Tuning fluorescence efficiency and white light emission in acridone derivatives with solvent polarity and aggregation-induced emission

Three acridone-ethynylbenzoate derivatives with bromine ( BrAcBz ), diphenylamine ( DPAAcBz ) and indoline ( IndAcBz ) donor groups were synthesised, and their electronic spectral and photophysical properties were investigated in different solvents and solvent mixtures. Fluorescence emission showed a dependence on the polarity of the medium, with locally excited (LE) and intramolecular charge transfer (ICT) emission bands/states found to be dependent on the donor and acceptor ability. In the case of BrAcBz , the spectral and photophysical properties are attributed to a LE state, with the absence of a CT state and dominance of radiationless decay processes, as indicated by the constant fluorescence lifetime of approximately 1 ns. In contrast, DPAAcBz and IndAcBz exhibit higher fluorescence efficiencies, mirrored by their quantum yields and lifetimes – ranging from 7 to 22 ns for DPAAcBz and 9 to 15 ns for IndAcBz – where both compounds show competition between radiative and radiationless decay processes. TDDFT quantum chemical calculations indicate that donor substitution enhances HOMO delocalisation throughout the molecule, extending from the donor group to the ethynylbenzoate moiety. In BrAcBz , the emission mainly arises from a LE state, consistent with the small spatial separation between the HOMO and LUMO and the solvent-independent emission behaviour observed. In mixtures of THF:water, a classical AIE effect is observed for BrAcBz whereas for DPAAcBz and IndAcBz a non-classical effect is found with high fluorescence quantum yield values in THF and in high-water-content mixtures and low in between. Bi-exponential fluorescence decays were observed for the aggregates, whereas single-exponential decays were obtained in good solvents. The presence of aggregates was further confirmed by dynamic light scattering (DLS). Mixtures of BrAcBz and IndAcBz in varying proportions were fine-tuned to achieve pure white light emission (WLE). • Three N -methylacridone-based compounds were synthesised and characterized. • Fluorescence tuned by solvent polarity and aggregation behaviour. • TDDFT supports charge transfer in donor-substituted acridone compounds. • White light emission achieved by tuning mixtures of two of the compounds.