Design and synthesis of an AIE-active naphthaldehyde hydrazone-based ligand: mechanochromic, acidochromic, and sensing studies

A new multifunctional Schiff base material, 1-((E)-(((Z)-3,5-difluoro-2-hydroxybenzylidene)hydrazineylidene)methyl)naphthalen-2-ol (L2), was synthesized through a simple one-step condensation of 2-hydroxy-1-naphthaldehyde hydrazone and 3,5-difluorosalicylaldehyde. The ligand was characterized by FT-IR, ¹H and ¹³C NMR, ESI-MS and single crystal X-ray diffraction. The crystal structure of L2 crystallizes in the monoclinic system with the space group P₁2₁/c₁. The compound exhibited aggregation-induced emission (AIE) and excited-state intramolecular proton transfer (ESIPT) characteristics, along with reversible mechanochromic and acidochromic behavior. L2 exhibited a rapid, sensitive, and reversible colorimetric as well as fluorometric “turn-off” response for Cu²⁺ ions in aqueous media, displaying a 1 : 1 binding stoichiometry and an exceptionally low detection limit of 0.71 µM. The sensor L2 was effectively applied for Cu²⁺ detection in an on-site detection kit. Moreover, bioimaging studies demonstrated its excellent cell permeability, low cytotoxicity, and efficient Cu²⁺ sensing within living cells.