Anti-Kasha fluorescence for polarity detection in cellular compartments

This work explores the development, photophysical study, and application of a new fluorescent molecule based on BODIPY for sensing intracellular polarity which behaves as an unimolecular ratiometric probe. The experimental data show that the emission spectra of this molecule feature two distinct peaks in different solvents: one stable at 525 nm and another that shifts between 656 nm and 690 nm depending on the solvent polarity. Theoretical calculations using Density Functional Theory (DFT) and Time-Dependent DFT confirm the nature of these dual emission from the S₂ and S₁ excited states, respectively, and their sensitivity to solvent polarity. The study demonstrates that this molecule exhibits anti-Kasha fluorescence, which can be tuned by the surrounding environment, making it a potential tool for polarity sensing. The application of this compound in live cells shows its rapid uptake and ability to differentiate between various cellular compartments based on different polarity, using the S1/S2 ratio from the two emission channels. This research provides valuable insights into the development of efficient and specific fluorescent probes for visualizing and monitoring intra-cellular polarity contributing to advancements in cellular biology and medicine.