Aggregation-Induced Emission or Hydrolysis by Water? The Case of Schiff Bases in Aqueous Organic Solvents

In the last two decades, several Schiff bases have been reported as AIEgens which remain nonemissive in organic solvents but show strong fluorescence in the presence of water. A methodical analysis involving 21 Schiff bases, including some of the reported molecules, shows that in the presence of water, the Schiff bases hydrolyze to yield the corresponding starting aldehydes and amines. The observed emission in the presence of water is found to be originated from the aggregation of the fluorogenic aldehydes and not of the original molecules. Thus, while the aggregation-induced emission (ATE) effect is valid for these systems, certainly, these Schiff bases cannot be termed as AIEgens. Notably, the observation that these aldehydes can act as AIEgens through their excimer emission is an important phenomenon with respect to the current understanding of AIEgens.