Energy transfer photosensitization

The most favorable process for energy transfer photosensitization involves exchange of energy from the excited triplet state of the photosensitizer to the excited triplet of the acceptor. Thermodynamic and life time considerations require that the triplet energy of the photosensitizer be greater than the triplet energy of the acceptor. 

while energy transfer does appear to occur for specific photosensitizer-onium salt combinations, it is of little practical use in photosensitized cationic polymerization due to the inefficiency of this process. 

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Fig. 4. Chemistry of poly(vinyl cinnamate) negative-acting resist. Initial light absorption by the photosensitizer is followed by energy transfer to produce a pendant cinnamate group in a triplet electronic state. This combines with a second cinnamate on another polymer chain, forming a polymer—polymer...

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

PCSs are systems of chromophores bound into a single macromolecule. Therefore, the study of processes of electronic excitation and energy transfer, as well as the investigation of the ways of deactivation of excited states, should lay a foundation for the understanding of such properties of PCSs as reactivity in photochemical transformations, photosensitizing and photoelectric activity, photoinitiated paramagnetism, etc. 

Photo-induced Diels Alder reaction occurs either by direct photo activation of a diene or dienophile or by irradiation of a photosensitizer (Rose Bengal, Methylene Blue, hematoporphyrin, tetraphenylporphyrin) that interacts with diene or dienophile. These processes produce an electronically excited reagent (energy transfer) or a radical cation (electron transfer) or a radical (hydrogen abstraction) that is subsequently trapped by the other reagent. 

Depending on the relative concentration of reactive substrate and dissolved molecular oxygen ( 02), RF is able to induce photosensitized oxidation of molecular targets by either Type I (electron-transfer) or Type II (energy-transfer) processes (Foote, 1991). In Type I... 

Zepp RG, PF Schlotzhauer, RM Sink (1985) Photosensitized transformations involving energy transfer in natural waters role of humic substances. Environ Sci Technol 19 74-81. 

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 ... 

A photosensitized dimerization of an isolated olefin, norbomene, has been reported by Scharf and Korte.<3) Irradiation in acetone or in the presence of acetophenone (Et = 74 kcal/mole) produced dimers (5) and (6) as major products. However, benzophenone (Et = 69 kcal/mole) failed to sensitize the reaction to (5) and (6), but in ether solution led to the quantitative formation of benzpinacol and in benzene to the oxetane (7) in 80% yield. Sensitizers of intermediate energy, such as xanthone (Et — 72 kcal/mole), demonstrated a competition between energy transfer to form triplet norbomene and cycloaddition to form the oxetane ... 

The photocycloaddition of an aldehyde or ketone with an olefin to yield an oxetane was reported by Paterno and Chieffi in 1909. 58> Contemporary studies on the synthetic utility and mechanistic features were initiated nearly 50 years later by Biichi et al. 59) Two review articles summarizing synthetic aspects of Paterno-Biichi reactions have been published 6.12)) and mechanistic studies have been reviewed several times. 6,38,60-62) The reaction involves the addition to olefin of a photo-excited carbonyl moiety. This circumstance makes it advantageous to review this reaction before a discussion of olefin-olefin additions, because the solution photochemistry of carbonyl compounds is probably better understood than any other aspect of organic photochemistry. Many of the reactions of carbonyl compounds have been elucidated during studies of the important phenomena of energy transfer and photosensitization. 63-65)... 

Whereas several l-aryl-4-nitroimidazoles are found to be good sensitizers for superoxide ion formations85 (Type I photooxidation), only l-phenyl-2-methyl-4-nitroimidazole 140 is a photosensitizer for singlet oxygen, i.e. by energy transfer of type II photooxidation (equation 70). 

Energy transfer in the photosensitization of silver halide photographic emulsions Optical sensitization, supersensitization and antisensitization. J. chem. Physics 19, 417 (1950-... 

A rule that affects energy transfers in photochemical reactions, particularly photosensitization processes. The total electron spin (/.c., the vectorial overall spin angular momentum of the system) does not change after the electronic energy transfer between an excited molecular entity and another molecular entity. 

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. 

Mechanism of Energy Transfer, in Mercury Photosensitization (Gunning... 

Figure 19.1 Mechanisms involved in sunlight-induced phototoxicity of drugs. Type 1 photosensitization (electron transfer) mainly generates singlet oxygen Oj", whereas type 2 reaction (energy transfer) leads to adduct formation or singlet oxygen...

Type 1 intrazeolite photooxygenation of alkenes has been also reported to give mainly allylic hydroperoxides (Scheme 42). In this process, the charge transfer band of the alkene—O2 complex within Na-Y was irradiated to form the alkene radical cation and superoxide ion. The radical ion pair in turn gives the allylic hydroperoxides via an allylic radical intermediate. On the other hand, for the Type II pathway, singlet molecular oxygen ( O2) is produced by energy transfer from the triplet excited state of a photosensitizer to 02. 

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