Emission anisotropy hindered rotations

A distinction should be made between free rotation and hindered rotation. In the case of free rotation, after a (5-pulse excitation the emission anisotropy decays from ro to 0 because the rotational motions of the molecules lead to a random orientation at long times. In the case of hindered rotations, the molecules cannot become randomly oriented at long times, and the emission anisotropy does not decay to zero but to a steady value, r (Figure 5.10). These two cases of free and hindered rotations will now be discussed. 

Fig. 5.10. Decay of emission anisotropy in the case of free and hindered rotations.

Equations (8.25) to (8.28) are no longer valid in the case of hindered rotations occurring in anisotropic media such as lipid bilayers and liquid crystals. In these media, the rotational motions of the probe are hindered and the emission anisotropy does not decay to zero but to a steady value rc0 (see Chapter 5). For isotropic rotations (rod-like probe), assuming a single correlation time, the emission anisotropy can be written in the following form ... 

---