Dexter transfer

If the charge distributions of the D and A overlap than a new class of interactions has to be considered, namely the exchange interaction between the electrons on D and on A. This type of energy transfer is called Dexter transfer [80, 96,98], Here we briefly outline the physical principles involved. 

The Dexter transfer probability is determined by the exchange integral ... 

In contrast, EET has been historically modelled in terms of two main schemes the Forster transfer [15], a resonant dipole-dipole interaction, and the Dexter transfer [16], based on wavefunction overlap. The effects of the environment where early recognized by Forster in its unified theory of EET, where the Coulomb interaction between donor and acceptor transition dipoles is screened by the presence of the environment (represented as a dielectric) through a screening factor l/n2, where n is the solvent refractive index. This description is clearly an approximation of the global effects induced by a polarizable environment on EET. In fact, the presence of a dielectric environment not only screens the Coulomb interactions as formulated by Forster but also affects all the electronic properties of the interacting donor and acceptor [17],... 

Many tests have been made of the validity of Forster-Dexter transfer in doped organic solids. A typical example is the classic solid solution study of anthracene and tetra-cene dissolved in naphthalene [26,166]. Energy originally absorbed by anthracene (donor) was shown to be transferred with high efficiency to tetracene (acceptor) by measuring the tetracene fluorescence. In these studies, the anthracene tetracene ratio was 1 1 and the mol fraction of the guests... 

Such a concentration dependence would te consistent either with a Foster or Dexter transfer mechanism whose rate is such that Stern-Volmer kinetic behaviour is obeyed or would be consistent with the alternative suggestion of scheme (3) where the excited state naphthalene chromophore experiences intramolecular diffusion controlled self quenching. 

The optical properties of polyfluorene make it an ideal system in which to investigate Dexter transfer. Fig. 10.41a shows the PA spectrum of an undoped PFO film, measured at 80 K.2 The PFO film was excited with a pump beam at... 

The spectrally narrow triplet PA band of PFO makes it possible to compare PFO triplet-state dynamics in the doped and undoped systems in order to look for evidence of Dexter transfer. The PtOEP triplet lifetime in a PFO matrix was determined by measuring the phosphorescence decay following photoexcitation. The phosphorescence decay of a blend containing 1 wt% PtOEP in PFO is inset in Fig. 10.42. The decay could be fit to a lifetime of 55 2/t,s, in excellent agreement with pulsed EL measurements.71... 

The lifetime of PFO triplet excitons is much longer than the phosphorescence lifetime of PtOEP in PFO. Dexter transfer from PFO to PtOEP should reduce the PFO triplet lifetime, because PtOEP molecules decay at a much more rapid rate (56 jis) than PFO triplets (0.3 - 3.2 ms). The solid line in Fig. 10.42 shows the frequency dependence of the PFO triplet PA signal in an 8 wt% doped PFO film, measured under identical conditions as for the undoped film. There is no difference in the frequency dependence of the PA of the two different samples. Thus, if there is any Dexter transfer from PFO to PtOEP, it must be exceedingly weak. This conclusion is supported by the fact that there was no evidence for phosphorescence originating from Dexter transfer of relatively long-lived PFO triplets. 

Dias FB, Knaapila M, Monkman AP, Burrows HD (2006) Fast and slow time regimes of fluorescence quenching in conjugated polyfluorene-fluorenone random copolymers The role of exciton hopping and dexter transfer along the polymer backbone. Macromolecules 39(4) 1598—1606... 

The energy transfer in this matrix occurs in different ways. They are (i) Forster type energy transfer, (ii) Dexter transfer (iii) Exciplex - excimer charge transfer and (iv)Trap assisted... 

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. 

An important optical process in organic materials concerns the energy transfer between molecules via excitons. Two mechanisms can be distinguished Forster and Dexter transfer. In the Forster mechanism, the energy released from the exciton upon its dissociation is nonradiatively transferred to a molecule, which in turn creates another exciton by a process previously described. This mechanism occurs between the donor and acceptor molecules and is a long-range distance process. Several conditions should be fulfilled to expect an efHcient energy transfer by the Forster mechanism ... 

Dexter transfer of triplet excitons from the host to the guest... 

Both Forster and Dexter transfers can also be used to gather the excited states for obtaining an optimized emission from devices [2]. These processes are essential in the photophysics of composites and will be discussed in detail in the following sections. 

Dexter transfer A similar expression determines the transfer rate for triplet excitons, namely,... 

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