Indole and indazole derivatives containing salicylaldimine as fluorescence materials for multi-stimuli response through assembling AIEgen in multiple states

Abstract

Organic functional materials with aggregation-induced emission (AIE) properties have been highly researched in recent years. However, there is still a significant void in research into indole (including indazole) groups with excellent optical properties. Aiming to exploit novel fluorescent materials based on indole (even those including an indazole fragment) with controllable properties between molecular structure and fluorescence characteristics, five compounds were designed and synthesized by introducing salicylaldehyde into diverse positions of a nitrogen-containing benzoheterocycle scaffold (indole or indazole). Fluorescence emission spectra exhibited different modification sites with a regular effect on the fluorescence emission that ranging from turquoise to orange was accompanied by displacement of the position of salicylaldehyde on the benzoheterocycle fragment. 4CN was obtained by exchanging the orientation of N and C atoms in the imine fragment from that in 4NC to explore the luminescence mechanism of derivatives of this scaffold. Furthermore, to solve the troubling aggregation-induced quenching (ACQ) phenomenon, a canonical conjugated structure (benzene) and AIE luminogen (triphenylamine) were introduced into the 1st position of compound 4NC, respectively. Co-modification with salicylaldehyde and triphenylamine into the indole scaffold gave 4NC-3 excellent fluorescence emission performance in aggregated states in solution and gel systems. AIE materials 4NC-1 and 4NC-3 could not only be applied as smart fluorescence sensors for the detection of Cu²⁺ ions with a limit of detection (LOD) of 17.30 nM in the range from 0 to 30 μM, but 4NC-1 could also serve as a multi-stimuli-responsive fluorescent smart material.