Aggregation-Caused quenching or Aggregation-Induced (Enhanced) Emission? The Decisive role of aggregation in solvents and their mixtures

An enhancement in the fluorescence quantum yield (FQY) of DMSO solutions of two carboxamide-based fluorophores — viz. 2-(4-methoxyphenyl)-5-amino-2H-1,2,3-triazole-4-carboxamide with piperidine (BSN1) and methylamino groups (BSN2) at the fifth position — upon the addition of water has been previously reported in the literature. Although the FQYs of BSN1 and BSN2 exhibited different trends as a function of water content, the dilution effect was attributed to the aggregation-induced enhanced emission (AIEE) phenomenon. However, either BSN1 or BSN2, in their solid phases, do not show fluorescence, suggesting the presence of aggregation-caused quenching (ACQ). Building on the molecular-level insights into the DMSO-water binary system from the prior study, the current work aims to clarify the origin of the AIEE phenomenon by investigating the intermolecular interactions between fluorophore molecules and between the fluorophores and the solvent. Vibrational spectroscopy and quantum mechanical calculations revealed that hydrogen bonds were formed between two dye molecules, between a dye molecule and a DMSO trimer, and among the dye molecule, DMSO trimer, and water. These interactions induced segregation, as evidenced by scanning electron microscopy (SEM) images, leading to an increase in FQY. Nevertheless, due to subtle structural differences between the two dyes, the morphological changes with varying water content exhibited different trends for BSN1 and BSN2, as reflected in UV–Visible spectroscopy and SEM. This work demonstrates that the increase in FQY upon adding water is not a monolithic characteristic of AIEE.