Excitation Energy Hopping in Multichromophoric Cyclodextrins

To illustrate the use of fluorescence anisotropy in the elucidation of energy transfer between identical fluorophores, multichromophoric cyclodextrins [29] similar to the CD-St described in Section 7.2.4.2 are considered, but now in the absence of a dye. 

In these multichromophoric cyclodextrins the fluorophores are randomly oriented. Excitation of one of the naphthoate fluorophores is followed by efficient dipole-dipole excitation energy transfer between the seven fluorophores, with a Forster radius of 14 A. This process is not detectable by fluorescence intensity measurements, as neither the intensity nor the decay law are affected by energy transfer between identical fluorophores (also called homotransfer. The dynamics of energy hopping are on the other hand reflected in the fluorescence anisotropy. To avoid depolarization by rotational motion of the fluorophores, experiments were conducted in a low temperature and optically clear rigid glass (9 1 ethanol-methanol at 110 K). 

The time-resolved anisotropy. Fig. 7.13, decays from an initial value of 0.291 to a constant value (0.042) that is reached in about 2 ns. The final value is exactly one- 

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