Singlet–Triplet\nInversion in Triangular Boron\nCarbon Nitrides

The discovery of singlet–triplet\n(ST) inversion\nin some\nπ-conjugated triangle-shaped boron carbon nitrides is a remarkable\nbreakthrough that defies Hund’s first rule. Deeply rooted in\nstrong electron–electron interactions, ST inversion has garnered\nsignificant interest due to its potential to revolutionize triplet\nharvesting in organic LEDs. Using the well-established Pariser–Parr–Pople\nmodel for correlated electrons in π-conjugated systems, we employ\na combination of CISDT and restricted active space configuration interaction\ncalculations to investigate the photophysics of several triangular\nboron carbon nitrides. Our findings reveal that ST inversion in these\nsystems is primarily driven by a network of alternating electron-donor\nand electron-acceptor groups in the molecular rim, rather than by\nthe triangular molecular structure itself.