Excitation energy transport in several β-cyclodextrins containing seven appended chromophores was studied theoretically and experimentally by steady-state and time-resolved fluorescence anisotropy. The absorption spectra compared to those of reference chromophores did not reveal significant interactions between the chromophores in the ground state, thus allowing us to assume a very weak coupling regime for energy transfer. The measured long time anisotropies were found to be in all cases close to one-seventh of the fundamental anisotropy, showing that the chromophores are randomly oriented. A realistic model in which the chromophores are in fixed positions but randomly oriented was developed to interpret the steady-state and time-resolved emission anisotropy data. A Monte-Carlo simulation based on the appropriate master equation allowed the calculation of the theoretical anisotropy decay in terms of reduced variables and parameters. The decay contains a wide spectrum of rate constants. A good fit to the experimental decays was obtained. Moreover, the nearest-neighbor distance recovered from the anisotropy and the steady-state anisotropy for all cyclodextrins (5−7 Å in all cases) are compatible with the nearest-neighbor distances expected from molecular modeling, which confirms the validity of the theoretical model.

The water-soluble β-cyclodextrin, CD-St, with seven steroidic naphthalene chromophores linked to the primary rim, can form inclusion complexes with a merocyanine dye (DCMOH) and an oxazine dye (Ox725); the stoichiometry is 2:1 (CD-St:dye). This system works as an antenna since the dye is surrounded by 14 chromophores. The efficiency of transfer from the antenna chromophores to the encased dye was found to be close to 100%. The dynamics of this heterotransfer and homotransfer (i.e., energy hopping among the antenna chromophores) was investigated by time-resolved fluorescence intensity and time-resolved fluorescence anisotropy experiments, respectively. The distribution of rate constants for homotransfer was recovered thanks to a previously described Monte Carlo simulation from which an average rate constant was calculated and found to be about 4 × 1011 s-1. This value is about 10 times faster than the rate constant for heterotransfer in the case of Ox725, and about three times faster than in the case of DCMOH. The results are discussed in terms of interchromophoric distances, mutual orientations and Förster radii.