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 YxSc3−xN@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.

0.3 band-pass 820-880 nm a In addition to the measureemmnts in the 820-880 nm range with PMT PMA 192, test PL lifetime measureemnt of Sc3N@C80 were also performed at λ>1000 nm with ID230 NIR single photon counter. These measurements did not reveal processes with different lifetimes and are thus not discussed.

DFT calculations
Computational details. Each structure was optimized in the S0 and T1 electronic states at the PBE level using the Priroda code 3, 4 with the implemented basis set of TZ2P quality with an effective core potential for Sc and Y atoms. Each unique conformer found in this screening was verified to be a true minimum by a Hessian calculation. Optimized coordinates of unique T1 conformers were then re-optimized in the S1 state at 5the TD-DFT level. Single point energy calculations at the PBE/def2-TZVPP level with ZORA scalarrelativistic corrections were then performed for all unique conformers in S0, S1, and T1 states using ORCA suite, which was also used for calculations of hyperfine tensors and g-factors in the triplet state. [5][6][7][8][9][10][11] Relative energies of all computed conformers in S0, T1, and S1 states are listed in Table S5, whereas Figures S2-S3 show isosurfaces of HOMO, LUMO, difference density Δρ(S0→S1), and spin density ρspin(T1) for selected conformers Table S5. Relative energies of YxSc3-xN@C80 conformers in S0, S1, and T1 electronic states (in eV) 29 Relative energies are referred to the lowest-energy conformers of the ground electronic state; "S0" denotes the energy of the S0 state in optimized S0 geometry, "S0{S1}" is the energy of the S0 state in the optimized S1 geometry, and "S0{T1}" is the energy of the S0 state in the optimized T1 geometry; ΔST is the adiabatic energy difference between S1 and T1, and ρspin(T1) is net Mulliken spin population of the M3N cluster in the triplet state 224 Δ-SCF denotes computations of the triplet state by ground-state DFT with spin multiplicity 2S+1 = 3. TD-DFT denotes computations of the triplet state energy with TD-DFT, using geometry optimized with Δ-SCF approach. The energies are referenced versus conf 1 in its S0 state (which gives excitation energies), or versus conf 3 in its T1 state (which gives relative energies of conformers). For the anions, electron affinity EA is computed as the energy difference of the optimized anion of a given conformer and the neutral Sc3N@C80, conf 1 in S0 state.   7,8,12 Computed parameters are: Principal values of A-tensor (Ax,Ay,Az) and their average (Aiso) in MHz, principal values of g-tensor (gx, gy, gz) and their average (giso), nuclear quadrupolar coupling |e 2 Qq/h| in MHz, and asymmetry parameter η.
Comparison of experimental and computed 45 Sc hfc constants shows a systematic underestimation of the experimental values by theory. Similar underestimation was also observed for the 45 Sc hfc constants in Sc3N@C80 − anion in the exhaustive study, 13 which used various density functional and basis sets. A possible reason of this underestimation is the influence of the dynamic effects on the experimental values, 14 but the failure of DFT approach to correctly describe the polarization of core s-electrons in Sc atoms is also a possibility.

Photoelectron spectroscopy
Photoelectron spectra of Sc3N@C80 − were obtained using a size-selective cryogenic photoelectron spectroscopy apparatus that couples an electrospray ionization (ESI) source and a temperature-controlled ion trap to a magnetic-bottle time-of-flight photoelectron spectrometer. 15 A small sample of Sc3N@C80 was dissolved in toluene and reduced by adding diluted tetrakis(dimethylamino)ethylene (TDAE) in CH3CN under a nitrogen-filled glovebox. The resulting ESI solution was ca. 5x10 -4 M. Sc3N@C80 − anions were directed by two rf-only quadruples into the cryogenic ion trap, where the ions were accumulated and cooled down to 12 K in order to eliminate vibrational hot bands and achieve optimal spectral resolution. The Sc3N@C80 − anions were mass-selected and then maximally decelerated (to minimize Doppler Broadening) before being photodetached with 266 nm (4.661 eV) or 355 nm (3.496 eV) photons. The laser was operated at a 20 Hz repetition rate with the ion beam off on alternating laser shots for background subtraction. Photoelectrons were collected at nearly 100% efficiency by the magnetic bottle and analyzed in a 5.2 m long electron flight tube. TOF photoelectron spectra were collected and converted to kinetic energy spectra calibrated using the known spectra of I − (ref 16 ) and OsCl6 2-(ref 17 ). The electron binding energy spectra were obtained by subtracting the kinetic energy spectra from the detachment photon energies used. The gas-phase electron affinity was directly measured from the 0-0 transition in the corresponding photoelectron spectrum.

Light-induced pulsed EPR
Saturated solutions of Y3N@C80, Y2ScN@C80 , YSc2N@C80 and Sc3N@C80 were prepared in toluene-d8 (Sigma-Aldrich) at ambient conditions (without degassing), filled in the W-band quartz tubes, and flashfrozen by immersing into the liquid nitrogen before insertion into the microwave cavity of the spectrometer.

Sample illumination in EPR/ENDOR experiments
The continuous irradiation in W-band EPR/ENDOR experiments was achieved with the Power Technology Inc. IQ1C laser (510 nm, 40mW). A pulsed Nd-YAG Innolas SpitLight Compact 400 laser equipped with OPO (1.5 mJ/pulse for W-band, 3 mJ/pulse for X-band at 488 nm) was used in time-resolved measurements.

W-band pulsed EPR measurements
W-band measurements were performed at 20 K using a Bruker ELEXSYS E680 spectrometer operating at about 94 GHz. All experiments were carried out with a homebuilt ENDOR microwave cavity. [18][19][20] Electron spin echo-detected (ESE) field-swept spectra were measured using the Hahn echo pulse sequence tp−τ− 2tp−τ−echo with tp = 20 ns and τ = 300 ns. In continuous irradiation experiments (510 nm, 40 mW), 20-100 echoes were accumulated depending on the S/N and integrated over 140 ns around their maximum at each field position. The pulse repetition time was set to 2 ms. In the time-resolved EPR measurements, 12 ns laser pulse and a 500 ns delay after laser flash (tDAF) preceded the Hahn echo sequence. Four echoes per field point were recorded with a repetition time of 2.5 s (laser repetition time). The decay of the polarized ESE signals was followed by incrementing the tDAF in the range of 0.5μs-900 ms.
The Mims-type ENDOR spectra were measured at 20 K under continuous irradiation of the sample (510 nm, 40 mW) using the Mims-type ENDOR sequence tp−τ−tp−tRF−tp−τ−echo, with an RF pulse applied during the time interval tRF. The experimental conditions were tp = 40 ns, tRF = 48 μs or 60 μs, and τ = 564 ns. All ENDOR spectra were recorded using the stochastic acquisition mode with two shots for each point, and the total number of scans was varied in the range 100−400 depending on the S/N. The Davies-type ENDOR spectra were measured at 20 K under continuous irradiation of the sample (510 nm, 40 mW) using the ENDOR sequence tinv−tRF−tp−τ−2tp−τ−echo. The experimental conditions were tinv = 100 ns, tp = 40 ns, tRF = 10 μs and τ = 1000 ns. Davies-type ENDOR spectra were recorded using the stochastic acquisition mode with two shots for each point, and the total number of scans was varied in the range 500−1000 depending on the S/N. The EPR and ENDOR spectra were analyzed by computer simulation within the Spin Hamiltonian framework sketched in the manuscript using the EasySpin 21 package running under Matlab. Figure S5. W-band (94 GHz) time-resolved ESE EPR spectra of the EMF triplets from a) Y3N@C80, b) Y2ScN@C80, d) YSc2N@C80 and d) Y3N@C80. The pale lines show the polarized spectra recorded directly after the laser flash while the full colors represent records after tDAF of few ms when the polarization already relaxed. The polarized spectra (pale lines) reflect the non-equilibrium population of triplet energy levels built during the intersystem crossing and include absorptive (A) and emissive (E) signals. The "relaxed" spectra (full colors) correspond to the thermal Boltzmann population of the triplet levels and are purely absorptive.