Theoretical Explorations of New Heteroaromatic Compounds with Inverted Singlet-Triplet Gaps for OLED Emitters

Abstract

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 (∆ST). This gap often restricts the efficiency of organic light-emitting diodes (OLEDs). Reducing ∆ST has the potential to significantly enhance internal quantum efficiency. One promising approach involves minimizing ∆ST to facilitate 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 ∆ST. Furthermore, basic structural designs within networks were developed and evaluated. The results reveal that these compounds possess significantly negative ∆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.