Aggregation-activated delayed and two-photon fluorescence in a charge-modulated pyridine-acridan luminophore

Molecular charge modulation offers a promising strategy for controlling the excited-state properties and aggregation behavior of organic luminophores. Nevertheless, the consequences of cationization strongly depend on the molecular structure. Herein, we investigate a twisted π-extended pyridine-dimethylacridan derivative (PyAcr) and its cationic pyridinium analogue (PyAcr+) to elucidate how neutral-to-cationic transformation affects their photophysical properties. Experimental characterization combined with theoretical calculations provides insight into the relationship between molecular structure, charge distribution, excited-state character, and emission behavior. PyAcr exhibits aggregation-induced delayed fluorescence (AIDF), accompanied by a moderate photoluminescence quantum yield of 45% and strong two-photon-excited fluorescence (TPEF) under near-infrared excitation. In contrast, conversion of PyAcr to PyAcr+ alters its aggregation behavior and results in substantially weakened emission, likely reflecting the combined effects of cation formation and the structural modification introduced upon quaternization. Both derivatives exhibit high thermal stability, with onset temperatures of thermal degradation above 360 °C, good electrochemical stability, and small singlet-triplet energy gaps of 0.07-0.13 eV. Overall, the results demonstrate that cationization alone does not necessarily enhance luminescence and that the interplay between molecular charge, electronic coupling, and aggregation is critical for controlling excited-state dynamics. These structure-property relationships provide useful guidelines for the rational design of environment-responsive luminophores, while PyAcr shows promise as a two-photon-excited probe for bioimaging.