The type II reaction involves electronic energy transfer from the triplet-excited photosensitizer to ground state molecular oxygen that is spin-matched, thereby forming excited singlet molecular oxygen while the photosensitizer is regenerated (Equation 2.14). The two types of singlet oxygen with different spectroscopic symmetry notations are :Ag and !Eg+. Their energies are, respectively, 92 and 155 kJ/mol higher than that of ground state oxygen 3Sg+. The :Ag state possesses a much longer lifetime and normally has a higher yield in biological systems than does !Eg+. Consequently, the :Ag state is the main consideration here. Because of the relatively small energy difference from the ground state, many compounds are capable of acting as sensitizers for singlet oxygen formation. For example, the dyes methylene blue and Rose Bengal have a triplet state energy of about 140 and 170 kJ/mol, respectively.
The production of :O2 has been reported to occur by energy transfer from the singlet- and triplet-excited states of the sensitizer, but that from the triplet excited state is highly preferred because singlet-triplet interaction is of very low probability. The lifetime of 1O2 is highly dependent on the solvent medium and the presence of scavengers or oxidizable acceptors; it was determined to be about 3.1 x 10-6 s in water (Rodgers and Snowden, 1982) and 50 to 100 x 10-6 s in lipid (Henderson and
Dougherty, 1992). A half-life in tissue was estimated to be less than 5 x 10-7 s (Patterson et al., 1990). Singlet oxygen might diffuse about 0.01 to 0.02 mm in a cellular environment (Moan et al., 1979).
Although the energy of :O2 is only 92 kJ/mol higher than that of ground state oxygen, its chemical reactivity is completely different because it is now spin matched with ground state molecules susceptible to oxidation. Thus, :O2 is capable of oxidizing a large variety of substances including biological cell components such as DNA, protein, and lipids. Because many sensitizers are in a reduced form, they also may act as substrates, giving fully oxidized products. As a consequence, many preparative organic chemical processes are carried out photochemically, with 1O2 the mediator.
Singlet molecular oxygen is deactivated by physical or chemical quenching agents. The two physical mechanisms are energy-transfer and charge-transfer quenching. The carotenoid pigments play an important role in the protection of biological systems, apparently because they are particularly efficient energy-transfer quenchers. Beta-carotene is the most studied member of this group. The extended conjugated p-system of b-carotene has triplet energies close to or below that of singlet oxygen so that collisional energy transfer occurs. Subsequently, the excited b-carotene decays by vibrational relaxation and no net chemical change is observed (Gorman and Rodgers, 1981).
Amines generally are capable of quenching singlet oxygen via a charge-transfer process, but may react chemically as well. The primary process is envisaged as formation of a complex between the electron-donating quencher and the electron-deficient oxygen species; the quenching rate constants correlate with the amine ionization potential. The resulting triplet complex dissociates with loss of energy by vibrational relaxation or forms oxidation products. Formation of products requires an abstractable hydrogen a to the nitrogen; N-methyl groups are particularly susceptible. Diazabicyclo-octane (DABCO) is unable to react chemically, presumably on steric grounds, but is an efficient physical quencher. Some phenols are also able to quench singlet oxygen by a mixture of physical and chemical processes, e.g., the 2,4,6-trisubstituted phenols used as antioxidants, BHT, and a-tocopherol.
Other chemical reactions or quenching of singlet oxygen rely on the fact that singlet oxygen is more electrophilic than ground state oxygen and therefore can react selectively with electron-rich regions of many molecules, e.g., olefins and aromatics. Some examples of the addition of singlet oxygen are given in Figure 2.9, including the ene-reaction in which an olefin possessing an allylic hydrogen atom forms allylic hydroperoxides, and endoperoxide formation by 1,4-addition to p-systems such as furan and anthracene derivatives. As with other oxidation reactions, the initial products are metastable and secondary reactions will occur, but on a slower time scale. Dioxetan formation occurs by singlet oxygen addition to olefins in which the double bond possesses an electron-donating heteroatom, generally N, O, or S; this leads ultimately to cleavage of the double bond in a way similar to the reaction of superoxide in Equation 2.17. The similarity leads to some controversy as to the mechanism of dioxetan formation (Gorman and Rodgers, 1981).
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