Mechanistic insights into the aggregation-induced emission of halogenated Schiff base fluorescent probes
Aadil A. Ahangar, Asif A. Malik, Ishtiyaq Ahmad, Aijaz A. Dar
Abstract
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 ( λ max , 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 .