Tuning Solid-State Emission via Alkoxy Chain Modulation: Switching ACQ to AIE by Restricting Intermolecular Motion (RIM)

Subtle substituent modifications can significantly alter the photophysical behavior of organic luminogens in the aggregated state. In this work, the influence of minor alkoxy substitution on emission properties is investigated using two closely related carbohydrazide–salicylaldehyde derivatives bearing methoxy (CDMS) and ethoxy (CDES) substituents. Both compounds exhibit nearly identical UV–visible absorption characteristics in solution, confirming comparable electronic structures of the chromophoric core. However, their emission behavior differs markedly depending on the molecular environment. CDMS is strongly emissive in solution but quenched in the solid state, whereas CDES shows weak solution emission and strong solid-state emission. Aggregation studies in THF/water mixtures further validate this divergence, revealing progressive fluorescence quenching for CDMS but significant emission enhancement for CDES. Single-crystal X-ray diffraction and Hirshfeld surface analyses show that CDES forms a packing arrangement with a higher density of intermolecular contacts and hydrogen-bond interactions, whereas CDMS exhibits closer π–π interactions within the crystal lattice. These findings demonstrate that even minimal alkoxy chain elongation can modulate intermolecular packing and switch the emission response from aggregation-caused quenching (ACQ) to aggregation-induced emission (AIE).