Substituent-driven ESIPT in Chromeno-pyrroloquinoline probe for differentiating normal and cancer cells.

Abstract

Despite significant advancements in the structural flexibility and functional diversity of fluorescent molecular sensors, the chromophores often require complex synthetic processes and are typically designed to perform only a specific function. Herein, we have demonstrated the unique features of fluorophores based on a fused coumarin-indole scaffold, which are synthetically available via a one-step reaction. Four fluorophores (ICH, ICEst, ICOMe, and ICNMe2) with varying substituents were synthesized and characterized. Subsequently, their response towards aggregation, solvent polarity, and viscosity was studied. Probe ICNMe2 exhibited aggregation-induced emission (AIE), while others displayed aggregation-caused quenching. The viscosity-sensitive nature of these fluorophores was evaluated using the Froster-Hoffman equation. ICNMe2 displayed the highest sensitivity towards polarity and polarity-independent viscosity. The plausible mechanism involved is intramolecular charge transfer (ICT) in probes ICH, ICEst, and ICOMe, whereas excited state intramolecular proton transfer (ESIPT) coupled ICT in the case of ICNMe2. Based on the distinct AIE-viscosity responses and large stokes shift (~175 nm), ICNMe2 was utilized for distinguishing normal (RAW 264.7) cells and cancer (A549) cells using confocal microscopy. Results demonstrated that ICNMe2 could effectively extend its photophysical activity in the cellular milieu with an enhanced emission in channel-1 (λem = 460-530 nm) for A549 compared to RAW 264.7 cells.