English

• 1. Introduction

  Single-molecule magnets (SMMs), molecules that possess a well-isolated magnetic bistable state with a substantial energy barrier (U_eff) to spin inversion and show slow relaxation of magnetization and magnetic hysteresis at a critical temperature (blocking temperature, T_B), have great potential applications in spintronics, high-density information storage and quantum computing [1–4]. Since the discovery of the famous Mn₁₂ compound [5], numerous experimental and theoretical studies have devoted in such transition metal clusters [6]. Until the revolutionary breakthrough came with the report of large U_eff in a D_4d-symmetric terbium-phthalocyanine SMM in 2003 [7], lanthanide ions have blossomed into ideal candidates for design of high-performance SMMs. Over the past decades, the introduction of Ln³⁺ ions, especially Dy³⁺ and Tb³⁺, has flourished in the SMM field, owing to their large magnetic moments and significant magnetic anisotropy [8,9]. Dy-SMMs are the most numerous because Dy³⁺ ion is a Kramers ion, retaining doubly-degenerate ground state irrespective of the ligand field symmetry. According to the crystal field (CF) theory, the CF multiplet structures depend on the coordination environment, including coordination geometry, charge distribution and type of donor atoms, and determine the magnetic anisotropy and magnetic relaxation dynamics. Therefore, carefully tailoring the coordination environment around Ln³⁺ ions is of great significance to the design of high-performance Ln-SMMs. Based on the electrostatic model [10], building a highly axial CF for oblate Dy³⁺ can maximize the anisotropy barrier to magnetization reversal and suppress the quantum tunneling of magnetization (QTM) efficiently. Numerous high-symmetric geometry Dy-SMMs have been reported successively [11–20], exhibiting impressive improvements in magnetic performance, such as vast increase of U_eff thanks to the combination of a strongly axial CF with almost negligible transverse CF components. Alternatively, introducing a predominant bond with high electronegativity into low-symmetry Dy complexes is another feasible approach to improve SMM properties [21–24]. Based on previous studies, the design criteria for Dy-SMMs with high U_eff has been well established and proven. The investigations of the link between molecular flexibility, vibration modes and magnetic relaxation may be the key to enhance “working temperatures” of SMMs further [25–29]. It demonstrated from calculation and experimental studies that constrained metal-ligand vibrational modes may improve relaxation dynamics significantly, especially in dysprosium metallocene SMMs [30–32].

  Aside from Ln-SMMs, significant efforts have been focused on the investigation of multifunctional molecular materials, which display various physical features combining magnetic properties with porosity [33], chirality [34], conductivity [35] or luminescence [36]. Such properties can coexist in a single component in perfect harmony or exhibit effective coupling or synergetic effects [37–40], providing the possibility of modulating one property with the other. In addition, an even more challenging task is the design of stimuli-responsive Ln-SMMs [41,42], whose magnetizations can be effectively controlled via external stimulus [41–43], such as pressure [44,45], solvent [46], electric potential [47] or light [48]. This may further widen the extent of their applications in information storage and molecular spintronics. Among these stimuli, light irradiation is a particularly attractive tool for this purpose because of its wildly available, environment friendly, non-invasive and mild nature, which may allow the preservation of crystallinity of complexes compared with chemical or thermal stimulus. Meanwhile, light can provide a unique opportunity for rapid, remote and precise control of magnetic characteristic with high spatial and temporal resolution. So far, several lanthanide SMMs with photoactive organic moieties displaying photoinduced magnetism regulation have been reported (Table 1). Given the significant influence of the crystal field structures and magnetic interactions on magnetic anisotropy and relaxation dynamics in lanthanide systems, the successful photo control of ligand structures or the stabilization of light-induced radicals may enable an efficient handle for spin manipulation. Therefore, a general approach to designing and synthesizing photo-tunable Ln-SMMs has been developed via introducing photoactive moieties, including dithienylethene derivatives with open and close forms [48–54], anthracene-containing ligands [55–59] and 1,2-bis(4-pyridyl)ethene(bpe) [60,61] with photocycloaddition, spiropyran-merocyanine with ring-opening and pericyclic reaction or cis-trans isomerization [62–64] and azo-based ligands with cis-trans isomerization [65,66], viologens [67–70] or macrocyclic [71,72] system with photogenerated radicals and polyoxometalates [73] with electron transfer. There are two types of photoactive systems mentioned above, one of which consists of common organic photoswitches capable of undergoing reversible structural changes between two isomers (Scheme 1) and the other is dominated by photoinduced electron transfer (PET) mechanism, containing both electron acceptors and donors to stabilize radicals for long-live charge separation. For the former, the photo-isomerization to alter the first coordination sphere of lanthanide ion is expected. However, the structural transformations in the solid state are severely constricted due to the close molecular stacking, which leads to poor isomerization dynamic and unsatisfactory photochromism. Thus, overcoming molecular crowding and increasing conformational freedom are of great significance to the effective modulation of magnetism. For the latter, the complexes contain suitable electron donors and acceptors, able to offer an efficient platform for the PET process and generate stable light-induced radicals, which not only give rise to naked-eye detectable color changes but tune the magnetic interactions and field strength of spin centers. Compared to those Ln-SMMs with photoisomerization, electron-transfer complexes display higher sensitivity, faster respond but produce less structural changes, even if the presence of radicals, which may allow the retention of crystallinity, access to establish the key structure-properties relationship. To date, the number of Ln-SMMs with light-triggered radicals is limited partly due to the monotonous type of ligands and poor chemical expansibility. It is not easy to control the CF symmetry and magnetic anisotropy as well, considering the flexible coordination patterns of lanthanide ions.

  Table 1

  

  Table 1.  Parameters for magnetic switching in lanthanide SMMs.

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  Classification	Compounds	Orbach parameters	τQTM (s)	Refs.
  DTE-based Ln-SMMs	1c	Ueff = 157.0 cm−1, τ0 = 7.70 × 10−7 s	3.37× 10−2	[48]
  	1o	Ueff = 157.0 cm−1, τ0 = 7.70 × 10−7 s	1.50 × 10−3
  	2c	Ueff = 193.0 cm−1, τ0 = 1.86 × 10−7 s	6.60 × 10−1	[54]
  	2o	Ueff = 193.0 cm−1, τ0 = 1.86 × 10−7 s	1.60 × 10−1
  	14	Ueff = 10.4 cm−1, τ0 = 2.39 × 10−7 s	−	[49]
  	14-UV	Ueff = 5.9 cm−1, τ0 = 9.30 × 10−6 s	−
  	15	Ueff = 8.4 cm−1, τ0 = 3.03 × 10−6 s	−
  	15-vis	Ueff = 9.3 cm−1, τ0 = 5.27 × 10−7 s	−
  	16	Ueff = 9.9 cm−1, τ0 = 1.90 × 10−8 s	4.53 × 10−4	[50]
  	16-UV (1500 Oe)	Ueff = 10.2 cm−1, τ0 = 7.25× 10−9 s	2.45 × 10−4
  	16-vis (1500 Oe)	Ueff = 9.8 cm−1, τ0 = 5.27 × 10−9 s	2.47 × 10−4
  	17 (1500 Oe)	Ueff = 10.0 cm−1, τ0 = 7.49 × 10−8 s	5.21 × 10−5
  	18o	Ueff = 10.1 cm−1, τ0 = 5.33 × 10−6 s	2.77 × 10−4	[53]
  	18c (1000 Oe)	Ueff = 11.9 cm−1, τ0 = 1.90 × 10−6 s	6.90× 10−3
  	19o	Ueff = 13.3 cm−1, τ0 = 5.20 × 10−7 s	−	[51]
  	19c	Ueff = 19.6 cm−1, τ0 = 1.02 × 10−6 s	1.51 × 10–4
  	19o-solution	Ueff = 5.0 cm−1, τ0 = 2.10 × 10−6 s	−
  	19c-solution	Ueff = 3.1 cm−1, τ0 = 4.20 × 10−6 s	1.33 × 10–4
  Merocyanine-based Ln-SMMs	3MC	Ueff = 150.2 cm−1, τ0 = 4.70 × 10−9 s	1.60 × 10−3	[62]
  	4MC-trans	Ueff = 24.6 cm−1, τ0 = 4.51 × 10−6 s	1.40 × 10−1	[63]
  	4MC-cis	Ueff = 22.9 cm−1, τ0 = 5.76 × 10−6 s	2.30 × 10−2
  	4solid	Ueff = 22.9 cm−1, τ0 = 1.24 × 10−5 s	1.00 × 10−1
  Anthracene-based Ln-SMMs	5	Ueff = 14.2 cm−1, τ0 = 1.50 × 10−8 s	3.94 × 10–4	[55]
  	5a	Ueff = 30.0 cm−1, τ0 = 5.12 × 10−9 s	1.04 × 10−3
  	6	Ueff = 192.6 cm−1, τ0 = 7.16 × 10−11 s	1.88 × 10−1	[59]
  	6a	Ueff = 96.5 cm−1, τ0 = 7.63 × 10−9 s	1.06 × 10−3
  	20	Ueff = 98.0 cm−1, τ0 = 1.10 × 10−9 s	1.30 × 10−2	[56]
  	20-UV	Ueff = 70.1 cm−1, τ0 = 1.00 × 10−9 s	1.40 × 10−3
  	20-R	Ueff = 106.3 cm−1, τ0 = 4.00 × 10−10 s	1.30 × 10−2
  	21	Ueff = 81.7 cm−1, τ0 = 1.41 × 10−8 s	−	[58]
  	21-UV	Ueff = 119.4 cm−1, τ0 = 5.07 × 10−11 s	−
  	22	Ueff = 33.6 cm−1, τ0 = 3.00 × 10−8 s	−
  		Ueff = 76.9 cm−1, τ0 = 1.22 × 10−9 s	−
  	22-UV	Ueff = 83.7 cm−1, τ0 = 8.40 × 10−10 s	−
  	23	Ueff = 38.8 cm−1, τ0 = 1.98 × 10−7 s	−	[57]
  	23-UV	Ueff = 80.5 cm−1, τ0 = 1.98 × 10−7 s	3.40 × 10−2
  Bpe-based Ln-SMMs	7	Ueff = 106.8 cm−1, τ0 = 2.08 × 10−12 s	4.06 × 10−3	[61]
  	7a	Ueff = 142.7 cm−1, τ0 = 5.54 × 10−10 s	3.26 × 10−3
  	24	Ueff = 38.3 cm−1, τ0 = 9.81 × 10−9 s	9.00 × 10−2	[60]
  	24-UV	Ueff = 33.3 cm−1, τ0 = 1.22 × 10−9 s	6.80 × 10−1
  Crown ether-based Ln-SMMs	9 (800 Oe)	Ueff = 12.8 cm−1, τ0 = 5.20 × 10−6 s	−	[72]
  	9a (800 Oe)	Ueff = 11.8 cm−1, τ0 = 7.50 × 10−6 s	−
  Macrocycle-based Ln-SMM	10	Ueff = 26.3 cm−1, τ0 = 4.70 × 10−7 s	−	[71]
  Viologens-based Ln-SMMs	11	off	−	[70]
  	11a	Ueff = 75.1 cm−1, τ0 = 1.50 × 10−9 s	−
  	12 (2000 Oe)	Ueff = 11.7 cm−1, τ0 = 8.40 × 10−6 s	−	[69]
  	12a	off	−
  	13 (2000 Oe)	Ueff = 8.4 cm−1, τ0 = 8.10 × 10−6 s	−
  	13a	off	−
  POM-based Ln SMM	25	Ueff = 13.9 cm−1, τ0 = 4.20 × 10−7 s	−	[73]
  14 [Cu2Tb2(L)2(NO3)2(dae-o)2]·2(n-BuOH); 14-UV (H2L= 1,3-bis((3-methoxysalicylidene)amino)propane, H2dae =1,2-bis(5-carboxyl-2-methyl-3-thienyl) perfluorocyclopentene). 15 {[CuTb(L)(n-BuOH)0.5]2(dae-c)3}·5(DMF)·4(n-BuOH)·2(H2O); 15-vis (H2L =1,3-bis((3-methoxysalicylidene)amino)propane, H2dae =1,2-bis(5-carboxyl-2-methyl-3-thienyl)perfluorocyclopentene). 16 [{Dy2(dae)3(DMSO)3(MeOH)}·10MeOH]n; 16-UV; 16-vis (H2dae=1,2-bis(5-carboxyl-2-methyl-3-thienyl)perfluorocyclopentene). 17 [{Ho2(dae)3(DMSO)3(MeOH)}·10MeOH]n; 17-UV (H2dae= 1,2-bis(5-carboxyl-2-methyl-3-thienyl)perfluorocyclopentene). 18o [Dy2(DTE)3(bipyridine)2(H2O)2]n; 18c (DTE= 1,2-bis(5-carboxyl-2-methyl-3-thienyl)perfluorocyclopemoietiesntene, bipyridine=2,2′-bipyridine). 20 [Dy(SCN)3(depma)2(4-hpy)2]; 20-UV; 20-R (depma=9-diethylphosphonomethylanthracen, 4-hpy= 4-hydroxypyridine). 21 [Dy2(SCN)4(L)2(dmpma)2(H2O)2]; 21-UV (HL= 2,6-dimethoxyphenol, dmpma= 9-dimethylphosphonomethylanthracen). 22 [Dy2(SCN)4(L)2(depma)2(H2O)2]; 22-UV (HL= 2,6-dimethoxyphenol, depma=9-diethylphosphonomethylanthracen). 23 [Dy2(SCN)4(L)2(dmpma)4]; 23-UV (HL= 4-methyl-2,6-dimethoxyphenol, dmpma=9-dimethylphosphonomethylanthracen). 24 [Dy0.055Y0.945(bpe)(H2O)4(NO3)2](NO3)2bpe (bpe =1,2-bis(4-pyridyl)ethene). 24-UV [Dy0.11Y1.89(tpcb)(H2O)8(NO3)4](NO3)22bpe·tpcb (bpe =1,2-bis(4-pyridyl)ethene, tpcb= tetrakis(4-pyridyl)cyclobutane). 25 [N(CH3)4]6K3H7[Dy(C4H2O6)(α-PW11O39)]2·27H2O.

  Scheme 1

  

  Scheme 1.  Photo-responsive units in Ln-SMMs with photoisomerization.

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  To sum up, the photochromic reactions in Ln-SMMs either bring about the structural changes or magnetic interactions variations, which result in the regulation of polarity, electronic properties, crystal packing or intermolecular interactions etc., providing a platform to tune their magnetic dynamics. While the scope of Ln-SMMs is increasing explosively, only a small percentage of those compounds exhibit photo-magnetism. In this review, we will focus on the main findings in the emerging research trends concerning the photo-responsive Ln-SMMs, providing future outlooks and perspectives in this field.

  2. Photo-responsive Ln-SMMs with photoisomerization

  2.1 DTE-based Ln-SMMs

  The diarylethene (DTE) derivatives, an ideal class of photochromic molecules, can undergo reversible coloration and decoloration reactions between two thermally stable isomers [74,75]. These well-designed compounds exhibit rapid response, high sensitivity and remarkable fatigue-resistant photochromic performance in solution and in the single-crystalline phase, along with electronic as well as geometrical structure changes, which bring about new applications for Ln-SMMs and may have access to modulate the magnetic properties reversibly. In 2009, a diarylethene ligand (1,2-bis(5-caxboxyl-2-methyl-3-thienyl)perfluorocyclopentene, DTE-COOH) was used as photoswitch and cluster linkages to construct [Mn₄] one dimensional (1D) chain complexes [76], obtaining the first phototunable SMM. Over the next few years, Yamashita and co-workers reported a series of photo-responsive chains of Ln-SMMs (Ln = Dy, Tb and Ho) using DTE-COOH as the photoactive unit [49–53]. Among these complexes, the photochromic ligand DTE-COOH underwent reversible cyclization and cycloreversion reactions upon the irradiation with ultraviolet (UV)/visible (vis) light. Accompanied by the color change, their magnetic relaxation times or effective barriers varied to some extent, because the light-induced isomerization contributes to a structural change in coordination environment of lanthanide metals. However, these compounds suffer some limitations, including unsatisfactory SMM performance and unclear magneto-structural relationships, due to the unfavorable CF structures and the disruption of a single crystallinity, which hinders the further research.

  In 2019, Norel and Bernot reported an air-stable, highly anisotropic phototunable chain of Dy-SMM [Dy(Tp^py)F(L1c)]PF₆, 1c (Tp^py = tris(3-(2-pyridyl)pyrazolyl)hydroborate, L1c = 1,2-bis(2-methyl,5-(4-pydidyl)-3-thienyl)perfluorocyclopentene), featuring DTE-based ligand as a photoactive unit [48]. Upon irradiation of green light, 1c can undergo a single-crystal-to-single-crystal (SC-SC) transformation, obtaining the closed form [Dy(Tp^py)F(L1o)]PF₆, 1o (Fig. 1), with a color change from dark blue to colorless and variation of unit cell parameters with the elongation in b and c lattices and shrinkage in a lattice. In order to explore the impact of photoisomerization on magnetic properties, static (dc) and dynamic (ac) susceptibility measurements have been performed for two isomers. 1o and 1c exhibited very similar magnetic behavior, yielding the same U_eff of 157 cm⁻¹ and τ₀ of 7.7 × 10⁻⁷s under zero dc field. In stark contrast, they displayed significant difference in quantum tunneling time (τ_QTM), which decreases from 0.0337 s for 1c to 0.0015 s for 1o, in good agreement of a wider hysteresis loop of 1c than 1o at all investigated temperatures. Thanks to the full transformation between closed and open states, accurate crystal structures can be determined. Combining with ab initio calculations, it enables to investigate magneto-structural correlations precisely, where the obvious differences in magnetic hysteresis behavior for 1c and 1o attributed to variations in dipolar or spin-photon coupling in each compound owing to the crystal packing modification. In 2023, this group synthesized a cyclic dinuclear Dy-SMMs [(Dy(Tp^py)F)₂(L2c)₂](BArF)₂, 2c using [Dy(Tp^py)F(pyridine)₂]PF₆ and 3-pyridyl terminated DTE photoswitches (L2c, 1,2-bis(2-methyl,5-(3-pydidyl)-3-thienyl)perfluorocyclopentene) as building units [54]. The light irradiation at 660 nm of 2c allowed the complete transformation to the open isomer, 2o. These two isomers possessed the identical U_eff of 193 cm⁻¹, because of their virtually identical coordination spheres, which is in good agreement with 1c and 1o. In stark contact with 1c and 1o, the hysteresis loops of 2c and 2o exhibited pretty much the same characteristics, which mainly comes from the less impact of the cycloreversion reactions on the quantum tunneling.

  Figure 1

  

  Figure 1.  (a) Open and closed forms of the photoactive bispyridyl dithienylethene units used as a bridging ligand in 1c and 2c. (b) Single-crystal structures of 1c and 1o (top), 2c and 2o (bottom). (c) Variable field magnetization data for 1c and 1o (top) and for 2c and 2o (bottom). Reproduced with permission [48]. Copyright 2020, American Chemical Society. Reproduced with permission [54]. Copyright 2023, the Royal Society of Chemistry.

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  2.2 Merocyanine-based Ln-SMMs

  The spiropyran (SP) is a famous photochromic molecule, whose photoswitching involves two-step continuous isomerization, beginning with the breakage of C–O bond to form a cis-isomer of zwitterionic merocyanine (MC), followed by a conformational inversion to the trans-isomer [77]. In 2016, Norel and Bernot reported the first structurally characterized MC-Dy complex [Dy(hfac)₂(L_MC)]I, 3_MC (hfac = hexafluoroacetylacetonate, L_MC = (E)-2-((bis(pyridin-2-ylmethyl)amino)methyl)-4-nitro-6-(2-(1,3,3-trimethyl-3H-indol-1-ium-2-yl)vinyl)phenolate), which showed a high effective barrier for magnetization reversal (150 cm⁻¹) and the opening of magnetic hysteresis loop at 1.8 K, due to the strongly axial anisotropy generated by a highly changed phenolate oxygen in MC ligand [62]. It is worth noting that yttrium analogue can undergo the isomerization to the spiropyran form with a release of Y(hfac)₂⁻ moiety in solution upon visible light irradiation.

  In 2018, the same group explored a modified complex [Dy(L_MC)(OTf)₂(H₂O)₂]Otf [63], 4_MC-trans, which can realize a reversible photoinduced partial isomerization of the trans-MC ligand to the cis-MC one (4_MC-cis) and maintain the metal-phenolate bond, different from the previously published 3_MC. Magnetic investigations on two isomers in frozen solution demonstrated that 4_MC-trans and 4_MC-cis displayed similar energy barrier while the photo-conversion accelerated the magnetic relaxation rates at low temperatures. In 2023, 3_MC was deposited onto the Au(111) surface via the drop-cast method in which partial compound isomerized to 3_SP (Dy(hfac)₂(L_SP) [64], spontaneously. The conformational transformation of 3_SP to 3_MC can be implemented by injecting tunneling electrons with an STM tip, which was confirmed through detecting the Kondo resonance of the radical form of 3_MC, theoretically corroborated by DFT calculations. These studies may pave the way for practical application in quantum information coding by using photo-responsive lanthanide SMMs.

  2.3 Anthracene-based Ln-SMMs

  As one of the most classical reactions in photochemistry, the photodimerization of anthracene and its derivatives can be considered as a paradigm of the photocycloaddition of unsaturated hydrocarbons [78,79]. The anthracene-containing ligands have potential to achieve a [4 + 4] cycloaddition reaction in a single crystal state. In 2018, Zheng's group has presented an air-stable anthracene-based dysprosium single-ion magnet (SIM), Dy(depma)(NO₃)₃(hmpa)₂ [55], 5 (depma, 9-diethylphosphono-methylanthracene; hmpa, hexamethylphosphoramide). The anthracene moieties underwent [4 + 4] dimerization upon the irradiation at 365 nm with retention of crystallinity and achieved depolymerization by heating at 100 ℃ or partially on irradiation at 254 nm (Fig. 2). The reversible photodimerization reactions caused dramatic structural changes leading to remarkable alteration of magnetic and luminescent properties of the complex. Upon irradiation, the SIM, 5 converted to a single-molecule magnet 5a with doubling of the barrier energy along with luminescent change from yellow-green to blue-white emission.

  Figure 2

  

  Figure 2.  (a) Single-crystal structures of 5 and 5a. (b) The frequency dependence of the out-of-phase χ′′ of the ac susceptibilities for 5 (left) and 5a (right). (c) Photoluminescence spectra and the CIE graph of 5, 5a and 5a heated. Reproduced with permission [55]. Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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  To explore more possibilities of the interplay between the photocycloaddition reaction and magneto-optic properties, Zheng's group synthesized a photo-responsive Dy-based SMM, [Dy(SCN)₂(NO₃)(depma)₂(4-hpy)₂] [59], 6 with the same photoswitch depma (Fig. 3). Compared to the previous effort, complex 6 has several significant features: (1) It displays significantly improved SMM property with the highest anisotropy barrier (U_eff = 193 cm⁻¹) among the known photo-responsive Ln-SMMs; (2) It undergoes a rapid light-induced SC-SC transformation to form a 1D polymer [Dy(SCN)₂(NO₃)(depma₂)(4-hpy)₂]_n, 6a which can be reversed by thermal annealing; (3) Accompanied by the structural transformation, the phototunable SMM properties can be observed with reduction of energy barrier from 193 cm⁻¹ for 6 to 96.5 cm⁻¹ for 6a and the narrowing of hysteresis loop. These works may shed light on the development of photo-responsive smart molecular materials.

  Figure 3

  

  Figure 3.  (a) Single-crystal structures of 6 and 6a. (b) The frequency dependence of the out-of-phase χ′′ of the ac susceptibilities for 6 (left) and 6a (right). (c) Photoluminescence spectra of 6, 6a and 6 heated. (d) Variable field magnetization data for 6 and 6a. Reproduced with permission [59]. Copyright 2023, Wiley-VCH GmbH.

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  2.4 Bpe-based Ln-SMMs

  In addition, the photoactive ligand 1,2-bis(4-pyridyl)ethene (bpe), and its derivatives have been extensively used to construct photo-responsive molecular materials due to their facility for photoinduced [2 + 2] cycloaddition [60,61,80,81]. For example, Tong's group synthesized a mononuclear Dy-based SMM (Hbpe)₂[Dy(bpe)(H₂O)(4-pyO)(NO₃)(SCN)₃]SCN [61], 7 where the photochemical [2 + 2] reaction took place between neighboring bpe ligands, giving the cycloaddition product through the SC-SC transformation. The photoinduced structural alteration influenced the Orbach process and Raman process, which leaded to the opposite influence on the energy barrier and relaxation time for these two isomers. The irradiated product 7a finally displayed an increased energy barrier and shortened relaxation time.

  Beyond that, several lanthanide complexes containing photochromic azo-based ligands [65,66] have been reported and their photochromism and magnetism were investigated separately. The photo-responsive SMM behaviors may deserve further investigation.

  3. Photo-responsive Ln-SMMs with electron transfers

  Recent several significant advances of photo-responsive Ln-SMMs are based on the photoisomerization reactions. However, the effective photoisomerization in the condensed phase not only require sufficient conformational freedom but a time-consuming process. In this respect, a photoinduced electron transfer (PET) process with stable radicals in Ln-SMMs may be free from the above-mentioned problems. Meanwhile, the radical bridging ligands with diffuse spin orbitals can penetrate the deeply buried 4f orbitals, which may facilitate the strong coupling between radicals and Ln³⁺ ions and further alter the magnetic dynamics.

  3.1 Crown ether-based Ln-SMM

  The first photochromic and photomagnetic example of 3d-4f hexacyanoferrates at room temperature was published in 2015 by Guo and coworkers, incorporating [Eu³⁺(18C6)(H₂O)₃]Fe³⁺(CN)₆·2H₂O (8), (18C6 = 18-crown-6) (Fig. 4) [68]. An efficient photoinduced electron transfer process can be performed from the crown to Fe³⁺, yielding long-lived charge-separated crown radicals, which has been confirmed by IR, UV–vis and EPR spectra. The photoinduced radicals coupled with Eu³⁺ centers with a strong antiferromagnetic interaction which resulted in a great decrease (33.5%) in magnetic moment at room temperature. Inspired by this work, the authors synthesized a dysprosium analogue [Dy³⁺(18C6)(H₂O)₃]Fe³⁺(CN)₆·2H₂O (9) [72], given that Dy³⁺ possesses large magnetic moment and strong magnetic anisotropy. Notably, this complex underwent a magnetic phase transition from the paramagnetic to ferromagnetic characteristic with a dramatic enhancement (20.9%) of magnetization at room temperature upon illumination.

  Figure 4

  

  Figure 4.  (a) Single-crystal structures of [Ln³⁺(18C6)(H₂O)₃]Fe³⁺(CN)₆·2H₂O (Ln = Eu(8) and Dy(9)). (b) UV–vis spectra for 8, 8a and 8b. Inset: photochromism of 8 at room temperature. (c) χ_MT versus temperature (T) for 8 and 8a. (d) EPR spectra of 9 and 9a. (e) UV–vis spectra for 9, 9a. Inset: photochromism of 9 at room temperature. (f) χ_MT versus temperature (T) for 9, 9a and 9b. Reproduced with permission [68]. Copyright 2015, American Chemical Society. Reproduced with permission [72]. Copyright 2021, Royal Society of Chemistry.

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  3.2 Macrocycle-based Ln-SMM

  In addition to lanthanide crown-ether-based complexes, a hexaazamacrocyclic dysprosium complex [Dy(L^N6)-(NO₃)₂](BPh₄) (10), (L^N6 =(3E,5E,10E,12E)-3,6,10,13-tetraaza-1,8(2,6)-dipyridinacyclotetradecaphane-3,5,10,12-tetraene) [82] with photogenerated radicals has been revealed for the first time by Tong's group in 2023 (Fig. 5). This compound underwent photochromism and photomagnetism under UV light irradiation due to the electron transfer from BPh₄⁻ to L^N6, generating the photoinduced radicals. But the photochromic product is sensitive to air and the dark color samples gradually turn into the initial state under an aerobic environment. The irradiated product 10a exhibited a faster relaxation rate compared to 10. This may be owing to the location of L^N6 radicals in the equatorial plane, which increases the transverse components of CF and facilitates the QTM process.

  Figure 5

  

  Figure 5.  (a) Single-crystal structure of 10. Variable temperature ac magnetic susceptibilities for (b) 10 and (d) 10a under a 2 kOe dc field. (c) Relaxation time ln(τ) vs. T⁻¹ plot for 10 (gray circle) and 10a (red circle). Reproduced with permission [71]. Copyright 2023, American Chemical Society.

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  3.3 Viologens-based Ln-SMMs

  Although the phototunable magnetic hysteresis and energy barriers have been realized in above mentioned complexes, the on/off single-molecule magnet behavior was not observed. In 2020, a chain complex [Dy₃(H-HEDP)₃(H₂-HEDP)₃]·2H₃-TPT·H₄-HEDP·10H₂O, 11 (HEDP = hydroxyethylidene diphosphonate; TPT = 2,4,6-tri(4-pyridyl)-1,3,5-triazine) was synthesized [70], which can undergo the reversible photochromism and photomagnetism at room temperature, induced by the photogenerated radicals via a PET mechanism. The photo-triggered on/off SMM behaviors was realized for the first time. Upon irradiation, the colorless compound can turn blue immediately (the blue sample named as 11a), which can be reversible by thermal annealing at 100 ℃ for 30 min (the decolor sample termed 11b). The photoinduced photolysis or isomerization can be excluded by the unchanged PXRD and IR spectra while UV–vis, luminescence and EPR spectra have confirmed that the PET process occurs from HEDP to TPT, generating the O^• and TPT^• radicals. In order to gain further insight to the influence of photochromism on magnetic relaxation dynamics, the dc and ac magnetic susceptibility measurements have been carried on 11, 11a and 11b. 11a presented a slight larger χT value (43.67 cm³ K/mol) than that (42.45 cm³ K/mol) of 11 at 300 K (Fig. 6), indicative of the generation of radicals at room temperature. Upon cooling, dc magnetic susceptibility curves of 11 and 11a showcased distinct thermal evolution. The χT value of 11 showed a sharp decline to 32.24 cm³ K/mol at 2 K, but that of 11a exhibited a steady increase to 52.67 cm³ K/mol, which suggests that the initial antiferromagnetic interactions at low temperatures convert to the ferromagnetic characteristic because of the formation of strongly coupled Dy³⁺−O^• species. Accordingly, 11a behaves as a typical zero-field SMM with energy barrier of 108.1 K while 11 displayed no temperature dependence in both χ′ and χ′′ components, which demonstrated that the strong ferromagnetic coupling suppressed QTM effectively. Furthermore, after decoloration, the dc and ac data of 11b returned to the initial state of the as-prepared samples 11, implying the reversibility of phototunable magnetic behavior. In 2022, this group synthesized two viologen-based lanthanide phosphonate coordinated polymers [Ln₃(H-HEDP)₂(H₂-HEDP)₂(H₃-HEDP)₂]·H₃-TPP·11H₂O (Ln = Dy (12) and Tb (13)) [69]. These complexes are able to undergo the reversible photochromic behaviors along with photogeneration of HEDP^• and H3-TPP^• radicals. As a result of antiferromagnetic coupling between Ln³⁺ and radicals, two complexes displayed a radical-triggered off SMM behavior via light illumination at room temperature, as the first case of radical-quenched SMM in molecule-based magnets.

  Figure 6

  

  Figure 6.  (a) Single-crystal structures of 11. (b) χ_MT versus temperature (T) for 11, 11a and 11b. Variable temperature ac magnetic susceptibilities for (c) 11 and (d) 11a. Reproduced with permission [70]. Copyright 2020, American Chemical Society.

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  3.4 POM-based Ln-SMM

  Inorganic–organic hybrid polyoxometalates (POMs) are versatile candidates for the design of multifunctional materials incorporating electrochromism, photochromism, magnetism, luminescence and so on. In 2019, Wang and collaborators published a series of photochromic, luminescent mono-lanthanide POMs [73], among which Dy analogue displayed a switchable luminescent behavior based on the reversible photochromism dominated by the PET mechanism. This complex also displayed a field-induced SMM behavior, but the synergistic effect between magnetism and photochromism has not been investigated.

  4. Conclusions and perspectives

  To data, several impressive advancements have been realized in the design of photoswitchable Ln-SMMs, including phototunable magnetic relaxation time, energy barrier, magnetic hysteresis even the on/off SMM behavior. However, these complexes suffer from one or more limitations including incomplete photochromism, poor reversibility, inapparent magnetic properties variations, unpreservable crystallinity upon irradiation leading to unclear magneto-structural correlations, unsatisfactory SMM properties and poor chemical modification. In terms of photo-isomerized Ln-SMMs, the photoinduced structural control may provide possibility for the remote manipulation of magnetic properties. The more significant structural changes are, the greater discrepancy of magnetic dynamics will be. However, a serious conflict between the sufficient conformational freedom for large structural changes and the close packing of photoswitches in solid state often causes forbidden or incomplete photoisomerization process. Thus, taking a delicate balance of rigidity and flexibility of ligands may help to high efficiency reaction and SC-SC structural transition in the crystalline state. Meanwhile, well-design photoswitches with highly charged coordinated atoms may facilitate more prominent changes of CF multiplet structures of Ln³⁺, contributing to more significant magnetic changes before and after optical radiation. Light irradiation is also the key factor to determine the efficiency of the photo reaction. For instance, UV light is strongly scattered, making deep penetration throughout the crystal difficult. The photoactive ligands where photoisomerization can occur entirely in the visible region even in near infrared region would therefore be highly desirable. The typical incomplete photoconversions may be also induced by the overlap of absorption region between two isomers, thus preventing one species-rich photostationary states. The drawbacks can be circumvented by chemical modification of photoswtiches so that the separation of absorption bands is large enough, allowing to isomerize both isomers selectively. In comparison with photo-isomerized lanthanide systems, the electron transfers photochromic families reveal the features of faster response and less structural changes. But a limited range of ligands without flexible and feasible modification may give rise to the unfavorable CF environment for lanthanide centers, leading to the unsatisfactory SMM properties. The development of more types of ligands are necessary for the construction of photo-responsive Ln-SMMs. For example, strategically, with easy modification and stable SMM characteristics, the popular Dy(Ⅲ) macrocyclic or crown ether complexes possessing hexagonal-bipyramidal (pseudo-D_6h symmetry) local coordination geometry, may be effective models to construct photo-responsive Ln-SMMs, whose high-performance SMM property and air-stable feature can offer great convenience for the fabrication of information storage devices. Therefore, the design of photoactive phenoxy or alkoxide units as axial ligands for above complexes is promising, because any subtle changes in these highly charged ligands may cause great influence on magnetic relaxations. In term of the optical controllable magnetism devices, the manipulation of magnetic states was predominantly realized via polarized light with the help of ultrafast light technologies. In order to utilize readily available and more energy-efficient natural light sources, such as sunlight, research focus has increasingly shifted toward magnetic heterostructures, like organic spin valves [83]. The development of photoresponsive molecular magnetic materials may provide more elemental motifs and inspirations for future development of devices. Whether from the fundamental aspect or the potential technological applications, the design and synthesis of photo-responsive Ln-SMMs with high-performance magnetic properties, good reversibility will undoubtedly become a research focus in the near future.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  CRediT authorship contribution statement

  Jinjiang Wu: Writing – original draft. Zhenhua Zhu: Writing – review & editing, Writing – original draft, Conceptualization. Jinkui Tang: Writing – review & editing, Supervision, Conceptualization.

  Acknowledgments

  This project was financially supported by the Fundamental Research Program of Shanxi Province (No. 202303021222126) and the National Natural Science Foundation of China (No. 92261103).

  
  
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• 

  Scheme 1  Photo-responsive units in Ln-SMMs with photoisomerization.

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  Figure 1  (a) Open and closed forms of the photoactive bispyridyl dithienylethene units used as a bridging ligand in 1c and 2c. (b) Single-crystal structures of 1c and 1o (top), 2c and 2o (bottom). (c) Variable field magnetization data for 1c and 1o (top) and for 2c and 2o (bottom). Reproduced with permission [48]. Copyright 2020, American Chemical Society. Reproduced with permission [54]. Copyright 2023, the Royal Society of Chemistry.

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  Figure 2  (a) Single-crystal structures of 5 and 5a. (b) The frequency dependence of the out-of-phase χ′′ of the ac susceptibilities for 5 (left) and 5a (right). (c) Photoluminescence spectra and the CIE graph of 5, 5a and 5a heated. Reproduced with permission [55]. Copyright 2018, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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  Figure 3  (a) Single-crystal structures of 6 and 6a. (b) The frequency dependence of the out-of-phase χ′′ of the ac susceptibilities for 6 (left) and 6a (right). (c) Photoluminescence spectra of 6, 6a and 6 heated. (d) Variable field magnetization data for 6 and 6a. Reproduced with permission [59]. Copyright 2023, Wiley-VCH GmbH.

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  Figure 4  (a) Single-crystal structures of [Ln³⁺(18C6)(H₂O)₃]Fe³⁺(CN)₆·2H₂O (Ln = Eu(8) and Dy(9)). (b) UV–vis spectra for 8, 8a and 8b. Inset: photochromism of 8 at room temperature. (c) χ_MT versus temperature (T) for 8 and 8a. (d) EPR spectra of 9 and 9a. (e) UV–vis spectra for 9, 9a. Inset: photochromism of 9 at room temperature. (f) χ_MT versus temperature (T) for 9, 9a and 9b. Reproduced with permission [68]. Copyright 2015, American Chemical Society. Reproduced with permission [72]. Copyright 2021, Royal Society of Chemistry.

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  Figure 5  (a) Single-crystal structure of 10. Variable temperature ac magnetic susceptibilities for (b) 10 and (d) 10a under a 2 kOe dc field. (c) Relaxation time ln(τ) vs. T⁻¹ plot for 10 (gray circle) and 10a (red circle). Reproduced with permission [71]. Copyright 2023, American Chemical Society.

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  Figure 6  (a) Single-crystal structures of 11. (b) χ_MT versus temperature (T) for 11, 11a and 11b. Variable temperature ac magnetic susceptibilities for (c) 11 and (d) 11a. Reproduced with permission [70]. Copyright 2020, American Chemical Society.

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  Table 1.  Parameters for magnetic switching in lanthanide SMMs.

  Classification	Compounds	Orbach parameters	τQTM (s)	Refs.
  DTE-based Ln-SMMs	1c	Ueff = 157.0 cm−1, τ0 = 7.70 × 10−7 s	3.37× 10−2	[48]
  	1o	Ueff = 157.0 cm−1, τ0 = 7.70 × 10−7 s	1.50 × 10−3
  	2c	Ueff = 193.0 cm−1, τ0 = 1.86 × 10−7 s	6.60 × 10−1	[54]
  	2o	Ueff = 193.0 cm−1, τ0 = 1.86 × 10−7 s	1.60 × 10−1
  	14	Ueff = 10.4 cm−1, τ0 = 2.39 × 10−7 s	−	[49]
  	14-UV	Ueff = 5.9 cm−1, τ0 = 9.30 × 10−6 s	−
  	15	Ueff = 8.4 cm−1, τ0 = 3.03 × 10−6 s	−
  	15-vis	Ueff = 9.3 cm−1, τ0 = 5.27 × 10−7 s	−
  	16	Ueff = 9.9 cm−1, τ0 = 1.90 × 10−8 s	4.53 × 10−4	[50]
  	16-UV (1500 Oe)	Ueff = 10.2 cm−1, τ0 = 7.25× 10−9 s	2.45 × 10−4
  	16-vis (1500 Oe)	Ueff = 9.8 cm−1, τ0 = 5.27 × 10−9 s	2.47 × 10−4
  	17 (1500 Oe)	Ueff = 10.0 cm−1, τ0 = 7.49 × 10−8 s	5.21 × 10−5
  	18o	Ueff = 10.1 cm−1, τ0 = 5.33 × 10−6 s	2.77 × 10−4	[53]
  	18c (1000 Oe)	Ueff = 11.9 cm−1, τ0 = 1.90 × 10−6 s	6.90× 10−3
  	19o	Ueff = 13.3 cm−1, τ0 = 5.20 × 10−7 s	−	[51]
  	19c	Ueff = 19.6 cm−1, τ0 = 1.02 × 10−6 s	1.51 × 10–4
  	19o-solution	Ueff = 5.0 cm−1, τ0 = 2.10 × 10−6 s	−
  	19c-solution	Ueff = 3.1 cm−1, τ0 = 4.20 × 10−6 s	1.33 × 10–4
  Merocyanine-based Ln-SMMs	3MC	Ueff = 150.2 cm−1, τ0 = 4.70 × 10−9 s	1.60 × 10−3	[62]
  	4MC-trans	Ueff = 24.6 cm−1, τ0 = 4.51 × 10−6 s	1.40 × 10−1	[63]
  	4MC-cis	Ueff = 22.9 cm−1, τ0 = 5.76 × 10−6 s	2.30 × 10−2
  	4solid	Ueff = 22.9 cm−1, τ0 = 1.24 × 10−5 s	1.00 × 10−1
  Anthracene-based Ln-SMMs	5	Ueff = 14.2 cm−1, τ0 = 1.50 × 10−8 s	3.94 × 10–4	[55]
  	5a	Ueff = 30.0 cm−1, τ0 = 5.12 × 10−9 s	1.04 × 10−3
  	6	Ueff = 192.6 cm−1, τ0 = 7.16 × 10−11 s	1.88 × 10−1	[59]
  	6a	Ueff = 96.5 cm−1, τ0 = 7.63 × 10−9 s	1.06 × 10−3
  	20	Ueff = 98.0 cm−1, τ0 = 1.10 × 10−9 s	1.30 × 10−2	[56]
  	20-UV	Ueff = 70.1 cm−1, τ0 = 1.00 × 10−9 s	1.40 × 10−3
  	20-R	Ueff = 106.3 cm−1, τ0 = 4.00 × 10−10 s	1.30 × 10−2
  	21	Ueff = 81.7 cm−1, τ0 = 1.41 × 10−8 s	−	[58]
  	21-UV	Ueff = 119.4 cm−1, τ0 = 5.07 × 10−11 s	−
  	22	Ueff = 33.6 cm−1, τ0 = 3.00 × 10−8 s	−
  		Ueff = 76.9 cm−1, τ0 = 1.22 × 10−9 s	−
  	22-UV	Ueff = 83.7 cm−1, τ0 = 8.40 × 10−10 s	−
  	23	Ueff = 38.8 cm−1, τ0 = 1.98 × 10−7 s	−	[57]
  	23-UV	Ueff = 80.5 cm−1, τ0 = 1.98 × 10−7 s	3.40 × 10−2
  Bpe-based Ln-SMMs	7	Ueff = 106.8 cm−1, τ0 = 2.08 × 10−12 s	4.06 × 10−3	[61]
  	7a	Ueff = 142.7 cm−1, τ0 = 5.54 × 10−10 s	3.26 × 10−3
  	24	Ueff = 38.3 cm−1, τ0 = 9.81 × 10−9 s	9.00 × 10−2	[60]
  	24-UV	Ueff = 33.3 cm−1, τ0 = 1.22 × 10−9 s	6.80 × 10−1
  Crown ether-based Ln-SMMs	9 (800 Oe)	Ueff = 12.8 cm−1, τ0 = 5.20 × 10−6 s	−	[72]
  	9a (800 Oe)	Ueff = 11.8 cm−1, τ0 = 7.50 × 10−6 s	−
  Macrocycle-based Ln-SMM	10	Ueff = 26.3 cm−1, τ0 = 4.70 × 10−7 s	−	[71]
  Viologens-based Ln-SMMs	11	off	−	[70]
  	11a	Ueff = 75.1 cm−1, τ0 = 1.50 × 10−9 s	−
  	12 (2000 Oe)	Ueff = 11.7 cm−1, τ0 = 8.40 × 10−6 s	−	[69]
  	12a	off	−
  	13 (2000 Oe)	Ueff = 8.4 cm−1, τ0 = 8.10 × 10−6 s	−
  	13a	off	−
  POM-based Ln SMM	25	Ueff = 13.9 cm−1, τ0 = 4.20 × 10−7 s	−	[73]
  14 [Cu2Tb2(L)2(NO3)2(dae-o)2]·2(n-BuOH); 14-UV (H2L= 1,3-bis((3-methoxysalicylidene)amino)propane, H2dae =1,2-bis(5-carboxyl-2-methyl-3-thienyl) perfluorocyclopentene). 15 {[CuTb(L)(n-BuOH)0.5]2(dae-c)3}·5(DMF)·4(n-BuOH)·2(H2O); 15-vis (H2L =1,3-bis((3-methoxysalicylidene)amino)propane, H2dae =1,2-bis(5-carboxyl-2-methyl-3-thienyl)perfluorocyclopentene). 16 [{Dy2(dae)3(DMSO)3(MeOH)}·10MeOH]n; 16-UV; 16-vis (H2dae=1,2-bis(5-carboxyl-2-methyl-3-thienyl)perfluorocyclopentene). 17 [{Ho2(dae)3(DMSO)3(MeOH)}·10MeOH]n; 17-UV (H2dae= 1,2-bis(5-carboxyl-2-methyl-3-thienyl)perfluorocyclopentene). 18o [Dy2(DTE)3(bipyridine)2(H2O)2]n; 18c (DTE= 1,2-bis(5-carboxyl-2-methyl-3-thienyl)perfluorocyclopemoietiesntene, bipyridine=2,2′-bipyridine). 20 [Dy(SCN)3(depma)2(4-hpy)2]; 20-UV; 20-R (depma=9-diethylphosphonomethylanthracen, 4-hpy= 4-hydroxypyridine). 21 [Dy2(SCN)4(L)2(dmpma)2(H2O)2]; 21-UV (HL= 2,6-dimethoxyphenol, dmpma= 9-dimethylphosphonomethylanthracen). 22 [Dy2(SCN)4(L)2(depma)2(H2O)2]; 22-UV (HL= 2,6-dimethoxyphenol, depma=9-diethylphosphonomethylanthracen). 23 [Dy2(SCN)4(L)2(dmpma)4]; 23-UV (HL= 4-methyl-2,6-dimethoxyphenol, dmpma=9-dimethylphosphonomethylanthracen). 24 [Dy0.055Y0.945(bpe)(H2O)4(NO3)2](NO3)2bpe (bpe =1,2-bis(4-pyridyl)ethene). 24-UV [Dy0.11Y1.89(tpcb)(H2O)8(NO3)4](NO3)22bpe·tpcb (bpe =1,2-bis(4-pyridyl)ethene, tpcb= tetrakis(4-pyridyl)cyclobutane). 25 [N(CH3)4]6K3H7[Dy(C4H2O6)(α-PW11O39)]2·27H2O.

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