Abstract
Metallacrowns (MCs) are a distinctive class of metallamacrocycles that serve as inorganic counterparts to the well-known crown ethers, in which the characteristic –[C–O–O]– repeating units are replaced by –[M–N–O]– motifs. Much like their organic analogues, the donor oxygen atoms within the MC framework can capture cations in the central cavity. The modifiable nature of MCs provides remarkable structural diversity, as both ring-forming ligands and metal ions can be varied to tune and modify the chemical and physical properties of corresponding MC. In this minireview, we focus specifically on metallacrowns bearing gallium(III) as the ring metal, highlighting their structural versatility as well as the properties and applications of GaIII-LnIII heterometallic MCs. We begin by introducing the first GaIII-based MC and trace the structural evolution of this family of MCs toward systems incorporating LnIII ions with intriguing photophysical properties that open avenues for diverse functional and applied uses.
Introduction
Metallacrowns (MC) are metal-rich macrocyclic frameworks described by Pecoraro and coworkers in 1989 (; ). Structurally similar to organic crown ethers, metallacrowns feature repeating –[M–N–O]–n units in place of methylene groups in a cyclic arrangement, with oxygen donor atoms oriented inward capable of binding metal cations within the central cavity (Figure 1A). This family of metallamacrocycles is constructed using multidentate ligands capable of coordinating several metal ions, typically a hydroximate-based ligand whose N-O donor components bind two Mn+ ions as part of a fused chelate ring. In addition to the ligand selection, (; ), several other factors such as the choice of ring metal (; ; ), counter-ion (), solvent (; ), and stoichiometry () should also be considered in the synthesis of MCs. The broad selection of the ring metals and ligands has led to a diversity of MC structures with different size (e.g., 9-MC-3 to 60-MC-20), composition, and 3-dimensional topology () described over more than 3 decades ().
FIGURE 1
In recent years, trivalent lanthanide ions (LnIII) have attracted considerable research interests owing to their luminescence and magnetic properties. The first example of a metallacrown incorporating an f-block metal ion was reported in 1996 based on efforts to design and develop agents capable of selectively binding rare earths over other hard metal ions such as Ca2+. This planar 15-MC-5 structure, featuring CuII as the ring metal, was shown to bind the uranyl cation (UO22+) preferentially over CuII and Ca2+ (). In the following years, an alternative planar 12-MC-4 with MnIII as ring metals and lanthanides as central guests (), as well as more complex metallacrowns containing MnIII in the ring positions and LnIII in the central metal positions (; ; ), were also reported. These and related structures were instrumental for the development of numerous lanthanide-containing metallacrowns with various applications in magnetism and molecular recognition (; ; ; ; ; ; ; ). In fact, owing to their rigid and preorganized coordination environments that place multiple metal centers in close proximity, MCs have been exploited to stabilize high-anisotropy LnIII ions. This has led to the development of several single-molecule magnets (SMMs) and single-ion magnets (SIMs) based on the MC platform (; ; ; ; ; ). As magnetism lies beyond the scope of this minireview, we direct interested readers to the cited literature for a more comprehensive discussions. Nevertheless, it was not until 2011 that the remarkable luminescence properties of lanthanide-containing metallacrowns were identified (). Lanthanide MCs have high thermodynamic stability and avoid typically limiting restrictions such as vibrational quenching from high energy oscillators and quenching solvent interactions () by offering a highly encapsulated environment. The hydroxamic acid ligands also serve as a built-in antenna that are photostable, with high extinction coefficients. While cryptates (e.g., [Ln⊂bpy.bpy.py (CO2Et)2]3+) () are well-known for their ability to surround the central lanthanide ion, expelling solvent molecules from the first coordination sphere, they often suffer from vibrational quenching. While offering high kinetic stability, their synthesis often requires multistep procedures, and their encapsulation is generally less rigid than that of MCs. β-diketonate complexes (e.g., [Eu(terpy)(hfac)3]) () exhibit high quantum yields, but suffer from poor photostability under UV light. Their open coordination environment leaves the lanthanide exposed to solvent, leading to solvent quenching and vibrational deactivation. Lanthanide-containing porous coordination polymers, such as lanthanide-based MOFs, offer high modularity and tunable luminescence properties due to the ability to host molecules within their network pores that can influence the lanthanide emission (). However, these extended porous frameworks often introduce guest-molecule interactions that can enhance non-radiative deactivation of the LnIII center luminescence, which limits their applications to Ln-based luminescent sensors and are of a sufficiently large size to fall outside the range for molecular applications (). One of the most important lanthanide-based luminescent materials, being used commercially, are known as Lumiphors, which are based on macrocyclic bi-functional chelators IAMs1, TAMs2, and HOPOs3. Their TbIII complexes, with high quantum yields (Φtotal ≥ 50%) and large extinction coefficients (≥20,000 M-1cm-1), are among the brightest lanthanide luminescent materials (Supplementary Table S1) ().
The first luminescent Ln-MC formed by self-assembly upon the reaction between picoline hydroximic acid (H2picHA), sodium hydroxide, zinc(II) triflate, and lanthanide(III) nitrate in methanol (LnIII-ZnII metallacrowns, where LnIII = Y, La, Sm, Eu, Gd, Dy, Tb, and Yb) (). Single-crystal X-ray analysis of the resulting LnZn16 metallacrowns revealed a unique “encapsulated sandwich” architecture. In this structure, two concave [12-MCZnIIN(picHA)-4] units sandwich an eight-coordinate LnIII ion, and the entire assembly is further encapsulated within the cavity of an outer [24-MCZnIIN(picHA)-8] ring (Figure 2G). This topology effectively excluded C-H oscillators from the proximity of the central lanthanide, with the nearest C-H bond located 6.7 Å away from the LnIII ion, and no solvent molecules bound to the central lanthanide. Photophysical measurements on YbIII and NdIII analogues exhibited bright luminescence, demonstrating that the unique MC architecture can enable near-infrared emission by protecting the lanthanide center from high-energy oscillators (). Building on their promising luminescence properties, the library of encapsulated sandwich LnZn16 metallacrowns was expanded by including isostructural analogues with quinaldine- (quinHA2-) () and pyrazine- (pyzHA2-) () hydroximate ligands to either shift the excitation wavelengths or increase water solubilities, respectively. In recent years, several research groups worldwide have also explored lanthanide luminescence in MC frameworks using other diamagnetic metals, such as AlIII and CdII, whose optical inertness is crucial for exploiting the intrinsic properties of the central LnIII ion (; ).
FIGURE 2
In this minireview, however, we focus on lanthanide-containing metallacrowns that use the closed-shell GaIII ion as the ring metal and salicylhydroximate (shi3-) as the ring-forming ligand, which also initially served as the antenna. The colorless and diamagnetic nature of the GaIII ion, together with its hard Lewis acid character that promotes exceptionally strong and kinetically stable bonds with the shi3- ligand, provides an ideal platform for investigating lanthanide luminescence. Additionally, the compatibility between the +3 charge of the ring metal and the deprotonation state of the ring ligand helps maintain charge balance within the MC framework.
Ln(III)/Ga(III)
The first example of a main group metal-based MC was reported in 1993, describing a dimeric gallium-based 12-MC-4 that captured one sodium ion between two 12-MCGaIIIN(shi)-4 metallamacrocycles and linked via four hydroxide bridges between GaIII with two additional sodium ions bound to the outer faces (Figure 1B) (). Despite this early report, main group MCs had remained relatively unexplored until the mid-2010s. The major breakthrough occurred in 2016 with the synthesis, structural characterization, and photophysical investigation of a new family of LnIII-GaIII metallacrowns. The resulting systems adopted the archetypical LnIII[12-MCGaIIIN(shi)-4](bz)4 framework (LnIII = Sm–Yb) and exhibited photophysical properties, with the ability to sensitize emission from many trivalent lanthanide ions including Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb. Notably, the DyIII and SmIII analogues emit in both the visible and near-infrared (NIR) regions, while the YbGa4 MC possesses rather a strong emission in the NIR with an observed quantum yield of 5.88% in the solid state (). Single-crystal structural analysis of the DyIII analogue revealed a pseudo-fourfold symmetry for the 12-MC-4 unit, composed of four crystallographically inequivalent GaIII ions bridged by four shi3- ligands in a nearly planar geometry (Figure 1C). In addition, the central DyIII ion is tethered to the MC ring by four benzoate (bz) ligands and adopts an eight-coordinate square antiprism geometry.
Interestingly, the size and flat shape of these Ln[12-MCGaIII-4](bz)4 molecules closely resemble that of G-tetrads that stabilize telomeric DNA. The poly G-rich regions at the end of chromosomes are stabilized by K+. Given the shape similarity, it was hypothesized that SmIIIGa4 or TbIIIGa4 might form adducts to these nucleic acid structures. This was demonstrated in buffered aqueous solution in which Stern–Volmer plots gave ∼10 μM affinities for GT-MC affinities. It was also shown that SmIIIGa4 caused destabilization of the G-tetrad indicating a shape and size recognition between these molecules (). This result indicated that MCs could serve as a probe for these important regulators of gene expression.
In 2019, upon reacting Ga(NO3)3, Dy(O2CMe)3, H3shi, Bu4NClO4, and piperidine in an 8:1:8:3:8 ratio in MeOH, Rentschler et al. reported a lanthanide containing complex that resembled the first gallium 12-MC-4 dimer reported in 1993 (). An X-ray structure of this dimeric MC revealed that one DyIII ion was encapsulated between two 12-MCGaIIIN(shi)-4 frameworks (). This structure formed a sandwich-like topology in which two 12-MC-4 units are connected to the central DyIII ion via the oxime oxygens of the shi3- ligands (Figure 1D). The authors focused their investigations on the magnetic properties of this compound and characterized it as a single-ion magnet (SIM) with an effective barrier (Ueff) of 39.00 K ().
In the same year, the entire series of [LnGa8(shi)8(OH)4]Na (LnIII = Pr, Nd, Sm−Yb, and Y), was synthesized in order to examine the luminescent properties of this molecular class (). X-ray single-crystal structural analysis of the NdGa8 analogue exhibited that each molecule consists of two [12-MCGaIIIN(shi)-4] units, with a composition identical to the previously presented MCs (Figure 1E) (; ; ). Photophysical studies revealed that these dimeric MCs are among the few molecular complexes capable of sensitizing f-f transitions of ten out of fifteen trivalent lanthanide ions, excluding LaIII (4f0), CeIII (impeded by 4f-5d transitions), PmIII (radioactive), GdIII (4f7), and LuIII (4f14). Notably, they can also sensitize the emissions of six LnIII ions across both the visible and NIR regions (PrIII, SmIII, DyIII, HoIII, ErIII, and TmIII) (Supplementary Table S1) ().
Another class of 12-MCGaIIIN(shi)-4 dimers was reported when David Chow reasoned that stable, self-assembled dimers derived from the corresponding LnIII [12-MCGaIIIN(shi)-4](bz)4 complexes (LnIII = Pr–Yb, excluding Pm) could be formed by substituting the benzoate anions with meta-directed dicarboxylate ligands. It was demonstrated that the introduction of a second MC unit using isophthalate “bridging” ligands (ip2-) enhanced solution stability while maintaining the photophysical properties of the parent monomeric MCs (). These dimeric and dianionic MCs exhibited large molar absorptivities, high quantum yields, and long luminescence lifetimes, particularly for the visible-emitting (TbGa4)2(ip2-)4 complex and the NIR-emitting (YbGa4)2(ip2-)4 in the solid state. The dimeric structure of (DyGa4)2(ip2-)4 is shown as Figure 1F. Notably, the four rigid ip2- ligands lock the two DyIII cations at a fixed distance of 7.224 (1) Å. This report laid the cornerstone for future discoveries, leading to the introduction of several variations of this structure type.
Armed with these solution-stable and highly luminescent dimeric compounds, which efficiently sensitized the emission of up to 10 different lanthanide ions, the next objective was to develop versatile functionalization strategies for these MCs. Fortunately, there are many examples of modified shi3- and ip2- ligands that can serve as precursors for these compounds. Both shi3- and ip2- were substituted with iodo groups (meta-iodo-shi (I-shi) and 5-iodoisophthalate (I-ip)) allowing the preparation of [Ln2Ga8 (shi)8(I-ip)4]2-, [Ln2Ga8(I-shi)8(ip)4]2-, and [Ln2Ga8(I-shi)8(I-ip)4]2-. The Sm2Ga8(shi)8(I-ip)4 is shown as Figure 2A (). The goal was to prepare bimodal near-infrared and X-ray computed tomography (XCT) imaging agents. These variations enabled systematic control over iodine content and location, allowing the authors to assess the impact of each modification on X-ray attenuation and NIR luminescence. The conclusion was that iodination did not diminish the emission intensity of the metallacrowns. There was a linear dependence on X-ray attenuation based on the degree of iodination, with [Ln2Ga8(I-shi)8(I-ip)4]2- showing a nearly 50-fold enhancement compared to the clinically administered iobitridol. Surprisingly, even [Ln2Ga8(shi)8(ip)4]2- was superior to this clinical agent because the X-ray cross section of the two lanthanide ions is much greater than iodine.
Because there are so many commercially available functionalized H3shi and H2ip molecules, both the ring and bridging positions can be modified using highly effective synthetic processes. For example, by exploiting the versatility of “click” chemistry, the dimeric MCs can be functionalized via the copper-catalyzed alkyne azide [3 + 2] cycloaddition (CuAAC) () and the thiol-maleimido coupling via Michael addition (). An alkyne substituent was installed on the shi3- (eshi3-) for the CuAAC reaction and photocatalytic radical couplings, while a maleimide group was added on the back of isophthalate (mip2-) for thiol couplings (). Structural analysis of the [Sm2Ga8(eshi)8 (ip)4]2- and [Nd2Ga8(shi)8(mip)4]2- analogues (Figures 2B,C) closely resemble that of the previously reported {Dy[12-MCGaIIIN(shi)-4]}2(ip)4 metallacrown (Figure 1F), indicating that adorning the periphery of the MCs does not perturb the basic structure of the dimer.
The preparation of dendrimeric MCs (DMCs) provides an instructive example of the power of having orthogonal coupling groups to attach side chains for functionalization (). By starting with cysteamine, one can alternately couple methyl acrylate and ethylene diamine to make dendrons of successively higher molecular weight and complexity. By the time one prepares generation 2 dendrons (G-2), symmetric subunits of significant molar mass are realized and these may be coupled to either the maleimide group, to provide substitution only at the mip2- bridges, to use photochemistry to induce radical coupling to eshi3- or, by subsequent coupling steps, first functionalize the eshi ligand followed by clicking a different generation of type of functional group to the mip ligand. These different approaches can lead to a wide variety of dendrimeric metallacrowns as shown schematically in Figure 2D. All of these structures yield dendrimeric metallacrowns that are highly luminescent, water soluble, and biocompatible.
Before expanding on the applications of the monomeric and dimeric metallacrowns discussed thus far, we will introduce a few more structures that have been prepared using gallium and lanthanides. The isostructural complexes [Ga4Ln2 (shi)4(Hshi2)2(H2shi)2]4- (where LnIII = Gd, Tb, Dy, Er, and Y) were prepared to examine their magnetic properties (). All complexes (except Er) exhibit antiferromagnetic exchange coupling with a diamagnetic ground state. The DyIII complex acted as a single-molecule magnet (SMM), exhibiting relaxation processes that are due to the excited ferromagnetic state and the uncoupled DyIII centers. The observed antiferromagnetic coupling in the DyIII complex was shown to be mainly due to an exchange mechanism, not dipolar interactions. These studies suggest that the two closely separated states could be considered an entry point for the formation of qubits using metallacrowns. Another interesting family of GaIII-LnIII metallacrowns was made upon direct reaction of salicylhydroxamic acid (H3shi) with GaIII and LnIII nitrates in a CH3OH/pyridine mixture (). Single-crystal X-ray diffraction analysis revealed two distinct types of structures, [LnGa4 (shi)4(H2shi)2(py)4(NO3)](py)2 (LnIII = Gd, Tb, Dy, and Ho) and [LnGa4(shi)4(H2shi)2(py)5](NO3)(py) (LnIII = Er, Tm, and Yb), depending on whether the nitrate counterion bounds to the central LnIII ion or not. While with early LnIII ions (Nd, Sm, and Eu) no pure MC products were obtained, the mid-series ions (Gd, Tb, Dy, and Ho) formed complexes in which the LnIII center is nine-coordinated (Figure 2E). Meanwhile, the complexes formed with the smallest LnIII ions of the series (Er, Tm, and Yb) yielded eight-coordinated lanthanides (Figure 2F). This family of MCs demonstrated that planar GaIII-based MCs are not a requirement for lanthanide luminescence and that the presence of protonated H2shi does not lead to significant vibrational quenching.
Applications of luminescent lanthanides
The unique electronic configurations of LnIII ions give rise to narrow emission bands, long luminescence lifetimes, and strong resistance to photobleaching. These properties make LnIII-based luminescence highly valuable for practical applications in biomedical imaging (; ; ), solar energy conversion (; ; ), light-emitting diodes (; ; ), and telecommunications (; ). Furthermore, NIR-emitting LnIII ions draw particular interest because of the transparency of biological tissue in this spectral range.
However, due to the symmetry forbidden nature of f–f transitions, most lanthanide(III) ions exhibit extremely low molar absorption coefficients, which in turn limits both the number of emitted photons and the overall detection sensitivity. As a result, direct population of excited states generally requires high-power laser excitation sources, a condition that is particularly unfavorable for biological imaging applications. To overcome this intrinsic limitation, organic chromophoric frameworks are commonly employed for the complexation and sensitization of lanthanide(III) ions via the so-called “antenna effect”, in which the lanthanide center is embedded into an environment with good light-harvesting and energy transfer properties. In this way, extremely large pseudo-Stoke shifts (the difference in energy where the antenna absorbs and the lanthanide emits) can be thousands of wavenumbers. Therefore, quantification of the emission is unlikely to be complicated by overlap with a broad excitation band. Another critical consideration in designing luminescent lanthanide materials is the presence of nearby C–H, O–H, or N–H bonds, whose high-energy vibrational overtones can easily couple with the small energy gaps between the emitting and first levels of the ground multiplet in NIR-emitting lanthanides and quench emission through non-radiative pathways. Such vibrations are commonly found in organic antennae, which limits their efficiency in sensitizing NIR-emitting lanthanides. This limitation is evident when comparing the highest reported overall quantum yields of lanthanide complexes containing C–H bonds, with NIR-emitting lanthanide complexes exhibiting relatively low yields of 3.8% for YbIII, 0.42% for NdIII, and 0.033% for ErIII (; ; ), whereas visible-emitting lanthanide complexes such as those of EuIII and TbIII can achieve quantum yields exceeding 80% (; ; ). Not only is it essential to isolate the LnIII ion from C–H, O–H, or N–H bonds, but it also necessary to ensure that paramagnetic ring metal ions, which have detrimental effect on radiative process and quench luminescence, are not present.
Metallacrowns provided an entry into the field of luminescent lanthanide complexes by avoiding all of the restrictions described above. The first example was the host(host-guest) self-assembled LnIII-ZnII metallacrowns (LnIII = Y, La, Sm, Eu, Gd, Dy, Tb, and Yb) introduced in 2011 (vide supra) (Figure 2G). Subsequently, this synthetic strategy was expanded using different hydroximate ligands in order to change excitation wavelengths and solubility (; ; ; ). Through these alterations, excellent human cell images could be obtained that distinguished necrotically vs apoptotically killed cells () and that could be used to simultaneously fix and image cells (). The Zn16 metallacrown scaffold perfectly addressed this quenching concern as the central Ln was shielded from vibrational quenching, since the closest C-H oscillator was >7 Å from the captured Ln atom. Similarly, the ZnII diamagnetic d10 electronic configuration removed concern of paramagnetic quenching. However, while these LnZn16 based metallacrowns exhibited large pseudo-Stokes shifts and were highly emissive in the Near IR, because their mechanism of Ln sensitization was achieved by intraligand charge transfer (ILCT), other lanthanides such as TbIII and SmIII did not emit in the visible region. Furthermore, selective functionalization of the molecule for targeting applications proved difficult, therefore, a different metallacrown scaffold was sought in an effort to broaden the utility of metallacrowns in luminescent applications. As described above, the GaIII based 12-MC-4 scaffold resolved the limitations of LnZn16.
Modern Light Emitting Diodes (LEDs) are constructed with multiple glass or polymer layers that each contain different chromophores that in coherent emission yield what our eyes see as white light (WLEs). These white light LEDs may be dichroic (blue/yellow), trichroic (red/green/blue) or tetrachroic (red/green/blue/cyan), with the latter being the most expensive to make but yielding the highest quality light. Efforts to prepare single molecules that are tri- or tetrachoric are heavily studied since using such materials would markedly decrease fabrication costs while ensuring that the white light integrity would not change (to more blue or yellow) due to differential photobleaching of the emitters. Lanthanides are promising for this application as they have multiple emissive lines that do not photobleach. Notably, DyIII is of particular interest since it possesses three main sharp emission bands 4F9/2→6H15/2 (blue), 4F9/2→6H13/2 (yellow), and 4F9/2→6H11/2 (red) in the visible region that allows for the development of single-component WLEs. While crystal field perturbations of lanthanide orbital energies are often neglected, it has been shown that small variations in molecular symmetry around DyIII can alter the relative emission intensities of these transitions yielding different shades of white light. Optical analysis of the DyIII-GaIII metallacrowns demonstrated that the relative intensities of these emissions could be controlled through structural variation imparted by different MCs on the DyIII environment. Workers in this field use the Commission Internationale de l’Eclairage (CIE 1931) chromaticity standards to assess the quality of the WLE with coordinates x = 0.33 and y = 0.33 corresponding to pure white light. The “D4” symmetric [DyGa8(shi)8(OH)4]Na (x = 0.309 and y = 0.334) was within the acceptable range for WLE, whereas the “C1” symmetric [DyGa4(shi)4(H2shi)2(NO3)] (x = 0.398 and y = 0.430) appeared pale yellow and intermediate coordinates were acquired for any of the “C4” symmetric monomeric [DyGa4(shi)4] (x = 0.352 and y = 0.393) or dimeric [DyGa8(shi)8] (x = 0.352 and y = 0.393) MCs containing benzoate or isophthalate, respectively (). Hence these molecules represented trichroic systems. Further tuning of the white light emission–from warm to neutral to cold white–to obtain tetrachoric emitters was achieved by varying the chromophoric building blocks. The strategy exploited controlling back energy transfer rates to introduce ligand-centered fluorescence in the blue spectral region while preserving the coordination geometry of DyIII (). New DyIII-GaIII MCs based on the [DyGa4(shi)4] or dimeric [DyGa8(shi)8] frameworks with various combinations of building blocks derived from substituted salicylhydroxamic (H3L’) (Cl, Br, I, OMe, Me) were prepared () and in some cases contained a ligand based fluorescent emission in the 370–420 nm region that could modify the CIE coordinates of the compounds. Thus, the new tetrachroic emitter shifted the [DyGa4(shi)4(bz)4] (x = 0.352 and y = 0.393) to [DyGa4(Cl-shi)4(bz)4] (x = 0.32 and y = 0.33) a near perfect WLE.
Back energy transfer rates can also be exploited in the field of molecular nanothermometry. The Ga8Ln2(shi)8(ip)4 MCs can be used as ratiometric luminescent molecular nanothermometers in the physiological temperature range. Using a 1:1 mixture of visible-emitting Tb2Ga8 and Sm2Ga8 molecules, they observed a luminescent molecular thermometer with interesting thermal sensitivity. At 328 K, the system exhibited a relative thermal sensitivity (Sr) of 1.9% K−1 and a minimal temperature uncertainty (dT) of less than 0.045 K, in an aqueous suspension of polystyrene nanobeads loaded with the corresponding MCs (). The performance was further improved by preparing a new series of dimeric Ln2Ga8L8(ip)4 metallacrowns (LnIII = Tb or Sm; L = salicylhydroxamic acid (H3shi), 5-methylsalicylhydroxamic acid (H3mshi), 5-methoxysalicylhydroxamic acid (H3moshi), and 3-hydroxy-2-naphthohydroxamic acid (H3nha)) and investigating the effect of ligand energy levels on the thermal dependence of lanthanide emission (). Optical measurements on the MC series demonstrated that the energy gap (ΔE) between the LnIII excited state and the ligand excited (T1) state is the primary factor controlling the thermal dependence of emission intensity via the back energy transfer mechanism, where a smaller ΔE leads to a greater decrease in emission intensity. However, when the energy difference between the ligand-centered singlet state (S1) and triplet state is small (below 3,760 cm-1), intersystem crossing between S1 and T1 (both S1→T1 and the reverse S1←T1) also contributes to the thermal behavior of metallacrowns. Eventually, both mechanisms affect LnIII-centered room-temperature quantum yields with values ranging from 2.07(6)% to 31.2(2)% for Tb2Ga8L8 and from 0.0267 (7)% to 2.27(5)% for Sm2Ga8L8 (). Functional molecular thermometers could serve the temperature range 100–375 K.
Finally, in vivo imaging is best achieved using molecules that can both be sensitized and emit at wavelengths that exceed 620 nm (∼16,000 cm-1) because these low energy photons can penetrate deeply into tissue. These requirements suggest that LnIII = Yb, Nd, and Er (NIR-II, 1000–1700 nm) would be ideal emitters; however, the H3shi ligands require excitation at much too high energies (>23,000 cm-1) to be practical. Hence, the next generation of luminescent MCs are appending organic or inorganic antenna to the back of the bridging isophthalic acid ligands, enabling absorption at longer wavelengths than the MC forming ligands.
An example of a metal-based antenna linked to the dimeric LnIII-GaIII metallacrowns uses RuII-centered metal-to-ligand charge transfer (MLCT) states of RuII polypyridyl complexes to sensitize the LnIII centers (). The [Ln2Ga8(shi)8(bpy2RuPhenMip)4](PF6)6 complexes (LnIII = Yb, Er, Nd, and Y) are the first examples of dimeric LnIII-GaIII MCs functionalized with appended transition metal complexes. Excitation of these molecules at ∼450 nm (22,000 cm-1) yielded NIR-II emission in DMSO/H2O mixtures (Supplementary Table S1). A very recent report has exploited the well-studied coumarin group of antenna (). Again, these chromophores have excitation energies >20,000 cm-1; however, they provide proof of principle that organic fluorophores can sensitize NIR-II emission in this molecular class. The diethylcoumarin derivative was once again attached to the Ln2Ga8shi8(mip)4 (Figure 2C) via thiol Click chemistry to the mip bridge. When excited at 420 nm, YbIII, NdIII, and ErIII could all be sensitized, although the relative efficiency was not as effective for YbIII. Despite this observation, the Nd derivative was very bright and, most important, was taken up by living HeLa cells and provided excellent images without showing toxicity. To evaluate the biocompatibility of the MCs, live HeLA cells were incubated with varying concentration of [Ln2Ga8shi8(C-mip)4]2- for 24 h. Cell viability remained above 0.95 across all tested concentration until 100 μg⋅mL-1, at which point the MC began to show cytotoxicity and viability dropped to 0.86 (2) (; ). While both of these examples provide the opportunity for longer wavelength excitation, future studies with additional, even lower energy absorbing, antenna are currently underway.
We hope that this brief review of metallacrowns has provided insight into a technologically useful and intellectually interesting class of metallamacrocycles. We expect new structural and compositional modifications of this fascinating molecular class will not disappoint researchers that work in this field.
Statements
Author contributions
SN: Writing – original draft, Writing – review and editing. VP: Supervision, Conceptualization, Writing – review and editing, Writing – original draft, Funding acquisition.
Funding
The authors declare that financial support was received for the research and/or publication of this article. VLP thanks the NSF for funding this research (CHE-2154116).
Acknowledgments
The authors would like to thank the numerous graduate students and postdocs that have participated in this work over the past 35 years. In addition, Stéphane Petoud and Svetlana Eliseeva in Orléans, Talal Mallah in Saclay, Luis Carlos in Aviero, and Matteo Tegoni in Parma all have contributed significantly to these studies. VLP thanks the NSF for funding this research (CHE-2154116).
Conflict of interest
Author VP discloses that he is one of the founders of VieWaves, a company that uses some of the technology described herein.
The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Supplementary material
The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fchbi.2025.1726839/full#supplementary-material
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Summary
Keywords
metallacrowns, supramolecular, lanthanide luminescence, nanothermometry, white-light emission, biological imaging
Citation
Naseri S and Pecoraro VL (2025) Metallacrowns: an ideal platform for accessing lanthanide luminescence. Front. Chem. Biol. 4:1726839. doi: 10.3389/fchbi.2025.1726839
Received
16 October 2025
Revised
21 November 2025
Accepted
27 November 2025
Published
18 December 2025
Volume
4 - 2025
Edited by
Craig C. McLauchlan, Illinois State University, United States
Reviewed by
Virender, Deenbandhu Chhotu Ram University of Science and Technology, India
Corey Martin, Savannah River National Laboratory (DOE), United States
Updates
Copyright
© 2025 Naseri and Pecoraro.
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*Correspondence: Vincent L. Pecoraro, vlpec@umich.edu
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