Sensitization electronic energy transfer

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 ... 

Daubendiek et al.55 have extended this technique to demonstrate electronic energy transfer at the gas-solid interface. They irradiated photosensitizers, in polycrystalline form, in the presence of m-piperylene vapor and found that the extent of isomerization was comparable to values obtained for the same sensitizer in a solution of piperylene. Their results indicated an energy difference of about 1 kcal between triplet states of sensitizers in solution and on the surface of organic crystals. 

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. 

In somewhat related work of Venturi et al., the dinuclear complex [(bpy)2Ru(MACl)Os(bpy)2]4+ was examined (Scheme 2). Luminescence spectral data show that the emission band of the Ru(II) unit is almost completely quenched with concomitant sensitization of the emission of the Os (II) unit. Electronic energy transfer from the Ru(II) to the Os(II) 3MLCT state takes place by two distinct processes (ken = 2.0 x 108 and 2.2 x 107s-1 at 298 K) [74],... 

It should be borne in mind that the physical process of electronic energy transfer is not the only one which can lead to sensitization and inhibition or quenching.81 It is therefore essential, when one assumes that energy transfer is responsible for such effects, to verify wherever possible that this is the case. 

Very elegant experiments unequivocally proving the occurrence of electronic energy transfer were performed in 1922 and 1923 by Carlo and Franck [14], When a mixed vapor of mercury and thallium was irradiated with the mercury line at 253.67 nm, the emission lines of thallium could be observed in addition to the anticipated fluorescence spectrum of mercury. Since thallium cannot absorb 253.67-nm light, it must have been sensitized by the excited mercury atoms in order to produce the green fluorescence... 

Sensitized biacetyl phosphorescence and sensitized decomposition occur in the vapour phase as a result of electronic energy transfer from benzene to biacetyl. Some fluorescence was also observed however, the ratio of phosphorescence to fluorescence was very high. In fluid solutions benzene and some other organic compounds sensitize the biacetyl fluorescence but not the phosphorescence (refs. 316-318), in contrast to the findings for the vapour phase. A satisfactory explanation for this difference is not available at present. 

Photoreduction of the herbicide paraquat dichloride in aqueous propan-2-ol is more efficient in the presence of a sensitizer such as benzophenone than on direct irradiation.84 Hyde and Ledwith84 propose that the paraquat cation radical is formed by electron transfer from ketyl radicals, in turn produced during the conventional photoreduction of the sensitizer ketone. The suggested mechanism is given in reactions (23)—(25), where PQ2+ is the paraquat dication. The reduction process therefore involves chemical sensitization, rather than electronic energy transfer. 

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