A simple Schiff base chemosensor 2-((2-(2,4-dinitro phenyl)hydrazono)methyl)-4-nitrophenol (L) has been developed as a colorimetric and fluorimetric ‘turn on’ sensor for fluoride (F⁻).

Two newly synthesized Schiff base molecules are herein reported as anion sensors. –NO₂ substituted receptor (P1) is comparatively more acidic and can sense F⁻, OAc⁻ and H₂PO₄ ⁻, whereas –CN substituted receptor (P2) is less acidic and is selective for F⁻ only. Reversible UV-Vis response for both receptors with F⁻ can mimic multiple logic gate functions, and several complex electronic circuits based on XNOR, XOR, OR, AND, NOT and NOR logic operations with ‘Write–Read–Erase–Read’ options have been executed. Interesting ‘turn on and off’ fluorescence responses were noticed for the receptors with F⁻. Intracellular F⁻ detection as a diagnosis of non-skeletal fluorosis was successful using a fluorescence microscope with Candida albicans (prokaryotic cell, a diploid fungus) and pollen grains of Tecoma stans (eukaryotic cell) incubated in 10⁻⁶ M fluoride-contaminated hand-pump water collected from Bankura, West Bengal, India. Furthermore, a solution test kit was fabricated for easy and selective detection of F⁻ in an aqueous solvent.

A novel colorimetric hydrazine-functionalized Schiff base chemoreceptor, NPMP, was synthesized following a simple one-step Schiff base condensation pathway. NPMP showed selective colorimetric change from faint yellow to yellowish orange in the presence of biologically ubiquitous fluoride (F⁻). It also showed a ‘turn off’ fluorescent response in the presence of F⁻ that could effectively distinguish it from all anions tested except acetate. Acetate (OAc⁻) caused a weak response, while other anions like chloride, bromide, iodide, phosphate, hydrogen sulfate and nitrate did not have any observable effect on the NPMP receptor (E)-4-nitro-2-((2-(perfluorophenyl)hydrazono)methyl)phenol. Recognition of F⁻ in the presence of NPMP can be explained in light of multiple H-bonding interactions, as well as acid–base interactions between host receptor and guest F⁻. Interestingly, NPMP also showed enormous potential as a staining agent in determining the presence of low levels of intracellular fluoride. Moreover, it was found that in NPMP⋯F⁻ solutions, incorporation of Hg²⁺ showed observable optical changes, revealing that this compound is a smart material. Optical responses of NPMP can mimic a molecular logic gate (INHIBIT gate). This can be interpreted as a combination of an AND gate with a NOT function. It also represented a potential ‘Write–Read–Erase–Read’ memory function reflecting multi-writing ability.

An efficient and versatile mechanochemical route to 2-aryl benzothiazoles and both 2-substituted and 1,2-disubstituted benzimidazole derivatives has been developed via a simple mortar–pestle grinding method. The mechanochemical agitation was found to be sufficient for smooth condensation between a variety of aromatic aldehydes and o-aminothiophenol/o-phenylenediamine followed by cyclization leading to the formation of the corresponding 1,3-benzazoles. The salient features of this new protocol are catalyst-free reaction, cleaner reaction profiles, absence of a work-up step, high yields, and short reaction times.