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 (<i>Δ</i>_ST). This gap often restricts the efficiency of organic light-emitting diodes (OLEDs). Reducing <i>Δ</i>_ST has the potential to significantly enhance internal quantum efficiency. A promising approach involves minimizing <i>Δ</i>_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 <i>ab initio</i> 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 <i>Δ</i>_ST. Furthermore, basic structural designs within networks were developed and evaluated. The results reveal that these compounds possess significantly negative <i>Δ</i>_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.