Recent advances on the construction of diarylethene-based supramolecular metallacycles and metallacages via coordination-driven self-assembly

Diarylethene as the backbone of metallacycles

In 2001, Murguly et al.^[29] reported the synthesis of the early example of diarylethene-based metallacycles and investigated their chiroptical behavior for nondestructive data processing. As shown in Figure 1, treatment of the chiral oxazoline-modified diarylethene ligand 1 or 2 with an equimolar amount of CuOTf·0.5C₆H₆ provided the rectangular [2+2] metallacycle Cu₂(1o)₂ or Cu₂(2o)₂, respectively. The metallacycles were well characterized by mass spectra and X-ray crystallography analysis. It was found that the metallacycle Cu₂(1o)₂ was a stereochemically pure specie, which was ascribed to the remote chiral induction of the oxazoline moieties and steric hindrance in the metallacycle. The ring-open form metallacycle with stereochemical purity could orientate the two methyl groups on the thiophene ring, thus leading to the ring-closed form metallacycle with stereochemical purity under UV irradiation. ¹H NMR experiments showed that the photoconversion yield was about 90% in the photostationary state, and it could be completely recovered under visible light irradiation. It should be noted that the photoconversion yield and diastereo-selectivity during the photochromic process was affected by the concentration of the ligands. A higher concentration (10^-3 mol/L) resulted in a high photoconversion yield and diastereo-selectivity, probably due to the disassembly of the metallacycles at low concentrations. Furthermore, optical rotary dispersion spectra during the photochromic process were performed, which indicated that the ring-open and ring-closed forms of the metallacycle Cu₂(1o)₂exhibited several different spectral regions (e.g., 450 and 475 nm) in rotation. In addition, the modulated optical rotation showed good fatigue-resistant property, thus providing possibility for nondestructive data processing.

Figure 1. (A) The synthetic route of the metallacycle Cu₂(1o)₂ and Cu₂(2o)₂. (B) Crystal structure of (S, S)-Cu₂(1o)₂. (C) The optical rotary dispersion spectra of 1o with CuOTf·0.5C₆H₆ (-) and 1c with CuOTf·0.5C₆H₆ (···). (D) The fatigue-resistant property of the optical rotation of 1o with CuOTf·0.5C₆H₆ at 450 nm (◆) and 475 nm (■) through altering UV and visible irradiation. This figure is used with permission from Murguly et al.^[29].

Light-triggered reversibly structural transformations play an important role in the construction of smart artificial machines. In 2012, Chen et al.^[30] reported the synthesis of Pt-based diarylethene hexagons for the first time and studied their photochromic behaviors. As shown in Figure 2, through the treatment of the diarylethene-containing 120° donor 3 with the classical 120° Pt acceptor 4 or 180° Pt acceptor 5, the [3+3] hexagon 6 or [6+6] hexagon 7 was readily obtained through coordination-driven self-assembly. The hexagons were well characterized with ¹H NMR spectra, ³¹P NMR spectra and ESI-TOF-MS. Furthermore, the photochromic property of the hexagons was investigated. As for 6, under 365 nm irradiation, a new absorption band appeared at 628 nm accompanied by an obvious color change from colorless to dark cyan, which was caused by the formation of ring-closed form diarylethene. The fluorescence of hexagon 6 was also decreased dramatically under 365 nm irradiation. Moreover, the absorption and fluorescence of 6 could be completely recovered under visible light irradiation, which could be repeated for 10 cycles without any obvious degradation, indicating its excellent fatigue-resistant property. In addition, the results of ¹H NMR and ³¹P NMR spectra also demonstrated the photochemical reaction could efficiently occur without disturbing the hexagon’s structure. More importantly, the photoconversion yield of hexagons 6 and 7 from the ring-open form to ring-closed form was almost quantitative, while, for ligand 3, it was just 88%. This finding suggests that the formation of coordination bond promoted the photochromic process in this study. Further DFT calculations indicated that the antiparallel conformation of the ligand was closer to 120°, which was more favorable for the formation of hexagons according to the principles of “symmetry interaction” and “directional bonding”^[31]. In addition, the hexagons with ligands in antiparallel conformation showed a lower energy (7.67 kcal/mol) than the parallel ones, thus demonstrating their good stability. This research realized the increased photoconversion yield through coordination induction, which laid foundation for the design and construction of new photochromic SCCs.

Figure 2. (A) The synthesis of the multi-bisthienylethene hexagons 6 and 7 and their structural transformations. Absorption (B) and emission (C) spectral changes of hexagon 6o upon irradiation at 365 nm. This figure is used with permission from Chen et al.^[30].

In 2019, Li et al.^[32] reported the conformation-dependent self-assembled behaviors of diarylethenes. The diarylethene-containing ligand 8 has two conformations, antiparallel (8o) and parallel (p-8), and only the antiparallel conformation 8o is photoactive [Figure 3]. Due to the steric hindrance of the bulky bisbenzo(thiadiazole) group, the two conformations of the ligand could be readily separated by silica-gel chromatography. X-ray crystal indicated that the parallel conformation p-8 adopted a 65° bend angle and the antiparallel 8o showed 155° bend angle. Further self-assembly experiments manifested that p-8 could form [2+2] rhombus 9 or [3+3] triangle 10 with 120° or 180° di-platinum acceptors, respectively, while [3+3] and [6+6] hexagons (11 and 12) were obtained for 8o, which was in agreement with the principles of “symmetry interaction” and “directional bonding”. By mixing p-8 and 8o together with 180° di-platinum acceptors, [3+3] triangles and [6+6] hexagons (10 and 12) were formed individually, conforming their self-sorting behavior. Furthermore, NMR spectra and UV-Vis and fluorescence spectra were conducted to study their photochromic property. The results indicate that the metallacycles with parallel conformation showed no photochromism and the antiparallel ones exhibited excellent photochromic property such as high photocyclization quantum yield (66.8% and 61.2% for 11 and 12, respectively) and good fatigue-resistant property. In addition, the cycloreversion quantum yields for the ring-closed metallacycles of 11 and 12 was 6.3% and 6.5%, respectively. Such a conformation-dependent self-assembled and photochromic behaviors of diarylethene-based metallacycles could be applied in developing smart materials for catalysis and precise drug delivery.

Figure 3. (A) The conversion relationship between the different diarylethene isomers. (B) Crystal structures of p-8, 8o and 8c. (C) The preparation of the metallacages 9 and 10 from p-8. (D) The preparation of the metallacages 11 and 12 from 8o. This figure is used with permission from Li et al.^[32].

Furthermore, in 2019, the two chiral conformers (P and M) of 8o were successfully separated. Similar to the racemate, the chiral diarylethene ligands could also assembly with 120° di-platinum acceptor to form [3+3] metallacycles P-11o or M-11o [Figure 4]^[33]. The metallacycles exhibited obvious CD signals similar to the ligands. Under UV irradiation, new signals appeared in the CD spectra, which were ascribed to the formation of ring-closed diarylethene. The CD signals could be repeated for 12 cycles without degradation, thus confirming the photo-controlled chirality switching within the metallacycles. In addition, the combined results from ¹H NMR, ¹³C NMR, ³¹P NMR and HPLC separation indicate the photochromic process of the three diarylethene subunits within the metallacycle was a conversion process rather than a step-by-step conversion. Furthermore, the metallacycle was doped into poly(D/L-lactic acid) to form a film. By taking advantage of the excellent photoreversibility of the optical rotation at 633 nm, non-destructive “writing”, “erasing” and “reading” were realized within the film, which showed potential applications in information encoding.

Figure 4. (A) Graphical representation of the construction of chiral metallacage P-11o form the chiral diarylethene ligand P-8o. (B) Photoisomerization between the two isomers P-11o and (R, R)-11c. (C) The CD spectral changes of P-11o under UV irradiation. (D) Schematic illustration of the non-destructive “writing”, “erasing” and “reading” with chiral metallacycles. This figure is used with permission from Li et al.^[33].

As for the construction of photoresponsive chirality-tunable metallacycles, except for the self-assembly of chiral diarylethene ligands with metal, integration of achiral diarylethene with chiral building blocks into one metallacycle is another choice. In 2018, a series of metallacycles containing both photochromic diarylethene subunits and chiral 1,1’-bi-2-naphthol (BINOL) subunits was prepared by Yang via coordination-driven self-assembly and their photoresponsive chirality-tunable property was investigated^[34]. As shown in Figure 5, by mixing two ligands, (S)-13 and 14 or 15, in acetone/H₂O at 55 °C for 8 h and subsequent anion change with KPF₆, two [2+2] rhombus (S)-16o and (S)-17o were readily obtained. The metallacycles were well characterized with NMR spectra and ESI-TOF-MS. The photochromic studies from UV and NMR spectra indicated the metallacycles showed obvious photochromism and moderate fatigue-resistant property. Interestingly, under irradiation at 365 nm, an increase in the intensity of CD signals at 240 and 360 nm for the metallacycle (S)-17o was observed, which was probably due to the dihedral angle changes of BONIL units induced by geometrical structure transformation of diarylethene units during photochromism. Moreover, the photoresponsive chirality regulation was fully reversible under UV and visible light irradiation, which might be useful for the construction of photoresponsive supramolecular systems for asymmetric catalysis and chiral recognition.

Figure 5. (A) Graphical representation of the synthesis of metallacages (S)-16o and (S)-17o and their structural transformations. (B) Absorption spectral changes of metallacycle (S)-17o under 365 nm irradiation. (C) CD spectral changes of metallacycle (S)-17o under 365 nm irradiation. This figure is used with permission from Wang et al.^[34].

It should be noted that, even though the above-mentioned diarylethene-based metallacycles exhibited excellent photophysical properties and ingratiating applications, they showed no fluorescence or weak fluorescence whether in the ring-open form or ring-closed form. The possible reason might be the competition between photochromic process and luminescence process, which thus hampered their further applications. In 2019, Qin et al.^[35] reported the construction of a series of diarylethene-based metallacycles with brilliant fluorescence through in situ light irradiation. As shown in Figure 6, treating the diarylethene ligands 18o and 19o bearing the 1,2-bis(2-ethyl-6-iodo-1-benzothiophen-1,1-dioxide-3-yl) perfluorocyclopentene group with 120° di-platinum acceptor afforded the two [3+3] hexagons 20o and 21o, respectively, almost quantitatively. The metallacycles were well characterized with NMR spectra and ESI-TOF-MS. DFT calculations indicated that the antiparallel configurations of the ligands were close to 120°, thus resulting in the facile construction of [3+3] hexagons. Under UV irradiation, the color of the solution for metallacycle 20o or 21o changed from colorless to yellow or red, respectively, accompanied by the appearance of a new absorption peak at 460 or 505 nm, respectively. More importantly, the metallacycles initially showed no fluorescence in solution but exhibited bright yellow or orange fluorescence under UV irradiation for 20o or 21o, respectively, which is different from most previous photochromic systems. Moreover, the ring-closed form metallacycles displayed relatively high quantum yields (Φ_F = 0.5), and the fluorescence wavelength and lifetime of the metallacycles were almost not affected by metal coordination. This study realized in situ construction of high-emissive metallacycles under irradiation, which could be helpful in developing supramolecular materials for super-resolution fluorescence imaging.

Figure 6. (A) Graphical representation of the self-assembled metallacycles 20o and 21o from the diarylethene ligands 18o and 19o and their structural transformations. (B) Absorption and fluorescent spectral changes of metallacycle 20o under 365 nm irradiation. This figure is used with permission from Qin et al.^[35].

Diarylethene attached to the periphery of metallacycles

Another way to construct diarylethene-based metallacycles is to attach the diarylethene units onto the periphery of metallacycles. In recent years, photoswitchable fluorescent systems have gained increasing attention due to their applications in super-resolution imaging, anti-counterfeiting and information storage. Qin et al.^[36] reported the synthesis of a heterometallic Ir-Pt macrocycle with photoswitchable luminescence behavior in 2019. Through treatment of the Ir complex-containing 60° dipyridinyl donor 22 and the diarylethene-containing 120° di-platinum acceptor 23o at 55 °C, the [2+2] rhombus 24o was readily obtained [Figure 7]. Under irradiation at 365 nm, a new absorption band at 560 nm appeared, accompanied by an obvious color change of the solution from light-yellow to purple due to the formation of ring-closed form diarylethene. Moreover, the fluorescence of the metallacycle 24o was dramatically declined, which was ascribed to the intramolecular Förster resonance energy transfer (FRET) between the Ir complex and ring-closed form diarylethene. In addition, the absorption and fluorescence of the metallacycle could be recovered under visible light irradiation and the metallacycle showed good fatigue-resistant property. DFT calculation indicated that the distance between the two building blocks was about 3.0 nm and the energy level was suitable, thus demonstrating the efficient photoswitchable FRET behavior within the metallacycle. Furthermore, visual experiments of the metallacycle 24o were conducted and the metallacycle exhibited obvious light-controlled luminescence switching both on filter paper and poly (vinylidene fluoride) film, which showed potential applications in photoswitchable molecular devices and information storage.

Figure 7. (A) Synthetic route of the dual-functional metallacycle 24o from 22 and 23o. (B) Absorption and emission spectral changes of metallacycle 24o under 365 nm irradiation. (C) Light-controlled luminescence switching on a filter paper and in the poly (vinylidene fluoride) film. This figure is used with permission from Qin et al.^[36].

Controllable generation of singlet oxygen (¹O₂) plays an important role in decreasing the side effects to normal tissues during photodynamic therapy. In 2019, a dual-stage metallacycle containing both porphyrin photosensitizer and diarylethene photochromic switch was reported by Yang’s group, which featured interesting light-controlled generation of ¹O₂^[37]. As shown in Figure 8, the [3+3] hexagon 26o was prepared by mixing a porphyrin-functionalized 120° dipyridinyl donor 25 and the diarylethene-containing 120° di-platinum acceptor 23o via coordination-driven self-assembly. The combined results from NMR spectra and absorption and fluorescence spectra manifest that the metallacycle 26o possessed excellent photochromic property and good fatigue-resistant property. Furthermore, the ¹O₂ generation ability of the metallacycle at different state was investigated by using singlet oxygen sensor green (SOSG) as the probe. It was found that the ring-open form metallacycle 26o showed efficient ¹O₂ generation, whereas the ring-closed form metallacycle 26c showed almost no ¹O₂ generation. The results from theoretical calculations reveal that such a behavior of controllable generation of ¹O₂ was ascribed to the photoswitchable energy transfer from porphyrin to diarylethene photoswitch. Furthermore, to explore the possibility of the applications in biological systems, the metallacycles 26o and 26c were encapsulated into an amphiphilic polymer mPEG-DSPE, leading to the formation of nanoparticles. The experimental results indicate that O-NPs (the ring-open form of nanoparticles) showed a higher ¹O₂ generation ability than C-NPs (the ring-closed form of nanoparticles). Further in vitro and in vivo experiments both indicated that O-NPs showed stronger anti-tumor effects under irradiation than C-NPs because of the higher ¹O₂ generation ability of O-NPs. Thus, light-controlled generation of singlet oxygen was achieved in the discrete supramolecular coordination complexes, which provided a blueprint for the development of SCCs for biological applications.

Figure 8. (A) Graphical representation of the light-controlled generation of ¹O₂ within the metallacycle 26 and the nanoparticles. (B) In vivo photodynamic therapy efficacy on HeLa tumor-bearing mice of the nanoparticles. This figure is used with permission from Qin et al.^[37].

Diarylethene-based metallacages via Pd-ligand coordination

Han et al.^[38] made significant contributions in the field of diarylethene-based metallacages. In 2013, they reported the preparation of a diarylethene-based metallacage for the first time, which exhibited light-induced guest uptake/release behavior. By reaction of the diarylethene-containing dipyridinyl ligand 27o and [Pd(CH₃CN)₄](BF₄)₂ in a 2:1 ratio in CD₃CN, the M₂L₄-type metallacage 28o was formed quantitatively [Figure 9]. The metallacage was well characterized with 1D and 2D NMR spectra and MS spectra. X-ray crystal structure revealed that the two {Pd(pyridine)₄} planes were not eventually congruent but led to the congruent shape of the metallacage, which might be due to the flexible conformation of diarylethene moiety. In addition, the metallacage 28o showed excellent photochromic property by monitoring the changes of NMR spectra and absorption spectra. The results of 2D diffusion-ordered spectroscopy (DOSY) NMR measurements indicate that the ring-open form and ring-closed form metallacages had different cavity sizes, 7.04 and 8.67 Å for 28o and 28c, respectively. The relatively smaller size of the ring-open form metallacage was probably due to the conformational flexibility of the ring-open form diarylethene and the rigid structure of the ring-closed form diarylethene. Furthermore, the host/guest behavior of the metallacage was investigated. It was demonstrated that dodecafluorododecaborate anion ([B₁₂F₁₂]^2-) could be efficiently encapsulated in the cage with bonding constants of K₂₉₃ = 3.2 × 10⁴ M^-1 and K₂₉₃ = 6.7 × 10² M^-1 for 28o and 28c, respectively. The stronger affinity of 28o for the guest was probably due to the conformational flexibility of ring-open form diarylethene, which exhibited an induction-fit effect, while the rigid structure of 28c was not able to adjust its cavity to match the guest. By taking advantage of the light-triggered structural transformation of the metallacage, photoinduced guest uptake/release was realized.

Figure 9. Photoisomerization of (A) the diarylethene ligand 27 and (B) the metallacage 28. (C) Cage 28o shows a much stronger affinity for G (G = [B₁₂F₁₂]^2-) than cage 28c.

Furthermore, in 2016, Han et al.^[39] reported light-controlled structural conversion between a self-assembled triangle and a rhombicuboctahedral sphere within diarylethene-based SCCs. As shown in Figure 10, by heating a mixture of the diarylethene-based dipyridinyl ligand 29o at the para-position with [Pd(CH₃CN)₄](BF₄)₂ in a 2:1 ratio in CD₃CN at 70 °C, the M₃L₆ three-membered-ring product 30 with a small amount of M₄L₈ four-membered-ring product was obtained, as confirmed with 1D and 2D NMR spectra and MS spectra. Under UV irradiation, the mixture of the triangle and square converted into a new specie with broad NMR spectra signals, which was confirmed to be M₂₄L₄₈ type rhombicuboctahedron 32c by CSI-MS spectra. The results from transmission electron microscopy (TEM), atomic force microscopy and grazing-incidence small-angle X-ray scattering (GISAXS) experiments indicate that this large metallacage 32c showed a diameter of about 6-7 nm, which was consistent with the result from 2D DOSY spectra (7.0 nm). Furthermore, the photoreaction dynamics of the metallacage was studied, which revealed that a very long time (15 h) was needed for the complete structural conversion from 30 to 32c. The possible reason is that the diarylethene units within the ring-open metallacage adopted a photoinactive parallel conformation and the tiny amount of free ligand in the mixture transferred into ring-closed form under irradiation, thus leading to a slow equilibrium shifting. In addition, the reverse conversion of M₂₄L₄₈ rhombicuboctahedron into the small M₃L₆ rings was not straightforward and the ring-closed form diarylethene in the rhombicuboctahedron recovered into the ring-open state, followed by the slow disassembly-reassembly into the small M₃L₆ rings. This study realized the light-controlled structural conversion between different self-assembled SCCs, which played an important role in developing SCC-based molecular machines. In addition, some other diarylethene-based metallacages were also constructed by Li et al.^[40-42] and exhibited interesting properties and functions.

Figure 10. Graphical representation of self-assembly and structural conversion between a self-assembled triangle and a rhombicuboctahedral sphere under light irradiation.

In 2018, Gu et al.^[43] realized light-controlled topology transformation in polymer networks by making use of diarylethene-based metallacages as crosslinks. Firstly, the two terminals of the PEG chains were functionalized with diarylethene-based dipyridinyl ligand to afford the polymer ligand 35o [Figure 11]. By treatment of the 35o with 1.0 eq. Pd(CH₃CN)₄(BF₄)₂ in CH₃CN, a dark-brown gel (o-gel) with the diarylethene in ring-open form was obtained. Under UV irradiation, the o-gel transformed into a dark-blue gel (c-gel) due to the ring-closure reactions of diarylethenes. It should be noted that discrete diarylethene-based metallacages constituted the crosslinks of the polymer gels. Similar to the polymer ligand 35o, the model diarylethene ligand without PEG chain exhibited structural conversion between small Pd₃L₆ triangles and large Pd₂₄L₄₈ rhombicuboctahedra under UV and visible light irradiation, respectively, as confirmed by NMR and ESI-TOF-MS spectra. Thus, the structural changes of the crosslinks led to a substantial change of the polymer topology networks of the polymer gel, which played an important role in the physical properties including storage modulus G’ and self-healing properties. For example, the c-gel showed 1.7 times higher G’ than the o-gel (8.3 and 14.1 kPa for o-gel and c-gel, respectively) due to more topological defects in the c-gel. In addition, the results of SAXS and DFT calculations reveal that the c-gel had a higher defect tolerance and less dynamics than o-gel, which was derived from the inherent high branch functionality of the c-gel. Thus, the o-gel exhibited remarkable self-healing properties, while the c-gel showed no self-healing under the same condition. Moreover, the polymer gel showed a moderate fatigue-resistant property, as confirmed by the shear modulus, SAXS and molecular calculations. This study successfully prepared a photoresponsive polymer gel by using diarylethene-based metallacages as crosslinks. The resulting o-gel and c-gel showed remarkable difference in storage modulus and self-healing properties due to the photoswitchable topology transformation in polymer networks under light irradiation.

Figure 11. (A) Schematic illustration of light-induced structural interconversion between two different network topologies. (B) Chemical structure of the photoresponsive polymer ligand 35. (C) Photographs of self-healing experiments of the polymer gel (o-gel and c-gel). This figure is used with permission from Gu et al.^[43].

Except for the induction of polymer chain on diarylethene ligand for the construction of photoresponsive gels, the diarylethene-based metallacage itself could also form supramolecular gels under suitable conditions. In 2015, Wei et al.^[44] reported a diarylethene-based M₂L₄ type metallacage with multiresponsivity. The metallacage 37 was similar to the previous reported one (28). Under UV and visible light irradiation, the metallacage 37 showed excellent photochromic property with full reversibility. Interestingly, the ring-open form metallacage 37o exhibited solution-to-gel transformation in common solvents such as DMF and DMSO at room temperature [Figure 12]. Further, the combined results from NMR spectra, SEM, TEM, dynamic light scattering and rheological tests indicate that the formation of gel was affected by temperature, light, coordination anions and shear force, thus endowing the gel with multi-stimuli-responsive property. The photo-induced solution-to-gel transformation was ascribed to the flexible conformation of the ring-open form diarylethene, which was beneficial for the multiple noncovalent interactions between the metallacage. In addition, the porous metallacage kept intact during the solution/gel transformation, which was helpful in developing new porous materials with multi-stimuli-responsivity.

Figure 12. The multi-stimuli-responsive behavior of the metallacage 37. This figure is used with permission from Wei et al.^[44].

Diarylethene-based metallacages through coordination with other metals

In 2019, Li et al.^[45] reported the controlled modulation of the magnetic properties within diarylethene-based metallacages. Treatment of the diarylethene ligand 38o with Fe(OTf)₂ under room temperature afforded the metallacage Fe²⁺₂(38o)₃[Figure 13]. X-ray crystallography analysis indicated that the diarylethene ligand within Fe²⁺₂(38o)₃was in the antiparallel conformation and the metallacage adopted a triple-stranded helical structure with an average Fe-N distance of 1.972 Å, suggesting its low-spin (LS) diamagnetic Fe^II state. Under irradiation at 365 nm, the ring-open form Fe²⁺₂(38o)₃converted into its ring-closed form Fe²⁺₂(38c)₃. Moreover, DFT calculations indicated that Fe²⁺₂(38c)₃exhibited a higher energy (48.5 kcal/mol) in the closed shell singlet state than Fe²⁺₂(38o)₃, further demonstrating their structural difference. In addition, a tetranuclear square complex Fe³⁺₂Fe²⁺₂(38o)₂was obtained through reaction of the ligand 38o with Fe(OTf)₂ and (TEA)[Fe^III(Tp*)(CN)₃]·H₂O at 320 K. X-ray crystal of Fe³⁺₂Fe²⁺₂(38o)₂revealed the parallel conformation of diarylethenes, which was photoinactive under UV irradiation. Moreover, the average of length of Fe-N bond in Fe³⁺₂Fe²⁺₂(38o)₂was 1.957 and 1.965 Å at 100 and 295 K, respectively, indicating its LS Fe^II state. It should be noted that Fe²⁺₂(38o)₃and Fe²⁺₂(38c)₃could transform into Fe³⁺₂Fe²⁺₂(38o)₂with the addition of [Fe^III(Tp*)(CN)₃]^- at 320 K. This finding demonstrates that Fe²⁺₂(38o)₃was dynamically stable, while Fe³⁺₂Fe²⁺₂(38o)₂was thermodynamically stable. Furthermore, the magnetic properties of these three metallacages were investigated by superconducting quantum interference device (SQUID) experiments. The results indicate that Fe²⁺₂(38o)₃was diamagnetic when the temperature was lower than 400 K and Fe²⁺₂(38c)₃showed paramagnetism and incomplete spin-crossover behavior (30% high-spin). Fe³⁺₂Fe²⁺₂(38o)₂ exhibited paramagnetism and complete spin-crossover behavior with a spin-transition temperature at about 353 K, corresponding to the results from variable-temperature IR and ⁵⁷Fe Mössbauer spectra. In addition, experiments on cyclic voltammetry measurements indicated both Fe²⁺₂(38o)₃ and Fe³⁺₂Fe²⁺₂(38o)₂ showed extremely high electrochemical stabilities. This study realized the construction of diarylethene-based metallacages with switchable magnetism, which is very helpful for developing SCCs for molecular magnetic devices in the future.

Figure 13. (A) Synthetic routes of the metallacages form the diarylethene ligands 38o and 38c. (B) Crystal structures of Fe²⁺₂(38o)₃ and Fe³⁺₂Fe²⁺₂(38o)₂ and optimized structures of Fe²⁺₂(38c)₃. (C) Magnetic properties of three metallacages. This figure is used with permission from Li et al.^[45].

Rare-earth metals can also be used to construct SCCs. In 2018, Cai et al.^[46] reported the construction of a chiral diarylethene-based metallacage via Eu coordination and studied its photochromic behavior. As shown in Figure 14, by reacting of the diarylethene-containing chiral ligand 39 with Eu(ClO₄)₃·6H₂O in a 3:2 ratio in CH₃CN, the Eu₂L₃ helicate 40o was obtained quantitatively, which was well characterized with NMR and ESI-TOF-MS spectra. In particular, CD experiments disclosed that the helicates showed extremely strong CD signals as compared to the weak CD signal of the ligands, which was ascribed to the chirality induction from the strong cooperative mechanical coupling effects within the helicates. DFT calculations revealed that the point-chirality of the ligands could be transferred to metal centers, which further led to the formation of helical chirality. In addition, the helicate showed excellent photochromic properties under light irradiation and the ring-closed form helicate also showed strong CD signals. However, stereo-controlled ring closure of the helicates failed, probably due to their relatively flexible structures, which resulted in fast ligand exchanges within the helicates. Moreover, the helicate exhibited a photoswitchable luminescence property. The luminescence of the typical Eu complex at 615 nm was enhanced under UV irradiation, thus offering an opportunity to develop erasable molecular devices.

Figure 14. (A) Synthesis of the helicates P-40o and M-40o. CD spectra of P-40o and M-40o (B) and P-40c and M-40c (C). (D) Excitation and emission spectra of the helicates. This figure is used with permission from Cai et al.^[46].

Subcomponent self-assembly is an important method for constructing multifunctional SCCs. Recently, Zhang et al.^[47] constructed a series of diarylethene-based metallacages via subcomponent self-assembly, which was then reduced to pure covalent-organic cages, and investigated their photochromic behavior and applications in multi-readout logic gate systems. As shown in Figure 15, the metallacages 44o-46o were obtained through the reaction of 41o, tris(2-aminoethyl) amine (42) and Zn(OTf)₂ or Cd(OTf)₂ in CH₃CN/CHCl₃. X-ray crystal structures disclosed that the metallacages both adopted triple-helicate structures with larger cavities for 45o and 46o. Further experiments on UV-Vis absorption spectra and NMR spectra demonstrated that the efficient photochromism could occur within the metallacages 44o-46o, but only one diarylethene subunit could be converted into the ring-closed form due to the spatial hindrance in the metallacages. Furthermore, the metallacages were reduced and demetalated with NaBH₄ to afford the covalent-organic cages 47o and 48o, which showed similar photochromic properties to the metallacages (only one diarylethene subunit could be converted into the ring-closed form). It should be noted that the photoisomerization dynamic rates from the ring-open form to ring-closed form of metallacages 44o-46o and organic cages 47o and 48o were largely decreased as compared to the ligand 41o, due to the steric hindrance of the cage structure. Host/guest test of the resultant covalent cages indicated that they could bind HSO₄^- or H₂PO₄^- anions through hydrogen bonding. The addition of anions decreased the photoconversion yield because of the restricted diarylethene skeletons. Furthermore, the metallacages and organic cages were doped with rare earth upconversion materials to form hybrid materials, and the hybrid materials exhibited potential for multi-readout logic gate applications.

Figure 15. Synthesis route of a series of metallacages 44o-46o via subcomponent self-assembly and subsequent reduction into covalent-organic cages 47o and 48o.