Study on the structural and photophysical properties of N-acyl amino acid europium complexes

2.1 Preparation of the complexes

First, the ligands were prepared by in-solution synthesis using alanine (≥99%) and ethylene, butyl, hexyl, octyl, decyl, and lauryl chloride (≥99%) compounds. Then several amino acid rare earth europium complexes were prepared according to the experimental method used by naren et al(Naren et al., 2009). A specific weight of each N-acyl amino acid ligand compound was dissolved in methanol, into which a KOH methanol solution was added to adjust the pH to about 6, and reacted for several hours under stirring. After that, the EuCl₃ methanol solution was added dropwise to adjust the pH value and also reacted for several hours. A white solid powder was finally obtained after purification with ethanol and acetone. The elemental analysis results of these amino acid rare earth complexes are as follows: Eu(C₅H₈NO₃)₃·2H₂O(abbreviated as Eu(ac-ala)₃·2H₂O): Anal: C 31.38, H 4.22, N 7.30 wt%. Theoretical: C 31.16, H 4.53, N 7.27 wt%. Eu(C₇H₁₂NO₃)₃·H₂O (abbreviated as Eu(but-ala)₃·H₂O):Anal: C 39.17, H 6.11, N 6.41 wt%. Theoretical: C 39.13, H 5.94, N 6.25 wt%. Eu(C₉H₁₆NO₃)₃·H₂O (abbreviated as Eu(hex-ala)₃·H₂O):Anal: C 45.26, H 7.04, N 5.76 wt%. theoretical: C 45.07, H 6.84, N 5.82 wt%. Eu(C₁₁H₂₀NO₃)₃·H₂O (abbreviated as Eu(oct-ala)₃·H₂O):Anal: C 48.97, H 7.68, N 5.17 wt%. Theoretical: C 48.81, H 7.65, N 5.18 wt%. Eu(C₁₃H₂₄NO₃)₃·H₂O (abbreviated as Eu(dec-ala)₃·H₂O):Anal: C 52.22, H 8.32, N 4.68 wt%. Theoretical: C 52.41, H 8.30, N 4.64 wt%. Eu(C₁₅H₂₈NO₃)₃·H₂O (abbreviated as Eu(dod-ala)₃·H₂O):Anal: C 55.09, H 8.84, N 4.28 wt%. Theoretical: C 55.53, H 9.06, N 4.38 wt%.

2.2 Test method for complex analyses

To investigate the structure and properties of the complex, the following tests were conducted(Iida et al., 2006; Shi et al., 2022; Zhang et al., 2021; Zhang et al., 2022): A Perkin Elmer Spectrum Two FT-IR instrument was used to obtain the infrared vibrational absorption spectra of the complexes in the 400–––4000 cm⁻¹. A BRUKER NMR600 NMR spectrometer was used for ¹H NMR spectral analysis. The solvent was DMSO‑d₆, and the structural information of the complex was inferred. The self-diffusion coefficient of complexes in methanol solution were obtained with Conductivity tester. The absorption spectra of the complexes were obtained with a Perkin Elmer Lambda 35 UV–Vis spectrometer in the wavelength range of 200–––700 nm, the solubility of the complex in methanol is the best, therefore a methanol solution of the complex was used for testing. The excitation and emission spectra of the complexes were obtained with an Edinburgh Instruments FS5 luminescence spectrophotometer, and the quantum yield and luminescence lifetime were determined.

2.3 Computational details

All calculations were performed with the GAUSSIAN 09 software package(Frisch et al., 2009). The ground state structure of the ligand compound molecules was optimized using the M062X functional and the 6-311G (d,p) basis set; theoretical infrared vibrational spectra were obtained through vibrational analysis. Based on the theoretical ground state structure, the PBE0 and M062X functionals and the 6-31G (d,p) basis set is used to simulate the excited state structure. Based on this excited state structure, the PBE0 and the M062X functionals and the def2TZVP basis set are used to obtain the excited state energy levels of the ligand compounds.

For the complexes, due to the large atomic number of the Eu³⁺ ion and the complexity of the internal 4f electronic layer, the Stuttgart pseudopotential basis set is introduced to simulate the theoretical structure of the complexes. The PBE0 functional and the ECP53MWB (for Eu³⁺) and 6-31G (d, p) (for C, O, N, and H) basis set was applied to determine the molecular structure and obtain the vibrational spectra.

¹H NMR of the ligand compounds and complexes were computed using the revTPSS functional and the pcSseg-1(de Oliveira et al., 2021) (for C,O,N and H) and ECP53MWB (for Eu³⁺).

2.4 Formula used

From the CIE coordinates (x, y) of the complexes, the color coordinates (u ', v') of each complex are determined using Equation(1)(Hooda et al., 2022):

The correlated color temperature(CCT) of the synthesized europium complexes was determined from Equation(2)(McCamy, 1992):

The value of color purity in the synthesized complexes was estimated using Equation(3)(Zapata-Lizama et al., 2020):

Where x_d(0.688) and y_d(0.331) represent the dominant points for red color and x_i(0.333) and y_i(0.333) signify the illumination points(Nehra et al., 2022).

Luminescence lifetime curves were fitted to a double exponential equation: $I_{(t)}=I_0+A_1\exp \left(-\frac{t}{{\tau }_1}\right)+A_2\mathrm{e}\mathrm{x}\mathrm{p}(-t/{\tau }_2)$ , where ${\tau }_1$ and ${\tau }_2$ are luminescence lifetimes, $A_1$ , $A_2$ , and $I_0$ are (amplitudes A and initial intensity I) fitting parameters. Then the final luminescence lifetimes of the six complexes were obtained by the equation for the mean lifetime: $\tau =(A_1{\tau }_1^2+A_2{\tau }_2^2)/(A_1{\tau }_1+A_2{\tau }_2)$ .

The coordination environment in the complexes around the Eu³⁺ ion was determined using the Judd-Ofelt theory. Photoluminescence data were utilized to determine the J-O parameters and transition rates. Radiative transition rates of the complexes were calculated by summation of the radiative rates (A_OJ) for ⁵D₀→⁷F_0-4. A_OJ was estimated by the relationship Equation (4)(De Sa et al., 2000):

The luminescence quantum efficiency of the complexes was determined using Equation (6)(Biju et al., 2006):

Judd-Ofelt intensity parameters (Ω₂ and Ω₄) were affected by the coordination field around the Eu³⁺ ion, which can be estimated by Equation (7)(Yu, 2020):

The fluorescence branching ratio that depicts the involvement of each transition as the percentage of radiative decay rate was calculated using Equation(8)(Righini and Ferrari, 2005):

3.1 Molecular structure analysis of the complexes

¹H NMR was used for the identification of the different chemical environments of hydrogen atoms in the rare earth molecular complexes based on the different resonance signal intensities and chemical shifts. The chemical shifts “δ” of the hydrogen atoms in the six ligands are as follows: H(ac-ala):¹H NMR (600 MHz, DMSO‑d₆) δ 13.22 (s, 1H), 8.43 (s, 1H), 3.92 (s, 1H), 1.41 – 1.40 (m, 6H). H(but-ala): ¹H NMR (600 MHz, DMSO‑d₆) δ 8.26 (s, 1H), 8.14 – 8.02 (m, 1H), 3.91 – 3.90 (m, 1H), 2.45 – 1.58 (m, 2H), 1.39 (s, 5H), 1.29 – 1.20 (m, 2H), 0.95 – 0.76 (m, 1H). H(hex-ala): ¹H NMR (600 MHz, DMSO‑d₆) δ 12.42 (s, 1H), 8.07 (s, 1H), 4.17 (s, 1H), 2.08 (s, 2H), 1.56 – 1.22 (m, 9H), 0.85 (s, 3H)。H(oct-ala): ¹H NMR (600 MHz, DMSO‑d₆) δ 12.42 (s, 1H), 8.06 (s, 1H), 4.18 (s, 1H), 2.08 (s, 2H), 1.48 (s, 2H), 1.31 – 1.26 (m, 2H), 1.25 (s, 5H), 1.24 (s, 4H), 0.86 (s, 3H)。H(dec-ala): ¹H NMR (600 MHz, DMSO‑d₆) δ 12.30 (s, 1H), 8.07 (s, 1H), 4.17 (s, 1H), 2.13 (s, 2H), 1.54––1.23 (m, 17H), 0.86 (s, 3H)。H(dod-ala): ¹H NMR (600 MHz, DMSO‑d₆) δ 12.29–––12.11 (m, 1H), 8.07 (s, 1H), 4.17 (s, 1H), 2.18 (s, 2H), 1.24 (s, 21H), 0.86 (s, 3H)。The chemical shifts of hydrogen atoms in the six Eu³⁺complexes are as follows: Eu(ac-ala)₃:¹H NMR (600 MHz, DMSO‑d₆) δ 7.76 (s, 1H), 4.38 – 4.01 (m, 1H), 2.13 – 2.07 (m, 1H), 1.48 – 1.00 (m, 3H), 0.63 – 0.27 (m, 2H). Eu(but-ala)₃:¹H NMR (600 MHz, DMSO‑d₆) δ 7.70 – 7.62 (m, 1H), 4.40 (s, 1H), 1.86 – 1.15 (m, 2H), 1.05 (q, J = 6.8 Hz, 3H), 0.66 (s, 5H). Eu(hex-ala)₃:¹H NMR (600 MHz, DMSO‑d₆) δ 8.04 (s, 1H), 4.20 – 4.18 (m, 1H), 1.41 (d, J = 7.2 Hz, 5H), 1.23 (t, J = 7.1 Hz, 6H), 0.62 (s, 3H). Eu(oct-ala)₃:¹H NMR (600 MHz, DMSO‑d₆) δ 5.82 (s, 1H), 4.23 (d, J = 185.8 Hz, 1H), 1.90 – 1.49 (m, 3H), 1.38 – 0.97 (m, 12H), 0.85 (t, J = 7.2 Hz, 3H). Eu(dec-ala)₃:¹H NMR (600 MHz, DMSO‑d₆) δ 6.06 (s, 1H), 4.45 – 4.02 (m, 1H), 1.70 (s, 2H), 1.46 – 0.95 (m, 17H), 0.85 (m, J = 7.0 Hz, 3H). Eu(dod-ala)₃:¹H NMR (600 MHz, DMSO‑d₆) δ 7.45 (s, 1H), 4.41 (s, 1H), 1.70 – 1.31 (m, 2H), 1.21 – 1.07 (m, 21H), 0.86 (d, J = 6.9 Hz, 3H). Fig. 1 shows the position of hydrogen atoms corresponding to each signal in the ¹H NMR spectra of the europium (III) complexes and ligand compounds. The two peaks highlighted in red in Fig. 1 are the solvent peak (2.49 ppm) and solvent water peak (3.3 ppm) of DMSO‑d₆, respectively. Comparing the ¹H NMR signals of the experimental and the theoretical spectra of the complexes, it can be seen that there is only a small difference in the chemical shifts of nonreactive hydrogens, proving the reliability of the theoretical structures obtained for the complexes. Furthermore, the chemical shift differences of the complexes relative to those of the individual ligands are significant, supporting the binding of europium ions to the ligands. In addition, it can be seen from the theoretical structure of the complexes shown in Fig. 1 that the ligands coordinate to the central Eu³⁺ through the bidentate chelation mode.

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Fig. 1

¹H NMR spectra of the ligands and europium complexes.

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Fig. 1

¹H NMR spectra of the ligands and europium complexes.

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Fig. 1

¹H NMR spectra of the ligands and europium complexes.

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Fig. 1

¹H NMR spectra of the ligands and europium complexes.

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Structural information of the rare earth complexes was obtained by FT-IR spectral analysis as shown in Fig. 2. There is a significant difference in the FT-IR spectra between the ligands and the complexes indicating that the ligands were bonding with Eu³⁺. It can be observed that the vibrational peaks of the –CONH-functional groups of the six europium complexes were distributed around 3090 cm⁻¹. The acyl chloride reacted with alanine to generate acyl amino acid ligand compounds, indicated by the fact that these vibrational peaks only appeared in the infrared spectra of solid samples. The functional group –CONH- is a secondary amide, which generally produces three absorption bands. They were the amide band I (C = O (CONH) which showed a strong absorption peak in the range of 1680–––1630 cm⁻¹), amide band II (the coupling between δNH and vC-N formed the amide band II due to the resonance absorption in the range of 1570–––1510 cm⁻¹), and amide band III (the coupling between δNH and vC-N formed the amide band III of the resonance absorption in the range of 1335–––1200 cm⁻¹). The complex is an organic carboxylate. Its –COO- functional group is a multi-electron conjugated system with strong absorption peaks in the wavenumber range of 1610–––1560 cm⁻¹ and 1440–––1350 cm⁻¹. They respectively correspond to the antisymmetric and symmetric stretching vibrations; the peak intensity of the symmetric stretching vibration was smaller than that of the antisymmetric stretching vibration(Ning and Ernst, 2014). In addition, the absorption peak near 535 cm⁻¹ belongs to the absorption band generated by O-Eu formed by the ligand binding to the rare earth ions. The lower the wave number, the higher the energy. The vibration peak of –CH₂- shifts towards the short wave number as the length of the carbon chain increases, indicating that the vibration of long carbon chains requires higher energy. And the vibration peak of O-Eu tends to shift towards higher wave numbers as the carbon chain grows, indicating that the binding force of short carbon chain complexes is stronger. The peaks corresponding to the respective functional groups are shown in Table 1.

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Fig. 2

FT-IR spectra of ligands and europium complexes.

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Fig. 2

FT-IR spectra of ligands and europium complexes.

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According to the infrared spectral analysis, the complex had characteristic functional groups such as –CONH- and n-alkyl groups. The hydrogen atom in –CONH- is an active hydrogen, and its characteristic signal was a blunt peak. Additionally, its oxygen atom is a strong electrophile, causing a decrease in the electron density around the nuclei of the hydrogen atoms and a further increase in its chemical shift. As a result, its characteristic peak appeared at about 5.5–––7 ppm. The methylene chemical shifts of the n-alkyl groups were relatively close, and the peaks were concentrated around 1.25 ppm. The top methyl group was the most obvious, showing triplets at about 0.87 ppm. However, the electron-affinity of the group affected the methylene hydrogen atom close to –CONH-, and its chemical shift increased as compared to the other methylene groups, moving to around 1.5 ppm(Ning and Ernst, 2014). According to the elemental analysis, ¹H NMR, and FT-IR analysis, the molecular structure and the photophysical properties of the target product could be determined.

3.2 Light absorption and luminescence capacity of the complexes

The energy transfer from ligands to rare earth ions is also known as the antenna effect(Wu et al., 2018). First, the energy absorbed by the ligand allows for their transition from the ground state to the excited singlet state level and then passes through the intersystem crossing (ISC) radiationless transition state to the excited triplet state level. The energy is transferred to the lowest excited state level of rare earth ions. Since the light absorption capacity of the complex is one of the main factors affecting the luminescence performance of the complex, UV–visible absorption spectra of the complex in methanol solution (0.01 mol/L) were taken and analyzed. The results are illustrated in Fig. 3. It can be observed that the light absorption capacity of the complex came primarily from the ligand and that the complex had good light absorption capacity in the range of 200–––300 nm, shown as two distinct absorption peaks. The above analysis shows that the complex contains –CONH- and the carboxyl functional group contains C = O, where the unpaired n electrons on both sides of the oxygen atom are excited to the antibonding orbital Π* of the C = O double bond after absorbing energy, thus generating an optical absorption peak(Jin, n.d.). In addition, the absorption spectra in the enlarged view of Fig. 3 show that there are some absorption bands with very low absorption intensity, among which the absorption peak at 394 nm can be attributed to the 4f-4f transition absorption of Eu³⁺. There was no significant difference in the absorption capacity of the six complexes with different chain lengths, with the exception of the Eu(oct-ala)₃ complex, which had a slightly stronger absorption capacity.

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Fig. 3

UV–vis spectrum of ligands and europium complexes (a) ligands (b) complexes.

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3.3 Luminescence spectral analysis of the solid complexes

The luminescence excitation spectral scan measures the total excitation wavelength-dependent luminescence of the samples(Jin, n.d.). The optimal excitation value of the luminescence emission spectrum can be obtained by the analysis of the excitation spectrum. Excitation spectra of EuCl₃ and the complexes were taken, and their analytical results are shown in Fig. 4. As shown in Fig. 4(a), the sharp absorption peaks of EuCl₃ at 298 nm (⁷F₀→⁵F₄), 318 nm (⁷F₀→⁵H₆), 362 nm (⁷F₀→⁵D₄), 384 nm (⁷F₀→⁵L₇), 394 nm (⁷F₀→⁵L₆), 416 nm (⁷F₀→⁵D₃), and 464 nm (⁷F₀→⁵D₂) were attributed to the 4f → 4f characteristic absorption peak of Eu3+(Khan et al., 2014). By comparing the EuCl₃ excitation spectra of EuCl₃ and the rare earth europium complexes, it was found that the difference between the excitation peaks of the complex and chloride was small, and the optimal excitation wavelength was still at 394 nm. The complex still produced luminescence in the range of 200–––250 nm, but the total luminescence intensity was weak, indicating that the ligand compound could transfer energy to Eu³⁺ only with low efficiency.

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Fig. 4

Excitation spectrum of EuCl₃(a) and europium complexes(b).

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The optimal excitation wavelength for each complex determined from the luminescence excitation spectrum was used for the luminescence emission spectral analysis. The results are shown in Fig. 5. The four major emission peaks of the Eu³⁺ complex were detected at 590 nm, 610 nm, 650 nm, and 698 nm. The corresponding electronic transitions of Eu³⁺ are ⁵D₀→⁷F₁, ⁵D₀→⁷F₂, ⁵D₀→⁷F₃, and ⁵D₀→⁷F₄, among which ⁵D₀→⁷F₁ is a magnetic dipole transition, and ⁵D₀→⁷F₂ is an electric dipole transition(Wu et al., 2015). Since this transition emission peak was the strongest, this peak was used as the detection peak in the luminescence lifetime test of the complex. The remaining emission peaks at 467 nm, 482 nm, 536 nm, 556 nm, and 571 nm correspond to the ⁵D₂→⁷F₂, ⁵D₂→⁷F₃, ⁵D₁→⁷F₁, ⁵D₁→⁷F₂ and ⁵D₀→⁷F₀ transitions of Eu³⁺(Lima et al., 2017; Wang et al., 2015; Huang et al., 2013; Jiang et al., 2012). A comparison of the luminescence intensities of the complexes and EuCl₃ proved that acyl amino acid ligands played a role in sensitizing Eu³⁺ characteristic luminescence. In addition, the ⁵D₀→⁷F₁ (590 nm) transition peak of Eu³⁺ was attributed to the magnetic dipole transition, and its intensity was not much affected by the coordination environment. However, the ⁵D₀→⁷F₂ (610 nm) transition is an electric dipole transition, and its intensity was quite sensitive to changes in the surrounding chemical environment. Therefore, the coordination environment of Eu³⁺ ions could be determined by comparing the two transition intensities. The general indicator for a centrosymmetric coordination environment is I_{(610 nm/590 nm)} < 8(Jin, (1068)). The luminescence intensities at 610 nm and 590 nm of the six Eu³⁺ complexes were compared, and the results are as follows: I_Eu(ac-ala)3 = 3.114; I_Eu(but-ala)3 = 2.664; I_Eu(hex-ala)3 = 3.254; I_Eu(oct-ala)3 = 3.585; I_Eu(dec-ala)3 = 4.442; I_Eu(dod-ala)3 = 4.160. The comparison showed that the six complexes Eu³⁺ were in a centrosymmetric position. According to Refs.(Shi et al., 2012; Binnemans et al., 1997), the Judd-Ofelt (J-O) intensity parameter Ω₂ (shown in Table3) significantly impacts the intensity of the ultra-sensitive transition (⁵D₀→⁷F₂), which is determined by the asymmetry of the coordination environment of rare earth ions. The increase in asymmetry will enhance the luminescence intensity of rare earth ions leading to a difference in the luminescence intensity of the complexes in Fig. 5(b).

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Fig. 5

Emission spectrum of EuCl₃ (a) and europium complexes (b).

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3.3.2

3.3.2 CIE color coordinates and color temperature

From the emission spectrum data of the complexes, chromaticity coordinates were analyzed, and the CIE diagram was drawn, as shown in Fig. 6. The color parameters for synthesized complexes are located within the red region of the CIE triangle. From the CIE coordinates (x, y) of the complexes, the color coordinates (u ', v'), the correlated color temperature(CCT) and the value of color purity in the synthesized complexes was estimated. Several optical parameters of the complexes are presented in Table 3.

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Fig. 6

Real object diagram of rare earth europium complex CIE and luminescence (a)Eu(ac-ala)₃; (b)Eu(but-ala)₃; (c)Eu(hex-ala)₃; (d)Eu(oct-ala)₃; (e)Eu(dec-ala)₃; (f)Eu(dod-ala)_3.

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Table 3 Various optical parameters for Europium complexes.

	(x,y)	(u’,v’)	CCT/K	CP/%
Eu(ac-ala)3	(0.551,0.331)	(0.375,0.507)	1860	65.07
Eu(but-ala)3	(0.633,0.342)	(0.434,0.527)	2544	89.62
Eu(hex-ala)3	(0.640,0.342)	(0.439,0.528)	2632	91.71
Eu(oct-ala)3	(0.639,0.340)	(0.441,0.527)	2677	91.39
Eu(dec-ala)3	(0.640,0.340)	(0.441,0.527)	2700	91.68
Eu(dod-ala)3	(0.648,0.343)	(0.445,0.530)	2723	94.10

The CCT is an imperative luminescence indicator related to the measurement of temperature and color of lighting sources. Their values for the complexes reflect their application significance. It can be seen from Table 3 that the color purity (CP) and CCT of the complexes increase with the length of the carbon chain of the ligand.

3.3.3

3.3.3 Luminescence lifetime, efficiency, and Judd-Ofelt parameters

The luminescence lifetime and quantum yield tests of the complexes were carried out to assess their luminescence lifetime and quantum yield which are important indicators for the luminescence performance of substances., The luminescence lifetime curve is shown in Fig. 7. Luminescence lifetime curves were fitted to a double exponential equation: ${\mathit{I}}_{(\mathit{t})}={\mathit{I}}_0+{\mathit{A}}_1\mathbf{exp}\left(-\frac{\mathit{t}}{{\mathit{\tau }}_1}\right)+{\mathit{A}}_2\mathbf{e}\mathbf{x}\mathbf{p}(-\mathit{t}/{\mathit{\tau }}_2)$ , where ${\mathit{\tau }}_1$ and ${\mathit{\tau }}_2$ are luminescence lifetimes, ${\mathit{A}}_1$ , ${\mathit{A}}_2$ , and ${\mathit{I}}_0$ are (amplitudes A and initial intensity I) fitting parameters. Then the final luminescence lifetimes of the six complexes were obtained by the equation for the mean lifetime: $\mathit{\tau }=({\mathit{A}}_1{\mathit{\tau }}_1^2+{\mathit{A}}_2{\mathit{\tau }}_2^2)/({\mathit{A}}_1{\mathit{\tau }}_1+{\mathit{A}}_2{\mathit{\tau }}_2)$ . The test results are shown in Table 4.

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Fig. 7

Luminescence lifetime diagram of rare earth europium complexes.

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Table 4 Experimental photophysical characteristics of Europium complexes.

	τ /ms	Arad (s−1)	Anrad (s−1)	η (%)	Ω2 (×10-20 cm2)	Ω4 (×10-20 cm2)
Eu(ac-ala)3	0.4	339.7	2041.3	14.3	0.31	0.27
Eu(but-ala)3	0.6	319.4	1267.9	20.1	0.29	0.27
Eu(hex-ala)3	0.7	319.1	1089.4	22.7	0.31	0.23
Eu(oct-ala)3	0.4	367.4	1958.2	15.8	0.36	0.26
Eu(dec-ala)3	0.7	406.8	1021.8	28.5	0.41	0.28
Eu(dod-ala)3	0.6	413.5	1253.7	24.8	0.38	0.35

The coordination environment in the complexes around the Eu³⁺ ion was determined using the Judd-Ofelt theory. Photoluminescence data were utilized to determine the J-O parameters and transition rates. Radiative transition rates, nonradiative rates, the fluorescence branching ratio that depicts the involvement of each transition as the percentage of radiative decay rate of the complexes, and several experimentally determined photophysical parameters such as decay time, and quantum efficiency, are presented in Tables 4 and 5.

Table 5 Contribution of various transitions to radiative rates.

/ %	β(5D0→7F0)	β(5D0→7F1)	β(5D0→7F2)	β(5D0→7F3)	β(5D0→7F4)
Eu(ac-ala)3	1.17	14.72	57.85	2.24	24.02
Eu(but-ala)3	0.69	15.65	56.57	1.66	25.42
Eu(hex-ala)3	0.81	15.67	59.26	2.01	22.25
Eu(oct-ala)3	0.82	13.61	62.6	1.85	21.12
Eu(dec-ala)3	0.84	12.29	63.96	2.11	20.79
Eu(dod-ala)3	1.04	12.09	58.96	2.18	25.61

3.4 Performance analysis of the complexes in the solution

3.4.1

3.4.1 The dissociation of metal complexes in the solution

The degree of aggregation/diffusion of the metal complexes in methanol solution was obtained by measuring the self-diffusion coefficient of the europium complexes. The decrease in the solute self-diffusion coefficient indicates an interaction between solute and soluteAziz et al., 1971. Fig. 8 shows the conductivity of the three europium complexes, Eu(ac-ala)₃, Eu(hex-ala)₃, and Eu(dec-ala)₃, in methanol as a function of the concentration of the complex. The magnitude of the conductivity in solution at lower concentration ranges of the europium complexes is in the order of Eu(ac-ala)₃ > Eu(dec-ala)₃ > Eu(hex-ala)₃. As the concentrations increase to about 0.002–––0.012 mol/L, the conductivities of all three Eu³⁺ complex solutions increase indicating that dissociation occurs in the complex solutions. When the concentration is higher than 0.014 mol/L, with increasing molar mass concentration, the conductivities of the Eu(ac-ala)₃ solution start to decrease. This is due to the aggregation behavior of metal ions and ligand ions in solution forming aggregate states(Naren et al., 2009; Naren et al., 2008). However, the complexes with longer carbon chains are less prone to form aggregate states.

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Fig. 8

Electric conductivities (σ) of Eu(ac-ala)₃, Eu(hex-ala)₃, and Eu(dec-ala)₃ in methanol as a function of the concentration (C) of the complex.

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3.4.2

3.4.2 The effect of the solution concentration on fluorescence performance

Fig. 9 shows the luminescence intensity of the complexes, (a) Eu(ac-ala)₃, (b) Eu(hex-ala)₃, and (c) Eu(dec-ala)₃. The luminescence intensity of all three complexes increases as the molar concentration increases. This is caused by an increase in the number of luminescent particles. The fluorescence lifetime of the complexes also varies with concentration, as shown in Fig. 10, indicating that as the concentration increases to about 0.002–––0.012 mol/L, the fluorescence lifetime of the Eu(ac-ala)₃ also increases. When the concentration is higher than 0.014 mol/L, as the molar concentration increases, the conductivities of the Eu(ac-ala)₃ solution start to decrease, as shown by the dissociation of metal complexes in the solution that form an aggregated state of Eu(ac-ala)₃ at 0.014 mol/L. At this time, the Eu³⁺ ions are aggregating, one of the excited Eu³⁺ ions (ion 1) transfers energy to another nearby ion (ion 2) through cross-relaxation, ion 1 returns to the ground state in a radiationless decay and ion 2 is excited to a higher energy level, which results in a decrease of the original radiation transition, manifested as a decrease in fluorescence lifetime(Meert et al., 2014). This trend of the fluorescence lifetime for Eu(hex-ala)₃ reaches a plateau and remains unchanged with further increasing concentration. It can be inferred that complexes with longer carbon chains are less prone to aggregation in solution (increasing repulsive forces with increasing hydrophobicity), and the concentration, where fluorescence quenching occurs, is higher.

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Fig. 9

Luminescence intensity of complexes as a function of concentration.

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Fig. 10

Fluorescence lifetime in solution as a function of concentration.

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3.4.3

3.4.3 Solvent effect on the fluorescence performance of complexes with different carbon chain lengths

In order to explore the solvent effects on the luminescence properties of the complexes with different carbon chain lengths, the luminescence spectrum of a methanol solution (0.01 mol/L) of the complexes was studied, and the results are shown in Fig. 11. A comparison of Fig. 11(a) with Fig. 4(b)shows that the optimal excitation wavelength of the complex solution was still 394 nm. Nevertheless, the total luminescence peak generated at 200–––250 nm by the energy transfer from the ligand compounds to the rare earth ion disappeared; instead, a smaller total luminescence peak appeared in the range of 250–––275 nm. This phenomenon can be attributed to the dissolution of the complex in methanol, where the methanol molecules formed a “solvent cage” around the complex. During the entire excitation and emission process, the solvent molecules would participate in the energy adjustment process(Jin, n.d.), causing a reduction in the energy level of the ligand compounds. Consequently, less excitation energy is required, leading to a redshift in wavelength. In addition, the above analysis shows that the energy transfer efficiency between the energy level of the ligand compound and Eu³⁺ was low, and the energy absorbed by the ligands was mainly dissipated through the rotation of their own functional groups and the vibration of the carbon chains. Furthermore, the complex molecules could rotate freely in the solution, which enhanced the energy dissipation of the complex molecules, leading to a decline in the total amount of luminescence generated by the energy transfer from the ligands to the Eu³⁺ ion. As shown in Fig. 11(b), no significant difference has been observed in the luminescence intensities of the Eu³⁺ complexes, which was different from the solid luminescence spectrum. As mentioned above, the asymmetric environment of Eu³⁺ increases the luminescence intensity of the Eu³⁺ ultra-sensitive transition, which led to the difference in the luminescence intensity of Eu³⁺ complexes in the solid spectra. However, in the solution spectra, the complex was dissolved in methanol. The methanol molecule coordinated with Eu³⁺, which changed the coordination environment of the complex(Kropp and Windsor, 1965), thus reducing the difference in luminescence intensity between the complexes. As can be seen from the luminescence curve in Fig. 11(b), the coordination environment of Eu(ac-ala)₃ in solution seemed to contribute to the transition from ⁵D₀→⁷F₄, resulting in a transition peak intensity that was higher than those of other complexes, and in a large change in the fluorescent color of the solution compared with that of the solid state (see Fig. 6(a)).

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Fig. 11

Luminescence spectra of europium complexes in methanol solution (a) excitation spectrum; (b) Emission spectrum.

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3.5 Variable temperature spectral analysis

The thermal stability of fluorescent substances is an important index for the service life and luminescent properties of LEDs. Generally, the use temperature of the LED is about 150 °C(Gu, 2019). Under the optimal excitation and emission wavelengths of the complex, the variable temperature spectra were studied in the range of 25–––160 °C, and the test results are illustrated in Fig. 12 below. It is evident that within the studied temperature range, the six complexes could be divided into three groups according to the trends of the luminescence intensity variation with temperature: (I) Eu(ac-ala)₃ showed a trend of first decreasing luminescence intensity, then increasing and then again decreasing; (II) the luminescence of Eu(oct-ala)₃ and Eu(dec-ala)₃ increased first and then decreased; (III) Eu(but-ala)₃, Eu(hex-ala)₃ and Eu(dod-ala)₃ declined first, then remained stable and then again decreased; the luminescence intensity of the complexes changed little in the low-temperature region (20–––60 °C). The unique energy level structure of Eu³⁺ is shown in Fig. 13(a). When the difference between adjacent energy levels of rare earth ions is small (500–––2000 cm⁻¹), photons at the adjacent energy levels can be added to saturate the atomic orbitals or removed with the change of temperature of the fluorescent substance. These adjacent energy levels are thermally coupled energy levels(Wang et al., 2015; Dramićanin, 2016). The ⁵D₀ and ⁵D₁ of Eu³⁺ were thermally coupled energy levels, consequently, their corresponding luminescence peak intensities would change with temperature. At high temperatures, the energy of two rare earth ions can be attenuated by cross-relaxation(Meert et al., 2014), as shown in Fig. 13(b). The photon absorption energy on ion 1 ⁵D₀ jumped to the ⁵D₁ level, and then ion 1 transferred its energy at the ⁵D₁ level to ion 2. Ion 1 returned to the ground state, and ion 2 was excited to the ⁵D₁ level, which enhanced the luminescence emission from ⁵D₁. Thus, with rising temperature, the ⁵D₁→⁷F_J transition increased, and the ⁵D₀→⁷F_J transition intensity decreased. As mentioned above, the 610 nm peak was attributed to the ⁵D₀→⁷F₂ transition, and the 534 nm peak to the ⁵D₁→⁷F₁ transition. Therefore, the intensities corresponding to the 610 nm and 534 nm peaks of the complex as a function of temperature were compared, to investigate the reasons for the change in the luminescence intensity of the complexes (Fig. 14).

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Fig. 12

Variable temperature spectrogram of the complexes:(a) Eu(ac-ala)₃; (b) Eu(but-ala)₃; (c) Eu(hex-ala)₃; (d) Eu(oct-ala)₃; (e) Eu(dec-ala)₃; (f) Eu(dod-ala)_3.

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Fig. 13

(a) Variable temperature energy transfer diagram of the Eu³⁺complex; (b) Schematic diagram of Eu³⁺cross relaxation.

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Fig. 14

Graphs showing the temperature-dependent luminescence intensity of the rare earth europium complexes at different wavelengths. (a)Eu(ac-ala)₃; (b) Eu(hex-ala)₃; (c) Eu(oct-ala)₃; (d) Eu(dod-ala)_3.

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The luminescence intensity curves of 610 nm and 534 nm peaks of the complex were increasing and decreasing at the same time, which seemed to be inconsistent with the trend of luminescence intensity variation at the thermally coupled energy level. In this regard, the author puts forward two points of view. (1) When the complex was in a low-temperature range, ⁵D₀ and ⁵D₁ had an uncoupling relationship, which increased the probability of non-radiative transition of the complex with rising temperature, and the luminescence intensity of the complex decreased. When the two energy levels reached the coupling temperatures, the two ions increased the transition probability of ⁵D₁ by cross-relaxation, resulting in an increase in the intensity of the 534 nm transition peak corresponding to ⁵D₁→⁷F₁. Since the energy difference between ⁵D₁ and ⁵D₀ was ΔΕ₁ = 1885 (+/-100) cm⁻¹, and the energy difference between ⁷F₂ and ⁷F₄ was 2064 cm⁻¹, the transition wavelength of ⁵D₁→⁷F₄ was similar to that of ⁵D₀→⁷F₂. Thus, when ⁵D₁ of Eu³⁺ was the main emission level, the ⁵D₁→⁷F₄ transition supplemented the missing ⁵D₀→⁷F₂ transition, leaving the 610 nm unchanged or enhancing its intensity. Whether the peak intensity was enhanced depended on whether the coordination environment of the complex was suitable for the ⁵D₁→⁷F₄ transition. As mentioned above, the ⁵D₀→⁷F₄ transition of Eu(ac-ala)₃ was enhanced in the solution. The coordination environment of this complex in a molten state was similar to that in solution, so its ⁵D₁→⁷F₄ transition was stronger, and its corresponding 610 nm peak was stronger as well, while the intensity of the 534 nm transition peak corresponding to ⁵D₁→⁷F₁ increased only slightly, as shown in Fig. 11(a). For the complexes whose coordination environment was not suitable for the ⁵D₁→⁷F₄ transition, the increase in its 610 nm peak intensity was small, whereas the increase in the 534 nm peak intensity corresponding to ⁵D₁→⁷F₁ was large, as shown in Fig. 14(b). (2) The second proposed mechanism is that the photon absorption energy on ion 1 ⁵D₀ jumped to the ⁵D₁ level and transferred energy to ion 2 through cross-relaxation. However, different from the above process, the energy on ion 1 ⁵D₁ was transferred to ion 2, causing the transition of photons on ion 2 from ⁵D₁ to ⁵D₂. As shown in Fig. 10(a), ΔΕ₂ (2153 cm⁻¹) was the energy level difference between ⁵D₁ and ⁵D₂. Although ΔΕ₂ > ΔΕ₁, the temperature supplemented the energy required for the photon transition, and the photon on ⁵D₁ jumped to ⁵D₂. The photon on ion 1 was deexcited to the ⁵D₀ level, and the energy on ion 2 ⁵D₂ was transferred back to the ion 1 ⁵D₀ level. At this time, there were “two” photons on the ⁵D₀ level, enhancing the transition peak corresponding to ⁵D₀, or keeping it unchanged. This process is similar to a photon avalanche (Chivian et al., 1979). Whether the corresponding transition peak was enhanced depended on the energy transfer from ⁵D₂ to ⁵D₀. According to the excitation spectrum of the solid shown above, the 464 nm (⁷F₀→⁵D₂) absorption peak could also generate the ⁵D₀→⁷F₂ (610 nm) transition (energy was transferred to ⁵D₀ by non-radiative transition). The total luminescence peak generated was determined by the energy transfer efficiency from ⁵D₂ energy level to ⁵D₀. Therefore, the intensity of I (464 nm/394 nm) was used to assess the probability of a ⁵D₀→⁷F₂ (610 nm) transition, and to measure the energy transfer efficiency from the ⁵D₂ energy level to ⁵D₀. The results are as follows: I_Eu(ac-ala)3 = 0.5017; I_Eu(but-ala)3 = 0.4086; I_Eu(hex-ala)3 = 0.3819; I_Eu(oct-ala)3 = 0.5170; I_Eu(dec-ala)3 = 0.6138; I_Eu(dod-ala)3 = 0.5359. It can be seen that the values of Eu(ac-ala)₃, Eu(oct-ala)₃, Eu(dec-ala)₃ and Eu(dod-ala)₃ were large, indicating that “more” ⁵D₀→⁷F₂ (610 nm) transitions were generated at the ⁵D₂ energy level. Therefore, within a certain temperature range, the ⁵D₀→⁷F₂ (610 nm) transition peak was enhanced or modified slightly with temperature.