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Article

Dysprosium Complexes Incorporating Halogen-Substituted Anthracene: Piezochromism and Single-Molecule Magnet Properties

by
Ye-Hui Qin
,
Qian-Qian Su
,
Song-Song Bao
and
Li-Min Zheng
*
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, China
*
Author to whom correspondence should be addressed.
Magnetochemistry 2025, 11(12), 102; https://doi.org/10.3390/magnetochemistry11120102
Submission received: 29 July 2025 / Revised: 8 November 2025 / Accepted: 11 November 2025 / Published: 21 November 2025
(This article belongs to the Special Issue Molecular Magnetism: A Themed Issue in Honor of Professor Dai-Zheng Liao on the Occasion of His 85th Birthday)
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Abstract

Lanthanide-based single-molecule magnets (Ln-SMMs) showing stimuli-responsive changes in photoluminescence (PL) and magnetic properties are attractive for their potential applications in information storage and molecular devices. In this work, we report two mononuclear complexes, namely, Dy(SCN)2(NO3)(Cl-depma)2(4-hpy)2 (Dy-Cl) and Dy(SCN)2(NO3)(Br-depma)2(4-hpy)2 (Dy-Br), where X-depma represents 10-X-9-diethylphosphinomethylanthracene (X = Cl, Br) and 4-hpy is 4-hydroxypyridine. Both contain face-to-face π-π-interacted anthracene rings and exhibit yellow-green excimer emission. Unlike the other related Dy–anthracene complexes without a halogen substituent, Dy-Cl and Dy-Br cannot undergo photocycloaddition reaction under UV-light irradiation. However, they exhibited remarkable grinding-induced changes in luminescence. Magnetic studies revealed that Dy-Cl and Dy-Br show SMM behavior under zero dc field with the effective energy barriers (Ueff/kB) of 259 K and 264 K, respectively. We also investigated the effect of pressure on the magnetic properties of Dy-Br and observed a reduction in the magnetization value, narrowing of the butterfly-shaped hysteresis loop, and acceleration of the magnetic relaxation under 1.09 GPa. The results demonstrate that introducing a halogen substituent into an anthracene group may pose significant influences on the photophysical and photochemical properties of the complexes. In addition, pressure may be a promising external stimulus to modulate the PL and SMM behaviors of Dy–anthracene complexes.

1. Introduction

Luminescent lanthanide-based single-molecule magnets (Ln-SMMs) have received great attention in recent years owing to their potential applications in high-density data storage, molecular devices, and sensors [1,2,3,4]. Of particular interest are those that undergo synergistic changes in magnetic and photoluminescence (PL) properties in response to external stimuli [5,6,7,8,9,10,11,12]. By utilizing reversible [4 + 4] photocycloaddition of anthracene groups, we and others have successfully isolated a series of lanthanide–anthracene complexes, including mononuclear [13,14,15] and polynuclear complexes [16,17,18,19,20] and coordination polymers [21,22], which exhibit photoresponsive PL and/or SMM properties. Introducing substituents into anthracene may alter its electronic structure and thus affect the photophysical properties of the complex [23,24,25]. The photochemical properties may also vary due to the different stacking patterns and electronic structures of the anthracene groups brought about by the substituents [26,27]. For example, Bardeen and co-workers found that 10-halogen (F, Cl, Br)-substituted 9-anthracenecarboxylate acid (9AC) showed a similar stacking mode, but only F-substituted 9AC exhibited photoreactivity [26]. Noting that brominated anthracene encapsulated in a metal–organic cage experienced a photocycloaddition reaction [28], we were curious whether the same would happen with lanthanide complexes containing halogen-substituted anthracene moieties.
Pressure is another promising stimulus for inducing alterations in luminescence and magnetic properties [29,30,31,32]. However, although numerous efforts have been devoted to the study of pressure-induced magnetic changes in molecular systems, especially the spin crossover compounds [33,34,35], investigations into pressure-induced magnetic changes in lanthanide complexes are still rare [36,37,38,39,40]. In particular, there are only a handful of examples of lanthanide complexes reported in the literature that exhibit both piezochromism and SMM behavior [40,41]. For lanthanide complexes containing π-π-interacted anthracene groups, pressure may change not only the stacking pattern of the anthracene groups but also the coordination environment of the lanthanide ion, thus modulating both the luminescent and magnetic properties, but similar studies have not been reported.
In this work, we report two new complexes Dy(SCN)2(NO3)(Cl-depma)2(4-hpy)2 (Dy-Cl) and Dy(SCN)2(NO3)(Br-depma)2(4-hpy)2 (Dy-Br), where X-depma represents 10-X-9-diethylphosphinomethylanthracene (X = Cl, Br) (Scheme 1). Their structures are very similar to the known compound Dy(SCN)2(NO3)(depma)2(4-hpy)2 (Dy-H, depma = 9-diethylphosphinomethylanthracene), in which the anthracene groups are face-to-face π-π-interacted [14]. All exhibit excimer emission arising from the anthracene pair and SMM behavior under zero dc field. But the PL and SMM properties of Dy-Cl and Dy-Br are slightly different from those of Dy-H. The main difference in the Dy-Cl and Dy-Br compounds compared to Dy-H, which undergoes photoinduced single-crystal-to-single-crystal (SC-SC) structural transformations, is that they do not experience photochemical reaction under 365 nm light irradiation. However, both Dy-Cl and Dy-Br show remarkable changes in photoluminescence caused by grinding. Magnetic studies on compound Dy-Br revealed a decrease in magnetization value under high pressure (1.09 GPa). These results indicate that the introduction of halogen substituents to anthracene significantly affects the photochemical properties of the resulting dysprosium complexes, as well as their photophysical and magnetic properties. More importantly, pressure can be a potential stimulus to modulate the PL and SMM properties of Dy–anthracene systems. This work provides useful clues for the design and synthesis of stimuli-responsive lanthanide complexes based on anthracene derivatives.

2. Experimental Section

2.1. General Information

10-Chloro-9-diethylphosphinomethylanthracene (Cl-depma) was synthesized from 10-chloro-9-anthracenecarboxaldehyde and characterized by 1H NMR (Figures S1 and S2) (see details in the Supporting Information) [42]. 10-Bromo-9-diethylphosphinomethylanthracene (Br-depma) was synthesized according to the literature [43]. CH2Cl2 and EtOAc were obtained from Yasheng Chemical Co., Ltd., Wuxi, China, and other starting reagents and solvents were obtained from Aladdin, Shanghai, China. All were used directly. All other starting reagents and solvents were obtained from commercial sources and used directly. The elemental analysis for C, H, and N was performed using an Elementar Vario Macro cube, Elementar Analysensysteme GmbH, Langenselbold, German. The infrared (IR) spectra were measured in the range 4000–400 cm−1 using KBr pellets on a Bruker Tensor 27 spectrometer, Bruker Corporation, Billerica, MA, USA. Powder X-ray diffraction (PXRD) data were recorded on a Bruker D8 advance diffractometer (Bruker Corporation, Billerica, MA, USA), with Cu-Kα radiation in a range of 5–50° at room temperature. The differential scanning calorimetry (DSC) measurements was were conducted on a Mettler DSC823e instrument (Mettler-Toledo, Zurich, Switzerland). The UV/Vis spectra were measured on a Perkin Elmer Lambda 950 UV/VIS/NIR spectrometer (PerkinElmer, Inc., Waltham, MA, USA), using powder samples. The steady fluorescence spectra were recorded using PerkinElmer LS55 spectrometer (PerkinElmer, Inc., Waltham, MA, USA). Time-resolved fluorescence and quantum yield measurements were performed on an Edinburgh FLS 980 (Edinburgh Instruments, Livingston, UK), at room temperature. The 1H NMR spectra were recorded on a BRUKER AVANCE III 400 MHz spectrometer (Bruker Corporation, Billerica, MA, USA). The dc and ac magnetic susceptibility data were collected on polycrystalline samples by a Quantum Design vibrating sample magnetometer (VSM) (Quantum Design Inc., San Diego, CA, USA). A drop of Paraffin smaller than 2 mg was added to avoid the movement and reorientation of samples during measurement. The dc susceptibilities were corrected for diamagnetic contributions of holder, paraffin, and sample.

2.2. Synthesis of Dy(SCN)2(NO3)(Cl-depma)2(4-hpy)2 (Dy-Cl)

Dy(NO3)3·6H2O (228 mg, 0.5 mmol) and KSCN (107 mg, 1.1 mmol) were dissolved in 10 mL CH3CN and stirred for 30 min. After filtration of the white precipitate, 1 mL of the filtrate was taken and 4-hydroxypyridine (4-hpy, 10.4 mg, 0.11 mmol), Cl-depma (37.0 mg, 0.1 mmol), and 5 mL CH3CN were added and stirred for about 20 min. After filtration, the clear solution was slowly evaporated for one week to give yellow crystals, the purity of which was verified by PXRD measurements (Figure S3). Yield: 35.0 mg (55%). Elemental anal. calcd (%): C, 47.80; H, 4.01; N, 5.64. Found (%): C, 47.45; H, 4.03; N, 5.50. IR (KBr): ν(C≡N): 2086, 2064 cm−1; ν(P-O): 1024, 1056, 1185 cm−1 (Figure S4).

2.3. Synthesis of Dy(SCN)2(NO3)(Br-depma)2(4-hpy)2 (Dy-Br)

Compound Dy-Br was synthesized using a method similar to Dy-Cl, except that the ligand Cl-depma was replaced with Br-depma (40.7 mg, 0.1 mmol). The purity was verified by PXRD measurements (Figure S3). The infrared spectrum of Dy-Br is also similar to that of Dy-Cl (Figure S4). Yield: 38.5 mg (56%). Elemental anal. calcd (%): C, 44.64; H, 3.75; N, 5.21. Found (%): C, 44.92; H, 4.05; N, 5.12. IR (KBr): ν(C≡N): 2086, 2064 cm−1; ν(P-O): 1025, 1054, 1185 cm−1 (Figure S4).

2.4. Single-Crystal X-Ray Crystallography

Single crystals were selected for data collection on a Bruker D8 diffractometer with a Metal Jet (Ga-Kα radiation, λ = 1.34139 Å). The data were integrated using the SAINT program [44]. Multiscan absorption corrections were applied. The structures were solved by intrinsic phasing methods and refined on F2 by full-matrix least squares using SHELXTL [45]. All the non-hydrogen atoms were refined anisotropically. All H atoms were placed in theoretical positions and refined isotropically. The residual electron densities were of no chemical significance.

2.5. High-Pressure Magnetic Measurement

High-pressure magnetic measurements were performed using a Quantum Design HMD high-pressure cell made of BeCu. The data were collected on polycrystalline samples by a Quantum Design vibrating sample magnetometer (VSM). A small amount of lead (<0.2 mg) was used as a pressure calibrant, and mineral oil served as the pressure-transmitting medium. Before each measurement, the magnet reset option was executed to eliminate any remnant field from the superconducting magnet. The background signal was collected at ambient pressure with samples loaded in the cell, and pressurized magnetic data was collected under pressure. The applied pressure was determined by measuring the superconducting transition temperature of the lead calibrant.

3. Results and Discussion

3.1. Crystal Structure

Single-crystal structural analysis shows that Dy-Cl crystallizes in the monoclinic P21/m space group at 193 K (Table 1 and Table S1). The asymmetric unit consists of half of a DyIII ion, one SCN−, 0.5 NO3−, one Cl-depma, and two halves of 4-hpy, respectively. A symmetry plane is present in the structure, where one 4-hpy ligand and NO3− ligand are located on this symmetry plane. Each DyIII ion is eight-coordinated with two N atoms (N3, N3A) from two SCN−, two O atoms (O6, O6A) from two Cl-depma ligands, two O atoms (O3, O4) from chelated NO3−, and two O atoms (O1, O2) from two 4-hpy, respectively, forming an eight-coordinated [DyO6N2] configuration (Figure 1a, Table S2). According to the continuous shape measure (CShM) analysis [46], the geometry of the [DyO6N2] polyhedron in Dy-Cl is a distorted triangular dodecahedron (CShM = 1.856, D2d) (Table S3). However, if we consider the chelating NO3− anion as occupying one position, the coordination geometry of the DyIII ion has an approximately D5h symmetry. The two oxygen atoms from 4-hpy (O1, O2) occupy the axial positions, while the remaining atoms occupy the equatorial plane. The Dy1−O/N bond lengths in the equatorial plane range from 2.337(4) to 2.572(5) Å, and the axial Dy1−O1 and Dy1−O2 bond lengths are 2.251(6) and 2.246(6) Å, respectively (Table 1 and Table S2). One of the two axial 4-hpy (N1) ligands is disordered over two sites. The protonated pyridine nitrogen atoms of 4-hpy are involved in the intermolecular hydrogen bonding with the uncoordinated nitrate oxygen atom (O5) and the sulfur atom (S1) from the NCS− anion (Table S4), forming a supramolecular layer in the ac plane (Figure S5a). Neighboring anthracene groups are face-to-face stacked through π−π interactions, thus connecting the supramolecular layers into a three-dimensional (3D) network (Figure 1b and Figure S5b). The center-to-center distance (dcc) and plane-to-plane distance (dpp) of the anthracene pair are 3.758 and 3.372 Å, respectively, which satisfy Schmidt’s rule for photodimerization [47]. The slip angle is 23.10° and the central C2−C9A distance (dC2-C9A) is 3.743 Å. In addition, the Cl···S distance between the S atom in SCN− and Cl atom in the Cl-depma ligand of another molecule is 3.378 Å, which is less than the sum of the van der Waals radius (3.55 Å) of one Cl atom and one S atom [48], indicating the presence of halogen bonds.
Compound Dy-Br crystallizes in the space group P21/n at 193 K (Table 1 and Table S1). The asymmetric unit consists of one DyIII ion, two SCN− ions, one NO3−, two Br-depma, and two 4-hpy. It has a similar mononuclear structure to Dy-Cl. However, the replacement of Cl by Br brought slight changes in structural parameters (Table 1 and Table S5). The [DyO6N2] polyhedron is close to a distorted triangular dodecahedron, with a CShM value (1.831) slightly smaller than that in Dy-Cl (CShM = 1.856) (Table S3). Again, the coordination geometry of DyIII can be viewed as having a pseudo-D5h symmetry if the chelating NO3− anion is considered to occupy one position. The axial Dy1–O1 and Dy1–O2 bond lengths are 2.260(4) and 2.235(5) Å, respectively, while the equatorial Dy1-O/N bond lengths are 2.328(3)–2.574(4) Å. The O1–Dy1–O2 angle is 163.7(2)°, close to that in Dy-Cl [163.9(2)°]. The Br···S halogen bonds are observed between the molecules with the distances of 3.337 and 3.346 Å, less than the sum of the van der Waals radius of Br and S (3.65 Å) [48]. These halogen bonds, together with the N–H···O(S) hydrogen bonds, connect the adjacent molecules, forming a 3D supramolecular network (Table S4, Figure S6).
Compared to the analog compound [Dy(SCN)2(NO3)(depma)2(4-hpy)2] (Dy-H), Dy-Cl and Dy-Br have several distinct structural features. First, there exist halogen bonds in the structures of Dy-Cl and Dy-Br, while not in Dy-H. The halogen bond in Dy-Br is stronger, with shorter Br···S distances (3.337 and 3.346 Å) than the Cl···S distance (3.378 Å) in Dy-Cl. We expect that the intermolecular halogen bonds will slightly alter the packing of the anthracene groups. Indeed, the slip angles, defined as the angle between the centroid–centroid line and the vertical line of the anthracene pair, follow the order Dy-Br (23.61/23.06°) > Dy-Cl (23.10°) > Dy-H (22.36°) (Figures S5c and S6c), which is consistent with the decreasing strength of the intermolecular halogen bonds. A similar trend is found for the center-to-center distance (dcc) of the anthracene pair. Notably, the plane-to-plane distance (dpp) of Dy-Cl is the shortest among the three complexes. Secondly, the coordination environments of the dysprosium ions are slightly different. The axial Dy1–O1 bond length in Dy-Cl [2.251(5) Å] is shorter than in Dy-Br [2.260(4) Å], but comparable to Dy-H [2.248(8) Å], whereas the axial Dy1–O2 bond length in Dy-Cl [2.246(5) Å] is longer than in Dy-Br [2.235(5) Å] and Dy-H [2.213(9) Å]. The equatorial Dy1–N bond lengths are elongated in Dy-Cl [2.450(5) Å] and Dy-Br [2.455(4) and 2.440(4) Å] compared to Dy-H [2.426(8) Å], potentially arising from the halogen bond between halogen atom of the anthracene ligand and sulfur atom of the NCS− anion. Thirdly, the Dy···Dy separation is expanded following the sequence Dy-Br (9.754 Å) > Dy-Cl (9.736 Å) > Dy-H (9.605 Å), consistent with the decreasing atomic radii of Br, Cl, and H. All these structural differences will be reflected by their optical and magnetic properties.
It is noteworthy that both 4-hpy ligands in Dy-Br are ordered at 193 K, unlike for Dy-Cl at the same temperature, in which one of the two 4-hpy ligands is disordered over two sites. To examine whether an order–disorder phase transition occurred during the temperature change, we performed DSC measurements on Dy-Br in the temperature range of 153–303 K. Unfortunately, after several attempts, we were unable to observe an endothermic or exothermic peak in the DSC curves, possibly due to the very small entropy change during the phase transition. Nevertheless, single-crystal structural analysis measured at 218 K revealed that one of the two 4-hpy in the structure of Dy-Br was disordered (Table S6, Figure 1e,f and Figure S7), akin to Dy-Cl. The fact that the order–disorder phase transition occurred above 193 K for Dy-Br but not for Dy-Cl and Dy-H should be related to their structures, but the underlying mechanism is not clear to us.

3.2. Photophysical Properties and External Stimulus Effects

To investigate the photophysical properties of compounds Dy-Cl and Dy-Br, we first measured their UV-vis diffuse reflectance spectra in the solid state. As shown in Figure S8, the two compounds display intense and broad absorption bands in the range of 200–600 nm, corresponding to the π*←π transitions of the X-depma and 4-hpy ligands. Furthermore, small and narrow absorption peaks appear at 754 and 800 nm for both compounds, originating from the characteristic 4f–4f transitions of the DyIII ion from the ground state (6H15/2) to the excited states (6F3/2 and 6F5/2).
When polycrystalline samples of Dy-Cl and Dy-Br were irradiated with 365 nm UV light, they showed bright yellow luminescence. We then measured the photoluminescence (PL) spectra of Dy-Cl and Dy-Br. Upon excitation at 365 nm, Dy-Cl and Dy-Br exhibited broad emission bands peaking at 550 nm and 535 nm, respectively (Figure 2), attributed to the excimer emission due to the face-to-face π−π-interacted anthracene pair. Compared to Dy-H (540 nm), the emission peaks of Dy-Cl and Dy-Br are either red-shifted or blue-shifted. According to the structural description, Dy-Cl has the shortest anthracene plane spacing (dpp) (3.372 Å) compared to Dy-Br (3.408 Å) and Dy-H (3.409 Å). This result suggests that the π−π-interactions between the anthracene groups in Dy-Cl are enhanced, which may account for the longest wavelength of the excimer emission peak in Dy-Cl. In contrast, although Dy-Br shows nearly the same dpp distance as that of Dy-H, the slip angle of the anthracene pair in Dy-Br (23.61/23.06°) is larger than that in Dy-H (22.36°), which may account for the blue-shift in the excimer emission in Dy-Br. We notice that the slip angle of the anthracene pair in Dy-Cl (23.10°) is also larger than that in Dy-H (22.36°), but the emission peak appears at a longer wavelength. Obviously, compared to the slip angle, the shortening of anthracene plane spacing plays a more important role in the red-shift in the excimer emission of Dy-Cl. Moreover, we measured the photoluminescent lifetimes of the two compounds. As expected, Dy-Cl displays the longest lifetime (54.1 ns) among the three compounds (Dy-Br: 7.11 ns; Dy-H: 41.7 ns). The quantum yield (PLQY) was also obtained as 10.31% for Dy-Cl, 1.76% for Dy-Br, and 12.69% for Dy-H. The extremely short fluorescence lifetime and low quantum yield of Dy-Br can be ascribed to the heavy atom effect of the Br atom.
Considering that the anthracene groups in Dy-Cl and Dy-Br are face-to-face π−π-interacted with separation satisfying Schmidt’s rule for the photocycloaddition reaction, we further studied the photochemical properties of the two complexes. We irradiated the two samples with 365 nm UV light (power density: ca. 100 mW cm–2) for 1 h, but observed almost no change in their PL spectra (Figure S9). The IR spectra of Dy-Cl and Dy-Br before and after light irradiation are also identical (Figure S10). The results indicate that Dy-Cl and Dy-Br cannot undergo photocycloaddition reactions, which is similar to the case of 10-X-9-anthracenecarboxylate acid (X = Cl, Br) in its solid state [26]. The reason for this could be two-fold: first, the larger size of the halogen atom increases the spatial repulsion around the anthracene ligand and prevents photocycloaddition reactions [26]; second, the presence of heavy atoms such as Br and Cl facilitates the formation of triplet states by intersystem crossing and is not conducive to the formation of a singlet excited state, which is the reactive species in the anthracycline photodimerization reaction [27].
Applying mechanical force may cause a change in the stacking mode of anthracene rings in compounds Dy-Cl and Dy-Br, thereby affecting their PL properties [49,50,51,52,53]. We next investigated the PL spectra of Dy-Cl and Dy-Br before and after grinding. Figure S11 shows clearly that after 5 min of grinding, the emission color of Dy-Cl (named as Dy-Cl-ground) changed from vivid yellow to yellow-green, and the emission color of Dy-Br (named as Dy-Br-ground) changed from yellow-green to blue-green. By heating the grinding products at 100 °C for 10 min (named as Dy-Cl-heat and Dy-Br-heat), the emission colors returned to their initial color. The PL spectra can more accurately display the changes in luminescence before and after grinding. As shown in Figure 3a,c, both Dy-Cl-ground and Dy-Br-ground exhibited broad bands with maxima blue-shifted to 535 nm and 490 nm, respectively, which are 15 nm and 45 nm less than the pristine samples. The blue-shift of the emission peak after grinding can be attributed to the formation of a more pronounced displaced stacking structure of the anthracene pair induced by external forces. After heating at 100 °C for 10 min, the PL spectra of Dy-X-heat matched those of Dy-X (X = Cl, Br), suggesting that the luminescence changes induced by grinding are reversible. Apparently, thermal treatment promotes the recovery of anthracene stacking to its initial state. The PXRD and IR measurements confirmed that compounds Dy-X (X = Cl, Br) experienced grinding-induced amorphization and thermo-induced crystalline-phase recovery (Figure 3b,d and Figure S12). We also performed five cycles of grinding/heating measurements on compounds Dy-Cl and Dy-Br. The PL spectra showed that the luminescence changes and recovery remained reversible throughout the entire process (Figures S13 and S14).
It is noteworthy that compound Dy-Br shows a significantly greater blue-shift after grinding than Dy-Cl. This phenomenon may be attributed to the fact that the dcc and dpp distances between the anthracene rings in Dy-Br are larger and the interactions are weaker, which makes it easier for them to be moved by mechanical forces, ultimately leading to a more pronounced blue-shift.

3.3. Magnetic Properties

Structural description has demonstrated that the introduction of a halogen atom causes slight changes in the coordination environment of the DyIII ion in Dy-Cl and Dy-Br. Since the magnetic properties of lanthanide complexes are sensitive to environmental changes, we studied their magnetic properties. Figure S15 shows the direct current (dc) magnetic susceptibilities of Dy-Cl and Dy-Br measured under a 1 kOe dc field in the temperature range of 2–300 K. At 300 K, the χMT values were 14.77 cm3 K mol−1 and 14.92 cm3 K mol−1 for Dy-Cl and Dy-Br, respectively, slightly larger than that of an isolated DyIII ion (14.17 cm3 K mol−1, 6H15/2, S = 5/2, L = 5, gJ = 4/3). Upon cooling, the χMT values slowly decreased until 10 K, at which point the values were 12.32 cm3 K mol−1 for Dy-Cl and 12.31 cm3 K mol−1 for Dy-Br. Below 10 K, the χMT value showed an upturn and reached a maximum at 5 K, which may be ascribed to the weak intermolecular ferromagnetic dipole–dipole interactions. Finally, below 5 K, the χMT values dropped sharply, reaching 8.89 cm3 K mol−1 for Dy-Cl and 8.75 cm3 K mol−1 for Dy-Br at 2 K. This dramatic decrease is attributed to the blocking of magnetic moment [54]. Notably, while 4f–4f interactions are typically negligible or exhibit only weak antiferromagnetic behavior in mononuclear systems, weak ferromagnetic dipole–dipole interactions have been observed in certain complexes. These include the Dy-H complex with the shortest Dy···Dy distance of 9.605 Å [14], a mononuclear complex [Dy(NTA)3L] with the shortest Dy···Dy distance of 9.729 Å [55], and a DyIII-CdII-phthalocyaninato sextuple-decker complex (Dy2Cd3) with a long intramolecular Dy···Dy distance of 13.047 Å and intermolecular Dy···Dy distance of 11.246 Å [56]. We also measured the field-dependent magnetization curves of the two compounds. The M values at 2 K and 70 kOe were 5.65 Nβ and 5.54 Nβ for Dy-Cl and Dy-Br, respectively, lower than the theoretical value of 10 Nβ for a single DyIII ion, ascribed to strong magnetic anisotropy (Figure S16a,c). The non-superposition of the M vs. H/T curves provides further evidence for the presence of strong magnetic anisotropy (Figure S16b,d). Notably, the M vs. H/T curves of both compounds at 2 K show a clear sigmoid shape, a phenomenon also observed in the single spin-based SMMs associated with the magnetic blocking [57]. The hysteresis measurements confirmed the magnetic blocking, with butterfly-shaped hysteresis loops for both compounds (Figure 4a and Figure S17). However, the coercive field and remnant magnetization were both zero at Hdc = 0 Oe, indicating the presence of strong quantum tunneling of magnetization (QTM).
We subsequently measured the alternating current (ac) magnetic susceptibility of Dy-Cl and Dy-Br to investigate their magnetic dynamics (Figure 5). Both the in-phase (χ’) and out-of-phase (χ”) ac susceptibilities exhibited strong frequency dependence under zero dc field below 16 K for Dy-Cl and 18 K for Dy-Br (Figure 5a,b,d,e), respectively, which is characteristic of SMM behavior. The Cole–Cole plots displayed a distinct semicircle, indicating a single-step magnetic relaxation process (Figure S18). These Cole–Cole plots can be fitted by a generalized Debye model to extract the relaxation time (τ) (Tables S7 and S8) [58]. The plot of ln(τ) versus T−1 could then be fitted using Equation (1), which accounts for the QTM, Raman, and Orbach relaxation processes, where Ueff represents the effective energy barrier for the Orbach process (Figure 5c,f). The best fit gave the parameters Ueff/kB = 259(12) K, τ0 = 2.77(222) × 10−11 s, C = 2.1(5) × 10−3 K−4.37 s−1, n = 4.37(10), and τQTM = 0.241(7) s for Dy-Cl, and Ueff/kB = 264(9) K, τ0 = 4.46(243) × 10−11 s, C = 9.1 × 10−4 K−4.80 s−1, n = 4.80(8), and τQTM = 0.361(8) s for Dy-Br.
τ − 1 = τ QTM − 1 + C T n + τ 0 − 1 − U eff k B T
Clearly, the Ueff/kB value of Dy-Cl (259 K) is slightly smaller than those of Dy-Br (264 K) and Dy-H (277 K). By comparing the structures of Dy-Cl and Dy-H, we found that the average equatorial Dy1-O/N bond lengths in Dy-Cl [2.447 Å/2.450(5) Å] are longer than those in Dy-H [2.437 Å/2.426(8) Å], suggesting that Dy-Cl has a weaker equatorial ligand field. In addition, the axial Dy1–O1 distance and O1–Dy1–O2 angle in Dy-Cl [2.251(5) Å, 163.9(2)°] are close to those in Dy-H [2.248(8) Å, 163.9(4)°]. However, the axial Dy1–O2 distance in Dy-Cl is significantly longer [2.246(5) Å vs. 2.213(9) Å in Dy-H]. Apparently, the axial Dy1–O2 distance plays a key role in determining the magnetic anisotropy of the material. Compound Dy-H, with a much shorter Dy1–O2 distance, shows a higher energy barrier than Dy-Cl with a longer Dy1–O2 distance. For Dy-Br, both the axial and equatorial Dy1–O/N bond lengths are elongated compared to Dy-H; this may explain why Dy-Br exhibits a lower energy barrier than Dy-H. Notably, applying an external field (0–500 Oe) cannot enhance the slow magnetic relaxation (Figures S19 and S20).
In addition to the different effective energy barriers of the Orbach process, we also found that the Raman process exponents of Dy-Cl (n = 4.37) and Dy-Br (n = 4.80) are different. Both theoretical and experimental studies have demonstrated that Raman relaxation is associated with low-energy phonons, which dominate at low temperatures [59]. Molecular vibrations beyond the ion’s first coordination shell play a significant role in spin–phonon coupling through the electrostatic polarization effect [60]. The fact that Dy-Br has a higher n value than Dy-Cl may be due to the presence of stronger halogen bonds between the molecules.

3.4. Effect of High Pressure on the Magnetic Properties of Dy-Br

As already described, both Dy-Cl and Dy-Br show grinding-induced emission color change due to the displaced stacking structure of their anthracene pairs. To examine whether an external pressure may affect their magnetic properties, we measured the magnetic hysteresis loops of the ground samples (Dy-Cl-ground and Dy-Br-ground) at 2 K. As shown in Figure S21, a pronounced narrowing of the loops was observed for the ground samples, in contrast to the pristine samples, indicating enhancement of QTM effect after grinding.
We then selected compound Dy-Br for high-pressure magnetic measurements using a pressurized BeCu device. By measuring the superconducting transition temperature of the lead calibrant, the pressure was determined to be 1.09 GPa (Figure S22). At 300 K, the χMT value under pressure was 11.97 cm3 K mol−1, much lower than the value of 14.92 cm3 K mol−1 at ambient pressure (Figure S23). The temperature dependence of χMT under pressure followed a trend similar to that at ambient pressure, reaching a minimum value of 7.67 cm3 K mol−1 at 2 K, which was also slightly lower than the value at ambient pressure (8.75 cm3 K mol−1). Magnetization curves at 2 K under both high pressure and ambient pressure exhibited a sigmoid shape (Figures S24 and S25). The magnetization value at 70 kOe was 4.40 Nβ under pressure, lower than the 5.54 Nβ observed at ambient pressure. Hysteresis loops at 2 K remained butterfly-shaped under pressure but were significantly narrower compared to those at ambient pressure (Figure 4b and Figure S26). Overgaard and co-workers found that for a pseudo-pentagonal bipyramidal DyIII complex, the axial O-Dy-O angle decreased under pressure, leading to a drop in the magnetization at zero field [38]. We speculate that the loss of magnetization in Dy-Br may be related to the distortion of the pseudo-pentagonal bipyramidal geometry of DyIII ions, leading to a decrease in magnetic anisotropy, as well as the shortening of the Dy···Dy distance, which enhances QTM and/or Raman relaxation processes [40].
The severe distortion of ac magnetic susceptibility at high frequencies (>100 Hz) arises from eddy currents induced in the BeCu device under alternating magnetic fields (Figure S27a,b). These eddy currents become particularly intense at elevated frequencies, generating substantial background signals that are challenging to fully compensate. Consequently, ac susceptibility data become unavailable in the higher-frequency regime. Additionally, this effect contributes to discontinuous jumps in data points observed at lower frequencies. Fortunately, for compound Dy-Br, the susceptibility data in the range of 2–12 K are still available in the lower-frequency range (<50 Hz) under high pressure. As shown in Figure S27b, the peak signals in out-of-phase ac susceptibility curves clearly shift towards lower frequencies upon cooling. The Cole–Cole plot exhibits a semicircular shape, indicating a single-step relaxation process (Figure S27c). The Cole–Cole plots can be fitted by a generalized Debye model to extract the relaxation time (τ) (Table S9). However, the plot of ln(τ) versus T−1 could not be adequately fitted by considering any combination of the QTM, Raman, and Orbach processes (Figure S27d). Nevertheless, a linear fit with the Orbach process using Equation (2) in the high-temperature region provides a reasonable approximation for estimating the effective energy barrier. The fit gives an energy barrier of 30(3) K, which is significantly lower than that obtained under ambient pressure [264(5) K]. This result suggests that the application of high pressure accelerates the magnetic relaxation and reduces the energy barrier in Dy-Br.
l n τ = l n τ 0 + ( U eff k B ) × 1 T

4. Conclusions

We report two new compounds, namely [Dy(SCN)2(NO3)(Cl-depma)2(4-hpy)2] (Dy-Cl) and [Dy(SCN)2(NO3)(Br-depma)2(4-hpy)2] (Dy-Br). Both contain anthracene rings with face-to-face π-π interactions and exhibit yellow-green excimer emission. The halogen substituent has an effect on the stacking modes of the anthracene pairs, such as the plane-to-plane distance and the slip angle of the anthracene rings. As a result, the emission peak in Dy-Cl is red-shifted by 15 nm compared to Dy-Br. Moreover, Dy-Cl shows a significantly longer fluorescence lifetime and higher quantum yield than Dy-Br, attributed to the heavy atom effect of Br. Interestingly, neither Dy-Cl nor Dy-Br undergoes the [4 + 4] photocycloaddition reaction, though the stacking of anthracene pairs in the two compounds meet the criteria for Schmidt’s rule. However, they show piezochromism upon grinding, with the emission peak blue-shifted by 15 nm for Dy-Cl and 45 nm for Dy-Br, respectively. This color change is reversible after thermal annealing. In addition, both compounds exhibit single-molecule magnet behavior, with energy barriers of 259 K for Dy-Cl and 264 K for Dy-Br. High-pressure magnetic studies on Dy-Br revealed a decrease in magnetization value and the acceleration of the magnetic relaxation at 1.09 GPa. This work offers new insights into the design and synthesis of stimulus-responsive anthracene-based single-molecule magnets.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/magnetochemistry11120102/s1. Scheme S1: Synthetic procedures for Cl-depma. Figure S1: The 1H NMR spectra of 10-Chloro-9-anthracenemethanol; Figure S2: The 1H NMR spectra of Cl-depma; Figure S3: The PXRD patterns for Dy-Cl and Dy-Br; Figure S4: The FT-IR spectra in the range of 4000 cm−1 to 400 cm−1 (a) and 1800 cm−1 to 600 cm−1 (b) of Dy-Cl and Dy-Br; Figure S5: (a) The 2D supramolecular layer in the ac plane in Dy-Cl; (b) the 3D supramolecular structure viewed from the bc plane in Dy-Cl; Figure S6: (a) The 2D supramolecular layer in the ac plane in Dy-Br (193K); (b) the 3D supramolecular structure viewed from the bc plane in Dy-Br (193K); Figure S7: (a) The 2D supramolecular layer in the ac plane in Dy-Br (218K); (b) the 3D supramolecular structure viewed from the bc plane in Dy-Br (218K); Figure S8: The solid-state UV-Vis absorption spectra of Dy-Cl (blue) and Dy-Br (red) at room temperature; Figure S9: The photoluminescence spectrum of Dy-Cl (a) and Dy-Br (b) before and after 1 h of 365 nm UV irradiation (λex = 365 nm); Figure S10: The FT-IR spectra in the range of (a) 4000 cm−1 to 400 cm−1 and (b) 1800 cm−1 to 600 cm−1 for compounds Dy-Cl and Dy-Br before and after 1 h of 365 nm UV irradiation; Figure S11: Photoluminescence photograph of the samples under 365 nm UV light. (a) Dy-Cl; (b) Dy-Cl-ground; (c) Dy-Cl-heat; (d) Dy-Br; (e) Dy-Br-ground; (f) Dy-Br-heat; Figure S12: (a) The FT-IR spectra in the range of 1800 cm−1 to 600 cm−1 for Dy-Cl, Dy-Cl-ground, and Dy-Cl-heat; (b) The FT-IR spectra in the range of 1800 cm−1 to 600 cm−1 for Dy-Br, Dy-Br-ground, and Dy-Br-heat; Figure S13: Normalized photoluminescence spectra of Dy-Cl-ground and Dy-Cl-heat (a) and Dy-Br-ground and Dy-Br-heat (b) within five cycles of grinding/heating; Figure S14: Peak wavelength of photoluminescence spectra of Dy-Cl-ground and Dy-Cl-heat (a) and Dy-Br-ground and Dy-Br-heat (b) in cyclic grinding/heating test; Figure S15: The temperature-dependent χMT plots for Dy-Cl and Dy-Br within the range of 2–300 K; Figure S16: (a, c) The isothermal magnetization as a function of H at the depicted temperatures for Dy-Cl (a) and Dy-Br (c). (b, d) The isothermal magnetization as a function of HT−1 at the depicted temperatures for Dy-Cl (b) and Dy-Br (d); Figure S17: The magnetic hysteresis loops for Dy-Cl measured at the indicated temperatures; Figure S18: The Cole–Cole plots of Dy-Cl (a) and Dy-Br (b); Figure S19: The in-phase (a) and out-of-phase (b) ac magnetic susceptibilities of Dy-Cl under a 0–500 Oe dc field at 2 K; Figure S20: The in-phase (a) and out-of-phase (b) ac magnetic susceptibilities of Dy-Br under a 0–500 Oe dc field at 2 K; Figure S21: The magnetic hysteresis loops for Dy-Cl and Dy-Cl-ground (a) and Dy-Br and Dy-Br-ground (b) at 2 K; Figure S22: Plots of DC moment of lead to determine pressure; Figure S23: The temperature-dependent χMT plots for Dy-Br within the range of 2–300 K under 1.09 GPa and 0 GPa; Figure S24: (a) The isothermal magnetization as a function of H at the depicted temperatures for Dy-Br under 1.09 GPa; (b) the isothermal magnetization as a function of HT−1 at the depicted temperatures for Dy-Br under 1.09 GPa; Figure S25: The isothermal magnetization as a function of H at 2 K temperatures for Dy-Br under 1.09 GPa and 0 GPa; Figure S26: The magnetic hysteresis loops for Dy-Br under 1.09 GPa, measured at the indicated temperatures; Figure S27: (a, b) The in-phase (a) and out-of-phase (b) ac susceptibilities for Dy-Br under 1.09 GPa; (c) the Cole–Cole plot of Dy-Br under 1.09 GPa; (d) the plot of lnτ vs. T−1 for Dy-Br under 1.09 GPa; Table S1: The cell and structural parameters of Dy-Cl, Dy-Br (193K), and Dy-Br (218K); Table S2: Selected bond lengths (Å) and angles (°) for Dy-Cl at 193 K; Table S3: Continuous Shape Measure (CShM) analyses of dysprosium geometries for Dy-Cl, Dy-Br (193K), and Dy-Br (218K) using the SHAPE2.1 software; Table S4: The parameters of H-bonding and halogen bonding for Dy-Cl, Dy-Br (193K), and Dy-Br (218K); Table S5: Selected bond lengths (Å) and angles (°) for Dy-Br (193K) measured at 193 K; Table S6: Selected bond lengths (Å) and angles (°) for Dy-Br (218K) measured at 218 K; Table S7: The fit parameters obtained from analyses of the ac susceptibilities of Dy-Cl under zero bias dc field; Table S8: The fit parameters obtained from analyses of the ac susceptibilities of Dy-Br under zero bias dc field; Table S9: The fit parameters obtained from analyses of the ac susceptibilities of Dy-Br under 1.09 GPa and zero bias dc field.

Author Contributions

Synthesizes, characterization, and writing, Y.-H.Q.; magnetic measurement and single-crystal structure analysis, Q.-Q.S.; single-crystal structure analysis, S.-S.B.; writing—review and editing, supervision, project administration, and funding acquisition, L.-M.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 22273037, 21731003).

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author (L.-M.Z.).

Acknowledgments

We sincerely thank Dawid Pinkowicz of Jagiellonian University of Poland for the valuable discussions related to high-pressure magnetic measurements.

Conflicts of Interest

The authors declare no conflicts of interest.

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Magnetochemistry 11 00102 sch001
Scheme 1. Molecular structures of Dy-X (X = Cl, Br).
Scheme 1. Molecular structures of Dy-X (X = Cl, Br).
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Magnetochemistry 11 00102 g001
Figure 1. (a,c,e) The molecular structures of Dy-Cl (a), Dy-Br (193K) (c), and Dy-Br (218K) (e); (b,d,f) fragments of the supramolecular chains formed by halogen bonds and face-to-face π-π interactions of anthracene rings in compounds Dy-Cl (b), Dy-Br (193K) (d), and Dy-Br (218K) (f).
Figure 1. (a,c,e) The molecular structures of Dy-Cl (a), Dy-Br (193K) (c), and Dy-Br (218K) (e); (b,d,f) fragments of the supramolecular chains formed by halogen bonds and face-to-face π-π interactions of anthracene rings in compounds Dy-Cl (b), Dy-Br (193K) (d), and Dy-Br (218K) (f).
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Figure 2. Normalized PL spectra of Dy-Cl, Dy-Br, and Dy-H (λex = 365 nm).
Figure 2. Normalized PL spectra of Dy-Cl, Dy-Br, and Dy-H (λex = 365 nm).
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Figure 3. (a) Normalized PL spectra (λex = 365 nm) and (b) PXRD patterns for Dy-Cl, Dy-Cl-ground, and Dy-Cl-heat; (c) normalized PL spectra (λex = 365 nm) and (d) PXRD patterns for Dy-Br, Dy-Br-ground, and Dy-Br-heat.
Figure 3. (a) Normalized PL spectra (λex = 365 nm) and (b) PXRD patterns for Dy-Cl, Dy-Cl-ground, and Dy-Cl-heat; (c) normalized PL spectra (λex = 365 nm) and (d) PXRD patterns for Dy-Br, Dy-Br-ground, and Dy-Br-heat.
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Figure 4. (a) The magnetic hysteresis loops for Dy-Br measured at the indicated temperatures; (b) the magnetic hysteresis loops for Dy-Br measured under 0 GPa and 1.09 GPa at 2 K.
Figure 4. (a) The magnetic hysteresis loops for Dy-Br measured at the indicated temperatures; (b) the magnetic hysteresis loops for Dy-Br measured under 0 GPa and 1.09 GPa at 2 K.
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Figure 5. (a,d) The in-phase ac susceptibilities for Dy-Cl (a) and Dy-Br (d); (b,e) the out-of-phase ac susceptibilities for Dy-Cl (b) and Dy-Br (e); (c,f) the plot of lnτ vs. T−1 for Dy-Cl (c) and Dy-Br (f).
Figure 5. (a,d) The in-phase ac susceptibilities for Dy-Cl (a) and Dy-Br (d); (b,e) the out-of-phase ac susceptibilities for Dy-Cl (b) and Dy-Br (e); (c,f) the plot of lnτ vs. T−1 for Dy-Cl (c) and Dy-Br (f).
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Table 1. The unit cell and structural parameters and PL and magnetic properties of Dy-Cl, Dy-Br, and Dy-H.
Table 1. The unit cell and structural parameters and PL and magnetic properties of Dy-Cl, Dy-Br, and Dy-H.
CompoundDy-ClDy-BrDy-H a
Cell parametersTemperature193 K193 K193 K
Crystal systemMonoclinicMonoclinicMonoclinic
Space groupP21/mP21/nP21/m
a/Å9.736(1)9.754(1)9.605(3)
b/Å25.723(1)25.833(1)25.654(10)
c/Å11.251(1)21.114(1)11.161(4)
β/°111.4(1)94.739(1)112.1(1)
V/Å32623.2(2)5302.0(4)2548.5(16)
Z242
CCDC number247593424759702233761
Structural parametersDy1-O(axial)/Å2.251(5)/2.246(5) 2.260(4)/2.235(5)2.248(8)/2.213(9)
Dy1-O(equatorial)/Å2.337(4)-2.572(5)2.328(3)-2.574(4) 2.319(7)-2.571(8)
Dy1-N/Å2.450(5)2.455(4)/2.440(4) 2.426(8)
O1-Dy1-O2(axial)/°163.9(2)163.7(2)163.9(4)
dDy–Dy/Å9.7369.7549.605
dcc/Å 3.758 3.8103.753
dpp/Å 3.372 3.4083.409
slip angle/° 23.10 23.61/23.0622.36
dC2–C9A/Å3.743-3.754
dC2–C28, dC9-C21/Å-3.819/3.811-
PL propertiesExcimer peak/nm550535540
Lifetime/ns54.17.1141.7
PLQY/%10.311.7612.69
Magnetic propertiesχMT/cm3 K mol−114.7714.9214.18
M/Nβ (70 kOe)5.655.545.22
Ueff/kB/K259(12)264(9)277
τ0/s2.77(222) × 10−114.46(243) × 10−117.16 × 10−11
C/K−n s−12.1(5) × 10−39.1(7) × 10−41.17 × 10−3
n4.37(10)4.80(8)4.42
τQTM/s0.241(7)0.316(8)0.188
a The parameters for Dy-H are cited from [14].
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Qin, Y.-H.; Su, Q.-Q.; Bao, S.-S.; Zheng, L.-M. Dysprosium Complexes Incorporating Halogen-Substituted Anthracene: Piezochromism and Single-Molecule Magnet Properties. Magnetochemistry 2025, 11, 102. https://doi.org/10.3390/magnetochemistry11120102

AMA Style

Qin Y-H, Su Q-Q, Bao S-S, Zheng L-M. Dysprosium Complexes Incorporating Halogen-Substituted Anthracene: Piezochromism and Single-Molecule Magnet Properties. Magnetochemistry. 2025; 11(12):102. https://doi.org/10.3390/magnetochemistry11120102

Chicago/Turabian Style

Qin, Ye-Hui, Qian-Qian Su, Song-Song Bao, and Li-Min Zheng. 2025. "Dysprosium Complexes Incorporating Halogen-Substituted Anthracene: Piezochromism and Single-Molecule Magnet Properties" Magnetochemistry 11, no. 12: 102. https://doi.org/10.3390/magnetochemistry11120102

APA Style

Qin, Y.-H., Su, Q.-Q., Bao, S.-S., & Zheng, L.-M. (2025). Dysprosium Complexes Incorporating Halogen-Substituted Anthracene: Piezochromism and Single-Molecule Magnet Properties. Magnetochemistry, 11(12), 102. https://doi.org/10.3390/magnetochemistry11120102

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