Two‐Step Synthesis of Blue Luminescent (Pyrrol‐3‐yl)‐1H‐(aza)indazoles Based on a Three‐Component Coupling–Cyclocondensation Sequence

Abstract

(Pyrrol‐3‐yl)‐1H‐(aza)indazoles can be efficiently prepared by a two‐step process that consists of a consecutive three‐component coupling–cyclocondensation synthesis to give ortho‐halo‐3‐acylpyrrol‐3‐yl‐substituted (hetero)arenes followed by a cyclization–condensation–SNAr sequence. Almost all derivatives display an intense blue emission upon excitation in the near UV with enormous Stokes shifts (6500–8300 cm–1) and considerable high fluorescence quantum yields (Φf = 0.28–0.46). Electronic transitions can be plausibly rationalized by TD‐DFT computations performed on DFT‐optimized geometries. Reversible protonation leads to static fluorescence quenching in narrow pH‐ranges, which qualifies the title compounds as favorable ON/OFF fluorescence switching systems.