Functionalizable 1H‐Indazoles by Palladium Catalyzed Aza‐Nenitzescu Reaction: Pharmacophores to Donor‐Acceptor Type Multi‐Luminescent Fluorophores

Abstract

Development of small‐molecule‐based multi‐luminescent fluorophores utilizing simple synthetic methodologies, as well as easily available starting materials, has gained much attention in recent years. Herein, we disclose an efficient protocol for the synthesis of N‐protected 1H‐indazole derivatives with diverse substituents at their 3‐ and 5‐positions via palladium‐catalyzed reactions of hydrazones and p‐benzoquinones. The obtained 1H‐indazole derivatives can be easily modified into donor‐acceptor (D−A)‐type chromophores (Indazo‐Fluors) with tunable emission properties in both solid and solution state. Owing to the extent of intramolecular charge transfer, Indazo‐Fluors exhibit positive solvatochromic emission spanning from blue‐green to orange‐red. Theoretical studies were undertaken to rationalize the observed trends in the optical properties of Indazo‐Fluors. Finally, a triethylene glycol (TEG) appended Indazo‐Fluor exhibiting a large Stokes shift and low cytotoxicity allowed us to use it for cell imaging.