Multifunctional properties existing in Ln–nitronyl nitroxide single-chain magnets

Abstract

Taking advantage of a nitronyl nitroxide radical ligand 8-QNNIT (1) (8-quinolyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) and Ln(hfac)₃·2H₂O (Ln^III = Tb 2 and Dy 3; hfac = hexafluoroacetylacetonate), two Ln–radical one-dimensional chains [Ln(hfac)₃(8-QNNIT)]_n were constructed. The magnetic properties, thermodynamics and optical behavior of both compounds were observed, exhibiting polyfunctionality of the 2p–4f system. Complexes 2 and 3 show typical single-chain magnet (SCM) properties with temperature-dependent relaxation peaks giving an effective energy barrier (U_eff/k_B) of 47.4 K (2) and 64.6 K (3). Luminescence spectra and ultravioletvisible (UV-vis) absorption spectra of the Tb compound, and UV-vis absorption spectra and thermodynamics properties of the Dy complex were investigated. As far as we know, the Dy complex is the first SCM with thermodynamics behavior and optical properties comparable to those of multifunctional magnetic materials. This study aims to provide a new way and field of vision for the construction of 2p–4f heterospin multi-functional materials.

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Introduction

Single-chain magnets (SCMs) are one-dimensional compounds with strong Ising-type magnetic anisotropy, significant intra-chain magnetic coupling between spin carriers and negligible inter-chain interaction, displaying slow magnetic relaxation behavior.¹ Based on the above factors, the incorporation of anisotropic metal ions and short bridges such as hydrone (H₂O), hydroxyl (OH⁻) and Br⁻, which can strengthen the magnetic interaction between adjacent spin carriers, is one of the effective chemical routes to synthesize single-chain magnets.^2–4 Recently, Yang et al. have obtained water and hydroxyl-extended lanthanide-based SCMs with a relatively high relaxation energy barrier.² On the other hand, conjunction with anisotropic 4f/3d ions and radical ligands, which will be able to transmit effectively magnetic interactions, can also produce SCMs successfully. One of the representative achievements is that Gatteschi and co-workers obtained the first single-chain magnet [Co(hfac)₂(NITPhOMe)]_n.⁵ Since then, several remarkable single-chain magnets involving a radical and 3d/4f/3d–4f metal ions have been reported.⁶ Lately, Cheng et al. employed an air-stable nitronyl nitroxide radical and Co²⁺ ions to achieve successfully a quasi-1D spin organization, with giant coercive fields up to 65 kOe.⁷

On the other hand, compared to an intensive study of the magnetic properties, thermodynamics behavior and luminescence properties of lanthanide–radical complexes have rarely been explored. Moreover, in recent years, molecules with multifunctional functionalities are currently of great interest in coordination chemistry. For example, Deun et al. prepared a series of 4f-based nanosheets for optical luminescence thermometry.⁸ And the dual functionality of an Yb compound for slow magnetic relaxation and luminescence thermometry was explored.⁹ Quite lately, tetranuclear lanthanide complexes based on a functionalized nitronyl nitroxide biradical, displaying slow magnetic relaxation behavior and luminescence performance, were obtained successfully.¹⁰ In addition, Vaz et al. constructed lanthanide compounds with a dppnTEMPO radical displaying two relaxation processes and the presence of two distinct luminescent centers.¹¹ However, to date, lanthanide–nitronyl nitroxide complexes with SCM propertyies, thermodynamics or/and optical behavior coexisting in single molecules have not been reported.

In this regard, to investigate multifunctional properties of 2p–4f heterospin SCMs, herein, we synthesize a new nitronyl nitroxide radical 8-QNNIT (1) (8-quinolyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) and utilize the radical ligand to achieve two heterospin 1D compounds, namely, [Ln(hfac)₃(8-QNNIT)]_n (Ln^III = Tb 2, Dy 3; hfac = hexafluoroacetylacetonate). Additionally, the magnetic, optical and thermodynamics properties of the 2p–4f system have been systematically investigated. Tb and Dy compounds display frequency-dependent out-of-phase signals indicating SCM behavior. Moreover, the Tb–radical chain shows the characteristic emission peaks of metal Tb^III ions. The heat capacity of the Dy compound is also studied in the paper. To our knowledge, this is the first report on nitronyl nitroxide-based Ln^III compounds with SCM properties and optical or/and thermodynamics behavior coexisting in a molecular entity.

Experimental section

General materials and physical measurements

All the reagents were used directly without any further refinement. Elemental analyses (for C, H, and N) were implemented on a PerkinElmer 240 elemental analyzer. FT-IR spectra were recorded in the region of 4000–400 cm⁻¹ on a Bruker TENOR 27 spectrometer. Magnetic data were recorded for samples 1–3 employing a Quantum Design SQUID VSM magnetometer, corrected for the diamagnetic 1–3 with Pascal's constants and these sample holders by measurement.¹² Employing an F-4500 fluorescence spectrophotometer, the excitation and emission spectra of the Tb complex were measured. Ultraviolet-visible (UV-vis) spectra were recorded through a TU-1901 spectrophotometer. The heat capacities were obtained using a Netzsch DSC 204 F1 analyzer in an N₂ atmosphere.

Preparation of [8-QNNIT] (1)

Radical 8-QNNIT (8-quinolyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide) was synthesized according to published literature.¹³ The 8-QNNIT radical was recrystallized in CH₂Cl₂/heptane (2 : 5) mixed solution to obtain purple strip crystals. Yield 83%. C₁₆H₁₈N₃O₂ (284.33 g mol⁻¹): calcd C 67.58, H 6.38, N 14.78; found C 67.32, H 6.39, N 14.50. FT-IR (KBr): 3411 (s), 2348 (m), 1625 (s), 1613 (s), 1221 (s), 1053 (s), 945 (m), 806 (m), 701 (m), 527(m), 506 (m) cm⁻¹.

Preparation of compounds 2 and 3

The synthetic reaction of both compounds was based on the following process. Ln(hfac)₃·2H₂O (0.01 mmol) (Ln = Tb 2 and Dy 3) in 16 mL of n-heptane was allowed to reflux for 3.5 hours, to which a solution of 8-QNNIT (0.0028 g, 0.01 mmol) in dichloromethane (4 mL) was introduced. The resulting solution was stirred for about 25 min to give a pink reaction mixture, and then the above solution was cooled to room temperature. A pink filtrate was left standing for 48 hours at ambient temperature, and pink strip crystals were generated.

[Tb(hfac)₃(8-QNNIT)]_n (2). Yield 65%. C₃₁H₂₁F₁₈N₃O₈Tb (1064.42 g mol⁻¹): calcd C 34.98, H 1.99, N 3.95; found C 34.67, H 1.71, N 3.86; FT-IR (KBr): 3420 (s), 1625(m), 1618 (m), 1343 (m), 1140 (s), 1074 (s), 958 (s), 859 (s), 776 (m), 548 (s), 525 (s) cm⁻¹.

[Dy(hfac)₃(8-QNNIT)]_n (3). Yield 60%. C₃₁H₂₁F₁₈N₃O₈Dy (1067.98 g mol⁻¹): calcd C 34.86, H 1.98, N 3.93; found C 34.71, H 1.58, N 3.74; FT-IR (KBr): 3419 (s), 1626 (m), 1616 (m), 1343 (m), 1141 (s), 1075 (s), 957 (s), 858 (s), 775 (m), 548 (s), 526 (s) cm⁻¹.

X-ray crystallography

Diffraction data of compounds 1–3 were gathered on a Rigaku Saturn diffractometer with Mo-Kα radiation (λ = 0.71073 Å) at 293 K for 1 and at 150 K for 2 and 3. Absorption correction was carried out via CrystalClear. The structures of the radical ligand and Tb and Dy compounds were solved by direct methods with the Olex2 program, and refined by F² by a full-matrix least-squares procedure using SHELXL-2014.¹⁴ H atoms attached to C atoms were placed in reasonable positions, and then refined with a riding model. Anisotropic refinement was imposed on other non-H atoms. CCDC 2024373–2024375 (1–3) contain the supplementary crystallographic data.† The crystal data and structure refinement of three compounds are listed in Table 1. The important bond lengths and angles of complexes 2 and 3 are summarized in Table 2 and Tables S1 and S2 (ESI†).

Table 1 Crystallographic data and structure refinement summary for 1–3

Complex	1	2	3
Empirical formula	C16H18N3O2	C31H21F18N3O8Tb	C31H21F18N3O8Dy
M r	284.33	1064.42	1067.98
T (K)	293(2)	150(2)	150(2)
Crystal system	Orthorhombic	Monoclinic	Monoclinic
Space group	Pca21	P21/c	P21/c
a/Å	15.2203(3)	22.868(5)	22.7736(11)
b/Å	8.09480(16)	16.664(3)	16.6481(8)
c/Å	11.9026(3)	22.826(5)	22.7247(11)
α/°	90	90	90
β/°	90	117.20(3)	117.2050(10)
γ/°	90	90	90
V/Å^3	1466.46(5)	7736(3)	7662.7(6)
Z	4	4	4
D calcd/g cm^−3	1.288	1.828	1.852
μ/mm^−1	0.703	1.966	2.090
θ/°	5.46–67.23	2.890–25.906	2.015–26.420
F(000)	604	4152	4160
Reflections collected	8594	78 446	83 997
Unique reflns/Rint	2494/0.0235	14 336/0.0789	15 709/0.0607
GOF (F^2)	1.055	1.058	1.086
R 1, wR2 (I > 2σ(I))	0.0308, 0.0829	0.0729, 0.1822	0.0613, 0.1656
R 1, wR2 (all data)	0.0323, 0.0837	0.1075, 0.2062	0.0837/0.1829

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Table 2 The important bond lengths [Å] and angles [°] for complexes 2 and 3

Complex	2	3
Ln–O(hfac)	2.345(6)–2.393(7)	2.322(5)–2.384(5)
Ln–O(rad)	2.333(6)–2.389(7)	2.316(5)–2.384(5)
Ln–O–N	137.2(6)–149.2(6)	137.5(4)–148.7(4)

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Results and discussion

Molecular structures

Complex 1 belongs to the orthorhombic crystal system. The molecular structure of the 8-QNNIT radical is given in Fig. 1. Two O(1)–N(2) and O(2)–N(3) distances are 1.2749(17) Å and 1.2772(16) Å, respectively, which are in line with the N–O contact in the radical (1.25 to 1.32 Å).¹⁵ The dihedral angle between the quinoline ring and five-membered heterocyclic rings of the radical is 78.86(1)°. In the intermolecular, the nearest NO–NO contact with the O⋯O distance is 4.5627(25) Å (Fig. S1, ESI†).

Fig. 1 Molecular structure of complex 1, where all the hydrogen atoms are omitted for clarity.

Crystallographic studies manifest that isostructural 2 and 3 are crystallized in monoclinic space group P2₁/c (Fig. 2 and Fig. S2, ESI†). Taking compound 3 for example, the compound displays a one-dimensional 4f-system, which is supported by 8-QNNIT radicals. Each Dy^III ion ligates two oxygen atoms of NO groups, and the rest of the coordination sphere is completed with six oxygen atoms by using three hfac⁻ coligands. Both crystallographically independent Dy(hfac)₃(8-QNNIT) moieties belong to different 1D chains. The SHAPE measure analysis¹⁶ gives these values (0.372 (Dy1) and 0.246 (Dy2)) for a triangular dodecahedron geometry (D_2d symmetry) (Fig. S5 and Table S3, ESI†). The Dy^III–O_hfac bond lengths are in the range of 2.322(5)–2.384(5) Å, while Dy^III–O_rad distances vary in the range of 2.316(5)–2.384(5) Å.

Fig. 2 Single-crystal X-ray diffraction structure of complex 3, in which H and F atoms are omitted for clarity.

As can be found, Dy–radical chains are well separated (Fig. 3), with the nearest Dy⋯Dy distance being 11.65 Å for 3, which can avert significant intermolecular interactions.

Fig. 3 Packing diagram of complex 3, where all the H and F atoms are omitted for clarity.

Magnetic properties

Direct current magnetic susceptibility measurements for complex 1 (Fig. 4) display that the experimental χ_MT value of 0.374 cm³ K mol⁻¹ (at 300 K) agrees with the theoretical value (S = 1/2, g = 2.0, C = 0.375 cm³ K mol⁻¹) for an 8-QNNIT radical. The χ_MT product remains almost constant as the temperature decreases to 30 K. Then, the value of χ_MT rises quickly upon further cooling, attaining 0.408 cm³ K mol⁻¹ at 2 K. Assuming isotropic Heisenberg interactions, the reported equation of the magnetic susceptibility in terms of ferromagnetically¹⁷ coupled (J) linear arrays of 1/2 spins is

χ_M = (C/T)[(1 + 5.7979916K + 16.902653K² + 29.376885K³ + 14.036981K⁵)/(1 + 2.7979916K + 7.0086780K² + 8.6538644K³ + 4.5743114K⁴)]^2/3

where K = J/2kT and zJ′ represents the magnetic interaction between uncoordinated NO groups.

Fig. 4 Plots of χ_MT and χ_M⁻¹versus T for compound 1. The solid line represents the best fitting.

The best fit values are J = 0.11 cm⁻¹, zJ′ = −0.009 cm⁻¹, C = 0.3755 emu K mol^−l, and g = 2.0. In addition, the reciprocal susceptibility vs. the temperature abides by the Curie–Weiss law (2–300 K) with Weiss constant θ = 0.152 K and Curie constant C = 0.40 cm³ K mol⁻¹, implying the existence of ferromagnetic interactions.

The observed positive values of J and θ may result from dominant ferromagnetic coupling, derived from the pseudo 1D chain (Fig. 5), in which the bridging sp² carbon atoms (C10) and the oxygen atoms (O1) from the NO groups of the neighboring 8-QNNIT radicals carry the opposite sign spin densities, following McConnell's law.¹⁷ The negative value of zJ′ arises from short NO–ON contacts and π⋯π interactions.

Fig. 5 The pseudo 1D chain of complex 1 presenting the alternating spin densities.

The static direct-current (dc) magnetic susceptibility properties for compounds 2 and 3 were measured under a dc field of 1 kOe from 300 to 2 K (Fig. 6). At 300 K, the χ_MT values are 12.27 cm³ K mol⁻¹ for 2 and 14.94 cm³ K mol⁻¹ for 3, which are slightly more than the theoretical values (12.19 cm³ K mol⁻¹ for 2 and 14.54 cm³ K mol⁻¹ for 3) for one non-interacting Ln^III ion (Tb^III: ⁷F₆, S = 3, L = 3, g = 3/2, C = 11.82 cm³ K mol⁻¹; Dy^III: ⁶H_15/2, S = 5/2, L = 5, g = 4/3, C = 14.17 cm³ K mol⁻¹) and one 8-QNNIT radical (S = 1/2, C = 0.375 cm³ K mol⁻¹).

Fig. 6 Plots of χ_MT–T for 2 (left) and 3 (right). Insets: ln(χ_MT)–1/T plots. The red solid lines represent the best fits.

Upon cooling, the χ_MT value of compound 2 decreases steadily to 7 K and the value of 3 remains almost constant above 100 K. Then χ_MT values go down, followed by an abrupt increase for both complexes, and finally drop to 6.07 cm³ K mol⁻¹ for 2 and 4.52 cm³ K mol⁻¹ for 3 at 2.0 K. At low temperature, the χ_MT values of both compounds display a degree of rise, suggesting the presence of nearest-neighbor heavy lanthanide metal–radical ferromagnetic coupling (Fig. 6).

For the Ising-like or anisotropic Heisenberg 1D chain, the value of χ_MT could follow an exponential behavior (χ_MT ≈ C_eff exp(Δ_ξ/T)), in which the energy needed to create a domain wall along the chain is represented by Δ_ξ.^1a For 2 and 3, the ln(χ_MT) vs. 1/T plot displays a linear region between 3.5 and 4.5 K with Δ_ξ = 1.35 K for 2 and between 4.5 and 6 K with Δ_ξ = 2.54 K for 3, verifying the 1D Ising-like character (Fig. 6, inset).

The isothermal field-dependent magnetization data for Tb and Dy compounds display continuous increases up to 4.86 and 6.66 Nβ at 2 K and 70 kOe (Fig. 7 and Fig. S6, ESI†) with the lack of high-field saturation, illustrating the crystal-field effects and low-lying excited states. Besides, the non-superposition of the M vs. H/T plots at different temperatures (2 K, 3 K and 5 K) is indicative of the presence of significant magnetic anisotropy (Fig. 7 and Fig. S7, ESI†).

Fig. 7 Field dependences of magnetization for compound 3 at temperatures of 2, 3, and 5 K. Insets: Plots of the reduced magnetization M versus HT⁻¹.

It is worth mentioning that the M vs. H curve of the Dy complex displays a sigmoid, suggesting a metamagnetic system. The dM/dH curve shows that a three-step field induced transition at critical fields of H₁ = 93 Oe, H₂ = 10 kOe, and H₃ = 15 kOe (Fig. 8).

Fig. 8 Plots of M vs. H and dM/dH vs. H for 3 at 2.0 K with a field scan rate of 200 Oe s⁻¹.

At around 93 Oe, a transition is to surmount relatively weak interchain antiferromagnetic couplings. The second metamagnetic step is related to the transformation of the system from the spin-canted phase to the spin-flop state.¹⁸ The third step shows that when the magnetic field is larger than the critical field, the intrachain antiferromagnetic interactions can be induced to transform to ferromagnetic couplings.¹⁹ Moreover, there is a narrow hysteresis at 2 K for the Dy complex (Fig. 9), indicating that a dramatic slowing down of the relaxation time does not occur as the temperature is lowered below critical temperature.

Fig. 9 M vs. H hysteresis profiles of complex 3 at 2.0 K.

Toward further understanding the spin dynamic behaviors of compounds 2 and 3, alternating current magnetic susceptibilities in the variable-temperature and variable-frequency patterns were implemented. In the absence of a dc field, out-of-phase (χ′′) signals display temperature- and frequency-dependence obviously, revealing the slow magnetization relaxation (Fig. 10 and Fig. S8–S13, ESI†). To rule out the possibility of glassiness, the parameter φ was evaluated (φ = (ΔT_p/T_p)/Δ(log f)).²⁰ The results show that the φ value is 0.191 for 2 and 0.090 for 3 to exclude the possibility of a spin-glass (0.01 < φ < 0.08).²¹

Fig. 10 Frequency dependence of the out-of-phase (χ′′) for 2 (left) and 3 (right) under zero dc field. The solid lines represent the best fitting.

The equation τ = τ₀ exp (Δ_τ/k_BT) was employed to analyze the relaxation time under 0 Oe DC fields (Fig. 11), giving τ₀ = 1.63(5) × 10⁻⁸ s, Δ_τ/k_B = 47.4(8) for compound 2 and τ₀ = 5.26(8) × 10⁻¹² s, Δ_τ/k_B = 64.6(1) K for compound 3, which correspond to the values reported for rad-4f SCMs.^6e,22 This energy barrier (Δ_τ) is made up of two parts, namely the correlation energy (Δ_ξ) and the blocking energy (Δ_A).²³ And the latter is about 46.05 K for 2 and 62.06 K for 3, which are related to the magnetic anisotropy of the 4f ion unit.

Fig. 11 Arrhenius plots for 2 (left) and 3 (right). The solid lines represent the best fitting.

The Cole–Cole plots (Fig. 12) can be fitted by the Debye model to afford α factors in the range of 0.14–0.19 for compound 2 with a middle distribution and 0.050–0.601 for compound 3 with a wide distribution.

Fig. 12 Cole–Cole plots for 2 (left) and 3 (right). The solid lines represent the best fitting.

Luminescence properties

The solution fluorescence of the Tb compound is recorded by using an excitation wavelength of 320 nm at room temperature in CH₂Cl₂ with an approximate concentration of 10⁻⁶ mol L⁻¹, showing four groups of characteristic emission peaks of the Tb^III ion at about 491, 546, 584 and 614 nm, being related to the ⁵D₄ → ⁷F_6,5,4,3 transitions, respectively.²⁴ The intensity of the ⁵D₄–⁷F₅ transition is stronger than those of other transitions, which reveals that the 8-QNNIT radical is propitious to sensitize green luminescence of Tb^III (Fig. 13). Furthermore, the green emission peak possesses a narrow full-width at half-maximum (FWHM) of about 10 nm during the ⁵D₄–⁷F₅ transition. The luminescence decay profile for the ⁵D₄–⁷F₅ emission is shown in Fig. S14 (ESI†). The lifetime value is 18.04 μs, manifesting a short-lived excited state.

Fig. 13 UV-vis absorption (purple line) and luminescence (green line, emission) spectra of compound 2 in dichloromethane.

Ultraviolet-visible (UV-vis) spectra

UV-vis spectra of the 8-QNNIT radical ligand, Dy(hfac)₃·2H₂O, and compound 3 in CH₂Cl₂ (1 × 10⁻⁷ mol L⁻¹) were investigated in the range of 230–600 nm at room temperature. In addition, the UV-vis spectroscopy test of complex 2 was performed under the same conditions.

As depicted in Fig. 14, the 8-QNNIT radical ligand shows three absorption bands around 238, 318 and 390 nm, on account of the π–π* and n–π* transitions of the aromatic nucleus of the radical. For Dy(hfac)₃·2H₂O, an intense absorption around 299 nm is observed. Complex 3 exhibits weak absorption bands at 237, assigned to the radical ligand, which is blue shifted slightly compared with the 8-QNNIT radical due to the influence of Dy^III ion coordination. And, the strong absorption of compound 3 around 299 nm is related to the hfac⁻ coligand. Besides, the wavelengths of the UV-vis absorption peaks of the Tb complex are 225 and 299 nm, respectively (Fig. 13).

Fig. 14 The UV-vis spectra of the 8-QNNIT radical ligand, Dy(hfac)₃·2H₂O, and complex 3 in CH₂Cl₂.

Heat capacity properties

The molar heat capacity measurement of complex 3 was carried out by DSC from 263.01 to 346.71 K. The average molar heat capacities are plotted in Fig. 15. The heat capacity (C_p,m) of compound 3 gradually increases with the increase of temperature, and there is no obvious thermal anomaly. The average values of C_p,m (the molar heat capacity) were analyzed by a polynomial equation at reduced temperature (X) via the least-squares method:^25,26

Fig. 15 Temperature dependence of molar heat capacities for compound 3. The solid line represents the best fitting.

in which X denotes the reduced temperature (X = [T − (T_max + T_min)/2]/[(T_max − T_min)/2]), T represents the experimental temperature, and T_max and T_min are the maximum and minimum temperatures (346.71 and 263.01 K).

Discussion and perspectives

To restrain interchain dipolar interactions of one-dimensional compounds and improve slow magnetic relaxation behavior, increasing the distance between interchain metal ions via the bulkier substituents of nitronyl nitroxide radicals is a productive way. Structural and magnetic parameters are shown in Table 3 for some reported 4f-nitronyl nitroxide 1D compounds with a similar structure. As illustrated in the table, compound {[DyCu(hfac)₅(NIT-Ph-p-OCH₂trz)]·0.5C₆H₁₄} displays a lower effective energy barrier (29.0 K) on account of the shorter distance between interchain Dy^III ions (9.75 Å). Moreover, with the distance between interchain 4f ions increasing, the energy barrier tends to increase gradually. For the current work, the nearest distance between interchain Dy^III ions is the longest with 11.98 Å, therefore interchain dipolar interactions can be ignored, exhibiting a relatively higher effective energy barrier (64.6 K). The bulkier quinoline substituent of the 8-QNNIT radical is conducive to increase the distance between interchains, and then cut down interchain dipolar interactions for optimizing magnetic behavior.

Table 3 Structural and magnetic parameters for reported 4f-nitronyl nitroxide 1D compounds with a similar structure

4f-Based 1D complex	The minimum distance between interchain 4f ions (Å)	Energy barrier (K)	Relaxation time (s)	Ref.
{[DyCu(hfac)5(NIT-Ph-p-OCH2trz)]·0.5C6H14}	9.75	29.0	6.1 × 10^−10	6d
[Dy(hfac)3NIT3BrPhOMe]n	10.13	39.8	2.43 × 10^−10	27
[Dy(hfac)3NITPh(OMe)2]n	11.53	70.3	8.68 × 10^−15	6c
[Dy(hfac)3(8-QNNIT)]n	11.98	64.6	5.26 × 10^−12	This work

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On the other hand, in the field of 4f-based hetero-spin complexes involving nitronyl nitroxide, compared with well-studied SMM and SCM behaviors, the thermodynamics behavior and optical properties of lanthanide complexes have rarely been documented and multifunctional molecular materials are still in their infancy (Table 4). As far as we know, [Ln(hfac)₃(8-QNNIT)]_n (Ln^III = Tb 2, Dy 3) is the first SCMs with thermodynamics behavior as multifunctional magnetic materials.

Table 4 Diverse properties for reported nitronyl nitroxide-based multifunctional molecular materials

Multifunctional molecular materials	Diverse properties			Ref.
[Tb(acac)3NIT2Py·0.5H2O]	Luminescence	SMM behavior		28
[Ln2(hfac)6(H2O)2(dppnTEMPO)] (Ln^III = Tb, Dy)	Luminescence	Field induced SMM behavior		9
Ln(hfac)3(NITPhOCF3)2 (Ln^III = Tb, Dy)	Thermodynamics	Slow magnetic relaxation		25
[{Ln(hfac)3}3{Cu(hfac)2}{NIT-Ph(OMe)2}4]n (Ln^III = Gd, Tb)	Magnetocaloric effect	Slow magnetic relaxation		29
[Ln(hfac)3(8-QNNIT)]n (Ln^III = Tb, Dy)	Optical property	SCM behavior	Thermodynamics	This work

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Lanthanide SCMs/SMMs as multifunctional molecular magnetic materials, such as luminescent magnets, magnetoelectric materials, conducting magnets, chiral magnets, magnetic refrigeration materials, etc., displaying specific functionalities, have gradually caused extensive concern in the molecular material field on account of their potential applications in magneto-luminescence sensing, luminescence thermometry,^8,9 photocatalysis,³⁰ single molecule detection, magnetic cooling technology, etc. Among them, photomagnets act as a kind of multifunctional magnetic material, in which the magnetic properties can be regulated or/and controlled by photons, arousing great interest of scientific research. From the point of view of practical application, photomagnets can be extended to the Ln–radical system for designing multifunctional materials involving nitronyl nitroxide, which may become the research focal point in the future.

Conclusions

In summary, we have successfully constructed a new nitronyl nitroxide radical and two 2p–4f heterospin one dimensional compounds with the radical ligand bridging Ln^III ions exhibiting obvious slow magnetic relaxation behavior under zero dc field. Interestingly, the thermodynamics behavior, optical properties and SCM behavior are characteristic of those exhibited by multifunctional materials, and the Dy complex is the first SCM with thermodynamics behavior. To our knowledge, this kind of multifunctional material has rarely been reported in the radical–Ln^III system. This work still sheds light on a new way to generate radical-based molecular multifunctional materials. Unremitting efforts are well underway for designing fascinating families of molecular-based multifunctional materials with interesting topological structures via a multidentate nitronyl nitroxide radical ligand.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work was supported by the Science and Technology Innovation Project of Shanxi Province (No. 2020L0650).

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2024373–2024375. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0tc04266h

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