352 LIGHT AND LIFE 



place (Fig. 29, equation 2) . The resulting vacancy, or chlorophyll 

 positive ion, can then migrate by hole diffusion, that is, election trans- 

 fer from normal chlorophyll, into the vacant orbital of the neighbor- 

 ing chlorophyll positive ion. This process is the one in the entire 

 sequence which most nearly resembles the properties of a semiconduc- 

 tor and permits the oxidizing point (the chlorophyll positive ion) to 

 separate from the reducing point (the electrons in the quinone or- 

 bitals) by a process which is very nearly temperature-independent. 

 The oxidizing point will make itself apparent as a chemical change, 

 finally, when it captures an electron from a suitable reducing agent, 

 in this case shown as a ferrocytochrome, thus producing a ferricyto- 

 chrome and regenerating chlorophyll (Fig. 29, equation 3) . 



It is conceivable that in order for the reduction of pyridine nucleo- 

 tide to occur, possibly through lipoic acid (51, 56), the quinone 

 must be in the form of a di-anion, in which case a second electron 

 transfer from an excited chlorophyll to a quinone negative ion radi- 

 cal, produced in equation 2 (Fig. 29) wdll take place. This clearly 

 will require somewhat greater energy than the first reaction, if only 

 to overcome the electrostatic repulsion of the pre-existing negative 

 charge. It is interesting to view these two steps as a possible means 

 of understanding the collaborational requirement of light of two 

 wavelengths (7000 A and 6500 A) which was mentioned earlier (52) . 



Another alternative would involve the transfer of the electron from 

 the donor (ferrocytochrome) to the excited chlorophyll as the first 

 act. This would lead to a chlorophyll negative ion radical in which 

 an electron has been placed in the lowest 77-orbital of the chlorophyll. 

 The migration would then have to occur in this form until the 

 acceptor site (cjiunone) is arrived at. 



We prefer the first formulation described above, since every effort 

 we have made to find either dark- or photo-induced electron transfer 

 from a donor to the neutral phthalocyanine in our phthalocyanine 

 model has failed. Beyond this, in practically every case in which it 

 has been determined, the charge migration in an organic molecular 

 crystal takes place via hole migration rather than via electron 

 migration (30) . 



Conclusion 



In sinimiary, then, we can see that while the solid state model 

 (phthalocyanine) allows an approach from a somewhat different 

 point of view, the net result is the same as what was sought, but so 

 far not found, when we looked at the solution chemistry of chloro- 



