Photolysis mechanism direct energy transfer

Based on the known chemistry of flavin photolysis reactions, it appears unlikely that thymine dimer cleavage occurs via a direct energy transfer mechanism (160). One proposal suggests that in the model reaction with 1-deazariboflavin, the thymine dimer radical anion is formed via electron donation from the excited sensitizer (164). Alternatively, electron abstraction by the excited flavin could occur, resulting in the thymine dimer radical cation (159, 160), although it is unlikely that reduced flavin would act as an electron acceptor. A schematic for this mechanism is illustrated in Scheme 33, where the initial formation of a sensitizer-dimer complex is consistent with the observed saturation kinetics. The complex is activated by excitation of the ionized sensitizer (pH > 7), and electron donation to the dimer forms the dimer radical anion and the zwitterionic, neutral 1-deazariboflavin radical (162). Thymine dimer radical would spontane-... 

Excited states may be formed by (1) light absorption (photolysis) (2) direct excitation by the impact of charged particles (3) ion neutralization (4) dissociation from ionized or superexcited states and (5) energy transfer. Some of these have been alluded to in Sect. 3.2. Other mechanisms include thermal processes (flames) and chemical reaction (chemiluminescence). It is instructive to consider some of the processes generating excited states and their inverses. Figure 4.3 illustrates this following Brocklehurst (1970) luminescence (l— 2)... 

Dektar and Hacker report on the photochemistry of diaryliodonium salts in the case of direct photolysis and in the case of energy transfer from a photosensitizer [26], The photoproducts and the reaction mechanisms are different in the two situations (Scheme 12.5). 

It consists of thioxanthone or 2-isopropylthioxanthone in combination with (2,4,6-trimethylbenzoyl)-diphenyl-phosphine oxide or bis(2,4,6-trimethyl-benzoyl)-phenylphosphine oxide. The mechanism of sensitization was reported to involve triplet-triplet energy transfer from the thioxanthones to die phosphine oxides. That is followed by formation of radicals through a-cleavage of the photoinitiators. Direct photolysis of the phosphine oxides results in an absorptive, chemically induced, d)mamic electron polarization due to the triplet mechanism of polarization of the substituted benzoyl. The phosphoras-centered radicals produced by a-cleavage of the photoinitiators are the same radicals that... 

The photodissociation of an adsorbed molecule may occur directly or indirectly. Direct absorption of a photon of sufficient energy results in a Franck-Condon transition from the ground state to an electronically excited repulsive or predissociative state. Indirect photodissociation of adsorbates, involving absorption of photons by the substrate, can take place via two processes. The first one is analogous to the process of sensitized photolysis in gases. The second one, also substrate mediated, implies the phototransfer of an electron from the substrate to an antibonding orbital of the adsorbate, i.e. charge transfer photodissociation. The basic principles of these two excitation mechanisms will be discussed later in this part. 

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