Energy hopping

Energy hopping among the peryleneimide chromophores, revealed by anisotropy decay times [30], occurs with a rate constant of 4.6x10 s E When three peryleneimide and one terryleneimide chromophores are attached to the dendrimer rim, energy transfer from the former to the latter units takes place with... 

Finally, energy migration, i.e. excitation energy hopping, in polymers, artificial antenna systems, photo synthetic units, etc. can be investigated by fluorescence polarization (see Chapter 9). 

A //-cyclodextrin bearing seven 2-naphthoyloxy chromophores, CD7(6), is a good model for studying the effect of the excitation wavelength on energy hopping among chromophores in well-defined positions, as in photo synthetic antennae. 

Figure B9.3.1 shows the parallelism between the increase in emission spectrum displacement and fluorescence anisotropy observed for the red-edge of most vibronic bands and especially for the 0-0 one. It can be interpreted in terms of inhomogeous spectral broadening due to solvation heterogeneity. The decrease in energy transfer that is observed upon red-edge excitation is evidence that energy hopping is not chaotic but directed toward lower energy chromophores, as in photosynthetic antennae.

Berberan-Santos M. N., Choppinet P., Fedorov A., Jullien L., Valeur B. (1999) Multichromophoric Cydodextrins. 6. Investigation of Exdtation Energy Hopping by Monte-Carlo Simulations and Time-Resolved Fluorescence Anisotropy, J. Am. Chem. Soc. 121, 2526-2533. 

Scheme 3 summarizes this problem with a minimum number of sites and competing processes. In this scheme, two sites, square-well type (X) and spherical-well type (Y), are available for the residence of reactant molecules (A). For the sake of convenience, molecules residing at sites X and Y are labeled Ax and AY. Excitation of these molecules gives rise to A and A. Photoreactivity of molecules excited in each site will be identical if they equilibrate between X and Y before becoming photoproducts. In media with time-independent structures, such as crystals, equilibration requires diffusion of molecules of A in media with time-dependent structures, such as micelles and liquid crystals, equilibration can be accomplished via fluctuations in the microstructure of the reaction cavities as well as translational motion of A (Scheme 4). An additional mechanism for site selective reactions or equilibration of A and A molecules can be achieved via energy migration (e.g., energy hopping, exciton migration, or Forster energy transfer).

Photosynthetic membranes also contain pigment-protein complexes that serve as antennas. When the antenna absorbs a photon, energy hops rapidly from complex to complex by resonance energy transfer until it is trapped in a reaction center. 

Analysis of the data showed the presence of a fast intramolecular photoinduced energy transfer process from pyrene -perylene to pyrene-perylene (ken 6.2 x 109 s 1) with a high yield (>90%), followed by efficient intramolecular electron transfer from pyrene-perylene to pyrene +-perylene (70%, ket 6.6 x 109s 1). Both processes occur from the pyrene unit to the perylene moiety. The Forster distance was calculated to be 3.4 nm and the corresponding donor-acceptor distance was calculated from the energy transfer rate as 0.9 nm. No indications for energy hopping between different pyrene moieties were observed. 

Energy hopping is a Forster-type process that is present in the multichromo-phoric dendrimer such as p-ClP4 and can be related to the spectral overlap as... 

Figure 1.15. Schematic representation of the efficiency of the energy hopping process present in / -ClP4 as a function of distances expressed in Eq. (4).

In these multichromophoric cyclodextrins the fluorophores are randomly oriented. Excitation of one of the naphthoate fluorophores is followed by efficient dipole-dipole excitation energy transfer between the seven fluorophores, with a Forster radius of 14 A. This process is not detectable by fluorescence intensity measurements, as neither the intensity nor the decay law are affected by energy transfer between identical fluorophores (also called homotransfer. The dynamics of energy hopping are on the other hand reflected in the fluorescence anisotropy. To avoid depolarization by rotational motion of the fluorophores, experiments were conducted in a low temperature and optically clear rigid glass (9 1 ethanol-methanol at 110 K). 

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