Breaking the trade-off between ΔEST and oscillator strength in hybrid LR/SR-CT compounds for enhanced TADF performance

Abstract

Achieving an ideal thermally activated delayed fluorescence (TADF) emitter requires balancing a minimal singlet–triplet energy gap (ΔE_ST) with a considerable oscillator strength (f), which are typically mutually exclusive. By strategically integrating a subsidiary short-range charge transfer (SRCT) with primary long-range charge transfer (LRCT), we have designed hybrid TADF emitters through quantum chemical calculations and numerical simulations. These compounds exhibit large singlet radiative rates (∼10⁷ s⁻¹), significantly higher than intersystem crossing rates, along with substantial reverse intersystem crossing (RISC) rates (up to ∼10⁷ s⁻¹) and sub-microsecond lifetimes for the delayed component. High internal quantum efficiencies (IQE = 100%) and photoluminescence quantum yields (PLQY > 90%) were achieved in our simulations. The designed compounds exhibit significant bond dissociation energies, indicating high stability and potentially long device lifetimes. This innovative approach of combining SRCT and LRCT effectively breaks the trade-off between ΔE_ST and f, offering a new pathway for developing highly efficient and stable TADF emitters.