A guideline for design of photoinduced electron transfer-based viscosity probe with rigid fluorophore-quencher configuration

Viscosity, as a key parameter of the microenvironment, is closely related to many functional disorders and diseases. Although many kinds of viscosity probes have been developed, the guidelines for the rational design of viscosity probes with rigid fluorophores based on photoinduced electron transfer (PET) are still lacking. By synthesizing and investigating a series of phenyl substituted rhodols with different energy gaps of the frontier molecular orbitals (ΔE), we found a guideline for design of PET-based viscosity probe with rigid fluorophore-quencher configuration. That is when ΔE < 0.6 eV, the viscosity probe would have good performances such as low fluorescence background, significant fluorescence turn-on ratios (100-fold), and high viscosity sensitivity. This guideline was successfully applied to the phosphine oxide rhodamine to design a NIR-viscosity probe. Further, probe 4-NO2-RHO was applied to detect viscosity changes in mitochondria and was used to study the correlation between mitochondrial pH and viscosity in living cells.