Multichromophoric cyclodextrins. 6. Investigation of excitation energy hopping by Monte-Carlo simulations and time-resolved fluorescence anisotropy

Excitation energy transport in several beta-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, showings 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 Angstrom in all cases) are compatible with the nearest-neighbor distances expected from molecular modeling, which confirms the validity of the theoretical model.