R. S. BECKER AND M. KASHA 37 



interaction is very sensitive to such effects, because of the high-power 

 dependence of the probabiHty of singlet-triplet transition on atomic 

 number (Z"*). 



Applied to the porphyrin-like molecules, this means that replace- 

 ment of magnesium ( Z = 12 ) by zinc ( Z = 30 ) should result in a 

 pronounced enhancement of the phosphorescence and a considerable 

 shortening of its mean lifetime. Replacement of magnesium by copper 

 (Z = 29) should have an additional strong effect due to the magnetic 

 field produced by the unpaired copper ti-electrons. Thus, Calvin and 

 Dorough (1947, 1948) found such effects in zinc and copper chlorin 

 and porphyrin derivatives. It is in fact an old observation in the por- 

 phyrin fluorescence field that the porphyrin derivatives of paramag- 

 netic metals such as iron and copper, are nonfluorescent. We expect 

 to find these strongly phosphorescent. 



In our laboratory we have made some preliminar)' investigations 

 on magnesium and copper phthallocyanines. The magnesium phthallo- 

 cyanine is strongly red fluorescent, while the copper phthallocyanine 

 shows no fluorescence. However, copper phthallocyanine shows a 

 strong infrared phosphorescence at 77° K, in conformity with expec- 

 tation. 



A different sort of magnetic or electrical effect is also possible for 

 heavy or paramagnetic atoms in the neighborhood of the excited 

 molecule. It has been demonstrated (Kasha, 1952) that heavy-atom 

 fluorescence quenchers induce the conversion of excited singlet energy 

 to triplet energy. It is undoubtedly true that similar quenching effects 

 by paramagnetic molecules such as O2 would be due to the same 

 mechanism. It may turn out that these field effects will have some 

 influence on the energy processes in chlorophylls in the photosynthetic 

 system. 



(3) Nature of Lowest Triplet State of Chlorophyll 



According to our discussion in Section IV, both 7r,7r and n,7r excited 

 singlet states may be present in the region of the red absorption band 

 of chlorophyll a and chlorophyll b. If this is so, and that can be 

 established rigorously by further experiment, we would be compelled 

 to consider that below the lowest excited singlet state there will be 

 at least two triplet states in both chlorophyll a and chlorophyll b, one 



