7-Azaspiroketal as a unique and effective auxochrome moiety: demonstration in a fluorescent coumarin dye and application in cell imaging

Replacement of electron-donating N,N-dialkyl groups with azacyclic groups has been described as a promising auxochrome to improve the low fluorescent quantum yields (phi F) of conventional fluorophores. However, the insufficient brightness and poor solubility of fluorophores pose a significant bottleneck for bioimaging. A new type of fluorescent coumarin dye was designed and synthesized using 7-azaspiroketals as the electron-donating auxochrome group. Such modifications resulted in high-quality coumarin fluorophores with improved solubility and brightness. To the best of our knowledge, this is the first report of using apex-N-substituted azaspiroketals as an electron donor group in any class of donor-acceptor dye. Furthermore, the successful application of these new dyes in biological imaging for living cells has been demonstrated. A new type of fluorescent coumarin dye was synthesized using 7-azaspiroketals as the electron-donating auxochrome group. Such modifications resulted in high-quality coumarin fluorophores with improved solubility and brightness.