Synthesis of an aggregation-induced emission (AIE) active salicylaldehyde based Schiff base: study of mechanoluminescence and sensitive Zn(II) sensing

Syntheses of multi-functional Aggregation-Induced Emission (AIE) active molecules in a simple manner have been drawing great attention in current luminescence materials research. In this report a simple diamine molecule (N-1-tritylethane-1,2-diamine(1)) is reacted with salicylaldehyde using a Schiff-base technique which results in a new AIE active organic molecule [2-((2-(tritylamino) ethylideneamino) methyl)phenol (2)]. Computational calculations support that the nature of the transition is intra-molecular charge transfer/twisted intramolecular charge transfer (ICT/TICT). The mechanism of AIE has been attributed to restricted intramolecular rotation (RIR). Packing diagrams support that the nature of the aggregation is J-aggregation. The compound, 2, exhibits an irreversible mechanoluminescence (ML) property with a drastic colour change from blue to green (lambda(max), 445 nm > 512 nm) upon grinding. However, it undergoes a reversible transition with the same colour change (blue -> green) through applying pressure axially (using a hydraulic press). The reversible transition is observed by lowering the temperature of 2 to that of liquid nitrogen. The causes of such transitions showing variations in the emission colour upon different triggers have been investigated. In addition, 2 has been successfully tested for the sensing of Zn(II) and shows a rare turn-on luminescence change, the mechanism behind which has been explored. The detection limit for Zn(II) is determined to be 0.064 ppm.