Figure 5

Basic structure of an engineered ratiometric dye system (A). A conventional fluorescent molecule that is not viscosity-sensitive serves as intensity reference (Ref) and is coupled to a molecular rotor through a linker structure. The fluorescent groups can be excited individually and emit fluorescence at their typical wavelength. However, when the reference unit is excited, it transfers some of its excited-state energy to the rotor via resonance energy transfer (RET), and dual emission from both reference and rotor can be observed. One possible chemical structure is shown in B. The reference is a coumarin (blue), and the rotor is a structure is based on an aniline motif rather than the tricyclic julolidine found in DCVJ.