Energy transfer, exciplex intermediate

The primary interaction of singlet oxygen, produced by energy transfer from the excited sensitizer, with the diene can give rise to an exciplet that then collapses to peroxide, to a 1,4-biradical or to a 1,4-zwitterion alternatively, the adduct is the result of a concerted action without the involvement of an intermediate. Detailed kinetic Diels-Alder investigations of singlet oxygen and furans indicate that the reactions proceed concertedly but are asynchronous with the involvement of an exciplex as the primary reaction intermediate [63]. 

In contrast to the dipole-dipole interaction, the electron-exchange interaction is short ranged its rate decreases exponentially with the donor-acceptor distance (Dexter, 1953). This is expected since, for the electron exchange between D and A, respective orbital overlap would be needed. If the energy transfer is envisaged via an intermediate collision complex or an exciplex, D + A—(D-------A)- D + A, then Wigner s rule applies there must be a spin com-... 

Indirect evidence for the formation of a complex intermediate in diffusional energy transfer processes may be provided by the measurement of kt from both donor quenching and acceptor sensitization at low temperatures where kQC kf = 1/T° in this case exciplex relaxation should reduce the quencher sensitization efficiency but leave the donor fluorescence quenching constant unchanged. 

Morteani et al. demonstrated that after photoexcitation and subsequent dissociation of an exciton at the polymer-polymer heterojunction, an intermediate bound geminate polaron pair is formed across the interface [56,57]. These geminate pairs may either dissociate into free charge carriers or collapse into an exciplex state, and either contribute to red-shifted photoliuni-nescence or may be endothermically back-transferred to form a bulk exciton again [57]. In photovoltaic operation the first route is desired, whereas the second route is an imwanted loss channel. Figure 54 displays the potential energy ciu ves for the different states. 

Photosensitized enantiodifferentiating reactions are synthetically attractive and mechanistically interesting photochemical processes. The chiral information of the sensitizer is transferred to the substrate by short-Hved interactions in the excited state (i.e., during the lifetime of an exciplex of a reaction intermediate and the chiral sensitizer that is involved in the reaction mechanism) hence, the chirahty is multiplied, and only catalytic amounts of the optically active sensitizer are required. The stabilization energy of an exciplex compared to the locally excited state and its lifetime are often found to be strongly dependent on both electronic and steric properties of its components. Chiral induction can be achieved by different stabilization energies or lifetimes of the exciplex between the sensitizer and the intermediates that lead to the enantiomeric photoproducts. The absence of other reaction pathways without intimate contact to the sensitizer and of racemization processes in the further course of the reaction mechanism is an additional requirement to ensure effective chirality transfer. 

Solvents not only have an influence on the ordering of excited states but may also alter the reaction pathway taken by intermediates. For example, while the quantum yield of addition of diphenylvinylene carbonate to 2,5-dimethyl-2,4-hexadiene in hexane is 0.5, that in acetonitrile is <0.001 (Scheme 15) that is, adduct is formed in hexane but not in acetonitrile. The reaction in both solvents proceeds via an exciplex. In nonpolar solvents, the exciplex yields the adduct while in polar solvents, electron transfer occurs, which leads to energy wastage. 

---