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.

Excited‐State Intramolecular Proton Transfer (ESIPT) for Optical Sensing in Solid State

“…As illustrated in Figure 1, the ESIPT fluorophores mentioned in this article include 2-(2'-hydroxyphenyl)benzimidazole (HBI), [12] 2-(2'-hydroxyphenyl) benzothiazole (HBT), [13][14][15][16][17][18][19][20][21][22] 2-(2'-hydroxyphenyl)benzoxazole (HBO), [23] N-salicylideneaniline, [24,25] 2-(1H-pyrazol-3-yl)phenol, [26,27] 2-(2H-1,2,3-triazol-2-yl) phenol, [28] and so on. [29][30][31][32][33][34][35][36] The various mechanisms of ESIPTrelated optical sensing in the solid state will be discussed and summarized below.…”

“…synthesized a series of (E)-N'-benzylidene-2-hydroxybenzohydrazide derivative named Sensor 4 series, for the detection of Co 2+ . [31] Sensor 4 series were shown to coordinate with Co 2+ in a 1:1 stoichiometry. This binding resulted in the deactivation of the ESIPT process, thereby quenching the fluorescence emission.…”

“…Reproduced with permission. [31] Copyright 2020, Wiley-VCH. in molecular sensing devices for biologically important amines with a low detection limit of 10 ppm (Scheme 5).…”

“…Figure 2. a) Molecular structure of Sensor 4. b) The selective recognition digital fluorescence images and fluorescence spectra for Co2+ . Reproduced with permission [31]. Copyright 2020, Wiley-VCH.…”

Coordination‐Driven Terpyridine‐Based Twisted Prisms with Tunable Emissions and Hierarchical Self‐Assembly Properties

“…In the past decades, tunable‐fluorescence materials have attracted extensive attention and exhibited a wide range of applications in biological imaging, [ 1 ] environmental sensing, [ 2 ] fluorescent probes, [ 3 ] and other fields, [ 4 ] due to their high sensitivity and stimulus responsiveness. [ 5 ] What is more, tunable‐fluorescence materials, which are affected by external factors, such as temperature, [ 6 ] force, [ 7 ] light, [ 8 ] and solvent, [ 9 ] and internal factors, such as donor–receptor, [ 10 ] π–π interaction, [ 11 ] and host–guest interaction, [ 12 ] play a key role for the development of light‐emitting materials. However, the studies on fluorescent‐tunable materials by hydrophobic interactions are scarce yet.…”