Lanthanide phosphonates with pseudo-D_5h local symmetry exhibiting magnetic and luminescence bifunctional properties

Abstract

Two lanthanide(III) phosphonates [Ln(notpH₄)(H₂O)]ClO₄·3H₂O [Ln = Dy(1), Ho(2)] in which the lanthanide ion has a pseudo-D_5h symmetry have been reported. Both show layer structures where the neighbouring lanthanide atoms are connected by a pair of O–P–O bridges. Magnetic studies reveal that field-induced slow relaxation can be observed in both cases. Complex 1 is of particular interest because it shows not only field-tunable dual relaxation processes originating from the single ion anisotropy as well as the spin collective effect, but also simultaneous emissions from the metal ion, the ligand and the radiative energy transfer from the ligand to metal. The emission of Dy^III can be correlated to the magnetic data.

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Introduction

Magnetic materials possessing multifunctions have been of intense interest due to their fundamental importance and potential applications in information storage and devices etc.^1–3 The lanthanide compounds are appealing in this aspect because they can exhibit characteristic luminescence with well-resolved emission bands and long lifetimes.⁴ Meanwhile such molecules can display large single-ion magnetic anisotropy arising from the large unquenched orbital angular momentum and strong spin–orbit coupling, thus providing promising systems for single-molecule magnets (SMMs).^5–8 So far, a number of lanthanide-based single-molecule magnets (Ln-SMMs) have been reported, some of which display rather high anisotropy energy barriers and blocking temperatures.^9–19 Nevertheless, most of these studies focus on the magnetic behaviors of the materials, and examples of emissive Ln-SMMs are still limited.^20–33 Moreover, the correlation of the relaxation mechanism and the luminescence spectra has been discussed only for a few Tb^III, Dy^III and Yb^III systems.^28–33

Since the f–f transitions are parity forbidden, the absorption coefficients of lanthanides are normally very low. In order to circumvent this drawback, organic ligands or coordination complexes with strongly absorbing chromophores can be used as “antenna” to transfer energy to the lanthanide ions. By taking advantage of the coordination capabilities of the phosphonate ligands towards lanthanide ions, we succeeded in isolating the compound DyIr₆(ppy)₁₂(bpp)₂(bppH)₄(CF₃SO₃)·8H₂O (ppy⁻ = 2-phenylpyridine, bpp²⁻ = 2-pyridylphosphonate).³⁴ It shows field-induced slow magnetic relaxation at low temperature as well as photoluminescence at room temperature originating from the iridium chromophore. The absence of luminescence from the central Dy^III ion indicates that the energy transfer from the iridium chromophore is not efficient. The 1,4,7-triazacyclononane-1,4,7-triyl-tris(methylenephosphonic acid) (notpH₆) can provide three nitrogen and nine oxygen donors to chelate and bridge the lanthanide ions into clusters or extended structures.³⁵ Recently, we found that a layered erbium-notp compound [Er(notpH₄)(H₂O)]ClO₄·3H₂O can display dual functions with the observation of field-tunable slow magnetization relaxation and near-IR emission at low temperature.³⁶ This compound also gives the first example of Er-SMMs that shows an optical and magnetic correlation. Considering that notpH₆ does not contain an obvious absorbing chromophore such as an aromatic ring, we are interested in studying the luminescence properties of the ligand itself and whether an efficient energy transfer from this ligand can be extended to the other lanthanide ions such as dysprosium and holmium.

In this paper, we report two new Ln-notp compounds, namely, [Ln(notpH₄)(H₂O)]ClO₄·3H₂O [Ln = Dy(1), Ho(2)]. Both display layered structures in which the Ln^III ions are connected by O–P–O bridges. Field-induced slow magnetic relaxation is observed in both cases at low temperature. Compound 1 is of particular interest because it shows not only field-tunable dual magnetic relaxation processes but also simultaneous emissions of the ligand, the Dy^III ion and the radiative energy transfer from the ligand to metal. A correlation between the magnetic and luminescence properties is also demonstrated for compound 1.

Experimental section

General methods

1,4,7-Triazacyclononane-1,4,7-triyl-tris(methylenephosphonic acid) (notpH₆)³⁷ and [Y(notpH₄)(H₂O)]ClO₄·3H₂O³⁶ were prepared according to the literature. All the other starting materials were of reagent grade quality and were obtained from commercial sources without further purification. Elemental analyses for C, H and N were carried out on a PE 240C analyzer. IR spectra were recorded with a TENSOR 27 Fourier transform infrared spectrometer. Thermal analyses were performed in nitrogen on a METTLER TOLEDO TGA instrument. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 Advance diffractometer. The lanthanide contents in the doped samples were analyzed using Inductively Coupled Plasma Optical Emission Spectrometry (ICP OES-Optima 5300DV, PE). The magnetic susceptibility data were obtained on polycrystalline or powdered samples using a Quantum Design SQUID VSM system. The data were corrected for diamagnetic contributions of both the sample holder and the compound obtained from Pascal's constants.³⁸

The photoluminescence properties were investigated using a modular double grating excitation spectrofluorimeter with a TRIAX 320 emission monochromator (Fluorolog-3, Horiba Scientific) coupled to a R928 Hamamatsu photomultiplier, using a front face acquisition mode. The excitation source was a 450 W Xe arc lamp. The emission spectra were corrected for detection and 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 emission decay curves of the ⁴F_9/2 (Dy³⁺) excited state were monitored with a Fluorolog TCSPC spectrofluorometer (Horiba Scientific) coupled to a TBX-04 photomultiplier tube module (950 V), 200 ns time-to-amplitude converter and 70 ns delay. The excitation source was a Horiba–Jobin–Yvon pulsed diode (NanoLED-390, peak at 390 nm, 1.2 ns pulse duration, 1 MHz repetition rate, and 150 ns synchronization delay).

Preparation of [Dy(notpH₄)(H₂O)]ClO₄·3H₂O (1)

A solution of Dy(ClO₄)₃·6H₂O (1 M, 0.400 mL) was added to a solution of notpH₆ (0.396 mmol, 0.163 g) in water (20 mL). HClO₄ (70%) was dropped into the mixture until pH = 1.0. The clear solution was left at room temperature for 2 days to afford colorless prismatic crystals in a yield of 62%. IR (KBr, cm⁻¹): 3457–2883 (br), 1638 (m), 1452 (w), 1171 (s), 1099 (s), 930 (m), 773 (m), 738 (w), 623 (m), 571 (m). Elemental analysis calcd (%) for C₉H₂₄N₃P₃O₁₄ClDy·3H₂O: C 14.54, H 4.07, N 5.65; found: C 14.22, H 4.10, N 5.55. Thermal analysis shows that the weight loss in the temperature range 25–85 °C is 9.5%, close to the release of four water molecules (calcd 9.7%). Between 100 °C and 270 °C, a plateau is observed in the TG curve (Fig. S1†).

Preparation of [Ho(notpH₄)(H₂O)]ClO₄·3H₂O (2)

Ho(OAc)₃·4H₂O (0.05 mmol, 0.020 g) was added to a solution of notpH₆ (0.05 mmol, 0.020 g) in water (10 mL). HClO₄ (70%) was dropped into the mixture until pH = 1.0. The clear solution was left at room temperature for 2 weeks to afford colorless prismatic crystals in a yield of 37%. IR (KBr, cm⁻¹): 3443–2876 (br), 1626 (m), 1496 (w) 1454 (w), 1431 (w), 1154 (s), 1099 (s), 932 (m), 769 (m), 742 (w), 624 (m), 568 (m). Elemental analysis calcd (%) for C₉H₂₄N₃P₃O₁₄ClHo·3H₂O: C 14.50, H 4.06, N 5.64; found: C 14.22, H 3.81, N 5.55. Thermal analysis shows that the weight loss in the 25–100 °C temperature range is 9.8%, in agreement with the value expected for the release of four water molecules (calcd 9.7%) (Fig. S1†).

Preparation of the diluted sample of 1a

A mixture of Dy(ClO₄)₃·6H₂O (1 M, 0.040 mL) and Y(ClO₄)₃·6H₂O (1 M, 0.360 mL) solutions was added to a solution of notpH₆ (0.394 mmol, 0.162 g) in water (20 mL). HClO₄ (70%) was dropped into the mixture until pH = 1.0. The clear solution was left at room temperature for 2 days to afford colorless prismatic crystals in a yield of 85% based on notpH₆. IR (KBr, cm⁻¹): 3457–2873 (br), 1631 (m), 1451 (w), 1171 (s), 1094 (s), 929 (m), 771 (m), 739 (w), 625 (m), 561 (m). Elemental analysis calcd (%) for C₉H₂₄N₃P₃O₁₄ClDy_0.103Y_0.897·3H₂O: C 15.96, H 4.46, N 6.20; found: C 16.27, H 4.51, N 6.14. The amount of Dy^III in 1a (10.3%) is confirmed by the inductively coupled plasma (ICP) measurements.

Crystallographic analyses

Data collection was carried out on a Bruker SMART APEXII CCD (for 1) or a Bruker SMART APEX Duo CCD (for 2) diffractometer equipped with graphite monochromated MoK_α (λ = 0.71073 Å) radiation at 296 K. The data were integrated using the Siemens SAINT program,³⁹ with the intensities corrected for the Lorentz factor, polarization, air absorption, and absorption due to variation in the path length through the detector faceplate. Empirical absorption and extinction corrections were applied. The structures were solved by direct methods and refined on F² by full-matrix least squares using SHELXTL.⁴⁰ All the non-hydrogen atoms were refined anisotropically. All the hydrogen atoms were put in calculated positions or located from the Fourier maps and refined isotropically with the isotropic vibration parameters related to the non-hydrogen atom to which they are bonded. The crystallographic data are given in Table 1, and selected bond lengths and angles are gathered in Table 2, respectively.

Table 1 Crystallographic data for compounds 1 and 2

Compound	1	2
a R 1 = ∑||F0| − |Fc||/∑|F0|; wR2 = [∑w(F0^2 − FC^2)^2/∑(FC^2)^2]^1/2.
Formula	C9H30DyN3O17P3	C9H30HoN3O17P3
M	743.22	745.65
Crystal system	Triclinic	Triclinic
Space group	P1̄	P1̄
a/Å	9.1503(10)	9.1535(8)
b/Å	9.2244(10)	9.2424(8)
c/Å	15.1607(16)	15.2450(13)
α/°	87.9626(18)	88.2040(10)
β /°	76.3602(17)	76.2920(10)
γ /°	75.8111(18)	75.8130(10)
Z	2	2
V/Å^3	1205.3(2)	1214.29(18)
D c/g cm^−3	2.048	2.045
F(000)	738	744
Goodness-of-fit on F^2	1.00	1.069
R 1, wR2^a [I > 2σ(I)]	0.0252, 0.0599	0.0229, 0.0558
R 1, wR2 (all data)	0.0274, 0.0605	0.0257, 0.0573
(Δρ)max, (Δρ)min/[e Å^−3]	0.81, −0.63	1.241, −0.980
CCDC number	1041404	1041405

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Table 2 Selected bond lengths [Å] and angles [°]^a

	1	2
a Symmetry transformations used to generate equivalent atoms: A: −x + 1, −y, −z + 1; B: −x, −y, −z + 1; C: −x, −y + 1, −z + 1.
Ln1–O1	2.257(3)	2.248(3)
Ln1–O8C	2.212(3)	2.204(3)
Ln1–O4	2.435(2)	2.430(2)
Ln1–O6B	2.257(3)	2.252(2)
Ln1–O7	2.263(3)	2.261(2)
Ln1–O2A	2.302(3)	2.305(3)
Ln1–O1W	2.483(3)	2.472(3)
O8C–Ln1–O6B	81.29(10)	81.40(9)
O8C–Ln1–O7	88.73(10)	88.02(9)
O6B–Ln1–O7	147.87(10)	147.54(10)
O8C–Ln1–O1	172.96(10)	172.64(9)
O6B–Ln1–O1	104.24(10)	104.45(9)
O7–Ln1–O1	84.28(10)	84.67(9)
O8C–Ln1–O2A	103.04(10)	103.37(9)
O6B–Ln1–O2A	75.24(10)	75.48(9)
O7–Ln1–O2A	136.89(10)	136.97(10)
O1–Ln1–O2A	82.77(10)	82.63(9)
O8C–Ln1–O4	97.20(10)	97.15(9)
O6B–Ln1–O4	76.85(9)	76.55(8)
O7–Ln1–O4	74.21(9)	74.44(8)
O1–Ln1–O4	80.07(10)	80.14(8)
O2A–Ln1–O4	142.30(9)	142.10(8)
O8C–Ln1–O1W	83.60(10)	83.58(10)
O6B–Ln1–O1W	136.39(9)	136.54(9)
O7–Ln1–O1W	71.66(10)	71.59(9)
O1–Ln1–O1W	94.99(10)	94.83(10)
O2A–Ln1–O1W	68.78(9)	68.78(9)
O4–Ln1–O1W	145.84(9)	145.98(8)

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Results and discussion

Structural description

Single crystal structure analyses reveal that compounds 1 and 2 are isostructural to [Ln(notpH₄)(H₂O)]ClO₄·3H₂O (Ln = Eu, Gd, Tb, Er, Y).^35,36 Thus compound 1 is selected for structural description. It crystallizes in the triclinic space group P1̄. The asymmetric unit contains one independent Dy^III ion, one notpH₄²⁻, one ClO₄⁻, and one coordinated and three lattice water molecules. The seven-coordinated Dy^III ion has a distorted pentagonal bipyramidal geometry with a pseudo-D_5h symmetry (Fig. 1), as determined by Continuous Shape Measure (CShM) analysis (Table S1†).⁴¹ The five equatorial positions are occupied by four phosphonate oxygens (O7, O4, O2A and O6B) and one water molecule (O1W) [Dy–O_eq 2.212(3)–2.483(3) Å]. The axial sites are filled with two phosphonate oxygens (O1 and O8C) [Dy–O_ax 2.212(3)–2.257(3) Å]. The average axial Dy–O distance (2.264 Å) is shorter than the equatorial ones (av. 2.348 Å), suggesting a compressed pentagonal bipyramidal geometry around the Dy^III ion. Equivalent {DyO₇} polyhedra are connected by {PO₃C} tetrahedra through corner-sharing, forming a two-dimensional brick-wall-like layered structure containing 24-member rings. The coordinated water molecules point toward the center of the rings, and form hydrogen bonds with the lattice water molecules (O2W, O3W) (Fig. 2). Extensive hydrogen bond interactions are also found among the triazamacrocyclic nitrogen atoms, the phosphonate oxygen atoms and the lattice water molecules.

Fig. 1 Building unit of structure 1. The ClO₄⁻ counterion, the lattice water molecules and all H atoms are omitted for clarity (thermal ellipsoids shown at 30% probability). Symmetry codes: A: −x + 1, −y, −z + 1; B: −x, −y, −z + 1; C: −x, −y + 1, −z + 1.

Fig. 2 (a) Packing diagram of structure 1 along the a-axis. All H atoms are omitted for clarity; (b) structure 1 packed along the c-axis. The ClO₄⁻ counterions and the lattice water molecules are omitted for clarity.

The static magnetic properties of 1–2

The temperature dependent molar susceptibilities were performed in the temperature range 1.8–300 K under an applied dc field of 1 kOe (Fig. 3). The room temperature χ_MT values are 14.12 emu K mol⁻¹ for 1 and 13.98 emu K mol⁻¹ for 2, respectively, which are consistent with the expected values of 14.17 emu K mol⁻¹ for one isolated Dy^III ion (⁶H_15/2, S = 5/2, L = 5, g = 4/3) and 14.07 emu K mol⁻¹ for the Ho^III ion (⁵I₈, S = 2, L = 6, g = 5/4). The susceptibility data obey the Curie–Weiss law over the high temperature region (>100 K). The Weiss constants (θ) are −11.2(5) K (for 1) and −10.8(2) K (for 2). In both cases, the χ_MT product decreases gradually on cooling and then more rapidly below ca. 25 K, which could originate from the thermal depopulation of the M_J sublevels and/or the presence of significant magnetic anisotropy.

Fig. 3 The χ_MT data were measured on powdered samples 1–2 under a dc magnetic field of 1 kOe; the solid lines represent the best fits using the CONDON package. The column bars represent the M_J sublevels of the ground state ⁶H_15/2 (for 1) and ⁵I₈ (for 2) obtained from dc, ac magnetic measurements and photoluminescence measurements.

The M versus H/T plots below 10 K show a linear increase in the magnetization at low fields and then more gradually above 7 kOe. The magnetizations at 70 kOe (5.35Nβ for 1, 5.61Nβ for 2 at 1.8 K) are not saturated, indicating the existence of magnetic anisotropy of the system and/or low-lying excited states (Fig. S6†). The non-superimposition of isothermal curves also confirms the presence of significant magnetic anisotropy.

To determine the splitting of the ground state of the lanthanide ions caused by the crystal field and predict the dynamic magnetic properties, we use the CONDON package to fit the dc magnetic susceptibilities for 1 and 2.^42,43 The Hamiltonian can be expressed as eqn (1):

Ĥ = Ĥ₀ + Ĥ_ee + Ĥ_SO + Ĥ_CF + Ĥ_M (1)

where Ĥ₀, Ĥ_ee, Ĥ_SO, Ĥ_CF and Ĥ_M account for the Hamiltonians of zero level energy, inter-electronic repulsion, spin–orbit effect, crystal field effect and Zeeman effect, respectively. For the term of the crystal field effect, the definition of ligand field parameters used here is that of Wybourne,⁴⁴ instead of the commonly used Steven parameters. In D_5h symmetry approximation, only the Wybourne notations B₀², B₀⁴, B₀⁶ have nonzero values; thus the crystal field operator of f electrons, omitting B₀⁰, reads:

Ĥ_CF = B₀²Ĉ₀²(i) + B₀⁴Ĉ₀⁴(i) + B₀⁶Ĉ₀⁶(i) (2)

A good fit of all the magnetic curves has been obtained from a unique set of CF parameters (Table S2†). The electronic fine structure can be obtained from the CF parameters (Fig. 3). For complex 1, the ground state doublet is a mixture of M_J = ±13/2 with M_J = ±3/2. The first excited state M_J = ±9/2 is a little close in energy at 12 cm⁻¹. The doublet M_J = ±15/2 which has the largest M_J values lies far from the ground state at 176 cm⁻¹ (Fig. 3). For complex 2, the ground state is a mixture of M_J = ±7 and M_J = ±3, while the first excited state is M_J = ±6 mixed with M_J = ±2 which lies 20 cm⁻¹ above the ground state. The doublet M_J = ±8 state shows the highest energy of Zeeman splitting at 372 cm⁻¹ (Fig. 3).

The dynamic magnetic properties of 1–2

To investigate the magnetization dynamics of 1, alternating current (ac) magnetic susceptibilities were measured. Neither peak nor frequency dependence is observed for the in-phase (χ^′_M) and out-of-phase (χ^′′_M) signals at zero field (Fig. S7†), possibly due to the quantum tunneling of magnetization (QTM). To suppress the QTM effect, the ac measurements were conducted at 2 K under an applied dc field of 0.5–3 kOe with the frequency scan 1–999 Hz (Fig. S8†). Indeed, well-resolved peaks appear in the plots of χ^′′_Mvs. frequency, indicating slow relaxation of magnetization (relaxation A). Interestingly, above 0.5 kOe, another peak appears in the low frequency region of χ^′′_M(ν) plots, associated with the second slower relaxation process (relaxation B). The peak shifts to lower frequency with increasing external field. The field-tunable multiple relaxation processes can be clearly identified in the Cole–Cole plots (χ^′′_Mvs. χ^′_M) with the appearance of two semicircles above 0.5 kOe (Fig. S9†). The frequency dependence of the χ^′′_M ac susceptibilities can be fitted by the generalized Debye model for the single relaxation process [eqn (3)]^45,46 or a linear combination of two modified Debye models for the double relaxation process [eqn (4)]:^47,48where χ_S is the adiabatic susceptibility, χ_T is the isothermal susceptibility, and τ is the average relaxation time of magnetization, and α parameter, which ranges between 0 and 1, quantifies the width of the τ distribution, and f_A represents the percentage of relaxation A. The fitting results are given in Table S5.† The α parameters are 0.14(1)–0.35(5) for relaxation A and 0.03(1)–0.16(2) for relaxation B. At 0.5 kOe, relaxation A is dominant with f_A = 0.83. The τ value increases with increasing field and reaches a maximum at 1.5 kOe, above which the relaxation time decreases again. For relaxation B, the τ value increases monotonically with increasing field. The weight ratio of relaxation B increases with increasing dc field, and becomes dominant with f_B = 0.75 at 3 kOe (Fig. 4). Thus 1.5 kOe is chosen as the optimum field.

Fig. 4 The ln(τ) vs. H plots for 1 (relaxations A and B) and 1a (relaxation A′) measured at 2 K. Solid lines are eye-guided. The dimensions of the symbols represent the percentage of the different relaxation processes.

The optimum field H_m = 1.5 kOe of relaxation A is a competition result of the QTM effect and direct moment reversal processes. Under a lower external dc field, the degeneracy of the ground ±M_J levels is split, reducing the degree of mixing and mitigating the quantum tunneling process; while under a higher dc field the emergence of the direct relaxation process between the non-degenerate ±M_J levels accelerated the relaxation process again. To model the field dependence of τ, eqn (5) was introduced:⁴⁹

The first term represents the direct process, while the second corresponds to the zero-field QTM. The results are shown in Fig. S10,† and the values of A, B₁ and B₂ are given in Table S6.†

The magnetic dynamics of compound 1 was investigated in detail under 0.5 kOe and 1.5 kOe dc fields. Fig. S11† shows the temperature dependence of ac signals, measured under 0.5 kOe dc field. Both χ^′_M and χ^′′_M signals are frequency dependent with the peaks appearing below 4 K. The ac susceptibility with frequency scan below 4 K shows dominant peaks in the high frequency region, corresponding to relaxation A (Fig. 5a). The maxima of the χ^′′_M(ν) peaks shift to lower frequencies upon cooling, confirming that the relaxation A is thermally activated. Notably, shoulders are also visible in the χ^′′_M(ν) curves below 3 K, corresponding to relaxation B. The weight ratio of relaxation A and B (f_A : f_B) increases from 0.81 : 0.19 at 1.8 K to 0.92 : 0.08 upon heating to 3.2 K (Table S7†). It is interesting that the dual relaxation processes become more significant when the external dc field is increased to 1.5 kOe (Fig. 5b). The intensity of the two peaks in χ^′′_M are comparable at 1.8 K (f_B = 0.52), but that of relaxation B decreases dramatically upon heating (f_B = 0.25 at 4 K) (Table S8†).

Fig. 5 Out-of-phase ac susceptibility vs. frequency at 0.5 kOe (a) and 1.5 kOe (b) dc field of 1.

The relaxation times can be extracted from the fitting of eqn (3) or eqn (4) (Tables S7 and S8†). For relaxation A, the temperature dependent relaxation time profiles can be fitted as a sum of the contributions of the quantum tunneling, direct, Raman, and Orbach relaxation mechanisms, shown in eqn (6):⁵⁰

The third term represents the Raman relaxation process which occurs via a virtual excited state with n = 9 for the Kramers ions. The last term represents the Orbach relaxation process which occurs via a real excited state, typically for SMM behavior. To avoid possible meaningless fits as a result of overparameterization, the values of A, B₁ and B₂ are taken from fits to the field-dependent τ data (Table S6†). The best fits to eqn (6) lead to extremely small C values (close to zero), indicating that the Raman relaxation can be negligible. Assuming that C = 0, the anisotropic energy barriers can be obtained as U_eff = 39.9 ± 1.1 K (τ₀ = 9.0 ± 0.9 × 10⁻¹⁰ s) for data measured at 0.5 kOe, and U_eff = 41.6 ± 1.3 K (τ₀ = 1.3 ± 0.3 × 10⁻⁹ s) for data measured at 1.5 kOe, respectively (Fig. 6, Table S6†). The C values are very small. Also, the energy barriers remain almost unchanged while the relaxation time increases in the whole temperature range by increasing the external dc field, in accordance with the reducing degree of mixing and mitigating the quantum tunneling process.

Fig. 6 The ln(τ) vs. T⁻¹ plots for 1 (relaxations A and B). Solid lines are either best fits (for relaxation A) or eye-guided (for relaxation B). The dimensions of the symbols represent the percentage of the different relaxation processes.

It is noted that the relaxation times of relaxation B are 1–2 orders of magnitude larger than those of relaxation A, and show weak temperature dependence at low temperature, suggesting a direct relaxation mechanism (Fig. 6). When the external field is 1.5 kOe, the temperature dependence of the relaxation time is visible above 3 K for relaxation B. The Arrhenius fit results in the energy barrier of U = 19.4 ± 2.4 K (τ₀ = 7 ± 4 × 10⁻⁵ s). Considering that the relative intensity of relaxation B is low in the high temperature region, the relaxation times extracted from the modified Debye models could not be quite reliable. To further explore the origin of the two relaxation processes in 1, the dynamic properties of the diamagnetically diluted sample 1a (10.3% Dy^III doped into a Y^III-based matrix) were investigated.

As shown in Fig. S19,† the χ^′′_M(ν) plots of 1a show well-shaped single peaks at 2 K under 0.1–3 kOe dc fields, associated with a single relaxation process (relaxation A′). The ln τ vs. H profile for 1a is similar to that for 1, but the relaxation time is much longer in the former case (6.46(13) ms for 1avs. 2.48(2) ms for 1 at 1.5 kOe) (Fig. 4). This can be explained by the suppression of the QTM effect upon dilution. The field dependence of τ can be fitted with eqn (5), and the derived parameters of A, B₁ and B₂ are given in Table S6.† The parameters are different from those obtained for compound 1, which may be related to the inefficient QTM via dipolar interactions upon dilution and other variables such as the crystal density, the speed of sound in the solid, and the strength of the interaction of the spin system with the phonons.⁵¹

The temperature and frequency dependent ac magnetic susceptibilities of 1a were measured under 1.5 kOe (Fig. S21–23†). The relaxation times extracted from the Debye model were fitted with eqn (6). The anisotropy barrier of the Orbach process is 38.2 ± 2.6 K (τ₀ = 3.1 ± 0.6 × 10⁻¹⁰ s), close to that for relaxation A in compound 1 (Fig. S24†). The results demonstrate that the relaxation A in compound 1 is of single-ion anisotropy origin. Noting that the relaxation B disappears in the diluted sample 1a, the weak magnetic interactions between the Dy^III ions must play an important role in the relaxation process.

The energy barrier for compound 1 is relatively smaller than other reported Dy-based SMMs in D_5h symmetry.^17,18 The weak easy-axis anisotropy could be explained by the coordination geometry of Dy^III ions in 1 which deviates significantly from an idealized five-fold symmetry. As a result, the axial crystal field Wybourne notation B₀², B₀⁴, B₀⁶ parameters could be reduced, and non-axial parameters (B_q^k, q ≠ 0) could have non-zero values, which may induce the QTM effect.

The observation of dual thermally activated relaxation processes with extremely different relaxation times is unusual in Ln-based SMMs. This phenomenon was first observed by Long et al. in studying the magnetic dynamics of U(Ph₂BPz₂)₃.⁵² They proposed that the faster relaxation is of single ion origin, while the very slow relaxation is attributed to a collective spin relaxation brought about by short-range intermolecular ordering. Another two examples showing similar behavior are a mononuclear Dy–TTF complex reported by Pointillart et al.⁵³ and [Er(notpH₄)(H₂O)]ClO₄·3H₂O described by us.³⁶

It is interesting to compare the magnetic behavior of compound 1 with that of [Er(notpH₄)(H₂O)]ClO₄·3H₂O in which the Er^III ions are cross-linked by O–P–O units.³⁶ According to Rinehart and Long, axial and equatorial ligand fields are required to stabilize the most oblate Dy^III and prolate Er^III ions, respectively.⁵⁴ The similar energy barriers obtained for compound 1 and its Er-analogue can be explained by the deviation of coordination spheres around the Dy^III or Er^III ions from the ideal pentagonal bipyramidal geometry in the two complexes (Table 3). Noting that both Dy^III and Er^III are Kramers ions, we are curious to know whether a similar field-induced multiple relaxation could occur in analogous compounds containing non-Kramers ions such as Ho^III (⁵I₈). Therefore the magnetic dynamics of compound 2 was investigated.

Table 3 The energy barriers (K) of 1 and its Er-analogue obtained from dc magnetic fitting (DC), ac magnetic fitting (AC) and photoluminescence fitting (PL)

Compound	DC	AC	PL
a Obtained at 1.5 kOe dc field. b Obtained at 1.0 kOe dc field.
1	17.2	41.4 ± 1.5^a	56.1 ± 9
[Er(notpH4)(H2O)]ClO4·3H2O	15.8	34.9^b	44.9 ^36

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Fig. S25† gives the ac susceptibilities of 2 measured at zero dc field showing neither peak nor frequency dependence. The application of an external field of 1.5 kOe switches on the χ^′′_M signals which become temperature dependent below 6 K (Fig. S27–29†). However no peaks are observed down to 1.8 K. The relaxation time τ can be extracted from the Debye model fitting the Cole–Cole plots at indicated temperatures. The anisotropic energy barrier is estimated to be 5.5 ± 0.2 K (τ₀ = 8.4 ± 0.6 × 10⁻⁷ s) according to the Arrhenius law (Fig. S30†). This value is much smaller than that of compound 1, attributed to the effective QTM through transverse anisotropy. Obviously, the field-induced dual relaxation process is not visible in compound 2.

Photoluminescence properties

Since the accuracy of the electronic structure of the ground states obtained from the CONDON package is limited due to the departure from the idealized D_5h geometry and non-negligible magnetic interactions in compound 1, the photoluminescence measurements were conducted to correlate the magnetic results.

The photoluminescence of 1 was investigated in the solid state at room temperature (Fig. S31†) and at 14 K (Fig. 7). Under direct intra-4f excitation (349 nm, ⁶H_15/2) the emission spectrum reveals the characteristic ⁴F_9/2 → ⁶H_15/2,_13/2,_11/2 (479, 571 and 661 nm, respectively) Dy^III lines, superimposed on a broad band with two components at 415 nm and 525 nm, also observed in the emission spectra of the metal-free notpH₆ ligand (Fig. S32†). A series of self-absorptions ascribed to intra-4f Dy^III transitions (marked with vertical lines) are also observed, pointing out the presence of radiative energy transfer (part of the emitted light is absorbed by the Dy^III ions and subsequently converted into f–f emission). Such radiative energy transfer has been previously observed and named the “inner filter” effect.⁵⁵ The self-absorptions are also clearly observed in the excitation spectra monitored within the ligand emission (415 nm, Fig. 7b) superimposed on a broad band peak in the UV region ascribed to the ligand excited states (Fig. S32†). The excitation spectrum monitored around the maximum intensity peak position of the Dy^III emission (571 nm) displays the ligand related bands, another component around 256 nm attributed to a ligand-to-metal-charge-transfer band (LMCT) and a series of Dy^III transitions between the ⁶H_15/2 level and the ⁴D_7/2, ⁴K_13/2,17/2, ⁶P_3/2,5/2, ⁴F_5/2–9/2, ⁴H_13/2,15/2, ⁴M_9/2, ⁴I_13/2, and ⁴G_11/2 excited states.⁵⁶

Fig. 7 (a) Emission and (b) excitation spectra (14 K) of 1 excited at 323 and 349 nm and monitored at 415 nm and 571 nm, respectively. The vertical lines identify intra-4f self-absorptions. (c) Room-temperature UV-visible reflectance spectrum.

These intra-4f lines are also observed in the UV-visible reflectance spectrum (Fig. 7c). We should note that the LMCT band is not observed in the excitation spectrum selectively monitored within the ligand emission at 415 nm (Fig. 7b) and in the reflectance spectrum of 1 (Fig. 7c) and in the excitation spectra of the isolated ligand (Fig. S32†). The larger relative intensity of the intra-4f lines indicates that the main excitation path for the Dy^III ions is through direct intra-4f excitation. In the diluted sample 1a, the absence of the self-absorptions suggests that the radiative ligand-to-Dy^III energy transfer is absent, possibly due to the low concentration of Dy^III (Fig. S34†).

The ⁴F_9/2 emission decay curves were monitored at 571 nm for 1 being well reproduced by a single exponential function, in good agreement with the presence of a single average Dy^III local environment (Fig. S36†). From the fit, the ⁴F_9/2 lifetime values are 4.59 ± 0.13 ns and 3.78 ± 0.09 ns, at 14 K and at room temperature, respectively. These values are lower than those measured for other Dy^III-based complexes reported in the literature,^20,23 which may be explained by the presence of OH oscillators from one coordinated water in the first coordination shell of 1 and/or concentration quenching.

In order to experimentally investigate the crystal field splitting of the ground state of the Dy^III ions in 1, the 14 K high-resolution emission spectrum in the spectral region of the ⁶F_9/2 → ⁶H_15/2 transition was acquired, as illustrated in Fig. 8.

Fig. 8 (a) High-resolution emission spectrum (14 K) of 1 excited at 349 nm. The multi-Gaussian function envelope fit (solid circles) and the components arising from the first ⁴F_9/2 Stark sublevel to the ⁶H_15/2 multiplet are also displayed. (b) Partial energy diagram of the ⁴F_9/2 → ⁶H_15/2 transition. (c) Regular residual plot (R² > 0.98) for a better judgment of the fit quality.

The maximum Stark splitting of the ⁶H_15/2 levels is eight, pointing out the presence of two transitions arising from “hot” bands involving the first Stark component of the ⁷F_9/2 level. The assignment of the transitions is proposed in Fig. 8b which allows us to establish the energy diagram of the Stark-sub-levels. The energy gap between the ground and first excited states of the ⁶H_15/2 crystal field splitting (ΔE, Fig. 8) is 56.1 ± 12.9 K (39.0 ± 9.0 cm⁻¹), which agrees well with the energy barrier extracted from the ac magnetic studies (41.4 K).

The photoluminescence properties of 2 were also investigated in the UV-visible range. The emission of Ho^III could not be observed due to the non-radiative deactivation processes from the emissive state of ⁵S₂.⁴ The ligand emission concomitant with the radiative energy transfer from ligand-to-Ho^III can be obviously observed in the excitation and emission spectra (Fig. S39 and S40†), in which several negative sharp peaks associated with the self-absorption of the Ho^III appear on the broadband of ligand excitation and emission. The positions of these peaks are well coinciding with the absorption spectra of 2, corresponding to the absorption from the ground state ⁵I₈ to excited states of (⁵G, ⁵D, ³G)₄ (287 nm), ³K₆ (334 nm), ³H₆ (360 nm), (⁵G, ³G)₅ (417 nm), ⁵G₆ (452 nm), ⁵K₈ (472 nm), ⁵F₃ (486 nm), ⁵F₄ (536 nm), ⁵S₂ (545 nm), by referring to Ho^III aqua ions.⁵⁶

Conclusions

Two lanthanide phosphonates [Ln(notpH₄)(H₂O)]ClO₄·3H₂O [Ln = Dy(1), Ho(2)] with layered structures are reported. Complex 1 exhibits field-tunable dual relaxation processes that originated from single ion anisotropy and the collective spin relaxation, respectively. The latter can be quenched under magnetic dilution. Complex 2 shows a field-induced single relaxation process with a very small energy barrier. Photoluminescence studies reveal that compound 1 shows simultaneous emissions from the metal and the ligand as well as the radiative ligand-to-metal energy transfer. The latter two are also visible in compound 2. More interestingly, the luminescence data of compound 1 can be correlated to the magnetic data. Obviously, compounds 1 and 2 provide new members to the small family of emissive molecular materials showing slow magnetization relaxation at low temperature. Further work is in progress to explore new materials combining magnetic and luminescence functions.

Acknowledgements

We acknowledge the financial support of this work by the National Basic Research Program of China (2013CB922102) and the Graduate Innovation Fund of Jiangsu Province (0205Y511).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Thermal analyses, PXRD patterns, additional magnetic and photoluminescence data. CCDC 1041404 and 1041405. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4qi00242c

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