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

Abstract

The inversion of the energies of the lowest singlet (S₁) and lowest 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, wave-function based ab initio computational methods were employed to explore the possibility of S₁–T₁ inversion in hexagonal polycyclic aromatic and heteroaromatic compounds. In these molecules, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are two-fold degenerate. The HOMO–LUMO transition gives rise to three singlet and three triplet excited states. While the singlet–triplet energy gap Δ_ST, defined as the energy difference between the S₁ state and the T₁ state, is clearly positive for benzene, it is predicted to be close to zero for borazine, the boron nitride analogue of benzene. Although Δ_ST decreases with increasing size of hexagonal polycyclic aromatics, it remains positive up to circumcoronene (19 rings). However, symmetry-preserving 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 Δ_ST. These findings establish the existence of a new family of boron carbon nitrides with inverted singlet–triplet gaps.