Fluorescence mechanism switching from ICT to PET by substituent chemical manipulation: Macrophage cytoplasm imaging probes

The lack of polarity sensing fluorophores with OFF-ON features when increasing the environment polarity has limited the monitoring of biological processes that involve an increase in local hydrophilicity. In this work, replacement of a hydroxyl group by a dimethylamino group transformed solvatochromic ICT naphthalimide- and quinolimide-based fluorophores into reversed solvatochromic ones, with higher emission in polar than in apolar environments. Excited-state dynamics studies, TD-DFT calculations, X-ray and NMR support the existence of a folded conformation for the 2-(dimethylamino)ethyl chain upon the imide ring in apolar solvents, where the dimethylamino group would quench the fluorescence by a PET effect, while in polar solvents the chain has an extended conformation, where the PET is hindered. These PET fluorophores have given rise to H₂O and pH sensors in organic solvents as well as to bright macrophage cytoplasm imaging probes.