Tuning frontier orbital energies of the TTBrM-PPTA radical <i>via</i> electron donor and acceptor substituents

The radical TTBrM-PPTA (<i>a</i>) exhibits a SOMO-HOMO inversion (SHI) electronic structure, in which the singly occupied molecular orbital (SOMO) lies below the highest doubly occupied orbital (HOMO). To investigate the modulation of this unique electronic structure, a series of TTBrM-PPTA-based radicals were designed by introducing various electron-donating (-N(CH₃)₂, -OCH₃, -OH and -C(CH₃)₃) and electron-withdrawing groups (-CF₃, -NO₂ and -CN) into the PPTA moiety. Density functional theory calculations reveal that these substituents can modulate the SOMO-HOMO gap and, in some cases, even alter the ordering of the frontier orbital energies. Overall, the SOMO remains unaffected upon substitution, while the electron-withdrawing groups reduce the α-HOMO, and the electron-donating groups raise it relative to the TTBrM-PPTA radical. Notably, substituents with lower adiabatic ionization potentials (AIPs), which are typically electron-donating groups, enhance SHI behavior by facilitating spatial separation between the HOMO and the LUMO. These findings demonstrate the effectiveness of the secondary electron-donating substituents in tuning frontier molecular orbital energy levels and provide a promising strategy for developing SHI-type luminescent organic radicals.