The SUMO (Singly Unoccupied Molecular Orbital)-LUMO (Lowest Unoccupied Molecular Orbital) Inversion Radicals

Herein, SUMO-LUMO inversion (SLI) radicals 2a–2c were designed by the combination of the tris(2,4,6-trichlorophenyl)methyl (TTM) radical and pyridinium derivatives (electron-withdrawing groups) for the first time. The energy of the LUMO lies below that of the SUMO, which deviated from the Aufbau principle as an alternative electronic configuration beyond the well-established SOMO-HOMO inversed system. Thus, for SLI radicals, the injection of one extra electron preferred to occupy the LUMO rather than the SUMO, giving diradicals, one of which 3 had been fully confirmed by single crystal analysis, VT-NMR and VT-EPR experiments, as well as DFT calculations. Furthermore, SLI radicals 2a–2c exhibit photoluminescent properties and good photostability under UV irradiation (t_1/2 > 24 days for 2c). This finding would open up new exploration on SLI radicals and their potential applications as photoelectromagnetic material.