Facile Synthesis of Phthalazinone/Isoindolinone Substituted Fluorescence Active Indolo[1,2-a]quinoxaline Derivatives via ANRORC and Fluorescence Turn Off Sensing of Fe2+

Synthesis of a series of indolo[1,2-a]quinoxaline derivatives substituted with phthalazinones/isoindolinones/carbonyl benzoylesters at C-4 position have been accomplished under open air rt/heating conditions following ANRORC (Addition of Nucleophile, Ring Opening and Ring Closure) mechanism. Initially condensation of 2-(1-indolyl)-aniline and ninhydrin generates a spirocyclic intermediate 5′H-spiro[indene-2,6′-indolo[1,2-a]quinoxaline]-1,3-dione, which upon reaction with various nucleophiles like hydrazine/phenylhydrazine, amines and alcohols affords C-4 substituted indolo[1,2-a]quinoxalines in high yield (up to ~88 %) via ANRORC. The photophysical investigation shows that the quinoxaline derivatives possess significant fluorescence property with high quantum yield (QY~11.0–17.0). The N-phenylphthalazinone substituted indolo[1,2-a]quinoxaline, a molecule structurally similar to 2,2′-bipyridine system, can efficiently and selectively detect Fe²⁺ through fluorescence turn off sensing.