Rational Design, Green Synthesis, and Initial Evaluation of a Series of Full‐Color Tunable Fluorescent Dyes Enabled by the Copper‐Catalyzed N‐Arylation of 6‐Phenyl Pyridazinones and Their Application in Cell Imaging

There is widespread interest in the application, optimization, and evolution of the transition‐metal‐catalyzed arylation of N‐heteroarenes to discover full‐color tunable fluorescent core frameworks. Inspired by the versatile roles of pyridazinone in organic synthesis and medicinal chemistry, herein, we report a simple and efficient copper‐catalyzed cross‐coupling reaction for the N‐functionalization of pyridazinones in neat water. To achieve the efficient conversion of pyridazinones and organic halides in aqueous phase, a series of copper‐salen complexes composed of different Schiff base ligands were investigated by rational design. A final choice of fine‐tuned hydrophilicity balanced with lipophilicity among the candidates was confirmed, which affords excellent activity towards the reaction of a wide range of pyridazinones and organic halides. More importantly, the products as N‐substituted pyridazinones were synthesized rationally by this methodology as full‐color tunable fluorescent agents (426–612 nm). The N2 position of pyridazinones was modified by different aryl group such as benzothiazole, N , N ‐dimethylaniline, 3‐quinoline, 4‐isoquinoline and 2‐thiophene, resulting in a series of full‐color tunable fluorescent reagents. Meanwhile, the effects of electron‐donating and electron‐withdrawing groups of the 6‐substituted phenyl ring have also been investigated to optimize the fluorescent properties. These fluorescent core frameworks were studied in several cell lines as fluorescent dyes. Different colors from blue to red were observed by using fluorescence microscopy and confocal microscopy.