Rational Perturbation of the Fluorescence Quantum Yield in Emission‐Tunable and Predictable Fluorophores (Seoul‐Fluors) by a Facile Synthetic Method Involving C—H Activation†

Fluorescence imaging enables the uniquely sensitive observation of functional- and molecular-recognition events in living cells. However, only a limited range of biological processes have been subjected to imaging because of the lack of a design strategy and difficulties in the synthesis of biosensors. Herein, we report a facile synthesis of emission-tunable and predictable Seoul-Fluors, 9-aryl-1,2-dihydrolopyrrolo[3,4-b]indolizin-3-ones, with various R(1) and R(2) substituents by coinage-metal-catalyzed intramolecular 1,3-dipolar cycloaddition and subsequent palladium-mediated CH activation. We also showed that the quantum yields of Seoul-Fluors are controlled by the electronic nature of the substituents, which influences the extent of photoinduced electron transfer. On the basis of this understanding, we demonstrated our design strategy by the development of a Seoul-Fluor-based chemosensor 20 for reactive oxygen species that was not accessible by a previous synthetic route.