Chiral Dy(III) Fluorescent Single-Molecule Magnet Based on an Achiral Flexible Ligand

Abstract

A novel multi-channel barcode module was developed by using chiral co-crystals which contain field-induced SMM behavior and different emission bands. The chiral co-crystals [Zn(H₂L)Dy(DBM)₂]₄(ClO₄)₄⋅9CH₃OH⋅H₂O (1a) and [Zn(H₂L)Dy(DBM)₂]₄(ClO₄)₄⋅8CH₃OH⋅0.5H₂O (1b) (H₄L = 2,2′-[1,2-ethanediylbis[(hydroxyethylimino)methylene]]bis[6-methoxy-4-methyl-phenol], HDBM = dibenzoylmethane) were obtained through one-pot reaction of Zn^II and Dy^III with the achiral ligands H₄L and HDBM. X-ray single crystal diffraction and CD spectroscopy confirmed that they are enantiomers crystallized in P4₃ (1a) and P4₁ (1b), both consisting of two ∆-[Zn(H₂L)Dy(DBM)₂]⁺ cations, two Λ-[Zn(H₂L)Dy(DBM)₂]⁺ cations and four (ClO₄)⁻ anions. The presence of Dy^III ions endow them with the property of field-induced slow magnetic relaxation. The relatively low energy barrier of 35.0(9) K for complex 1 may be due to the poor axiality of the ligand field caused by the long Dy-O_phenoxy bond lengths and the small O_phenoxy-Dy-O_phenoxy bond angles. Moreover, when the organic ligands H₄L (λ_ex = 350 nm) and Dy^III (λ_ex = 420 nm) are excited, different emission spectra are observed.

1. Introduction

Barcodes, as an important medium for information storage, play a crucial role in transportation, education and national security [1,2,3]. The ever-increasing demand for information security calls for higher requirements on the coding ability of barcodes [4,5,6,7]. Combining multiple storage technologies to construct multi-channel barcode modules is an effective strategy to improve information storage capability [8]. Circular dichroism (CD) is the differential absorption of left- and right-handed chiral species, which is related to the ground states of the chiral structures [9,10], and has important applications in optical storage. Single-molecule magnets (SMMs) are special metal-organic complexes that can switch between “0” and “1” in the direction of magnetic field at a certain temperature, which can be used for magnetic storage and have a larger storage capacity compared with conventional magnets [11,12,13,14,15,16]. Therefore, chiral species combining CD and SMM behavior form a family of potential two-channel magneto-optical storage materials. In recent years, luminescent materials with multiple emission bands have been widely used in barcodes [8,17,18,19,20,21,22]. For example, Zhao et al. designed multicolor photonic barcodes using 1D Ln-MOF multi-block heterostructures [20]. Su et al. established a dual-channel barcode module by Pr^III-MOF crystals [21].

Based on the above considerations, we are interested in designing a three-channel barcode module with multiple emission bands through chiral SMMs. According to our previous report, a chiral Co^IIDy^III SMM with the energy gap of 89.9 K was obtained based on the achiral ligands 2,2′-[1,2-ethanediylbis[(hydroxyethylimino)methylene]]bis[6-methoxy-4-methyl-phenol] (H₄L) and dibenzoylmethane (HDBM) [23]. H₄L is a luminescent ligand and the Dy^III ion can emit light in the visible range. In order to generate and maintain multiple emission bands, the Co^III ion is replaced with the Zn^II ion in this paper.

Herein, we propose a novel multi-channel barcode module based on a chiral SMM which contains two different emission bands upon excitation of the metal ion and organic ligand. As shown in Scheme 1, the as-prepared chiral co-crystals [Zn(H₂L)Dy(DBM)₂]₄(ClO₄)₄⋅9CH₃OH⋅H₂O (1a) and [Zn(H₂L)Dy(DBM)₂]₄(ClO₄)₄⋅8CH₃OH⋅0.5H₂O (1b) are enantiomers crystallized in P4₃ (1a) and P4₁ (1b) with mirror-symmetric CD spectra and allow for the definition of one channel of the barcode. Moreover, the presence of Dy^III ions gives the kryptoracemate (hidden racemate) [24] complex 1 the property of field-induced SMM behavior with the relatively low energy barrier of 35.0(9) K, which can also be defined as the other one channel of the barcode. Furthermore, complex 1 emits different emission spectra upon excitation of the organic ligand H₄L (λ_ex = 350 nm) and the metal ion Dy^III (λ_ex = 420 nm), further increasing the barcoding channel. This study not only provides some valuable information for the design of novel chiral complexes, but also opens up a new way to construct multi-channel barcodes.

2. Results

2.1. Crystal Structures

H₄L, HDBM, Zn(ClO₄)₂⋅6H₂O, Dy(ClO₄)₃⋅6H₂O and triethylamine was dissolved in a MeOH/MeCN mixture, and the resultant solution was slowly evaporated at room temperature, giving rise to the concurrent crystallization of [Zn(H₂L)Dy(DBM)₂]₄(ClO₄)₄⋅9CH₃OH⋅H₂O 1a or [Zn(H₂L)Dy(DBM)₂]₄(ClO₄)₄⋅8CH₃OH⋅0.5H₂O 1b. The crystallographic data (

Table 1. Crystal data for complexes 1a and 1b.

Complexes	1a	1b
formula	C225H262Cl4Dy4N8O66Zn4	C224H257Cl4Dy4N8O64.5Zn4
formula weight	5187.95	5146.90
T/K	173(2)	173(2)
crystal system	tetragonal	tetragonal
space group	P43	P41
a, b/Å	29.7479(2)	29.7491(2)
c/Å	25.3023(3)	25.3017(2)
α, β, γ/°	90	90
V/Å3	22,391.0(4)	22,392.2(3)
Z	4	4
ρcalc/g⋅cm−3	1.539	1.527
μ/mm−1	8.583	8.562
F(000)	10,192	10,500
data/restraints/parameters	29,484/102/2724	34,446/103/2666
GOF on F2	1.019	1.013
Rint	0.0253	0.0253
R1, wR2 [I ≥ 2σ (I)]	0.0338, 0.0860	0.0420, 0.1092
R1, wR2 (all data)	0.0357, 0.0872	0.0442, 0.1111
Flack parameter	−0.0025(14)	0.0012(16)
CCDC	2,209,058	2,209,059

) show that (1a) and (1b) crystallize in the chiral space groups P4₃ and P4₁ with the Flack factor of −0.0025(14) and 0.0012(16), respectively, indicating that 1a and 1b are chiral (hidden racemate). The powder X-ray diffraction (PXRD) patterns of the products are in good agreement with the simulated diffraction patterns of 1a and 1b (Figure S1), which further indicates that there is no other impurity phase. The calculated formulas of 1a or 1b were consistent with the results of CHN element analysis for the kryptoracemates. As shown in Figure 1, the achiral flexible ligand H₄L was synthesized by the reaction of 2-methoxy-4-methylphenol, N,N′-Bis(2-hydroxyethyl)ethylenediamine and formaldehyde through a Mannich condensation reaction [25]. The detailed self-assembly process of complexes 1a and 1b is as follows: Firstly, H₄L chelates with Zn²⁺ through two N atoms, two O_hydroxy atoms and two O_phenoxy atoms to form Zn(H₂L); secondly, Zn(H₂L) is connected to Dy³⁺ through two O_phenoxy atoms and two O_ether atoms, and then the generated [Zn(H₂L)Dy]³⁺ moiety is linked to two DBM⁻ to form a [Zn(H₂L)Dy(DBM)₂]⁺ cation with ∆- or Λ-configuration around Dy(III); finally, four [Zn(H₂L)Dy(DBM)₂]⁺ cations and four ClO₄⁻ anions self-assemble into mirror-symmetric 1a or 1b. The existence of ClO₄⁻ anion was confirmed by the peak at ∼1087 cm⁻¹ in the IR spectrum.

The solid-state CD spectra of 1a and 1b were measured with two individual single crystals by several attempts because the crystal shapes of 1a and 1b are indistinguishable. Two opposite CD signals are successfully obtained, as shown in Figure 2. For 1a, negative CD signal is detected at 265 and 385 nm, and positive CD signal is detected at 420 nm. In contrast, the CD signal for 1b is positive at 265 and 385 nm, and negative at 420 nm. The near-perfect mirror-image CD spectra in the range of 200–600 nm confirm the optical purity of 1a and 1b. The UV-vis spectra of complexes 1a and 1b in the solid state have obvious broad absorption in the range of 200–470 nm, which is basically consistent with the CD peaks.

Chiral co-crystals formed from the same number of ∆-configurations and Λ-configurations are rare. Complexes 1a and 1b are the first reported enantiomers which consist of two ∆-configurations and two Λ-configurations. As shown in Figure 3, for 1a, Zn1-Dy1 and Zn4-Dy4 are of Λ-configurations, Zn2-Dy2 and Zn3-Dy3 are of ∆-configurations and the ∆- and Λ-ZnDy moieties are diastereomers. In contrast, for 1b, Zn1-Dy1 and Zn4-Dy4 are ∆-configurations, and Zn2-Dy2 and Zn3-Dy3 are Λ-configurations, and similarly the ∆- and Λ-ZnDy moieties are diastereomers. The ∆- and Λ-ZnDy moieties in 1a and 1b are enantiomers (Figure 3). Here, only the crystal structure of Zn1-Dy1 in 1a is selected as the example to be described. Specifically, two O_phenoxy atoms and two N atoms of H₂L²⁻ occupy the equatorial position of octahedral Zn atom with the Zn-O_phenoxy bond distances of 2.005(4) Å and the Zn-N bond distances of 2.088(5) and 2.096(5) Å. Two O_hydroxy atoms of H₂L²⁻ are situated at the axial position with the O_hydroxy-Zn-O_hydroxy angle of 172.55° and Zn-O_hydroxy bond lengths of 2.239(4) and 2.290(5) Å. The Dy atom is eight-coordinate with two O_phenoxy atoms and two O_ether atoms from H₂L²⁻, and four O_carbonyl atoms from two BDM⁻. The Dy-O_ether bond distances are 2.468(4) and 2.497(4) Å, and the Dy-O_carbonyl bond distances of 2.266(4)–2.394(4) Å are within the normal range [26,27]. The Dy-O_phenoxy bond distances are 2.307(4) and 2.268(4) Å, and the O_phenoxy-Dy-O_phenoxy bond angle is 71.48(13)°. The coordination geometry of Dy1 in complex 1a calculated by SHAPE software is close to D_4d and D_2d, and the deviation parameters are 1.674 and 1.615 (Table S1), respectively. Zn and Dy atoms are connected by two O_phenoxy atoms with a Zn1-Dy1 distance of 3.350 Å. H₂L²⁻ and BDM⁻ are connected through strong intramolecular hydrogen bond with O_hydroxy-H-O_carbonyl angles of 125.7 and 160.2°, and O_hydroxy-O_carbonyl distances of 2.740 and 2.795 Å.

2.2. Magnetic Properties

Since complexes 1a and 1b are mirror-related kryptoracemates with similar magnetic properties, only the magnetic properties of complex 1 (a mixture of 1a and 1b) are measured. As shown in Figure 4, the room temperature χ_MT value for complex 1 is 56.98 cm³ K mol⁻¹, which is consistent with the theoretical value of 56.68 cm³ K mol⁻¹ for four Dy^III ions (g = 4/3, ⁶H_15/2) and four diamagnetic Zn^II ions. Upon decreasing the temperature, the χ_MT value was almost unchanged in the range of 300–100 K, and slowly decreased to the lowest value of 44.92 cm³ K mol⁻¹ in the range of 100–8 K. The plot of 1/χ_M vs. T in the range of 8–300 K conforms to the Curie–Weiss law with θ = –4.54 K (Figure S2), which is due to the existence of crystal-field splitting of Dy(III) in complex 1. However, as the temperature continues to decrease to 2 K, the χ_MT value slightly increases to 46.38 cm³ K mol⁻¹, which may be attributed to the intermolecular ferromagnetic interaction. The M–H curve of complex 1 at 2 K rapidly increases from 0 to 20.38 Nβ, and then slowly increases to the maximum value of 25.26 Nβ, which is much lower than the theoretical saturation value of 40 Nβ for four Dy^III ions due to the magnetic anisotropy of the Dy^III ion. In addition, the M–H curves at 2, 3, 4, 5 and 6 K are not overlapped, which again confirms the existence of magnetic anisotropy [28].

In order to study the dynamic magnetic relaxation of complex 1, the temperature dependence of ac magnetic susceptibility was investigated in an oscillating field of 2.5 Oe. As shown in Figure S3, the plot of χ″ versus H for complex 1 at 997 Hz shows that 600 Oe is the best dc field, while the plot of χ″ vs. T shows no peak at 600 Oe. Then, the dc fields of 0, 1500 and 2000 Oe were selected, respectively, to obtain the plots of χ″ vs. T (Figure S4). The plot of χ″ vs. T at 1500 Oe shows a more obvious peak than that of a 2000 Oe dc field, therefore, a dc field of 1500 Oe was applied to suppress the quantum tunnelling of magnetization (QTM). Under the dc field of 1500 Oe, both the in-phase χ′ and the out-of-phase χ″ exhibit obvious frequency dependence in the range of 2–8 K (Figure S5 and Figure 5a), which indicates the field-induced slow magnetic relaxation of complex 1. As shown in Figure 5b, the scatter plot of lnτ vs. T⁻¹ was derived from the peak temperature values of the χ″ vs. T plots at the frequencies of 100, 250, 499 and 997 Hz, which was fitted according to the Arrhenius’ law of lnτ = lnτ₀ + U_eff/kT (τ₀ is the relaxation time, U_eff/k is the relaxation energy barrier), resulting in τ₀ = 3.9(4) × 10⁻¹¹ s and U_eff/k = 35.0(9) K. A τ₀ value within the range of 10⁻¹²–10⁻⁶ s further indicates that complex 1 is a typical field-induced SMM. The relatively low energy barrier may be due to the poor axiality of the ligand field: the Dy-O_phenoxy bond length is in the range of 2.268–2.321(4) Å, while the O_phenoxy-Dy-O_phenoxy bond angle is in the range of 70.12.48(13)–72.17(14)°, which is far away from the perfect 180°. As shown in Figure S6, the Cole–Cole plots show a characteristic double magnetic relaxation for complex 1, which can be fitted to obtain α₁ = 0.163–0.251 and α₂ = 0.299–0.348 (Table S2), which correspond to two crystallographically independent Dy^III ions [29]. Of course, theoretical calculations could provide more accurate information on the relaxation mechanism of such ZnDy SMMs, which deserves future investigations.

2.3. Luminescent Properties

The luminescent properties of complex 1 in MeCN were investigated, as shown in Figure 6a. Upon excitation at 350 nm, complex 1 exhibits a broad band similar to H₄L, which can be attributed to the characteristic peak of the H₂L²⁻ moiety in 1. However, the band appears at 420 nm for complex 1 and 402 nm for H₄L, and the emissive intensity of complex 1 is nearly 85 times weaker than that of H₄L. The weakened intensity may be due to the inner-filter effect (IFE) in complex 1 in the presence of the overlap between the emission and the visible absorption at ca. 402 nm (Figure 2). As shown in Figure 6b, the excitation spectrum of complex 1 was recorded by monitoring the characteristic emission band of Dy^III ions at 478 nm. The peaks at 402 and 420 nm originate from the intra-ligand π→π * transitions, while the peak at 432 nm is due to the f→f * transition of the Dy^III ion from the ⁶H_15/2 ground state to the ⁴G_11/2 excited state [30]. Therefore, the emission spectrum of complex 1 was recorded upon the excitation of 420 nm. There are two peaks at 462 and 478 nm, corresponding to the ⁴I_15/2→⁶H_15/2 and ⁴F_9/2→⁶H_15/2 transitions of Dy^III [31,32].

3. Materials and Methods

3.1. Materials

Methanol (MeOH), acetonitrile (MeCN), 2-methoxy-4-methylphenol, N,N′-Bis(2-hydroxyethyl)ethylenediamine, formaldehyde, dibenzoylmethane (HDBM), Zn(ClO₄)₂⋅6H₂O, Dy(ClO₄)₃⋅6H₂O and triethylamine were purchased from commercial sources and used without further purification.

3.2. The Preparation of Complex 1

H₄L was prepared according to the method we reported previously [23]. The mixture of H₄L (45 mg, 0.1 mmol), HDBM (45 mg, 0.2 mmol), Zn(ClO₄)₂⋅6H₂O (37 mg, 0.1 mmol) and 96 μL Dy(ClO₄)₃⋅6H₂O aqueous solution (50%, 0.1 mmol) was dissolved in acetonitrile (5 mL) and methanol (10 mL). Then, triethylamine (100 μL) was slowly added and stirred for 3 min. Subsequently, the above solution was filtered and slowly evaporated without disturbance at room temperature. Colorless needle crystals of complex 1 were obtained after 2 days. Yield: about 74%. Two types of crystal structures were obtained by single crystal X-ray diffraction analysis on several randomly selected single crystals. Anal. calcd (%) for C₂₂₅H₂₆₂Cl₄Dy₄N₈O₆₆Zn₄ (1a): C, 52.09; H, 5.09; and N, 2.16. Anal. calcd (%) for C₂₂₄H₂₅₇Cl₄Dy₄N₈O_64.5Zn₄ (1b): C, 52.27; H, 5.03; and N, 2.18. Found for 1a + 1b: C, 52.9; H, 5.0; and N, 2.4. Main IR peaks (KBr, cm⁻¹): 1593 (s), 1544 (vs), 1521 (vs), 1481 (vs), 1452 (m), 1390 (vs), 1315 (m), 1250 (m), 1087 (vs), 816 (m), 728 (m), 694 (w), and 617 (m).

3.3. Physical Measurements

IR spectra was recorded on a Beijing Rayleigh WQF-510A FTIR spectrometer. CHN element analysis was tested on a thermal Flash EA 1112 elemental analyzer. Powder X-ray diffraction (PXRD) was measured on a Rigaku D/max 2500 diffractometer. Circular dichroism (CD) spectra were measured on a JASCO J-1500 spectrometer. Luminescence spectra were tested on a Hitachi F-7000 fluorescence spectrometer. Magnetic measurements were carried on a SQUID MPMS-XL5 magnetometer, and the Pascal’s constants were used to correct the diamagnetism. Single-crystal X-ray diffraction was performed on a Rigaku Super Nova diffractometer. The crystal structures were solved and refined by OLEX-2 software. To resolve the severe disorder of the lattice solvent molecules of complexes 1a and 1b, the SQUEEZE function of PLATON software was applied. For complex 1a, 104 electrons in the hole of 509 ų correspond to 5CH₃OH and 1H₂O for each formula (100 electrons). For 1b, 133 electrons in the hole of 822 ų correspond to 7CH₃OH and 0.5H₂O for each formula (131 electrons).

4. Conclusions

In summary, we have developed a novel multi-channel barcode module based on chiral co-crystal complexes 1a and 1b, which contain field-induced SMM behavior and different emission bands. Complexes 1a and 1b are the first chiral kryptoracemate complexes composed of four independent binuclear Zn-Dy groups, two of which are ∆-configuration and two are Λ-configuration. These two chiral complexes have mirror-related structures and mirror-symmetric CD spectra. Complex 1 shows field-induced SMM behavior with an energy barrier of 35.0(9) K. Moreover, luminescence measurements show that complex 1 exhibits different luminescence emissions under the excitation of UV or visible light. The present work provides some valuable information for the construction of novel chiral complexes from achiral ligands, and describes a new strategy for creating multi-channel barcode modules for high-density information storage.