Mechanisms for photosensitization

The success of the energy transfer theory in correlating data for a number of systems with sensitizers of widely varying structure, particularly aromatic hydrocarbons for which bond formation between donor and acceptor does not appear to be a reasonable process, led to its acceptance as the general mechanism for photosensitized olefin isomerization. However, there are special cases in which another mechanism of sensitization is operative. 

SCHEME 3.3 Mechanism for photosensitized carbon-carbon bond cleavage. 

Figure 3.62. Mechanism for photosensitized generation of singlet oxygen using TP excitation.

Scheme 4.19 Different mechanism for photosensitized oxygenation reactions...

Specifically, direct and indirect methods of photosensitization were investigated to extend the range of spectral sensitivity of diaryiiodonium salts. Much of the initial effort was concentrated on classical triplet energy transfer as a direct photosensitization method. However, this type of photosensitization proved to be rather inefficient and, consequently, the direction of the focus of the research turned toward electron transfer photosensitization. Electron transfer photosensitization provides a practical means to extend the photosensitivity of diaryiiodonium salts from the mid-UV well into the visible region of the spectrum. Shown in Scheme 22 is the proposed generalized mechanism for photosensitization as applied to diaryiiodonium salts. 

Based on the work of several investigators, three different mechanisms for photosensitization have been found to be operative and are discussed in the following sections. 

The transfer of energy must proceed with net conservation of spin. In the usual case, the acceptor molecule is a ground-state singlet, and its reaction with the triplet state of the sensitizer will produce the triplet state of the acceptor. The mechanism for triplet photosensitization is outlined below ... 

Irradiation of l,l,3,3-tetraphenyl-5-methyl-l,4-hexadiene gives the two products shown below. When the reaction is carried out by photosensitization, B is not formed. Suggest mechanisms for the formation of A and B. What other products might have been expected Can you rationalize their absence ... 

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 ... 

Once internalized within the RPE, there must be a mechanism for carotenoid transport to photoreceptors. The RPE metabolizes lipids from phagocytosed POS and provides a constant supply of lipids to photoreceptors for the synthesis of new discs and molecular renewal of lipids within existing discs (Strauss, 2005). Thus there is a constant transfer of lipids from the RPE to photoreceptors. It has been shown in the rabbit and monkey that intraveneous administration of lipophilic benzopor-phyrin bound to LDLs results in an efficient delivery of the fluorescent photosensitizer not only to the RPE but also to photoreceptors this occurs within 20 min following injection (Haimovici et al., 1997 Miller et al., 1995). 

The term photosensitizer was originally used to refer to the second pathway, especially when it involved energy transfer, but that distinction has become blurred. The mechanism for photoinitiation in a reaction system is not always clear-cut and may involve both pathways. Photosensitizer is now used to refer to any substance that either increases the rate of photo-initiated polymerization or shifts the wavelength at which polymerization occurs. 

The Schenck Mechanism. Two mechanisms were originally proposed for photosensitized olefin reactions. One suggested that sensitization occurred by exchange energy transfer from an excited donor (S) to yield ground state donor and excited state acceptor.24,136... 

Later on a lot of research into the photosensitivity of the polymer matrix with various inserted molecules was carried out. A great deal of attention was focused on molecule doped polycarbonates. The photoconduction mechanism in such systems has many common features with the same mechanism for doped PVC. 

Oxidation of DMS to DMSO and DMSO. DMS is chemically and biochemically oxidized to dimethylsulfoxide (DMSO). Mechanisms for the in situ oxidation of DMS to DMSO in seawater have received little attention, even though this may be an important sink for DMS. Hydrogen peroxide occurs in surface oceanic waters (22) and is produced by marine algae (98). It may participate in a chemical oxidation of DMS, since peroxide oxidizes sulfides to sulfoxides (991. Photochemical oxidation of DMS to DMSO occurs in the atmosphere and DMSO is found in rain from marine regions (681. DMS is also photo-oxygenated in aqueous solution to DMSO if a photosensitizer is present natural compounds in coastal seawater catalyzed photo-oxidation at rates which may be similar to those at which DMS escapes from seawater into the atmosphere (1001. 

Various mechanisms for the initiation of cationic polymerization are feasible. Photosensitizer radical cations (PS+ ) may react directly with the monomer M ... 

For discussions on the mechanism of photosensitized reactions, the reader is referred to the publications by Schenck 10193 and Hammond et a/.204 206 See also Foote and Wexler164 and Saltiel.207... 

A similar electron transfer mechanism has been proposed for photosensitized electron transfer catalysis of the Mukaiyama-aldol reaction of aldehydes and ketones with enol silanes [301], Photoinduced electron transfer from enol silanes to a monocationic -bridged porphyrin [302, 303] leads to the production of a... 

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