Positional Isomerism and Chlorine-Driven Crystal Packing Effects in Acridinium-Based Organic Ionic Pairs for TADF

Three precisely engineered acridinium-based ionic pairs─acridinium 5-chlorosalicylate (Ac5ClSA), acridinium salicylate (AcSA), and acridinium 3-chlorosalicylate (Ac3ClSA) ─ contain each a well-defined donor-acceptor (D--A+) arrangement. This deliberate molecular design enables efficient thermally activated delayed fluorescence (TADF) for organic light-emitting diodes (OLEDs) applications. These acridinium-based ionic pairs exemplify how the positional isomerism of the chlorine substituent tunes solid-state fluorescence, by offering low ΔEST, different molecular packings that control charge transfer characteristics, and enabling emission color tuning in the region red to green. Their suitability for TADF was theoretically predicted through density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, which demonstrated decoupled HOMO–LUMO orbitals with ΔEST < 0.026 eV. The lifetime measurements confirm the presence of delayed fluorescence, with distinct lifetimes for triplet and singlet excitons, confirming the TADF. All three ionic pairs exhibit exciplex-type excited states, driven by electrostatic interactions between the anionic salicylates and the cationic acridinium ion. FT-IR analysis confirmed the presence of an NH+ bond at 2331 cm–1 for all three ionic pairs, confirming the proton transfer from salicylic to acridine. Ac5ClSA demonstrated the highest photoluminescence quantum yield (PLQY) of 13%. Furthermore, all the ionic pairs were doped with PMMA polymer with 10, 20, and 30% concentrations to optimize the emission property and to study electrostatic interactions. Cyclic voltammetry (CV) results aligned well with DFT-calculated HOMO values, demonstrating the intramolecular and intermolecular charge transfer (ICT) characteristics. The thermal stability of ionic pairs is high as expected, due to their ionic nature. This study establishes that the ionic pairs are promising candidates for next-generation TADF and deepens our understanding of structure–property relationships that guide the rational design for controlled emission characteristics.