Triplet Forster mechanism

The mixed triplet quenching mechanism just described implies the formation of mixed dimers AD and AD. This is not an unreasonable proposition because Birks and Christophorou48 have already demonstrated the formation of mixed singlet excited dimers by the Forster— Kasper mechanism ... 

The Dexter mechanism can be thought of as electron tunneling, by which one electron from the donor s LUMO moves to the acceptor s LUMO at the same time as an electron from the acceptor s HOMO moves to the donor s HOMO. In this mechanism, both singlet-singlet and triplet-triplet energy transfers are possible. This contrasts with the Forster mechanism, which operates in only singlet states. 

Note that in the case where the Forster mechanism concerns an organic species transferring its energy to the lanthanide center to which it is bound, this is often referred to as the antenna effect and the ligand is sometimes called a sensitiser. Such an effect becomes efficient provided that the organic chromophore possesses a triplet excited state close to but at least 1700 cm-1 above that of the lanthanide emissive state (Parker and Williams, 1996). 

There is another context in which long-range overlap independent interactions are important, namely, dipole-dipole (or Fdrster ) coupling between dipole-allowed electronically excited states of chromophores. Since the Forster contribution to the coupling has a power law (R ) distance dependence, and, as will be shown below, the TB and TS interactions fall off exponentially with increasing distance, the Forster mechanism dominates, except for systems with short bridges. For the corresponding triplet states there is no counterpart to the Forster mechanism, and TB and TS interactions dominate. 

The overlap integrals J (v) which are essential for a Forster long-range dipole-dipole transfer mechanism 56> were obtained from the benzaldehyde phosphorescence spectrum and the absorption spectra of the fluorescers used. A linear relation was observed between the square root of the overlap integrals and the number of excited singlet fluorescer molecules produced per molecule of benzaldehyde triplet, which is a strong argument for the essential role of the proposed transfer mechanism. 

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. 

The distance between the chromophores Is approximately 21 8 as measured on molecular models and no ground state Interaction could be observed. In dioxane, the singlet energy transfer occurs by a Forster-type mechanism. On the basis of this mechanism, the distance can be calculated from the transfer efficiency. For [16a] a value of 21.211.6 X was found, while for [I6b] one obtained a value of 16.7 1.4 X. This latter discrepancy was explained on the basis of preferential orientation of the chromophores. That this phenomenon Indeed plays an Important role, was proved by Leermakers (59,60) In a series of indole alkaloids. The same author (59) also Illustrated the absence of triplet-triplet transfer at low temperature in compound [16b]. 

Furthermore, the diffusion coefficients for triplet exciton migration extracted from this three-dimensional intramolecular model were nearly the same as those obtained using the conventional kinetic equation (i.e. eq 1). The hopping frequencies for triplet exciton migration in PVCA for these three models are summarized in Table 111(20). Neither the electron exchange mechanism(21) nor the Forster... 

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