Luminescent Ir(III)–Ln(III) coordination polymers showing slow magnetization relaxation

Abstract

A series of iridium(III)–lanthanide(III) bimetallic coordination polymers were successfully obtained by hydrothermal reactions based on the Ir unit [Ir(ppy)₂(Hdcbpy)], namely, [Ln{Ir(ppy)₂(dcbpy)}₂(NO₃)(H₂O)_n]·solvent (Ir₂Ln, n = 5, Ln = Gd and Dy; and n = 4, Ln = Er and Yb) and [Ln₂(OH){Ir(ppy)₂(dcbpy)}₄(NO₃)(H₂O)₄]·solvent (Ir₄Ln₂, Ln = Gd, Dy, Er, and Yb). Compounds Ir₂Ln show chain structures in which the lanthanide ions are bridged by the Ir(ppy)₂(dcbpy)⁻ ligands through the carboxylate groups. Compounds Ir₄Ln₂ have double-chain structures, in which the dimers of Yb₂(OH) are connected by the Ir(ppy)₂(dcbpy)⁻ ligands. Magnetic studies reveal that Ir₂Dy and Ir₄Dy₂ exhibit slow magnetic relaxation behaviour in zero dc field, which is characteristic of single molecule magnets, but Ir₂Er, Ir₄Yb₂, Ir₄Er₂ and Ir₄Yb₂ require a field to induce such characteristics. Moreover, since the iridium(III) moiety can serve as an “antenna” to effectively sensitize the f–f transitions of the lanthanide ions, near-infrared luminescence has been observed for the Yb^III and Er^III complexes. As far as we know, this is the first observation of SMM behaviour and NIR luminescence in iridium(III)–lanthanide(III) bimetallic coordination polymers.

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Introduction

The exponential growth of studies related to multifunctional coordination polymers integrating several physical and chemical properties into one single crystalline structure has become a hot topic of research over the last two decades.¹ Magneto-luminescent molecular materials belong to one family of those multifunctional materials which are gaining increasing attention due to their potential applications in several areas such as quantum computing, medical imaging and sensors.^2,3 One of the most effective strategies to combine magnetism with luminescence is the synthesis of lanthanide complexes which exhibit not only large unquenched orbital angular momentum and strong spin–orbit coupling that are ideal for the construction of single-molecule magnets (SMMs),^4,5 but also excellent luminescence properties including narrow line-like emission, high quantum yield and long lifetime.^2,6 Since the direct excitation of a lanthanide ion requires a Laporte forbidden f–f transition, indirect excitation via light-harvesting antennas or sensitizing chromophores is generally employed.⁷ In comparison with the traditional organic chromophores being used as sensitizing groups,⁸ using transition metal complexes allows more design flexibility, offering longer emission lifetime and extends the optical excitation window to lower energy region.⁹ Their promising characteristics have therefore attracted substantial attention in recent years. Lanthanide ions that are near-infrared (NIR) emitters (Nd^III, Yb^III, Er^III, and Pr^III) have been involved in most of the work in this area and are useful in optical communication, medical imaging and biological labelling.^9,10

Among the transition metals, the Ir^III complexes with energy levels modifiable through the cyclometalated ligands stand out as sensitizers for NIR emission from lanthanide ions.¹¹ The advantage is that the excitation is done in the visible region of the spectrum instead of the destructive UV light normally needed for organics. In 2008, De Cola and co-workers first reported a tetrametallic iridium–ytterbium complex showing NIR emission from Yb sensitized by the Ir complex.¹² Since then, several Ir^III–Ln^III bimetallic complexes showing NIR emission have been reported.¹³ Furthermore, only three Ir^III–Ln^III (Ln = Yb, Er, and Nd) coordination polymers exhibiting NIR luminescence were reported by Luo et al. in 2014, which successfully extended the dimension of luminescent Ir^III–Ln^III system.¹⁴

On the contrary, luminescent Ir^III–Ln^III single molecule magnets (SMMs) have been scarcely described. The only examples, as far as we are aware, are Ir₆Ln (Ln = Dy, Er, and Yb) clusters which have been reported by us and exhibit simultaneously slow magnetization relaxation from Ln^III and luminescence dominated by Ir^III.¹⁵ However, no efficient energy transfer between Ir and Ln was observed.

Owing to our long-term interest in magneto-optical bifunctional materials,^15,16 herein we report two new series of Ir^III–Ln^III coordination polymers based on the metalloligand Ir(ppy)₂(Hdcbpy) (Scheme 1), namely, [Ln{Ir(ppy)₂(dcbpy)}₂(NO₃)(H₂O)_n]·solvent (Ir₂Ln, n = 5, Ln = Gd and Dy; and n = 4, Ln = Er and Yb) and [Ln₂(OH){Ir(ppy)₂(dcbpy)}₄(NO₃)(H₂O)₄]·solvent (Ir₄Ln₂, Ln = Gd, Dy, Er, and Yb). Ir₂Ln show chain structures, in which the Ln^III ions are cross-linked by the Ir(ppy)₂(dcbpy)⁻ ligands via the two carboxylate groups. For Ir₄Ln₂, dimers of Ln₂(OH) are observed, which are further connected by the carboxylate groups of the Ir(ppy)₂(dcbpy)⁻ ligands, forming a ladder-like chain structure. Interestingly, both single molecule magnetism and Ir → Ln energy transfer accompanied by near-infrared emission were demonstrated to co-exist in Ir₂Ln and Ir₄Ln₂ (Ln = Er and Yb).

Scheme 1 The molecular structure of the metalloligand Ir(ppy)₂(Hdcbpy).

Results and discussion

Crystal structures

Compounds [Ln{Ir(ppy)₂(dcbpy)}₂(NO₃)(H₂O)_n]·solvent (Ir₂Ln, n = 5, Ln = Gd and Dy; and n = 4, Ln = Er and Yb) and [Ln₂(OH) {Ir(ppy)₂(dcbpy)}₄(NO₃)(H₂O)₄]·solvent (Ir₄Ln₂, Ln = Gd, Dy, Er, and Yb) were synthesized by solvothermal reactions of Ir(ppy)₂(Hdcbpy) and Ln(NO₃)₃·6H₂O in H₂O/MeOH using different concentrations of NaOH and temperatures. Powder XRD measurements revealed that Ir₂Ln (Ln = Gd, Dy, Er, and Yb) and Ir₄Ln₂ (Ln = Gd, Dy, Er, and Yb) are isostructural compounds. However, single crystals of good quality were obtained only for compounds Ir₂Dy, Ir₂Er, Ir₂Yb and Ir₄Yb₂. The crystallographic data for these four compounds are given in Table 1.

Table 1 Crystallographic data for compounds Ir₂Dy, Ir₂Er, Ir₂Yb and Ir₄Yb₂

	Ir2Dy	Ir2Er	Ir2Yb	Ir4Yb2
a R 1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = [∑w(Fo^2 − Fc^2)^2/∑w(Fo^2)^2]^1/2.
Formula	C68H54N8O13DyIr2	C140H122N18O35Er2Ir4	C140H122N18O35Yb2Ir4	C280H236N34O61Yb4Ir8
M	1738.09	3719.87	3731.43	7282.77
Crystal system	Triclinic	Triclinic	Triclinic	Monoclinic
Space group	P1̄	P1̄	P1̄	P21/c
a (Å)	8.5559(13)	8.6129(14)	8.5775(16)	36.569(3)
b (Å)	18.370(3)	18.343(3)	18.360(3)	8.6347(5)
c (Å)	23.830(4)	23.903(4)	23.823(4)	43.991(3)
α (^o)	70.638(3)	70.841(4)	70.887(5)	90
β (^o)	80.107(3)	80.828(4)	81.179(6)	112.317(2)
γ (^o)	77.866(3)	77.440(3)	77.676(5)	90
V (Å^3)	3433.3(9)	3465.9(10)	3449.0(11)	12 850.4(14)
Z	2	1	1	2
ρ calcd (g cm^−3)	1.681	1.782	1.797	1.882
μ (mm^−1)	5.009	5.106	5.270	5.653
F (000)	1684.0	1812.0	1816.0	7076.0
R int	0.0495	0.0482	0.0432	0.0617
T max, Tmin	0.634, 0.350	0.629, 0.350	0.621, 0.350	0.728, 0.360
R 1 ^a, wR2 ^b [I > 2σ(I)]	0.1105, 0.2755	0.0795, 0.1847	0.0485, 0.1178	0.0844, 0.1952
R 1 ^a, wR2 ^b (all data)	0.1273, 0.2852	0.1093, 0.2019	0.0621, 0.1244	0.1012, 0.2036
GoF on F^2	1.110	1.031	0.998	1.022
(Δρ)max, (Δρ)min (e Å^−3)	5.05, −8.18	4.86, −6.94	2.22, −1.97	5.32, −3.22
CCDC	1956646	1956647	1956648	1956649

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Ir₂Ln are isomorphous compounds that crystallize in the triclinic P1̄ space group, though the numbers of the coordination water molecules are slightly different (n = 5 for Ln = Gd and Dy; and n = 4 for Ln = Er and Yb). Therefore, Ir₂Yb is selected as a representative for the description of the structures. The asymmetric unit of Ir₂Yb consists of two Ir(ppy)₂(dcbpy)⁻ ligands, one Yb^III, one NO₃⁻, four coordinated water molecules, two MeOH and 0.5 lattice water molecule. As shown in Fig. 1a and b, the Yb atom is seven-coordinated by three carboxylate oxygen atoms from three ligands (O1, O3A, and O5) and four coordination water molecules. One of the coordination water molecules is disordered over two sites (O4W and O4W′) (Fig. 1b). Continuous Shape Measure (CShM) analysis¹⁷ suggests that the geometries of the two possible [YbO₇] polyhedra are slightly deviated from an ideal capped trigonal prism (CShM = 1.17) and pentagonal bipyramid (CShM = 1.25), respectively. The Yb–O bond lengths are in the range of 2.191(7)–2.452(18) Å, and the O–Yb–O angles are in the range of 67.4(5)–149.8(3)°.

Fig. 1 (a) The building unit of the structure of Ir₂Yb. (b) Coordination environments of the Yb^III ion. (c) The single chain structure of the compound Ir₂Yb. The ppy ligands are omitted for clarity. (d) The double-chain structure of Ir₂Yb. (e) Packing diagram of the structure of Ir₂Yb viewed along the a-axis. All H atoms except those attached to the oxygen atoms are omitted for clarity.

Two Ir(ppy)₂(dcbpy)⁻ ligands are crystallographically distinguished. One ligand (Ir1) adopts a bi-dentate coordination mode by using two of its four carboxylate oxygen atoms (O1 and O3), and connects the neighbouring Yb centres into an infinite chain running along the a-axis. The other ligand (Ir2) adopts a mono-dentate coordination mode and binds the Yb atom through its carboxylate oxygen atom O5 (Fig. 1c). The Yb⋯Yb distance within the chain is 8.58 Å. The chain is polar with the Ir(ppy)₂(dcbpy)⁻ ligands sitting on one side and the coordination water molecules on the other side. Two equivalent chains are connected through extensive hydrogen bonding interactions between the coordination water molecules, forming a supramolecular double chain which is non-polar (Fig. 1d). The shortest interchain Yb⋯Yb separation is 6.20 Å. The double chains are stacked along the a-axis with the space between the double-chains filled with lattice water and methanol molecules and nitrate anions (Fig. 1e). Extensive H-bonds are observed among the aromatic groups, the non-coordinated carboxylate oxygen atoms, the coordination and lattice water molecules and the nitrate anions. A three-dimensional supramolecular network is thus constructed.

Compounds Ir₂Dy and Ir₂Er show similar 3D supramolecular structures made up of 1D chain (Fig. S3 and S4†). However, a significant difference is found in the coordination environment of Dy^III in Ir₂Dy. Unlike the seven-coordinated lanthanide ion in Ir₂Yb and Ir₂Er (Fig. S4†), the Dy atom is eight-coordinated by three carboxylate oxygen atoms from three ligands (O1, O3A, and O5) [Dy–O: 2.270(16)–2.376(16) Å] and five coordinated water molecules (O1W, O2W, O3W, O4W, and O5W) [Dy–O: 2.332(19)–2.48(3) Å] (Fig. 2). This can be related to the lanthanide contract effect, the Dy^III ion with larger ionic radius shows higher coordination number. The geometry of Dy^III is best described as a slightly distorted triangular dodecahedron (D_2d) with a CShM value of 0.94.

Fig. 2 (a) The building unit of the structure of Ir₂Dy. (b) The coordination environment of the Dy^III ion.

The compound Ir₄Yb₂ crystallizes in the monoclinic space group P2₁/c. Its asymmetric unit contains four Ir(ppy)₂(dcbpy)⁻ ligands, two Yb^III, one OH⁻, one NO₃⁻, four coordinated water molecules, four free MeOH molecules and two and a half lattice water molecules. As shown in Fig. 3, both the crystallographically distinct Yb atoms have seven coordinated environments, viz. four carboxylate oxygen atoms, one bridging coordinated OH⁻ and two oxygen atoms from water (Fig. 3b). Both Yb^III ions adopt pentagonal bipyramidal geometry, as suggested by the CShM values S(D_5h) = 1.80 (Yb1) and S(D_5h) = 0.96 (Yb2). The neighbouring [YbO₇] polyhedra are linked via the OH⁻ group, forming a dimer of {Yb₂(OH)(COO)₈(H₂O)₄} (Yb–O–Yb = 159.6(1)° and Yb⋯Yb = 4.3962(11) Å). All of the Ir(ppy)₂(dcbpy)⁻ ligands adopt a bi-dentate coordination mode, unlike those in Ir₂Ln, and connect the {Yb₂(OH)(COO)₈(H₂O)₄} dimers into a one-dimensional double-chain structure (Fig. 3c). The intrachain Yb⋯Yb distance between the dimers is 8.63 Å. The double chains are stacked in the lattice. The lattice solvent molecules and the nitrate anions occupy the inter-chain spaces, and are involved in the extensive H-bond network.

Fig. 3 (a) The building unit of the structure of Ir₄Yb₂. The ppy ligands are omitted for clarity. (b) Coordination environments of the Yb^III ions. (c) The double-chain structure of the compound Ir₄Yb₂. (d) Packing diagram of the structure of Ir₄Yb₂ viewed along the b-axis. All H atoms except those attached to the oxygen atoms are omitted for clarity.

Although single crystals of good quality were not obtained for compounds Ir₂Gd, Ir₄Gd₂, Ir₄Dy₂ and Ir₄Er₂, the unit cell parameters were confirmed by Pawley fitting of their powder diffraction patterns using TOPAS 5.0 program (Table S8, Fig. S5–S10†).

Obviously, the structure of Ir₄Yb₂ is significantly different from that of Ir₂Yb. A careful analysis reveals, however, that the double chain in Ir₄Yb₂ can be viewed as two single chains in Ir₂Yb connected by the hydroxyl groups. The involvement of an OH⁻ group in Ir₄Yb₂ can be explained by the more basic reaction condition used for the synthesis of Ir₄Yb₂.

Notably, Ir^III–Ln^III coordination polymers containing the same ligand Ir(ppy)₂(Hdcbpy) are still rare. The only examples, as far as we are aware, are Ln[Ir(ppy)₂(dcbpy)]₂(OH)·H₂O (Ln = Gd, Yb, Er, and Nd)¹⁴ and Y[Ir(ppy)₂(dcbpy)]₂(OH).¹⁸ These compounds possess identical chain structures that are remarkably different from those of Ir₂Yb and Ir₄Yb₂, in which the dimers of edge-sharing {LnO₇} or {YO₇} pentagonal bipyramid via two OH⁻ groups are further connected by the carboxylate groups in the metalloligand. Therefore compounds Ir₂Ln and Ir₄Ln₂ provide new structural types of the Ir^III–Ln^III coordination polymers based on Ir(ppy)₂(Hdcbpy).

Magnetic properties

Variable temperature dc magnetic susceptibilities of Ir₂Ln and Ir₄Ln₂ (Ln = Gd, Dy, Er, and Yb) were measured in the temperature range of 2–300 K in an applied field of 1 kOe. Plots of χ_MT versus T (χ_M is the molar magnetic susceptibility per Ln^III ion) for these compounds are shown in Fig. 4. At 300 K, the χ_MT values are 7.73 and 7.84 cm³ K mol⁻¹ for Ir₂Gd and Ir₄Gd₂, respectively, which are close to the expected value of 7.88 cm³ K mol⁻¹ for an isolated Gd^III ion (S = 7/2, L = 0, ⁸S_7/2, J = 7/2, g = 2). Upon cooling, the χ_MT value of Ir₂Gd decreases continuously in the range of 300–50 K and then decreases smoothly to the minimum value of 6.88 cm³ K mol⁻¹ at 2 K. In contrast, for compound Ir₄Gd₂, the χ_MT value decreases to 20 K followed by a rapid drop to the minimum value of 5.91 cm³ K mol⁻¹ at 2 K. The low temperature feature could be attributed to the occurrence of the Gd^III⋯Gd^III antiferromagnetic exchange interactions, and is given by the negative values of the Weiss constants (θ) as −0.04 K for Ir₂Gd and −0.54 K for Ir₄Gd₂ (Fig. S11†). The stronger antiferromagnetic interactions in Ir₄Gd₂ may be due to short distances of the Gd^III ions in the dimer connected via the OH⁻ group. The field dependence of the magnetization at 2 K shows a rapid increase at low field and then slowly reaches the corresponding maximum values of 6.67Nβ per Gd for Ir₂Gd and 6.80Nβ per Gd for Ir₄Gd₂ at 70 kOe, which are close to the theoretical saturation value of 7Nβ for an isolated Gd^III ion.

Fig. 4 (a) Temperature dependence of χ_MT per Gd on cooling in a field of 1 kOe for Ir₂Gd and Ir₄Gd₂. Inset: M vs. H plots of the two compounds at 2 K. (b) Temperature dependence of χ_MT per Ln on cooling in a field of 1 kOe for Ir₂Ln and Ir₄Ln₂ (Ln = Dy, Er, and Yb).

The dc magnetic susceptibilities of compounds Ir₂Ln and Ir₄Ln₂ (Ln = Dy, Er, and Yb) were measured under the same conditions. As shown in Fig. 4b, all the compounds show a similar behaviour. The room temperature χ_MT values per Ln are 13.65 (Ir₂Dy), 14.44 (Ir₄Dy₂), 11.60 (Ir₂Er), 11.98 (Ir₄Er₂), 2.53 (Ir₂Yb) and 2.45 (Ir₄Yb₂) cm³ K mol⁻¹, which are consistent with the theoretical values of 14.17 (Dy^III, ⁶H_15/2), 11.48 (Er^III, ⁴I_15/2) and 2.57 (Yb^III, ²F_7/2) cm³ K mol⁻¹. The decrease in the χ_MT value on lowering the temperature is attributed to the thermal depopulation of the excited M_J sublevels and/or the weak antiferromagnetic exchange interactions between the metal centres.¹⁹ The magnetization values of compounds Ir₂Ln and Ir₄Ln₂ at 2 K and 70 kOe are significantly smaller than the expected saturation values for the corresponding lanthanide ions, revealing the existence of a significant magnetic anisotropy²⁰ which is further confirmed by the non-superimposed M vs. H/T plots at different temperatures (Fig. S12 and S13†).

In order to assess the effect of coordination configuration on the dynamic magnetic properties, the alternative-current (ac) magnetic susceptibilities of Ir₂Ln and Ir₄Ln₂ (Ln = Dy, Er, and Yb) were measured using a Quantum Design PPMS in the frequency range of 10–10 000 Hz. Under the zero dc field, only Ir₂Dy and Ir₄Dy₂ showed clear frequency dependence of in-phase (χ′) and out-of-phase (χ′′) susceptibilities below ca. 11 K, indicating a zero-field SMM with slow magnetization relaxation (Fig. 5a and c). However, the peak value of χ′′ did not change significantly with increasing temperature, indicating that the magnetic relaxation mainly originated from the quantum tunnelling process.²¹ The Cole–Cole plots can be well fit by the generalized Debye model to extract the relaxation time (τ) (Tables S9 and S14†).²² The distribution parameters (α) are in the range of 0.34–0.37 for Ir₂Dy and in the range of 0.25–0.45 for Ir₄Dy₂, suggesting multiple relaxation processes with broad distribution of the relaxation times. Moreover, the ln τ vs. T⁻¹ curve can be fit by considering the Orbach and quantum tunnelling process using eqn (1),²³ and determining the parameters U_eff = 5.2 K, τ₀ = 1.6 × 10⁻⁵ s, and τ_QTM = 3.8 × 10⁻⁵ s for Ir₂Dy (Fig. 5b), and U_eff = 23.5 K, τ₀ = 1.17 × 10⁻⁶ s, and τ_QTM = 7.34 × 10⁻⁴ s for Ir₄Dy₂ (Fig. 5d), respectively.

τ⁻¹ = τ_QTM⁻¹ + τ₀⁻¹ exp(−U_eff/kT) (1)

τ⁻¹ = τ_QTM⁻¹ + AT + τ₀⁻¹ exp(−U_eff/kT) (2)

Fig. 5 Out-of-phase (χ′′) ac susceptibilities of Ir₂Dy (a) and Ir₄Dy₂ (c) under zero dc field. Plots of ln(τ) vs. 1/T for Ir₂Dy (b) and Ir₄Dy₂ (d).

In order to suppress the QTM and get further insight into the magnetic relaxation dynamics of complexes Ir₂Dy and Ir₄Dy₂, we obtained the ac magnetic susceptibilities under an applied dc bias field of 2 kOe. Variable-frequency ac magnetic susceptibilities of Ir₂Dy are shown in Fig. S15 and S16.† With increasing temperature, the position of χ′′ peak did not change below 5.4 K, indicating a complicated relaxation process. The Cole–Cole plots were generated using the frequency dependent ac susceptibilities in the temperature region of 5.4–7.5 K, and can be well fit by employing the sum of two modified Debye functions. For the fast relaxation (FR) process, the low value of the relaxation width (α₁ ≈ 0, Table S10†) suggests a narrow distribution of relaxation times, and the temperature independence of ln τ also indicates that only the quantum tunnelling mechanism is involved.²⁴ However, the slow relaxation (SR) processes show a linear temperature dependence of ln τ corresponding to an Orbach relaxation mechanism,²⁵ leading to the parameters U_eff = 38.1 K and τ₀ = 2.93 × 10⁻⁶ s according to the Arrhenius law (Fig. S16d†).

For compounds Ir₄Dy₂, the frequency-dependent χ′′ exhibits more clear peaks than in zero-dc field and the positions of maxima change from high to low frequency as the temperature is decreased. Although there are two crystallographic sites differing in environments in Ir₄Dy₂, no dual magnetization relaxation processes were distinguished. It is possible to fit the Cole–Cole plots with only one relaxation process (Fig. S18 and 19, Table S12†).²⁶ Furthermore, the temperature dependent relaxation time profiles can be fit as a sum of the contributions of the Orbach, quantum tunnelling, and direct mechanisms (eqn (2)).²⁷ The best fit led to the following parameters: U_eff = 21.4 K, τ₀ = 2.89 × 10⁻⁶ s, τ_QTM = 7.98 × 10⁻⁴ s and A = 193.9 K⁻¹ s⁻¹. Obviously, the parameters of Orbach and quantum tunnelling processes do not change significantly under an applied field, but the direct process induced by the applied field results in the decrease of the magnetic relaxation rate.

The magnetization dynamics of compounds Ir₂Er and Ir₄Er₂ were also explored. Unfortunately, under zero external field, neither peaks nor frequency dependence of χ′ and χ′′ was found at 2 K, indicating the presence of significant QTM effect. When an external dc field was applied, both χ′ and χ′′ were found and they showed frequency-dependence, which are typical of a field-induced SMM behaviour (Fig. S20–S23, Tables S13 and S14†). As shown in Fig. S21 and S23,† the maximum of χ′′ gradually shifted to high frequencies with increasing temperature, while no maximum was observed above 2.2 K. The relaxation time τ can be extracted by fitting the Cole–Cole plots using the generalized Debye model assuming a single relaxation process. The ln τ values increase linearly with temperature in the range of 1.8–2.6 K, and an Arrhenius law fitting results in the energy barriers of 13.6 K (τ₀ = 2.72 × 10⁻⁸ s) for Ir₂Er and 11.8 K (τ₀ = 6.68 × 10⁻⁸ s) for Ir₄Er₂.

The temperature-dependent magnetic dynamics of Ir₂Yb and Ir₄Yb₂ were also investigated under an external dc field (Fig. 6, S24–S27, Tables S15 and S16†). For Ir₂Yb, the frequency scan of χ′′ exhibited single peaks below 6 K and the maxima gradually shifted to high frequencies upon heating. By assuming a single relaxation process, the relaxation time τ can again be extracted by fitting the Cole–Cole plots using the generalized Debye model. The ln τ vs. T⁻¹ curve can be reasonably fit by using eqn (2), leading to the parameters U_eff = 24.4 K, τ₀ = 2.88 × 10⁻⁷ s, and A = 687.9 K⁻¹ s⁻¹. However, two distinct peaks at low temperatures were observed in the temperature-dependent magnetic dynamics of Ir₄Yb₂, indicating dual relaxation processes by accounting for the different combinations of contributions from the direct and Orbach relaxation processes. The ln τ vs. T⁻¹ profile can be fit in the whole temperature range,²⁸ and the resulting parameters are U_eff = 22.2 K, τ₀ = 2.03 × 10⁻⁷ s and A = 8678 K⁻¹ s⁻¹ for the FR process, while U_eff = 28.3 K, τ₀ = 2.5 × 10⁻⁸ s and A = 682 K⁻¹ s⁻¹ for the SR process. The observation of dual magnetization relaxation processes in Ir₄Yb₂ could result from the presence of two crystallographic sites with two different environments in the structure. A similar phenomenon was also observed in a few other Yb dimer complexes.²⁹

Fig. 6 Frequency dependence of out-of-phase (χ′′) ac susceptibilities at different temperatures for Ir₂Yb (a) and Ir₄Yb₂ (d) under an external dc field. The Cole–Cole plots of the ac susceptibilities for Ir₂Yb (b) and Ir₄Yb₂ (e). Plots of ln(τ) vs. 1/T for Ir₂Yb (c) and Ir₄Yb₂ (f).

To the benefit of the D_5h symmetry, especially the Kramers Dy^III and Er^III ions, the observation of high energy barrier to moment reversal in Ln–SMMs was reported by several groups.^30–32 Therefore, a small anisotropy barrier in Ir–Ln bimetallic complexes is not expected. This may be explained by the orientation of the easy-axis anisotropy deviating significantly from an idealized C₅ axis, which has been observed in other lanthanide complexes.³³

Optical properties

The UV-vis spectra of the free metalloligand and Ir–Ln coordination polymers in the solid state were transformed from the diffuse reflectance spectra using the Kubelka–Munk equation: F(R) = (1 − R)²/2R. As shown in Fig. S28,† the absorptions in the high energy region (250–330 nm) are associated with the π–π* transition of the aromatic ligands (Hdcbpy and ppy) and the intense absorption bands at 330–450 nm can be explained by the mixed spin-allowed ¹LLCT (ligand-to-ligand charge-transfer) and ¹MLCT (metal-to-ligand charge-transfer). The lower energy absorptions (ca. 450–550 nm) originate from the spin forbidden ³MLCT and are common in cyclometalated iridium derivatives.³⁴ The absorption profiles of all Ir–Ln complexes show a similar electronic transition to the Ir^III unit. However, the most significant difference is that weak absorption peaks located at 755 nm (13 245 cm⁻¹) and 654 nm (15 290 cm⁻¹) were observed for Ir₂Dy and Ir₂Er, respectively, corresponding to the f–f transitions of the Dy^III ions (⁵H_15/2 to ⁶F_3/2) and Er^III ions (⁴I_15/2 to ⁴F_9/2).³⁵ Although the structures are different, a similar phenomenon was also observed for Ir₄Dy₂ and Ir₄Er₂.

Upon excitation in the visible region at 375 nm, Ir(ppy)₂(Hdcbpy) exhibits a strong emission band at 605 nm, corresponding to the ³MLCT transition as a consequence of the heavy-atom effect. As is to be expected, the solid-state luminescence spectra of compounds Ir₂Ln and Ir₄Ln₂ (Ln = Gd, Dy, Er, and Yb) are similar to that of the free metalloligand, indicating that the phosphorescence of polymers arises from the Ir(III) unit (Fig. 7 and Table 2). However, the emission maxima of Ir–Ln compounds were blue-shifted with respect to the parent metalloligand, which may be attributed to the deprotonation of the carboxylate groups upon coordination, leading to a decrease in the intermolecular H-bond interactions in the polymers.³⁶

Fig. 7 (a) The emission spectra of Ir(ppy)₂(Hdcbpy) and Ir₂Ln in the solid-state (λ_ex = 375 nm). (b) The emission spectra of Ir(ppy)₂(Hdcbpy) and Ir₄Ln₂ (Ln = Gd, Dy, Er, Yb) in the solid-state (λ_ex = 375 nm). (c) The solid-state NIR emission spectra of the Ir–Yb (left) and Ir–Er (right) complexes (λ_ex = 500 nm).

Table 2 Magnetic and photophysical parameters of Ir₂Ln and Ir₄Ln₂ (Ln = Gd, Dy, Er, and Yb)

	dc field	Relaxation processes	Fitting parameters	λ em/nm	τ/μs	QY/%
Ir(ppy)2(Hdcbpy)	—	—	—	605	6.60	4.22
Ir2Gd	—	—	—	598	8.69	5.83
Ir4Gd2	—	—	—	578	7.74	8.08
Ir2Dy	0 kOe	Orbach, QTM	U eff = 5.2 K, τ0 = 1.62 × 10^−5 s, τQTM = 3.80 × 10^−5 s	583	4.79	6.00
	2.0 kOe	QTM (FR)	τ QTM = 2.25 × 10^−5
		Orbach(SR)	U eff = 38.1 K, τ0 = 2.93 × 10^−6 s
Ir4Dy2	0 kOe	Orbach, QTM	U eff = 23.5 K, τ0 = 1.17 × 10^−6 s	575	0.66	11.94
			τ QTM = 7.34 × 10^−4 s
	2.0 kOe	Orbach, QTM;	U eff = 21.4 K, τ0 = 2.89 × 10^−6 s
		direct	τ QTM = 7.98 × 10^−4 s, A = 193.9 K^−1 s^−1
Ir2Er	1.0 kOe	Orbach	U eff = 13.6 K, τ0 = 2.72 × 10^−8 s	584	6.37	0.68
				1538	1.40	—
Ir4Er2	1.5 kOe	Orbach	U eff = 11.8 K, τ0 = 6.68 × 10^−8 s	588	5.47	1.08
				1538	1.37	—
Ir2Yb	1.0 kOe	Orbach, direct	U eff = 24.4 k, τ0 = 2.88 × 10^−7 s, A = 687.9 K^−1 s^−1	590	2.23	5.60
				975	0.92	—
Ir4Yb2	1.5 kOe	Orbach, direct (FR)	U eff = 22.2 K, τ0 = 2.03 × 10^−7 s	582	1.24	7.91
			A = 8647.8 K^−1 s^−1	975	0.86	—
		Orbach, direct (SR)	U eff = 28.3 K, τ0 = 2.46 × 10^−8 s, A = 682.6 K^−1 s^−1

---

Further research on the photoluminescence efficiencies and time-resolved luminescence decay of the Ir^III unit and Ir–Ln compounds was carried out, and the data are summarized in Table 2. Owing to the mismatch in the energy levels, the Gd^III ions cannot accept energy from the triplet state of the Ir(III) unit,³⁷ so the photoluminescence efficiencies of Ir₂Gd and Ir₄Gd₂ can be used as the reference for the other Ir–Ln (Ln = Dy, Yb, and Er) complexes. The quantum yield increases slightly from 4.22% to 5.83% (Ir₂Gd) and 8.08% (Ir₄Gd₂), which is attributed to the rigidity of the coordination framework that reduces self-quenching and the possibility of nonradiative transitions.³⁸ In addition, the long lifetime compared to the parent linker (6.60 μs) also implies that no d–f energy transfer occurs in Ir₂Gd and Ir₄Gd₂ (Fig. S29†), while for the other luminescent Ir–Ln (Ln = Dy, Er, and Yb) coordination polymers, the lifetimes were shortened to varying degrees, which indicates that the energy effectively transfers from the light-harvesting Ir unit to the lanthanide centres. Moreover, according to the emission spectrum, the triplet energy level of Ir(ppy)₂(Hdcbpy) is 16 600 cm⁻¹, which is suitable for sensitizing the NIR lanthanide ions Yb^III and Er^III, but is too low to sensitize the low-lying excited state (⁴F_9/2, 21 000 cm⁻¹) of Dy^III ions.^35,39

In order to detect whether a lanthanide ion can be sensitized by the metalloligand Ir(ppy)₂(Hdcbpy), upon irradiation at 500 nm, the near-infrared emission spectra of Ir–Ln (Ln = Er and Yb) compounds were examined in the solid-state. For the Yb^III complexes, the emission bands occurring at 978 nm can be assigned to the ²F_5/2 → ²F_7/2 transition, while in Ir₄Yb₂, the emission peak shows a shoulder at 1005 nm, indicating that the local symmetry environment around the metal centre is affected by crystal-field splitting,⁴⁰ which is in agreement with the two independent Yb^III ions in the single crystal structure. The emission spectra of the Er^III complexes show an emission peak at around 1530 nm, which is attributed to the ⁴I_13/2 → ⁴I_15/2 transition.⁴¹ Moreover, the luminescence decay is well fit to a bi-exponential curve, affording lifetimes of 0.92 μs for Ir₂Yb, 0.86 μs for Ir₄Yb₂, 1.40 μs for Ir₂Er and 1.37 μs for Ir₄Er₂ (Fig. S30 and S31†). Noting that the quantum yields of Ir₂Er (ϕ = 0.68%) and Ir₄Er₂ (ϕ = 1.08%) in the solid state are markedly lower than those of the other Ir–Ln (Ln = Gd, Dy, and Yb) complexes, the energy transfer between the metalloligand and erbium could be more efficient. This can be explained by the effective energy overlap between the ³MLCT state of Ir(ppy)₂(Hdcbpy) and the f–f excited states of erbium.

Conclusion

In this work, we report a series of iridium(III)–lanthanide(III) bimetallic coordination polymers with single-chain (for Ir₂Ln) and double-chain (for Ir₄Ln₂) structures. Magnetic studies demonstrate that complexes Ir₂Dy and Ir₄Dy₂ exhibit SMM behaviour in zero dc field, while complexes Ir₂Er, Ir₂Yb, Ir₄Er₂ and Ir₄Yb₂ show field-induced slow magnetic relaxation. The complex Ir₄Yb₂ is of particular interest because of the field-induced dual relaxation processes, which may originate from the two different coordinated geometries of lanthanide ions. Moreover, in the complexes Ir₂Er, Ir₂Yb, Ir₄Er₂ and Ir₄Yb₂, the Ir unit can act as a good light-harvesting antenna to effectively sensitize Ln-based near-infrared (NIR) luminescence by energy transfer from the Ir unit to the Ln³⁺ ions. As far as we know, this is the first observation of SMM behaviour and NIR luminescence coexisting in iridium(III)–lanthanide(III) bimetallic CPs.

Experimental section

Materials and physical measurements

All the starting materials were of reagent grade and were obtained from the commercial sources and used without further purification. Powder X-ray diffraction (PXRD) data were obtained using a Bruker D8 ADVANCE X-ray powder diffractometer (Cu-K_α) at room temperature. Infrared spectra were recorded on a Bruker Tensor 27 spectrometer using KBr pellets in the range of 400–4000 cm⁻¹. Thermogravimetric analysis (TGA) was performed under a nitrogen atmosphere in the temperature range of 25–600 °C at a heating rate of 5 °C min⁻¹ using a METTLER TOLEDO TGA/DSC 1 instrument. Elemental analyses for C, H and N were performed with a PerkinElmer 240C elemental analyzer. The UV-vis spectra were recorded on a PerkinElmer Lambda 950 UV/Vis/NIR spectrometer using powder samples. The steady fluorescence spectra in the visible region were obtained using a PerkinElmer LS55 fluorescence spectrometer and a Xenon lamp as the excitation source. The emission lifetime and the absolute luminescent quantum yields in the solid state at room temperature were determined using a HORIBA FM-4P-TCSPC spectrometer. The steady state near infrared (NIR) emission spectra were recorded on an Edinburgh FLS980 fluorescence spectrometer equipped with a Hamamatsu R5509-72 photomultiplier. The magnetic susceptibility data were obtained using Quantum Design SQUID MPMS-XL7, VSM and PPMS magnetometer for freshly prepared samples. The diamagnetic contributions of the samples were estimated from Pascal's constants.

Synthesis of [Ln{Ir(ppy)₂(dcbpy)}₂(NO₃)(H₂O)_n]·solvent (Ir₂Ln, n = 5, Ln = Gd and Dy; n = 4, Ln = Er and Yb)

Compounds Ir₂Ln (Ln = Gd, Dy, Er, and Yb) were synthesized under similar optimized experimental conditions. A typical procedure for the preparation of [Yb{Ir(ppy)₂(dcbpy)}₂(NO₃)(H₂O)₄]·2CH₃OH·0.5H₂O (Ir₂Yb) is described below. Ir(ppy)₂(Hdcbpy) (7.4 mg, 0.01 mmol) was suspended in 2 mL of distilled water and dissolved in 5 μL of aqueous NaOH (1 M). Then Yb(NO₃)₃·6H₂O (9.34 mg, 0.02 mmol) in 7 mL of MeOH was added. After keeping the vial at 60 °C for 2 days, orange rod crystals were obtained. The crystalline product of Ir₂Yb was separated by filtration and washed with distilled water and a minimum amount of methanol. Yield: 29%. Elemental analyses (%) calcd for C₇₀H₆₁YbIr₂N₉O_17.5, C, 45.06; H, 3.29; N, 6.76; found: C, 45.09; H, 3.12; N, 6.74. IR (KBr, cm⁻¹): 3384 (m), 3105 (m), 3062 (m), 3043 (m), 1606 (s), 1585 (s), 1546 (m), 1477 (s), 1423 (s), 1380 (s), 1317 (w), 1288 (m), 1269 (m), 1234 (m), 1031 (w), 783 (m), 759 (s), 738 (w), 730 (w), 696 (w).

For [Gd{Ir(ppy)₂(dcbpy)}₂(NO₃)(H₂O)₅]·2CH₃OH·0.5H₂O (Ir₂Gd): yield: 18%. Elemental analyses (%) calcd for C₇₀H₆₁GdIr₂N₉O_17.5, C, 45.45; H, 3.32; N, 6.81; found: C, 44.89; H, 3.27; N, 6.81. IR (KBr, cm⁻¹): 3355 (m), 3107 (w), 3059 (m), 2925 (m), 1631 (s), 1585 (s), 1544 (s), 1476 (s), 1421 (s), 1367 (s), 1270 (m), 1233 (m), 1160 (w), 864 (m), 781 (m), 758 (m), 694 (m), 670(w).

For [Dy{Ir(ppy)₂(dcbpy)}₂(NO₃)(H₂O)₅]·2CH₃OH·0.5H₂O (Ir₂Dy): yield: 41%. Elemental analyses (%) calcd for C₇₀H₆₁DyIr₂N₉O_17.5, C, 45.32; H, 3.31; N, 6.79; found: C, 43.93; H, 3.26; N, 6.63. IR (KBr, cm⁻¹): 3423 (s), 3107 (w), 3049 (w), 1610 (s), 1546 (m), 1477 (s), 1427 (s), 1379 (s), 1313 (m), 1274 (m), 1234 (m), 1161 (w), 1062 (w), 1031 (w), 761 (m), 734 (w), 698 (w), 628 (w), 603 (w), 567 (w).

For [Er{Ir(ppy)₂(dcbpy)}₂(NO₃)(H₂O)₄]·2CH₃OH·0.5H₂O (Ir₂Er): yield: 37%. Elemental analyses (%) calcd for C₇₀H₆₁ErIr₂N₉O_17.5, C, 45.20; H, 3.30; N, 6.77; found: C, 44.76; H, 3.24; N, 6.83. IR (KBr, cm⁻¹): 3375 (m), 3103 (m), 3043 (m), 1606 (s), 1585 (s), 1546 (m), 1514 (w), 1477 (s), 1421 (s), 1377 (s), 1286 (m), 1269 (m), 1232 (m), 1161 (w), 1031 (w), 781 (m), 759 (s), 738 (w), 729 (w), 696 (w).

Thermal analyses reveal that the weight losses at 90 °C are 4.2% (for Ir₂Dy), 4.4% (for Ir₂Er) and 3.9% (for Ir₂Yb) (Fig. S32†), which is in agreement with the release of lattice solvents (calcd 3.9% for 2CH₃OH + 0.5H₂O). The weight loss for Ir₂Gd (5.6%) is higher than those for others, suggesting that one coordination water molecule could also be released below this temperature (calcd 4.9%). Between 90 and 300 °C slow weight losses occur corresponding to the partial removal of the coordination water molecule. Above 300 °C, the organic ligands start to decompose and the framework structures collapse.

Synthesis of [Ln_2{Ir(ppy)₂(dcbpy)}₄(NO₃)₂(H₂O)₄]·solvent (Ir₄Ln₂; Ln = Gd, Dy, Er, and Yb)

Compounds Ir₄Ln₂ (Ln = Gd, Dy, Er, and Yb) were synthesized under similar optimized experimental conditions. A typical preparation procedure for [Yb₂(OH){Ir(ppy)₂(dcbpy)}₄(NO₃)(H₂O)₄]·2CH₃OH·2.5H₂O (Ir₄Yb₂) is as follows. Ir(ppy)₂(Hdcbpy) (7.4 mg, 0.01 mmol) was suspended in 2 mL of distilled water and dissolved in 10 μL of aqueous NaOH (1 M). Then Yb(NO₃)₃·6H₂O (9.34 mg, 0.02 mmol) in 7 mL of MeOH was added. An immediate precipitate of a green powder was obtained. After keeping the vial at 80 °C for 2 days, orange rod crystals were obtained. The crystalline product of Ir₄Yb₂ was separated by filtration and washed with distilled water and a minimum amount of methanol. Yield: 23%. Elemental analyses (%) calcd for C₁₃₈H₁₁₀N₁₇Yb₂Ir₄O_28.5, C, 46.33; H, 3.10; N, 6.66; found: C, 45.82; H, 3.16; N, 6.83. IR (KBr, cm⁻¹): 3400 (s), 3103 (m), 3043 (m), 1608 (s), 1546 (m), 1477 (s), 1427 (s), 1379 (s), 1272 (w), 1232 (m), 1161 (w), 1031 (w), 866 (w), 784 (m), 761 (m), 734 (w), 698 (w), 630 (w).

For [Gd₂(OH){Ir(ppy)₂(dcbpy)}₄(NO₃)(H₂O)₄]·2CH₃OH·2.5H₂O (Ir₄Gd₂): yield: 15%. Elemental analyses (%) calcd for C₁₃₈H₁₁₀N₁₇Gd₂Ir₄O_28.5, C, 46.74; H, 3.13; N, 6.71; found: C, 45.97; H, 3.48; N, 6.99. IR (KBr, cm⁻¹): 3357 (s), 3105 (m), 3042 (m), 1604 (s), 1583 (s), 1562 (m), 1544 (s), 1476 (s), 1420 (s), 1365 (s), 1317 (m), 1269 (w), 1235 (m), 782 (m), 729 (w), 694 (w), 629 (w).

For [Dy₂(OH){Ir(ppy)₂(dcbpy)}₄(NO₃)(H₂O)₄]·2CH₃OH·2.5 H₂O (Ir₄Dy₂): yield: 26%. Elemental analyses (%) calcd for C₁₃₈H₁₁₀N₁₇Dy₂Ir₄O_28.5, C, 46.61; H, 3.12; N, 6.70; found: C, 45.74; H, 3.21; N, 6.68. IR (KBr, cm⁻¹): 3429 (m), 3107 (m), 3053 (m), 1610 (s), 1548 (m), 1477 (s), 1425 (s), 1379 (s), 1271 (w), 1234 (w), 1162 (w), 1033 (w), 761 (s), 734 (w), 698 (w), 669 (w), 628 (w), 559 (w), 426 (w).

For [Er₂(OH){Ir(ppy)₂(dcbpy)}₄(NO₃)(H₂O)₄]·3CH₃OH·2.5H₂O (Ir₄Er₂): yield: 28%. Elemental analyses (%) calcd For C₁₃₉H₁₁₄N₁₇Er₂Ir₄O_29.5, C, 46.40; H, 3.19; N, 6.62; found: C, 45.50; H, 3.57; N, 6.48. IR (KBr, cm⁻¹): 3402 (m), 3107 (m), 3045 (m), 1608 (s), 1546 (m), 1477 (s), 1427 (s), 1380 (s), 1271 (w), 1234 (w), 1161 (w), 1031 (w), 784 (m), 761 (m), 734 (w), 698 (w), 630 (w).

Thermal analyses reveal that the weight losses at 100 °C are 2.8% for Ir₄Gd₂, Ir₄Dy₂ and Ir₄Yb₂ (Fig. S33†), which is in agreement with the release of two CH₃OH and 2.5H₂O molecules (calcd 3.0–3.1%). The weight loss for Ir₄Er₂ is 4.1%, corresponding to the release of three CH₃OH and 2.5 H₂O molecules (calcd 3.9%). The number of solvent molecules for Ir₄Yb₂ is less than that obtained from the single crystal structural determination, suggesting that the lattice solvents are partially released at room temperature. The weight losses between 100 and 300 °C are attributed to the partial release of the coordination water molecules, and above 300 °C the structure begins to collapse.

X-ray crystallographic analyses

Suitable single crystals were selected for indexing and obtaining intensity data using a Bruker SMART APEX D8 diffractometer equipped with a graphite-monochromated Mo Kα radiation source (λ = 0.71073 Å) at 150 K.⁴² Adsorption corrections were carefully performed for all data. The structures were solved by direct methods and refined on F² by full-matrix least squares using SHELXTL.⁴³ All non-hydrogen atoms were located in the Fourier maps and were refined with anisotropic thermal parameters. All H atoms were either placed in the calculated positions or found in the F-maps and refined isotropically.

In the case of Ir₂Dy, the residual electron densities in the solvent-accessible void due to heavily disordered solvent molecules and nitrate anions were determined using the PLATON/SQUEEZE program.⁴⁴ In cases of Ir₂Er, Ir₂Yb and Ir₄Yb₂, the lattice solvent molecules and nitrate anions were found in the Fourier map. The hydrogen atoms of disordered water molecules were not placed. Quite large (2.2–5.3 e A³) residual electron density peaks were located near the Ln or Ir atoms in the corresponding structures, and could not be reasonably modelled with the disordered atoms. The crystallographic data and refinement details are listed in Table 1. The selected bond lengths and angles for Ir₂Dy, Ir₂Er, Ir₂Yb and Ir₄Yb₂ are given in Tables S1–S7.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the National Key R&D Program of China (2017YFA0303203, 2018YFA0306004) and the National Natural Science Foundation of China (21731003) is acknowledged. MK is funded by the CNRS (France).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Synthetic details, structural data, IR, PXRD, TG. CCDC 1956646–1956649. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9qi01504c

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