One-Pot Synthesis of Highly Fluorescent Pyrido[1,2-a]indole Derivatives through CH/NH Activation: Photophysical Investigations and Application in Cell Imaging

We describe a straightforward strategy for the synthesis of strongly fluorescent pyridoindoles by Pd‐catalyzed oxidative annulations of internal alkynes with C‐3 functionalized indoles through CH/NH bond activation in a one‐pot tandem process. Mechanistic investigations reveal the preferential activation of NH indole followed by CH activation during the cyclization process. Photophysical properties of pyridoindoles exhibited the highest fluorescence quantum yield of nearly 80 %, with emission color varying from blue to green to orange depending on the substructures. Quantum mechanical calculations provide insights into the observed photophysical properties. The strong fluorescence of the pyrido[1,2‐a]indole derivative has been employed in subcellular imaging, which demonstrates its localization in the cell nucleus. Shining indoles in the nucleus: A range of substituted pyrido[1,2‐a]indoles were synthesized by facile Pd‐catalyzed oxidative annulations of internal alkynes with C‐3 functionalized indoles in a one‐pot process (see scheme). Detailed photophysical characterizations coupled with quantum mechanical calculations revealed the dependence of the fluorescence color and quantum yields on the substructure. Pyrido[1,2‐a]indoles are found to be interesting candidates for subcellular imaging.