Mechanistic insights into the aggregation-induced emission of halogenated Schiff base fluorescent probes

Design of the molecular structures and control of the crystal packing reas critical for the development of solid-state and aggregation-induced emitters (AIEgens). Here in, we report the rational synthesis of hetero-halogenated Schiff bases 1–3 to understand the role of the salicylaldehyde nature and position of halogens on crystal packing and properties. Solids 1 and 3, obtained by condensation of homo-halogenated salicylaldehyde utilized an aggregate through in-plane halogen bonding and out-of-plane π-π interactions, with similar packing symmetry and space group P-1. 2 formed from hetero-halogenated salicylaldehyde forms out-of-plane halogen bonding interactions to form J-aggregates with poor slipped π-overlap. As a consequence of the presence of π-interactions, 1 and 3 are non-emissive in the solid state, while 2 is an orange-yellow emitter. All solid forms are non-emissive in solution state and exhibit aggregation-induced emission (AIE) to form green-emitting suspensions (λmax, φ: 505 nm, 54 % (1); 518 nm, 48 % (2) and 510 nm, 47% (3)). The rare AIE behaviour of dual state non-emissive 1 and 3 is attributed to the restriction of free rotations of the molecules and relocking of the intramolecular hydrogen bond in non-planar molecular conformation, due to sudden precipitation and hydrogen bonding by water molecules. The proposed explanation for emission switching of AIEgen forms is supported by powder-diffraction (P-XRD), scanning-electron-microscopy (SEM) and thermo-gravimetric (TGA) studies. Hirshfeld studies provide a quantitative estimate of the contribution of halogen atoms towards molecular packing. The non-emissive solutions of the Schiff bases have been utilized for sensing of ammonia and the tendency has been extended for the detection of basic amino acids.