Easily Accessible Schiff Base ESIPT Molecules with Tunable Solid State Fluorescence: Mechanofluorochromism and Highly Selective Co2+ Fluorescence Sensing

Abstract

Schiff base molecules of 1–4 ((E)-N′-benzylidene-2-hydroxybenzohydrazide derivatives) showed structure controlled solid state fluorescence tuning, reversible stimuli induced fluorescence switching and highly selective Co2+ sensing. The strong intramolecular H-bonding between imine and amine facilitates excited state intramolecular proton transfer (ESIPT) and large red shifted solid state fluorescence. Positional isomers and substitutional change tuned the solid state fluorescence (460 to 605 nm). The substituents and positions control stimuli-induced fluorescence switching. ESIPT fluorophores showed strong fluorescence in DMF (Φf=0.057 to 0.35 compared to quinine sulphate). ESIPT fluorophores (2–4) exhibited highly selective fluorescence sensing of Co2+ ions without significant interference from other metal ions. Concentration dependent studies of fluorophores show the detection limit (LOD) of 54 nM that is much lower than the Environmental Protection Agency (EPA) guideline value (1.7 μM) in drinking water. Solid state fluorescence of ESIPT fluorophores have been made use to fabricate solid fluorescence assay (PVA polymer composite thin films) and demonstrated highly selective fluorescence sensing of Co2+ upon dipping into aqueous metal ions solution. Thus, the present studies demonstrate the versatility of Schiff base molecules for developing ESIPT fluorophores for stimuli responsive materials and highly selective fluorescence sensing both in solution and solid state.