Figure 6

Emission profile of the ratiometric dye system in Figure 5. The fluorescence fingerprint (A) shows the emission intensity as a function of both excitation and emission wavelength. Three distinct emission peaks emerge. The first is caused by the reference fluorophore (Ref), the second by the molecular rotor where the excitation of the rotor molecule takes place through resonance energy transfer from the reference (RET), and the third is emitted from the rotor after direct excitation of the rotor (ROTOR). B shows the emission maxima in solvents of different viscosity. It can be seen that the reference emission is widely independent of the viscosity, and small variations can be attributed to refractive index changes. Conversely, rotor emission increases with viscosity in a power-law fashion, irrespective of whether the rotor is excited directly or through RET.