Construction of stable luminescent donor–acceptor neutral radicals: a theoretical study

Abstract

Stable luminescent donor–acceptor (D–A˙) radical molecules with theoretically 100% internal quantum efficiency in emission have recently attracted a lot of attention, especially in light-emitting diodes. However, radicals are intrinsically highly reactive, short-lived and unstable; stabilization of organic radicals is of crucial importance. Here, we conduct a systematic theoretical study of luminescent D–A˙ neutral radicals, aiming to elucidate the relationship between the stability and the structural properties of radicals in terms of thermodynamic, kinetic, and electrochemical stabilities based on radical stabilization energy (RSE), the percentage of buried volume (%V_Bur) and ionization potential (IP), respectively. With these stability parameters and a newly proposed index to measure the whole stability of radicals, PTM-based radicals have the highest overall stability among them and the stability of BTM-based radicals is the most enhanced one after substituent modification. Moreover, a small dihedral angle and a short bond length between the donor and the acceptor can improve the thermodynamic stability of radicals; large donor groups will effectively fill the spatial vacancy of the radical center to improve the kinetic stability; and weak donors prevent oxidation and reduction of radicals and improve electrochemical stability. Thus, based on these structure–property relationships, five highly stable neutral radical molecules by introducing bulky weak donor substituents are designed, among which two molecules performed particularly well. This study not only provides an efficient method to evaluate and design highly stable luminescent radical molecules but also reveals important guidelines for the exploration of stable radicals.