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Review

Metal-Organic Cages Based on Phosphorescent Organometallics

by
Yunliang Yu
1,2,3,
Xiaoxia Wang
1,
Yuliang Liu
2,* and
Chao Zou
2,*
1
School of Materials Science and Engineering, Dongguan University of Technology, Dongguan 523808, China
2
Functional Coordination Material Group, Frontier Research Center, Songshan Lake Materials Laboratory, Dongguan 523808, China
3
School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, China
*
Authors to whom correspondence should be addressed.
Inorganics 2023, 11(11), 436; https://doi.org/10.3390/inorganics11110436
Submission received: 10 October 2023 / Revised: 28 October 2023 / Accepted: 30 October 2023 / Published: 14 November 2023
(This article belongs to the Special Issue Phosphorescent Materials Based on Organometallic Complexes)
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Abstract

:
During the last two decades, metal-organic cages (MOCs) have been extensively investigated and well documented. Meanwhile, phosphorescent MOCs have emerged as a kind of new MOC material but have not been given much attention. The diversity of their structures and their flexibility of self-assembly result in various luminescent behaviors. Additionally, their special photoactive properties are quite attractive in the background of photochemistry and worthy of discussion. Here, we would like to introduce the recent development of phosphorescent MOCs, including their structures, syntheses, photophysical properties and possible applications. This minireview may hopefully inspire the development of novel phosphorescent MOCs and also facilitate promising applications.

Graphical Abstract

1. Introduction

Porous materials, such as metal-organic frameworks (MOFs), have drawn extensive attention in the last few decades [1,2]. In addition, metal-organic cages (MOCs) or metal-organic polyhedrons (MOPs) have also aroused the growing interest of chemists as they have a similar structure to MOFs along with discrete self-assembly, well-defined shapes, extensive porosity and a superior solubility to that of MOFs [3,4,5,6,7,8]. Since the pioneering work of modern MOCs reported by Saalfrank in 1988 [9], a number of efforts have been devoted to the library of MOCs by Stang [10], Fujita [11], Raymond [12], Cotton [13], Cook [14], Mirkin [15], Hosono and Kitagawa [16], Nitschke [17] and others. A variety of excellent works about these delicate cages have emerged in rapid succession since 2008 (Figure 1, blue column). Among these cages, photoactive cages have recently become a research focus. The high concentration of chromophores and tunable cavities in luminescent MOCs results in host-guest optoelectronic interactions, which have various potential applications, such as in imaging, nonlinear optics, sensors and photocatalysis [18]. However, works on luminescent MOCs are still sparse (Figure 1, shadow column).
Luminescence usually includes fluorescence and phosphorescence. Fluorescence is produced by the radiative electronic transition from the lower vibrational energy level of an excited singlet (S1) to the ground state (S0). Phosphorescence is generated from the excited triplet state (T1) to the ground state (S0). According to the mechanism of luminescence in metal-organic porous materials, it commonly results from direct metal-centered emissions (usually examined in lanthanide metals), organic ligand excitation (especially from highly conjugated ligands) and charge transfer, including metal-to-ligand charge transfer (MLCT) and ligand-to-metal charge transfer (LMCT). Moreover, the guest molecules in porous materials can also cause luminescence [19,20,21].
Since the rational design and synthesis of MOF materials by researchers, luminescent MOF materials have made great strides. Although people do not often distinguish between the luminescence mechanisms (fluorescence, phosphorescence, etc.) of MOF materials in this field, long-lived luminescent MOF materials (mostly phosphorescence), especially those built with classic ruthenium(II) and iridium(III) metal organic units, combining their adjustable pore structures, have been developed for wide applications in photocatalysis, biological imaging and photodynamic therapy, among other fields [19,20,21]. MOF materials cannot be dissolved in a solution in a discrete state, which limits their processing and efficient application. However, as porous MOC materials are built by metal organic coordination bonds and their molecularly structured discrete cages can be dissolved in solvents, they hold promise for solving these problems.
Even though luminescent MOC materials are often reported in the literature, the issue of not distinguishing and discussing their luminescence mechanisms (fluorescence, phosphorescence, etc.) still exists. While a number of fluorescent MOCs have been developed by many research groups [22,23], less attention has been paid to the phosphorescence of MOCs. Phosphorescence is widely observed in d-block metal-organic cages, in particular, those containing d6, d8 and d10 electronic configurations of second and third-row transition metal ions. These heavy metal atoms not only strengthen spinorbit coupling which could result in efficient intersystem crossing (ISC) from S1 to T1 manifolds under photoexcitation, but also relax the spin selection rule which could improve the spin forbidden transition from T1 to S0 [24]. Zysman-Colman and his colleagues presented photoactive coordination cages incorporating d-block ruthenium(II) and iridium(III) transition metal complexes in 2018 [18]. In this minireview, we will mainly focus on MOCs derived from phosphorescent organometallic skeletons, especially those incorporating d-block transition metal complexes, and provide an outlook for the basic research and applications of these phosphorescent MOCs. The classification of phosphorescent MOCs mentioned in this minireview is based on the metal type and photophysical properties (Table 1).

2. Metal-Organic Cages Based on Phosphorescent Organometallics

2.1. Phosphorescent MOCs Incorporating d6 Metal Ion

As important luminophores, phosphorescent transition-metal complexes with a d6 electronic configuration, especially ruthenium(II) and iridium(III) complexes, whose excited states include metal-to-ligand charge transfer (MLCT), exhibit superior photophysical, thermal and chemical stabilities [40]. Therefore, phosphorescent d6 transition-metal complexes are usually used for the self-assembly of phosphorescent MOCs.
In 2017, Campagna, Hanan and their colleagues reported a photo-induced luminescent tetraruthenium square, MOC-Ru-1, assembled from four terpyridine and four bipyridine ligands with Ru(II) ions as nodes (Scheme 1) [25]. MOC-Ru-1 possesses the typical 3MLCT emission of Ru(II) polypyridine complexes at 745 nm with an emission lifetime (τPL) of 135 ns (40 times greater than that of its mononuclear precursor) and a quantum yield (ΦPL) of 1% in acetonitrile solution at room temperature (Figure 2). This case offers a new route for the assembly of supramolecules via photochemical techniques.
Another ruthenium(II) cage (MOC-Ru-2) with Ru(II) polypyridine complexes as nodes was synthesized by Cook and co-workers through the coordination of 2,4,6-tris(4-pyridyl)-1,3,5-triazine (TPT, 2) with cis-bis(2,2′-bipyridine)ruthenium(II) Ru3 (Scheme 2) [26]. They investigated the photophysical properties of MOC-Ru-2 in acetonitrile at two different temperatures. The λmax of emissions was 577 nm and 689 nm at room temperature and 77 K, respectively, with a very low ΦPL value of less than 0.1% and biexponential excited state lifetimes (τPL) of 2 ns and 790 ns. There is still a red shift of about 0.35 eV (~112 nm) and a biexponential decay in the 77 K emission spectrum compared with those of the room temperature emission, which probably accounted for the existence of two separate but thermally equilibrated triplet excited states, 3ML(bpyπ*)CT and 3ML(TPTπ*)CT (Figure 3). The thermal population of this high-energy 3ML(bpyπ*)CT did not happen at 77 K, and the lower-energy 3ML(TPTπ*)CT became mainly populated. It is the two separate triplet excited states that give the cage its unique photophysical properties, which encourages the design of new MOCs.
Besides being metal nodes, ruthenium(II) complexes can also be used as ligands for the construction of MOCs. In 2014, Su’s group designed and synthesized a nanosized heteronuclear homochiral Pd6Ru8 metal-organic cage (MOC-Ru-3, Scheme 3) [27,41]. A series of functional and practical applications using this MOC were developed for chiral separation [41], photochemical hydrogen production [42,43], biological imaging [44] and enantioselective photocatalysis [45,46,47]. The single-crystal X-ray diffraction of this cage exhibited a truncated octahedral, or more accurately, a rhombododecahedron structure with eight metalloligands with Ru4 comprising the eight faces and six PdN4 planes truncating the six vertices of the octahedron. The enantiomers Δ- or Λ-MOC-Ru-3 were generated following four steps. Starting with the chiral induction agent, K2{Sb2[(+)-tartrate]2}·3H2O, the racemate [Ru(phen)3]2+ was first isolated into a pair of enantiomers (Δ- or Λ-[Ru(phen)3]2+). After oxidizing into Δ- or Λ- [Ru(phendione)3]2+, Δ- or Λ-Ru4 metalloligands were obtained from Δ- or Λ[Ru(phendione)3]2+ reacting with 3-pyridinecarboxaldehyde. The final step was the coordination of Δ- or Λ-Ru4 metalloligands and Pd2+ ions to form homochiral Δ- or Λ-MOC-Ru-3, respectively. The circular dichroism (CD) spectra of homochiral Δ- or Λ-MOC-R-3 were measured to monitor their chirality (Scheme 3). Compared with the free Ru4 metalloligand, MOC-Ru-3 displayed a similar absorption and emission spectrum in DMSO (Figure 4). The emission maxima at ca. 610 nm stems from Ru(phen)3-centerd triplet 3MLCT states with a τPL of 601 ns for Ru4 and 484 ns for MOC-Ru-3. A strong π-π* transition from the intraligand (at 290 nm) in the ultraviolet region and a broad peak corresponding to the metal-to-ligand charge transfer (1MLCT from Ru to Phen) in the 400–550 nm visible region were observed. The molar absorptivity was intensified at every wavelength, which indicated that the assembly of multiple chromophoric RuL3 units could form a good light absorber. However, the emission intensity of MOC-Ru-3 was decreased by ~32% because of the intramolecular charge transfer from RuL3 to the Pd(pyridine)4 moieties. In addition, both the DFT calculations and ultrafast transient absorption spectrum demonstrated the electronic structure of MOC-Ru-3. At the beginning, the electron was excited from the [Ru(phen)3]2+ chromophore at 400 nm to the 1MLCT state, then it transferred into the 3MLCT state of the phenanthroline via intersystem crossing (ISC). Followed by the intraligand charge transfer (ILCT) process from phen to biim-py, the excited state relaxation occurred and subsequently arrived to the Pd catalytic center through a much slower process of ligand-to-metal charge transfer (LMCT) (Scheme 4).
Various applications, especially in photocatalysis, have been developed with this multifunctional MOC. The 1,4-coupling of 3-bromo-2-naphthol in a stereochemical control was realized via photoinduction (Scheme 4a) [45]. Under Δ- or Λ-MOC-Ru-3, a major R- or S-1,4′-bis(2-naphthol) product was obtained with both 56% ee, respectively. Another photocatalytic reaction, the selective intermolecular [2 + 2] cycloaddition of α,β-unsaturated carbonyl compounds, was also achieved by MOC-Ru-3 (Scheme 4b) [46]. This highly efficient and robust cage catalyst could acquire syn-HH diastereomers with an excellent diastereoselectivity (up to 99%) and substrate tolerance. Recently, the same group continued to further study photocatalytic reactions with MOC-Ru-3 and reported the Meerwein arylation of alkenes with controllable E/Z selectivity using this cage (Scheme 4c) [47]. The desired products were obtained with a high stereoselectivity (E/Z up to 99/1). Moreover, major Z-isomers could be formed through the control of the solvent.
Beves and his colleagues also designed Ru-Pd metal-organic cages (MOC-Ru-4) with an inert [Ru(tpy)2]2+ block (tpy = 2,2′,6′,2″-terpyridine) decorated with pendant pyridyl rings, which were capable of coordinating to square planar Pd(II) complexes (Scheme 5) [28]. Only a single major species with a tetrameric structure in solution was generated in the reaction of the Ru ligand complex Ru5 with two equivalents of Pd(dppp)(OTf)2 (dppp = 1,3-diphenylphosphinopropane) in nitromethane at room temperature (Scheme 5a, MOC-Ru-4a). A box-like structure (MOC-Ru-4a crystallized in the P 1 ¯ space group) with dimensions of ca. 21 × 21 × 32 Å was observed with Pd(II) centers at each end to construct near-perfect squares (with Pd-Pd-Pd angles of 86.0–92.8° and Pd⋯Pd distances of 13.2–13.4 Å). [Ru(tpy)2] units were located at the center of the cage with alternating Ru⋯Ru distances of 11.82 Å and 8.78 Å. The analogous reaction by using the Ru ligand complex Ru6 obtained a trimeric structure instead of a tetrameric structure (Scheme 5b). These cages have similar photophysical properties with corresponding Ru ligand complexes. The maxima of the 1MLCT absorption of Ru ligand complexes Ru5, Ru6, MOC-Ru-4a and MOC-Ru-4b were at λmax = 490 nm (Figure 5a). And the 3MLCT emission spectra were weak at 640 nm with short mono-exponential excited state lifetimes of 1.26 ± 0.01 ns and 1.21 ± 0.01 ns, respectively (Figure 5b).
Another phosphorescent Ru-Pd heteronuclear metal-organic cage (MOC-Ru-5) was assembled featuring the metalloligand Ru7 [Ru(dtbubpy)2(qpy)]2+ (qpy is 4,4′:2′,2″:4″,4‴-quaterpyridine and dtbubpy is 4,4′-di-tertbutyl-2,2′-bipyridine) and Pd2+ ions by Zysman-Colman’s group in 2018 [29]. It is a square arrangement with two ligands doubly bridging adjacent Pd(II) centers in a crown-like fashion, disposing of the four palladium ions (Scheme 6). Different from those of Ruqpy (λPL = 674 nm, ΦPL = 7.3%), the emission properties of MOC-Ru-5 in DCM (λPL = 710 nm, ΦPL = 6.9%) were broader and had a greater red shift owing to the coordination of the Lewis acidic PdII ions to the ruthenium complex (Figure 6). The πqpy* orbital level involved in the emission could be stabilized by their coordination, leading to the lower energy of the triplet state. The emission of Ruqpy and MOC-Ru-5 decayed biexponentially with lifetimes (τPL) of 324 and 1047 ns, 151 and 700 ns, respectively (Figure 6), which indicated the Pd(II) ions in MOC-Ru-5 did not adversely affect the photophysical properties of Ru ligand complexes.
Although plenty of Ru-Pd cages have been reported, the emissive nature of these cages is often partially or completely quenched by Pd(II) ions due to non-emissive charge-transfer states between the Ru and Pd centers [18]. Thus, a feasible strategy is to separately append the functionalized units as a pendant ligand at the exohedral [48] or endohedral [49] faces of the assembled cage for electronically isolating the emissive Ru(II) ligand complexes from the Pd(II) ions. In 2016, Crowley, Gordon and their colleagues synthesized [Pd2L4] cages with photoactive ruthenium complexes attached at the exohedral face (Scheme 7, MOC-Ru-6) [30]. The exo-functionalized ligands were obtained by using the functional tolerant group “click” reaction [50] of copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC). The photophysical properties of MOC-Ru-6a and 6b (ruthenium-based 3MLCT emissions; λPL = 620 and 638 nm; ΦPL = 0.2% and 2.6%; τPL = 20 and 659 ns, respectively) were almost retained with their parent Ru ligand complexes Ru8 (λPL = 620 and 638 nm; ΦPL = 0.2% and 6.5%; τPL = 21 and 943 ns, respectively) (Figure 7). This strategy offered a practicable way to keep the functionalization of the parent system in the assembled cages without compromising their photophysical properties.
Later, analogous exo-functionalized [Pd2L4]12+ cages were demonstrated with Ru(II) pyridine complexes by Casini, Kühn and co-workers (Scheme 8) [31]. The pendant Ru(II) terpyridine and Ru(II) bipyridine moieties were connected by an amide bond to form the ligands Ru10 and Ru11, which mixed with [Pd(MeCN)4](BF4)2 in DMSO at room temperature, leading to MOC-Ru-7a and 7b. A series of exhaustive spectroscopy analyses were carried out on these cages to study their photophysical properties. The absorption spectra of MOC-Ru-7 exhibited a range of bands from 250 to 350 nm, which were caused by the strong π-π* transitions of the highly conjugated ligands. Moreover, there is an additional band in the vis region (MOC-Ru-7a at 495 nm and MOC-Ru-7b at 460 nm) of the UV-vis spectra. The cages displayed interesting emissive properties, which, by varying the molecular structure of the ligand backbone, could increase or decrease the luminescence of the cages. MOC-Ru-7a and the corresponding Ru ligand did not show any emissive properties due to the disruption of the chromophoric system in the excited state by the torsion of the amide bond. To avoid bending the amide bond, an alkyl spacer as an electronic separation was used between the ruthenium moiety and the amide bond of the bis(pyridyl) ligand. Upon irradiation at 260 nm, the Ru ligand Ru11 and cage MOC-Ru-7b emitted strong orange phosphorescence at λPL = 640 nm with unusually high ΦPL values of 88% and 66%, respectively (Figure 8).
Apart from ruthenium(II) complexes, cyclometalated iridium(III) complexes, which also have a d6 electronic configuration, showed the ability to modulate their emission energy in the visible spectrum and usually were phosphorescent and chemically thermodynamically stable [51]. Therefore, iridium complexes as luminescent moieties have been employed for the assembly of metal-organic cages.
In 2012, the first 3D luminescent and enantiopure Ir(III) octahedral capsule MOC-Ir-8 ([(Ir(ppy)2)6(tcb)4](OTf)6 (tbc = 1,3,5-tricyanobenzene, 12)) was described by Lusby’s group (Figure 9) [32]. Firstly, the enantiomers Λ- and Δ-[(Ir(ppy)2Cl)2] were obtained from resolving rac-[(Ir(ppy)2Cl)2] Ir13 via the chromatographic resolution of D- and L-serine complexes and their transformation to Λ- and Δ-[(Ir(ppy)2OTf)2] by AgOTf. Then tcb was added into Λ- or Δ-[(Ir(ppy)2OTf)2] to generate corresponding Λ- or Δ-MOC-Ir-8, a truncated octahedron with triflates located in each of the octahedron windows. MOC-Ir-8 emitted orange in a tetrachloroethane (TCE) solution. The emission of Λ- or Δ-MOC-Ir-8 was broad and there was a red shift at 575 nm with ΦPL values of 4% as compared with those of a reference complex ([Ir(ppy)2(PhCN)2]OTf (λPL = 525 nm, ΦPL < 0.1%)) (Figure 10).
Subsequently, Zysman-Colman and co-workers revealed the first homochiral red-emitting metal-organic cages [Ir8Pd4]16+, Δ-, Λ- and rac-MOC-Ir-9, self-assembled by two enantiopure [52] and racemic Ir(III) ligand complexes, Ir14 and Pd2+ ions (Scheme 9) [33]. The photophysical properties of all enantiomers of the Ir(III) ligand complexes Ir14 and MOC-Ir-9 were distinguishable either in DCM solution or in polymethyl methacrylate (PMMA)-doped films. The emission profiles of Δ-, Λ- and rac-MOC-Ir-9a and Δ-, Λ- and rac-MOC-Ir-9b had red shifts at 655 nm and 561 nm in degassed DCM, respectively, with lower ΦPL values of 5% and 14%, as well as shorter τPL values of 202 ns and 825 ns, compared with those of the corresponding Δ-, Λ- and rac-Ir14 (λPL = 620 and 527 nm; ΦPL = 14% and 34%; τPL = 300 and 1000 ns, respectively). While in PMMA thin films, the emissions of Δ-Ir14a, Δ-Ir14b, Δ-, Λ- and rac-MOC-Ir-9a and Δ-, Λ- and rac-MOC-Ir-9b had blue shifts at 564 nm, 518 nm, 643 nm and 531 nm, respectively, with higher ΦPL values of 28%, 41%, 10% and 16% and longer multi-exponential τPL values than those in DCM (Figure 11).
In 2017, Zysman-Colman, Hardie and their colleagues demonstrated that two metal-organic cages, [(Ir(ppy)3)3(CTV)2]3+ MOC-Ir-10, could be self-assembled by the chiral tripodal ligands cyclotriveratrylene (CTV, 15) and rac-[Ir(ppy)2-(NCMe)2] Ir16 (Scheme 10) [34]. MOC-Ir-10 contained three pseudo-octahedrally coordinated Ir(III) centers, including two ppy ligands (Ir16) and two pyridyl groups from two CTV ligands (15) as bridges connecting with the Ir(III) centers in cis-arrangements. Notably, homochiral self-sorting could be observed during the process of self-assembly. Although there were 12 potential cage stereoisomers, only two enantiomeric cages (MM-ΛΛΛ and PP-ΔΔΔ) were generated with slow self-sorting. The emission properties of MOC-Ir-10 were examined in DCM solution, as powders and in PMMA-doped films (Figure 12). The MOC-Ir-10a powder had a red shift (λpL = 648 nm) higher than that in DCM solution (λmax = 604 nm) with low ΦPL values of around 1% in both media and short bi-exponential emission decays (τPL = 55 and 203 ns as a powder; τPL = 59 and 129 ns in DCM). Meanwhile, in the film, there was a blue shift at ΦpL = 514 nm with a higher photoluminescence quantum yield (ΦPL = 5.5%) and longer bi-exponential excited state lifetime (τPL = 634 ns and 2319 ns) (Figure 12a). For MOC-Ir-10b, a vibronic 3LC emission was more predominant, leading to a greater blue shift emission than that of MOC-Ir-10a. Simultaneously, MOC-Ir-10b possessed similar energies in DCM, as a powder and in PMMA-doped films at 516 nm with enhanced ΦPL values (15%, 1.6% and 10%, respectively) and bi-exponential τPL values (523 and 887 ns; 141 and 1175 ns; and 688 and 3042 ns, respectively) (Figure 12b).
The same researchers reported a similar structure, Ir2L3, as above with CTV-type ligands bearing azo-aromatic linkers [53]. Interestingly, the E/Z photoisomerization of the pyridyl-azo-phenyl groups in these cages was observed by photoirradiation with a high-powered laser, and reversible isomerization occurred with blue light. Despite the weak emission of these cages, there was still a deep blue light at ca. 450 nm.
In 2019, Su group presented a heteronuclear Ir8Pd4 metal-organic cage, MOC-Ir-11, of the form of the bipodal Ir(III) ligand complex [Ir(ppy)2(qpy)(BF4)] Ir17 (qpy = 4,4′:2′,2″:4″,4‴-quaterpyridine; ppy = 2-phenylpridine) and Pd(II) ions (Scheme 11) [35]. The bipodal Ir(III) ligand complex Ir17, bearing two coordinated 2-phenylpyridine units, as well as one coordinated bipyridine linker with two pendants’ pyridine (Py), was mixed with Pd(CH3CN)4(BF4)2 at 80 °C to obtain the cubic-barrelled, heteronuclear Ir8-Pd4 MOC-Ir-11. The photophysical properties of MOC-Ir-11 were investigated in the DMSO solution. The photoluminescent excitation and emission spectra of MOC-Ir-11 were similar compared to those of its corresponding Ir(III) ligand complex Ir17 with excitation λpL values of 470 and 430 nm and emission λpL values of 658 and 654 nm, respectively (Figure 13). Their excited state lifetimes (τPL) in solution of 141 and 158 ns (MOC-Ir-11 and Ir(III) ligand complex Ir17, respectively) were longer than those in the solid state with quantum yields (ΦPL) of 3.5% and 11%, respectively, at room temperature. Upon visible light irradiation, this cage exhibited one-photon and two-photon excited deep red emissions and a large singlet oxygen quantum yield, which offer possibilities for organelle-targeted cell imaging and photodynamic therapy (PDT).
The d6 metal ion Os(II) coordinated with ligands in its d-d state possessed a high amount of energy but could not provide a thermally accessible deactivation pathway, leading to the luminescence of the lowest-energy 3MLCT state. An octanuclear heterometallic coordination cage [L12(Ma)4(Mb)4]16+, MOC-Os-12 was obtained via self-assembly by the pyrazolyl-pyridine ligand 18 with Os(II) and Cd(II) (Scheme 12) [36]. The photophysical properties of MOC-Os-12 in MeCN had a broad emission at λmax = 625 nm with a ΦPL of 2.5% and biexponential lifetimes (τPL) of 156 ns (minor) and 73 ns (major).
As d6 metal-organic complexes naturally have six-coordinate octahedral building blocks, whether as node metals or skeleton components as mentioned previously, it is easier to construct phosphorescent porous structural units based on the principle of symmetrical ligands. In comparison, d8 and d10 metal-organic units adopt square planar and linear coordination modes, with axial open coordination spaces. It is relatively more difficult to establish a structurally stable 3D porous cage while retaining the luminescent properties of phosphorescent metal-organic units without quenching, and there are also fewer reported examples in the literature.

2.2. Phosphorescent MOCs Incorporating d8 Metal Ion

Due to having planar geometry with an open axial site (defined as the z-axis), d8 transition-metal complexes are apt to generate dimers (or aggregates) in the ground state or excimers in the excited state via intermolecular metal-metal and ligand-ligand interactions, resulting in metal-metal-to-ligand charge transfer (1/3MMLCT) excited states [24]. For examples, Pt(II) complexes, because of the presence of Pt(II)⋯Pt(II) interactions, usually display unique photophysical, self-assembly properties as well as conductivity [54,55], which may provide unique pathways for assembling phosphorescent cages.
In 2011, Castellano, Sallé and co-workers synthesized a series of Pt(II) square planar metallocycles (MOC-Pt-13) via the self-assembly of bridging pyridine ligands (Pt18) containing phenyleneethynylene moieties offering a ligand-localized triplet pathway (intramolecular charge transfer sensitization) for excited-state decay, contributing to their phosphorescence (Scheme 13) [37]. The phosphorescent emissions of MOC-Pt-13b and 13c centered between 524 and 527 nm with ΦPL values of 16.7% and 11.0% as well as τPL values of 15.5 μs and 17.1 μs, respectively (Figure 14).
In 2017, Cook and co-workers reported two phosphorescent tetrahedral cages in the form of bimetallic Pt-Zn, [ZnII4Pt6](PF6)8 (MOC-Pt-14) and [FeII4Pt6](OTf)8 (MOC-Pt-15), which were coordinated via Zn(NO3)2·6H2O or Fe(OTf)2 and the linker Pt(PE3)2(C≡C-bpy)2 Pt19 (Scheme 14) [14]. Four transition-metal nodes as vertices containing bipyridyl ligands from six Pt ligand complexes (Pt19) as edges composed a tetrahedral cage. The molar absorption coefficient of MOC-Pt-15 at λmax = 402 nm was significantly higher than that of MOC-Pt-14 and the Pt ligand complex Pt19 (λmax = 393 and 360 nm, respectively). The emission spectra of both cages and the Pt ligand complex Pt19 were examined in an N2-degassed solution at 25 °C (Figure 15). All of compounds exhibited very week fluorescence and differing phosphorescence values. The phosphorescence emission of MOC-Pt-4 centered at 545 nm with a ΦPL of 10% and τPL of 95 μs from an IL 3π*C≡C–bpy → 1πC≡C–bpy transition. Meanwhile, the weak phosphorescence quantum yield of MOC-Pt-15 was less than 0.1%, probably due to nonradiative competition from emissions from [Fe] nodes, introducing a low-energy 5MFeC state [56].
The strong intermolecular Pt⋯Pt interaction [57] among Pt(II) luminophores could promote aggregations and emissions through the aggregation-induced phosphorescent emission (AIPE) effect [58]. In 2020, Li’s group devised the giant, discrete metallosupramolecular concentric hexagon MOC-Pt-16 via the self-assembly of Pt(II) bzimpy Pt20 (bzimpy = 2,6-bis(benzimidazole-2′-yl)pyridine) motifs with 2,2′:6′,2″-terpyridine (tpy) and Cd(II) (Scheme 15) [38]. The Pt(II) bzimpy motif contained four parts, a terminal alkynyl group for installing the Pt(II) motif with a stable and rigid Pt(II)-alkynyl bond to connect the other parts, four tpy arms at the sides for the coordination of Cd(II) to connect the other motifs, multiple hydrophilic ethylene glycol chains on the periphery and a long alkyl chain (C12) into the interior for manipulating the aggregation via balancing the whole hydrophobicity or hydrophilicity. The final metallosupramolecular MOC-Pt-16 was assembled by six Pt(II) bzimpy motifs and twelve Cd(II), where the size was larger than 10 nm and the molecular weight was beyond 26,000 Da. The emission spectra of the Pt(II) bzimpy motif Pt20 and MOC-Pt-16 were similar due to their similar original emitting excited states in acetonitrile solutions at room temperature (Figure 16). Both of the emission wavelengths were at λPL = 567 nm with τPL values of 270 and 218 ns, respectively. In N2-deoxygenated acetonitrile/water solvent (λex = 370 nm), MOC-Pt-16 displayed a significant improvement in terms of its AIPE compared with that of the Pt(II) bzimpy motif via synergistically combining the AIPE from Pt(II) luminophores and AIE properties from Cd(II)-tpy moieties.

2.3. Phosphorescent MOCs Incorporating d10 Metal Ion

Complexes of d10 transition metals, especially coinage metal complexes such as Cu(I), Ag(I) and as Au(I), either with or without metallophilic interactions, could lead to special spectroscopic properties in the absorption and emission spectra [54,55]. Moreover, the d10-d10 metal-metal interactions in gold(I) complexes were stronger and thus were named as “aurophilic” by Schmidbaur [59], always rendering them phosphorescent.
In 2019, Balch’s group demonstrated the mono-bromo box [Au6(Triphos)4Br](SbF6)5·6(CH2Cl2) MOC-Au-17 and the dibromo box [Au6(Triphos)4Br2·H2O](SbF6)4·4(CH2Cl2) MOC-Au-18 by the synthesis of [Au3(Triphos) 2](SbF6)3 salt in dichloromethane mixed with potassium bromide (the equivalent of one for the mono-bromo box and twice as much for the dibromo box) in methanolic solution [39]. The mono-bromo box MOC-Au-17 included a bromide ion disordered over two sites and two dichloromethane molecules. The dibromo box MOC-Au-18 had a similar shape to that of the mono-bromo box. Interestingly, despite there being no aurophilic interactions in these gold boxes because of the dispersion of the Au(I) ion, phosphorescence was produced from the three-coordinate gold(I) complexes [60]. The excitation and emission spectra of MOC-Au-17 and MOC-Au-18 were at an excitation λmax of 359 nm and an emission λmax of 485 nm and an excitation λmax of 372 nm and an emission λmax of 496 nm, respectively, with emission lifetimes of 10 μs and 12 μs, respectively, resulting from the phosphorescence (Figure 17).
Stang’s group reported the host-guest coordination cage had an enhanced absorptivity and prolonged triplet state lifetime [61]. The host-guest coordination cage possessed a broadened visible light absorption range compared with that of the guest molecular coronene and MOC because of the encapsulation-induced core-to-cage charge transfer. Under visible light irradiation, the microenvironment inside the MOC promoted an electrostatic interaction between the light-induced excited electron of the coronene and the positive charge of the node of the MOC. Then the transition-metal node promoted intersystem cross-coupling and attenuated nonradiative decay. These processes provided an enhanced phosphorescence and a prolonged triplet lifetime.

3. Conclusions and Prospect

There is no doubt that phosphorescent metal-organic cages have aroused increasing interest among chemists due to their unique luminescent properties, such as their varied emissive triplet excited states, long lifetimes and large Stokes shift. As multifunctional materials, they have been preliminarily applied in chemical catalysis, biomedicine, sensors, molecular recognition and electro-optical devices. The rapid growth of research on luminescent metal-organic cages in recent decades has established fundamental guiding principles for the design and self-assembly of discrete metal-organic cages. Although considerable efforts have been devoted to the research of luminescent metal-organic cages, it is still at the exploration stage. In this minireview, we have summarized the recent works on phosphorescent MOCs, including their structures, luminescence properties and potential applications, aiming to offer some guidance on their synthesis and a greater understanding of their photochemical/physical features.
However, recently relatively few phosphorescent MOCs have been reported, and their structures and functions are also relatively simple and targetless due to the instability of MOCs resulting from relatively weak and reversible coordination bonds [3]. Thus, in general, efficient and concise assembled strategies need to be fully developed for further exploration. Furthermore, the coordination of phosphorescent ligands and metal nodes result in the quenching of luminescence via nonradiative deactivation, leading to a low-photoluminescence quantum yield, which is also a challenge to be solved. Some organic linkers with high-conjugated π electrons, such as pyrene- and naphthalene-based organic linkers and heterometal-organic ligands with special phosphorescent properties, are expected to be potential candidates for the construction the phosphorescent MOCs. Moreover, the encapsulation of photoactive guests in the cages to excite phosphorescence is also another attractive method.
Considering the unique luminescence properties of phosphorescent MOCs, comprehensive experimental studies will need to focus on screening their phosphorescent properties, such as their wavelength, lifetime and quantum yield, for the establishment of a phosphorescent MOC database. Simultaneously, an in-depth understanding of and studies on the mechanism of phosphorescent MOCs, including the origin of their phosphorescence and the relationship between their structure and luminescence, will contribute to the improvement and optimization of their functions and properties.
Currently, most applications of phosphorescent MOCs are limited to the lab scale, such as those in photocatalysis, biomedicine and optoelectronic materials. The regulation of the relationship between their structure and properties via the suitable choice of components (for phosphorescence, reversible redox activity, etc.) is key, along with the delicate control of pore size. Moreover, host-guest interactions will also offer a novel method for the assembly of MOCs. Recently, there has been significant progress in the development of phosphorescent systems based on first-row transition-metal organic complexes [62]. However, there are no reports on the use of such organometallic skeletons to construct MOC materials. This may also be a future research and exploration direction. Future development will focus on the new synthetic exploration, efficient assembly and specific photoactivity of MOCs. It is anticipated that more and more new phosphorescent MOCs will be available for practical applications following continuing efforts in this field.

Author Contributions

Conceptualization, Y.L. and C.Z.; investigation, Y.Y. and Y.L.; writing—original draft preparation, Y.Y. and Y.L.; writing—review and editing, Y.L., X.W. and C.Z. All authors have read and agreed to the published version of the manuscript.

Funding

Y.Y., Y.L. and C.Z. acknowledge support from the National Natural Science Foundation of China (21901109, 22302136) and Guangdong Basic and Applied Basic Research Foundation (2020A1515011448) as well as start-up funding from Songshan Lake Materials Laboratory (Y1D1061C11).

Data Availability Statement

No new data were created.

Conflicts of 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.

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Figure 1. The annual growth of publications in the research of metal-organic cages in Web of Science on the topics of “metal-organic cages” and “luminescent metal-organic cages”, respectively.
Figure 1. The annual growth of publications in the research of metal-organic cages in Web of Science on the topics of “metal-organic cages” and “luminescent metal-organic cages”, respectively.
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Scheme 1. The luminescent tetraruthenium square MOC-Ru-1 [25].
Scheme 1. The luminescent tetraruthenium square MOC-Ru-1 [25].
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Figure 2. Absorption, excitation and emission spectra of MOC-Ru-1 in acetonitrile solution at room temperature (adapted from data of [25]).
Figure 2. Absorption, excitation and emission spectra of MOC-Ru-1 in acetonitrile solution at room temperature (adapted from data of [25]).
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Scheme 2. Coordination-driven self-assembly of the truncated tetrahedral cage MOC-Ru-2 [26].
Scheme 2. Coordination-driven self-assembly of the truncated tetrahedral cage MOC-Ru-2 [26].
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Figure 3. Absorption and emission at 298 K (red; excitation at 430 nm) and 77 K (blue, excitation at 430 nm) of MOC-Ru-2 in acetonitrile (adapted from data of [26]).
Figure 3. Absorption and emission at 298 K (red; excitation at 430 nm) and 77 K (blue, excitation at 430 nm) of MOC-Ru-2 in acetonitrile (adapted from data of [26]).
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Figure 4. UV-vis absorption and emission spectrum of Ru4 and 1/8 RuL3 concentration of MOC-Ru-3 in DMSO (adapted from data of [42]).
Figure 4. UV-vis absorption and emission spectrum of Ru4 and 1/8 RuL3 concentration of MOC-Ru-3 in DMSO (adapted from data of [42]).
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Scheme 3. The synthetic route, UV-vis absorption and CD spectra (in H2O) of homochiral Δ- or Λ-MOC-Ru-3 (adapted from data of [27,41]).
Scheme 3. The synthetic route, UV-vis absorption and CD spectra (in H2O) of homochiral Δ- or Λ-MOC-Ru-3 (adapted from data of [27,41]).
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Scheme 4. Photocatalytic reactions: (a) The 1,4-coupling of 3-bromo-2-naphthol in a stereochemical control; (b) the selective intermolecular [2 + 2] cycloaddition; (c) E/Z selective Meerwein arylation, catalyzed by MOC-Ru-3 with the multi-channel electron transfer routes between Ru and catalytic Pd metal centers [45,46,47].
Scheme 4. Photocatalytic reactions: (a) The 1,4-coupling of 3-bromo-2-naphthol in a stereochemical control; (b) the selective intermolecular [2 + 2] cycloaddition; (c) E/Z selective Meerwein arylation, catalyzed by MOC-Ru-3 with the multi-channel electron transfer routes between Ru and catalytic Pd metal centers [45,46,47].
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Scheme 5. Self-assembly of (a) MOC-Ru-4a and (b) MOC-Ru-4b [28].
Scheme 5. Self-assembly of (a) MOC-Ru-4a and (b) MOC-Ru-4b [28].
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Figure 5. (a) Absorption spectra of Ru5, Ru6, MOC-Ru-4a and 4b; and (b) the 3MLCT emission lifetime peaks of Ru5, Ru6, MOC-Ru-4a and 4b, together with Instrument Response Function (IRF) (adapted from data of [28]).
Figure 5. (a) Absorption spectra of Ru5, Ru6, MOC-Ru-4a and 4b; and (b) the 3MLCT emission lifetime peaks of Ru5, Ru6, MOC-Ru-4a and 4b, together with Instrument Response Function (IRF) (adapted from data of [28]).
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Scheme 6. The synthesis and structure of MOC-Ru-5 viewed down to the crystallographic a- (top) and c-axes (bottom) [29].
Scheme 6. The synthesis and structure of MOC-Ru-5 viewed down to the crystallographic a- (top) and c-axes (bottom) [29].
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Figure 6. Normalized emission spectra and emission decays of Ru7 (black line) and MOC-Ru-5 (red line) in degassed DCM at 298 K (adapted from data of [29]).
Figure 6. Normalized emission spectra and emission decays of Ru7 (black line) and MOC-Ru-5 (red line) in degassed DCM at 298 K (adapted from data of [29]).
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Scheme 7. Synthesis of [Pd2L4]4+ cages with photoactive ruthenium complexes [30].
Scheme 7. Synthesis of [Pd2L4]4+ cages with photoactive ruthenium complexes [30].
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Figure 7. Absorption (solid lines) and emission (dash lines) spectra of Ru8, Ru9, MOC-Ru-6a and MOC-Ru-6b (adapted from data of [30]).
Figure 7. Absorption (solid lines) and emission (dash lines) spectra of Ru8, Ru9, MOC-Ru-6a and MOC-Ru-6b (adapted from data of [30]).
Inorganics 11 00436 g007
Inorganics 11 00436 sch008
Scheme 8. Synthesis of the [Pd2L4]12+ MOC-Ru-7 [31].
Scheme 8. Synthesis of the [Pd2L4]12+ MOC-Ru-7 [31].
Inorganics 11 00436 sch008
Inorganics 11 00436 g008
Figure 8. Absorption (solid lines) and emission (dash lines) spectra of Ru10, Ru11, MOC-Ru-7a and MOC-Ru-7b (adapted from data of [31]).
Figure 8. Absorption (solid lines) and emission (dash lines) spectra of Ru10, Ru11, MOC-Ru-7a and MOC-Ru-7b (adapted from data of [31]).
Inorganics 11 00436 g008
Inorganics 11 00436 g009
Figure 9. The structure of Λ- and Δ-MOC-Ir-8 and CD spectra of Λ-Ir13, Δ-Ir13, Λ-MOC-Ir-8 and Δ-MOC-Ir-8 (adapted from data of [32]).
Figure 9. The structure of Λ- and Δ-MOC-Ir-8 and CD spectra of Λ-Ir13, Δ-Ir13, Λ-MOC-Ir-8 and Δ-MOC-Ir-8 (adapted from data of [32]).
Inorganics 11 00436 g009
Inorganics 11 00436 g010
Figure 10. Absorption (solid lines) and emission (dash lines) spectra of [Ir(ppy)2(PhCN)2]OTf, Λ- and Δ-MOC-Ir-8 (adapted from data of [32]).
Figure 10. Absorption (solid lines) and emission (dash lines) spectra of [Ir(ppy)2(PhCN)2]OTf, Λ- and Δ-MOC-Ir-8 (adapted from data of [32]).
Inorganics 11 00436 g010
Inorganics 11 00436 sch009
Scheme 9. Self-assembly between the Ir(III) ligands and Pd2+ ions for synthesis of MOC-Ir-9 [33].
Scheme 9. Self-assembly between the Ir(III) ligands and Pd2+ ions for synthesis of MOC-Ir-9 [33].
Inorganics 11 00436 sch009
Inorganics 11 00436 g011
Figure 11. Normalized emission spectra of Ir(III) ligand complexes (a) Δ-Ir14a, Δ-MOC-Ir-9a and (b) Δ-Ir14b and Δ-MOC-Ir-9b in deaerated DCM and in PMMA thin films (adapted from data of [33]).
Figure 11. Normalized emission spectra of Ir(III) ligand complexes (a) Δ-Ir14a, Δ-MOC-Ir-9a and (b) Δ-Ir14b and Δ-MOC-Ir-9b in deaerated DCM and in PMMA thin films (adapted from data of [33]).
Inorganics 11 00436 g011
Inorganics 11 00436 sch010
Scheme 10. Self-assembly of MOC-Ir-10 by chiral tripodal ligands and rac-[Ir(ppy)2-(NCMe)2] [34].
Scheme 10. Self-assembly of MOC-Ir-10 by chiral tripodal ligands and rac-[Ir(ppy)2-(NCMe)2] [34].
Inorganics 11 00436 sch010
Inorganics 11 00436 g012
Figure 12. Normalized photoluminescence spectra of (a) MOC-Ir-10a and (b) MOC-Ir-10b in DCM solution, as powders and in PMMA-doped films (adapted from data of [34]).
Figure 12. Normalized photoluminescence spectra of (a) MOC-Ir-10a and (b) MOC-Ir-10b in DCM solution, as powders and in PMMA-doped films (adapted from data of [34]).
Inorganics 11 00436 g012
Inorganics 11 00436 sch011
Scheme 11. Synthesis of MOC-Ir-11 [35].
Scheme 11. Synthesis of MOC-Ir-11 [35].
Inorganics 11 00436 sch011
Inorganics 11 00436 g013
Figure 13. The photoluminescent excitation (solid lines) and emission spectra (dash lines) of MOC-Ir-11 and Ir(III) ligand complex Ir17 (adapted from data of [35]).
Figure 13. The photoluminescent excitation (solid lines) and emission spectra (dash lines) of MOC-Ir-11 and Ir(III) ligand complex Ir17 (adapted from data of [35]).
Inorganics 11 00436 g013
Inorganics 11 00436 sch012
Scheme 12. The synthesis of MOC-Os-12 (orange line represented octanuclear heterometallic coordination cage) [36].
Scheme 12. The synthesis of MOC-Os-12 (orange line represented octanuclear heterometallic coordination cage) [36].
Inorganics 11 00436 sch012
Inorganics 11 00436 sch013
Scheme 13. Synthesis of MOC-Pt-13 metallocycles [37].
Scheme 13. Synthesis of MOC-Pt-13 metallocycles [37].
Inorganics 11 00436 sch013
Inorganics 11 00436 g014
Figure 14. (a) Absorption (solid lines) and emission (dash lines) spectra of Pt18. (b) Absorption (solid lines) and emission (dash lines) spectra of MOC-Pt-13 (adapted from data of [37]).
Figure 14. (a) Absorption (solid lines) and emission (dash lines) spectra of Pt18. (b) Absorption (solid lines) and emission (dash lines) spectra of MOC-Pt-13 (adapted from data of [37]).
Inorganics 11 00436 g014
Inorganics 11 00436 sch014
Scheme 14. Synthesis of MOC-Pt-14 and MOC-Pt-15 [14].
Scheme 14. Synthesis of MOC-Pt-14 and MOC-Pt-15 [14].
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Inorganics 11 00436 g015
Figure 15. Molar absorption coefficients and normalized phosphorescent emission spectra of Pt19, MOC-Pt-14 and MOC-Pt-15 (adapted from data of [14]).
Figure 15. Molar absorption coefficients and normalized phosphorescent emission spectra of Pt19, MOC-Pt-14 and MOC-Pt-15 (adapted from data of [14]).
Inorganics 11 00436 g015
Inorganics 11 00436 sch015
Scheme 15. Synthesis of Pt ligand complex and self-assembly of MOC-Pt-16 [38].
Scheme 15. Synthesis of Pt ligand complex and self-assembly of MOC-Pt-16 [38].
Inorganics 11 00436 sch015
Inorganics 11 00436 g016
Figure 16. Normalized emission spectra (a) in N2-deoxygenated acetonitrile and (b) in N2-deoxygenated acetonitrile/water solvent of Pt(II) bzimpy Pt20 motif and MOC-Pt-16 (adapted from data of [38]).
Figure 16. Normalized emission spectra (a) in N2-deoxygenated acetonitrile and (b) in N2-deoxygenated acetonitrile/water solvent of Pt(II) bzimpy Pt20 motif and MOC-Pt-16 (adapted from data of [38]).
Inorganics 11 00436 g016
Inorganics 11 00436 g017
Figure 17. The excitation and emission spectra of (a) MOC-Au-17 and (b) MOC-Au-18 (adapted from data of [39]).
Figure 17. The excitation and emission spectra of (a) MOC-Au-17 and (b) MOC-Au-18 (adapted from data of [39]).
Inorganics 11 00436 g017
Table 1. Summary of phosphorescent MOCs based on the metal type and photophysical properties.
Table 1. Summary of phosphorescent MOCs based on the metal type and photophysical properties.
EntryMOCs(Metallo) LigandNode Precursorλem (nm) aΦPL (%)τPL (ns)Applications
1MOC-Ru-1 [25]
[Ru4] (PF6)6		-Inorganics 11 00436 i004
Ru1		7451135- g
2MOC-Ru-2 [26]
[L4 Ru6]Cl12		Inorganics 11 00436 i001
2		Inorganics 11 00436 i005
Ru3		577
689 b		<0.12, 790- g
3MOC-Ru-3 [27] [Ru8Pd6] (BF4)28Inorganics 11 00436 i002
Ru4		[Pd(MeCN)4](BF4)2610 c- g484Photocatalysis and bio-imaging
4MOC-Ru-4a [28]
[Ru4Pd8] (PF6)24		Inorganics 11 00436 i003
Ru5		Inorganics 11 00436 i006
640- g1.21- g
5MOC-Ru-5 [29]
[Ru8Pd4] (BF4)24		Inorganics 11 00436 i007
Ru7		[Pd(MeCN)4] (BF4)2710 d6.9151 and 700 h- g
6MOC-Ru-6a [30]
[Ru4Pd2] (BF4)12		Inorganics 11 00436 i008Inorganics 11 00436 i009
Ru8		[Pd(MeCN)4] (BF4)2638 e2.6659- g
7MOC-Ru-7b [31]
[Ru4Pd2] (BF4)12		Inorganics 11 00436 i010Inorganics 11 00436 i011
Ru11		[Pd(MeCN)4] (BF4)2640 c66- g- g
8MOC-Ir-8 [32]
[L4Ir6] (OTF)6		Inorganics 11 00436 i01212Inorganics 11 00436 i013
Ir13		575 f4- g- g
9MOC-Ir-9 [33]
[Ir8Pd4] (BF4)16		Inorganics 11 00436 i014
Ir14		[Pd(MeCN)4](BF4)29a 655, d
9b 569 d 		9a 5,
9b 14		9a 202
9b 825		- g
10MOC-Ir-10 [34]
[L2Ir3] (PF6)3		Inorganics 11 00436 i015
15		Inorganics 11 00436 i019
Ir16		10a 604, d
10b 485 d		10a 1,
10b 15		10a
59 and 129, h
10b
523 and 887 h		- g
11MOC-Ir-11 [35]
[Ir8Pd4] (BF4)16		Inorganics 11 00436 i016
Ir17		[Pd(MeCN)4](BF4)2658 c3.5141Bio-imaging and
photodynamic therapy
12MOC-Os-12 [36]
[L12(Os)4(Cd)4] (ClO4)16		Inorganics 11 00436 i017OsCl3·3H2O
Cd(ClO4)2·6H2O		6252.5156 and 73 h- g
13MOC-Pt-13b [37]
[Pt2Pt2] (OTf)4		Inorganics 11 00436 i018
Pt18b		Inorganics 11 00436 i02052716.715,500- g
14MOC-Pt-14 [14]
[Pt6Zn4] (PF6)8		Inorganics 11 00436 i021
Pt19		Zn(NO3)2·6H2O5451095,000- g
15MOC-Pt-16 [38]
[Pt6Cd12] (PF6)36		Inorganics 11 00436 i022
Pt20		Cd(NO3)2567- g218- g
16MOC-Au-17 [39]
[L4Au6Br] (SbF6)5		Inorganics 11 00436 i023Chloro(tetrahydrothiophene) gold
KBr		485- g10- g
a In MeCN, at 298 K; b at 77 K; c in DMSO; d in DCM; e in DMF; f in tetrachloroethane; g Not mentioned in the literature; and h bi-exponential photoluminescence decay.
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Yu, Y.; Wang, X.; Liu, Y.; Zou, C. Metal-Organic Cages Based on Phosphorescent Organometallics. Inorganics 2023, 11, 436. https://doi.org/10.3390/inorganics11110436

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Yu Y, Wang X, Liu Y, Zou C. Metal-Organic Cages Based on Phosphorescent Organometallics. Inorganics. 2023; 11(11):436. https://doi.org/10.3390/inorganics11110436

Chicago/Turabian Style

Yu, Yunliang, Xiaoxia Wang, Yuliang Liu, and Chao Zou. 2023. "Metal-Organic Cages Based on Phosphorescent Organometallics" Inorganics 11, no. 11: 436. https://doi.org/10.3390/inorganics11110436

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