Design of non-Aufbau organic radicals based on TTM/TTBrM core

Abstract

Non-Aufbau organic radicals with SOMO-HOMO inversion (SHI) have been attracting significant interest from both theoretical and experimental perspectives. The unusual orbital ordering in these systems makes their electronic properties apart from the more common case, where the unpaired electron is filled in the SOMO which is higher in energy than HOMO. In this work, we designed a series of radical systems based on well-known TTM/TTBrM core, functionalized by various electron donor groups (PDMAC, PPTA, PPXZ, BFCz, PA and PT) to achieve SHI-type radicals. Using density functional theory, it was demonstrated that these electron donor groups can modulate the order of SOMO and HOMO, and all radical systems follow the general illustration of SHI that the α-HOMO pair of matching spin orbitals is higher in energy than the SOMO, and is below the corresponding SUMO in non-Aufbau electronic structures. Moreover, all radical systems exhibit fluorescence radiative rate (k_r) approaching approximately 10⁷ s⁻¹. Ab Initio Molecular Dynamics (AIMD) simulation further indicates that all radical systems are stable at high temperatures. Our calculated results show that the formation of SHI-type radical systems is related to the nature of electron donor units. Consequently, PDMAC, PPTA, PPXZ, BFCz, PA, and PT are reliable choices for designing SHI-type radical emitters. Our work thus establishes molecular guidelines for designing SHI-type radical systems and expands their applicability to organic light-emitting diode emitters.