Multichromophoric cyclodextrins. 8. Dynamics of homo- and heterotransfer of excitation energy in inclusion complexes with fluorescent dyes

The water-soluble beta -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 x 10(11) 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 Forster radii.