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

Abstract

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 a key step. These bimane-related fluorescent compounds with λex = 338–400 nm and λem = 522–644 nm exhibit interesting and promising optical properties, including a remarkably large Stokes shift (150–268 nm), positive solvatochromism, and strong dependence of emission intensity on pH in a range of pH 2–10. The benzotriazolide- and N-succinimidyl ester-functionalized compounds were successfully used as fluorescent probes for protein labelling.