Synthesis of Triazolocoumarins and Differences in the Fluorescence Properties of Their Isomers

Despite the simple structures of 3,4‐fused tricyclo coumarins, their fluorescence properties have not been studied extensively. We have synthesized triazolo[c]coumarins using Huisgen cycloaddition and investigated their fluorescence properties. The Huisgen cycloaddition was found to proceed in excellent yields when subjected to heating under solvent‐free conditions. As is generally known, fluorescence is enhanced with an increase in the electron‐donating group at the 7‐position. The 7‐NEt2‐triazolo[c]coumarin derivatives showed useful fluorescence, and their relative fluorescence quantum yields in aprotic polar organic solvents were high. It revealed that fluorescence properties of triazolo[c]coumarins depended on the regio isomer. The electron distribution was hardly extended to alkyl side chain on triazole from the molecular orbital calculation by density functional theory (DFT) and time‐dependent DFT (TD‐DFT). This result suggests that the trend of fluorescence properties is similar to the present result even when various substituents are introduced on triazole.