Thermally Activated Delayed Fluorescence in a Y3 N@C80 Endohedral Fullerene: Time-Resolved Luminescence and EPR Studies

Covalency versus magnetic axiality in Nd molecular magnets: Nd-photoluminescence, strong ligand-field, and unprecedented nephelauxetic effect in fullerenes NdM2N@C80 (M = Sc, Lu, Y)

Nd-based nitride clusterfullerenes NdM2N@C80 with rare-earth metals of different sizes (M = Sc, Y, Lu) were synthesized to elucidate the influence of the cluster composition, shape and internal strain on the structural and magnetic properties. Single crystal X-ray diffraction revealed a very short Nd-N bond length in NdSc2N@C80. For Lu and Y analogs, the further shortening of the Nd-N bond and pyramidalization of the NdM2N cluster are predicted by DFT calculations as a result of the increased cluster size and a strain caused by the limited size of the fullerene cage. The short distance between Nd and nitride ions leads to a very large ligand-field splitting of Nd3+ of 1100-1200 cm-1, while the variation of the NdM2N cluster composition and concomitant internal strain results in the noticeable modulation of the splitting, which could be directly assessed from the well-resolved fine structure in the Nd-based photoluminescence spectra of NdM2N@C80 clusterfullerenes. Photoluminescence measurements also revealed an unprecedentedly strong nephelauxetic effect, pointing to a high degree of covalency. The latter appears detrimental to the magnetic axiality despite the strong ligand field. As a result, the ground magnetic state has considerable transversal components of the pseudospin g-tensor, and the slow magnetic relaxation of NdSc2N@C80 could be observed by AC magnetometry only in the presence of a magnetic field. A combination of the well-resolved magneto-optical states and slow relaxation of magnetization suggests that Nd clusterfullerenes can be useful building blocks for magneto-photonic quantum technologies.

Single-Molecule Magnet with Thermally Activated Delayed Fluorescence Based on a Metallofullerene Integrated by Dysprosium and Yttrium Ions

It is vital to construct luminescent single-molecule magnets (SMMs) and explore their applications in quantum computing technique and magneto-luminescence devices. In this work, we report a luminescent single-molecule magnet with thermally activated delayed fluorescence (TADF) based on metallofullerene DyY₂N@C₈₀. DyY₂N@C₈₀ was constructed by integrating dysprosium and yttrium ions into a fullerene cage. Magnetic results suggest that DyY₂N@C₈₀ exhibits magnetic hysteresis loops below 8 K originating from the Dy³⁺ ion. Moreover, DyY₂N@C₈₀ exhibits TADF originating from the Y³⁺-coordinated carbon cage, whose luminescence peak positions and peak intensities can be obviously influenced by Dy³⁺. Furthermore, a supramolecular complex of DyY₂N@C₈₀ and [12]Cycloparaphenylene ([12]CPP) was then prepared to construct a single-molecule magnet with multiwavelength luminescence. The effects of host-guest interaction on photoluminescence properties of DyY₂N@C₈₀ were disclosed. Theoretical calculations were also employed to illustrate the structures of DyY₂N@C₈₀ and DyY₂N@C₈₀⊂[12]CPP.

Metallofullerene photoswitches driven by photoinduced fullerene-to-metal electron transfer

We report on the discovery and detailed exploration of the unconventional photo-switching mechanism in metallofullerenes, in which the energy of the photon absorbed by the carbon cage π-system is transformed to mechanical motion of the endohedral cluster accompanied by accumulation of spin density on the metal atoms. Comprehensive photophysical and electron paramagnetic resonance (EPR) studies augmented by theoretical modelling are performed to address the phenomenon of the light-induced photo-switching and triplet state spin dynamics in a series of Y _x Sc_3-x N@C₈₀ (x = 0-3) nitride clusterfullerenes. Variable temperature and time-resolved photoluminescence studies revealed a strong dependence of their photophysical properties on the number of Sc atoms in the cluster. All molecules in the series exhibit temperature-dependent luminescence assigned to the near-infrared thermally-activated delayed fluorescence (TADF) and phosphorescence. The emission wavelengths and Stokes shift increase systematically with the number of Sc atoms in the endohedral cluster, whereas the triplet state lifetime and S₁-T₁ gap decrease in this row. For Sc₃N@C₈₀, we also applied photoelectron spectroscopy to obtain the triplet state energy as well as the electron affinity. Spin distribution and dynamics in the triplet states are then studied by light-induced pulsed EPR and ENDOR spectroscopies. The spin-lattice relaxation times and triplet state lifetimes are determined from the temporal evolution of the electron spin echo after the laser pulse. Well resolved ENDOR spectra of triplets with a rich structure caused by the hyperfine and quadrupolar interactions with ¹⁴N, ⁴⁵Sc, and ⁸⁹Y nuclear spins are obtained. The systematic increase of the metal contribution to the triplet spin density from Y₃N to Sc₃N found in the ENDOR study points to a substantial fullerene-to-metal charge transfer in the excited state. These experimental results are rationalized with the help of ground-state and time-dependent DFT calculations, which revealed a substantial variation of the endohedral cluster position in the photoexcited states driven by the predisposition of Sc atoms to maximize their spin population.

Turning On the Near-Infrared Photoluminescence of Erbium Metallofullerenes by Covalent Modification

The photoluminescence of lanthanide ions inside fullerenes is usually very weak due to the quenching effect of the fullerene cage. In the case of Er@C₈₂, the near-infrared emission from the Er³⁺ ion is completely quenched by the C₈₂ fullerene cage. It remains challenging to turn on the photoluminescence of Er@C₈₂ and other monometallofullerenes. In this work, we adopt a covalent modification strategy to alter the electronic structure of the fullerene cage for sensitizing the near-infrared emission of Er³⁺ ions in metallofullerenes Er@C_2n (2n = 72, 76, and 82). After covalent modification with trifluoromethyl, phenyl, or dichlorophenyl groups, the erbium metallofullerenes exhibit photoluminescence at 1.5 μm, which is the characteristic emission of the Er³⁺ ion. Particularly, the otherwise nonfluorescent metallofullerene Er@C₈₂ is transformed into fluorescent derivatives by using this strategy. The photoluminescence from the Er³⁺ ion is ascribed to energy transfer from the fullerene cage to the Er³⁺ ion. According to theoretical calculations, the sensitization of the Er³⁺ ion by the fullerene cage is associated with the large HOMO-LUMO gap and the closed-shell electronic structure of the metallofullerene derivatives. This work provides useful guidance for the design and synthesis of new fluorescent metallofullerenes.

Thermally Activated Delayed Fluorescence in a Y3 N@C80 Endohedral Fullerene: Time-Resolved Luminescence and EPR Studies

The endohedral fullerene Y3 N@C80 exhibits luminescence with reasonable quantum yield and extraordinary long lifetime. By variable-temperature steady-state and time-resolved luminescence spectroscopy, it is demonstrated that above 60 K the Y3 N@C80 exhibits thermally activated delayed fluorescence with maximum emission at 120 K and a negligible prompt fluorescence. Below 60 K, a phosphorescence with a lifetime of 192±1 ms is observed. Spin distribution and dynamics in the triplet excited state is investigated with X- and W-band EPR and ENDOR spectroscopies and DFT computations. Finally, electroluminescence of the Y3 N@C80 /PFO film is demonstrated opening the possibility for red-emitting fullerene-based organic light-emitting diodes (OLEDs).