Acid-responsive fluorescent compounds based on nitro-group-substituted L-shaped pentacycles, pyrrolo[1,2-a][1,8]naphthylidines.

Acid-responsive fluorescent compounds were prepared by introducing a nitrophenyl group to L-shaped pentacycles with a pyrrolo[1,2-a][1,8]naphthylidine backbone. These compounds show almost no fluorescence under neutral conditions, but emit green to orange fluorescence upon addition of trifluoroacetic acid. Acid titration experiments and NMR spectroscopy, plus DFT calculations, show that formation of a pyridinium cation species is responsible for the appearance of fluorescence.