 11). The shortest wavelength
Figure 7. Excitation and emission spectra of the Tb(III) complex(a) and [Tb0.990Tm0.010 L5(ClO4)2](ClO4)ꢀ3H2O (b) in the solid state at room temperature.
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Luminescence 2011

Fluorescence enhancement in benzyl benzoylmethyl sulphoxide complexes
ligand could only absorb radiant energy and transfer it to Tb(III)
ion, which led to the luminescence enhancement of Tb(III) in the
doped complexes.
Conclusion
A sulphoxide ligand and a series of highly luminescent com-
plexes were synthesized and characterized. Their photophysical
properties were studied using ultraviolet spectra, fluorescence
excitation and emission spectra, fluorescence lifetimes and
phosphorescence spectra. The terbium complex emitted charac-
5
teristic fluorescence of Tb(III) ions, and the lifetime of the D4
Figure 9. Fluorescence fitted curve of the Tb(III) complex.
excited states of [TbL5(ClO4)2](ClO4)ꢀ3H2O was 625.21 ms. The
complex showed the best luminescence properties when the
mole ratio Tb(III):Tm(III) was 0.990:0.010. The intensity of fluores-
cence could be increased to 390%.
The introduction of the inert Tm(III) ions could influence the
fluorescence intensity of [TbL5(ClO4)2](ClO4)ꢀ3H2O obviously.
The fluorescence intensity of the doped complexes [(TbxTmy)L5
(ClO4)2](ClO4)ꢀ3H2O could be increased by Tm(III) ions in our sys-
tem. The triplet state energy level of the ligand L matched well
with Tb(III). Therefore, we could say that sulphoxide complexes
were one important group for strong sensitized luminescence
of RE(III) ions.
The luminescent materials which we synthesized had good
solubility. The materials had practical value as luminescent
security marking materials.
Figure 10. Fluorescence fitted curve for the [Tb0.990Tm0.010 L5(ClO4)2](ClO4)ꢀ3H2O
Acknowledgements
complex.
This work was financially supported by the National Natural Sci-
ence Foundations of China (Research Project No. 20861005), the
Natural Science Foundation of Inner Mongolia (Grant No. 2010
MS 0204) and the Inner Mongolia Yili Industrial Group horizontal
joint project.
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