Energy transfer, hopping mechanism

Spacers with energy levels or redox states in between those of the donor and acceptor may help energy or electron transfer (hopping mechanism). Spacers whose energy or redox levels can be manipulated by an external stimulus can play the role of switches for the energy- or electron-transfer processes.141... 

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).

The two most important depolarization mechanisms that give rise to a time-dependent anisotropy are fluorophore rotation and energy transfer. Energy transfer leads to depolarization as the hopping of excitation from one fluorophore to another, when not parallel, is equivalent to an angular displacement. 

First, consider the simplest case in which the sensitizer-activator interaction is treated as equivalent to the sensitizer-sensitizer interaction so the exciton becomes trapped only when it happens to hop onto an activator site. Also let us use as an example a simple cubic lattice of sensitizers and assume an electric dipole-dipole interaction as the mechanism causing the energy transfer. The hopping time is represented by th and the probability of host fluorescence per time of one step is a. The fraction of lattice sites which are traps is Cj and the probability of luminescence emission from a trapped exciton per time of one step is (3. The probability of host luminescence at the nth step in the random walk is... 

Direct evidence of energy migration in the different films was found in polarized fluorescence emission studies. The fluorescence anisotropy indicated that intramolecular energy migration (with a Dexter hopping mechanism) existed in both the solution and spin-cast films. Further energy migration and transfer studies were... 

Third, the energy transfer mechanism could be the Dexter hopping mechanism, which relies more on the direct orbital overlap, and the Forster dipole-dipole energy transfer, which is more influenced by the relative orientations and the distance between the donors and acceptors. The Forster transfer is demonstrated to be much faster than Dexter transfer in the conjugated polymers. 

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