Design and Synthesis of an Eu-Based β-Diketone-Sensor for the Detection of Al³⁺ Ions

Abstract

A new β-diketone, 2,2,2-trifluoro-1-(4-hydroxy-2H-chromen-3-yl)ethanone (4-TFC), which contains a trifluorinated alkyl group and a rigid chromanone, has been designed and synthesised successfully, and utilized for the synthesis of a new stable Eu³⁺ ion-based 4-TFC lanthanide complex (1). The central Eu³⁺ ion is eight-coordinated by two O/N atoms from ancillary ligands and six oxygen atoms provided by three 4-TFC ligands. In particular, the complex (1) can detect Al³⁺ ions with high selectivity and sensitivity in a moment and can be seen by the naked eye clearly under 365 nm. The experimental data can be explained with the calculated transitions quite well.

1. Introduction

There are plenty of demands for novel fluorescent probes in the detection of metal ions because of their threat to human health and environment [1,2,3,4,5,6]. Among these, a common example is the presence of aluminum in food additives. Millions of people in the world are suffering from senile dementias. Scientists have found that Alzheimer’s disease and aluminum are closely related [7]. Also, they have found that aluminum causes damage in the human brain, heart, liver, kidney, and DNA [8]. Among the detection technologies, fluorometric methods have inspired an unprecedented wave of interest, not only in view of their structural diversity and aesthetic appeal, but also due to their distinguishing advantages, i.e., selectivity and non-invasiveness, high sensibility, easy operation, simplicity, short response time, and visualization [9,10,11,12]. Luminescent lanthanide complexes on the basis of β-diketones can generate strong luminescent signals with visible emission colors, which means that these types of complexes can be developed as promising chemical sensors [13,14]. The β-diketone ligand class is developing as one of the crucial “antennas” according to high harvest emissions by reason of the effectiveness of the energy transfer from this type of ligand to metal ions [15,16,17]. Following this idea, Eu³⁺ and 2,2,2-trifluoro-1-(4-hydroxy-2H-chromen-3-yl) ethanone (4-TFC) were chosen for the synthesis of a β-diketone complex in order to achieve the transition metal ion Al³⁺ based on the following reasons. (i) The coordination geometries of the transition metal ions always can be predicted, which is the decisive factor in designing the expected complexes with significant properties [18,19]; (ii) The 4-TFC ligand with low symmetry can increase the structural complexity, which promotes the synthesis of the complex [20]; (iii) The lanthanide complexes exhibit bright photoluminescence upon excitation in ligand-centered absorption bonds, and the process of detection is clearly visible under 365 nm [21,22].

In previous studies, Pradyut Ghosh [23] and Chen Yu-Chie [24] developed related substances for detecting Al³⁺ which have excellent properties. Probe 1H was synthesized by Pradyut Ghosh; its detection limit can be achieved at 0.5 nM. Glutathione-capped gold nanoclusters were fabricated by Yu-Chie Chen; they can detect Al³⁺ at 100 nM in water within 10 min. Compared with these substances, our complex is different in that it contains lanthanide metal ions Eu³⁺ which have high thermal stability (Figure S6) even when changed under fluorescence color from red to bright blue in seconds (Figure S10). We tested five varieties of vermicelli in order to detect their Al³⁺ content by using complex 1, and the results corresponded well with ICP tested. Moreover, the detection speed of complex 1 was very fast and the fluorescence phenomenon was easy to observe under 365 nm by the naked eye. The remarkable luminescence properties reveal that this complex could be an effective sensor for Al³⁺ ions. The absorptions of Eu and Al complexes were calculated with the time-dependent density functional theory (TD-DFT). The methods and calculations support the experimental data and explain them clearly.

2. Results and Discussion

2.1. Synthesis of 2,2,2-Trifluoro-1-(4-hydroxy-2H-chromen-3-yl)ethanone (4-TFC)

As shown in Scheme 1, 4-chromanone and ethyl trifluoroacetate were added into THF, and the mixed solution was stirred for 30 min. To this solution, NaH was added under an inert atmosphere and stirred for 24 h at room temperature. The reaction mixture was cooled to room temperature, then poured into ice water and acidified to pH 2−3 using hydrochloric acid (2 M). The final mixture was filtered and dried in a vacuum. Recrystallization from petroleum ether gave green flake crystals (2.6 g, 79%) (Figure S1). Elemental analysis (%) calcd for C₁₁H₇F₃O₃ (244.17): C, 54.06; H, 2.87. Found: C, 54.04; H, 2.28. IR (KBr, cm⁻¹): 2924 (w), 1611 (s), 1555 (w), 1484 (m), 1299 (w), 1237 (m), 1181 (m), 1142 (s), 1031 (w), 992 (m), 782 (m), 762 (m), 716 (w), 602 (s), 528 (w). (Figure S2). 1H NMR (400 MHz, DMSO) δ = 7.83 (d, J = 6.8, 1H), 7.66 (t, J = 7.1, 1H), 7.18 (t, J = 7.5, 1H), 7.08 (d, J = 8.4, 1H), 5.09 (s, 2H). (Figure S3). ESI-TOF m/z = 244 (M⁻) (Figure S4).

2.2. Synthesis of Eu(4-TFC)₃(phen) (1)

The ligand 4-TFC was synthesized, as shown in Scheme 2. It was prepared by stirring solutions of 4-TFC. Then, NaOH was added to CH₃OH for 30 min. To this methanol solution, EuCl₃·6H₂O was added dropwise, and the mixture was stirred for 24 h at room temperature. Then, two molar nitrogen donors were added into this solution for 10 h at 65 °C. Finally, 200 mL water was added to this mixture, and the precipitate thus formed was filtered, washed with water, and dried in the air. Single crystals were obtained in about one week by recrystallization from DCM/hexane. Eu(4-TFC)₃(phen) was achieved with a yield: 86%. Elemental analysis (%) calcd for C₄₅H₂₆EuF₉N₂O₉ (1061.65): C, 50.91; H, 2.47; N, 2.64 Found: C, 51.23; H, 2.38; N, 2.51. IR (KBr)ν_max: 1632 cm⁻¹ (s, v _C=O), 1316 cm⁻¹ (s), 1258 cm⁻¹ (s), 1137 cm⁻¹ (s, v _C-F), 763 cm⁻¹ (m, v _CF3) (Figure S5).

2.3. X-ray Crystallography

The single crystal X-ray structural analysis revealed that complex 1 crystallizes in the orthorhombic space group Pbca and the asymmetric unit of 1 consists of one europium (III) atom, one 1,10-phen ligand and three 4-TFC ligands. As shown in Figure 1, the Eu1 atom is coordinated by six β-diketone oxygen atoms, Eu1–O2 2.366(3) Å, Eu1–O3 2.355(3) Å, Eu1–O5 2.311(3) Å, Eu1–O6 2.380(3) Å, Eu1–O8 2.347(3) Å, and Eu1–O9 2.389(3) Å, from three 4-TFC ligands and two nitrogen atoms, Eu1–N1 2.585(3) Å and Eu1–N2 2.595(3) Å, from one 1,10-phen ligand to build a distorted hexahedron coordination geometry. The crystal data and refinement details are summarized in

Table 1. Relevant crystal data for ligand 4-TFC and complex Eu(4-TFC)₃(phen).

Empirical Formula	C45H26EuF9N2O9	C11H7F3O3
Formula weight	1061.64	244.17
Color	Colorless	Green
Cryst syst	Orthorhombic	Monoclinic
Space group	P b c a	P 21/c
Temperature (K)	293(2) K	293(2) K
a (Å)	20.5707(18)	11.927
b (Å)	19.0486(17)	7.523
c (Å)	21.4827(19)	13.044
α (deg)	90	90
β (deg)	90	119.09
γ (deg)	90	90
V (Å3)	8417.8(13)	1022.8
Z	8	4
ρ (g cm3)	1.675	1.586
μ (mm−1)	1.590	0.150
F (000)	4208	496
R1 [I > 2σ(I)]	0.0393	0.0478
wR2 [I > 2σ(I)]	0.0966	0.1347
R1 (all data)	0.06371	0.0715
wR2 (alldat)	0.1129	0.1555
GOF on F2	1.097	1.015
CCDC	1443763	1524762

.

2.4. UV-Vis Spectra Analysis

The ultraviolet-visible (UV-Vis) absorption spectra of the ligand 4-TFC and the complex Eu(4-TFC)₃(phen) were measured in DMF solution at the concentration of 1 × 10⁻⁵ mol/L in the range of 200–450 nm at room temperature. Figure 2 shows that the maximum UV-Vis absorption for 4-TFC is at a wavelength of 360 nm, attributed to the singlet–singlet π-π* transition of the aromatic rings. Compared with the maximum absorption wavelength of 4-TFC, the maximum absorption wavelength of Eu(4-TFC)₃(phen) is around 365 nm, in which the maximum absorption wavelength undergoes a red-shift from 360 nm to 365 nm, attributed to the perturbation of the Eu³⁺ ion. Meanwhile, with the maximum absorption band of 4-TFC and Eu(4-TFC)₃(phen) at 360–365 nm, we can reasonably infer that the Eu³⁺ ion of the complex has almost no influence on the singlet excited state of the ligand. The absorption intensity of the complex is about three times as great as the ligand in the UV-Vis spectra at the concentration of 1 × 10⁻⁵ mol/L. This result agrees well with the presence of three ligands in each complex.

2.5. Excitation Spectra and Emission Spectra

Figure 3 shows the excitation spectra and emission spectra of complex Eu(4-TFC)₃(phen). The excitation wavelength at 360 nm is mainly attributed to the singlet–singlet π-π* electron transition of 4-TFC ligand. The emission spectra of complex Eu(4-TFC)₃(phen) lying in the range of 550–700 nm coincide with the characteristic emission bands of the Eu³⁺ ions corresponding to the ⁵D₀ → ⁷F_J (J = 0–4) transitions. Among these, the strongest emission is at λ = 612 nm for the electric dipole transition ⁵D₀ → ⁷F₂, and its corresponding intensity is very sensitive and easy to change as the coordination environment changes. This very intense ⁵D₀ → ⁷F₂ peak is responsible for the brilliant red emission of the complex, which indicates the presence of a single major chemical environment around the Eu³⁺ ion.

2.6. Luminescent Properties

In order to explore the sensing ability of complex 1 through its luminescence, a solution (1 × 10⁻² mol/L) of complex 1 was added into DMF including Na⁺, Fe³+, Ga³⁺, Cr³⁺, K⁺, Ag⁺, Mg²⁺, Mn²⁺, Ni²⁺, Pb²⁺, Zn²⁺, and Cd²⁺ (molar ratio 1:2) (Figure 4), respectively. Every M(NO₃)_x in DMF was sonicated for 20 min to ensure that the metal ions were homogeneous in the solution before being detected. Competitive Al³⁺ metal ions were selected for quenching the luminescence in seconds, while other metal ions showed no distinct interference with the emission by the displayed luminescence. The individual effects on the emission of luminescence between Al³⁺ and M^x+ ions indicate that complex 1 can be regarded as a promising luminescent probe for Al³⁺ ions.

Exploring the influence level of different competitive metal ions (Na⁺, Fe³+, Ga³⁺, Cr³⁺, K⁺, Ag⁺, Mg²⁺, Mn²⁺, Ni²⁺, Pb²⁺, Zn²⁺ and Cd²⁺) on the luminescence intensities could be considered a significant part in the process of checking the specificity of complex 1 towards Al³⁺ ions. The experimental results shown in Figure 5 reveal that complex 1 still maintains a high degree of selectivity towards Al³⁺ ions, and the presence of other metal ions do not have an effect on the detection of Al³⁺ ions by complex 1. The detection speed is still in the range of seconds. A wide range of blue shift in fluorescence spectra and an obvious color conversion from red to blue were observed in DMF solution gave a better understanding of the selective ability of complex 1. With confidence in these indicative results, we can draw the conclusion that complex 1 is an effective fluorescent sensor for the selective detection of Al³⁺ ions in DMF solution.

The concentration of Al³⁺ ions towards the luminescence intensity was further investigated in DMF under 365 nm. To investigate the fluorescent conversion process of Al³⁺ ions from unequal concentration, complex 1 was dispersed in different concentrations of Al³⁺ ions in DMF solution. As illustrated in Figure 6, the luminescence intensity increased gradually with the increase of the concentration until a saturation point was reached. Furthermore, the luminescence intensity of complex 1 in different concentrations of Al³⁺ ions could be clearly observed in the absorption spectrum, until the probe reached a concentration less than 2 × 10⁻⁶ mol/L.

We purchased five kinds of vermicelli from different street hawkers to be used as samples for detecting Al³⁺. These vermicelli samples had not been tested by the Food Safety Authority. Every 1.000 g sample after drying was treated with hydrogen peroxide and concentrated nitric acid in Teflon cans. The Teflon beaker was gently shaken to ensure that the sample was well dispersed. Then, the sample was placed in a microwave digestion device and digested for 30 min at 180 °C. After completely cooling, the solution was removed and transferred into a 100-mL volumetric flask, diluted with water to a total volume of 100 mL, and mixed well [25]. The solution was then diluted 20 times with water. Sample 6 was the control group, which only contained 2 × 10⁻⁶ mol/L Al³⁺ ion solution, an was limited to contain no more than 1.85 × 10⁻⁶ mol/L Al³⁺ after our digestion procedure by the Chinese Food Safety Authority. (It must be pointed out that the concentration is limited to no more than 3.70 × 10⁻⁶ mol/L under normal conditions, but in order to achieve a color change we had to reach the detection limit of 2 × 10⁻⁶ mol/L of complex 1). We used complex 1 to detect the remaining samples in the color tube. After adding the complex to samples 2 and 6, there was an obvious color conversion from red to blue under 365 nm. At the same time, the quantitative analysis of Al³⁺ of the five types of vermicelli was performed by ICP-MS analysis using the standard method [26]. The results of the ICP-MS analysis are shown in

Table 2. The contents of Al³⁺ ions in vermicelli detected by ICP-MS.

Sample	Color	Diameter (mm)	Concentration (mol/L)	After Adding Complex 1 (365 nm)
Sample 1	gray	1.4	0.81 × 10−6	red
Sample 2	translucent	2.9	5.10 × 10−6	red to blue
Sample 3	gray	3.3	1.21 × 10−6	red
Sample 4	gray	1.8	0.58 × 10−6	red
Sample 5	translucent	2.8	1.27 × 10−6	red
Sample 6	---	---	2.00 × 10−6	red to blue
water	---	---	---	red

. All samples were tested at least three times and averages are reported.

For the detection of metal ions, an Eu(4-TFC)₃(phen) probe was used to detect Al³⁺ ion selectively. In parallel, a series of methods for detecting were prepared under identical conditions to confirm if only Eu(4-TFC)₃(phen) has specific detection properties for Al³⁺ ions at the concentration of 1 × 10⁻² mol/L, as shown in Figure 7 and

Table 3. Different color of the mixture under 365 nm.

Sample	Complex	4-TFC	Phen	Eu3+	Al3+	Color
a		add				green
b		add			add	green
c		add	add		add	green
d			add		add	colorless
e		add		add		green
f			add	add		red
g		add	add	add		red
h			add	add	add	red
i		add		add	add	green
j	add					blue

. All of these (a–j) added the ligand into DMF firstly, and then added the metal ions into the corresponding solution. All solutions underwent a color conversion in seconds and the colors remained stably for a long time. Results of the experiments show that the ligand 4-TFC and the auxiliary ligand phen cannot have an effect on the detection of distinguishing Al³⁺ ions. However, the color of the light can be changed from red to blue rapidly by adding Al³⁺ ions into the solution of Eu(4-TFC)₃(phen). Therefore, it is essential to synthesize the complex Eu(4-TFC)₃(phen) for detecting Al³⁺ ions. In addition, complex 1 displayed remarkable specificity toward acetone (Figures S7 and S8).

The ligand 4-TFC belongs to the β-diketone ligands, which is the first class of studied ligands. The advantage of these is that they are easier to synthesize and modify. In lanthanide β-diketone complexes, energy migration between chromophores and acceptors is a necessary procession in lighting, and the extensively accepted energy transfer mechanism is proposed by Crosby et al. In their complex, there is a high efficiency of energy transfer from the β-diketone ligand with a high absorption coefficient to Eu³⁺. Thus, the complex has a very high luminous efficiency. Due to the existence of F ions that have high electronegativity in the functional group R, the sensitizing effect of CF₃ in the terminal functional group R is the strongest. Generally, the sensitization pathway of the europium luminescence is an intramolecular energy transfer. The energy from the light source can cause ligands to transfer from a ground state to an excited singlet state, after an intersystem cross to the triplet state. Subsequently, the energy transfers from the triplet state to the ⁵D_J of the Eu³⁺ ions, then crosses the intersystem to the emitting ⁵D₀ state. Finally, the Eu³⁺ ions emit when the ground state occurs.

2.7. Computational Studies

The optimized results of the geometry structures of complexes are presented in Figure 8 and

Table 4. Selected bond length and angles for optimized geometry structures of complexes.

Bond Length (Å)	Bond Length (Å)	Bond Angles (°)	Bond Angles (°)
Eu(1)–O(2) 2.3761	Al(1)–O(2) 1.9136	O(2)–Eu(1)–O(3) 70.2499	O(2)–Al(1)–O(3) 89.9886
Eu(1)–O(3) 2.3813	Al(1)–O(3) 1.9189	O(5)–Eu(1)–O(6) 69.9090	O(5)–Al(1)–O(6) 89.1116
Eu(1)–O(5) 2.3845	Al(1)–O(5) 1.9226	O(8)–Eu(1)–O(9) 70.9363	O(8)–Al(1)–O(9) 90.0988
Eu(1)–O(6) 2.3783	Al(1)–O(6) 1.9255	N(1)–Eu(1)–N(2) 61.2205	N(1)–Al(1)–N(2) 31.6322
Eu(1)–O(8) 2.3964	Al(1)–O(8) 1.9003
Eu(1)–O(9) 2.3370	Al(1)–O(9) 1.9193
Eu(1)–N(1) 2.6966	Al(1)–N(1) 4.6949
Eu(1)–N(2) 2.6852	Al(1)–N(2) 5.2663

. It is noted that the Eu³⁺ ion in the complex is ligated to six oxygen atoms from the three 4-TFC and two nitrogen atoms from 1,10-phenanthroline, resulting in a distorted square antiprism geometry. However, the Al³⁺ ion in the complex is ligated to six oxygen atoms from the three 4-TFC, forming a six-coordinated structure, and the 1,10-phenanthroline molecule is far away from the Al³⁺ ionic. The ionic radii of Al³⁺ ions is 0.535 Å, which is much smaller than the ionic radii of Eu³⁺ ions, 0.947 Å. It is clear that the affinity between O and Al atoms calculated from the bond dissociation energies is larger than that between O and Eu atoms. Resultingly, the Al atom is nearer to the O atoms in the Al complex than in the Eu complex. The bond lengths of the Al–O bonds are smaller than those of the Eu–O bonds, and the Al atoms are much closer to the O atoms than Eu atoms in the complex. As shown in Table 4, the bond lengths of the Al–O bonds in the range of 1.9003–1.9255 Å are shorter than those of the Eu–O bonds in the range of 2.3370–2.3964, which means the corresponding bonding strength of Al–O is much stronger than that of Eu–O in the complexes. That indicates that the Eu³⁺ ion can be replaced by the Al³⁺ ion easily during the ion determination.

The results of the theoretical calculations for the absorption properties of the complexes are shown in Figure 9 and Figure S9 and

Table 5. Absorptions of Eu and Al complexes calculated with the TD-DFT method.

Complexes	Sn	Confign	CI Codff	E/nm (eV)	Oscillator	Assignment
Eu complex	S6	H-2 → L	0.2741	362.17 (3.14)	0.1253	LMCT/MLCT/MC
	S7	H-1 → L + 2	0.3350	276.44 (4.49)	0.2128	ILCT/LMCT/MLCT/MC
	S8	H → L + 3	0.2709	270.48 (4.58)	0.4099	ILCT/LMCT/MLCT/MC
	S9	H → L + 4	0.3649	269.09 (4.61)	0.3189	ILCT/LMCT/MLCT/MC
Al complex	S7	H-3 → L + 1	0.2465	446.62 (2.78)	0.1685	ILCT/LMCT/MLCT
	S8	H-4 → L6	0.4417	270.08 (4.59)	0.2344	ILCT/LMCT/MLCT
	S9	H-1 → L + 2	0.4838	266.22 (4.66)	0.4058	LMCT/MLCT

. The HOMO and LUMO levels for the ground and singlet excited states of the Eu³⁺ complex were −0.21 eV and −0.08 eV, −0.31 eV and 0.05 eV, respectively. The electronic cloud distribution of HOMO in the ground and singlet excited states localizes at the three 4-TFC group, while that of LUMO localizes at 1,10-phenanthroline. As shown in Table 5, the lowest excitation energy of the Eu³⁺ complex calculated by TD-DFT is 3.14 eV. The absorption transition is mainly due to the ligand-to-metal charger transfer (LMCT), metal-to-ligand charger transfer (MLCT), and metal-centered (MC) transitions. Other high-energy absorption transitions are mainly due to the intraligand charge transfer (ILCT or π → π*), LMCT, MLCT, and MC transitions. In comparison with the absorption spectra of the Eu³⁺ complex, the lowest excitation energy of the Al³⁺ complex calculated by TD-DFT is 2.78 eV, which is mainly due to the ILCT, LMCT, and MLCT transitions. It is notable that the S9 high-energy absorption transitions are mainly due to LMCT and MLCT according to the selected frontier molecular orbits of the Al³⁺ complex, there is not ILCT transition in the absorption transitions (Figure S9).

3. Materials and Instrumentation

Eu₂O₃ (99.99% purity) was purchased from Ji Nan Rare Earth Chemical Plant (Shandong, China). Sodium hydride (60%, A. R.) and 1,10-phenanthroline monohydrate (99%, A. R.) were bought from Sigma-Aldrich (Beijing, China). 4-chromanone and ethyl trifluoroacetate were obtained from Sun Chemical Technology (Shanghai, China) Co., Ltd. EuCl₃·6H₂O was synthesized according to the literature by dissolving lanthanide oxide in a slight excess of hydrochloric acid. Other reagents and solvents were commercially available and used without further treatment. Fourier transform infrared (FT-IR) spectra were obtained on a Perkin-Elmer spectrometer. One spectrophotometer employed KBr disks in the range of 4000−400 cm⁻¹. Ultraviolet (UV) spectra were recorded on a Perkin-Elmer Lambda 35 spectrometer. Thermal analysis was conducted on a Perkin-Elmer STA 6000 with a heating rate of 10 °C min⁻¹ in a N₂ temperature range from 30 °C to 800 °C. 1HNMR spectra were recorded on a Bruker Avance (III) 400 MHz spectrometer in DMSO solution. Excitation and emission spectra were measured by an Edinburgh FLS 920 fluorescence spectrophotometer. The data were analyzed by software supplied by Edinburgh Instruments. Diffraction intensity data were collected on a Bruker SMART APEX II X-ray diffractometer with graphite monochromated Mo Kα radiation (l = 0.71073 Å). All the data were collected at a temperature of 18 ± 3 °C. ICP-MS data were recorded on an ICP-MS Perkin Elmer SCIEX ELAN 5000.

APEX2 and SAINT programs [27,28] were used for preliminary determination, the determination of integrated intensities, and the unit cell refinement of the crystals. The structures were solved with SHELIXS-2014/7 and refined with SHELIX-L2014/7 [29].

All computations were performed using the Gaussian03 code [30]. The initial geometry of the Al³⁺ complex was obtained by replacing the Eu³⁺ ion with an Al³⁺ ion. The geometry structures in the ground and excited states were optimized with B3LYP functional (Becke’s three-parameter nonlocal exchange functional) and unrestricted B3LYP (UB3LYP) methods, respectively. In the Al³⁺ complex calculations, the LANL2DZ basis sets with the effective core potentials (ECPs) of Hay and Wadt were used for Al atoms, and 6-31G(d) basis sets were used for other atoms. In the Eu³⁺ complex calculations, the ECP52MWB basis sets with the effective core potentials (ECPs) were used for Eu atoms, and 6-31G(d) basis sets were used for other atoms. Time-dependent density functional theory (TD-DFT) was employed to calculate the absorption properties on the basis of the optimized geometry structure in the excited states. The lowest spin-allowed singlet-singlet transitions, up to a wavelength of 245 nm, were taken into account while calculating the optical absorption spectra.

4. Conclusions

In summary, we synthesized a new dual-functional β-diketonate ligand 4-TFC, incorporating a trifluorinated alkyl group, a rigid chromanone, and 1,10-phen. Further systematic investigations on the photophysical properties suggested that the designed Eu(4-TFC)₃(phen) complex is a sensitive and effective probe to sense metal cations and small molecules. DFT calculations suggested that the average bond length (Al−O) around aluminum in the resulting Al³⁺ complex is much smaller than the corresponding average bond length (Eu−O) of complex 1. The calculation data demonstrate that the Eu³⁺ ion can be replaced by the Al³⁺ ion easily during the ion determination [31,32]. The luminescence intensities of complex 1 plays an essential role in detecting the Al³⁺ ion. Moreover, complex 1 also shows strongly selective luminescence for acetone. Therefore, the newly-designed β-diketone 4-TFC europium complex may be useful in more realistic conditions in the future.