Many fluorescent dyes change their photophysical properties with changes in their environment. Examples include fluorescent dyes that are sensitive to the environment's polarity, pH, presence of ions, temperature or electrostatic potential. A group of fluorophores whose photophysical mechanism is excited- state intamolecular charge transfer (ICT) have been termed "molecular rotors" because of their additional capability to form twisted conformations. Conformational changes are associated with changes in the ICT dipole, and twisting during the excited state changes the dye's fluorescence emission.
Intramolecular twisting depends on the environment, most notably, microviscosity. Molecular rotors have therefore been established as nanoscale, real-time viscosity reporters with the potential to exceed mechanical viscometers in precision, however, confounding factors that influence measured intensity need to be eliminated. Lifetime spectroscopy and engineered ratiometric dyes present possible solutions.
Molecular rotors have also been reported to show sensitivity towards fluid shear stress. The underlying photophysical mechanism is not yet understood, but two competing possible explanations involve polar-polar interaction and the formation of photoisomers. Data in support of both hypotheses exist, and it is likely that no single mechanism fully explains shear-sensitivity. From the application side, however, molecular rotors provide unsurpassed sensitivity at low flow rates combined with high spatial resolution.