Dexter mechanism

The occurrence of energy transfer requires electronic interactions and therefore its rate decreases with increasing distance. Depending on the interaction mechanism, the distance dependence may follow a 1/r (resonance (Forster) mechanism) or e (exchange (Dexter) mechanisms) [ 1 ]. In both cases, energy transfer is favored by overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor. 

Understand that nonradiative singlet-singlet transfer can occur through either a dipole (Forster) mechanism, which can operate over long distance, or an exchange (Dexter) mechanism, which involves close approach. 

Calixarene containing a dioxotetraaza unit, PET-18, is responsive to transition metal ions like Zn2+ and Ni2+. Interaction of Zn2+ with the amino groups induces a fluorescence enhancement according to the PET principle. In contrast, some fluorescence quenching is observed in the case of Ni2+. PET from the fluorophore to the metal ion is a reasonable explanation but energy transfer by electron exchange (Dexter mechanism) cannot be excluded. 

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. 

As well as returning to the ground state by radiative or radiationless processes, excited states can be deactivated by electronic energy transfer. The principal mechanisms for this involve dipole-dipole interactions (Forster mechanism) or exchange interactions (Dexter mechanism). The former can take place over large distances (5 nm in favourable cases) and is expected for cases where there is good overlap between the absorption spectrum of the acceptor and the emission spectrum of the donor and where there is no change in the spin... 

Figure 3.36 The Dexter mechanism of energy transfer through simultaneous electron exchange.

Dexter mechanism Energy transfer by radiative emission... 

Fig. 5.3 Energy transfer according to the Dexter mechanism (schematic)...

The electron exchange (Dexter) mechanism which involves the overlap of wavefunctions of the donor and acceptor groups. This is a short-range excitation transfer mechanism that operates by... 

For all the following considerations it is an important fact that within the CC of interest mutual chromophore wave function overlap and electron exchange effects among different chromophores do not take place (absence of the Dexter mechanism). Therefore, we may assume the orthogonality relation Pvnf Pnb) — A/ ,i A,.h to be valid, where A, / - /A,) denotes the electronic... 

The second type, radiationless energy transfer, is more efficient. There are two different mechanisms used to describe this type of energy transfer the Forster and Dexter mechanisms. 

The Dexter mechanism is a nonradiative energy transfer process that involves a double electron exchange between the donor and the acceptor (Fig. 12).16 Although the double electron exchange is involved in this mechanism, no charge separated-state is formed. 

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. 

In these graphs a Bohr radius value (L) of 4.8 A (the value for porphyrin) is used in the Dexter equation 33.18 Also, the solid lines correspond to hypothetical situations in which only the Forster mechanism operates the dotted lines are hypothetical situations for when the Dexter mechanism is the only process.18 The curved lines are simulated lines obtained with equation 32 (Forster) or 33 (Dexter) but transposed onto the other graph (i.e., Forster equation plotted against Dexter formulation and vice versa). 

FIGURE 26. Some donor-bridge-acceptor systems by which energy transfer occurs through both Forster and Dexter mechanisms. (Modified from Ref. 78.)... 

Coulombic or dipole-dipole interaction (Forster) and double electron exchange (Dexter) mechanisms. 

Through-bond energy transfer was also observed for porphyrin systems (regardless whether it occurs via a Forster or a Dexter mechanism). Through-bond energy transfer was reported for the rhodium meso-tetraphenylporphyrin-tin (2,3,7,13,17,18-hexamethyl-8,12-diethylcorrole), which exhibits a Rh-Sn bond... 

---