MULTICHROMOPHORIC CYCLODEXTRINS .3. INVESTIGATION OF DYNAMICS OF ENERGY HOPPING BY FREQUENCY-DOMAIN FLUOROMETRY

A beta-cyclodextrin labeled with seven naphthoyloxy chromophores was s tudied by steady-state and time-resolved fluorescence spectroscopy in order to get information on the dynamics of energy hopping between chr omophores. The steady-state fluorescence anisotropy was recorded as a function of excitation wavelength in a mixture of methanol and ethanol at 110 K (rigid glass). The fluorescence anisotropy decay was obtaine d under the same conditions by the multifrequency phase-modulation tec hnique upon excitation at 290 nm. The data were analyzed and interpret ed on the basis of a theoretical model involving a unique rate constan t for energy hopping between nearest neighbors. In particular, this mo del predicts a long-time leveling-off of the emission anisotropy at 1/ 7th of the fundamental anisotropy, which is confirmed by both steady-s tate and time-resolved data and thus indicates that there is no prefer red mutual orientation between the chromophores. As regards the rate o f energy hopping, an average value of 2 x 10(9) s(-1) can be deduced f rom the comparison between the theoretical and experimental decays. Th is value is shown to be consistent with a dipole-dipole mechanism of e nergy transfer.