Biradical triplet chain

The degradation process has a free radical mechanism. It is initiated by free radicals P that appear due to, for example, hydroperoxide decomposition induced thermally or by trace amounts of metal ions present in the polysaccharide. One cannot exclude even direct interaction of the polysaccharide with oxygen in its ground triplet state with biradical character. Hydroperoxidic and/or peracid moieties are easily formed by oxidation of semiacetal chain end groups. The sequence of reactions on carbon 6 of polysaccharide structural unit that ultimately may lead to chemiluminescence is shown in Scheme 11. 

The decay of the biradical produces ketone molecule in the triplet state, which is an emitter of light [222], The CL intensity was proved to be propotional to the rate of chain initiation, which is equal to the rate of chain termination. The observed luminescence spectra were found to be identical with the spectra of the subsequent ketone in the triplet state. The intensity of CL (/chi) produced by oxidized hydrocarbon is the following ... 

Wagner and Sakamoto [95] have studied the triplet decay of para-alkeny-loxyacetophenones with a substituent (X = Cl, CN, OMe, Me) at the position ortho to the alkenyloxy side chain. They propose that the cycloaddition reaction involves charge-transfer interaction between donor double-bond and acceptor triplet benzene first, followed by biradical formation (Scheme 18). 

Pyrolysis of acetylene to a mixture of aromatic hydrocarbons has been the subject of many studies, commencing with the work of Berthelot in 1866 (1866a, 1866b). The proposed mechanisms have ranged from formation of CH fragments by fission of acetylene (Bone and Coward, 1908) to free-radical chain reactions initiated by excitation of acetylene to its lowest-lying triplet state (Palmer and Dormisch, 1964 Palmer et al., 1966) and polymerization of monomeric or dimeric acetylene biradicals (Minkoff, 1959 see also Cullis et al., 1962). Photosensitized polymerization of acetylene and acetylene-d2 and isotopic analysis of the benzene produced indicated involvement of both free-radical and excited state mechanisms (Tsukuda and Shida, 1966). 

Magnetic field effects show a dependence of the modulated triplet yields for photogenerated biradicals of the type )n-2O" upon the methylene chain length SI. There is also an effect on the yield micellized triplet radical pairs2 2... 

Similarly, photogeneration of other reducing radicals results in reduction of diaryliodonium salts. Hydrogen abstraction from ethers and alcohols by ketone triplets [97], formation of 1,4-biradical by reaction of ketones with unsaturated compounds such as acrylates [97,103], all yield radicals which reduce diaryliodonium salts. Hydrogen atom abstraction from ethers and amines provides a chain process for iodonium salt decomposition (Scheme 6), wherein quantum yields as high as 5 have been reported [60a]. 

The mechanism of crosslinking in this system is well established. The UV quanta (340-360 nm) are absorbed by the BP molecules and excited to a singlet state (S) which is short-lived and rapidly reverts to the triplet state (T) by intersystem crossing. The T state is a rather long-lived biradical which abstracts hydrogen from surrounding molecules, e.g. polyethylene chains, which gives macroradicals on the chains (formula 1). 

Exploratory computations [12] indicate that both the triplet biradical and the singlet zwitterion are energetically accessible. Either of these two species could serve as the intermediate for polymerization to red phosphorus, which comprises chains of just such open tetrahedra [9, Chap. 2]. 

---