A novel Brønsted acidic ionic liquid 3-sulfonic acid 1-imidazolopyridinium hydrogen sulfate [Simp]HSO4 was synthesized via the reaction of caffeine with chlorosulfonic acid and was characterized by studying FT-IR, 1H NMR, 13C NMR, UV–Vis, and fluorescence spectra. Then, the prepared ionic liquid was utilized as an efficient catalyst for the preparation of 2H-indazolo[2,1-b]phthalazine-1,6,11(13H)-trione derivatives through one-pot, multi-component condensation of aldehydes, phthalhydrazide, and dimedone under solvent free conditions. The high Brønsted acidic function of this catalyst originates principally from the dual hydrogen bonding abilities of the SO3H group.
