Excitons Dexter

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. 

Scheme 2. In this photophysical scheme it was proposed that M, and D interact by the generally accepted exciton diffusion mechanism. M was considered to be an isolated naphthalene chromophore wliich can transfer energy into M with a transfer rate characterized by the rate coefficient kt- Reverse transfer from M to M was considered unimportant for the following reason. Exciton diffusion is expected to be very efficient within sequences of naphthalene chrcmiophoies within the chain comprising the M sites. In view of the reduced lifetime of M relative to M and of the delocalised nature of the energy within extended chromc hore sequences which increases the effective sqiaiation of M and M, M to M enei transfer by Foster or Dexter mechanisms is dimini ed relative to the to Mf process.

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... 

Knox, R.S. Theory of Excitons, Academic Press, New York, 1963 Dexter, D.L.,... 

The absence of an absorption cross section for the exciplex means that it cannot be excited optically. Instead, an exciplex is formed by complexation of a ground-state molecule with an excited-state molecule, i.e. by Dexter-type energy transfer from a bulk exciton. Figure 2.10 plots the photoluminescence excitation spectra of the PFB, the F8BT, and the exciplex emission, all measured from the same 50 50 PFB F8BT blend. The PLE signature of the exciplex is a superposition of those of the two excitons. Hence, the exciplex is excited via energy transfer from the two bulk excitons. 

Fig. 11.6 Scheme illustrating various transfer processes from the host to the guest electron (hole) transfer from the host LUMO (HOMO) to the guest LUMO (HOMO), Forster-type energy transfer between singlet states and Dexter-type energy transfer from the host to the guest triplet state. The nature of the excitonic state is simplified for better clarity. 

Another mechanism for the quenching of fluorescence or phosphorescence depends upon the possibility that the energy of the excited state (exciton) can be transferred to a molecule (acceptor) by a non-radiative mechanism. The two most probable such mechanisms are those of the dipole—dipole interaction and the exchange effect (Forster [12] and Dexter [13]) which may extend 2—5 and lnm, respectively. Except in very mobile solvents, the dipole—dipole interaction completely dominates diffusion and makes it difficult to observe the quenching behaviour characteristic of a diffusive process. The exchange effect is much less strong and is more comparable with diffusive quenching. Marshall et al. 

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