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. 2025 Nov 17;4(3):229–239. doi: 10.1021/prechem.5c00102

Precise Control of Homochiral Multifunctional Dy2 Single-Molecule Magnets by Changing the Positions of the Two Heterocyclic Nitrogen Atoms in the Schiff Base Ligands

Cai-Ming Liu †,*, Yi-Quan Zhang ‡,*, Xiang Hao †, Xi-Li Li §
PMCID: PMC13014321  PMID: 41890410

Abstract

Two pairs of homochiral Dy2 zero-field single-molecule magnets (SMMs) derived from 3-(perfluorobutyryl)-(+)/(−)-camphor (D-Hpfc/L-Hpfc) were assembled by using two hydrazone Schiff bases with the same molecular formula but different heterocyclic rings (pyrazine vs pyrimidine) as bridging ligands. This provides an excellent opportunity to precisely regulate the homochiral multifunctional SMMs by altering the positions of the two heterocyclic nitrogen atoms in the Schiff base ligands. [Dy2(LSchiff1)2(D-pfc)2(MeOH)2] (D-1)/[Dy2(LSchiff1)2(L-pfc)2(MeOH)2] (L-1) [H2LSchiff1 = {2-[(E)-(3,5-difluoro-2-hydroxyphenyl)­methylidene]­diazanyl}­(pyrazin-2-yl)­methanone] is crystallized in the P2 1 space group, while [Dy2(LSchiff2)2(D-pfc)2(MeOH)2] (D-2)/[Dy2(LSchiff2)2(L-pfc)2(MeOH)2] (L-2) [H2LSchiff2 = {2-[(E)-(3,5-difluoro-2-hydroxyphenyl)­methylidene]­diazanyl}­(pyrimidin-4-yl)­methanone] is crystallized in the P1 space group; and only the octagon coordination configuration was observed for the Dy3+ ions of D-1/L-1, while both the octagon and the heptagonal pyramid coordination configurations were observed for the Dy3+ ions of D-2/L-2. As a result, D-1/L-1 shows single magnetic relaxation, while D-2/L-2 displays double magnetic relaxation, though intramolecular ferromagnetic interactions exist in both D-1/L-1 and D-2/L-2. The ab initio calculations were carried out to explain their magnetic properties. Moreover, the magneto-optic effects of D-1/L-1 and D-2/L-2 were detected by their magnetic circular dichroism spectra. Furthermore, these two pairs of enantiomers exhibit strong third harmonic generation (THG).

Keywords: single-molecule magnet, multifunctional complex, third-order nonlinear optical material, magnetic circular dichroism, ab initio calculation, Dy(III) complex, chiral β-diketone, hydrazone Schiff base

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Introduction

As nanosized molecules with magnetic bistability and potential applications in high-density information storage, quantum computing, and other fields, single-molecule magnets (SMMs) have attracted extensive attention. − Through the continuous efforts of scientists, the research on SMMs has made great progress. , Interestingly, when chirality is introduced into SMMs, they can be given some additional physical properties, such as second-order nonlinear optics, − ferroelectricity, − magnetochiral dichroism (MChD), − circularly polarized luminescence (CPL), − etc., and the resulting chiral SMMs naturally become a class of nanoscale multifunctional molecular materials, which have bright application prospects in the fields of multidimensional intelligent responsive micro/nano devices. However, the synthesis of SMMs is inherently difficult, and it is necessary to overcome the natural tendency of racemization when self-assembling in solution to obtain chiral SMMs; therefore, the design and synthesis of chiral SMMs, especially zero-field chiral SMMs, still face great challenges.

With the exception of a few chiral SMMs that are obtained through spontaneous resolution of chirality by the coordination of achiral ligands, ,, the majority of chiral 4f SMMs and chiral 3d-4f SMMs are assembled from chiral ligands directly. − The commonly used chiral ligands include Schiff bases and their reduced Schiff bases, ,,− nitrogen-containing heterocyclics, , phosphoric acids, phosphates, carboxylic acids, − and β-diketonates. − Furthermore, the chiral ligands that are not derived from chiral carbon atoms, such as helical ligands , and axial chiral ligands, − can be used to assemble chiral SMMs as well. Although such homochiral SMMs have been developed in recent years, there is still a lack of systematic research on the same type of homochiral SMMs, especially on the precise control of chiral SMMs at the atomic level; such studies can not only expand new chiral SMMs but, more importantly, can also have implications for the development of high-performance chiral SMMs. In this study, we used two new hydrazone Schiff bases (H2LSchiff1 = {2-[(E)-(3,5-difluoro-2-hydroxyphenyl)­methylidene]­diazanyl}­(pyrazin-2-yl)­methanone; H2LSchiff2 = {2-[(E)-(3,5-difluoro-2-hydroxyphenyl)­methylidene]­diazanyl}­(pyrimidin-4-yl)­methanone, Scheme ) with the same molecular formula but different heterocyclones (pyrazine vs pyrimidine) as bridging ligands and homochiral β-diketonates (3-(perfluorobutyryl)-(+)/(−)-camphor (D-Hpfc/L-Hpfc)) as chiral terminal ligands to assemble homochiral 4f SMMs, obtaining two pairs of Dy2 enantiomers: [Dy2(LSchiff1)2(D-pfc)2(MeOH)2] (D-1)/[Dy2(LSchiff1)2(L-pfc)2(MeOH)2] (L-1) and [Dy2(LSchiff2)2(D-pfc)2(MeOH)2] (D-2)/[Dy2(LSchiff2)2(L-pfc)2(MeOH)2] (L-2). D-1/L-1 and D-2/L-2 have the same molecular formula but the positions of the two heterocyclic nitrogen atoms in the Schiff base ligands are different. The two pairs of enantiomers show zero-field SMM properties, but are crystallized in different chiral space groups and display different magnetic relaxation behaviors. Furthermore, they exhibit obvious magnetic circular dichroism effects and strong third harmonic generation (THG). Notably, the strongly electron-withdrawing fluorine atoms on the Schiff base ligands and the homochiral β-diketonate ligands not only help to increase the energy barrier of SMMs but also have a beneficial effect on third-order nonlinear optical properties.

1. Hydrazone Schiff Base Bridging Ligands H2LSchiff1 and H2LSchiff2.

1

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Results and Discussion

Synthesis

The hydrazone Schiff base ligands and homochiral ligands may be used to assemble homochiral Dy2 SMMs by a ‘mixed ligand’ strategy. ,, In this study, we first synthesized two new hydrazone Schiff base ligands (H2LSchiff1 and H2LSchiff2, Scheme ) by condensation reactions of 3,5-difluoro-2-hydroxybenzaldehyde with two hydrazides, pyrazinoic acid hydrazide, and pyrimidine-4-carbohydrazide. The two hydrazone Schiff base ligands H2LSchiff1 and H2LSchiff2 have the same formula, but the two nitrogen atoms in the heterocyclics are at different positions. They were then used as the bridging ligands and 3-(perfluorobutyryl)-(+)/(−)-camphor was used as the chiral terminal ligand to react with Dy­(CF3SO3)3 and LiOH·H2O in MeOH–CH2Cl2 or MeOH−DMF, assembling two new pairs of Dy2 enantiomers: D-1/L-1 and D-2/L-2 (for experimental procedures, see the Supporting Information). The molecular structures of these two pairs of Dy2 enantiomers contain the same Dy3+ cations, the same methanol terminal ligands, and the same homochiral β-diketonate terminal ligands, differing only in the positions of the two nitrogen atoms in the heterocycle of the Schiff base ligands. This provides an excellent opportunity to adjust their SMM properties at the atomic level. Notably, these assembly reactions are performed at room temperature to avoid the rapid racemization that commonly occurs in chiral complexes at high temperatures.

Crystal Structures

Although the two pairs of Dy2 enantiomers have the same formula, L-1/D-1 is crystallized in the P2 1 space group, while L-2/D-2 is crystallized in the P1 space group (Table ). The Flack values of L-1/D-1 and L-2/D-2 are all close to zero (Table ), which indicates that their single crystals are enantiomerically pure. Because the crystal structures of enantiomers are mirror-symmetrical, we only need to describe the structures of the L-configurations in detail. As shown in Figure , L-1 is composed of two Dy3+ ions, two LSchiff12– bridging ligands, two L-pfc– terminal ligands, and two coordinated MeOH molecules. In the crystal structure of L-1, two Dy3+ ions are connected by two inverse LSchiff12– bridging ligands to form a rough plane, and each Dy3+ ion is further coordinated by an L-pfc– terminal ligand and a methanol molecule in opposite directions to complete a structure having a pseudosymmetric center, with a Dy···Dy distance of 3.948 Å. In the [Dy2O2] core, the mean Dy–O bond distance is 2.349 Å (Table S1), and the average Dy–O–Dy bond angle is 112.0° (Table S1).

1. Crystal Data and Structural Refinement Parameters for D-1, L-1, D-2, and L-2 .

  D- 1 L- 1 D- 2 L- 2
formula C54H48Dy2 C54H48Dy2 C54H48Dy2 C54H48Dy2
F18N8O10 F18N8O10 F18N8O10 F18N8O10
F W 1636.00 1636.00 1636.00 1636.00
crystal system monoclinic monoclinic triclinic triclinic
space group P2 1 P2 1 P1 P1
a[Å] 9.96391(9) 9.95801(9) 10.1137(3) 10.0826(3)
b[Å] 21.6490(2) 21.6428(2) 10.4642(3) 10.4221(3)
c[Å] 13.97027(12) 13.97590(12) 15.9689(3) 15.9166(3)
α[°] 90.00 90.00 104.216(2) 104.305(2)
β[°] 101.6002(8) 101.5813(8) 94.483(2) 94.182(2)
γ[°] 90.00 90.00 112.045(3) 112.360(3)
V[Å3] 2951.95(5) 2950.75(5) 1490.95(8) 1472.60(8)
Z 2 2 1 1
ρcalc[g·cm–3] 1.841 1.841 1.822 1.845
μ[mm–1] 14.477 14.483 14.331 14.510
T[K] 170 170 170 100
λ [Å] 1.54184 1.54184 1.54184 1.54184
reflections collected 38168 42017 45060 25897
unique reflections 11896 11962 11187 8481
observed reflections 11423 11714 10518 8367
parameters 842 842 844 843
GoF[I ≥ 2σ (I)] 1.021 1.014 1.110 1.051
R 1[I ≥ 2σ (I)] 0.0428 0.0300 0.0397 0.0490
W R 2[I ≥ 2σ (I)] 0.1040 0.0786 0.1073 0.1284
Flack value –0.002(2) –0.0002(17) –0.003(4) 0.004(4)
CCDC 2479560 2479561 2479562 2479563
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1.

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Crystal structure of L- 1; all H atoms are omitted for clarity.

Each Dy3+ ion has a [N2O6] coordination sphere, which is bonded by one Ophenoxide atom, one Nimine atom, and one Ocarbonyl atom of one LSchiff12– bridging ligand; one Npyrazine atom and one Ocarbonyl atom of the other LSchiff12– bridging ligand; two O atoms of one L-pfc– terminal ligand; and one OMeOH atom. The exact coordination configurations could be analyzed by SHAPE-2.1 software, giving the octagon for both Dy1 and Dy2, with large CShM values of 31.003 for Dy1 and 29.603 for Dy2 (Table S2), suggesting a large degree of deviation for the ideal D 8h symmetry.

As expected, the crystal structure of D-1 is mirror-symmetrical to that of L-1 (Figure ). However, the Dy1···Dy2 separation distance of 3.894 Å in D-1 is a bit shorter than that in L-1 (3.948 Å).

2.

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Mirror symmetry of (a) L -1 and (b) D -1.

As shown in Figure , L-2 consists of two Dy3+ cations, two LSchiff22– anions, two L-pfc– anions, and two coordinated MeOH molecules. Similar to L-1, two inverse LSchiff22– ligands bridge two Dy3+ ions to generate an approximate plane, and an L-pfc– terminal ligand and a MeOH molecule complete the coordination of each Dy3+ ion in opposite directions. A structure with a pseudosymmetric center is thus formed; the Dy···Dy distance is 3.948 Å, the same as in L-1. In the [Dy2O2] core of L-2, the mean Dy–O bond distance is 2.303 Å (Table S1), which is slightly smaller than that of L-1 (2.349 Å), while the average Dy–O–Dy bond angle is 115.0 ° (Table S1), a little larger than that of L-1 (112.0 °).

3.

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Crystal structure of L- 2; all H atoms are omitted for clarity.

Each Dy3+ ion in L-2 is coordinated by one Npyrimidine atom and one Ocarbonyl atom from one LSchiff22– bridging ligand; one Ophenoxide atom, one Nimine atom, and one Ocarbonyl atom supplied by the other LSchiff22– bridging ligand; two O atoms from one L-pfc– anion; and one OMeOH atom. The coordination configurations were also analyzed by SHAPE-2.1 software, giving the heptagonal pyramid for Dy1, with a large CShM value of 30.883 for C 7v (Table S3), and the octagon for Dy2, with a large CShM value of 30.093 for D 8h (Table S3). Notably, two coordination configurations of the Dy3+ ions exist in L-2, while only one exists in L-1, which may affect their SMM properties.

The crystal structure of D-2 is mirror-symmetrical to that of L-2 (Figure ). The Dy1···Dy2 separation distance in D-2 is 3.954 Å, which is slightly longer than that in L-2 (3.948 Å).

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Mirror symmetry of L -2 (a) and D -2 (b).

Magnetic Properties

We are only studying the magnetism of L-1 and L-2 here because enantiomers often have very similar magnetic properties. Their variable-temperature dc susceptibility was first measured. As shown in Figure , the χ M T values at 300 K of L-1 (28.33 cm3Kmol–1) and L-2 (28.26 cm3Kmol–1) are both very close to the calculated value of 28.34 cm3Kmol–1 for two uncoupled Dy3+ ions. The χ M T-T curves of L-1 and L-2 both decrease slowly with the decrease of temperature, then increase sharply after reaching the minimum value in the low-temperature region, and finally increase to 40.93 cm3Kmol–1 for L-1 and 44.86 cm3Kmol–1 for L-2 at 2 K. Such a trend of the χ M T-T curve in the hook pattern is the result of the combined action of two factors: the depopulation of M j levels of the Dy3+ ion and the intramolecular Dy3+···Dy3+ ferromagnetic interaction. Similar cases have also been observed in other Dy2 SMMs derived from hydrazone Schiff base ligands. ,,,, Moreover, the field-dependent magnetization at 2, 4, and 6 K of L-1 and L-2 was measured as well, and no coincidence was found in their M-H/T curves (Figures S1 and S2), which implies that there are magnetic anisotropy and/or low-lying excited states in L-1 and L-2.

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Plot of χ M T versus T of L-1 and L-2 (H dc = 1000 Oe).

Analysis of the ac susceptibility of L-1 indicates that it is a zero-field SMM. As shown in Figure a, at 0 Oe, the χ″-T curves of L-1 have frequency-dependent peaks from 100 to 1399 Hz. On the other hand, its χ″-ν curves at 0 Oe also have obvious temperature-dependent peaks in the range of 2–10.5 K (Figure b), according to which the relationship between lnτ and 1/T could be extracted (Figure c). The obtained lnτ-1/T curve follows the formula τ–1 = τQTM –1 + CT n + τ0 –1  exp­(−U eff /kT), with τQTM = 0.0711, n = 3.95, C = 0.05857 s–1 K–3.95, U eff/k = 76.9 K, and τ0 = 1.0 × 10–6 s. The n value of 3.95 of L-1 is within the normal range for Dy SMMs (2 ≤ n ≤ 9), and its τ0 value (1.0 × 10–6 s) is a common value for SMMs as well. The Cole–Cole curves at 0 Oe of L-1 present as (incomplete) semicircles (Figure d), which could be fitted with a generalized Debye model, , obtaining α values of 0.03982–0.12998; these small α values suggest that the relaxation time distribution of L-1 is narrow. In addition, no obvious hysteresis loop could be discovered for L-1 at 1.9 K (Figure S3).

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χ″ versus T curves for L -1 (H dc = 0 Oe) (a); χ″ versus ν curves for L -1 (H dc = 0 Oe) (b); plot of ln­(τ) versus 1/T for L -1 (H dc = 0 Oe) (the solid line represents the best fitting with QTM + Raman + Orbach) (c); and Cole–Cole plot measured from 2 to 10.5 K for L-1 (H dc = 0 Oe) (the solid lines represent the best fitting with a generalized Debye model) (d).

We also examined the effect of a 1500 Oe dc magnetic field on the magnetic relaxation performance of L-1 (Figure S4). At 1500 Oe, the frequency-dependent χ″-T curves of L-1 have the first peak at a lower frequency (10 Hz, Figure S4a) than at 0 Oe (100 Hz, Figure a). However, its temperature-dependent χ″-ν curves at 1500 Oe have the first peak at a higher temperature (4 K, Figure S4b) than at 0 Oe (2 K, Figure b). The lnτ versus 1/T plot of L-1 (Figure S5) based on its χ″-ν curves at 1500 Oe could be fitted by τ –1 = CT n + τ0 –1  exp­(−U eff /kT), yielding n = 5.0, C = 0.00631 s–1 K–5.0, U eff/k = 87.6 K, and τ0 = 4.7 × 10–7 s. Compared with the zero field, the U eff/k value of L-1 increased (from 76.9 to 87.6 K) and the τ0 value decreased (from 1.0 × 10–6 to 4.7 × 10–7 s) after being applied with a dc magnetic field of 1500 Oe, which could be ascribed to the partial or complete suppression of the quantum tunneling of magnetization (QTM) by the dc magnetic field. This is also seen in other zero-field SMMs. ,,,

The ac susceptibility of L-2 reveals that it is also a zero-field SMM, but exhibits richer characteristics with respect to L-1. As shown in Figure a, the χ″-T curves of L-2 at 0 Oe display frequency-dependent peaks over a wider frequency range (from 10 to 1399 Hz) than L-1 (from 100 to 1399 Hz), and shoulder peaks appear at high frequencies. Notably, the χ″-ν curves of L-2 at 0 Oe not only have temperature-dependent peaks in the range of 2–12 K (Figure b) but can also show the appearance of two peaks in the intermediate-temperature region (4–6 K), clearly indicating that there is double magnetic relaxation: the fast relaxation phase (FR) in the low-frequency zone and the slow relaxation phase (SR) in the high-frequency zone. Based on the χ″-ν curves of L-2 at 0 Oe, two lnτ versus 1/T curves could be extracted for FR and SR, respectively (Figure c); both could be fitted with the formula τ –1 = τQTM –1 + CT n + τ0 –1  exp­(−U eff /kT), giving τQTM = 0.00464, n = 3.05, C = 4.1877 s–1 K–3.05, U eff/k = 78.1 K, and τ0 = 3.5 × 10–8 s for FR as well as τQTM = 1.4805, n = 6.04, C = 0.00016 s–1 K–6.04, U eff/k = 180.6 K, and τ0 = 3.7 × 10–10 s for SR. The U eff/k value of L-2 at 0 Oe of 180.6 K is larger than those of other homochiral Dy2 SMMs derived from hydrazone Schiff base ligands and homochiral β-diketonate ligands (≤173.8 K). The Cole–Cole curves at 0 Oe of L-2 also show the characteristics of double magnetic relaxation (Figure d), which could be fitted with the sum of two modified Debye functions, − giving α1 values of 0.126–0.186 and α2 values of 0.00015–0.0859. The α2 values of L-2 are obviously smaller than the α1 values, suggesting that the relaxation time distribution of SR is narrower than that of FR in L-2. Additionally, no hysteresis loop could be found for L-2 at 1.9 K (Figure S6).

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χ″ versus T curves for L -2 (H dc = 0 Oe) (a); χ″ versus ν curves for L -2 (H dc = 0 Oe) (b); plot of ln­(τ) versus 1/T for L -2 (H dc = 0 Oe) (the solid line represents the best fitting with QTM + Raman + Orbach) (c); and Cole–Cole plot measured from 2 to 12 K for L-2 (H dc = 0 Oe) (the solid lines represent the best fitting with the sum of two modified Debye functions) (d).

At 1500 Oe, the frequency-correlated χ″-T curves of L-2 show more obvious double magnetic relaxation characteristics than at 0 Oe, and bimodals appear in the range of 50 to 1399 Hz (Figure S7a). The χ″-ν curves at 1500 Oe for L-2 also exhibit temperature-dependent peaks in the range of 2–12 K (Figure S7b), but the intermediate-temperature region with two peaks becomes 4.5–7 K. The two lnτ versus 1/T plots (Figure S8), extracted from the χ″-ν curves at 1500 Oe of L-2, were fitted by τ –1 = CT n + τ0 –1  exp­(−U eff /kT), yielding n = 2.86, C = 2.3183 s–1 K–2.86, U eff/k = 85.8 K, and τ0 = 1.1 × 10–8 s for FR as well as n = 5.88, C = 0.00024 s–1 K–5.88, U eff/k = 197.8 K, and τ0 = 1.2 × 10–10 s for SR. Similar to L-1, at 1500 Oe, the U eff/k values of L-2 became larger with respect to 0 Oe (from 78.1 to 85.8 K for FR and from 180.6 to 197.8 K for SR), and its τ0 values became smaller with respect to 0 Oe (from 3.5 × 10–8 to 1.1 × 10–8 s for FR and from 3.7 × 10–10 to 1.2 × 10–10 s for SR).

Theoretical Analysis of Magnetic Properties

Complete active space self-consistent field (CASSCF) on two individual Dy3+ fragments according to the X-ray determined geometries of either L-1 or L-2 (Figure S9) has been carried out on the OpenMolcas program and the SINGLE_ANISO program (see the Supporting Information for details). − Table S4 lists the calculated energy levels, g (g x , g y , g z ) tensors, and the predominant m J values of the lowest eight Kramers doublets (KDs) of individual Dy3+ fragments for L-1 and L-2. As presented in Table S5, the ground KDs for L-1 and L-2 are mostly composed of m J = ±15/2; this results in small transversal magnetic moments for them, whereas the first excited KDs for L-1 and L-2 consist of several m J states severely, leading to the large transversal magnetic moments in the first excited KDs for L-1 and L-2.

The corresponding magnetization blocking barriers of individual Dy3+ fragments for L-1 and L-2 are shown in Figure S10, where the transversal magnetic moments in their ground KDs are all smaller than 0.5 × 10–1 μB, and the quantum tunneling of magnetization (QTM) values in their ground KDs are thus all suppressed at low temperatures. The transversal magnetic moments in the first excited states of individual Dy3+ fragments for L-1_Dy1, L-1_Dy2, L-2_Dy1, and L-2_Dy2 are 2.25 × 10–1, 2.03 × 10–1, 1.07, and 1.87 μB, respectively. Their transversal magnetic moments are all larger than 0.5 × 10–1 μB, thus allowing a fast QTM in their first excited KDs. The calculated energy barriers of L-1_Dy1, L-1_Dy2, L-2_Dy1, and L-2_Dy2 are 125.3, 114.9, 171.7, and 163.3 cm–1, respectively, which are larger than the experimental U eff values of 53.4 cm–1 at 0 Oe for L-1 as well as 54.3 cm–1 (FR) and 125.5 cm–1 (SR) at 0 Oe for L-2; they are even larger than the experimental U eff values at 1500 Oe: 60.9 cm–1 for L-1 as well as 59.6 cm–1 (FR) and 137.5 cm–1 (SR) for L-2. These results indicate that in addition to QTM, there are other factors such as Raman magnetic relaxation, anharmonic phonons, etc., which make the experimental U eff values unable to reach the theoretical U eff values. −

Although the magnetic anisotropies of L-1 and L-2 mainly originate from individual Dy3+ ions, the magnetic interactions between the Dy3+ ions generally have a certain effect on magnetic relaxation. Through comparison of the measured and computed magnetic susceptibilities for L-1 and L-2, the exchange coupling constants of J̃ exch and the intermolecular interaction zJ′ could be fitted by the program POLY_ANISO. −

The parameters from Table were calculated with respect to the pseudospin S̃ Dy = 1/2 of the Dy3+ ion. The total coupling parameters J̃ total (dipolar and exchange) are included into fitting the dc magnetic susceptibilities. The calculated and experimental χM T versus T plots of L-1 and L-2 are shown in Figure S11, where both of the fits are close to the experiments at low temperatures. The Dy3+–Dy3+ interactions in L-1 and L-2 are both ferromagnetic according to the obtained positive J̃ total values in Table within the Lines model. We further calculated the exchange energies, the energy differences between each exchange doublet Δ t , and the main values of g z for the lowest four exchange doublets of L-1 and L-2 (Table S6). In the ground exchange states, the g z values of L-1 and L-2 are 39.880 and 39.138, respectively, which confirm the Dy3+–Dy3+ ferromagnetic interactions in L-1 and L-2. The main magnetic axes on the Dy3+ ions of L-1 and L-2 are indicated in Figure , where the included angle between the magnetic axes on Dy1 and Dy2 in L-1 and L-2 are 6.14 and 1.26°, respectively. Their easy axes are almost parallel to each other and to the vector connecting the two Dy3+ ions. This results in very weak transversal components of the dipolar field induced by the Dy3+ ions on each other, thus significantly reducing the efficiency of the tunneling mechanism, which is otherwise often active in concentrated lanthanide-based SMMs. Obviously, the smaller angle of 1.26° in L-2 leads to the weaker transversal components of the dipolar field induced by the Dy3+ ions with respect to L-1.

2. Fitted Exchange Coupling J̃ exch, Calculated Dipole–Dipole Interaction J̃ dip, and Total Constant J̃ total between Dy3+ Ions in L-1 and L-2 (cm–1)­ .

  L-1 L-2
J̃ exch 0.25 0.25
J̃ dip 4.55 4.83
J̃ total 4.80 5.08
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a

The intermolecular interaction zJ′ of L-1 and L-2 was fitted to −0.02 cm–1

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Calculated orientations of the local main magnetic axes on the Dy3+ ions of L-1 (a) and L-2 (b).

Circular Dichroism (CD) Spectra and Magnetic Circular Dichroism (MCD) Properties

The circular dichroism (CD) spectra of D-1/L-1 in CH2Cl2 solution (0.1 g/L) in the range of 250–500 nm and D-2/L-2 in DMF solution (0.1 g/L) in the range of 280–550 nm were studied at room temperature, which confirm their chiral optical activities and enantiomer characteristics (Figures a anda). The CD spectra of both D-1/L-1 and D-2/L-2 are mirror-symmetrical, with D-1/L-1 having two strong peaks at 297 and 415 nm and with D-2/L-2 having two strong peaks at 316 and 421 nm. From Figures b andb, it can be seen that the UV absorption peaks are located at 314 and 408 nm for D-1/L-1 and at 323 and 415 nm for D-2/L-2, so the Cotton effect peaks at 297 nm for D-1/L-1 and 316 nm for D-2/L-2 should be attributed to the π–π* transition of the homochiral β-diketone ligands (D-pfc–/L-pfc–), while the Cotton effect peaks at 415 nm for D-1/L-1 and 421 nm for D-2/L-2 are caused by the n–π* transition of the CN groups of the bridging ligands (LSchiff12–/LSchiff22–).

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CD spectra (H dc = 0 and ±1.6 Oe, 5 mm optical path) (a) and UV spectra of L-1/D-1 in a CH2Cl2 solution (c = 0.1 g/L) in the range of 250–500 nm at room temperature (b).

10.

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CD spectra (H dc = 0 and ±1.6 Oe, 5 mm optical path) (a) and UV spectra of L-2/D-2 in a DMF solution (c = 0.1 g/L) in the range of 280–550 nm at room temperature (b).

To investigate the magneto-optic effects of D-1/L-1 and D-2/L-2, their MCD spectra at ±1.6 T were measured, where the positive and negative signs indicate that the direction of the magnetic field is parallel to and antiparallel to the direction of incidence of polarized light, respectively. As shown in Figures a and a, the strength of their CD signals changes under the action of external magnetic fields, and even the position of their peaks may be shifted. It can be seen that the D-1 NS (+1.6 T) curve and the L-1 SN (−1.6 T) curve as well as the D-1 SN (−1.6 T) curve and the L-1 NS (+1.6 T) curve are roughly mirror-symmetrical; similarly, the D-2 NS (+1.6 T) curve and the L-2 SN (−1.6 T) curve, and the D-2 SN (−1.6 T) curve and the L-2 NS (+1.6 T) curve are also roughly mirror-symmetrical. The pure MCD spectra of D-1/L-1 and D-2/L-2 were calculated by MCD = [CD­(+1.6 T) – CD­(−1.6 T)]/(2 × 1.6), with the strongest MCD signals at 309 nm for D-1 and 307 nm for L-1 (Figure S12) and at 310 nm for D-2 and 309 nm for L-2 (Figure S13).

Because the strongest MCD signals are caused by the π–π* transition of the homochiral β-diketone ligands (D-pfc–/L-pfc–), the g (CD) and g (MCD) values at the corresponding Cotton effect peaks were calculated. According to g (CD) = Δε(CD)/ε and g (MCD) = Δε(MCD)/ε, the g (CD) values are 3.2 × 10–4 for D-1 at 297 nm and −3.5 × 10–4 for L-1 at 297 nm and 3.1 × 10–4 for D-2 at 316 nm and −3.0 × 10–4 for L-2 at 316 nm, while the g (MCD) values are −6.2 × 10–5 for D-1 at 297 nm and −7.0 × 10–5 for L-1 at 297 nm and −4.4 × 10–5 for D-2 at 316 nm and −4.3 × 10–5 for L-2 at 316 nm (supposed B = ±1 T). On the other hand, for chiral molecules, the magneto-optical Faraday effects may be assessed using the percentage increase in the CD signal intensity caused by a magnetic field; such a percentage increase may be expressed as Δε(MCD)/Δε(CD), which is also equal to g (MCD)/g (CD) due to g (MCD) = g (CD)Δε(MCD)/Δε(CD). The Δε(MCD)/Δε(CD) values at 1.6 T are −30.7% for D-1 at 297 nm and 32.9% for L-1 at 297 nm and −22.8% for D-2 at 316 nm and 22.8% for L-2 at 316 nm. Their corresponding |Δε(MCD)/Δε(CD)| values are relatively large for the π–π* transition zone, ,, suggesting that the magneto-optical Faraday effects of D-1/L-1 and D-2/L-2 are relatively strong at room temperature.

Nonlinear Optical Properties

Compared with the reference KDP, L-1 and L-2 have only a very weak SHG intensity at room temperature. However, L-1 and L-2 have significantly stronger THG intensities than the reference α-SiO2 (Figure ). The THG intensity of L-1 (68.1 × α-SiO2) is comparable with that of L-2 (65.0 × α-SiO2). The χ(3) value of the reference α-SiO2 is known to be 214 pm2V–2, according to which the χ(3) values of L-1 and L-2 could be calculated to be 1766 and 1725 pm2V–2, respectively. These χ(3) values are large in Ln­(III) complexes ,, and can be attributed to the presence of many strongly electron-absorbing fluorine atoms in the chiral β-diketonate ligands. , Furthermore, the LSchiff12– and LSchiff22– bridging ligands also contain fluorine atoms, which form push–pull electron-conjugated systems, being conducive to the high THG performance as well. As is well known, third-order nonlinear optical materials play a wide-ranging role in fields such as optical communications, laser manufacturing, and optical limiting. , Therefore, L-1 and L-2, as well as their enantiomers (D-1 and D-2), may have potential applications in these areas.

11.

11

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THG spectra of crystalline samples of L-1, L-2, and α-SiO2 under excitation at λ = 1550 nm (T int = 0.5 s).

Conclusions

In summary, 3-(perfluorobutyryl)-(+)/(−)-camphor and two new hydrazone Schiff bases with different heterocyclic rings were utilized to assemble two pairs of Dy2 enantiomers, [Dy2(LSchiff1)2(D-pfc)2(MeOH)2] (D-1)/[Dy2(LSchiff1)2(L-pfc)2(MeOH)2] (L-1) and [Dy2(LSchiff2)2(D-pfc)2(MeOH)2] (D-2)/[Dy2(LSchiff2)2(L-pfc)2(MeOH)2] (L-2). The molecular structures of these two pairs of Dy2 enantiomers differ only in the positions of the two nitrogen atoms on the heterocycle of the Schiff base ligand, leading to their crystallization in different chiral space groups. More importantly, although ferromagnetic interactions exist in both L-1 and L-2, L-1 behaves as single magnetic relaxation, while L-2 shows double magnetic relaxation due to the difference of the coordination configurations of Dy3+ ions in L-1 and L-2. These magnetic properties are theoretically computed by the ab initio calculations. In addition, the two pairs of enantiomers exhibit obvious magnetic circular dichroism effects and strong third harmonic generation (THG). This work demonstrates that the magnetism of homochiral 4f SMMs can be effectively tuned by altering the positions of the two nitrogen atoms in the heterocycle of the Schiff base ligand. Such an atomic-level precise modulation provides greater possibilities and space for the design and synthesis of homochiral multifunctional SMMs.

Supplementary Material

pc5c00102_si_001.pdf (2.5MB, pdf)
pc5c00102_si_002.cif (1.2MB, cif)
pc5c00102_si_003.cif (1.3MB, cif)
pc5c00102_si_004.cif (1.4MB, cif)
pc5c00102_si_005.cif (916.2KB, cif)

Acknowledgments

This work was supported by the National Natural Science Foundation of China (22271289 and 21871274).

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/prechem.5c00102.

  • Experimental procedures; selected bond lengths (Å) and angles (°) of L-1 and L-2; Dy3+ ion geometry analysis by SHAPE-2.1 software for L-1 and L-2; M versus H/T and hysteresis loop plots for L-1 and L-2; plots of χ″ versus T and χ″ versus ν at 1500 Oe for L-1 and L-2; plots of ln­(τ) versus 1/T at 1500 Oe for L-1 and L-2; computational details; calculated individual Dy3+ fragments of L-1 and L-2; calculated energy levels (cm–1), g (g x , g y , g z ) tensors, and predominant m J values of the lowest eight Kramers doublets (KDs) of individual Dy3+ fragments for L-1 and L-2 using CASSCF/RASSI-SO with OpenMolcas; wave functions with a definite projection of the total moment | m J > for the lowest eight KDs of individual Dy3+ fragments for L-1 and L-2; magnetization blocking barriers of individual Dy3+ fragments for L-1 and L-2; calculated (red solid line) and experimental (black square dot) data of the magnetic susceptibilities of L-1 and L-2; exchange energies E (cm–1), the energy difference between each exchange doublet Δ t (cm–1), and the main values of g z for the lowest two exchange doublets of L-1 and L-2; and MCD spectra of D-1/L-1 and D-2/L-2 (PDF)

  • D-1 (CIF)

  • L-1 (CIF)

  • D-2 (CIF)

  • L-2 (CIF)

The authors declare no competing financial interest.

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Associated Data

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Supplementary Materials

pc5c00102_si_001.pdf (2.5MB, pdf)
pc5c00102_si_002.cif (1.2MB, cif)
pc5c00102_si_003.cif (1.3MB, cif)
pc5c00102_si_004.cif (1.4MB, cif)
pc5c00102_si_005.cif (916.2KB, cif)

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