Enhancement of Thermal Sensitivity of MOF-5 Doped with Eu³⁺ and Addition of Mn²⁺

Abstract

This work investigates the potential of Eu³⁺ and Eu³⁺-Mn²⁺ doped metal-organic frameworks (MOFs) of the MOF-5 type as a temperature sensor (298-393 K). The MOF-5 were synthesized via reflux and characterized using X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and spectroscopic techniques. The emission spectra of the MOFs were obtained in response to temperature variations to define the thermometric configurations and sensor performance. The MOF-5 containing only Eu³⁺ showed low thermal sensitivity in the temperature range between 298-393 K, with a maximum sensitivity of 0.12% K^-1. In turn, the addition of Mn²⁺ to MOF-5 (MnEu-MOF-5) was examined in a specific increase in thermal sensitivity close to 8% K^-1 estimated at the temperature of 303 K. Computational calculations were important for the structural elucidation of MOF-5 with Eu³⁺ and understanding the energy transfer processes involved. The results show that the incorporation of Mn²⁺ into EuMOF-5 may be a promising candidate for application as a luminescent thermometer, since it presented high sensitivity for temperature measurement close to 303 K.

Keywords:
MOFs; europium; manganese; luminescent thermometers; thermal sensitivity

Introduction

Metal-organic frameworks (MOFs) are an attractive option for combining metal ions coordinated with organic ligands. In recent decades, interest in these materials has increased and has been sparked by the diverse combinations of cations and organic ligands, which allows these porous materials to have various applications (catalysis, thermal sensors, biomedical, technological, and environmental applications).¹ This potential was recently recognized with the Nobel Prize in Chemistry (2025) awarded to laureates Susumu Kitagawa, Richard Robson, and Omar M. Yaghi, who in the 1990s presented the first groundbreaking studies of these porous materials.²

In these materials, the metal centers, ligands, ions, or guest molecules (located in the pores) are potential sources of light emission, in which luminescence can occur via sensitization of the metal by ligands, metal-ligand charge transfers, or host-guest interactions that can influence energy transfer processes (ET).³

The combination of MOFs with lanthanide ions (Ln³⁺) presents undemanding luminescence characteristics, since these ions have specific electronic, magnetic, and optical properties that are affected by the chemical environment due to the shielding of the 4f electrons by the 5s and 5p electrons. Furthermore, Ln³⁺ ions can also form an advantageously higher number of integration bonds than transition metal (TM) ions, offering more divergent development geometries for bonding with organic ligands in the construction of MOF networks and forming materials with greater structural stability.⁴

Ln-MOFs are promising materials because they exhibit narrow-band luminescence across a broad range in the visible or near-infrared (NIR) spectral regions, with enhanced intensity through energy transfer (ET) from the excited state of the ligand to the lanthanide ion. This favors the application of these materials such as temperature sensors, catalysts, adsorption/separation, and analyte identification.⁴,⁵

In this context, luminescent compounds obtained from Ln³⁺-doped MOFs allow a diverse combination of ligands and ions for application as temperature sensors, with tunable luminescence properties that differentiate them from conventional organic and inorganic materials. Covering a wide temperature range, cryogenic (T < 100 K), physiological (298-323 K) and high temperature (up to 473 K) values.³,⁶

MOFs doped with Ln³⁺, mainly Eu and Tb, have been widely reported as luminescent thermometers, achieving satisfactory results.⁷ Among the reported materials, MOF-5 first reported by Yaghi and co-workers⁸ is already known for different applications, such as gas sorption, dyes, drug loading, thermal sensor and catalysts. Its structure is composed of an octahedral zinc-based cluster [Zn₄O]⁶⁺, surrounded by organic ligands of terephthalic acid (1,4-benzenedicarboxylic acid) to form microporous cubic 3D structures.⁹ Xia et al.¹⁰ explored the Eu and Tb doped MOF-5 as a temperature sensor, obtaining promising results of thermal sensitivity (1.8% K^-1) at high temperature 473 K.

In turn, the combination of Ln³⁺ and TM in MOFs is less explored compared to inorganic materials such as temperature sensors. Among transition metals, Mn²⁺ with electronic configuration d⁵ is one of the most reported in luminescent thermometry, due to the high susceptibility of the emission intensity of Mn²⁺ (T₁ → ⁶A₁) as a function of temperature, which provides luminescent thermometers with high relative sensitivities.¹¹-¹³

In general, the spectral position of the ⁴T₁ → ⁶A₁ emission band of Mn²⁺ depends on crystal field strength and coordination. Broad emission bands can be observed between 480 and 700 nm, with two bands centered at 515 nm and 575-600 nm, corresponding to tetrahedral and octahedral ion coordination, respectively. Octahedral coordination is favored with a higher Mn²⁺ concentration.¹⁴,¹⁵ The synergy between Mn²⁺ and Eu³⁺ is known and discussed mainly in inorganic luminescent thermometers, involving energy transfer processes between the ⁴T₁ level of Mn²⁺ and the emitting levels of Eu³⁺ (⁵D₀ and ⁵D₁), favoring orange (⁵D₀ → ⁷F₁) and red (⁵D₀ → ⁷F₂) emission.¹⁶,¹⁷

Therefore, we propose to investigate the effect of manganese (Mn²⁺) addition to Eu³⁺-containing MOF-5 on the emission spectrum and energy transfer processes in enhancing luminescence. In this work, we evaluated the potential thermal sensitivity of the EuMOF-5 and MnEuMOF-5 to temperature variation.

Experimental

Materials and reagents

Eu³⁺ nitrate was obtained by pre-treating the metal oxide: Eu₂O₃ (Aldrich 99.99%) in an acidic medium (HNO₃, Alphatec 67%). The following reagents were used without prior treatment: terephthalic acid, C₈H₆O₄ (Sigma-Aldrich 98%), N,N-dimethylformamide, HCON(CH₃)₂, (Dynamics 99.8%), Zn(NO₃)₂.6H₂O (Dynamic 93%), Mn(NO₃)₂.4H₂O (Neon 97%).

Synthesis of EuMOF-5 and MnEuMOF-5

MOF-5 doped with the metal ions (Eu³⁺ and Eu³⁺-Mn²⁺) was synthesized via the reflux method.¹⁰ In this approach, the salts of Zn(NO₃)₂.6H₂O (5.6 mmol) and Eu(NO₃)₃ (5% relative to Zn(NO₃)₂) were dissolved in 20 mL of dimethylformamide (DMF) in a three-necked flask and stirred for 30 min. After dissolving the salts, the 1,4-benzenedicarboxylic acid (H₂BDC) solution (2.4 mmol) dissolved in 20 mL of DMF were added to the flask. The mixture was then heated under reflux at 125 ºC for 4 h. The material formed was cooled naturally, washed with DMF and chloroform, repeating the process 3 times, and after washing it was dried under vacuum. MOF-5 doped with Mn and Eu was synthesized using the same procedure. The proposed concentration for doping MOF-5 with Mn²⁺ and Eu³⁺ was kept at 5% in relation to the amount of Zn²⁺, with the concentration of Mn²⁺ being higher than that of Eu³⁺: 90 and 10%, respectively.

Characterization of EuMOF-5 and MnEuMOF-5

X-ray diffraction (XRD) of EuMOF-5 and MnEuMOF-5 were performed on a Bruker model D8 Advance. Under the conditions of 40 kV and 30 mA, step size 0.02 θ, time per step 1.0 s with angle 2θ ranging from 5° to 50°. Fourier-transformed infrared (FTIR) spectrophotometer from PerkinElmer was obtained using model Spectrum 400, between 4000 and 400 cm^-1, with spectral resolution of 2 cm^-1 and 64 accumulations. Thermogravimetry (TGA) analyses were performed on a Shimadzu thermobalance, model TA 60 DTG-60H, using an alumina sample holder, 100 mL min^-1 flow, N₂ atmosphere and heating rate of 10 °C min^-1 up to 800 °C. The morphology of the precursor materials was studied by scanning electron microscope (SEM), TESCAN - MIRA 3 scanning electron microscope, with 10 kV acceleration. The emission and excitation spectra were measured on a Horiba Jobin Yvon Fluorolog-3 ISA spectrofluorometer, equipped with a model FL-1039/40 monochromator, with a 450 W xenon lamp, a Hamamatsu R928P photomultiplier and a 150 W pulsed xenon lamp. Emission spectra were measured using 2 and 1 nm slits for excitation and emission, respectively, 0.5 increment and 365-720 nm sweep. Thermal characterizations were carried out using a heating plate connected to a CT-HP model temperature controller.

Results and Discussion

Structural characterization

The XRD pattern of EuMOF-5, Figure 1, shows reflections characteristic of MOF-5 near 2θ = 6.8°, 9.6°, 13.7° and 15.3° which correspond to the (002), (022), (004) and (024) crystal planes, respectively. These reflections correspond to the cubic phase, with lattice parameters a = b = c = 25.88 Å; α = β = γ = 90º, and space group Fm-3m, according to the structural and crystallographic data consulted from the information contained in crystallographic information file number 282411 in the Cambridge Crystallographic Data Centre (CCDC) database. The peaks intensity of EuMOF-5 differs from the intensities expected for the cubic MOF-5 diffraction pattern, which must be attributed to the greater distortion of the structure caused by the addition of Eu³⁺ with a larger ionic radius and higher coordination number.

Figure 1
X-ray diffraction patterns of MOFs: MnEuMOF-5, EuMOF-5 and simulated MOFs.

Additionally, the presence of unindexed peaks suggested the formation of other crystal structures, and alongside the usual MOF-5 cubic structure, another phase of this MOF was detected. This structure is reported as a trigonal system structure deposited in Cambridge Crystallography Data Centre (CCDC) number 645079, and the lattice parameters of this are: a = b = 18.382 Å; c = 44.97 Å; α = β = 90º; γ = 120º, with R-3m space group. Other structure detected was the MOF EuBDC reported in CCDC number 891471, with triclinic symmetry and lattice parameters a = 11.0552 Å; b = 11.0713 Å; c = 17.6488 Å; α = 93.0840º; β = 104.429º; γ = 119.032º.

In the MnEuMOF-5 material, reflections near 2θ = 6.8°, 9.6° also show the formation of a MOF-5 type phase. Conventionally, in MOF-5 the zinc is coordinated in tetrahedral form (ZnO₄), and these tetrahedrons form a porous 3D coordination polymer joined by the bond with the terephthalate anion, which formed a cubic cell.¹⁸

The XRD pattern in the EuMOF-5 material (Figure 1) shows an additional low-intensity reflection near 8.8° which can be attributed to small distortions in the symmetrical structure of the MOF.¹⁹ This break in symmetry can be explained by repulsions and distortions caused by the addition of Eu³⁺, a cation with a larger ionic radius (1.07 Å) and charge compared to Zn²⁺ (0.74 Å).

Chen et al.²⁰ report that the appearance of this small reflection (8.8°) can be attributed to the hydrolysis of zinc species, Zn(OH)₂, in the MOF structure.¹⁸ However, the formation of these species cannot be confirmed from the characterizations performed in the present study, since the thermogravimetric and FTIR results are not sufficient to identify this phase. The characteristic absorption band of Zn-OH is evidenced at 3600 cm^-1 and in this case, it was not identified in Figure 2, the thin band close to 3608 cm^-1 in this study is attributed to the combination of the water molecule with a metallic center.¹⁷ On the other hand, additional low-intensity reflections characteristic of the formation of the [Ln₂(BDC)₃(DMF)₂(H₂O)] phase is seen in Figure 1, consulted based on information from CCDC 891471.²¹ The formation of this additional phase was optimized using the MOPAC program (MOPAC2016, version 21.273, Stewart Computational Chemistry, Colorado Springs, CO, USA), and the proposed structures agree with the theoretical and experimental luminescence data presented in this study.

Figure 2
FTIR (ATR) spectra of the MOFs: MnEuMOF-5, EuMOF-5, MOF-5.

The XRD data was analyzed in GSAS-II (version 5280, Robert Von Dreele; Brian H. Toby, Argonne National Laboratory, Lemont, IL USA, 2013), and the method of intensity extractions of Le Bail et al.²² was conducted to index the peaks in the phases of MOF-5 trigonal and EuBDC MOF (Figure S1, Supplementary Information (SI) section). A preliminary Rietveld Refinement was conducted to elucidate the structure of the trigonal MOF 5 (Figure S2, SI section), once the intensities obtained experimentally do not match with the intensity of peaks reported by the structure deposited in CCDC. Although the result of Rietveld refinement is not satisfactory and needs further improvement (goodness of fit factor (GOF) = 11.88, weighted residual (Rw) = 12.71%), the difference of intensity in the peaks suggest that the atomic positions, occupations or thermal parameters need adjustments. However, the Le Bail intensity extraction conducted in both phases resulted in good peak fitting (GOF = 4.16, Rw = 4.99%), confirming that the structures obtained have the unit cell size and symmetry equal to the ones reported in CCDC.

Based on this understanding, powder XRD analysis and the reaction procedure, the starting geometries were proposed from the crystallographic structure CCDC 891471, which has two distinct Eu³⁺ sites, both octacoordinated in a distorted square antiprism with BDC, H₂O and DMF.²¹ On the Eu³⁺ (Eu1) site, coordination is observed with six oxygen atoms from the carboxylate groups, six different monodentate coordination ligands from BDC, one oxygen atom from DMF and one water molecule (Figure 3a). On the other Eu³⁺ site (Eu2), there is coordination with six oxygen atoms from the carboxylate groups of four different monodentate coordination ligands and one bidentate coordination ligand from BDC, one oxygen atom from DMF and one water molecule (Figure 3b). It should be noted that the Eu³⁺1 and/or Eu³⁺2 systems may be in the pore cavity and/or on the surface of MOF-5. These proposed structures agree with experimental and theoretical luminescence data.

Figure 3
Proposed molecular structures for the systems Eu³⁺: (a) Eu1 (b) Eu2. The proposed systems were optimized using MOPAC software.

The reflection near 6.8º is less intense when compared to the MOF-5 pattern (Figure 1), although there is agreement with the position of this diffraction peak. The low intensity at 2θ ≃ 6.8° is associated with the amount of solvent or other molecules that may be present in the pores of the MOF.¹⁷ In addition, the presence of zinc species trapped in the cavities and interpenetration of the network are also reported as causes of the pronounced variations in the intensities of the diffraction peaks.

The intensity of the reflection near 9.6° is independent of the number of molecules present in the pores of the MOF, and the appearance of splits in this peak indicates that the structure shows a slight distortion of cubic symmetry. The presence of these splits in the reflection near 9.6° reinforces the occurrence of the formation of additional phases containing Eu³⁺ with BDC, H₂O and DMF.

In turn, the MOF containing MnEu show more reflections characteristic of segregated phases of the Mn-1,4-BDC type CCDC 663975.²³,²⁴ The characteristic reflections of MOF 5 are also observed near 2θ ≃ 6.8° and 9.6°, suggesting the formation of the phase.²⁵,²⁶ In the material containing MnEu, it is possible to see mixtures of phases with reflections typical of MOF-5 and additional reflections of the [Ln(BDC)₃(DMF)₂(H₂O)] or Mn (BDC)₃(DMF)₂ type, suggesting greater competition between Mn²⁺ and Eu³⁺ in the formation of segregated phases with the ligand.

Although it only provides information about the morphology of the material, the appearance of the cubic shape in the SEM image (Figure 4) is another indication of the formation of MOF-5, since it agrees with the cubic topology characteristic of the formation of this MOF and with the structure determined by XRD.¹⁰,²⁰ In turn, the MnEuMOF-5 material showed the formation of more irregular cubes compared to EuMOF-5.

Figure 4
SEM images of MOFs: (a) EuMOF-5 (10 µm), (b) EuMOF-5 (20 µm), (c) MnEuMOF-5 (10 µm), (d) MnEuMOF-5 (20 µm).

The images shown in Figure 4 reinforce that the Eu³⁺ doped MOF-5 has a smooth surface, and the cubes formed are micrometric in size. The materials obtained in this study were close to 50 μm in size, which agrees with the size range reported for this type of MOF.¹⁹,²⁷,²⁸

In the materials obtained, the presence of five elements was identified: C, O, Zn, Mn and Eu: C, O, Zn, Mn and Eu (Table S1, SI section), in which it is possible to highlight the higher concentration of Mn²⁺ compared to Eu³⁺ in EuMnMOF-5.

The FTIR spectra of the MOFs: EuMOF-5 and MnEuMOF-5 (Figure 2) show bands characteristic of the terephthalate ligand. The symmetrical and antisymmetric vibrations of the carboxyl groups (COO^-) correspond to the absorption bands around 1577 and 1370 cm^-1, respectively, and indicate the coordination of the carboxyl group with a metal center. The absorption bands in the 1230 and 950 cm^-1 range correspond to the in-plane bending vibrations of the C-H groups in the benzene ring, while the peaks in the 800 to 700 cm^-1 regions correspond to the out-of-plane bending vibrations of the C-H groups. In turn, the bands located in the range of 517 and 576 cm^-1 are attributed to the vibrations of the coordinated metal centers M-O, in which M may correspond to the metal ions present in EuMOF-5 or MnEuMOF-5 (Zn²⁺, Eu³⁺, Mn²⁺).¹⁸,¹⁹

The band around 1668 cm^-1 can be attributed to the ν(C=O) stretching vibration of the coordinated DMF molecule. The band is slightly shifted compared to the ν(C=O) stretching vibration band of a free DMF (1657 cm^-1). The presence of this band in the EuMOF-5 and MnEuMOF-5 materials corroborates the additional reflections shown in the XRD patterns (Figure 1), in which Eu³⁺ is coordinated with BDC, H₂O and DMF.

The broader band seen in the materials between 3400 3100 cm^-1 can be attributed to the sample’s O-H stretching vibrations of water molecules. A thin band is also observed around 3608 cm^-1 which is associated with the combination of the water molecule with a metal center.¹⁹ In general, materials spectra agreed regarding the shapes positions of the ligand absorption bands, suggesting that the BDC^2- ligand is coordinated and the structures are similar.

The mass loss profile of the MOFs obtained in this study, Figure 5, shows similar behavior to the mass loss of MOF-5. The first mass loss occurs in the temperature range around 100-200 ºC, related to removing DMF molecules and water from the MOF.¹⁰,²⁹

Figure 5
Thermogravimetric curves of EuMOF-5, MnEuMOF-5 and MOF-5.

MOFs show thermal stability from 200 up to 400 ºC, at which point the solvent molecules have already been removed. The second mass loss event is associated with the decomposition of the MOF-5 structure, which occurs from 400 ºC up to around 550 ºC. From 550 ºC onwards, there is no longer any variation in mass with increasing temperature due to the formation of metal oxides.¹⁸,¹⁹

Photophysical properties

The luminescent properties EuMOF-5 were evaluated after excitation with UV light at 320 nm since this wavelength showed the highest luminescence intensity of the ⁵D₀ → ⁷F₂ transition of Eu³⁺ (Figure 6). The broad band shown in the excitation spectrum is attributed to the S₀ → S₁ (π-π*) transitions of the HBDC^1- anion, the sensitization of the BDC ligand to the metal center is observed in the wavelength range measured. In this MOF, the emission spectrum of Eu³⁺ is evidenced due to the transition from ⁵D_1,0 to the ⁷F_J levels, where J = 0, 1, 2, 3 and 4.

Figure 6
EuMOF-5 excitation and emission spectra. Measurements were performed at room temperature on powder samples.

When excitation is performed in 394 nm, region characteristic of europium absorption (⁵L₆ ← ⁷F₀), a discrete broad band is observed near 430-540 nm, associated with residual phosphorescence. The presence of this band near 430 540 nm may be an indication that at this wavelength the energy transfer between the ligand and the metal ion is less efficient, when compared to the other excited emission spectra in the broadband region of the ligand (300, 310 and 320 nm), where the broadband is not evident, or even due to the energy transfer from the ⁵L₆ → T₁ level.

In Figure 6, only one peak was detected for the ⁵D₀ → ⁷F₀ transition. This material contains only one type of symmetry site for Eu³⁺, where the Eu³⁺ ion is in a C_n, C_nv, or C_s site symmetry.³⁰,³¹ The lifetime (τ) = 0.55 ms, measured at room temperature (Table 1) of the emission from the EuMOF-5 sample, was adjusted using a monoexponential decay model: ExpDecay1 from OriginPro (version 9.0, OriginLab Corporation, Northampton, Massachusetts, USA, 2012) (R² > 0.999).

Table 1
Lifetimes (τ) in 10^-3 s, Judd-Ofelt intensity parameters Ω₂ and Ω₄, radiative (Arad) and nonradiative (A_nrad) decay rates, intrinsic quantum efficiency (η) and quantum yield () for the Eu³⁺ systems obtained from luminescence measurements and comparison with theoretical values of the proposed structures

Sample	τ / (10-3 s)	Ω2 / (10-20 cm2)	Ω4 / (10-20 cm2)	Arad / s-1	Anrad / s-1	η / %	/ %
EuMOF-5 experimental	0.55	9.63	6.42	449.61	1372.88	24.67	-
Eu1	-	9.63	6.40	435.50	1386.99	23.90	23.61
Eu2	-	9.64	6.41	435.86	1386.63	23.92	23.62
MnEuMOF-5 experimental	0.48	13.3	10.07	424.8	1755	20.2	-

The experimental lifetime calculated from a monoexponential fit can be justified, according to the kinetic model discussed by Carneiro Neto et al.³²,³³ in which when energy transfer between structurally distinct sites occurs faster than radiative decay, the excited populations equilibrate before emission, producing a single observable time constant. Thus, even if the theoretical model predicts two sites with slightly different binding fields, the coupling efficiency between them and the similarity of their electronic environments can result in experimentally indistinguishable emission, translated into a monoexponential decay.

This observation is consistent with the behavior of the intensity parameters Ω₂ and Ω₄, which reflect the local symmetry and polarizability of the ligand field. Small variations in the calculated values of these parameters between the two sites suggest subtle structural differences, but these are insufficient to generate a significant energy separation in the ⁵D₀ → ⁷F_J transitions. Thus, the Eu³⁺ emission may be dominated by a single effective photophysical environment, whose contribution overwhelms the theoretically predicted local variations. Thus, the results consolidate that EuMOF-5 presents two effectively distinct crystallographic sites, but with coupled photophysical properties, which leads to a homogeneous luminescent behavior and a characteristic single-site emission in the spectrum.

The distortion in the symmetrical environment of Eu³⁺ can be inferred from the luminescence intensity ratio (R) between the electric dipole ⁵D₀ → ⁷F₂ and magnetic dipole ⁵D₀ → ⁷F₁ transitions. In this case, R = (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁) is around 5.45. This value, greater than 1, suggests that the arrangement of the effective charges in the vicinity of the Eu³⁺ ions have low crystal field symmetry.³⁰,³⁴ It is important to note that the significant area of the ⁵D₀ → ⁷F₁ transition contributed to the reddish orange coloration in the luminescence.³⁵,³⁶ The ⁵D₀ → ⁷F₂ hypersensitive transition (around 612 nm) is the most intense, which gives the color a characteristic red hue in materials with Eu³⁺ ion systems present.

Table 1 presents experimental (obtained from the emission spectra) and theoretical values of the quantum yield (φ_Ln^Ligand) radiative (A_rad) and non-radiative (A_nrad) rates and of the Judd-Ofelt intensity parameters (Ω₂ and Ω₄), where good agreement is observed. The calculations were performed using the LUMPAC software,³⁷ the Judd-Ofelt parameters Ω₂ and Ω₄ are strongly correlated with the symmetry around the Eu³⁺ ion.³⁸ The systems presented in Figure 3 present theoretical values of Ω₂ and Ω₄ close to those obtained experimentally, contributing to justify the structure proposed. The quantum efficiency values obtained for the Eu³⁺1 and Eu³⁺2 systems were 23.60 and 23.66%, respectively, calculated from experimental luminescence data.

In the MnEuMOF-5 material shown in Figure 7, a higher luminescence intensity of the ⁵D₀ → ⁷F₂ transition of Eu³⁺ is observed under excitation at 300 nm, the region of highest intensity in the excitation spectrum. However, the luminescent properties against temperature variation were investigated from excitation at 320 nm, to use the same excitation conditions in the materials with and without Mn²⁺. The broad emission band observed between 365 490 nm in the materials containing Mn²⁺ presents a shift to the blue region, when compared to MOF-5 (black line), indicating that adding Mn²⁺ influences the change in the emission spectrum. Distortion in the symmetry around the Eu³⁺ ion for the MOF containing Mn²⁺, being an indication of the influence of the addition of this TM. The broad band evidenced at (365-490 nm) MnEuMOF-5 is attributed to ligand emission (π-π* transitions).

Figure 7
Excitation and emission spectra of MnEuMOF-5. Measurements were performed at room temperature on powder samples.

In the emission spectrum of MnEuMOF-5, the characteristic transitions from the excited ⁵D_1,0 level of Eu³⁺ to the ⁷F_J levels, where J = 0, 1, 2, 3, and 4, are observed. In this material, the distortion in the symmetric environment of Eu³⁺ inferred from the luminescence intensity ratio (R) of the forced electric dipole ⁵D₀ → ⁷F₂ and magnetic ⁵D₀ → ⁷F₁ transitions resulted in a value close to 3.89, reinforcing that the Eu³⁺ ion is in a site of low symmetry C_n, C_nv, or C_s. On the other hand, the difference in the values of Ω₂ and Ω₄ obtained experimentally and in the Eu1 and Eu2 systems indicate more significant distortion in the symmetry around the Eu³⁺ ion for the MOF containing Mn²⁺, being an indication of the influence of the addition of this TM.

Energy transfer processes

The energy transfer and back transfer rates between the ligands and Eu³⁺ were quantified by the theoretical model developed by Malta³⁹ and the calculations were conducted using the LUMPAC software. According to this model, energy transfer rates can be determined from the sum of two terms that are(multipolar mechanism) and (exchange mechanism) detailed in equations S7, S8 and S9 in the SI section.

The calculated excited state energy values of the ligands are detailed in Table S1 (SI section). The triple levels obtained theoretically agree with the experimental values determined by luminescence spectroscopy, considering the energy of the zero-phonon line (Table S1). As illustrated in Figures 8a-8b, these T₁ states have higher energy than the main emitter state of Eu³⁺ (⁵D₀), confirming the prediction of intramolecular energy transfer in these systems.

Figure 8
Energy level diagrams showing the possible intramolecular energy transfer channels of the Eu³⁺ systems: (a) Eu1 and (b) Eu2.

Luminescence in Eu³⁺ complexes can be affected by the energy difference between the lowest triplet state (T₁) of the ligand and the ⁵D_1,0 emitter state of the Eu³⁺ ion. According to Latva et al.,⁴⁰ an ideal Ligand → Eu³⁺ energy transfer process requires an energy difference ∆E (T₁ - ⁵D_1,0) in the range of 2500-4000 cm^-1. In the Eu1 and Eu2 systems proposed to explain the bond between Eu³⁺ and the ligand, the calculated values of these differences are 6006.50 cm^-1 for Eu1 and 5836.70 cm^-1 for Eu2. Although these values exceed the range established by Latva’s empirical rule,⁴⁰ the complexes demonstrated good photoluminescence under UV irradiation (Figure S2, SI section), with considerable quantum efficiency values, as shown in Table 1.

The T₁ states are mainly responsible for energy transfer in the Eu1 and Eu2 complexes, especially through the T₁ → ⁵D₁ and T₁ → ⁵D₀ channels. In this sense, the rates found for the Eu1 and Eu2 complexes are quite close, with the triplet states positioned above the ⁵D₁ and ⁵D₀ levels of the Eu³⁺ ion. However, the quantum efficiency of these complexes is reduced due to the presence of coordinated water molecules, which act as a vibrational deactivator of the excited states of the Eu³⁺ ion.⁴¹ The luminescence attenuation can also be confirmed by the high non-radiative rate in the complexes,⁴² as shown in Table 1.

The absorption spectra of the ligands, estimated by the INDO/S-CIS (Intermediate Neglect of Differential Overlap/Spectroscopic Configuration Interaction Single) method, showed absorption maxima at 240.7 and 246.4 nm for the Eu1 and Eu2 systems, respectively, corresponding to the π-π* transitions of the ligands (Figures S3-S4, SI section). The main orbitals involved in these transitions are HOMO+15 (π) → LUMO-7 (π*) for Eu1 and HOMO+11 (π) → LUMO-6 (π*) for Eu2 (Figures S5 S6, SI section). The calculated energy transfer and back transfer channels, as well as their respective rates, are shown in Figure 8.

For the MnEuMOF-5 material, the band shift observed in Figure 7 towards the blue region can be justified by the competition in the ET processes between the metal ions (Eu³⁺ and Mn²⁺) and the organic ligand, since the appearance of the broad band of the ligand reinforces that the ET for the Eu³⁺ ions did not occur as efficiently as in the MOF without Mn²⁺. In this case, the ET to the ⁵D_1,0 emitting level of Eu³⁺ can occur from the ligand triplet level and from the ⁴T₁ level of Mn²⁺.

The sensitization of Eu³⁺ by Mn²⁺ is shown in Figure 9, where Mn²⁺ ions can transfer nonradiative energy to Eu³⁺ from the lowest excited state ⁴T₁.¹⁶ The Mn²⁺ emitting level can be sensitized from the ligand triplet that is close to the ⁴T₁ level, in which case it is essential to consider that the energy back transfer between the Mn²⁺ emitting level and the ligand triplet has a contribution to the occurrence of the broad band between 365 and 490 nm.

Figure 9
Representative diagram of the energy levels of the ligand and ions: Eu³⁺ and Mn²⁺.

Thermometric characterization

Our MOFs were evaluated as potential candidates for temperature sensors, and the investigation was conducted exploring a temperature range that includes the physiological range (298-323 K) up to a maximum of 393 K. The thermometric parameter in the MOF containing Eu³⁺ (EuMOF-5) was defined from the ratio between the transitions: ⁵D₀ → ⁷F₂ / ⁵D₀ → ⁷F₁. In contrast, in the MOF containing Eu³⁺ and Mn²⁺ the ratio between the hypersensitive transition of Eu³⁺ (⁵D₀ → ⁷F ₂) and the broad band of the ligand was used.

Figures 10a and 10b show the behavior of the emission spectrum of the MOF (EuMOF-5) as a function of temperature, in which it is possible to demonstrate that the ⁵D₀ → ⁷F₂ transition presented a discrete and progressive increase in luminescence as the temperature varied. Due to this noticeable variation in the emission intensity up to a maximum of 47% of the initial intensity at a temperature close to 393 K, the MOF’s thermometric parameter and performance were investigated. The estimated lifetime values by monitoring the emission at 612 nm (Table S2, SI section) also show an increase in their values, suggesting a favoring of intramolecular ET with increasing temperature.

Figure 10
(a) Emission spectrum in the temperature range 229-393 K, with excitation at 320 nm; (b) dependence of the emission intensity ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₁ with temperature variation.

The expression used to fit the curve of the thermometric parameter (∆), Figure 11a, as a function of the temperature measured in Kelvin (K), was a linear function of Origin (R² = 0.99247) in which the temperature (T) is correlated with the thermometric parameter (equation 1). It is worth noting that the ⁵D₀ → ⁷F₁ transition was used because it does not present significant intensity variations in response to temperature changes.

Figure 11
(a) Luminescence intensity ratio I_5D0→7F2/I_5D0→7F1 as a function of temperature and curve fit; (b) relative thermal sensitivity temperature variation.

(1) $$\frac{{\mathrm{I}}_{5\mathrm{D}0\to 7\mathrm{F}2}}{{\mathrm{I}}_{5\mathrm{D}0\to 7\mathrm{F}1}}=3.46294+0.00571\times \mathrm{T}$$

The performance of the MOFs as candidates for temperature sensors was evaluated in terms of relative thermal sensitivity (Sr), which allows the comparison of these materials with other luminescent thermometers reported in literature. The curve shown in Figure 11 was obtained from the derivative of the thermometric parameter, according to equation 2, when the thermometric parameter (∆) is defined by the luminescence intensity ratio (LIR):

(2) $$S_r(\mathrm{\%})=\frac{1}{\mathrm{LIR}}\left|\frac{\partial \mathrm{LIR}}{\partial \mathrm{T}}\right|\times 100$$

The Sr curve for EuMOF-5 (Figure 11b) demonstrates that this MOF presents low sensitivity to temperature variations in the range of 298-393 K, in which the maximum Sr presented a value close to 0.12% K^-1 at the temperature of 298 K. In this case, the MOF containing only Eu did not demonstrate to be an efficient temperature sensor, when we compare its performance with the MOFs containing the ions Eu³⁺ and Tb³⁺ or Eu³⁺ and TM reported in the literature,¹¹,⁴³ with Sr values > 1% K^-1.

To improve the performance of EuMOF-5 against temperature variation, the MOF containing Mn²⁺ and Eu³⁺ was evaluated, since the significantly larger absorption cross section of transition metal (TM) ions compared to Ln³⁺ makes them promising candidates as Ln³⁺ sensitizers.

The synergy between Mn²⁺/Mn⁴⁺ and Eu³⁺ ions is well explored in the literature, mainly in the description of inorganic materials applied to luminescent thermometry, presenting promising results when relative sensitivity, through the relationship of Eu/Mn emission intensities, where this combination promoted Sr values > 1% K^-1.¹²,⁴³,⁴⁴ However, the combination between Mn-Eu ions in MOFs in luminescent thermometry is little explored.¹³ The spectral position of the ⁴T₁ → ⁶A₁ emission band of Mn²⁺ ions depend on the strength of the crystalline field acting on the Mn²⁺ ions and its coordination varies between different host materials, arising the interest of the present study.

In this context, the absence of the emission band in the green region reinforces that Mn²⁺ ions do not replace Zn²⁺ sites in tetrahedral coordination with BDC^2-. The emission spectrum for MnEuMOF-5 measured with a temperature variation between 298-393 K are presented in Figure 12. The temperature range investigated corresponds to the same one used to define the thermometric parameter of the MOF containing only Eu³⁺ (EuMOF-5). It is possible to observe the presence of a broad band between 365-490 nm, which undergoes slight change in intensity with the temperature variation. The appearance of the wide band is attributed to the presence of Mn²⁺ in the MOF, since the band was not observed in the absence of Mn²⁺.

Figure 12
(a) Emission spectrum in the temperature range 229-393 K, with excitation at 320 nm; (b) dependence of the emission intensity with temperature variation.

The ⁵D₀ → ⁷F₂ transition shows higher luminescence intensity with increasing temperature up to 393 K, with a maximum increase in intensity of approximately 52% of the initial intensity at a temperature of 298 K (Figure 12). The observed behavior is like the luminescence intensity progression profile of the ⁵D₀ → ⁷F₂ transition for the material without Mn²⁺.

The thermometric parameter (Figure 13) was determined from the ratio between the ⁵D₀ → ⁷F₂ transition of Eu³⁺ and the broad band of the ligand observed between 365-490 nm. The curve obtained was fitted by an exponential function: ExpDec2 from Origin (R² = 0.9929). The best fit of the curve was not achieved by a single exponential law, due to the multicentric character of the ions in the lattice and the probable defects in the lattice influenced by Mn.¹¹,⁴⁵ The ∆ can be described empirically by equation 3 in which temperature (T) is correlated with $\frac{{\mathrm{I}}_{\mathrm{Eu}}}{{\mathrm{I}}_{\text{Ligand}}}$.

Figure 13
(a) Luminescence intensity ratio I_5D0→7F2/I_Ligand as a function of temperature and curve fit; (b) relative thermal sensitivity.

(3) $$\frac{{\mathrm{I}}_{\mathrm{Eu}}}{{\mathrm{I}}_{\text{Ligand}}}=(-2.439\times {10}^{36}){\mathrm{e}}^{(-\frac{\mathrm{T}}{3.39017})}+(-1.21587){\mathrm{e}}^{(-\frac{\mathrm{T}}{94.86214})}+0.12746$$

The performance of MnEuMOF-5 was evaluated regarding relative thermal sensitivity (Figure 13), from the derivative of the thermometric parameter. In this case, it was possible to observe a considerable improvement in the thermal sensitivity of the MOF with Mn²⁺, when compared with the EuMOF-5 material at the same temperature. The highest thermal sensitivity was observed in a narrow temperature range, which encompasses part of the physiological range, in which the highest value reached close to 8% K^-1 was estimated at the temperature close to 303 K. The MOF also presented a satisfactory result of repeatability in 5 heating and cooling cycles (Figure S10, SI section), with a minimum temperature of 298 K and a maximum of 333 K. The maximum temperature considered in the reproducibility tests was 333 K, due to the low Sr values presented at higher temperatures, close to 393 K.

The improvement in the thermal sensitivity of MnEuMOF-5 is, according to our interpretation, due to the addition of Mn²⁺ in a higher concentration, since the higher concentration of Eu in the MOF did not demonstrate efficient sensitivity to temperature variation. To evaluate the mechanism responsible for the improvement in the sensitivity of the MOF to temperature variation, Figure 9 shows the diagram with the energy levels of the ligand, Eu³⁺ and Mn²⁺. In this case, the presence of the Mn²⁺ ion causes greater competition between the intramolecular ET channel, and the ET between the triplet level of the ligand and the ⁵D₀ emitter level is less efficient, which justifies the presence of the broad band of the ligand, undergoing a slight decrease in intensity with increasing temperature. The Eu³⁺ ions can be sensitized from the ⁴T₁ level of Mn²⁺, a mechanism that can be favored by gradually increasing the temperature.

The high sensitivity (Sr) of the MOF with Mn²⁺ (MnEuMOF-5) showed a promising result for ratiometric temperature detection, showing potential for temperature detection in the narrow temperature range between 303 K and close to 310 K, since with the progressive increase in temperature the Sr value drops considerably, indicating that the efficiency for a narrow temperature range, which approaches the physiological range.

The highest Sr value achieved in this study contributes to application as potential candidates for luminescent thermometers, as well as the MOFs reported in Table 2 consisting of Ln³⁺(Eu and Tb), Ln³⁺ (Eu³⁺) and TM (Mn²⁺) inorganic materials or Ln³⁺ TM and organic ligands: 2,4-difluoro-benzoic acid (2,4-DFBA), 1,10-phenanthroline (phen), 1-4-benzendicarboxylate (BDC), 1,3-benzene-dicarbo-xylate (1,3-BDC), 1,3,5-benzenetricarboxylic acid (BTC), H₂BDC-OH = 2-hydroxyterephthalic acid, H₂BDCNH₂ = 2-aminoterephthalic acid.

Table 2
Thermometric performance of compounds reported in the literature consisting of Ln³⁺ or Ln³⁺TM and organic ligands

Material	Doping	Sr / (% k-1)	Temperature / K	Reference
([Tb2-yEuy(2,4DFBA)6phen2])	y = 0.0244	2.3	290	Chen et al.47
EuyTb1-y(BDC)(H2O)Cl]	y = 0.0143	0.28	298-473	Miao et al.48
Tb1-yEuy -BDC-F4	y = 0.01	0.76	50-300	Zhou et al.49
EuyTb10-yNaLa(1,3-BDC)	y = 0.086	1.0	77-250	Mansar and Serier-Brault50
Eu-BTC/Zn2GeO4:Mn	Eu/Mn = 0.4	4.1	293-363	Zhang et al.11
EuyTby [Zn4O(BDC)3]	y = 2.5	1.8	303-473	Xia et al.10
Tb1-yEuy(BDC)1,5(H2O)2	y = 0.01	0.14	300-320	Cadiau et al.51
[Tb1- yEuy(CH3COO)(1,3-bdc)(H2O)]	y = 0.150	0.44	150-350	Trannoy et al.52
EuyTb1-y-BTC	y = 0.37	0.68	313-473	Wang et al.53
EuyTb1-yBDC-OH EuyTb1-yBDC-NH2	y = 0.01	2.24 2.49	313-473	Xia et al.54
Mn2+:Zn2SiO4 -Eu3+:Gd2O3	Mn:Zn = 6:94	3.05	303-323	Huang and Chen12
Eu3+y:MnWO4	y = 0.015	1.56	77-373	Peixoto et al.55
Zn1,98- yGeO 4:0,02Mn2+	y = 0.01-0.12	1.96	293-353	Yao et al.44
GdPO4:Mn2+, Eu3+	Eu:Mn = 1:10	8.88	305	Maciejewska et al.43
EuMOF-5	Eu = 5	0.12	298	this work
MnEuMOF-5	Mn:Eu = 90:10	8.0	303	this work

In this context, it is possible to highlight the proximity of the Sr value obtained in the present study when compared with the work of Maciejewska et al.,⁴³ which uses an inorganic matrix with a higher concentration of Mn²⁺ (10%) in relation to Eu³⁺ (1%) at a temperature of 305 K. In this study, the emission intensity of the ⁵D₀ → ⁷F₂ transition in the material doped only with Eu showed virtually no change in temperature. However, the emission of Mn²⁺ was susceptible to thermal quenching. The high concentration of Mn²⁺ relative to Eu³⁺ showed greater thermal sensitivity, since the Mn²⁺ concentration below 10% was less susceptible to temperature changes.

In turn, Zhang et al.¹¹ used a combination of MOF (Eu-BTC) and Zn²GeO⁴:Mn²⁺. In these materials, it was shown that the intermediate ratio of 0.4 between Mn²⁺ and Eu³⁺ presented a more satisfactory result of thermal sensitivity. The thermometric parameter is strongly influenced by the luminescence of Mn²⁺, which presented marked thermal extinction with temperature variation. As in the present work, it is observed that the ⁵D₀ → ⁷F₂ transition is little influenced. The combination of MnMOF with Eu³⁺ was reported by Zhang et al.⁴⁶ The authors emphasize that the fluorescence performance of the Eu³⁺ doped Mn²⁺ MOF is superior to that of MnMOF. In this material, no characteristic Mn²⁺ band was observed in the emission spectrum, and the addition of Eu³⁺ influences the disappearance of the ligand band via energy transfer processes to the metal ion.

Conclusions

To contribute to the development of luminescent thermometers, this work investigated the potential of Eu³⁺ and Eu³⁺-Mn²⁺ doped MOFs, coordinated with the terephthalate ligand (1,4-BDC), as temperature sensors. In this perspective, MOF-5 luminescent thermometers were prepared, in which an additional phase formed from the coordination of Eu³⁺ with [Ln₂(BDC)₃(DMF)₂(H₂O)] was also identified, which was confirmed by the congruence of experimental and theoretical luminescence data. Although the materials showed the formation of additional segregated phases, the cubic morphology was preserved. However, the EuMOF-5 material did not present satisfactory results for application as a potential temperature sensor, with very low thermal sensitivity values. The addition of a higher concentration of Mn²⁺ to the MOF resulted in an environment with more significant distortion for the Eu³⁺ coordination, due to the formation of an additional phase with reflections characteristic of Mn(BDC)₃(DMF)₂, observed in the diffraction pattern. Furthermore, the addition of TM clearly influenced the ET processes between the ligand and Eu³⁺, since the characteristic band of the ligand was observed in the emission spectrum, indicating that in this material, competition in the ET processes occurs between the metal ions (Eu³⁺ and Mn²⁺) and the organic ligand, making the ligand-Eu transfer channel less efficient. The ratio between the broadband and the ⁵D₀ → ⁷F₂ transition of MnEuMOF-5 was used to define the thermometric parameter over temperature variation, since the typical red-wave emission of Mn²⁺ cannot be clearly defined in the emission spectrum of this study. The investigation of the thermal sensitivity of the MOF containing Mn²⁺ and Eu³⁺ showed superior results for a narrow temperature range, reaching a maximum thermal sensitivity close to 8 % K^-1 at 303 K. In summary, this work demonstrates that the combination of Mn²⁺ and Eu ions led to an improvement in thermal sensitivity in a temperature range between 303 and 310 K, since above this temperature the sensitivity effect rapidly decreases. MnEuMOF-5, therefore, has potential as a temperature sensor, specifically focused on a narrow investigation range.

Supplementary Information

Supplementary Information

Supplementary data are available free of charge at http://jbcs.sbq.org.br as PDF file.

Acknowledgments

The authors acknowledge FACEPE (IBPG-1179-1.06/19), CNPq, Brazilian agencies, and Prof Hermi F. Brito for allowing access to his laboratory in the Institute of Chemistry-USP and help with the experimental data.

Data Availability Statement

All the data from this research is available in the text.

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Edited by

• Editor handled this article:
  Juliano Alves Bonacin (Associate)

Publication Dates

• Publication in this collection
  09 Jan 2026
• Date of issue
  2026

History

• Received
  19 Sept 2025
• Published
  19 Nov 2025