Achieving high color purity in multi-resonance thermally activated delayed fluorescence emitters through a substitution-driven design strategy

Abstract

To improve the visual quality and develop high-resolution displays, organic light-emitting diodes (OLEDs) with high color purity have garnered increasing attention. The color purity of OLEDs, which is determined by the full width at half-maximum of emission spectra, is associated with the vibronic coupling of emitters between the ground and emitting states. In this work, detailed theoretical analyses of the reorganization energy, which can characterize the strength of vibronic coupling, were conducted to clarify the color purity variations of B, O-doped polycyclic aromatic compounds with multi-resonance thermally activated delayed fluorescence (MR-TADF). The calculated results reveal that alterations in the bond length make the main contribution to the reorganization energies of these highly conjugated aromatic molecules. It is found that the origin of the large reorganization energy can be elucidated from the perspective of molecular orbitals (MOs). Moreover, the reorganization energy variations among these molecules can be rationalized by MO distribution. Based on these findings, we propose two substitution-driven design strategies to improve color purity. The first strategy involves introducing a neutral phenyl group to delocalize the orbital distribution, thereby weakening the bonding or antibonding character in frontier molecular orbitals of the bonds with a large reorganization energy. The second strategy entails the substitution at the position related to large reorganization energy with electron-donating or electron-withdrawing groups, thus decreasing the bond order difference between the MOs involved in the transition.