Abstract
A β-cyclodextrin labeled with seven naphthoyloxy chromophores was studied by steady-state and time-resolved fluorescence spectroscopy in order to get information on the dynamics of energy hopping between chromophores. 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 obtained under the same conditions by the multifrequency phase-modulation technique upon excitation at 290 nm. The data were analyzed and interpreted on the basis of a theoretical model involving a unique rate constant for energy hopping between nearest neighbors. In particular, this model predicts a long-time leveling-off of the emission anisotropy at 1/7th of the fundamental anisotropy, which is confirmed by both steady-state and time-resolved data and thus indicates that there is no preferred mutual orientation between the chromophores. As regards the rate of energy hopping, an average value of 2 × 109 s-1 can be deduced from the comparison between the theoretical and experimental decays. This value is shown to be consistent with a dipole−dipole mechanism of energy transfer.
Highlights
Excitation energy hopping between chromophores is an important process occurring in the antennae pigments of photosynthetic units[1] and in antenna-based photomolecular devices.[2]
In the first two papers of this series,[3,4] we focused our attention on the photophysical properties of various -cyclodextrins labeled with 7 or 14 2-naphthoyloxy chromophores
The value of k is surprisingly in very good agreement with that estimated by Forster’s theory, despite the latter being a crude approximation because the distributions of distances and mutual orientations of the chromophores have not been taken into account
Summary
Excitation energy hopping between chromophores is an important process occurring in the antennae pigments of photosynthetic units[1] and in antenna-based photomolecular devices.[2] For the understanding of fundamental aspects of energy hopping, it is of interest to study supramolecular systems containing a limited number of chromophores that are well spatially defined In this respect, -cyclodextrins labeled with chromophores offer distinct advantages.[3,4,5] They are able to include species in their cavity, allowing to study the antenna effect, i.e., energy transfer from the antenna chromophores to an included acceptor.[6]. In a rigid glass, a decrease of energy transfer was observed upon red-edge excitation as a result of inhomogeneous broadening due to solvation heterogeneity; energy hopping was shown to be not chaotic but directed toward lower-energy chromophores
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