2-Acyl-1-aryl-6,7-dihydro-1H,5H-pyrazolo[1,2-a]pyrazole derivatives: Versatile fluorescent probes with remarkably large Stokes shift

A series of thirteen 2-acyl-6,7-dihydro-1H,5H-pyrazolo[1,2-a]pyrazole derivatives were synthesized by using a copper-catalyzed azomethine-imine-alkyne cycloaddition as key step. These bimane-related fluorescent compounds with λex = 338–400 nm and λem 522–644 exhibit interesting promising optical properties, including remarkably large Stokes shift (150–268 nm), positive solvatochromism, strong dependence emission intensity on pH in range 2–10. The benzotriazolide- N-succinimidyl ester-functionalized successfully used probes for protein labelling.