118 RADIATION BIOLOGY 



triplet states, also at their neighbors, the branch carbon atoms. (The 

 mixing with the By one-electron wave function should increase the branch- 

 carbon density again, but this mixing, which was strong in porphin, will 

 be relatively small in the tetrahydro compound because of the loss of 

 square symmetry.) This is, then, a relatively favorable situation for the 

 loss of protons from the saturated carbons, especially since the resultant 

 conjugated system become larger. 



We may infer that dihydroporphins must have an intermediate behav- 

 ior and so will be unusual in their simultaneous capacity for photoreduc- 

 tion — adding protons to one side ring in their phosphorescent state — and 

 for photooxidation — giving up protons from the other side ring — at least 

 wherever the available excitation energy of about 40 kcal/mole is suf- 

 ficient for the reaction. These calculations might be somewhat related to 

 the unique biological role of these compounds as photocatalysts. Prob- 

 ably the successful quantitative treatment by Longuet-Higgins (1950b, c) 

 of proton-addition problems in amino aromatics from theoretical electron 

 densities could be adapted to the present problem, and the additional 

 directing effects of the ring substituents, which we have neglected here, 

 could also be considered. 



Calvin and Dorough (1948) showed convincingly that zinc tetraphenyl- 

 chlorin in its phosphorescent state could give its excess hydrogen atoms 

 to oxygen and to quinones and be converted to the porphin. A related 

 process may be the reversible photochemical interaction between chloro- 

 phyll and quinone at low temperatures in rigid glassy solvents, which was 

 found by Linschitz and Rennert (1952). Since this process should be 

 most probable in the Qy state, the result is not in disagreement with our 

 provisional labehng of the phosphorescent state as ^Qy. It would be 

 interesting to find out whether porphin in its phosphorescent state would 

 give the reverse reaction, as supposed here, and, for a substituted porphin, 

 on which rings the added hydrogen atoms would go. It would also be 

 interesting to know whether the chlorin in its phosphorescent state can 

 take hydrogen atoms from a suitable donor before it has lost any. 



REFERENCES 



American Petroleum Institute (1947) Research Project 44: Ultraviolet absorption 



spectrograms, No. 180, cyclooctatetraene. Spectroscopy Laboratory, Mass. Inst. 



Tech. June 30, 1947, Natl. Bur. Standards, Washington. 

 (1948) Research Project 44: Ultraviolet absorption spectrograms, No. 207, 



cyclooctatetraene. U.S. Bur. Mines, Bartlesville, Okla. July 31, 1948, Natl. 



Bur. Standards, Washington. 

 Aronoff, S. (1950) The absorption spectra of chlorophyll and related compounds. 



Chem. Revs., 47: 175-195. 

 Barany, H. C, E. A. Braude, and M. Pianka (1949) Light absorption. VII. Azines 



and related systems. A comparison of the C=C and C=N chromophores. J. 



Chem. Soc, 1898-1902. 



