Hydration‐Gated Photoinduced Electron Transfer in BODIPY Dyes

: Fluorescent probes based on photoinduced electron transfer (PET) are widely used to image intracellular environments. Here we report donor-acceptor dyads based on the BODIPY chromophore in which PET is selectively activated upon hydration. Replacing fluorine groups at boron with triethylene glycol (TEG) chains confers aqueous solubility and modulates the electronic properties of the boron-coordinated core, suppressing PET in polar organic solvents while enabling efficient charge separation in water. This switching arises from hydrogen bonding at the alkoxy oxygens, which strengthens the effective acceptor character of the BODIPY, supported by electrochemistry, transient absorption spectroscopy, and DFT calculations. The mechanism is general across dyads incorporating various aromatic donors and BODIPY substitution patterns. The lead compound, 5-TEG, displays hydration-dependent fluorescence lifetimes spanning a 6 ns range. In fluorescence lifetime imaging microscopy (FLIM) of living cells, tumor spheroids, and intestinal organoids, 5-TEG provides high brightness and lifetime contrast, enabling segmentation of subcellular compartments and reporting on microenvironmental differences. These results establish hydration-gated PET as a tunable photophysical mechanism for designing fluorescence probes with lifetime-encoded readouts in complex biological environments.