An air-stable high-performance single-molecule magnet operating as a luminescent thermometer below its blocking temperature

Abstract

The field of molecule magnets has advanced significantly in recent years. Yet, key challenges persist for their practical applications, such as achieving higher blocking temperatures and maintaining precise temperature control in air-stable magnets. This work addresses aspects related to both challenges. Thus, it presents the air-stable hexagonal bipyramidal compound {[Dy(L^N6en)(OSiPh₃)₂](BPh₄)}·1.5CH₂Cl₂ (1·1.5CH₂Cl₂) and its diluted analogue {[Dy_0.1Y_0.9(L^N6en)(OSiPh₃)₂](BPh₄)}·1.5CH₂Cl₂ (1@Y·1.5CH₂Cl₂). Their high axiality, achieved by reducing equatorial charge, enables magnetic behaviour with energy barriers higher than 1500 K and blocking temperatures based on hysteresis (T^H_B) of 12 and 40 K, respectively. Hence, 1@Y·1.5CH₂Cl₂ is the SMM with the highest T^H_B reported among air-stable uncapsulated molecule magnets. Besides, both complexes show temperature-dependent luminescence. Remarkably, 1@Y·1.5CH₂Cl₂ stands out as the pioneering example of a bifunctional molecule magnet and luminescent thermometer with both functionalities active below its T^H_B. This breakthrough makes it possible to monitor the temperature of a molecule in the range where it exhibits remanent magnetization for the first time. Moreover, this molecular material presents by far the best magnetic characteristics (U_eff and T^H_B) of any SMM luminescent thermometer reported to date. Experimental magnetic and luminescent data are analysed using theoretical calculations. Notably, luminescence is interpreted via coupled cluster methods, offering a more sophisticated alternative to the traditional time-dependent DFT approach.

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Introduction

Multifunctionality is the future goal of new smart materials. In this context, the development of magneto-luminescent molecular materials is a scientific field of growing interest. Single molecule magnets (SMMs) are very promising due to its potential applications in technologically significant areas such as magnetic storage¹ and quantum computing.^2,3 However, one of the primary challenges to overcome before such technological applications become feasible is attaining their precise temperature control, which is essential for maintaining the magnetized state and preserving magnetic memory. Nevertheless, conventional thermometers are not suitable for measuring temperature at the nanoscale. Therefore, constructing microdevices based on SMMs with integrated thermometric capabilities could overcome this limitation. In this regard, obtaining SMMs with temperature-dependent luminescence represents a key issue.^4,5 Achieving this end will allow SMMs to function as in situ thermometers, enabling continuous self-monitoring of their temperature, thus avoiding the loss of magnetic behaviour resulting from excessive heating. Since the first compounds displaying dual SMM and luminescent thermometer characteristics, described in 2019,^6–9 the number of additional compounds with such bifunctional nature reported to date remains relative low.^10–29 Since then, there have been substantial enhancements in the magnetic and thermometric properties of these molecular nanomaterials. Accordingly, a few compounds have been described that show thermometric character in part of the temperature range where they present slow relaxation of magnetization in the absence of an external field.^{7,14–16,21–23} However, it is worth noting that none of these complexes behaves as a luminescent thermometer below the blocking temperature of the SMM.

Consequently, another major challenge in this research field is the obtaining of air-stable SMMs of high blocking temperature. In this regard, it should be noted that the blocking temperature based on magnetic hysteresis (T^H_B) of 80 K, established with an organometallic mononuclear SMM,³⁰ has not been surpassed, but matched with another dinuclear mixed valent dysprosium metallocene.³¹ Nevertheless, the air instability of these organometallic compounds is challenging for their practical utility. This inconvenience has led to an active search for air-stable SMMs with improved properties. In this way, a dysprosium azafullerene with a 100-seconds blocking temperature of 45 K (T^H_B = 39 K) was recently reported,³² but the current record of T^H_B for unencapsulated SMMs is 36 K, which is held by a dysprosium complex with pentagonal bipyramid geometry.³³ In recent years, other air-stable mononuclear SMMs with hexagonal bipyramid geometry have been obtained, with energy barriers exceeding 2400 K,³⁴ and blocking temperatures up to 20 K.³⁵ However, within SMMs that in turn behave as luminescent thermometers, the energy barriers and blocking temperatures do not exceed 944 K and 8 K, respectively.²³

Developing lanthanoid luminescent thermometers with exceptionally high-energy barriers, leading to substantial crystal field (CF) splitting of both ground and excited state multiplets, presents a significant challenge. This arises because the thermal sensitivity of Boltzmann-type luminescent thermometers hinges on the energy gap between the thermally coupled excited states. Large energy gaps can compromise high relative thermal sensitivity across the entire temperature range where slow relaxation occurs.³⁶ Consequently, a fundamental obstacle remains in the development of molecular magnets that function as luminescent thermometers within their operational regime.

One potential way to circumvent this theoretical limitation would be to base the luminescent thermometric behaviour of the magnet not on the metal, but on the ligand. This would involve designing lanthanoid complexes with luminescent ligands that in turn predetermine a suitable geometry to favour high anisotropy. This strategy could lead to the improvement of the parameters U_eff and T^H_B in SMMs with luminescence thermometry, as well as to the achievement of the performance of both mentioned magnetic and optical properties in the same temperature range.

Building on these assumptions, we analyse the thermometric properties of the high-performance SMM {[Dy(L^N6en)(OSiPh₃)₂](BPh₄)}·1.5CH₂Cl₂ (1·1.5CH₂Cl₂). This analysis is based on ligand triplet emission coupled with Dy³⁺ luminescence. In addition, we also present here the bifunctional analogue {[Dy_0.1Y_0.9(L^N6en)(OSiPh₃)₂](BPh₄)}·1.5CH₂Cl₂ (1@Y·1.5CH₂Cl₂), which exhibits Dy³⁺-based slow relaxation of the magnetization and ligand-based luminescence thermometry. Both compounds are molecule magnets that function as optical thermometers, with magnetic properties surpassing those of any SMM with luminescence thermometry described so far. Notably, 1@Y·1.5CH₂Cl₂ is the first SMM capable of operating as a luminescent thermometer below its blocking temperature. Thus, 1@Y·1.5CH₂Cl₂ shows bifunctionality up to 40 K.

Results and discussion, experimental

Synthesis and structural characterization

{[Dy(L^N6en)(OSiPh₃)₂](BPh₄)}·1.5CH₂Cl₂ (1·1.5CH₂Cl₂) was isolated from previously reported {[Dy(L^N6en)(CH₃COO)₂](NO₃)}·2H₂O,²⁵ through a modified version of a published method,^37,38 as illustrated in Fig. 1.

Fig. 1 Reaction scheme for the isolation of 1 from related {[Dy(L^N6en)(CH₃COO)₂](NO₃)}. Solvate molecules are omitted for clarity.

Recrystallisation of the isolated solid by diffusion of diethyl ether into a dichloromethane solution of the complex at ca. 5 °C allowed to collection of colourless single crystals of {[Dy(L^N6en)(OSiPh₃)₂](BPh₄)}·1.5CH₂Cl₂ (1·1.5CH₂Cl₂). This compound is stable in air, as no significant changes were observed in the experimental powder X-ray diffractogram (Fig. S1†) or in its elemental analyses after one year. Moreover, no noticeable changes in colour or appearance were observed upon exposure to air over the same period.

The diluted {[Dy_0.1Y_0.9(L^N6en)(OSiPh₃)₂](BPh₄)}·1.5CH₂Cl₂ (1@Y·1.5CH₂Cl₂) complex was similarly obtained, by mixing dysprosium and yttrium species in the adequate ratios (see ESI†). The composition and purity of the yttrium species was checked by elemental analysis and ¹H NMR spectroscopy (ESI†). The Dy : Y proportion for 1@Y·1.5CH₂Cl₂ was corroborated by micro X-ray fluorescence techniques.

The experimental data for the X-ray diffraction studies of 1·1.5CH₂Cl₂ are summarised in Table S1.† The asymmetric unit of 1·1.5CH₂Cl₂ contains two crystallographically different, but chemically equivalent, halves of the cationic complex [Dy(L^N6en)(OSiPh₃)₂]⁺, whose metal atoms are sited on inversion centres, and a whole [BPh₄]⁻ anion. Consequently, the two whole complex molecules, which will be called 1a and 1b, are respectively generated by the symmetry operations (−x + 2, −y + 2, −z) and (−x + 1, −y + 1, −z + 1). Ellipsoid diagrams of the cations of these two molecules are shown in Fig. 2 and S2,† respectively. Moreover, the asymmetric unit includes disordered molecules of dichloromethane as solvate, summing up to 1.5 molecules per complex unit. In addition to the disorder shown by the solvates, for cation 1b (which contains Dy2), some atoms (N21, N23 and the contiguous ethylene chains) are also disordered on two sites (occupation sites 0.51 : 0.49).

Fig. 2 Balls and sticks diagram for the cation [Dy(L^N6en)(OSiPh₃)₂]⁺ in the {[Dy(L^N6en)(OSiPh₃)₂](BPh₄)} complex 1a, showing Dy1 as a polyhedron. Only heteroatoms of the asymmetric unit and those corresponding to the coordination sphere are labelled.

The cationic complex [Dy(L^N6en)(OSiPh₃)₂]⁺ displays an N₆O₂ coordination environment, similar to those observed for other octacoordinated dysprosium complexes derived from N₆ macrocycles (Table S2†),³⁹ with {[Dy(L⁸)(Ph₃SiO)₂](BPh₄)}³⁸ (L⁸ = (2E,6E,9E,13E)-2,7,9,14-tetramethyl-3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane-2,6,9,13-tetraene) being particularly alike, since the main difference with 1 is the presence of methyl groups on the imine carbon atoms of L⁸. Calculations of the degree of distortion of the DyN₆O₂ core relative to an ideal polyhedron of eight vertexes made for 1 with the SHAPE program^40,41 (Table S3†), reveal a hexagonal bipyramid (hbp) geometry. The neutral N₆ donor macrocycle lies in the equatorial plane, with both triphenylsilanolate anions occupying opposite apical positions with a perfect O-Dy-O angle of 180°. With such low distortion of the polyhedron, Dy1 is contained in the calculated plane formed by the six N-donor atoms, while these latter ones only protrude ca. 0.15 Å from the calculated plane. It is remarkable that the deviation from an ideal hbp is also notably smaller for 1 (Table S2†) than for the mentioned {[Dy(L⁸)(Ph₃SiO)₂](BPh₄)},³⁸ since its ChSM value is 2.163, while this value is between 1.12 and 1.44 for 1. The main geometric parameters of this compound are listed in Table S4,† with typical values, including both coordination Dy–O and Dy–N distances, which are within the usual ranges found for other related compounds.³⁹

Powder X-ray diffraction measurements for 1·1.5CH₂Cl₂ (Fig. S1†) reveal that the isolated product was obtained with high purity, as no additional peaks were observed in the experimental diffractogram. Furthermore, the diffractogram of the complex was remeasured after one year, demonstrating that both patterns are equal. These results confirm the air stability of the compound, without detecting any sign of decomposition.

In addition, the powder X-ray diffractogram for the diluted analogue 1@Y·1.5CH₂Cl₂ (Fig. S3†) agrees with a very similar structure to that of 1·1.5CH₂Cl₂.

Magnetic studies

Direct-current (dc) magnetic susceptibility measurements for 1·1.5CH₂Cl₂ were recorded in the 2–300 K temperature range. The χ_MT vs. T graph is shown in Fig. S4.† The χ_MT value at 300 K is 14.4 cm³ K mol⁻¹, which is close to the expected one for an uncoupled Dy³⁺ ion at room temperature (14.17 cm³ K mol⁻¹). The experimental curve remains basically constant up to 15 K, and then it drops sharply, to reach a value of 4.8 cm³ K mol⁻¹ at 2 K. This great drop seems to indicate magnetic blocking more than the presence of large crystal field splitting.³⁵ In fact, ab initio calculations (vide infra), which take into account the large crystal field, do not show such sharp drop at low T (Fig. S4†). The field dependence of the reduced magnetisation at 2 K at the maximum applied field of 5 T tends to 5.5 Nμ_β, suggesting a highly anisotropic Dy³⁺ ion with an Ising-like ground doublet.⁴² Moreover, the magnetisation graph shows a sigmoidal shape at low field, which is often found for high-performance SMMs.^35,43

The dynamic magnetic properties of 1·1.5CH₂Cl₂ were also studied, and they show out-of-phase frequency dependence of the susceptibility as a function of the temperature up to 100 K (Fig. 3).

Fig. 3 Frequency dependence of for 1·1.5CH₂Cl₂ at H_dc = 0 at frequencies ranging from: (a) 0.1 to 1500 Hz; (b) 50 to 10 000 Hz. Solid lines are fits to the data.

In addition, the Cole–Cole plots (Fig. S5†) display curves with α parameters in the range 0.3–0.1 (Table S5†), indicative of various relaxation processes.^37,44 The presence of multiple relaxation mechanisms is also supported by the dependence of the relaxation time τ with temperature in the 2–100 K range (Fig. 4), which shows varying behaviour across different temperature ranges. Thus, below 10 K, the relaxation is almost independent of temperature and this behaviour points out to a quantum tunnelling of magnetisation (QTM) mechanism. Above this temperature, the shape of the curve is in agreement with the presence of thermally activated processes.³²

Fig. 4 Dependence of the relaxation rate with temperature for 1·1.5CH₂Cl₂ (red solid circles) and 1@Y·1.5CH₂Cl₂ (blue solid circles). The red and blue solid lines account for the best fits considering Orbach, Raman and QTM relaxation processes.

Hence, attempts were made to fit this plot considering all the possible relaxation mechanisms except the Direct one (Orbach, Raman, and QTM), according to eqn (1), individually or grouped.

The best fit of the data (Fig. 4) is obtained considering the three relaxation processes, and this renders the parameters U_eff = (1528 ± 6) K [(1062 ± 4) cm⁻¹], τ₀ = (6.9 ± 0.3) × 10⁻¹¹ s, C = (8.5 ± 0.1) × 10⁻⁴ s⁻¹ K⁻ⁿ, n = (3.2 ± 0.2) and τ_QTM = (0.2 ± 0.1) s. The n value can seem quite low, but considerations of both the acoustic and optical phonons in magnetic dynamics can explain the deviation from the predicted value for the Kramers ion (n = 9).^45,46

The comparison of these data with those obtained for some magneto-structurally characterised mononuclear dysprosium SMMs with N₆ macrocycles and triphenylsilanolate as ligands (Table S2†)^35,38,43,47 allows to draw some conclusions. Thus, comparison between 1·1.5CH₂Cl₂ and {[Dy(L⁸)(Ph₃SiO)₂](BPh₄)}³⁸ clearly shows that removing an electron-donating group from the N₆ macrocyclic ligand, such as the methyl one, significantly improves the energy barrier for the spin reversal. From this, 1·1.5CH₂Cl₂ has a barrier of ca. 1500 K, while for {[Dy(L⁸)(Ph₃SiO)₂](BPh₄)} U_eff is ca. 1100 K.³⁸ The absence of the methyl groups in 1·1.5CH₂Cl₂ results in a more perfect hbp polyhedron. However, this less deviated geometry does not explain by itself the improved results, as Table S2† evidences. Accordingly, this work deepens in the fact that the elimination of electron-donating substituents in the equatorial plane is a key factor in enhancing the behaviour of SMMs, as previously inferred.³⁹

Variable-field magnetisation measurements revealed the trends in magnetic relaxation behaviour. The data were recorded by scanning the field between −3 and 3 T, employing an average sweep rate of 10 mT s⁻¹. At low temperatures, the magnetisation value experiences a steep decline close to H = 0 (Fig. 5a) but with open curves, which also suggests the existence of fast QTM relaxation. This relaxation mechanism is common among dysprosium hexagonal bipyramidal SMMs (Table S2†). In spite of this, the complex shows open thin hysteresis till 12 K (Fig. 5a and b). The blocking temperature T^H_B was established by observing the last temperature at which the remanent magnetisation decreases, a criterion commonly used^32,48 to establish the T^H_B values in different SMMs. The low values of magnetisation and coercive field cannot be attributed to artefacts due to errors in measures, given that the field was corrected using a palladium standard measured under the same conditions.

Fig. 5 (a) Magnetic hysteresis measurements for 1·1.5CH₂Cl₂ collected at 2–15 K with an average sweep rate of 10 mT s⁻¹. (b) Expansion of the region near zero field at 10–15 K. (c) Magnetic hysteresis measurements for 1@Y·1.5CH₂Cl₂ collected at 2–43 K with an average sweep rate of 10 mT s⁻¹. (d) Expansion of the region near zero field at 37–43 K.

In addition, zero-field-cooled (ZFC) and field-cooled (FC) magnetic susceptibility curves show divergence up to 10 K (Fig. S6†), supporting the hysteresis results.

An yttrium magnetically diluted mixed compound {[Dy_0.1Y_0.9(L^N6en)(OSiPh₃)₂](BPh₄)}·1.5CH₂Cl₂ (1@Y·1.5CH₂Cl₂) was also synthesized in order to improve the magnetic blocking. The relaxation times (Fig. 4) extracted from the vs. ν graph (Fig. S7†) for 1@Y·1.5CH₂Cl₂ closely overlap with those of 1·1.5CH₂Cl₂ above 20 K (Fig. 4). However, below 20 K, both curves differ significatively and the one for 1@Y·1.5CH₂Cl₂ shows a gradual but more pronunced drop of the τ⁻¹ values with temperature compared to 1·1.5CH₂Cl₂. This could suggets a reduction of the QTM. Nevertheless, the Cole–Cole plots (Fig. S8†) and the dependence of τ with temperature still agree with various relaxations mechanism. Thus, the best fitting of the relaxation times for the diluted complex renders the parameters U_eff = (1531 ± 5) K [(1064 ± 2) cm⁻¹], τ₀ = (4.3 ± 0.1) × 10⁻¹² s, C = (5.7 ± 0.5) × 10⁻⁵ s⁻¹ K⁻ⁿ, n = (3.8 ± 0.2) and τ_QTM = (0.6 ± 0.2) s. Accordingly, it seems that the dilution partially suppresses the QTM.⁴⁹

The hysteresis cycles recorded for 1@Y·1.5CH₂Cl₂ also show thin hysteresis, with open loops up to 40 K (Fig. 5c and d). Consequently, the diminishment of the QTM in the diluted sample seems to result in a significant improvement in the blocking temperature. However, the observed enhancement might also be partly driven by changes in the Raman relaxation mechanism, the mitigation of which generally leads to an improvement in the magnetic properties.⁵⁰

FC/ZFC measurements for 1@Y·1.5CH₂Cl₂ also reveal divergences in the curves up to 40 K (Fig. S9†), though no clear peaks are present. A similar absence of peaks was noted by Coronado in the encapsulated stable molecule magnet with T^H_B of 39 K, which also exhibits thin magnetic hysteresis.³²

Therefore, the FC/ZFC measurements also support the hysteresis results. Consequently, to the best of our knowledge, the blocking temperature for 1@Y·1.5CH₂Cl₂ exceeds the reported highest T^H_B of 36 K (sweep rate 20 mT s⁻¹) for uncapsulated, air-stable SMMs, establishing a new record.

Ab initio calculations

Multiconfigurational SA-CASSCF calculations⁵¹ with the SINGLE_ANISO code,⁵² as implemented in Orca 5.0.3,^53–55 have been performed to gain more insight into the magnetic properties of 1. The atom coordinates of both complex units present in the crystal structure have been employed. In the case of 1b, due to the disorder of the experimental geometry, two models have been created (1b_1 and 1b_2, see computational details in ESI and Fig. S10†). The results obtained for the three independent calculated molecules are similar, so we have included here only the results for 1a. Meanwhile, 1b results are collected in the ESI.† For the Dy³⁺ ion, the ground state ⁶H_15/2 is formed by eight Kramers Doublets (KDs). For 1a the KDs span in an energy range of about 1450 cm⁻¹ (Table 1). Analysis of the KDs and their g-factors reveals a very large easy-axis anisotropy in the complex (Tables 1, S6 and S7†).

Table 1 Information of the eight low-lying KDs calculated at CASSCF level. Relative energies (in cm⁻¹), components of the g-tensor, and tilting angle (θ) of the g_zz component of the corresponding excited state KD with respect to the ground state KD for 1a

Energy (cm^−1)	gxx	gyy	gzz	θ (°)
0	0.00025	0.00027	19.883
519.9	0.066	0.068	16.973	4.712
941.6	0.007	0.081	14.004	7.198
1206.6	1.343	1.532	9.238	12.346
1296.3	1.597	8.440	10.993	85.867
1323.6	1.148	6.988	11.265	86.580
1375.9	1.315	2.223	7.640	88.050
1443.1	0.563	7.319	12.239	80.777

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The ground state KD is m_J = |± 15/2〉, which is highly anisotropic, and it is characterised by a large axial g component and a small transverse one. The anisotropy g_zz axis for 1a lies mostly along the O–Dy–O direction, as shown in Fig. 6a.

Fig. 6 (a) Ball and stick representation of the molecular structure for 1a with the calculated orientation of the main magnetic axis of the ground Kramers doublet. Dysprosium, silicon, oxygen, nitrogen and carbon are represented in cyan, yellow, red, light blue and grey, respectively. Hydrogen atoms are omitted for clarity. (b) State energies as a function of their average magnetic moment, M, along the main anisotropy axis for 1a obtained with SINGLE-ANISO. The dashed green arrows correspond to the quantum tunnelling mechanism of ground or excited states, dashed purple arrow shows the hypothetical Orbach relaxation process. The solid red arrow indicates the transition between the ground and excited Kramers doublets, and the dashed red arrow the excitation pathway to the ground state with the reversed spin. The values close to the arrows indicate the matrix elements of the transition magnetic moments (above 0.1 an efficient spin relaxation mechanism is expected).

The first and second excited KDs are also highly anisotropic with large axial g components. Additionally, for the first and second excited KDs, the g_zz axis are aligned close (tilting angles smaller than 8°) to the ground state one, decreasing the probability of relaxation through those states. Moreover, the ab initio blocking barrier has been computed (Fig. 6b), suggesting that relaxation occurs through the third excited state, located around 1200 cm⁻¹, close to the experimental value of 1062 cm⁻¹.

Quite similar results were observed for 1b (see Tables S6, S7, Fig. S11 and S12†), demonstrating that ab initio calculations support the experimentally found energy barrier.

Photoluminescence and luminescence thermometry

To assess the potential of the prepared SMMs as thermographic materials, the temperature-dependent solid-state emission spectra of 1·1.5CH₂Cl₂ and 1@Y·1.5CH₂Cl₂ were acquired upon excitation at 364 and 461 nm, respectively (corresponding to the maxima observed in their excitation spectra, Fig. S13†). A significant influence of dilution on the luminescence was observed, as discussed below.

For 1·1.5CH₂Cl₂, the emission spectrum (Fig. S14a†) exhibits characteristic Dy³⁺ intra-4f transitions (⁴F_9/2 → ⁶H_11/2, ⁴F_9/2 → ⁶H_13/2, and ⁴F_9/2 → ⁶H_15/2) at approximately 659, 590, and 488 nm, respectively. These transitions, however, display low relative intensity and are only observed up to 145 K. Additionally, a broad band in the UV-visible spectral range (380–550 nm) attributed to ligand-based emission²⁵ dominates the spectra across the entire temperature range. At low temperatures, this band is significantly intense and quenches as the temperature rises (Fig. S14a†).

To explore this temperature-dependent behaviour for luminescence thermometry, the ratio of the integrated areas I₁ (ligand), I₂ (⁴F_9/2 → ⁶H_15/2), and I₃ (⁴F_9/2 → ⁶H_13/2), Δ₁ = (I₂ + I₃)/I₁, was employed as the thermometric parameter (Fig. S14b and c†). As temperature increases, Δ₁ decreases, indicating the potential of 1·1.5CH₂Cl₂ as a luminescent thermometer in the 13–120 K range. A phenomenological linear calibration curve was fitted to the data (Fig. S14c†), with the corresponding parameters summarized in Table S8.† The thermometric performance was further evaluated using the relative thermal sensitivity (S_r) and temperature uncertainty (δT), the standard figures of merit for luminescent thermometers (see ESI for details†).⁵⁶ A maximum S_r of 3.5% K⁻¹ and a minimum δT of 3.0 K at 120 K were obtained (Fig. S14d, e and Table S8†). These results show a modest thermometric performance. However, they also validate the viability of 1·1.5CH₂Cl₂ as a functional luminescent thermometer, capable of providing approximate temperature monitoring within the studied range, despite this range being above the magnetic blocking temperature of 1·1.5CH₂Cl₂, 12 K (as it occurs for all reported SMMs). Even so, this is the SMM with the highest effective energy barrier and blocking temperature for which luminescent thermometry has been observed, the previous records being at 994 K and 8 K, respectively.²³

Since 1@Y·1.5CH₂Cl₂ exhibits a blocking temperature of 40 K, its temperature-dependent luminescence was also evaluated and, as discussed below, the results demonstrate that it is the first SMM operating as a luminescent thermometer below its blocking temperature.

The emission spectra of 1@Y·1.5CH₂Cl₂ show a broad emission band centred at 575 nm (Fig. 7a), indicative of ligand-based luminescence. Although this band may appear red-shifted compared to that of the undiluted complex 1·1.5CH₂Cl₂, the shift arises from the use of a different excitation wavelength (461 nm) to maximize emission. When both spectra are recorded under identical excitation conditions (364 nm), the broad emission bands are centred at the same wavelength (Fig. S15†), confirming that no actual red shift occurs upon dilution.

Fig. 7 (a) Temperature-dependent emission spectra of 1@Y·1.5CH₂Cl₂ excited at 461 nm. (b) Temperature dependence of the thermometric parameter (Δ₂) for 1@Y·1.5CH₂Cl₂. The line represents the best linear fit (r² > 0.98) to the experimental data (fit details provided in Table S8†). (c) Temperature dependence of S_r for 1@Y·1.5CH₂Cl₂. (d) Temperature dependence of δT for 1@Y·1.5CH₂Cl₂.

In the diluted sample, with only 10% of the metal ion being Dy³⁺, the signal from Dy³⁺ completely vanishes, thus leaving only the ligand emission observable.¹³ Therefore, as the spectra are dominated by a ligand-band emission, the signal is not expected to be influenced by the coercive magnetic field retained in the sample once it is magnetized. Furthermore, excitation at 461 nm prevents efficient energy transfer from the ligand to the Dy³⁺ ion. As a result, the electronic energy levels of the Dy³⁺ centre should remain unaffected by the irradiation, preserving the single-molecule magnet behaviour. Consequently, this selective excitation is particularly advantageous, as it allows for luminescence thermometry to be performed without disturbing the magnetic relaxation dynamics. In contrast to systems where excitation can lead to changes in the population of the 4f electronic states of the lanthanoid ion -thus potentially altering its magnetic anisotropy or inducing changes in the relaxation processes-, this approach minimizes such risks by avoiding direct excitation of Dy³⁺. Hence, the separation of optical and magnetic pathways in 1@Y·1.5CH₂Cl₂ supports the potential for simultaneous operation of both functionalities.

In an effort to elucidate the underlying mechanism of the emission spectra, time-dependent density functional theory (TDDFT) calculations were performed on the X-ray structure of 1·1.5CH₂Cl₂ using hybrid correlation and exchange functionals like B3LYP⁵⁷ or PBE0.⁵⁸ However, these functionals, commonly used for such studies, yielded unrealistic results, indicating degeneracy of the excited singlet and triplet states.⁵⁹ This well-documented issue is often encountered in systems exhibiting long-range charge transfer excitations.^59,60 To better describe long-range interactions, long-range corrected functionals (e.g. CAM-B3LYP⁶¹) or double-hybrid functionals (e.g. SCS/SOS-wB2PLYP⁶²) are often employed. While these functionals partially alleviate the degeneracy issue for some optimised geometries of the first and fundamental excited states, they still yield unreasonable excitation energies significantly higher than experimental values.

To overcome these limitations, coupled-cluster calculations were performed using the STEOM-DLPNO-CCSD (similarity transformed equation of motion approach combined with the domain-based local pair orbital implementation of coupled-cluster singles and doubles) method^54,63–68 implemented in Orca 5.0.3.^53–55 This methodology does not present the problems of the TDDFT with the charge transfer excitations but is computationally demanding, limiting its applicability to smaller systems, preventing the calculation of the complete lanthanoid complex. Thus, the ligands L^N6en, Ph₃SiO⁻ and the counterion BPh₄⁻ were analysed separately. The STEOM-DLPNO-CCSD results (Table S9†) show that the lowest singlet excitation originates from the axial Ph₃SiO⁻ ligand (309 nm), while the lowest-energy triplet emission arises from the equatorial ligand (423 nm). These results qualitatively agree with experimental observations, suggesting a possible charge transfer between the two ligands, consistent with the difficulties encountered in TDDFT calculations. They also suggest that BPh₄⁻ is unlikely to participate directly in a charge transfer process with the ligands, given that its first singlet excited state appears at significantly higher energy (261 nm). While the STEOM-DLPNO-CCSD calculations provide a reasonable description, considering the limitations of calculating the complete system, a quantitative reproduction of experimental data remains challenging. Accordingly, the temperature dependence of the emission spectra of 1@Y·1.5CH₂Cl₂ can be explained by two distinct ligand-based components. To quantify these contributions, we employed a deconvolution technique with two Gaussian components (Fig. S16a†). The resulting fitting parameters (peak energy, width, and integrated area) are summarized in Table S10 in ESI.† Notably, while peak energy and width remain constant, the integrated area of each component (A₁ and A₂) decreases with increasing temperature (Fig. S16b†). This suggests a temperature-dependent variation in the relative intensities of these ligand-based emissions. Therefore, to demonstrate the applicability of 1@Y·1.5CH₂Cl₂ as a ratiometric luminescent thermometer, we employed the ratio of the integrated areas of the two Gaussian components (Δ₂ = A₁/A₂, Fig. 7b). In the absence of a theoretical framework for this ratio, a phenomenological linear calibration curve was established (parameters in Table S8 of the ESI†) and the thermometric performance was evaluated using S_r and δT values. 1@Y·1.5CH₂Cl₂ operates as a luminescent thermometer in the 13–325 K temperature range. While the maximum S_r (0.2% K⁻¹) and minimum δT (1.0 K), both at 325 K, are not exceptional (Fig. 7c and d), they are comparable to other broadband emission materials.⁵⁶

Despite this, the relatively low uncertainty (below 1.7 K across the entire temperature range) allows for temperature monitoring of cryogenic magnets, indicating that 1@Y·1.5CH₂Cl₂ is a luminescent thermometer below its T^H_B (40 K). Thus, 1@Y·1.5CH₂Cl₂ represents a dual magneto-optical molecule, capable of self-monitoring its temperature, and this advancement helps address the challenge of temperature monitoring in molecular magnetic nanomaterials where conventional thermometers are impractical, in order to avoid their loss of magnetization.

Conclusions

This work, which reports a high-performance mononuclear SMM (1·1.5CH₂Cl₂) and its yttrium diluted analogue (1@Y·1.5CH₂Cl₂), unveils the first SMM capable of self-monitoring temperature within the range where it acts as a magnet. Significantly, 1@Y·1.5CH₂Cl₂ is an air-stable SMM that exhibits a blocking temperature of 40 K, the highest T^H_B among uncapsulated air-stable molecule magnets reported to date. The integration of luminescence thermometry and magnet performance in this diluted material, with a high energy barrier (>1000 cm⁻¹), is achieved through a meticulous design that separates the origin of the magnetic properties, which relies on Dy³⁺ ions, from the luminescence, that is based on the emitting ligands. This approach diverges from conventional SMMs, where the lanthanoid ion contributes to both functions, often limiting the potential for high-performance luminescent magnets. Our strategy, exemplified by 1@Y·1.5CH₂Cl₂, overcomes this limitation, and provides a route for non-invasive optical interrogation of the material, enabling temperature readout without compromising the magnetic state. This separation of optical and magnetic pathways supports the simultaneous operation of both functionalities. Besides, 1@Y·1.5CH₂Cl₂ holds the distinction of being the SMM with the highest energy barrier and blocking temperature reported to date while simultaneously enabling self-temperature monitoring through luminescence thermometry.

Rigorous theoretical calculations, including innovative coupled cluster methods for luminescence analysis (instead of the more commonly used time-dependent DFT ones), provide robust support for the experimental magnetic and luminescent data.

As a result, this work sets a new standard for designing SMMs with enhanced integrated luminescence thermometry. Future research will focus on refining this strategy through ligand optimization to achieve even higher operational temperatures. Subtle modifications to the ligand can influence the coordination environment of the lanthanoid ions, resulting in increased magnetic anisotropy, and the material's luminescent properties, enhancing overall performance and functionality.

Author contributions

Conceptualization: M. F., J. C.-V., E. R., L. D. C.; methodology: J. C.-V., C. G.-B., J. S.-M., S. G.-C.; validation: M. F., E. R., L. D. C.; formal analysis: J. C.-V., M. F., A. M. G.-D., S. G.-C., E. R., C. D. S. B.; investigation: J. C.-V., C. G.-B.; resources: M. F., E. R., L. D. C.; visualization: J. C.-V., C. G.-B., S. G.-C., C. D. S. B.; data curation and supervision: M. F., E. R., L. D. C.; writing – original draft preparation: M. F., J. C.-V., E. R., S. G.-C., L. D. C.; project administration: M. F., L. D. C.; funding acquisition: M. F., E. R., L. D. C.

Data availability

The data supporting this article have been included as part of the ESI.†

Crystallographic data for 1·1.5CH₂Cl₂ have been deposited with The Cambridge Crystallographic Data Centre (CCDC 2262113†).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by Universidade de Santiago de Compostela (2024-PU027-1), Consellería de Cultura, Educación, Formación Profesional e Universidades, Xunta de Galicia (convenio 2023-1298), by Portuguese funds through the FCT/MCTES (PIDDAC) (UIDB/50011/2020, https://doi.org/10.54499/UIDB/50011/2020; UIDP/50011/2020, https://doi.org/10.54499/UIDP/50011/2020; and LA/P/0006/2020, https://doi.org/10.54499/LA/P/0006/2020 projects of the CICECO-Aveiro Institute of Materials), and by Ministerio de Ciencia, Innovación y Universidades (PID2021-122464NB-I00 and Maria de Maeztu CEX2021-001202-M). This article is based upon work from COST Action CA22131, supported by COST (European Cooperation in Science and Technology). J. C. V. also thanks Xunta de Galicia for his postdoctoral fellowship (ED481B-2022-068). We also acknowledge the Generalitat de Catalunya for the 2021-SGR-00286 grant and E. R. for an ICREA Academia grant. We thank CSUC and BSC supercomputer centres for the computational resources.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2262113. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5qi01113b

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