Type II mechanism

Attaching the ketone groups to the polymer backbone is more efficient on a chain scission/ketone basis because some of the light energy that the pendent ketone absorbs leads direcdy to chain scission via the Norrish type II mechanism, as well as photooxidation via the Norrish type I mechanism (see... 

As a side reaction, the Norrish type I reaction is often observed. The stability of the radical species formed by a-cleavage determines the Norrish type 1/Norrish type II ratio. For example aliphatic methyl ketones 10 react by a Norrish type II-mechanism, while aliphatic tcrt-butyl ketones 11 react preferentially by a Norrish type I-mechanism. 

Norrish type II mechanism for photo-oxidation of polyolefins. 

Nitroimidazoles (such as metronidazole and misonidazole) can enhance the porphyrin sensitized Type I photooxidations83 that is, electron transfer from the sensitizer to the oxygen molecule is facilitated to give more of the superoxide ion (Scheme 6). The Type II mechanism operates by energy transfer from the sensitizer to afford the singlet oxygen84. 

In other cases fullerene antibacterial action takes place after photoirradiation of fulleropyrrolidinium salts. It is not yet clear if the photodynamic action implies the participation of superoxide and hydroxyl radicals (type I mechanism) or singlet oxygen (type II mechanism) but the efficacy is really interesting with the death of more than 99.9% of bacterial and fungal cells and a special selectivity for microbes over mammalian cells (Tegos et al., 2005). Also a sulfobutyl fullerene derivative is able to inhibit environmental bacteria after photoirradiation and it exerts its action on E. coli even if incorporated in coated polymer (Yu et al., 2005). 

Grossweiner LI, Patel AS, Grossweiner JB (1982) Type I and type II mechanisms in the photosensitizing lyses of phosphatidylcholine liposomes by hematoporphyrin. Photochem Photobiol. 36 159-167. 

Many compounds sensitize biomolecules to damage by UVA (320-380 nm) and visible light. Two general mechanisms of sensitization are encountered. The Type I mechanism involves electron or hydrogen transfer from the target molecule to the photosensitizer in its triplet state. If 02 is present, this can be reduced to 02 by the reduced sensitizer. In the Type II mechanism, the excited sensitizer is quenched by 02, which is excited to the singlet state (typically A"g) and attacks the target molecule. Photosensitization is exploited in photodynamic therapy (PDT) for the destruction of cancerous or other unwanted cells. 

A very similar theory has been derived by Morrison and co-workers [158, 159], save that explicit expressions were used for the rate constants, derived from Gerischer s theory as discussed above, and a type II mechanism was postulated for the system investigated. The final expression took the form... 

Various 4-(arylazo)phenols and naphthols have been photooxidized using 02( A ) involving a type II mechanism . l,l -Binaphthol undergoes enantioselective oxidation (5.2% ee) when the chiral complex A-[Ru(4,4 -dimenthoxycarbonyl-2,2 -bipyridine)3] + is used as the photocatalyst . 

The photooxidation of oxopurines such as caffeine, theophylline, theobromine, and 1,3,7-trimethyluric acid using Rose Bengal as sensitizer occurs by a type II mechanism. 3-Methyl-5-(methylamine)-l,5-dehydrohydantoin has been characterised as a reaction product, and evidence is presented which suggests that the initial exciplex formed between 02( Ag) and the oxopurine evolves into a zwitterionic transition state. 

C.S. Foote (1987). Type I and Type II mechanisms of photodynamic action. In J.R. Heitz, K.R. Downum (Eds), Light-activated Pesticides (pp. 22-38). American Chemical Society, Washington, DC. 

Under this irradiation condition, 02, a molecular species of active oxygen with killing activity against microorganisms, was generated from the dye (Type II mechanism). Emission spectra of O2 generated from dyes under irradiation with a green laser (532 run) were measured by an NIR emission spectrometer that was developed and made in our laboratory (Data not shown). 

L.I. Grossweiner, A.S. Patel, J. Grossweiner (1982). Type I and Type II mechanisms in the photosensitized lysis of phosphatidylcholine liposomes by hematoporphyrin. Photochem. PhotobioL, 36, 159-167. 

Further confirmation of the important effect of solid-phase transitions in polymer photochemistry was reported by Dan and Guillet (29). They studied the quantum yields of chain scission, c >s, as a function of temperature in thin solid films of vinyl ketone homo- and copolymers. For polymers where the Norrish type-II mechanism was possibici large increases in n were observed at and above the glass transition T. Figure 8 shows this effect in a styrene copolymer containing about 5% phenyl vinyl ketone (PVK). Below Tg, )s is about 0.07, but at Tg it rises to about 0.3, a value similar to that observed for photolysis in solution at 2S°C. A similar effect was observed with poly (methyl methacrylate-co-methylvinyl ketone) (PMMA-MVK) and PVK homopolymer. 

When polyiphenyl vinyl ketone) is irradiated by ultraviolet light of 365 nm, it degrades and exhibits a loss of molecular weight [558]. No benzaldehyde was found among the products. That excludes a Norrish Type I reaction and indicates that the polymer cleaves by a Norrish Type II mechanism [558]. 

Aliphatic ketones, which decompose according to a Norrish type II mechanism, however, do not yield free radicals ... 

Monocyclic 3-lactams (411), containing a sulphur substituent at C(4) can be formed in variable yields from thioimides (410) via a photochemical Norrish Type II mechanism involving y-hydrogen abstraction by the thiocarbonyl group. 

However, the subsequent steps in the mechanism, in which active oxygen species are formed, are less well understood. T vo mechanisms have been proposed these are termed the Type I and Type II mechanisms. In the Type I mechanism the excited triplet sensitizer is involved in redox reactions leading to electron transfer and the formation of superoxide... 

The H/D exchange proceeds via type II mechanism (R ) for aU three types of sites and corre-... 

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