Computational design of efficient near-infrared TADF emitters with hot-exciton characteristics

Abstract

Developing near-infrared (NIR) TADF emitters is challenging due to the inherent energy gap law. In this work, we designed a set of twelve donor–acceptor1–acceptor2 (D–A1–A2) type NIR pure organic emitter molecules that contain a strong donor (triphenylamine (TPA)), a cyano group substituted anthrathiadiazole (AZ) unit as A1, and fused aromatic/heterocyclic molecules as the A2 unit. The strength of the acceptor part A2 is altered by introducing electron-withdrawing groups (–H, –F, –CN). We studied their geometrical, electronic, and excited state properties using Density functional theory (DFT) and time-dependent DFT methods. Comprehensive analysis of excited state properties obtained from computational methods such as the energy gap between singlet and triplet excited states (ΔE_ST), spin–orbit coupling (SOC) values, the nature of singlet and triplet excited states, rates of reverse intersystem crossing (k_RISC), and radiative and non-radiative emissions (k_r and k_nr) are conducted to acquire insights into the NIR emission in the studied molecules. Our calculated results show that the molecules should possess hybrid localized and charge transfer (HLCT) character dominated by Frenkel-type excitation (LE) in the lowest singlet excited state to achieve faster k_r. Furthermore, the strong donor, AZ unit, and moderate acceptor A2 unit provide smaller energy gaps between singlet and triplet states with reasonable SOC values in the higher excited states. In our study, we identify multiple hot-exciton channels to up-convert dark triplet excitons into bright singlet excitons, which might improve the exciton utilization efficiency.