Structure and luminescent property of a Sm³⁺ complex containing benzoyltrifluoroacetone and 1,2-bis[(anthracen-9-ylmethyl)amino]ethane ligands

Physical sciences | Chemistry
Vietnam Journal of Science,
Technology and Engineering20 June 2021 • Volume 63 number 2
Introduction
β-diketonate complexes are among the most thoroughly 
explored rare earth coordination compounds. This is 
mainly due to the fact that they are easily synthesized, 
readily available from commercial sources, and are 
utilized in many applications ranging from magnetism to 
photoluminescence [1, 2]. The narrow and strong emission 
of rare earth ions with β-diketonates makes them applicable 
in optical and electroluminescent devices as well as in 
luminescence sensors for cations and anions. However, 
due to “Laporte-forbidden” f-f transitions, emissions from 
the direct excitation of lanthanide ions are infeasible 
[3]. Benzoyltrifluoroacetone (HTFPB) is a commercially 
available and efficient sensitizer that is able to transfer 
excited energy to rare earth ions. Due to the suitable 
triplet energy level of TFPB, a so-called “antenna effect’’ 
is produced that turns on lanthanide emissions. Typically, 
the synthesis of lanthanide β-diketonates in the first step 
involves two water molecules in the coordination sphere 
of the central metal ion. Subsequent displacement of the 
coordinated water by ancillary chelating ligands with various 
electronic structures may lead to a fine tuning of lanthanide-
centered emissions [4-6]. It has been well documented 
that bispyridine, phenanthroline, and many pyridine-based 
ligands are able to turn on the emission of the central metal 
ion due to additional sensitizer effects [7, 8]. In this study, 
1,2-bis[(anthracen-9-ylmethyl)amino]ethane (BAAE2), a 
ligand with a low-lying triplet state, has been utilized to 
construct a tris β-diketonate complex [9-12]. BAAE2 is a 
potentially good bidentate ligand as the trivalent rare-earth 
ions are Lewis acids that preferentially form complexes 
with nitrogen donor bases. In the following discussions, the 
main attention will be focused on the syntheses, structures, 
and luminescent properties of Sm3+ complexes containing 
TFPB and BAAE2 ligands.
Experimental
Synthesis of ligands and complexes
Synthesis of BAAE2 ligand:
Step 1: Synthesis of BAAE1 [13]
BAAE1 was synthesized via a condensation reaction 
between ethylenediamine and anthracene-9-carcbadehyde, 
which is depicted in Scheme 1.
Structure and luminescent property of a Sm3+ complex 
containing benzoyltrifluoroacetone 
and 1,2-bis[(anthracen-9-ylmethyl)amino]ethane ligands
Thi Hien Dinh1*, Minh Hai Nguyen2
1Faculty of Chemistry, Hanoi National University of Education
2Faculty of Chemistry, University of Science, Vietnam National University, Hanoi
Received 1 April 2021; accepted 31 May 2021
*Corresponding author: Email: dth0104@gmail.com
Abstract: 
The synthesis, structure, and luminescent properties of a samarium(III) complex (A2) containing 
benzoyltrifluoroacetone (HTFPB) and 1,2-bis[(anthracen-9-ylmethyl)amino]ethane (BAAE2) ligands are herein 
reported. The structure of A2 has been elucidated by infrared spectroscopy and single crystal X-ray diffraction. X-ray 
crystallographic analysis demonstrated that A2 has a mononuclear structure with a formula of Sm(TFPB)3(BAAE2) 
in which Sm3+ ion is coordinated to six O-atoms from three TFPB ligands and two N-atoms from one ancillary 
ligand (BAAE2). UV-Vis data show that A2 strongly absorbs in the region of 220-400 nm. Nonetheless, A2 gives poor 
emission due to a quenching effect of the anthracenyl moiety.
Keywords: anthracene, β-diketone, rare earth complex.
Classification number: 2.2
DOI: 10.31276/VJSTE.63(2).20-24
Physical sciences | Chemistry
Vietnam Journal of Science,
Technology and Engineering 21June 2021 • Volume 63 number 2
Scheme 1.
A solution of anthracene-9-caboxaldehyde (0.400 g; 
1.94 mmol) in 12 ml DMF/CH3OH (v/v, 1:5) was added 
to ethylenediamine (0.067ml, 0.97 mmol) in methanol and 
the mixture was refluxed for 4 h with constant stirring. 
After the solution had cooled to room temperature, a yellow 
precipitate was formed and collected by vacuum filtration. 
The product was washed by a few drops of DMF, a large 
amount of methanol, and air-dried. The yield was 0.364 g 
(86%).
Step 2: Synthesis of BAAE2 ligand [13]
The synthetic procedure of ligand BAAE2 was based on 
a reaction reducing ligand BAAE1 by NaBH
4
 in methanol 
as described in Scheme 2.
Scheme 2.
BAAE1 (0.396 g, 0.907 mmol) was dissolved in a 
mixture of CH2Cl2 (30 ml) and CH3OH (15 ml) to obtain a 
yellow solution. A solution of NaBH
4
 (0.527 g, 13.9 mmol) 
in methanol (3 ml) was added under stirring to the mixture. 
The solution was stirred overnight at room temperature to 
give a yellow solid. The product was washed several times 
with distilled water, finally with diethyl ether, and air-dried. 
The yield was 0.320 g (80%).
Synthesis of the complexes:
Synthesis of Sm(TFPB)3(H2O)2 complex (A1) 
Sm2O3 (0.070 g, 0.2 mmol) was dissolved in HCl at 
50oC, then distilled water was added and heated at 100oC to 
form SmCl3. A solution of NaOH (0.048 g, 1.2 mmol) and 
HTFPB (0.259 g, 1.2 mmol) in MeOH (15 ml) was added 
dropwise under stirring to a solution of SmCl3 in MeOH 
(15 ml). The mixture was stirred at room temperature until 
a white solid completely formed. The product was washed 
by a large amount of CCl
4
 and air-dried. The yield was 88%. 
Synthesis of Sm(TFPB)3BAAE2 complex (A2)
A2 was achieved by reacting A1 with BAAE2 ligand in 
chloroform-methanol solvent mixture (Scheme 3).
F3C
O
O
Sm
OH2
OH2
3
BAAE2
Sm
OC
C O
F3C
N
H CH2
CH2HN
H2C
3
H
A1 A2
H2C
Scheme 3.
A solution of BAAE2 (0.397 g, 1 mmol) in CHCl3 (15 
ml) was added dropwise under stirring to a solution of A1 
(0.831 g, 1 mmol) in MeOH (15 ml). The mixture was 
stirred at room temperature for about 1 h until a yellowish 
precipitate formed. The solvent was removed in vacuum and 
the resulting solid was then washed with n-hexane. After 
drying under vacuum, a pale-yellow powder was obtained. 
The product was crystallized in EtOH/CH2Cl2 (v/v, 1:1) and 
the yield was 74%. 
Measurements
The IR spectra of A2 was measured with a FT-IR 8700 
infrared spectrophotometer (4000-400 cm-1) in KBr pellets 
at Institute of Chemistry, Vietnam Academy of Science and 
Technology. 
Single crystal X-ray diffraction data of the complex A2 
was collected on the X-ray diffractometer (Bruker D8 Quest) 
at 298 K at the Faculty of Chemistry, University of Science, 
Vietnam National University, Hanoi. Structure solution and 
refinement were performed with OLEX2 programs.
Absorption spectra of the ligands and the complexes 
were measured in dichloromethane at room temperature on 
Cary 5000 UV/Vis spectrometer at Faculty of Environmental 
Chemistry, Hanoi National University of Education. 
Emission spectra of the complexes were measured on 
Hitachi Fluorescence Spectrophotometer F-7000.
Results and discussion
Infrared spectroscopy
The infrared spectrum of the complex Sm(TFPB)3BAAE2 
(A2) is shown in Fig. 1. Typical absorption bands of the 
complexes and ligand are shown in Table 1.
Physical sciences | Chemistry
Vietnam Journal of Science,
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Table 1. Typical absorption bands of the complexes and ligand (cm-1).
Compounds νO-H νC-Caroma νC-F νC=O νC-N νSm-O
HTFPB 3450 3030 1191 1600 - -
BAAE2 - 3040 - - 1100 -
A1 3300 3035 1170 1608 - 557
A2 - 3054 1295 1609 991 562
The IR spectrum of A1 exhibits the typical broad 
absorption in the region 3000-3500 cm-1, which proposes 
the presence of water molecules coordinated to the central 
ion Sm3+. In contrast, the absence of the broad bands in 
the region of 3000-3500 cm-1 for A2 suggests that water 
molecules in A1 have been displaced by the nitrogen 
donor of BAAE2 ligand. The respective νC-F vibration of 
A2 is found at 1295 cm-1 that, compared to the starting 
material, is shifted to a somewhat higher frequency. The 
absorption at 1600 cm-1, which is typical for C=O sketch 
in the HTFPB ligand, is red-shifted to 1609 cm-1 in A2 
[2]. In addition, the absorption band responsible for νSm-N 
at 506 cm-1 in A2 confirms the complexation of Sm3+ ions 
with BAAE2 ligands through nitrogen atoms. The change 
in absorption frequency of νC=O compared with free ligands 
and the emergence of νSm-N absorption in the low frequency 
prove that HTFPB and BAAE2 ligands are present in the 
coordination sphere of Sm3+. 
Single crystal X-ray diffraction
The structure of A2 was determined by single crystal 
X-ray diffraction (Fig. 2). Selected bond lengths and angles 
are provided in Table 2. Crystal data and data collection 
parameters for the complex are given in Table 3.
The structure of the complex reveals a coordination 
number of eight in the central metal ion in which Sm3+ 
is bonded to six oxygen atoms from three TFPB and two 
nitrogen atoms from the BAAE2 ligand. The bond lengths 
of Sm1-O are 2.355-2.418 Å. The bond lengths of Sm3+ 
with two nitrogen atoms of BAAE2 are 2.599-2.658 Å. 
The O-Sm1-O bond angles are nearly the same and in the 
range of 69.58-70.7o, which is longer than that of N-Sm1-N 
(67.26o). The C-N bond lengths (1.465-1.491 Å) in the 
complex were found to be longer than a C-N single bond 
(1.472 Å). This confirms the delocalization of π electrons 
in the chelate ring upon complexation of BAAE2. The C-C 
bond length in the diketonate of C2 is 1.359-1.430 Å, which 
is shorter than the C-C bond length (1.54 Å) but longer than 
that of C=C (1.34 Å). Similarly, the C-O bond length in the 
diketone of A2 is 1.247-1.268 Å and it is also shorter than 
the bond length of C-O but longer than that of C=O. This 
confirms the delocalization of π electrons in the β-diketonate 
upon complexation between Sm3+ and TFPB ligands. The 
coordination of Sm3+ with BAAE2 ligands through two 
nitrogen atoms forms a five-membered chelate ring. 
Fig. 1. The infrared spectrum of A2.
Fig. 2. Molecular structure of A2.
Physical sciences | Chemistry
Vietnam Journal of Science,
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Table 2. Selected bond lengths and angles for A2.
Bond lengths/Å
Sm1-O1 2.418(5) O6-C14 1.249(8)
Sm1-O2 2.355(5) N1-C1 1.491(9)
Sm1-O3 2.370(5) N1-C2 1.465(9)
Sm1-O4 2.368(5) N2-C3 1.486(7)
Sm1-O5 2.374(5) N2-C4 1.478(8)
Sm1-O6 2.357(5) N2-C5 1.480(8)
Sm1-N1 2.599(6) C2-C3 1.351(12)
Sm1-N2 2.658(6) C6-C7 1.394(12)
O1-C6 1.259(9) C7-C8 1.373(12)
O2-C8 1.273(9) C9-C10 1.418(11)
O3-C9 1.258(9) C10-C11 1.365(10)
O4-C11 1.247(7) C12-C13 1.496(10)
O5-C12 1.268(7) C13-C14 1.412(10)
Bond angles/o
O1-Sm1-O2 69.58(18)
O3-Sm1-O4 70.7(2)
O5-Sm1-O6 70.34(17)
N1-Sm1-N2 67.26(17)
C1-N1-C2 112.6(5)
N1-C2-C3 110.4(5)
C2-C3-N2 111.9(5)
C3-N2-C4 109.4(5)
C4-N2-C5 107.5(5)
C5-N2-C3 108.9(5)
Table 3. Crystal data and structure refinement for A2.
Formula C50H50N6O6F9Sm
Mw/g.mol-1 1046.12
Crystal system monoclinic
a/Å 10.7101(10)
b/Å 23.075(2)
c/Å 23.458(2)
α/° 90
β/° 102.355(3)
γ/° 90
Volume/Å3 5663.0(9)
Space group P21/c
Z 4
ρcalcg/cm3 1.227
μ/mm-1 1.107
Reflections collected 33163
Independent reflections 10312 [Rint=0.1464, Rsigma=0.1418]
Data/restraints/parameters 10312/741/721
R1/wR2 [I≥2σ (I)] R1=0.0641, wR2=0.1364
GOF 0.996
UV-Vis absorption spectroscopy
To determine the photophysical properties of the 
compounds, we measured absorption spectra of ligands and 
Sm3+ complexes in the region of 200-800 nm in a CH2Cl2 
solvent. The absorption spectra of A1, A2, HTFPB, and 
BAAE2 are displayed in Fig. 3.
Fig. 3. Absorption spectra of A1, A2, HTFPB, and BAAE2 in 
CH2Cl2 at room temperature.
The spectra highlight strong absorptions in the region 
of 220-400 nm for the HTFPB and BAAE2 ligands as well 
as the A1 and A2 complexes. The broad bands observed 
at 327 and 328 nm are assigned to singlet-singlet π-π* 
transition in β-diketonate moiety [14]. These absorption 
bands are shifted slightly to the longer wavelength region 
compared with that of free HTFPB (325 nm), which hints at 
the perturbation of Sm3+ upon complexation [7]. The bands 
at a lower wavelength around 260 nm are anthracene-based 
π-π* electronic transitions. The auxiliary ligand BAAE2 is 
also absorbed at ultraviolet wavelengths. The lanthanide f-f 
transitions are not allowed, which makes absorption due to 
Sm3+ ions imperceptible in the spectra of A1 and A2. 
Photoluminescence spectroscopy
The photoluminescence spectra of A1 and A2 were studied 
using an excitation wavelength of 365 nm. The emission 
spectra are shown in the Fig. 4. Despite the quenching effect 
of O-H stretches, A1 gives a strong orange color and narrow 
band emission. This might be due to the very efficient 
sensitization of TFPB to Sm3+. Meanwhile, A2 is much less 
emissive than A1 but gives the same pattern of emission 
bands. We assume that the low-lying triplet energy level 
of the anthracenyl core in BAAE2 leads to intramolecular 
energy transfer from excited Sm3+. The emission lines at 565, 
603, 651, and 710 nm are assigned to the 4G5/2→6FJ (J=1/2-
9/2) transitions of Sm3+. The strongest emission band 
centered at 651 nm stems from the 4G5/2→6F7/2 transition. 
Physical sciences | Chemistry
Vietnam Journal of Science,
Technology and Engineering24 June 2021 • Volume 63 number 2
Fig. 4. pL spectra of A1 (a red line), A2 (a blue line) complexes. 
Conclusions 
Samarium (III) complexes containing TFPB and BAAE2 ligands were 
synthesized. The structure of A2 was definitively determined by X-ray diffraction and 
revealed a five-membered chelate ring of BAAE2 with Sm3+ ions. The results also 
described that Sm3+ in A2 adopts a coordination number of eight as it is bonded to six 
oxygen atoms from three TFPB ligands and two nitrogen atoms of BAAE2. UV-vis 
results confirm the strong absorptions produced by the β-diketonate and anthracenyl 
fragments. The A1 and A2 complexes both display Sm3+-centered orange emissions in 
which that of A2 is much weaker due to triplet-triplet energy transfer arising from the 
anthracenyl ring of BAAE2. Attempts to disrupt the aromaticity of the anthracenyl 
ring in order to switch on Sm3+ emissions in A2 are presently being made in our 
laboratory. 
ACKNOWLEDGEMENTS 
This work was completed with financial support from the Ministry of Education 
and Training of Vietnam, under the project B2018-SPH-49. 
COMPETING INTERESTS 
The authors declare that there is no conflict of interest regarding the publication 
of this article. 
REFERENCES 
[1] B. Song, G. Wang, M. Tan, J. Yuan (2006), “A europium(III) complex as an 
efficient singlet oxygen luminescence probe", J. Am. Chem. Soc., 128, pp.13442-
13450. 
[2] Y. Wang, H. Wang, X. Zhao, Y. Jin, H. Xiong, J. Yuan, J. Wu (2017), “A β-
diketonate-europium(iii) complex-based fluorescent probe for highly sensitive time-
gated luminescence detection of copper and sulfide ions in living cells", New J. Chem., 
41, pp.5981-5987. 
-20
0
20
40
60
80
100
120
500 550 600 650 700 750
 In
te
ns
ity
 (a
u)
Wavelength (nm) 
PL SPECTRA OF A1, A2 COMPLEXES 
Fig. 4. PL spectra of A1 (a red line), A2 (a blue line) complexes.
Conclusions
Samarium (III) complexes containing TFPB and 
BAAE2 ligands were synthesized. The structure of A2 was 
definitively determined by X-ray diffraction and revealed a 
five-membered chelate ring of BAAE2 with Sm3+ ions. The 
results also described that Sm3+ in A2 adopts a coordination 
number of eight as it is bonded to six oxygen atoms from 
three TFPB ligands and two nitrogen atoms of BAAE2. UV-
Vis results confirm the strong absorptions produced by the 
β-diketonate and anthracenyl fragments. The A1 and A2 
complexes both display Sm3+-centered orange emissions 
in which that of A2 is much weaker due to triplet-triplet 
energy transfer arising from the anthracenyl ring of BAAE2. 
Attempts to disrupt the aromaticity of the anthracenyl ring 
in order to switch on Sm3+ emissions in A2 are presently 
being made in our laboratory.
ACKNOWLEDGEMENTS
This work was completed with financial support from 
the Ministry of Education and Training of Vietnam, under 
the project B2018-SPH-49.
COMPETING INTERESTS
The authors declare that there is no conflict of interest 
regarding the publication of this article.
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