Harnessing ligand design to develop primary and self-calibrated luminescent thermometers with field-induced single ion magnet behaviour in Dy³⁺ complexes

Abstract

Novel complexes {[Dy(L^N6en)(OAc)₂](NO₃)}·2H₂O (1·2H₂O) and {[Dy(L^N6prop)(OAc)₂](NO₃)}·CHCl₃ (2·CHCl₃), containing partially flexible symmetric N₆ macrocycles, are reported. We explore the influence of the spacer length between two symmetrical N₃ rigid moieties of the ligand on their structural, magnetic, and luminescence properties. Crystallographic analysis reveals the presence of Dy³⁺ ions in distorted tetradecahedral (1·2H₂O) or bicapped square antiprism (2·CHCl₃) environments. This underscores the increased flexibility of the L^N6prop ligand, resulting in greater distortion of the N₆ macrocycle plane. Both complexes exhibit single-molecule magnet behaviour under an optimal field of 2000 Oe, with 2·CHCl₃ displaying the highest U_eff value of 127 K, despite its less planar N₆ macrocycle. Luminescence measurements indicate that the ratio between the integrated intensity of the ligands and that of the the Dy^{3+ 4}F_9/2 → ⁶H_13/2 transition can define secondary luminescent thermometers. Maximum relative thermal sensitivity values of 2.3 (1·2H₂O) and 5.1% K⁻¹ (2·CHCl₃) are achieved. Furthermore, deconvolution of the ⁴F_9/2 → ⁶H_15/2 transition in 2·CHCl₃ supports the previous determination of the energy barrier for magnetic relaxation. This permits the demonstration of the first example of a Dy³⁺ primary luminescent thermometer based on two thermally coupled Kramer's doublets of the ⁴F_9/2 level. Remarkably, 2·CHCl₃ is also the first self-calibrated luminescent thermometer with magnetic relaxation operating within the 86–211 K range, showcasing its potential in precise temperature sensing applications.

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Introduction

Lanthanoid metal ions intrinsically have unique properties that make them particularly attractive for their use in luminescent materials and single-molecule magnets (SMMs).¹ Thus, they show marked single-ion anisotropy due to the combined contributions from strong spin–orbit coupling and the crystal field effect, which is much weaker but critical from the point of view of SMM attributes.² In fact, according to Long's theory³ the axial arrangement of negatively charged donors around oblate lanthanoid ions, such as Dy³⁺, is the most efficient way to generate high-performance SMMs, with record blocking temperatures of up to 80 K.⁴ These molecular magnetic nanomaterials show great potential in magnetic storage, as they would allow data banking and processing at unprecedented speeds.⁵

Lanthanoid ions also exhibit unique luminescent properties due to their f–f electronic transitions, namely in the trivalent state (Ln³⁺).⁶ This has led to the development of luminescent molecular materials with applications in disparate fields, including bioimaging,⁷ chemical sensing,⁸ and optical thermometry.⁹ From a theoretical perspective, combining luminescence and molecular magnet properties provides a powerful tool for understanding magnetic behaviour. High-resolution emission spectra of Ln³⁺ complexes reveal the energy of different electronic states, which directly correlates with the energy barrier for magnetization reversal.¹⁰

In addition to its theoretical significance, the combination of these two properties holds substantial practical promise. It offers a viable solution to a significant challenge faced in the integration of molecular magnets into next-generation data storage devices – temperature control.¹¹ Accordingly, SMMs could function as luminescent thermometers, enabling self-monitoring of temperature in these nanomaterials.^12,13 Currently, there are several examples of mono-, di-, or homopolynuclear Nd³⁺,¹⁴ Dy³⁺,^12,15–20 Tb³⁺,²¹ Ho³⁺,²² or Yb³⁺ complexes,^23–25 as well as heteronuclear Yb³⁺Co³⁺,^26,27 Dy³⁺Co³⁺,^28,29 or Dy³⁺Eu³⁺ compounds,³⁰ in addition to some diluted Ln³⁺Y³⁺ complexes^31,32 that exhibit both slow relaxation of magnetization and luminescence thermometry (Table 1).

Table 1 Excitation wavelength (λ), temperature range (ΔT), maximum relative sensitivity (S_m), and corresponding temperature (T_m) of the Ln³⁺-based thermometers with slow relaxation of the magnetization reported so far

Ln^3+-SMM thermometer	U eff (K)/Hdc (kOe)	Thermometric parameter	λ (nm)	ΔT (K)	S m (% K^−1)	T m (K)	Ref.
TTA = 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione; H2bbpen = H2bbpen=N,N′-bis(2-hydroxybenzyl)-N,N′-bis(2-methylpyridyl)ethyl-enedi-amine; H3L^1,1,4 = N4O3 aminophenol; acac^− = acetylacetonate; PyAm = 2-amidinopyridine; PmAm = 2-amidinopyrimidine; bpm = 2,2′-bipyrimidine; tfac^− = 1,1,1-trifluoroacetylacetonate; tba^− = thiobenzoate; phen = phenantroline; bpyO2 = 2,2′-dipyridyl N,N′-dioxide; 4-benpyo = 4-benzyloxy-pyridine N-oxide; HL = 8-hydroxyquinoline; hfac^− = hexafluoroacetylacetonate; H2valdien = N1,N3-bis(3-methoxysalicylidene)diethylenetriamine; TPPO = triphenylphosphine oxide; hmpa = hexamethylphosphoramide.a Two components of the ^4F3/2 → ^4I9/2 transition.b Two components of the ^4F9/2 → ^6H13/2 transition.c Ligand (T1 → S0) and Dy^3+ (^4F9/2 → ^6H13/2) emission bands.d Two spectral ranges of the ^4F9/2 → ^6H15/2 transition.e Two components of the ^4F9/2 → ^6H15/2 transition.f Ligand and Dy^3+-related emission bands.g ^5D0 → ^7F4 transition upon 590 (^7F1 → ^5D0) and 612 nm (^7F2 → ^5D0) excitations.h Two spectral ranges of the ^5D4 → ^7F6 transition.i ^5I8 → ^5F1 (446 nm) and ^5I8 → ^5G6 (456 nm).j ^5I8 → ^5F1 (446 nm) and ^5I8 → ^5G6 (459 nm).k Two spectral ranges ascribed to the ^2F5/2 → ^2H7/2 transition.l Sr obtained from the fitting procedure of the lifetime values according to a Mott–Seitz model.m Ligand band.
[Nd(TTA)3(MeOH)2]	19.7/1.0	Intensity ratio	392	75–300	0.2^a	75	14
			360	10–300	0.02^a	125
[Dy(bbpen)Cl]	942.4/0		312	12–300	0.3^b	255	18
[Dy4(H3L^1,1,4)2(OAc)6]·CH3OH	39.7/0		358	11–295	1.6^c	295	17
[Dy(acac)3(H2O)2]·H2O	137/0		364	5–290	5.4^b	33	16
[Dy(acac)3(PyAm)]	Not Orbach/1.4		328	12–323	2.0^b	12	15
[Dy(acac)3(PmAm)]	Not Orbach/0		328	12–340	0.8^b	82
[Dy(acac)3(bpm)]	309/0		330	30–140	1.5^b	70	19
[Dy2(bpm)(tfac)6]	Not Orbach/0		405	5.4–85.5	1.8^d	5.4	12
			330	90–300	0.5^d	90
			330	298–398	3.3^c	298
[Dy(tba)3phen]	136.8/0		329	10–50	1.1^b	50	20
[DyCo(CN)6(bpyO2)2(H2O)3]·4H2O	86.5/1.0		375	11–200	1.8^b	70	28
					0.2^e	90
{[Dy4Co4(CN)24(4-benpyo)17(H2O)] 7H2O}n	Not Orbach/2.0		464	16–350	2.3^f	240	29
{(NEt4)2[(Ph4Si4O8)2Dy0.87Eu3.12(NO3)6(EtOH)(MeCN)2]}·4MeCN	16.3/1.0		590/612	293–373	1.2^g	293	30
{[Tb(H2O)2][Co(CN)6]·2.7H2O}@Y	Not Orbach/2.0		320	8–78	2.6^h	18.3	31
{Tb[Co(CN)6]}	854/3.0		310	6–42	5.3^h	15.9
{Tb[Co(CN)6]}@Y	859/3.0			6–96	5.4^h	14.7
{[Ho(4-pyridone)4(H2O)2][Co(CN)6]}@Y	Not Orbach/0.15		370	25–140	6.9^i	28	22
				60–195	5.8^j	58
{[Ho(4-pyridone)4(H2O)2][Rh(CN)6]}@Y	Not Orbach/0.1			25–130	2.7^i	42
				55–205	4.5^j	69
{[Ho(4-pyridone)4(H2O)2][Ir(CN)6]}@Y	Not Orbach/0.1			35–144	4.6^i	37
				70–197	5.3^j	61
[Yb2(valdien)2(NO3)2]	65.6 /1.0		390	80–320	1.1^k	130	23
[Yb(H3L^1,1,4)]·2MeOH	Not Orbach/1.6		360	25–300	0.2^k	300	24
[Yb(TPPO)3(NCS)3]	nr/1.5		320	10–300	1.5^k	150	25
[Yb(TPPO)3(NCSe)3]	nr/1.5		360	10–300	1.7^k	150
{(H5O2)2(H)[Yb(hmpa)4][Co(CN)6]2}	Not Orbach/1.0		320	50–175	4.6^k	50	26
[YbCo(CN)6(bpyO2)2(H2O)3]	Not Orbach /1.25		315	125–275	0.1^k	180	27
[Tb2(bpm)(acac)6]	34.3/1.0	Lifetime	350	83–283	4.9^l	250	21
[Tb2(bpm)(tfac)6]	nr		330	83–413	5.9^l	300
[Tb2(bpm)(hfac)6(H2O)2]	nr		350	83–323	2.4^l	320
[NaDy0.02Y0.98(μ2-L)4]n	68.0/0	Single intensity band	350	80–300	0.8^m	300	32

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Exploring the intricate interplay of factors that jointly affect both properties is a research topic still in its infancy. Therefore, in this work, leveraging our experience in luminescence thermometry of Ln³⁺ complexes with slow relaxation of magnetization,^{16,17,24,27,28} we investigate the impact of the aliphatic chain as a spacer between the N₃ units of a symmetric hexadentate N₆ ligand on the magnetic and thermometric properties of its Dy³⁺ derivatives. Moreover, we investigate the temperature determination using both secondary (requiring previous calibration) and primary (not requiring previous calibration) luminescence thermometry, comparing the performance of both approaches and discussing their ability to accomplish self-calibration of the luminescent thermometers.

Results and discussion

Synthetic procedures

The macrocyclic complexes {[Dy(L^N6en)(OAc)₂](NO₃)}·2H₂O (1·2H₂O) and {[Dy(L^N6prop)(OAc)₂](NO₃)}·CHCl₃ (2·CHCl₃) were obtained in a classical template synthesis, from the reaction of Dy(OAc)₃·4H₂O, Dy(NO₃)₃·6H₂O, ethylene- or propylendiamine and 2,6-pyridinedicarboxaldehyde in adequate molar ratios, as shown in Scheme 1. This procedure is similar to that previously described for the isolation of {[Dy(L^N6en)(NO₃)₂](NO₃)}.³³ In both cases, the complexes were obtained by refluxing the mixtures in methanol. The concentration of the reaction solutions leads to the isolation of oils, the recrystallisation of which in CHCl₃ layered with diethyl ether yields 1·2H₂O and 2·CHCl₃ as single crystals.

Scheme 1 Synthetic route for the isolation of {[Dy(L^N6en)(OAc)₂](NO₃)} and {[Dy(L^N6prop)(OAc)₂](NO₃)}. Solvate molecules are omitted for clarity.

These were characterized not only by X-ray diffraction studies but also by elemental analysis and IR spectroscopy (Fig. S1†). Thus, the IR spectra of both compounds show characteristic bands of the iminic and pyridinic C=N bonds at ca. 1660 and 1590 cm⁻¹, respectively.³⁴ In addition, both complexes show typical bands that can be attributed to the acetate ligands at ca. 1540 and 1450 cm⁻¹,³⁵ as well as a strong band at ca. 1330 cm⁻¹, in agreement with the presence of the nitrate counterion.^35,36

X-ray diffraction studies

Fig. S2† and Fig. 1 show ellipsoid diagrams for complexes 1·2H₂O and 2·CHCl₃, respectively, and their main distances and angles around the Dy³⁺ ions are summarised in Table S1.† Both complexes are quite similar to the previously reported {[Dy(L^N6cet)(OAc)₂](OAc)}·9H₂O, where L^N6acet is the Schiff base resulting from the condensation of acetophenone and ethylendiamine,³⁷ and to {[Dy(L^N6en)(NO₃)₂](BPh₄)}.³³

Fig. 1 Ellipsoid diagram (50% probability) for the cation [Dy(L^N6prop)(OAc)₂]⁺ in 2·CHCl₃.

1·2H₂O and 2·CHCl₃ also show many similarities between them, and for this reason, they will be described together. Their unit cell contains [Dy(L^N6x)(OAc)₂]⁺ (x = en or prop) cations, NO₃⁻ anions, and different solvates (two water molecules per complex in 1·2H₂O and one CHCl₃ molecule per complex in 2·CHCl₃). In the [Dy(L^N6x)(OAc)₂]⁺ cations, the semi-rigid Schiff base ligands L^N6en or L^N6prop act as neutral hexadentates, occupying the equatorial plane in the vicinity of Dy³⁺. The pyridine N-atoms are the ones that deviate the least from the mean calculated N₆ plane, and the deviation is even smaller for 1·2H₂O than for 2·CHCl₃, with the Dy³⁺ ion standing out very little from this theoretical plane (Table S2†). Nevertheless, the imine nitrogen atoms protrude significantly from this plane. Accordingly, these N-atoms are ca. 0.5 Å above or below the plane (2 above and 2 below) for 1·2H₂O, and ca. 0.73 Å for 2·CHCl₃, indicating that the longer aliphatic chain favours a greater distortion of this equatorial plane formed by the N₆ donor, as could be expected.

The coordination sphere of the Dy³⁺ ion is fulfilled by two acetate anions, acting as bidentate chelate donors, which leads to coordination number 10 around Dy³⁺, with a N₆O₄ environment. The calculations of the grade of distortion of the DyN₆O₄ core relative to an ideal polyhedron of ten vertexes with the SHAPE program³⁸ reveal a largely distorted geometry, between a tetradecahedron (as in the related {[Dy(L^N6en)(NO₃)₂](BPh₄)}³³) and a staggered dodecahedron for 1·2H₂O, and between a bicapped square antiprism and a sphenocorona for 2·CHCl₃ (Table S3†). Accordingly, the length of the spacer between the imine moieties has a profound impact on the geometry of the complex, which will obviously have significant repercussions for the magnetic and luminescence properties. The main angles and distances of the Dy³⁺ ion (Table S1†) are comparable with those previously reported for this kind of compound,^33,37 and do not deserve further consideration.

Finally, the mononuclear units [Dy(L^N6en)(OAc)₂]⁺ in 1·2H₂O establish hydrogen bonds with the water solvates, in such a way that pseudodimers are formed (Fig. 2), with an intermolecular Dy⋯Dy distance of 8.9404(6) Å. In the case of 2·CH₃Cl, the chloroform solvate seems to be held in the cell by weaker interactions, and no hydrogen bonds are observed, but a quite strong interaction between Cl12 and O2, with a distance of 2.82 Å, is observed (Fig. S3†). These interactions lead to longer intermolecular Dy⋯Dy distances of 9.8809(6) Å.

Fig. 2 Hydrogen bond scheme for 1·2H₂O, showing the pseudo-dimer arrangement.

In addition, powder X-ray diffraction measures for raw samples of 1·2H₂O and 2·CHCl₃ were performed (Fig. S4†). These studies show that the isolated products and the solved single crystals are the same complexes and that they have been obtained with high purity, given that no other peaks can be observed in the experimental diffractograms.

Magnetic properties

Direct-current (dc) magnetic susceptibility measurements were recorded for 1·2H₂O and 2·CHCl₃ as a function of the temperature. The graphs of χ_MT vs. T are shown in Fig. 3. The χ_MT values for the complexes at 300 K are 14.2 cm³ mol⁻¹ K in both cases, which are very close to the expected value for one uncoupled Dy³⁺ ion at room temperature (14.17 cm³ mol⁻¹ K).³⁹ The experimental curves continuously decrease until 2 K, reaching χ_MT values close to 10.5 cm³ mol⁻¹ K. This drop in the curves can be mainly ascribed to thermal depopulation of the excited Kramer's doublets (KDs), which arises from the split of the ground term by crystal field effects.

Fig. 3 (a) χ_MT vs. T and (b) M/Nμ_Bvs. H at 2 K for 1·2H₂O and 2·CHCl₃.

The magnetic field dependence of the reduced magnetisation at 2 K at the maximum applied field of 7 T is 5.8Nμ_B for 1·2H₂O and 5.3Nμ_B for 2·CHCl₃ (Fig. 3). These values are lower than the theoretical saturation characteristic of an isolated Dy³⁺ ion (10Nμ_B), but quite close to the value of 5Nμ_B for a highly anisotropic Dy³⁺ ion with an Ising-like ground doublet.²

The dynamic magnetic properties of the two mononuclear complexes were also studied, but none of the compounds show out-of-phase peaks of the susceptibility (χ′′_M) as a function of the temperature or the frequency above 2 K, as occurs in {[Dy(L^N6cet)(OAc)₂](OAc)}·9H₂O,³⁷ and {[Dy(L^N6en)(NO₃)₂](BPh₄)}.³³ This can be a consequence of the quantum tunnelling of the magnetization (QTM).

Accordingly, the alternating-current (ac) data were newly recorded in the presence of an optimum field of 2000 Oe (Fig. S5†), and now χ′′_M for 1·2H₂O and 2·CHCl₃ shows frequency and temperature dependence, with clear peaks (Fig. 4 and Fig. S6†), in agreement with single ion magnet (SIM) behaviour. Besides, the Cole–Cole plots (Fig. S7†) display curves with a wider distribution of the relaxation times for 1·2H₂O (α parameter in the range 0.45–0.53) than for 2·CHCl₃ (α values in the range 0.33–0.29). The relaxation dynamics were examined by the temperature dependence of the relaxation time (Fig. 5). The data deviate from linearity, indicating that the curve cannot be adequately reproduced by a simple Orbach model, as anticipated given the α parameter values. Hence, we have tried to fit these plots considering all the possible relaxation processes (Orbach, Raman, direct, and QTM), according to eqn (1), individually or grouped.

τ⁻¹ = τ₀⁻¹e^{−U_eff/k_BT} + CTⁿ + AT + τ_QTM⁻¹ (1)

Fig. 4 Frequency dependence of χ′′_M for 1·2H₂O (a) and 2·CHCl₃ (b) in a dc-applied field of 2000 Oe at different temperatures.

Fig. 5 Dependence of the relaxation time with temperature for 1·2H₂O and 2·CHCl₃ in a dc applied field of 2000 Oe. The blue and red solid lines account for the best fit (r² > 0.99) considering Orbach plus Raman relaxation processes.

The best fits of the data are obtained in both cases considering Orbach and Raman relaxation processes, and these render the parameters U_eff = 46.3(1) K (32.2 ± 0.7 cm⁻¹), τ₀ = 4.0(2) × 10⁻⁸ s, C = 0.4(1) s⁻¹ K⁻ⁿ, and n = 5.2(5) for 1·2H₂O, and U_eff = 127.0(4) K (88.3 ± 0.2 cm⁻¹), τ₀ = 1.3(1) × 10⁻¹³ s, C = 2.2(6) × 10⁻³ s⁻¹ K⁻ⁿ, and n = 8.7(1) for 2·CHCl₃.

The energy barrier between the ground and first excited state for 2·CHCl₃ was also calculated by luminescence spectroscopy (100.2 cm⁻¹, vide infra), and, accordingly, these studies support the thermally activated Orbach relaxation pathway. However, it is not possible to perform these spectroscopic calculations for 1·2H₂O, as detailed below, but its barrier is similar to the one obtained for {[Dy(L^N6en)(NO₃)₂](BPh₄)} (37.8 K).³³ In spite of this, the relaxation dynamics of 1·2H₂O were also analysed excluding the Orbach mechanism, and the data can be simply fitted with a Raman process (Fig. S8†), rendering the values C = 0.13(2) s⁻¹ K⁻ⁿ and n = 6.5(1), comparable with those observed for {[Dy(L^N6en)(NO₃)₂](BPh₄)}. Nevertheless, multiple attempts to fit the τ⁻¹vs. T curve for 2·CHCl₃ without considering the Orbach process were unsuccessful, which also supports the proposed Orbach plus Raman relaxation pathways. Comparing the energy barriers of 1·2H₂O, 2·CHCl₃ and the related {[Dy(L^N6en)(NO₃)₂](BPh₄)} complex reveals that the exchange between bidentate chelate nitrate and acetate ligands has minimal impact on U_eff, while the chain length between the N₃ entities of N₆ macrocyclic ligands significantly affects magnetic relaxation. This observation suggests that a greater barrier should be expected for 1·2H₂O and {[Dy(L^N6en)(NO₃)₂](BPh₄)}, due to their less distorted N₆ planes, and geometries closer to a pseudo-hexagonal bipyramid. However, U_eff is unexpectedly higher for 2·CHCl₃. Accordingly, it seems that the less semirigid ligand induces the highest energy barrier for the spin reversal. Additionally, as anticipated,⁴⁰ the nature of the counteranion appears to have no influence on U_eff.

Photoluminescence

Excitation spectra for both compounds were recorded while monitoring the ⁴F_9/2 → ⁶H_13/2 transition at 570 nm, revealing both sharp and broadband features (Fig. S9†). These features were assigned to transitions from the Dy³⁺ ions and organic ligands, respectively. The presence of the latter contribution indicates ligand-sensitized emission.

Fig. 6 depicts the temperature dependence of the emission spectra of 1·2H₂O and 2·CHCl₃, recorded using excitation wavelengths of 324 nm and 326 nm, respectively, in agreement with the maxima observed in the excitation spectrum. We observe the characteristic Dy^{3+ 4}F_9/2 → ⁶H_{11/2, 13/2, 15/2}intra-4f transitions within the visible spectral region. At all examined temperatures, the emission spectra exhibited a broad band in the UV-visible spectral region, attributed to the ligands. This band is notably more intense in 1·2H₂O than in 2·CHCl₃. This behaviour has been previously observed for Er³⁺ derivatives with the same macrocyclic ligands.⁴¹ Moreover, the energy of the triplet state for both ligands was estimated, and they do not differ significantly in energy (∼25 063 cm⁻¹).⁴¹

Fig. 6 Temperature dependence (11–350 K) of the emission spectra of (a) 1·2H₂O (excited at 324 nm) and (b) 2·CHCl₃ (excited at 326 nm). The insets depict enlarged views of the ligand and Dy³⁺ f–f transitions.

However, despite having the same energy for the donor state, minor structural differences may result in shifting donor–acceptor distances, i.e. the centroid of the ligand excited state (T₁ or S₁) to the Ln³⁺ ion. A shorter distance, in turn, may lead to a faster energy transfer rate.⁴² For instance, it is known that the excited states of the ligand are localized near an electronic delocalization structure, as discussed elsewhere.^42–46 This may imply variations in the energy transfer rates from the excited states (in this case, the T₁) of the macrocyclic ligand L^N6en or L^N6prop to Dy³⁺. Qualitatively, it is clear that the ligand-to-metal energy transfer is more effective in 2·CHCl₃, which can be justified based on structural differences, even when this difference is slight. In 2·CHCl₃, the donor–acceptor distance of the triplet state is shorter (Fig. S10†), leading to higher energy transfer rates and a more efficient “antenna” effect.

For 2·CHCl₃, we deconvoluted the ⁴F_9/2 → ⁶H_15/2 transition to extract more detailed information (Fig. S11†). As Dy³⁺ is a Kramer's ion, the maximum crystal field splitting of a level into KDs should be J + 1/2. The presence of at least 9 components, exceeding the anticipated 8 KDs for the ⁶H_15/2 level (attributed to the low symmetry Dy³⁺ local group), points out that the first excited KD of the ⁴F_9/2 level is populated (even at 11 K), resulting in the formation of what is commonly referred to a hot band. The deconvolution permits calculation of the relative energy of each KD, as depicted in Fig. S12.† The results reveal an energy gap between the first two KDs of the ⁶H_15/2 ground level of 100.2 ± 0.3 cm⁻¹ (referred to as ΔE₁). The energy gap between the first two KDs of the ⁴F_9/2 emitting level (referred to as ΔE₂) is 98 ± 0.3 cm⁻¹.

As mentioned above, the value found, 100.2 ± 0.3 cm⁻¹, is similar to but slightly higher than the energy barrier observed in the ac magnetic studies (88.3 ± 0.2 cm⁻¹). Comparable differences between magnetic and luminescent calculated barriers were found for previously reported complexes,⁴⁷ thus supporting the interpretation of the magnetic results. On the contrary, for 1·2H₂O, the overlap between the emissions of the ligands and Dy³⁺ ions precluded the deconvolution (Fig. 6a), and, therefore, the calculation of the energy gaps between m_j states.

Luminescence thermometry

The ratio between the integrated intensity of the ligand (I_L) and that of the dominant Dy^{3+ 4}F_9/2 → ⁶H_13/2 transition (I_F) is defined as the thermometric parameter (Δ) and is used to assess the thermometric properties of 1·2H₂O and 2·CHCl₃ (Fig. S13 and section 4 of the ESI† for details). The difference in the temperature dependence of I_L and I_F in both complexes underscores the influence of the spacer length between the two symmetrical N₃ rigid moieties of the ligand on the luminescence of these compounds.

Fig. 7 shows the temperature dependence of Δ for both compounds, which exhibits a linear decrease. For 2·CHCl₃, the operating range as a luminescent thermometer encompasses 11–286 K, while for 1·2H₂O it is narrower (11–211 K). For temperatures higher than 286 and 211 K, respectively, the variations in Δ are of the same order of their uncertainties and, thus, 1·2H₂O and 2 CHCl₃ are out of the thermometric operating range.⁴⁸

Fig. 7 Temperature dependence of the thermometric parameter (Δ) for (a) 1·2H₂O and (b) 2·CHCl₃. The lines represent the best linear fits (r² > 0.99) to the experimental data (fit details in Table S5†). Temperature dependence of (c) S_r and (d) δT for 1·2H₂O and 2·CHCl₃.

The thermometric performance of the two compounds was assessed through the typical figures of merit for luminescent thermometers: S_r and δT^49,50 (Fig. 8 and ESI† for details). Whereas 1·2H₂O achieves a maximum S_r value of 2.3% K⁻¹, 2·CHCl₃ has 5.1% K⁻¹. The corresponding minimum temperature uncertainty values are 0.1 K and 0.2 K, respectively, at the indicated temperatures.

Fig. 8 (a) Comparison between the temperature values measured using a reference silicon thermometer (y) and calculated using eqn (2) (x) for 2·CHCl₃. The solid line is a guide for the eyes corresponding to y = x. The horizontal error bars are the temperature uncertainty on the calculated temperature, calculated using eqn (S10).† The uncertainties in the measured temperature (not represented) are 0.5 K (12–30 K), 0.25 K (30–60 K), and 0.15 K (60–300 K). (b) Dependence of the calibrated temperature (y) with the calculated value (x). The solid line is a guide for the eyes corresponding to y = x. The horizontal and vertical error bars are the temperature uncertainty on the calculated temperature, calculated using eqn (S10) and (S6),† respectively.

In addition to the previously discussed intensity ratio between the ligand and the ⁴F_9/2 → ⁶H_13/2 transition, the concept of a primary luminescent thermometer can also be applied in 2·CHCl₃. Primary luminescent thermometers eliminate the need for prior calibration procedures, a major advantage in luminescence thermometry.⁵¹ Indeed, 2·CHCl₃ is the first example of a Dy³⁺ primary luminescent thermometer based on the intensity ratio between transitions from the lowest energy state or second lowest energy state of the excited ⁴F_9/2 level to the ⁶H_15/2 ground state multiplet.⁵¹ The other reported example of a Dy³⁺ primary luminescent thermometer with slow magnetic relaxation operating in the presence of a magnetic field up to 3.6 T is based on the classical Mott–Seitz model.¹⁶

Beyond Dy³⁺, primary luminescent thermometers have also been explored in Yb³⁺/Er³⁺-doped upconverting materials,^51–53 and Yb³⁺/Pr³⁺ co-doped fluoride phosphate glasses.⁵⁴ This technique uses the ratio between the intensities arising from two thermally coupled levels of the Ln³⁺ ions that are directly related to the absolute temperature, T, through a well-established equation:

where k_B is the Boltzmann constant, ΔE₂ is the energy gap between the barycenters of the thermally coupled levels (Fig. S11†), and Δ′ is the thermometric parameter ascribed to the two Ln³⁺ thermally coupled KDs of the ⁴F_9/2 level, with Δ′₀ being the value at the temperature T₀. Usually ΔE₂ and Δ′₀ are determined independently of the thermal calibration procedure, the first by a deconvolution procedure involving the emission spectra for determining the energy gap value and the second by an extrapolation of Δ′ when external heating is negligible.⁵¹

Taking advantage of the distinct temperature dependence of the deconvolution of the ⁴F_9/2 → ⁶H_15/2 transition, the energy gap between the fundamental and first level (hot band) of the ⁴F_9/2 state was estimated to be ΔE = 98 ± 0.3 cm⁻¹ (Fig S12†). The excitation source does not induce local discernible temperature increments, ensuring that the Δ′₀ value could be determined for any given temperature value.

To illustrate that 2·CHCl₃ is a luminescent primary thermometer, we begin by plotting ln(Δ′) versus 1/T to identify the temperature range for which eqn (2) is valid (i.e., a linear relation between ln(Δ) and 1/T, Fig. S14†). We conclude that a linear relation is observed for 86 ≤ T ≤ 211 K. Thus, we set T₀ = 211 K and Δ′₀ = 0.1477, where T₀ corresponds to the value that minimizes the absolute deviation between the calculated and measured temperatures. Next, we apply eqn (2) and find that the temperature value predicted by the primary thermometer fits well with the temperature determined experimentally by the calibration procedure up to 211 K. The deviation between predicted and measured values can be quantified using the mean absolute deviation, which is 5% for the primary thermometer (Fig. S15†). The S_r and δT for the primary 2·CHCl₃ luminescence thermometer (Fig. S16†) are comparable with those of the secondary thermometer, the best values being 1.8%·K⁻¹ and 2 K, respectively, at 86 K.

Additionally, and as an added benefit, in 2·CHCl₃ the primary molecular thermometer can be used to calibrate the secondary one, offering an extra advantage of this compound. Fig. 8b illustrates the calibration of the ratio between the integrated intensity of the ligand and that of the Dy^{3+ 4}F_9/2 → ⁶H_13/2 transition (secondary thermometer) using the temperature calculated using eqn (2) (primary thermometer). Notably, the uncertainty associated with the calculated temperature values is comparable with that of the calibrated temperature. A previous study on a poly(methylmethacrylate) film containing Er³⁺- and Tm³⁺-doped upconverting nanoparticles has demonstrated the use of a luminescent primary thermometer based on the Er^{3+ 2}H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions to calibrate a secondary thermometer based on the ¹G₄ → ³H₆ (Tm³⁺) and the ⁴S_3/2 → ⁴I_15/2 (Er³⁺) transitions.⁵⁵

Conclusions

The structural features and the magnetic and luminescence properties of mononuclear Dy³⁺ complexes derived from symmetrical N₆ semirigid macrocycles are strongly influenced by the spacer between the two N₃ rigid moieties. This is evident in the different coordination geometries observed for {[Dy(L^N6en)(OAc)₂](NO₃)}·2H₂O (1·2H₂O) (tetradecahedron) and {[Dy(L^N6prop)(OAc)₂](NO₃)}·CHCl₃ (2·CHCl₃) (bicapped square antiprism). These different environments also impact the magnetic properties of the complexes, which behave as SIMs in the presence of a dc field, with 2·CHCl₃ exhibiting the highest energy barrier.

The luminescence measurements show that the ratio between the integrated intensity of the ligands and that of the Dy^{3+ 4}F_9/2 → ⁶H_13/2 transition can be used to develop secondary luminescent thermometers with maximum relative thermal sensitivity values of 2.3 (1·2H₂O) and 5.1% K⁻¹ (2·CHCl₃). The minimum temperature uncertainties are, respectively, 1.3 (at 211 K) and 0.6 K (at 286 K). These values are comparable with those already reported for analogous thermographic SIMs (Table 1), and also show the better thermometric characteristics of 2·CHCl₃ with respect to 1·2H₂O.

The deconvolution of the ⁴F_9/2 → ⁶H_15/2 transition in 2·CHCl₃ provides further support for the determination of the energy barrier for magnetic relaxation. Moreover, this deconvolution enables the demonstration of the first example of a Dy³⁺ primary luminescent thermometer based on two thermally coupled KDs of the ⁴F_9/2 level. Additionally, 2·CHCl₃ is the first self-calibrated luminescent thermometer with magnetic relaxation, operating within the 86–211 K range. The calibration of the luminescent secondary thermometer closely aligns with the calculated temperature of the primary thermometer, with observed uncertainties in calculated temperature values comparable with the calibrated temperatures. As the intensity ratio between the fundamental and first excited KDs of the ⁴F_9/2 level and the ground KD of the ⁶H_15/2 multiplet is calibrated through eqn (2), it eliminates the need for recurrent calibration procedures whenever the system operates in new conditions. Therefore, 2·CHCl₃ is unique among its class, and it not only contributes to increasing the very small number of SMMs with primary luminescence thermometry, but also represents a significant advancement in the various mechanisms of luminescent thermometers using Dy³⁺ as the emitter.

Experimental

Materials and general methods

All chemical reagents were purchased from commercial sources and used as received without further purification. Elemental analyses of C, H, and N were performed on a Themoscientific Flash Smart analyser. FT-IR spectra were recorded on a Varian FT–IR 670 spectrometer equipped with an attenuated total reflectance (ATR) accessory, in the 500–4000 cm⁻¹ range.

Syntheses

The Dy³⁺ complexes were obtained by variation of a method previously reported.⁴⁰ The synthesis of {[Dy(L^N6en)(OAc)₂](NO₃)}·2H₂O (1·2H₂O) exemplifies the procedure.

{[Dy(L^N6en)(OAc)₂](NO₃)}·2H₂O (1·2H₂O). 2,6-Pyridinedicarboxaldehyde (0.270 g, 2.000 mmol) was dissolved in dry methanol (40 mL). Then, dysprosium acetate tetrahydrate (0.274 g, 0.667 mmol), dysprosium nitrate hexahydrate (0.152 g, 0.333 mmol), and ethylenediamine (99%, 0.142 mL, 2.100 mmol) were added. After 10 min of stirring, the reaction was refluxed for 24 hours, giving an orange solution. Removing the solvent under vacuum produced an orange oil. The oil was dissolved in chloroform (20 mL), and the solution was layered with diethyl ether and kept in the fridge (at ∼5 °C). After a few days, single crystals of [Dy(L^N6en)(OAc)₂](NO₃)·2H₂O (1·2H₂O) were isolated. The crystals were filtered and dried in air. Yield: 0.366 g (53%). Elemental analysis calcd for C₂₂H₂₈DyN₇O₉ (697.00): C 37.91, N 14.07, H 4.05%. Found: C 38.17, N 13.85, H 4.18%. IR (ATR, [small nu, Greek, tilde]/cm⁻¹): 1534 (COO), 1594 (C=N_py), 1659 (C=N_imine), 3415 (OH).

{[Dy(L^N6prop)(OAc)₂](NO₃)}·CHCl₃ (2·CHCl₃). 2,6-Pyridinedicarboxaldehyde (0.270 g, 2.000 mmol) in dry methanol (40 mL), dysprosium acetate tetrahydrate (0.274 g, 0.667 mmol), dysprosium nitrate hexahydrate (0.152 g, 0.333 mmol), 1,3-diaminopropane (99%, 0.178 mL, 2.100 mmol). Recrystallisation as in 1·2H₂O yielded single crystals of [Dy(L^N6prop)(OAc)₂](NO₃)·CHCl₃ (2·CHCl₃). Yield: 0.224 g (29%). Elemental analysis calcd for C₂₅H₂₈DyCl₃N₇O₇ (808.40): C 37.10, N 12.12, H 3.59%. Found: C 37.33, N 12.16, H 3.71%. IR (ATR, [small nu, Greek, tilde]/cm⁻¹): 1539 (COO), 1592 (C=N_py), 1655 (C=N_imine), 3440 (OH).

Single X-ray crystallographic refinement and structure solution

The crystal data and details of refinement are given in Table S4.† Data were collected at 100 K on a Bruker D8 VENTURE PHOTON III-14 diffractometer, employing graphite monochromatised Mo-Kα (λ = 0.71073 Å) radiation. Multi-scan absorption corrections were applied using the SADABS routine.⁵⁶ The structure was solved by standard direct methods employing SHELXT,⁵⁷ and then refined by full-matrix least-squares techniques on F² using SHELXL,⁵⁸ from the program package SHELX-2018.^57,58 All atoms different from hydrogen were anisotropically refined, while H atoms were typically included in the structure factor calculations in geometrically idealised positions. However, to reveal the hydrogen bonding scheme, hydrogen atoms belonging to water solvate in 1·2H₂O were located in the corresponding Fourier map. In this case, they were freely refined.

Powder X-ray diffraction studies

The powder diffractograms for 1·2H₂O and 2·CHCl₃ were recorded in a Philips diffractometer with a control unity type “PW1710”, a vertical goniometer type “PW1820/00” and a generator type “Enraf Nonius FR590”, operating at 40 kV and 30 mA, using monochromated Cu-K_α (λ = 1.5418 Å) radiation. A scan was performed in the range 2 < 2θ < 30° with t = 3 s and Δ2θ = 0.02°. LeBail refinement was obtained with the aid of HighScore Plus Version 3.0d.

Magnetic measurements

Magnetic susceptibility dc and ac measurements for microcrystalline samples of 1·2H₂O and 2·CHCl₃ were carried out with a PPMS Quantum Design susceptometer. The dc magnetic susceptibility data were recorded under a magnetic field of 1000 Oe in the temperature range 2–300 K. Magnetisation measurements at 2.0 K were recorded under magnetic fields ranging from 0 to 70 000 Oe. Diamagnetic corrections were estimated from Pascal's Tables. Alternating current (ac) susceptibility measurements at zero dc field were performed with an oscillating ac field of 3.5 Oe, and ac frequencies of 8000 and 10 000 Hz in the 2–20 K temperature range. ac measurements at 3 K were also recorded under magnetic fields from 250 to 3000 Oe, with an oscillating ac field of 3.5 Oe, and frequencies ranging from 50 to 1500 Hz. New ac data were acquired for 1·2H₂O and 2·CHCl₃ under a dc field of 2000 Oe, and with frequencies ranging from 50 to 10 000 Hz.

Luminescence measurements

The temperature-dependent emission and excitation spectra of 1·2H₂O and 2·CHCl₃ (in the solid state) were acquired in a Fluorolog3® spectrofluorometer (FL3-2T, Horiba) with a modular double grating excitation monochromator (fitted with a 1200 grooves per mm grating blazed at 330 nm) and a TRIAX 320 single emission monochromator (fitted with a 1200 grooves per mm grating blazed at 500 nm, reciprocal linear density of 2.6 nm mm⁻¹), coupled with a photomultiplier (R298, Hamamatsu), using the front face acquisition mode. The excitation source was a 450 W Xe arc lamp. The emission spectra were corrected for detection and the optical spectral response of the spectrofluorimeter, and the excitation spectra were corrected for the spectral distribution of the lamp intensity using a photodiode reference detector. The temperature was controlled using a helium-closed cycle cryostat, a vacuum system (4 × 10⁻⁴ Pa), and an autotuning temperature controller (Lakeshore 330, Lakeshore) with a resistance heater. The temperature was measured using a silicon diode cryogenic sensor (DT-470-SD, Lakeshore) with accuracies of ±0.5 K (12–30 K), ±0.25 K (30–60 K), and ±0.15 K (60–340 K).

Thermometric measurements

The thermometric performance of the materials was determined using MatLab®. The integrated areas of the transitions and the corresponding errors together with the thermometric parameters were determined as detailed in section 4 in the ESI.† The deconvolution of the ⁴F_9/2 → ⁶H_13/2 Dy³⁺ transition was implemented in OriginLab^© as detailed in the same section.

Author contributions

Conceptualization: J.C.-V., L.D.C., M.F.; methodology: J.C.-V., C.G.-B, M.A.H.-R., J.S.-M.; validation: L.D.C., M.F.; formal analysis: J.C.-V., A.M.G.-D., C.D.S.B., M.F.; investigation: J.C.-V., C.G.-B; resources: L.D.C., M.F.; visualization: J.C.-V., C.D.S.B.; data curation and supervision: L.D.C., M.F.; writing – original draft preparation: J.C.-V., L.D.C., C.D.S.B, M.F.; writing – review & editing: J.C.-V., C.D.S.B., L.D.C., M.F.; project administration: L.D.C., M.F.; funding acquisition: J. C.-V., L.D.C., M.F. All authors have read and agreed to the published version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was developed within the scope of the project CICECO – Aveiro Institute of Materials, UIDB/50011/2020, UIDP/50011/2020, and LA/P/0006/2020, financed by national funds through the FCT/MCTES (PIDDAC). This article is also based upon work from COST Action CA22131, supported by COST (European Cooperation in Science and Technology). The authors acknowledge fruitful discussions with Dr Albano N. Carneiro Neto (University of Aveiro) concerning ligand-to-metal energy transfer. J. C. V. also thanks Xunta de Galicia for his postdoctoral fellowship (ED481B-2022-068).

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Footnotes

† Electronic supplementary information (ESI) available: Structural, magnetic, photoluminescence and thermometric characterization. CCDC 2238908 and 2238909. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi02374e

‡ Current address: Departamento de Física, Universidad de La Laguna, Apdo. Correos 456, E-38200 San Cristóbal de La Laguna, Santa Cruz de Tenerife, Spain.

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