Forster mechanism, energy migration

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 Forster mechanism is also known as the coulombic mechanism or dipole-induced dipole interaction. It was first observed by Forster.14,15 Here the emission band of one molecule (donor) overlaps with the absorption band of another molecule (acceptor). In this case, a rapid energy transfer may occur without a photon emission. This mechanism involves the migration of energy by the resonant coupling of electrical dipoles from an excited molecule (donor) to an acceptor molecule. Based on the nature of interactions present between the donor and the acceptor, this process can occur over a long distances (30—100 A). The mechanism of the energy transfer by this mechanism is illustrated in Figure 11. 

If we assume that the Forster mechanism Is responsible for energy migration, then the average hopping time can be estimated from... 

The long lifetime of lower excited states favors the efficient transfer of electron excitation energy from molecules of the solvent to molecules of the solute, provided the emission spectrum of the donor overlaps the absorption spectrum of the acceptor. The mechanism of the nonradiative transfer of electron excitation energy was found by Forster and Dexter39-40. The distance over which the energy is transferred may be as large as 20-30 A. The migration of electron excitation also occurs in... 

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