Electronic energy transfer mechanism

The electronic energy transfer mechanism has become one of the most cful processes in photochemistry. It has wide applications as a mechanistic tool and in photochemical synthesis. It allows photosensitiza-of physical and chemical changes in the acceptor molecule by the... 

In all four cases, the assignment of the electron/energy transfer mechanisms have further experimental support from continuous and flash photolysis studies. Formation of Ru(III) and reduced acceptor ions can be observed in the first three cases and cage escape yields determined. For Eu(III) and Cr(III), in spite of the similarity in the redox potentials (E == - 0.43V and -0.41V respectively) and self-exchange rate constants, marked difference exists in the quenching behaviour. 

Photodegradation of copolymer of methyl methacrylate and 2[2"-hydroxy 5 vlnyl-phenyl] 2H-benzotrla-zole has been Investigated In order to determine the changes In the chemical composition of the surface of the copolymer on photooxldatlon. An electronic energy transfer mechanism has been postulated In order to Interpret the observed photochemical changes In the polymer. Preliminary examination of the photophysical properties of the chromophore provides support for such a mechanism. 

In this chapter we shall first outline the basic concepts of the various mechanisms for energy redistribution, followed by a very brief overview of collisional intennoleciilar energy transfer in chemical reaction systems. The main part of this chapter deals with true intramolecular energy transfer in polyatomic molecules, which is a topic of particular current importance. Stress is placed on basic ideas and concepts. It is not the aim of this chapter to review in detail the vast literature on this topic we refer to some of the key reviews and books [U, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, and 32] and the literature cited therein. These cover a variety of aspects of tire topic and fiirther, more detailed references will be given tliroiighoiit this review. We should mention here the energy transfer processes, which are of fiindamental importance but are beyond the scope of this review, such as electronic energy transfer by mechanisms of the Forster type [33, 34] and related processes. 

Not all sensitized photochemical reactions occur by electronic energy transfer. Schenck<77,78) has proposed that many sensitized photoreactions involve a sensitizer-substrate complex. The nature of this interaction could vary from case to case. At one extreme this interaction could involve a-bond formation and at the other extreme involve loose charge transfer or exciton interaction (exciplex formation). The Schenck mechanism for a photosensitized reaction is illustrated by the following hypothetical reaction ... 

The principal mechanisms by which electronic energy transfer occurs are shown in Figure 6.8. 

Figure 6.8 The principal mechanisms of electronic energy transfer. All three mechanisms require overlap between the fluorescence spectrum of the donor and the absorbance spectrum of the acceptor...

Mechanisms of Intramolecular Electronic Energy Transfer in Bichromophoric Molecular Systems Solution and Supersonic Jet Studies, Chem. Rev. 96, 1953—1976. 

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... 

Dye sensitization plays an important role in photography. The sensitization mechanism for ZnO-materials as used in electro-photography is obviously in complete correspondence with these electrochemical experiments as shown for single crystals under high vacuum conditions by Heiland 56> and for imbedded ZnO-particles by Hauffe 57). Even for silver halides where electron injection as sensitization mechanism has been questioned by the energy transfer mechanism 58> electrochemical experiments have shown that the electron injection mechanism is at least energetically possible in contact with electrolytes 59>. Silver halides behave as mixed conductors with predominance of ionic conductivity at room temperature. These results will therefore not be discussed here in any detail since such electrodes are quite inconvenient for the study of excited dye molecules. 

Two main models are usually discussed for the mechanism of the spectral sensitization. The excitation of the sensitizer by absorbed light and electron transfer from the excited sensitizer to the semiconductor is the first model. The alternative mechanism consists of the transfer of the excitation energy from the sensitizer to the semiconductor. This energy is used for photogeneration of the charge carriers in the sensitized photoconductor. In the first case the excited singlet level of the sensitizers has to be located above the conduction band of the semiconductor for realization of the electron transfer. For hole transfer the basic sensitizer level has to be located lower than the valence band of the sensitized photoconductor. The energy transfer mechanism does not need a special mutual location of the semiconductor and sensitizer levels. 

These rules also predict the nature of photoproducts expected in a metal-sensitized reactions. From the restrictions imposed by conservation of spin, we expect different products for singlet-sensitized and triplet-sensitized reactions. The Wigner spin rule is utilized to predict the outcome of photophysical processes such as, allowed electronic states of triplet-triplet annihilation processes, quenching by paramagnetic ions, electronic energy transfer by exchange mechanism and also in a variety of photochemical primary processes leading to reactant-product correlation. 

A very important bimolecular deactivation process is the electronic energy transfer (ET). In this process, a molecule initially excited by absorption of radiation, transfers its excitation energy by nonradiative mechanism to another molecule which is transparent to this particular wavelength. The second molecule, thus excited, can undergo various photophysical and photochemical processes according to its own characteristics. 

It has been assumed so far that the sensitizer acts by an energy-transfer mechanism, but in some cases other modes of interaction may occur. It is possible that electron transfer takes place to give the radical anion or the radical cation of the alkene, which is the species that subsequently isomerizes. This is likely to be the case in the chlorophyll-sensitized isomerization of vitamin A acetate, which is used commercially to obtain the required all-trans isomer 12.8) from the mixture of Isomers resulting from the synthesis. Unlike triplet-sensitized reactions, electron-transfer isomerizations frequently lead to a predominance of the most thermodynamically stable isomer. 

Photoinduced Electron Transfer. Monolayer organizates are particularly suited for the investigation of photoinduced electron transfer, since the molecules are fixed and the distance between the planes at which the donor and the acceptor molecules, respectively, are located can be well defined. Therefore, complex monolayers have been arranged in order to study the distance dependence of electron transfer in these systems (2, 20). This strategy has also been used to elucidate the relative contributions of electron injection and energy transfer mechanisms in the spectral sensitization of silver bromide (21). 

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