Theoretical explorations of new heteroaromatic compounds with inverted singlet–triplet gaps for OLED emitters

Hund's multiplicity rule asserts that the energy of the lowest excited singlet state (S₁) is invariably higher than that of the lowest excited triplet state (T₁) in organic compounds, leading to a positive singlet–triplet energy gap (Δ_ST). This gap often restricts the efficiency of organic light-emitting diodes (OLEDs). Reducing Δ_ST has the potential to significantly enhance internal quantum efficiency. A promising approach involves minimizing Δ_ST, facilitating reverse intersystem crossing (RISC) from T₁ to S₁ without requiring thermal activation. In this study, we present a novel class of organic compounds exhibiting inverted singlet–triplet gaps (IST), where T₁ lies above S₁, enabling RISC to occur without thermal excitation. A series of heteroaromatic compounds was designed by substituting C–C bonds in aromatic hydrocarbons with symmetrically arranged B–N groups in hexagonal patterns. Employing ab initio computational methods, we examined their electronic properties and assessed their potential for S₁–T₁ inversion. Frontier molecular orbitals were analyzed to support our findings regarding Δ_ST. Furthermore, basic structural designs within networks were developed and evaluated. The results reveal that these compounds possess significantly negative Δ_ST values, validating the existence of a novel category of boron–nitride-based heteroaromatics with inverted singlet–triplet gaps. This breakthrough paves the way for the development of highly efficient OLED materials, promising both enhanced performance and extended longevity.