Enhanced broadband yellow emission from charge-transfer states in a TADF organic ionic pair

Organic ionic pairs offer a promising route to thermally activated delayed fluorescence (TADF) by enabling small singlet-triplet energy gaps (ΔE _ST) and tunable charge-transfer (CT) interactions. However, balancing between HOMO-LUMO decoupling with moderate SOC and suppression of nonradiative decay remains a challenge. In this work, an electrostatically bound ionic pair, acridinium p-nitrobenzoate (AcPNB) exhibits a well-defined donor-acceptor (D-A) pair, and its physicochemical properties were studied. Single-crystal analysis reveals slipped π-π stacking (3.4 Å) and a slightly twisted D-A geometry (dihedral angle of -11.86°), while strong N-H⋯O and C-H⋯O interactions combinedly stabilize CT states. Density functional theory (DFT) calculations reveal decoupled frontier molecular orbitals, with a small ΔE _S1T1 of 6 meV and a SOC of 2.4 cm^-1. The AcPNB exhibits atypical broad photoluminescence (PL) emission centred at 578 nm, attributed to CT emission. The prompt and delayed lifetimes in ns and µs, together with non-monotonic temperature-dependent PL confirm a charge-transfer excited state and characteristics of the TADF mechanism. The cyclic voltammetric studies experimentally confirm the charge-transfer process, and this value relates to the DFT calculated HOMO value. The melting transition occurs at 171 °C, indicating strong ionic interactions against thermal decomposition. These findings give insights into the ionic pair-based TADF organic emitter for OLED applications by exploiting the advantages of decoupled HOMO-LUMO, SOC, and low ΔE _ST.