Excitons Forster

For dilute solutions of essentially independent donor and acceptor molecules the Forster or resonance interaction is quite important in molecular aggregrates and in molecular crystals exciton interactions are likely to be important. When the interaction is strong the excitation is not localized on the donor or acceptor but is spread over both. If larger aggregrates are involved, the excitation can be spread over many molecules/33-3 This can easily be seen for the case of a dimer where the donor and acceptor are... 

Baldo et al. [ 164] used the platinum complex of 2,3,7,8,12,13,17,18-octaethyl-21 //,23//-porphine (PtOEP, 66) as efficient phosphorescent material. This complex absorbs at 530 nm and exhibits weak fluorescence at 580 nm but strong phosphorescence from the triplet state at 650 nm. Triplet transfer from a host like Alq3 was assumed to follow the Dexter mechanism. Dexter-type excitation transfer is a short-range process involving the exchange of electrons. In contrast to Forster transfer, triplet exciton transfer is allowed. 

In other white light devices, blue, green and red emitters are combined. Kido et al. [169, 170] designed multilayer systems using 6 (TPD) for blue, metal-chelate complexes for green and red emission, respectively. Similar devices have been developed by other groups, using Forster transfer or exciton confinement for the creation of the three primary colors [171, 172]. Exciplex emission was... 

Fine Exciton-Leitung absorbierter Lichtenergie wurde auch anderweitig er-ortert Etwa beim Chlorophyll des Chloroplasten (Forster 1946 und 1948), bei den Alkalihalogeniden (Apker und Taft 1952) sowie schlieGlich auch beim Brom-silber selbst (Seitz 1951). 

Loi MA, Toffanin S, Muccini M, Forster M, Scherf U, Scharber M (2007) Charge transfer excitons in bulk heterojunctions of a polyfluorene copolymer and a fullerene derivative. Adv Funct Mater 17 2111... 

Given the density of Chi in PSI, it is immediately obvious that any detailed description of energy transfer in this system will need to handle both weakly coupled (Forster-type) and strongly coupled (excitonic) systems self consistently. It is also evident that discerning rate limiting steps and optimizing principles may not be straightforward. 

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).

In the Forster mechanism, the energy transfer upon excitation may take place between excited molecular entities separated by distances R, which are considered as spatially fixed Frenkel excitons. It is described in terms of resonant interaction between their transition dipole moments, which decreases as Rr6. 

Several theories have been developed to explain how energy absorbed by one molecule is transferred to a second acceptor molecule of the same or a different species. At first sight exciton theory,20 66 which accounts for excitation transfer in molecular aggregates or crystals and the Davydov splitting effects connected with it, appears to bear little relationship to the treatment of long-range resonance transfer as developed, for example, by Forster.81-32 However, these theories can be shown to arise from the same general considerations treated at different well-defined mathematical limits.33-79... 

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