English

• INTRODUCTION

  In 1985, the discovery of fullerene C₆₀ opened a new field of carbon-based materials.^[1] Researchers then proposed the possibility of confining atom or even small molecule in the internal cavity of fullerene. The first metallofullerene, La@C₆₀, was observed in the same year that C₆₀ was reported^[2]. Then in 1991, macroscopic lanthanide metallofullerene of La@C₈₂ was synthesized.^[3] These new compounds were called endohedral metallofullerenes (EMFs). Since then, various metallofullerenes have emerged. Due to their unique electronic and geometrical structures, metallofullerenes attract much attention from chemists, physicists and materials scientists. Then the first trimetallic nitride endohedral metallofullerene, Sc₃N@C₈₀, was discovered in 1999 and has gained widespread attention for its high yield and structural stability.^[4] As research continued, the family of metal nitride endohedral fullerenes expanded rapidly. Inspired by this discovery, metal carbide endohedral fullerenes, metal oxide endohedral fullerenes, metal sulphide endohedral fullerenes and metal carbonitride endohedral fullerenes have been discovered one after another. The discovery of these new endohedral clusterfullerenes has taken the development of metallofullerenes to a new level.^[5] Metallofullerenes not only have the physical and chemical properties of carbon cages, the endohedral metals but also bring richer electronic, optical and magnetic properties, making metallofullerenes important for applications in quantum information science, biomedicine, energy transformation and electronic devices.

  Luminescence has been a subject of extensive scientific interest for centuries. The color, intensity and lifetime of light-emitting materials have attracted much attention of researchers, leading to the development of applications in many fields such as lighting, displays, energy, sensing and biomedical imaging.^{[6, 7]} Fullerenes exhibit unique photophysical and photochemical properties due to their curved structures and the presence of π-electrons. The luminescent properties of hollow fullerenes C₆₀ and C₇₀ have long been investigated.^[8-10] In addition, some endohedral metals such as lanthanides and actinides encapsulated in fullerene carbon cages often result in better fluorescence properties than those of hollow fullerenes. At the same time, the luminescence of the endohedral metals is highly sensitive to the inner cluster and outer carbon cage, so the diverse combination of endohedral metals and carbon cages allows metallofullerenes to exhibit varied luminescence properties. Metallofullerenes are predominantly near-infrared luminescent due to their strong absorption in the visible region. So far, Erand Tm-based metallofullerenes have been reported to exhibit near-infrared luminescence derived from the f-f transition of the endohedral metal atoms.^[11-18] In the visible region, most metallofullerenes do not emit light or emit light very weakly. Apart from actinide metallofullerenes such as Th@C_3v-C₈₂^[19] and U₂@I_h-C₈₀^[20], the wellknown metallofullerene with luminescence in the visible region is the yttrium-based metallofullerenes. For example, Y₃N@C₈₀ is significantly more luminescent than the other luminescent metallofullerenes mentioned above. Y₃N@C₈₀ is also the first metallofullerene found to be with thermally activated delayed fluorescence (TADF) properties.^[21]

  TADF was discovered as the second long-lived emission in addition to phosphorescence. In recent years, TADF materials have received widespread attention as the key to achieving high efficiency in organic light-emitting diodes (OLEDs) devices due to their ability to make full use of electronically generated excitons, which can theoretically achieve 100% internal quantum efficiency.^[6] TADF properties rely on the existence of a very small energy gap between the lowest singlet states (S₁) and triplet excited states (T₁), which allows T₁ excitons to be transferred back to S₁ excitons by reverse intersystem crossing (RISC).^[22] Distinct from the prompt fluorescence, TADF is characterized by a marked increase in the lifetime of T₁. TADF materials have attracted a lot of attention from scientists worldwide because of their potential in next-generation display and solid-state lighting technologies. Compared to hollow fullerene C₇₀, which also has TADF properties, Y₃N@C₈₀ holds a smaller S₁-T₁ energy gap (ΔE_ST) and stronger fluorescence emission. Therefore, Y₃N@C₈₀ offers the possibility of using itself as TADF materials for applications such as OLEDs.^[21] Since the study of the luminescent properties of yttrium-based metallofullerenes is still in its infancy, in this feature article we mainly outline the progress of research in the luminescent properties of yttrium-based metallofullerenes and provide a perspective on the development.

  LUMINESCENT PROPERTIES OF YTTRIUMBASED METALLOFULLERENES

  Thermally Activated Delayed Fluorescence in Y₃N@C₈₀. As an important member of the yttrium-based metallofullerenes family, Y₃N@C₈₀ has been widely studied. Y₃N@C₈₀ consists of three yttrium and one nitrogen ion encapsulated in a C₈₀ fullerene cage, where the yttrium exists in the Y³⁺ state, forming a Y₃N cluster. Stable Y₃N@C₈₀ structure is obtained by transferring six electrons from the Y₃N cluster to the C₈₀ cage, and its electronic structure can be expressed as (Y₃N)⁶⁺@(C₈₀)^6-.^{[23, 24]} Figure 1 shows the optimized structure of Y₃N@C₈₀. Similar to the Sc₃N cluster in the well-known endohedral metallofullerene Sc₃N@C₈₀, the Y₃N cluster remains planar in the C₈₀ cage and is located in the "equatorial" plane of the spherical carbon cage with each Y atom facing a hexagon of the cage. It has been reported in the literature that the absorption of Y₃N@C₈₀ begins near 750 nm.^[25] Although most metallofullerenes do not emit strong light due to the strong absorption of fullerene cage in the visible range, there are still a few metallofullerenes that exhibit photoluminescence (PL) in the visible region. Typically, Y₃N@C₈₀ exhibits obvious fluorescence in the visible region due to the synergistic effect of the internal Y³⁺ ion and the carbon cage.

  Figure 1

  

  Figure 1.  Optimized molecular structure of Y₃N@C₈₀.

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  In addition, it is found that Y₃N@C₈₀ shows thermally activated delayed fluorescence (TADF) properties. TADF is a thermally activated re-luminescence process of triplet state excitons, as shown in Figure 2, which illustrates the TADF mechanism. The molecule reaches a higher excited singlet state upon excitation, after which it undergoes an internal transition to the lowest excited singlet state S₁. From S₁ it can return directly to the ground state S₀ by radioluminescence, called prompt fluorescence, which is a very fast process with a short lifetime. It can also undergo a non-radiative intersystem crossing (ISC) to the triplet state T₁, ending with phosphorescence emission and exhibiting a much longer lifetime. In theory, for T₁ excitons, a direct T₁-S₁ transition is forbidden, since the energy gap (ΔE_ST) between S₁ and T₁ in TADF materials is very small (typically less than 0.5 eV). With the help of thermal activation, the long-lived T₁ exciton returns to the S₁ state via reverse intersystem crossing (RISC) and radiates to produce fluorescence, i.e., TADF, which is delayed compared to the direct luminescence of S₁ and has a longer lifetime than prompt fluorescence. Moreover, since the lumine-scence is derived from S₁, the luminescence spectra of prompt fluorescence and TADF overlap entirely. Therefore, TADF materials can effectively utilize triplet excitons for luminescence.^{[6, 21]} Theoretically, TADF materials can achieve 100% quantum efficiency and are of great value for the third-generation organic electroluminescent materials. Although the TADF phenomenon has been known since 1960, ^[26] it was not until the early 21st century that TADF materials were used.^{[6, 27, 28]} While TADF is still a rare phenomenon in the fullerene field, only C₇₀ has distinct TADF properties due to its small ΔE_ST (0.25-0.3 eV) and long intrinsic phosphorescence lifetime.^{[9, 29, 30]}

  Figure 2

  

  Figure 2.  Schematic presentation of the luminescence mechanism with delayed fluorescence.^[31] Reprinted with permission from ref. [31]. Copyright 2021 Royal Society of Chemistry.

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  The emission spectrum of Y₃N@C₈₀ shows two main emission peaks at 701 and 739 nm, as shown in Figure 3 (for clarity of presentation, they are referred to as P1 and P2, respectively). The intensity of the P1 emission peak reaches a maximum at 120 K. From room temperature to 120 K, its PL intensity gradually increases with cooling. As the temperature drops below 120 K, the PL intensity decreases with decreasing temperature until the temperature drops to 60 K, under which this P1 emission peak disappears completely. Under 60 K, the disappearance of the P1 emission peak indicates that Y₃N@C₈₀ displays almost no prompt fluorescence. In contrast, the P2 emission peak continues to increase with falling temperature, and gradual dominance of the phosphorescence located at P2 under low temperature was deduced. Furthermore, below 120 K, the lowering of temperature causes a reduced RISC efficiency, resulting in a lower delayed luminescence yield as well as a gradual dominance of the phosphorescence. Under 60 K, only the phosphorescence peak remains and exhibits a lifetime of up to 192 ms. It is also clear from theoretical calculations that the highest occupied molecular orbital (HOMO) of Y₃N@C₈₀ is positioned at the poles, while the lowest unoccupied molecular orbital (LUMO) is mainly distributed near the hexagonal shape towards which each Y atom is oriented. The S₀→S₁ excitation of the molecule is mainly a HOMO-LUMO transition, a process that can be seen as the transfer of electrons from the poles to the equator within the molecule. It is this spatial separation of the "donor" and "acceptor" that gives the molecule its lesser ΔE_ST.^[21] This also provides the theoretical basis for subsequent reasonable predictions of whether fullerenes have TADF properties.

  Figure 3

  

  Figure 3.  Luminescence spectra of Y₃N@C₈₀ in polystyrene film were measured at 10 K steps during cooling from (a) 290 to 120 K, and (b) from 120 to 20 K (λ_exc = 405 nm); (c) Temperature profiles of the relative integral emission intensity (black dots) and relative peak intensities at 701 and 739 nm (dashed lines); the values are referred to 290 K, ranges with different emission mechanisms are marked (P = phosphorescence, TADF = thermally activated delayed fluorescence); (d) Luminescence lifetimes of Y₃N@C₈₀ in polystyrene at different temperatures (the inset shows the same on a logarithmic scale), the blue line is the fitted curve.^[21] Reprinted with permission from ref. [21]. Copyright 2017 Wiley-VCH.

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  In addition, Y₃N@C₈₀ fluorescence has low quantum yields (QY) at room temperature with an experimentally measured fluorescence lifetime of 240 ns^[32], which is short compared to the lifetimes of typical rare earth metal ions (Er³⁺, Yb³⁺, Y³⁺, and so on, which are generally in the microsecond to millisecond range).^[11] In general, fullerenes typically exhibit low QY due to moderate radiative decay and rapid ISC. The participation of TADF properties allows for an effective increase in QY, with quantum yields of up to 8% at high temperatures for C₇₀ and a maximum QY of 22% at 120 K for Y₃N@C₈₀.^{[9, 21]}

  Compared to hollow fullerene C₇₀, Y₃N@C₈₀ has a smaller ΔE_ST value (0.09 eV^[21]), which makes its TADF more effective. Moreover, as the first metallofullerene discovered with TADF properties, its PL properties are also far superior to those of hollow fullerenes. This has been discovered to considerably expand the application of fullerenes in the field of light emitting materials, making it viable to apply fullerenes in OLEDs.

  Modulation of the Luminescence Properties of Y₃N@C₈₀

  Modulation from the Inside of Carbon Cage. Since its first discovery in 1999, ^[4] trimetallic nitride template (TNT) has been used to develop the most numerous metallofullerene species, whose advantages such as high yield and structural stability have attracted many researchers. With the intensive research on trimetallic nitride endohedral metallofullerenes, mixed-metallic nitride endohedral fullerenes have also been synthesized. This method of encapsulating multiple metals inside a single carbon cage brings more diverse properties to metallofullerenes. For example, several mixed-metallic nitride endohedral fullerenes were synthesized to modulate the cage structures, electronchemical behaviors, and magnetic properties. Based on this method, the luminescent properties of yttrium-based metallofullerenes can be finely adjusted by the replacement of metal yttrium.

  There was a report that the gradual replacement of the Y atom in Y₃N@C₈₀ with Sc atom can result in significant changes for the luminescence properties (Figure 4).^[31] This series of mixed metal nitride endohedral fullerenes, Y_xSc_3-xN@C₈₀ (x = 0-3), have similar electronic structures, (Y_xSc_3-xN)⁶⁺@(C₈₀)^6-, and their luminescence properties contain the TADF. Briefly, Y₂ScN@C₈₀ and YSc₂N@C₈₀ show a similar change trend of temperature-dependent PL to Y₃N@C₈₀ with transition temperatures both at 120 K. The difference is that the luminescence of Sc₃N@C₈₀ reaches a maximum at 60 K. In addition, both the PL emission wavelength and the Stokes shift gradually increase with the increased numbers of Sc atoms in the endohedral clusters, while the PL lifetime and ΔE_ST subsequently decrease. The regular variation of the luminescence properties of metallofullerenes with the increasing number of internal Sc atoms provides a profitable experimental basis for subsequent modulation of the lumine-scence properties of metallofullerenes by similar means. Also, all Y_xSc_3-xN@C₈₀ molecules have a very small ΔE_ST of less than 0.1 eV. It can be proposed that TADF may be a general phenomenon in metallofullerenes. However, many metallofullerenes have very weak PL intensity and only can be detected under very low temperature.

  Figure 4

  

  Figure 4.  Left: molecular structure of Y_xSc_3-xN@C₈₀ (x = 0-3) (Y – green, Sc – magenta, N – blue, C – light grey). Middle: the luminescence (lum) and low-energy absorption (abs) spectra measured in toluene solution at room temperature. (Asterisks mark an instrumental artefact appearing as a negative peak near 950 nm.) Right: the luminescence spectra at three selected temperatures (270 K; the temperature at which PL has the highest intensity; the lowest temperature studied), λ_exc = 405 nm.^[31] Reprinted with permission from ref. [21]. Copyright 2017 Wiley-VCH.

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  Recently, our group synthesized a new molecule, DyY₂N@C₈₀, by replacing the Y atom in Y₃N@C₈₀ with a dysprosium (Dy) atom, achieving the modulation of luminescence properties. Dy³⁺ causes stronger interactions between the carbon cage and endohedral cluster, and lowers the transition temperature for the TADF changes to 100 K. As shown in Figure 5, the intensity of the fluorescence emission peak near 700 nm gradually increases as the temperature increases from 80 to 100 K. At 100 K, the TADF is dominated. The small ΔE_ST (0.095 eV) in DyY₂N@C₈₀ enables it to exhibit TADF properties. DyY₂N@C₈₀ retains the TADF nature, exhibiting a long lifetime at low temperatures similar to that of Y₃N@₈₀, 145.6 ms for the former and 155.3 ms for the latter. Nevertheless, the fluorescence QY was reduced from 5.85% to 0.59% due to the influence of Dy³⁺.^[33] These results reveal that some metal ions could greatly influence the TADF properties of yttrium-based metallofullerenes, and the changing mechanism should be further studied.

  Figure 5

  

  Figure 5.  (a) Temperature-dependent steady-state PL spectra of DyY₂N@C₈₀; (b) PL peak intensity of DyY₂N@C₈₀ at varied temperatures (λ_exc = 405 nm).^[33] Reprinted with permission from ref. [33]. Copyright 2021 American Chemical Society.

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  The introduction of different endohedral metals can keep the TADF properties of metallofullerenes and regulate the temperature at which the TADF is initiated, but it can also have an impact on the luminescence properties of the molecules, such as shorter fluorescence lifetimes and lower QY. There is plenty of room for development and research on the luminescence of metallofullerenes, especially yttrium-based metallofullerenes, such as the introduction of other metals to modulate the luminescence of yttrium-based metallofullerenes.

  Modulation from the Outside of Carbon Cage. In addition to replacing the internal metal, modifications on fullerene cage can also be employed to modulate luminescence of the yttrium-based metallofullerene. As mentioned above, metallofullerene molecules have nanoscale structure, which gives them possible applications in the field of single-molecule devices. Attaching a single Y₃N@C₈₀ to a gold nanoparticle can upgrade its PL intensity by two orders of magnitude.^[32] It can be seen from Figure 6, as the distance between the gold nanoparticle and the center of a single Y₃N@C₈₀ is narrowed, the more pronounced fluorescence enhancement occurs. The gold nanoparticles act as a simple optical antenna, resulting in a significant increase in excitation rate and quantum efficiency. This result shows a method to strengthen the luminescence of yttrium-based metallofullerenes, in which the surface-interface effect can be designed to modulate the emission. This method would not break the cage symmetry and is suitable to fabricate single-molecule devices. Except for the gold, other metals need to be explored to modulate the emission and the mechanism should be studied as well.

  Figure 6

  

  Figure 6.  PL from a single Y₃N@C₈₀ molecule as a function of separation between molecule and gold nanoparticle. Single Nile blue molecule as a comparison. Dots are experimental data and the solid line is a fit according to a dipole model.^[32] Reprinted with permission from ref. [32]. Copyright 2010 American Chemical Society.

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  Oligo (phenylene ethynylenes) (OPEs) owe their novel opto electronic properties and topology to their regularly arranged π-conjugated structures, and they have attractive applications in the construction of materials for optoelectronics, energy and biology. The Y₃N@C₈₀ liquid crystal derivatives obtained by covalently bonding two OPE units to a Y₃N@C₈₀ carbon cage as highly efficient concentrator antennas exhibit outstanding PL properties.^[34] The PL properties of this derivative are remarkably better than those of the C₆₀ derivative, and the fluorescence emission near 700 nm is greatly enhanced compared to pristine Y₃N@C₈₀. Furthermore, in the deoxygenated solution the QY is high, reaching 8%. The fluorescence lifetime is also longer, with a lifetime 20 times higher than that of pristine Y₃N@C₈₀ at room temperature. These results show that there is large room for the growth of luminescence of yttrium-based metallofullerenes though cage modification and intermolecular interaction.

  Besides, modulation of the PL of yttrium-based metallofullerenes also can be achieved through host-guest interactions. Metallofullerenes are good electron acceptor and are able to form the supramolecular complex with many bent π-conjugated molecules through concave-convex π-π interactions. In particular, Cycloparaphenylene (CPP) nanocycles are able to induce molecular orientation, alter assembly behaviors and modulate spin states when assembled with Y₂@C₇₉N, owing to their well-matched size.^[35] This implies the great potential of such systems for fullerene recognition and property modulation. Therefore, our group also attempted to modulate the PL properties of metallofullerene DyY₂N@C₈₀ through host-guest interactions between ^[12]Cycloparaphenylene (^[12]CPP) and metallofullerenes. Notably, the DyY₂N@C₈₀⊂^[12]CPP complex holds a similar variable temperature PL evolution as DyY₂N@C₈₀, with the TADF on temperature remaining at 100 K. Nevertheless, the PL peak in the 700-750 region is blue-shifted compared to DyY₂N@C₈₀. Moreover, the DyY₂N@C₈₀⊂^[12]CPP complex also maintains the luminescence of ^[12]CPP and has dual-emission.^[33] Most of the supramolecular macrocycles have excellent luminescent properties, and the introduction of such functional molecules can be used to achieve the modulation of luminescent materials and to obtain molecules with dual-emission. In addition, the weak non-covalent bonding interactions between such macrocycles host and guest can achieve covalent bonding effects in a simple manner. And the TADF-based dual-emission materials would bring about a more promising prospect for OLEDs, probing, and sensing in the future.^{[36, 37]} These results show that the host-guest interaction can be employed to effectively tune the luminescence of yttriumbased metallofullerenes. These supramolecular complexes would produce more functions.

  FUNCTIONS AND APPLICATIONS

  Since the 1980s, research into optical sensors has been gradually intensifying. Optical chemical sensors have attracted extensive attention because they can continuously record the concentration of chemical species and physical parameters (e.g. pressure, temperature, etc.). Among many optical methods used for sensing, fluorescence has received special attention because of its high sensitivity and versatility.^[7] The fluorescence of Y₃N@C₈₀ is very sensitive to the external environment. In deoxygenated toluene solution, Y₃N@C₈₀ shows PL with a QY of 1.7% and the lifetime of 0.95 ms, but the QY decreases threefold after contact with air. And in iodobenzene, the QY of Y₃N@C₈₀ reduces slightly due to the influence of external heavy atoms.^[21] Y₃N@C₈₀ liquidcrystal derivatives are also highly sensitive to oxygen in solution, ^[34] which makes them promising for applications in optical sensors.

  Furthermore, improving the fluorescence QY of luminescent substances has been a long-standing effort for researchers, and TADF is considered to be the most promising luminescent material for current applications in OLEDs, imaging as well as sensing. In the field of fullerenes, QY of up to 8% at high temperatures for C₇₀ and up to 22% at 120 K for Y₃N@C₈₀ are based on TADF properties. As shown in Figure 8, in a simple OLED device prepared by embedding Y₃N@C₈₀ into polyfluorene (PFO) as the active material, a new emission signal at 708 nm was observed in the PFO film due to the electroluminescence of Y₃N@C₈₀.^[21] It is shown that with appropriate optimization and QY enhancement, Y₃N@C₈₀ can be made available for red or near-infrared emitting OLEDs.

  Figure 7

  

  Figure 7.  Left: Chemical structures of Y₃N@C₈₀-based (1) and C₆₀-based (2) dyads and their common malonate intermediate dOPE. Right: PL spectra of 1 (-), 2 (- - -), dOPE (-•-•), and Y₃N@C₈₀ (••••••) in deaerated toluene solution at 298 K. (λ_exc = 325 nm).^[34]

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  Figure 8

  

  Figure 8.  Principle of the OLED device (left) and comparison of the electroluminescence of the PFO and PFO: Y₃N@C₈₀ films (right) at room temperature.^[21] Reprinted with permission from ref. [21]. Copyright 2017 Wiley-VCH.

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  In addition, there are partially filled 4f orbitals in the Dy³⁺ ion. When Dy replacing the Y in Y₃N@C₈₀, it brings single-molecule magnet (SMM) properties to yttrium-based metallofullerenes.^[33] SMMs can act like tiny magnets at certain low temperatures and switch between states of "0" and "1". Therefore, SMMs have promising applications in data storage^[38] and quantum computing technology^[39]. DyY₂N@C₈₀ exhibits SMM behavior below 8 K and also holds TADF properties. The combination with luminescent properties makes it possible for SMMs to read or write information more efficiently. The coupling of SMMs and PL has also become an emerging trend, which offers the possibility of designing new types of luminescent SMM devices. Considering the fact that metallofullerenes are able to encapsulate multiple metal atoms within a fullerene cage, they are an excellent carrier for achieving multi-functional molecules.

  The introduction of scandium atoms into the luminescent yttrium-based metallofullerene also brings intriguing properties to them.^[31] Upon photoexcitation, the spin density on the Sc atoms in the endohedral clusters is increased gradually with the number of Sc atoms, contributing to a gradual broadening of the electron paramagnetic resonance (EPR) signal and a shift of its central position towards lower g values. And a regular signal divergence appears in YSc₂N@C₈₀ and Sc₃N@C₈₀. With theoretical calculations, it is evident that only the molecular structure of Y₃N@C₈₀ does not change upon photoexcitation, while the clusters in the remaining three EMFs all undergo rotation. The charge transfer from the fullerenes to the clusters and the spin density on the metal atoms can be the driving force to construct optical switches at the nanoscale.

  CONCLUSION AND OUTLOOK

  This article briefly summarizes the recent research on the PL properties of yttrium-based metallofullerenes, with a focus on the TADF properties of Y₃N@C₈₀ and its derivatives. Combined with theoretical calculations, it is clear that the spatial separation of HOMO and LUMO on the fullerene cage leads to a relatively small ΔE_ST in Y₃N@C₈₀ and endows it with TADF properties. The substitution of Y atoms in the endohedral Y₃N cluster enables the modulation of the PL properties, resulting in varied PL properties for kinds of yttrium-based metallofullerenes. On the other hand, the introduction of optical antennas, covalent modifications on the cage and non-covalent interactions between host and guest also play a role in the modulation of PL properties for yttriumbased metallofullerenes.

  Furthermore, yttrium-based mixed metallofullerenes, such as Y₂ScN@C₈₀, YSc₂@C₈₀ and DyY₂N@C₈₀, bring intriguing properties such as photo-induced structural transformation and SMMs while retaining TADF properties, providing platforms for the design and synthesis of multi-functional molecular materials. As gold nanoparticles can increase the PL intensity and QY of yttrium-based metallofullerenes, this finding can promote the applications as high QY is essential in most cases. The TADF properties of yttrium-based metallofullerenes make it possible to use them in red or near-infrared emitting OLED devices.

  Although several works have been carried out, there are still many possibilities to be explored. There may also be some new metallofullerenes with TADF properties that have not yet been discovered. There is still further work to be done in this field, and we are convinced that there are surprises waiting to be discovered in the luminescence properties for yttrium-based metallofullerenes. In addition, the low yield is still a major problem that restricts their wide application, and how to improve the yield of metallofullerenes is also a direction that researchers are constantly working on.

  
  ACKNOWLEDGEMENTS: We thank the National Natural Science Foundation of China (52022098, 51972309, 51832008). T. Wang particularly thanks the Youth Innovation Promotion Association of Chinese Academy of Sciences (Y201910). COMPETING INTERESTS
  There are no conflicts to declare.
  Full paper can be accessed via http://manu30.magtech.com.cn/jghx/EN/10.14102/j.cnki.0254-5861.2022-0105
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  Figure 1  Optimized molecular structure of Y₃N@C₈₀.

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  Figure 2  Schematic presentation of the luminescence mechanism with delayed fluorescence.^[31] Reprinted with permission from ref. [31]. Copyright 2021 Royal Society of Chemistry.

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  Figure 3  Luminescence spectra of Y₃N@C₈₀ in polystyrene film were measured at 10 K steps during cooling from (a) 290 to 120 K, and (b) from 120 to 20 K (λ_exc = 405 nm); (c) Temperature profiles of the relative integral emission intensity (black dots) and relative peak intensities at 701 and 739 nm (dashed lines); the values are referred to 290 K, ranges with different emission mechanisms are marked (P = phosphorescence, TADF = thermally activated delayed fluorescence); (d) Luminescence lifetimes of Y₃N@C₈₀ in polystyrene at different temperatures (the inset shows the same on a logarithmic scale), the blue line is the fitted curve.^[21] Reprinted with permission from ref. [21]. Copyright 2017 Wiley-VCH.

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  Figure 4  Left: molecular structure of Y_xSc_3-xN@C₈₀ (x = 0-3) (Y – green, Sc – magenta, N – blue, C – light grey). Middle: the luminescence (lum) and low-energy absorption (abs) spectra measured in toluene solution at room temperature. (Asterisks mark an instrumental artefact appearing as a negative peak near 950 nm.) Right: the luminescence spectra at three selected temperatures (270 K; the temperature at which PL has the highest intensity; the lowest temperature studied), λ_exc = 405 nm.^[31] Reprinted with permission from ref. [21]. Copyright 2017 Wiley-VCH.

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  Figure 5  (a) Temperature-dependent steady-state PL spectra of DyY₂N@C₈₀; (b) PL peak intensity of DyY₂N@C₈₀ at varied temperatures (λ_exc = 405 nm).^[33] Reprinted with permission from ref. [33]. Copyright 2021 American Chemical Society.

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  Figure 6  PL from a single Y₃N@C₈₀ molecule as a function of separation between molecule and gold nanoparticle. Single Nile blue molecule as a comparison. Dots are experimental data and the solid line is a fit according to a dipole model.^[32] Reprinted with permission from ref. [32]. Copyright 2010 American Chemical Society.

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  Figure 7  Left: Chemical structures of Y₃N@C₈₀-based (1) and C₆₀-based (2) dyads and their common malonate intermediate dOPE. Right: PL spectra of 1 (-), 2 (- - -), dOPE (-•-•), and Y₃N@C₈₀ (••••••) in deaerated toluene solution at 298 K. (λ_exc = 325 nm).^[34]

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  Figure 8  Principle of the OLED device (left) and comparison of the electroluminescence of the PFO and PFO: Y₃N@C₈₀ films (right) at room temperature.^[21] Reprinted with permission from ref. [21]. Copyright 2017 Wiley-VCH.

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