Excited-state singlet-triplet inversion in hexagonal aromatic and heteroaromatic compounds

The inversion of the energies of the lowest singlet (S1) and triplet (T1) excited states in violation of Hund’s multiplicity rule is a rare phenomenon in stable organic molecules. S1-T1 inversion has significant consequences for the photophysics and photochemistry of organic chromophores. In this work, ab initio computational methods were employed to explore the possibility of S1-T1 inversion in hexagonal polycyclic aromatic and heteroaromatic compounds. Although the singlet-triplet energy gap ΔST = ES1 – ET1 decreases with increasing size of hexagonal polycyclic aromatics, it remains positive up to kekulene (19 rings). However, symmetric substitution of C-C pairs by B-N groups in the interior, keeping the conjugation of the outer rim intact, results in compounds with robustly negative ΔST. The non-overlapping pattern of the densities of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is identified as the decisive criterion for S1-T1 inversion. These findings establish the existence of a new family of boron carbon nitrides with inverted singlet-triplet gaps.