Synthesis and Optical Performance of terbium complexes with octanoyl amino acids - Arabian Journal of Chemistry (2025)

2.1

2.1 Preparation of the rare-earth amino acids

TbCl₃·7H₂O (≧99.9%) and three amino acids (≧98%), alaninate, phenylalaninate, and serinate, were purchased from Aladdin Industrial Corporation. CH₃OH (≧99.5%), CH₃CH₂OH (≧99.7%), NaOH (≧96%), H₂SO₄ (≧98%), (CH₃CH₂)₂O (≧99.5%), and CH₃COCH₃ (≧99.5%) were purchased from China Zhiyuan Chemical Reagent Co., Ltd, Tianjin. All aqueous solutions were prepared using Milli-Q water.

Three octanoyl amino acid ligands and the new octanoyl amino acid terbium complexes were synthesized according to similar methods as referenced (Bert et al., 1999).

The synthesis process of ligands is as follows: a 5M sodium hydroxide solution was first dripped into an aqueous solution of alanine to adjust the pH value of the solution. When the pH of the solution became stable, Octanoyl chloride was slowly dripped into the solution to obtain a pH of 10. After 12h of reaction, then sulfuric acid was added dropwise to reach a constant pH value of 4. Finally, purification was carried out using n-hexane and petroleum ether solvent to obtain the octanoyl amino acid ligands.

To synthesize the octanoyl amino acid terbium complexes, 3mol of synthetic octanoyl amino acid was dissolved in 80mL methanol and 0.89M potassium hydroxide solution was added to adjust the pH value of 10. Then 1mol of terbium chloride was added to the solution for 12h of reaction. After purifying the product with ethanol and acetone, a white powder was obtained. The water content of the rare-earth complexes was determined by Karl Fischer titration.

2.2

2.2 Characterization of the rare-earth amino acid complexes

¹H NMR analysis of the complexes was performed using a BRUKER NMR 600 (600MHZ) spectrogram at ambient temperature in dimethylsulfoxide (DMSO). FT-IR spectra were obtained using the KBr pellet technique on the PerkinElmer Spectrum Two. The spectrum range is 400–4000cm⁻¹, and the functional group information of the complexes was obtained by analyzing the spectrum.

In order to verify the long-range and short-range ordered/disordered structure information of the complexes, a Philips PW-1830 X-ray diffractometer (Cu-kα, λ=0.154184nm) was used to measure the complexes by wide-angle X-ray diffraction at room temperature, and the range of measurement was 6–80°. A Rigaku Ultima IV X-ray diffractometer was used for small-angle X-ray measurement, and the measurement range was 0.5–6°.

The microstructures of the complexes were observed using a Hitachi S-3400N scanning electron microscope. Polarizing microscopy was carried out using an OLYMPUS CX31 polarizing microscope. UV–Vis absorption spectra were obtained using a Lambda 35 UV–Vis Spectrometer (Perkin Elmer). The emission wavelength and fluorescence lifetime of the complex were detected using an EX-1000 fluorescence spectrometer with an excitation wavelength of 268nm.

2.3

2.3 Computational details of the rare-earth amino acids

The results of ¹H NMR, elemental analysis, and FT-IR analysis were used as the PBE1PBE/gen method in DFT to determine the structure of complexes (Frisch et al., 2013). The four elements, C, O, N and H, belong to a 6-31g* group, and the central element Tb is in the MWB54 group.

2.4

2.4 Preparation of the LED devices with terbium complexes

The LED devices are made of terbium complexes powder mixed with encapsulated transparent silica gel and added to the chip. The chip is a 365nm near-ultraviolet chip. The packaged chip is put into the oven to dry at about 60 °C. After drying, connect the LED device to the power supply, and observing its luminescent properties.

3.1

3.1 ¹H NMR and elemental analysis

The content ratio of hydrogen atoms bound by various functional groups in Tb(oct-ala)₃·H₂O, Tb(oct-phe)₃·H₂O and Tb(oct-ser)₃·3H₂O were examined using ¹H NMR. The NMR results show that the chemical shifts of the ligands observed for H(oct-ala) (Fig. 1 (a)) include δ 12.24 (s, 1H), 8.05 (d, J=7.3Hz, 1H), 4.23–4.10 (m, 1H), 2.20–2.05 (m, 2H), 1.52–1.42 (m, 3H), 1.24 (d, J=7.2Hz, 10H), and 0.88–0.84 (m, 3H). The chemical shift of 1.52–1.42 (m, 3H) corresponds to the —CH₃ peak on the side chain. The chemical shifts of H(oct-phe) (Fig. 1 (c)) obtained were δ 12.40 (s, 1H), 8.08 (d, J=8.2Hz, 1H), 7.28–7.17(m, 6H), 4.44–4.39 (m, 1H), 3.07–3.02 (m, 1H), 2.85–2.80 (m, 1H), 2.30–1.90 (m, 3H), 1.52–1.34 (m, 3H), 1.20–1.17 (m, J=7.3, 3.6Hz, 4H), and 0.87–0.84 (m, 4H). The peak at δ 7.28–7.17 is the characteristic peak of a benzene ring. For H(oct-ser) (Fig. 1 (e)), the following chemical shifts were obtained: δ=12.47 (s, 1H), 7.94 (d, J=7.9, 1H), 4.95 (s, 1H), 4.30–4.24 (m, 1H), 3.64 (m, 2H), 2.14 (m, 2H), 1.49 (m, 2H), 1.26 (m, 8H), 0.86 (m, 3H). Fig. 1 shows the ¹H NMR spectra of the rare-earth complexes (Fig. 1 b, d, f). The chemical shift differences of the rare-earth complexes relative to those of the ligands are significant. The paramagnetic properties of terbium induce the chemical shifts to lower fields (Zhang, 2009). Therefore, ¹H NMR cannot accurately determine the hydrogen content in the rare-earth complexes. Further elemental analysis needs to be carried out.

The elemental analysis results of the three terbium complexes are shown in Table 1. The theoretical calculations of the three complexes are smaller than the experimental results. The structures of the three terbium complexes were determined using the elemental analysis results combined with the ¹H NMR results.

3.2

3.2 FT-IR analysis

In order to obtain further information on the main functional group of the terbium complexes, IR spectroscopy analysis was performed. Fig. 2 shows the FT-IR spectra of the three terbium complexes. FT-IR is an effective method to identify the chemical information of the functional groups. The molecular structures of the terbium complexes were determined by combining the results of FT-IR, ¹H NMR with elemental analysis. In Tb(oct-ala)₃·H₂O, Tb(oct-phe)₃·H₂O, and Tb(oct-ser)₃·3H₂O, the stretch vibration peaks of —CH₃, —CH₂, —CH are in the range of 2800–3000cm⁻¹. IR spectrum of Tb(oct-ala)₃·H₂O showed a peak at 1379cm⁻¹, which is a characteristic peak of side-chain —CH₃. For Tb(oct-phe)₃·H₂O, a characteristic peak of benzene ring at 696cm⁻¹ was observed. For Tb(oct-ser)₃·3H₂O, peaks at 3272cm⁻¹ and 1032cm⁻¹ are the —OH stretch vibration. There are two sharp peaks at 1548cm⁻¹ and 1426cm⁻¹ and a broad peak at 2500–3200cm⁻¹, indicating the formation of a carboxylic acid. The results indicate that Tb³⁺ is linked to octanoyl amino acid ligands and forms coordination bonds with rare-earth ions replacing H in —COOH (Zhang, 2009; Li et al., 2019; Ning, 2002). It is worth noting that a broad stretching of the vibration absorption of H₂O appears in the range of 3500–3300cm⁻¹. The out-of-plane bending vibration of the crystalline water molecules and the in-plane swing vibration of coordinated water molecules were observed at 555cm⁻¹ and 920cm⁻¹, respectively; This confirms that there exist both coordinated water molecules and crystal water molecules in the complexes (Chen et al., 2016).

3.3

3.3 XRD analysis

In order to investigate the long-range/short-range structure of the rare-earth complexes, XRD measurements were carried out. The XRD results are shown in Fig. 3. Wide-angle XRD (WAXD) spectra of the three terbium complexes (Fig. 3 (1)) ranged from 6–80° show broad diffusion peaks between 15–25° (Jia et al., 2019). No sharp crystalline diffraction peaks was observed, which indicates that there exists long-range disorder and a rather disordered amorphous structure presents.

The detecting range for small-angle XRD (SAXD) analysis is 0.5–6° (Fig. 3 (2)). The spectra show sharp peaks at small angles, which is characteristic of periodic short-range ordering (Fonollosa et al., 2004). The sharp peaks of Tb(oct-phe)₃·H₂O at 2θ=4.5° and Tb(oct-ser)₃·3H₂O at 2θ=3.5° indicate that these two complexes exhibit short-range ordering in the small angle range (0.5–6°). However, Tb(oct-ala)₃·H₂O shows a wide peak in the small angle range (0.5–6°), which indicates a tendency of a transition from short-range order to disorder and is more inclined to the formation of a glass state.

The average distance of adjacent atoms in the rare-earth complexes is calculated using the Bragg equation: $2\mathrm{d}\sin \theta =n\lambda$. The calculation results are shown in Table 2. The average distance between adjacent atoms of Tb(oct-phe)₃·H₂O in three rare-earth complexes is the smallest.

3.4

3.4 Surface topography analysis

XRD results showed that Tb(oct-phe)₃·H₂O and Tb(oct-ser)₃·3H₂O have the characteristics of the long-range disorder and short-range order, while Tb(oct-ala)₃·H₂O has the tendency of long-range disorder and short-range disorder. To characterize the morphology of Tb(oct-ala)₃·H₂O, Tb(oct-phe)₃·H₂O, and Tb(oct-ser)₃·3H₂O, SEM experiments were carried out. The SEM images are shown in Fig. 4. The SEM results illustrated that all the complexes examined have irregular shapes (Ehab and Abdelrahman, 2019). Furthermore, the three ligands were not observed to have liquid crystal properties and showed specific rod-like and needle-like structures (Zhang et al., 2019). During the scanning process, the sample melted and deformed due to the temperature increase caused by the electron beam focusing, so the microstructure could only be observed roughly.

To further examine the optical properties of the three states, such as amorphous, crystalline, and liquid crystal state, the microscopic surface morphology of the three complexes were observed using polarized light microscopy below, at, and above the room temperature.

The Polarized Light Microscopy images of the Tb(oct-ala)₃·H₂O powder sample are shown in Fig. 5. The results show that Tb(oct-ala)₃·H₂O (Fig. 5) is a compound that can easily form transparent and uniform glassy compounds. The Tb(oct-phe)₃·H₂O sample show characteristics of optical anisotropy. As the temperature increased to 160°C, the sample began to flow and presented optical homogeneity. As temperature decreased to 25°C, little optical anisotropy appeared. In summary, Tb(oct-phe)₃·H₂O has liquid crystal properties. The Tb(oct-ser)₃·3H₂O (Fig. 5) exhibits optical anisotropy at 25°C, but gradually exhibits optical homogeneity as the temperature increases and showed optical anisotropy under gradual cooling. In conclusion, Tb(oct-ser)₃·3H₂O has better liquid crystal properties than Tb(oct-phe)₃·H₂O.

3.5

3.5 UV–Vis analysis

The UV–Vis absorption spectra of three rare-earth complexes, Tb(oct-ala)₃·H₂O, Tb(oct-phe)₃·H₂O and Tb(oct-ser)₃·3H₂O, with different concentrations were measured in the range of 200–400nm in both ethanol solution. It is well known that the absorbance of rare-earth complexes is very small and the absorption of organic ligands is larger in the ultraviolet, visible region. As shown in Fig. 6(a-c), the absorption intensity of the complexes increased as the concentration increased. An absorption peak appeared at about 210nm, which is due to the presence of —COOR group in the sample. The —COOR group directly connects with a heteroatom, which has an unpaired electron pair, producing n→π* transition. In Fig. 6 (b), in addition to the absorption peak at about 210nm, there is another absorption peak at 258nm, and the intensity of these two peaks is larger than that observed in Fig. 6 (a) and (c). This is due to the presence of benzene ring chromophore group, which causes a π–π* transition by conjugated effect, resulting in increased absorption. With the increase of concentration, the absorption intensity of the peak at 258nm increased gradually (Yang, 2006). This deviates significantly from the Beer-Lambert law. For the reason that the concentration of the terbium complex is above 0.01mol/L. When the concentration is high, the average distance between the absorbing particles decreases and their molar absorption coefficients change due to the interaction of the charge distribution between the particles, resulting in a deviation from Beer's law.

From the diffuse reflectance of the thick sample, we can deduce the absorption spectrum of the complex samples. Then, whether the absorption of the compound is strong or weak, and whether it has a blue or redshift could be determined. Ultra Violet-Visible-Near Infrared (UV–vis–NIR) diffuse reflectance spectroscopy provides information on molecular vibrations, which has advantages for the simple test method, fast speed measurement, and loss of compound samples.

The ultraviolet–visible diffuse scattering of solid powders of three ligands and rare-earth complexes were measured in the range of 220–400nm. The results are shown in Fig. 7. The shape and position of the peaks of the ligands and complexes were the same, except that the range of absorption peaks of rare-earth complexes increased more apparently than the ligands did, and the intensity of absorption of the complexes were slightly larger than the ligands. This could be attributed to the f-f transition of energy transfer to terbium after the ligands absorb light and rare earth ions also have some absorption, resulting in the increase in the range of the absorption peak (Bünzli and Piguet, 2006; Werts, 2005).

3.6

3.6 Luminescence analysis

Fig. 8 (a), (b), and (c) show the emission spectra of the Tb(oct-ala)₃·H₂O, Tb(oct-phe)₃·H₂O, and Tb(oct-ser)₃·3H₂O complexes, respectively, at various concentrations in ethanol solution and powder form. The emission intensity of the three complexes in ethanol solution increased as the solute concentration increased. The luminescence spectra of the three powder complexes are shown in Fig. 8 (d). Tb(oct-phe)₃·H₂O has better luminescence intensity than the other two complexes. The emission spectra bands of these three complexes are sharp, and the half-peak width is narrow (less than10nm). In Fig. 8, the four peaks at 488, 542, 582, and 620nm correspond to the characteristic fluorescence peaks of Tb³⁺ (⁵D₄→⁷F₆), (⁵D₄→⁷F₅), (⁵D₄→⁷F₄), and (⁵D₄→⁷F₃), respectively (Li et al., 2018; Yang et al., 2018; Li et al., 2013). It is apparent that the strong, sharp emission line of terbium complexes occurs near 542nm. The enhancement of Tb³⁺ emission is due to the octanoyl amino acid ligands. The polarity of Tb³⁺ is increased under the influence of the ligands, so that the electric dipole transition of ⁵D₄→⁷F₄ increased (Li et al., 2013). The emission spectra and chromaticity diagrams showed the characteristic fluorescence emission of terbium ion. The ligand has strong absorption in the ultraviolet region. It can effectively transfer excited state energy to the central ion through a non-radiative transition, thereby sensitizing the rare-earth ion to emit light and improving the luminous intensity of the rare-earth ion. Fig. 9 shows the pictures of the solid complex powders and ethanol solutions of the three terbium complexes irradiated by an ultraviolet lamp at 254nm. The three complexes show bright green after an ultraviolet lamp at 254nm. The powdered complex has a much brighter brightness than the complex in ethanol solution. When considering practical applications, the application of solid powder can maximize its optical performance.

Through the emission spectra of terbium complexes at different concentrations in an ethanol solution, the corresponding International Commission on Illumination (CIE) chromaticity diagrams are as shown in Fig. 10. In Fig. 10 (a-c), the locations of visible light are in the green area however, slightly different. A small peak appeared at 619nm in the emission spectrum. This peak was in the range of red light. Fig. 10 shows that the different concentration of terbium complexes in ethanol solutions has a slight effect on the color location, but for Tb(oct-phe)₃·H₂O (Fig. 10 (b)), the color location did not change as the concentration of the complex changed, due to ethanol being a strong polar solvent. The polarity of the solvent has a larger transition of n→π* than π–π*. There is an n→π* transitions in the ethanol solution of terbium complexes. However, in Tb(oct-phe)₃·H₂O (Fig. 10 (b)), there is a transition from π–π*, which makes the color shift of Tb(oct-phe)₃·H₂O (Fig. 10 (b)) least affected by the transition (Liu, 2007). It is concluded that Tb(oct-phe)₃·H₂O has stable chroma display at different concentrations of ethanol solutions.

The luminescence lifetime was determined by the excitation wavelength of 268nm and the emission wavelength of 544nm. The fluorescence lifetime diagram is shown in Fig. S1.

After fitting the curves exponentially, the equation (Ma et al., 2019) with a double-exponential peak shape was obtained.$$\mathrm{y}(\mathrm{t})=y_0+A_1\mathrm{e}\mathrm{x}\mathrm{p}(-t/{\tau }_1)+A_2\exp \left(-t/{\tau }_2\right)$$where y(t) shows the relationship between fluorescence intensity and time; y₀ is the lifetime of Poisson noise; τ₁ and τ₂ are the lifetimes of each component; A₁ and A₂ are the intensity amplitudes at t=0. The average lifetime of τ is as follows.$$\tau =\frac{A_1{\tau }_1^2+A_2{\tau }_1^2}{A_1{\tau }_1+A_2{\tau }_2}$$

The calculated fluorescence lifetime of three complexes are shown in Table 3.

The fluorescence lifetime is the time required to reduce the fluorescence intensity of the molecule to 1/e of the maximum fluorescence intensity after the excitation of the light pulse (Jin, 2018). The fluorescence lifetime of the powder sample of Tb(oct-phe)₃·H₂O is the largest which is consistent with the previously reported results (Ma et al., 2019). The fluorescence lifetime of three terbium complexes in ethanol solution is longer than that of the powder samples, and the values of fluorescence lifetime tend to be stable with the increase in concentration. When terbium complexes are dissolved in ethanol, due to the solvation kinetics, after light absorption, the solute is excited. The dipole moment of the solute changes, disturbing the solvent molecules around the solute and surrounding the solvent molecules (Li et al., 2018). The process of reorientation of the excited state solute increases the time a rare-earth complex is in the excited state and prolongs the fluorescence lifetime. The quantum yields of terbium complexes powder are also presented in Table 3. It can be seen overall that the changing rule of the QY is the same as the fluorescence lifetime and the value are less than 20%, this is consistent with other Tb (III) complexes (Harris et al., 2015; Laurent et al., 2014; Murase et al., 2013). Since the H(oct-phe) ligand has the benzene ring functional group, has the large conjugate structure, can transfer the energy more efficiently (Jiang, 2006). Furthermore, benzene ring chromophore group can absorb more energy in the ultraviolet range, the Tb(oct-phe)₃·H₂O has the highest QY (16.58%) and fluorescence lifetime (1.213) of three terbium complexes.

3.7

3.7 Theoretical calculations

To further examine the experimental results, we used density functional theory (DFT) (Parr and Weitao, 1989) to optimize the structure of the terbium complexes, analyze the molecular electrostatic potential, and calculated the infrared and ¹H NMR spectra of the terbium complexes. The DFT calculations were performed using Gaussian 09 (Frisch et al., 2013).

First, the structure optimization of the terbium complex and the calculation of the molecular vibrational frequency were performed to obtain a stable configuration. Then, the molecular electrostatic potential surface diagram (Fig. 11) and FT-IR spectrum (Fig.S2) were plotted using the results of the calculations. The molecular electrostatic potential surface diagram (Fekry et al., 2020) revealed that there is a positive electrostatic potential around the central atom terbium. The larger the electrostatic potential, the darker the color in the corresponding picture is. Comparing the calculated infrared spectrum with the experimental IR spectrum, we observed that the positions and shape of the main characteristic peaks were consistent. Finally, the ¹H NMR chemical shifts of the complex were calculated using the B792/gen method in DFT. The results are shown in Fig. S3. The peaks from the solvent were not included in the calculated ¹H NMR spectrum. In the spectrum, the displacement of H in the complex appeared mainly in the low field, which is consistent with the experimental results. The calculated FT-IR and ¹H NMR spectra of the complexes revealed that the structure of the complex we synthesized was the structure of the product we expected.

3.8

3.8 LED devices

The green LED device (Fig. 12) is made of ultraviolet chip (λem=365nm) combined with green-emitting phosphor (terbium complexes). It can be seen from the Fig. 12 that the LED device at 3.5V voltage is significantly brighter than 3V. Green light is one of the three primary colors, and the fact that terbium complex can be made into a green LED device proves that it can be used in white LEDS. The terbium complex synthesized in this paper has the advantages of simple synthesis method, clean environmental protection, low toxicity and so on, and applying in LED devices has a broad market prospect.