' L I B R A ^ Y t ^ 



The Electronic Spectra of Chlorophylls 

 and Related Molecules 



RALPH S. BECKER, Department of Chemistry, University of Houston, 



Houston, Texas 



The present discussion will deal only with recent advances in the 

 knowledge of the absorption and emission spectra of the chlorophylls 

 and their derivatives. The nature of the triplet state, the general 

 significance of quantum yields, the effect of electric and magnetic 

 fields, and the like have been discussed elsewhere. 



ABSORPTION SPECTRA AND DEDUCTIONS 



The interpretation of the fluorescence activation of the chlorophylls 

 by Livingston, Watson, and McArdle (3) has been previously dis- 

 cussed ( 1 ) . However, there are several additional factors to be pointed 

 out. 



Freed and Sancier (5) have shown, from absorption data, the 

 presence of various solvates and temperature-dependent isomers of 

 chlorophylls and related compounds. From these data, it was deter- 

 mined that if solvation occurs at the magnesium atom of chloro- 

 phylls, it also occurs at or near the two corresponding hydrogen 

 atoms of pheophytin. Russian workers (6) emphasized that the differ- 

 ence in activation of fluorescence between pheophytin and chlorophyll 

 was due to the fact that hydration of magnesium was necessary. Thus, 

 the fluorescence of pheophytin, which contains no magnesium, was 

 insensitive to the presence or absence of water vapor. Although pre- 

 viously indicated, with consideration of the work of Freed, it now 

 may be emphasized that the presence of the magnesium may well 

 cause the perturbation of the electronic system of the chlorophyll 

 molecule necessary for the facts concerning fluorescence to become 

 observable. Consequently, the observed results concerning the 

 effects of the presence or absence of water on the fluorescence may be 

 only coincidental facts of interest accompanying the real cause of the 



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