Dyes with Segmental Mobility: Molecular Rotors

Molecular rotors are fluorescent molecules that are characterized by the ability to form twisted states through the rotation of one segment of the structure with respect to the rest of the molecule. Intramolecular rotation changes the groundstate and excited-state energies, and molecular rotors deexcite from the twisted state either without photon emission or with a different wavelength than from the LE state. Intramolecular rotation is strongly dependent on the solvent. Solvent polarity, hydrogen bond formation, isomerization, excimer formation, and steric hindrance are predominant forms of solvent–fluorophore interaction. Of highest importance is steric hindrance, because it links the solvent’s microviscosity to the formation rate of TICT states, which, in turn, determines the spectral emission. For this reason, molecular rotors have found a wide range of applications as fluorescent sensors of microviscosity and solvent free volume. Application examples include bulk viscosity measurement, probing dynamics of polymer formation, protein sensing and probing of protein aggregation, and microviscosity probing in living cells.