Fluorescence Anisotropy and Electronic Transition

The origin of FA is the polarization of electronic transition of molecules to each transition is associated a vector called transition moment (see Sect. 1.5) which as a given orientation with respect to the molecular structure, In Fig. 6.15, the absorption transitions Sq — 5i and 5o — S2 of perylene are depicted. Transition 5i — Sq responsible for the fluorescence is almost parallel to 5q — 5i. In general, when the deactivation of an excited state takes place radiatively, the emitted photon is polarized parallel to the transition moment. Hence, if a single molecule is observed, the polarization of the emitted light is parallel to the direction defined by the transition responsible for the fluorescence. 

From the practical point of view, fluorescence measurements are carried out on a large number of molecules simultaneously molecules which are randomly 

This value, indicated as Tq is called fundamental anisotropy. It is worth noticing that the maximum possible value for this parameter is 0.4 and it can be observed in 

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