Analysis of the effect of viscosity in the design of fluorescent molecular rotors

Abstract

Fluorescent molecular rotors have gained popularity over the last 15 years. They are compounds with unique properties based on two moieties linked by a π-conjugated bridge. One moiety is called the stator and has a larger moment of inertia, while the other moiety is called the rotor or rotator, a fluorophore that can form a twisted internal charge transfer state (TICT) after photoexcitation, as it exhibits two different competitive pathways of energy loss towards the ground state: fluorescence emission and non-radiative deexcitation.FMRs are widely used as viscosity probes due to their viscosity-sensitive fluorescence behaviour. In a low viscosity medium, the rotation of the rotor is not hindered and induces the non-radiative relaxation of the excitation energy, resulting in the quenching of the fluorescence intensity. In a higher viscosity medium, the rotation is more hindered and fluorescence intensity is enhanced. The sensitivity to solvent viscosity can be quantified using the established relationship between the viscosity (ƞ) of the medium and the fluorescence intensity contrast (I/I0) or fluorescence quantum yield (ϕFL), known as the Förster-Hoffmann equation where x is the slope of the plot and is related to the viscosity sensitivity of the FMR, and C is an experimental constant related to the dyeThe structure of a fluorescent molecular rotor directly influences its ability to sense local viscosity. A classification of the main families of fluorescent molecular rotors that are going to be studied has been made: julolidine-based, BODIPY-based, cyanine-based and rhodamine-based FMRs. Their structural design and sensitivity to viscosity will be reviewed to determine the best sensors of each dye family, and a comparison will be made to find out the best options as viscosity sensors.