A Possible Energy-Transfer Mechanism

As has been pointed out earlier, the kinetic results (ft = k6/k9 independent of the nature of the sensitizer) as well as the observed product distributions obtained in photosensitized oxygenation reactions and in (chemically prepared) singlet oxygen oxygenation reactions (see p. 45, for example) leave but little doubt that 102(1Ag or 1S9+) is the common oxygenating species.  

This process, however, can only occur if the energy separation between 1S1 and 3Si is at least 22.6 kcal/mole required for the excitation of3S,-02 into its lowest excited singlet state, 

According to the energy separation, AE = lsi — 3 reaction (34) is energy forbidden with rose bengal and chlorophyll. However, 

The problems posed by chlorophyll and anthracenes may allow for some speculation about the mechanism of the energy-transfer process. 

Introducing another contact pair, 1X, which arises from interaction of the singlet ground-state light absorber, 1S0, with either +02 or we get what we may call an extended Tsubomura-Mulliken diagram (Fig. 3). 

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