Magneto-optical response and luminescence properties of lanthanide–titanium–oxo clusters Eu₂Ti₇ and Sm₂Ti₇

Abstract

The study of magneto-optical effects based on the f–f emission and absorption of lanthanide ions has attracted considerable interest. In this work, we present a series of isostructural lanthanide–titanium–oxo clusters (LTOCs) Ln₂Ti₇ (Ln = La, Sm, Eu) using 3,5-di-tert-butylbenzoic acid as the ligand. A detailed comparison of the luminescence properties of Sm₂Ti₇ and Eu₂Ti₇ shows that Eu₂Ti₇ displays superior luminescence intensity, higher color purity red light, longer lifetime, and significantly higher quantum yield. These properties, along with its high stability in solution, make Eu₂Ti₇ an excellent candidate for magnetic circularly polarized luminescence (MCPL) studies. Under an external magnetic field, Eu₂Ti₇ exhibited strong MCPL signals, with the maximum |g_MCPL| value being 0.04 T⁻¹ from the ⁵D₀ → ⁷F₄ transition. In contrast, the weaker luminescence of Sm₂Ti₇ rendered MCPL analysis ineffective; however, its strong near-infrared absorption allowed for magnetic circular dichroism (MCD) studies. The MCD spectra of Sm₂Ti₇ revealed significant signals corresponding to f–f transitions in the 900–1600 nm range, with the maximum |g_MCD| value observed at 1102 nm. This work provides valuable insights into the magneto-optical properties of Ln-based clusters, emphasizing the role of energy-level analysis for further research into their potential applications in magneto-optical devices.

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10th anniversary statement

It is a great honor to contribute to the 10th anniversary collection of this esteemed journal. I had the privilege of publishing a paper on lanthanide-oxo clusters in the Emerging Investigator themed collection of Inorganic Chemistry Frontiers in 2016 (Inorg. Chem. Front., 2016, 3, 320–325). Over the years, this journal has provided an invaluable platform for reporting groundbreaking research and encouraging collaborations within China, across Asia, and internationally. Inorganic Chemistry Frontiers has played a vital role in advancing the field of inorganic chemistry. I look forward to the journal's continued growth and influence.

Introduction

Most trivalent lanthanide ions possess a rich array of uniquely arranged electron energy levels, which result in characteristic f–f absorption and emission transitions.¹ These features make Ln(III) highly valuable in spectroscopy, particularly in the unique magneto-optical properties.² An external magnetic field can induce optical activity and magneto-optical effects in both achiral and optically inactive substances, such as the Faraday effect, Zeeman effect, and polar Kerr effect.³ The interaction of light with matter in an external magnetic field is particularly noteworthy because it can lead to magnetic circular dichroism (MCD) in the ground state and magnetic circularly polarized luminescence (MCPL) in the excited state. This phenomenon arises from the Zeeman splitting in their degenerate ground and excited states of achiral and optically inactive substances.⁴

The design and synthesis of molecules containing Ln(III) with well-defined structure are essential for studying the magneto-optical effect of these ions.⁵ In the selected molecular model for investigating the magneto-optical behavior of Ln(III), the MCPL and MCD signals of Ln(III) can be obtained by choosing suitable ligands, regulating the coordination environment of Ln(III), and minimizing interference with f–f transitions.⁶ The 3d–4f clusters have the characteristics of multiple metal centers, allowing for synergistic interactions between different metal ions.⁷ However, 3d–4f clusters with both absorption and emission magneto-optical effects are extremely rare. Current studies of MCPL in lanthanide systems have mostly focused on mononuclear complexes,⁸ with the exception of the Ln₂₀ cluster,⁹ and investigations into MCD in the near-infrared range remain limited.¹⁰

Lanthanide–titanium–oxo clusters (LTOCs) represent a distinct class of 3d–4f clusters. Their core structures, due to the presence of Ti(IV) with strong Lewis acidity, allow the metal ions to be commonly interconnected via O²⁻ bridges, which helps mitigate the negative impact of O–H stretching vibrations on Ln(III) luminescence.¹¹ By selecting efficient photosensitive ligands, LTOCs with excellent luminescence properties can be achieved.¹² These clusters have recently emerged as promising candidates for Ln(III)-based luminescent materials. Furthermore, since Ti(IV) lacks d-electrons, there are no d–d transitions in the absorption spectra, ensuring no interference with the f–f transitions of the lanthanide ions.¹³ LTOCs provide new alternatives for the study of MCPL and MCD.

Herein, based on bis-tert-butyl modified carboxylic acid ligands, we report a series of lanthanide–titanium–oxo clusters, formulated as Ln₂Ti₇(μ₃-O)₆(L)₁₄(EtO)₈ (Ln₂Ti₇, Ln = La, Sm, Eu; HL = 3,5-di-tert-butylbenzoic acid). The introduction of tert-butyl groups enhances the ligand sensitization effect and improves the solubility of the cluster. Luminescence studies reveal that Eu₂Ti₇ exhibits superior luminescence properties compared to Sm₂Ti₇, with a longer lifetime and higher quantum yield. The excellent solubility and stability of the cluster in dichloromethane make it suitable for further MCPL study of Eu₂Ti₇ and near-infrared MCD study of Sm₂Ti₇. Notably, achiral Eu₂Ti₇ displays distinct and well-defined CPL signals in the external magnetic field. The energy level analysis prompted the study of the MCD of Sm₂Ti₇ in the near-infrared region, which also yielded a signal with a near-ideal peak shape. This work marks the first investigation of both emission and absorption magneto-optical effects within the same 3d–4f cluster system.

Experimental section

Materials and physical measurements

All reagents were of analytical grade, commercially sourced, and were used without further purification. The crystallographic data were collected on a Rigaku Oxford diffraction XtaLAB synergy diffractometer with micro-focus sealed X-ray Cu Kα radiation (λ = 1.54184 Å) at 100 K. The excitation and emission spectra of the room-temperature luminescence, the time-resolved PL decay curves and the absolute quantum yields were measured on the steady-state and transient fluorescence spectrometer (FLS-1000, Edinburgh). Dynamic light scattering analysis was conducted using the laser particle size analyzer (Malvern, ZSU3100). The MCPL and MCD spectra were measured on the JASCO CPL-300 and JASCO J-1700 equipped with a JASCO PM-491 compact permanent magnet (1.6 T).

Synthesis of Ln₂Ti₇

Clusters Ln₂Ti₇ (Ln = La, Sm, Eu) were synthesized through solvothermal crystallization by reacting Ln(OAc)₃·xH₂O, Ti(OⁱPr)₄, and 3,5-di-tert-butylbenzoic acid in ethanol. The solid reactants were fully dissolved by sonication in the sealed vials, followed by solvothermal reactions at 80 °C for 24–48 h. The colorless block crystals were obtained without cooling to room temperature, with yields exceeding 40% (calculated based on Ln(OAc)₃·xH₂O).

Results and discussion

Single crystal structures

Single-crystal X-ray diffraction (SCXRD) analysis revealed that the three obtained clusters, Ln₂Ti₇(μ₃-O)₆(EtO)₈(L)₁₄ (Ln₂Ti₇, Ln = La, Sm, Eu; HL = 3,5-di-tert-butylbenzoic acid), are electrically neutral and isostructural clusters, though their cell parameters differ. La₂Ti₇ and Sm₂Ti₇ crystallize in the triclinic crystal system with the space group P1̄, while Eu₂Ti₇ crystallizes in the monoclinic crystal system with the space group C2/c. To illustrate their structural features, we discuss in detail only the molecular structure of La₂Ti₇. As shown in Fig. 1a, the metal-oxo core [La₂Ti₇(μ₃-O)₆]²²⁺ is stabilized by 14 L⁻ anions and 8 EtO⁻ anions. The metal-oxo core consists of two eight-coordinated La³⁺ ions and seven six-coordinated Ti⁴⁺ ions, connected by 6 μ₃-O²⁻ bridges (Fig. 1b and c). Interestingly, the previously reported cluster LaTi₄, obtained using benzoic acid ligands, features a [LaTi₄(μ₃-O)₃]¹³⁺ metal-oxo core (Fig. S2†).¹⁴ The La₂Ti₇ can be envisioned as two [LaTi₄(μ₃-O)₃] units sharing a Ti⁴⁺ ion in a “hand in hand” arrangement. In each [LaTi₄(μ₃-O)₃] unit, the four Ti⁴⁺ ions form a non-linear circular arc, with the La³⁺ surrounded by Ti⁴⁺ bridged through O²⁻ bridges. The bond distances and angles (Tables S5–S12†) are consistent with literature values. Notably, the molecular model of Ln₂Ti₇ lacks OH⁻ bridges and features fourteen photosensitive ligands, making it suitable for luminescence studies.

Fig. 1 (a) Crystal structure of La₂Ti₇. (b) Coordination configurations of La³⁺ ion and Ti⁴⁺ ion. (c) Ball-and-stick depiction of metal-oxo core [La₂Ti₇(μ₃-O)₆]²²⁺.

Luminescence properties

Sm₂Ti₇ and Eu₂Ti₇ were selected for luminescence studies due to energy levels of Sm(III) and Eu(III) favorable match with the chosen photosensitive ligands. Room-temperature luminescence measurements of Sm₂Ti₇ and Eu₂Ti₇ at room temperature were performed, with the results presented in Fig. 2. Fig. 2a displays the excitation and emission spectra of Sm₂Ti₇, which exhibits Sm(III) characteristic emission peaks at 562 nm (⁴G_5/2 → ⁶H_5/2), 597 nm (⁴G_5/2 → ⁶H_7/2), 644 nm (⁴G_5/2 → ⁶H_9/2), and 706 nm (⁴G_5/2 → ⁶H_11/2) upon irradiation at 339 nm or 405 nm (⁶H_5/2 → ⁶P_3/2).¹⁵Fig. 2b presents the excitation and emission spectra of Eu₂Ti₇, where irradiation at 339 nm or 395 nm (⁷F₀ → ⁵L₆) leads to characteristic Eu(III) emissions at 580 nm (⁵D₀ → ⁷F₀), 593 nm (⁵D₀ → ⁷F₁), 618 nm (⁵D₀ → ⁷F₂), 650 nm (⁵D₀ → ⁷F₃) and 701 nm (⁵D₀ → ⁷F₄).¹⁶ In Fig. 2c, the relative emission intensities of Sm₂Ti₇ and Eu₂Ti₇ are compared, along with a reference for emission peak widths. The luminescence of Sm₂Ti₇ is primarily dominated by two f–f transitions: ⁴G_5/2 → ⁶H_7/2 and ⁴G_5/2 → ⁶H_9/2, whereas the luminescence of Eu₂Ti₇ is predominantly driven by the ⁵D₀ → ⁷F₂ transition. The emission peak widths of Sm₂Ti₇ are broader compared to Eu₂Ti₇, indicating higher color purity for Eu₂Ti₇.

Fig. 2 (a) Excitation and emission spectra of Sm₂Ti₇. (b) Excitation and emission spectra of Eu₂Ti₇. (c) Relative emission intensities of Sm₂Ti₇ and Eu₂Ti₇. (d) Physical photographs and CIE color coordinates of Sm₂Ti₇ and Eu₂Ti₇. (e) Excited-state decay curves of Sm₂Ti₇ and Eu₂Ti₇. (f) Quantum yield results of Sm₂Ti₇ and Eu₂Ti₇.

Under daylight, the crystal samples of Sm₂Ti₇ and Eu₂Ti₇ appear colorless, but exhibit red luminescence when exposed to ultraviolet light. The CIE color coordinates for Sm₂Ti₇ are (x = 0.6179, y = 0.3815), and for Eu₂Ti₇ are (x = 0.6632, y = 0.3365). Both coordinates fall within the orange-red region of the CIE chromaticity diagram, consistent with the observed luminescence. Notably, there is a visible difference in luminous intensity between the two samples, with Eu₂Ti₇ exhibiting significantly stronger emission than Sm₂Ti₇ (Fig. 2d). The excited-state decay curves, presented in Fig. 2e, indicate lifetimes of 72.6 μs (χ² = 1.1564) for Sm₂Ti₇ and 1.618 ms (χ² = 1.0182) for Eu₂Ti₇.

The photo-luminescence process in lanthanide coordination system generally involves three stages: energy absorption, energy transfer to the Ln(III) emission level, and the subsequent f–f transitions that produce luminescence. Energy absorption occurs primarily through photosensitive ligands or intrinsic f–f transitions of Ln(III). Different absorption modes correspond to distinct energy transfer pathways.¹⁷ In the first case, upon the ligand's absorption of energy, the energy populates the excited singlet energy level (S_n) of the ligand. Subsequently, it relaxes to the lowest singlet energy level (S₁) of the ligand. Then, intersystem crossing (ISC) takes place, leading to the energy populating the lowest triplet energy level (T₁) of the ligand. Afterwards, energy is transferred from the T₁ level to the excited state energy level of Ln(III). Once the energy reaches the emission energy level of Ln(III), radiative emission occurs. In the second case, Ln(III) absorbs energy via its intrinsic f–f transition, attaining a high excited state energy level of Ln(III). Subsequently, the energy relaxes to the emission level of Ln(III) through a non-radiative decay process and then radiative emission takes place. In the excitation spectra of Sm₂Ti₇ and Eu₂Ti₇, strong and broad excitation peaks are attributed to ligand-based energy absorption, while strong but narrow excitation peaks arise from direct f–f transitions. Quantum yields (QYs) for Sm₂Ti₇ and Eu₂Ti₇ were measured for the two excitation modes at their strongest excitation wavelengths (Fig. 2f). Under ligand-sensitized excitation (339 nm), the QYs of Sm₂Ti₇ and Eu₂Ti₇ are 2.62% and 49.68%, respectively. For Sm₂Ti₇, when excited by the strongest f–f transition ⁶H_5/2 → ⁶P_3/2 (excitation wavelength at 405 nm), the QY is 2.30%. For Eu₂Ti₇, when excited by the strongest f–f transition ⁷F₀ → ⁵L₆ (excitation wavelength at 395 nm), the QY is 86.14%.

Luminescence studies reveal that Eu₂Ti₇ exhibits stronger emission intensity, higher color purity red light, longer luminescence lifetime and higher luminescence efficiency, compared to Sm₂Ti₇ (Table 1). The difference in luminescence performance between Eu₂Ti₇ and Sm₂Ti₇ can be understood by analyzing their energy levels. When considering the ligand sensitization pathway, the emission levels of Sm(III) and Eu(III) are quite close (⁴G_5/2: 17 924 cm⁻¹; ⁵D₀: 17 286 cm⁻¹). This suggests that the ligand absorption and energy transfer processes leading to the emission levels should be similar in both Sm₂Ti₇ and Eu₂Ti₇. However, the lower QY of Sm₂Ti₇ points to significant energy loss during the transition from the emission level (⁴G_5/2) to the lower levels (⁶H_J). A comparison of energy level diagrams reveals that Eu(III) transitions directly from the ⁵D₀ level to the ⁷F_J levels without intermediate excited states. In contrast, Sm(III) has additional intermediate states (⁶F_J) between the ⁴G_5/2 and ⁶H_J levels. These intermediate states allow for non-radiative decay, contributing to energy loss and negatively impacting the luminescence of Sm(III). Interestingly, f–f transitions from the ground state (⁶H_5/2) to these intermediate excited state levels were observed in the diffuse reflection spectrum of Sm₂Ti₇, showing strong absorption in the near-infrared region (Fig. S9c†).

Table 1 Results of the luminescence properties of Ln₂Ti₇

Ln2Ti7	f–f transitions & emission peaks	CIE	Lifetime	QY (Ex)
Sm2Ti7	^4G5/2 → ^6HJ	(x = 0.6179, y = 0.3815)	72.6 μs	2.3% (405 nm), 2.6% (339 nm)
	562 nm (J = 5/2), 597 nm (J = 7/2), 644 nm (J = 9/2), 706 nm (J = 11/2)
Eu2Ti7	^5D0 → ^7FJ	(x = 0.6632, y = 0.3365)	1.618 ms	86.1% (395 nm), 49.7% (339 nm)
	580 nm (J = 0), 593 nm (J = 1), 618 nm (J = 2), 650 nm (J = 3), 701 nm (J = 4)

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Stability in solution

Eu₂Ti₇ was chosen as a representative to validate the stability of Ln₂Ti₇ in dichloromethane (CH₂Cl₂). Dynamic light scattering (DLS) analysis revealed an average particle size of Eu₂Ti₇ at 2.1 ± 0.5 nm (Fig. 3a), consistent with the theoretical particle size determined by single crystal structure analysis. To further assess the stability of Eu₂Ti₇, emission spectra were characterized at various concentrations, ranging from 10^–2 to 10^–5 M. The large solubility range benefits from the modification of the tert-butyl groups on the ligands. The results showed that the Eu₂Ti₇ solution emits red light under ultraviolet irradiation, with emission peaks consistent with the characteristic Eu(III) transitions observed in the solid state (Fig. 3b). As the concentration decreased, the emission intensity weakened, but the spectral shape and peak positions remained unchanged. This consistency suggests that Eu₂Ti₇ retains its structural integrity in CH₂Cl₂. Interestingly, after allowing the CH₂Cl₂ solution of Eu₂Ti₇ to evaporate, colorless block crystals formed. SCXRD analysis confirmed that these crystals were Eu₂Ti₇, but with a change in space group from C2/c to P1̄ (Fig. 3c), likely due to the incorporation of CH₂Cl₂ molecules into the crystal lattice (Fig. S1c†). The recrystallized of Eu₂Ti₇ also highlights the solubility and stability in CH₂Cl₂.

Fig. 3 (a) Dynamic light scattering analysis of Eu₂Ti₇. (b) Emission spectra of Eu₂Ti₇ solutions at various concentrations. (c) The space group transitioned from C2/c to P1̄ after recrystallization of Eu₂Ti₇.

Magneto-optical response

Given the remarkable luminescence properties and high stability of Eu₂Ti₇, the magnetic circularly polarized luminescence (MCPL) properties were studied in CH₂Cl₂ solution under an external magnetic field of 1.6 T. A CH₂Cl₂ solution of Eu₂Ti₇ with a concentration of 1 × 10^–3 M was prepared. As illustrated in Fig. 4a, Eu₂Ti₇ displays distinct MCPL spectra (⁵D₀ → ⁷F_J) upon excitation at 328 nm (Fig. S7†), with significant CPL signals (ΔI) observed in the wavelength ranges of 585–605 nm (J = 1), 605–630 nm (J = 2) and 675–715 nm (J = 4), corresponding to the overall emission (I). The signals are primarily dominated by Faraday A-term.¹⁸ In contrast, the Eu₂Ti₇ in solution did not show the CPL signal in the absence of an external magnetic field, confirming that the external magnetic field induces the CPL signal of achiral Eu₂Ti₇. The MCPL spectra under the N-up and S-up magnetic fields are nearly mirror images of each other.

Fig. 4 (a) The MCPL (upper) and PL (lower) spectra of Eu₂Ti₇ in solution state under the 1.6 T external magnetic field. (b) Correspondence between g_lum value and wavelength in MCPL of Eu₂Ti₇. (c) Energy level diagram of emission and absorption f–f transitions for Sm(III). (d) The near-infrared MCD (upper) and absorption (lower) spectra of Sm₂Ti₇ in solution state under the 1.6 T external magnetic field.

To quantitatively analyze the MCPL results for different f–f transitions of Eu₂Ti₇, the luminescence asymmetry factor g_lum (g_lum = ΔI/I) is used.¹⁹ In MCPL, the g_lum value should be normalized by the magnetic flux density (B) of the external magnetic field, giving g_MCPL = g_lum/B (T⁻¹). The g_lum values for Eu₂Ti₇ are presented in Fig. 4b, with the peak values summarized in Table S3.† For chiral Eu(III)-contained systems without an external magnetic field, the largest g_lum value typically occurs during the magnetic dipole transition ⁵D₀ → ⁷F₁. However, in Eu₂Ti₇, the magnetic field has the greatest influence on the electric dipole transition ⁵D₀ → ⁷F₄, followed by the magnetic dipole transition ⁵D₀ → ⁷F₁, and then the electric dipole transition ⁵D₀ → ⁷F₂. The maximum |g_MCPL| for the ⁵D₀ → ⁷F₄ transition is observed at 709 nm with a value of 4.0 × 10^–2 T⁻¹. For the ⁵D₀ → ⁷F₁ transition, the maximum |g_MCPL| is 1.5 × 10^–2 T⁻¹ at 586 nm. Lastly, for the ⁵D₀ → ⁷F₂ transition, the maximum |g_MCPL| is 0.8 × 10^–4 T⁻¹ at 625 nm. In contrast, accurate MCPL analysis for Sm₂Ti₇ was not feasible due to its weak emission intensity and the interference of background noise. This highlights the necessity of strong luminescence intensity as a prerequisite for effective MCPL measurement.

Although the suboptimal luminescence of Sm₂Ti₇ hinders obtaining a clear MCPL signal, its strong absorption (arising from ⁶H_5/2 → ⁶F_1/2–11/2 transitions) in the near-infrared region is of highly significant for magnetic circular dichroism (MCD) studies. As shown in Fig. 4d, the CD signals (ΔA) for Sm₂Ti₇ were recorded at the external magnetic field strength of 1.6 T, corresponding to the overall absorption (A). When the magnetic field direction was reversed, the CD signals maintain the magnitude but exhibit a mirrored distribution. The MCD results were further analyzed by calculating the g_MCD factor, defined as g_MCD = g_abs/B (where g_abs = ΔA/A is the absorption asymmetry factor²⁰). For Sm₂Ti₇, the maximum |g_MCD| value is 2.1 × 10^–2 T⁻¹ at 1102 nm, corresponding to the electric dipole transition ⁶H_5/2 → ⁶F_9/2. Additionally, for the electric dipole transition ⁶H_5/2 → ⁶F_11/2, the magnetic dipole transition ⁶H_5/2 → ⁶F_5/2 and the magnetic dipole transition ⁶H_5/2 → ⁶F_3/2, the maximum |g_MCD| values of these transitions are on the order of 10^–2 T⁻¹.

Conclusion

In conclusion, we report a series of isostructural LTOCs Ln₂Ti₇ (Ln = La, Sm, Eu), using 3,5-di-tert-butylbenzoic acid serving as the ligand. A detailed comparative analysis of the luminescence properties revealed that Eu₂Ti₇ exhibits significantly superior luminescence characteristics compared to Sm₂Ti₇, including stronger emission intensity, higher quantum yield, and longer lifetime. We also conducted a study of the magnetic circularly polarized luminescence (MCPL) of Eu₂Ti₇, demonstrating its responsiveness to external magnetic fields and showcasing its potential for magneto-optical applications. The energy-level analysis explaining the luminescence differences between Eu₂Ti₇ and Sm₂Ti₇ further motivated our research into the magnetic circular dichroism (MCD) properties of Sm₂Ti₇, particularly in the near-infrared region. This study not only contributes to the understanding of magneto-optical effects in lanthanide-based clusters but also emphasizes the importance of analyzing the energy levels of Ln(III) ions for advancing magneto-optical research.

Data availability

All relevant data are within the manuscript and ESI.† The data are available from the corresponding author on reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful for the financial support from the National Natural Science Foundation of China (Grant No. 92161104, 92161203, and 92361301).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2392362–2392364 and 2392375. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi02645d

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