Fig. 4. Schematic diagrams showing the TD-DFT-calculated energy levels of the singlet (S₁) and triplet (T_n) states (PBA-MeO). Note that H and L refer to the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), respectively, and only the T_n states lower than S₁ are shown. The conformations were derived from single-crystal diffraction data. The blue arrows represent the probable ISC processes occurring from the S₁ state to the T_n state, while the black arrows refer to transitions to the other triplet states available, but with a larger ΔE_ST (>0.4 eV).