Synthesis of triazolo[c]coumarins 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 ragio 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.