Formation of singlet oxygen in photosensitized systems

It has been found that oxygen efficiently quenches the fluorescence of many organic molecules [Refs. 60, 68, 81, 92, 486, 534]. The following energetically allowed mechanisms for oxygen quenching of molecules excited to the singlet state can be written as 

The oxygen quenching of triplet state molecules has been the subject of numerous experimental and theoretical studies [Refs. 60, 81, 326, 331, 332,379,486,654]. The processes expected to be involved in the quenching of triplet state molecules are 

Kearns et al. [326, 327, 339] and Tsubomura and Mulliken [621] assume the formation of intermediate contact complexes, or complexes of the charge transfer type, between the excited molecule of the sensitizer and molecular oxygen. These complexes take part in the transfer of the excitation energy from the sensitizer molecule to the oxygen molecule. 

The problem as to which of these two mechanisms of singlet oxygen formation is acutally involved is still controversial. A number of authors maintain that the lifetime of the excited singlet state, 10-8 s, is too short for the triplet—triplet transfer reaction. 

In systems where high concentrations of excited singlet and triplet state molecules are present, along with a large concentration of, 02(1 Ag), the following spin-aliowed processes are energetically possible  

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