An efficient Brønsted acid ionic liquid catalyzed synthesis of novel spiro1,2,4-triazolidine-5-thiones and their photoluminescence study

•Novel Bronsted acid ionic liquid an efficient Green catalyst.•Synthesis of novel indenoquinoxalino tethered spiro-1,2,4-triazolidine-5-thiones.•Mild reaction conditions, high yields, recyclability and thermal stability.•Fluorescent and UV active. We have synthesized a novel Bronsted acidic ionic liquid, 1-(2-hydroxyethyl)-1-(4-sulfobutyl)piperidin-1-ium hydrogen sulfate, [HEPiPYBSA]+HSO4− and explored its catalytic efficiency for synthesis of indenoquinoxalone tethered spiro-1,2,4-triazolidine-5-thiones from reaction of 11H-[1,2-b]quinoxalin-11-one and thiosemicarbazide. The most stable geometries of synthesized ionic liquid (IL) [HEPiPYBSA]+ HSO4− were obtained through systematically optimization by the DFT theory at B3LYP/6-31G* level. A photoluminescence study of the synthesized spiro-1,2,4-triazolidine-5-thiones revealed a remarkable fluorescent activity. The advantages of the present method are a reusable hydrophilic green catalyst, mild reaction conditions, use of benign solvent system, short reaction span, high atom economy and wide substrate scope. [Display omitted]