MULTICHROMOPHORIC CYCLODEXTRINS .2. INHOMOGENEOUS SPECTRAL BROADENING AND DIRECTED ENERGY HOPPING

A beta-cyclodextrin bearing seven 2-naphthoyloxy chromophores is a good model for the study of the effect of the excitation wavelength on energy hopping among chromophores in well-defined positions, as in photosynthetic antennae. Absorption spectra, emission spectra, and excitation polarization spectra were recorded in a propylene glycol-dioxane glass at 200 K. Comparison is made with a bis(naphthoate) bichromophoric molecule. The parallelism between the increase of emission spectrum displacement and fluorescence anisotropy observed for the red edge of most vibronic bands, and especially for the 0-0 one, is established for the first time. It can be interpreted in terms of inhomogeneous spectral broadening due to solvation heterogeneity. The decrease of energy transfer that is observed upon red-edge excitation is evidence that energy hopping is not chaotic but directed toward lower-energy chromophores.