38 LUMINESCENCE SPECTROSCOPY OF MOLECULES 



triplet of an n,7r type, and one triplet of a 7r,7r type. In addition, in 

 chlorophyll b, an additional triplet of an n,7r type may result from an 

 n-electron of the formyl group in its triplet excitation to a Tr-molecular 

 orbital. 



This discussion may appear rather complicated, but fortunately the 

 questions raised are accessible to experimental investigation. 



(4) Role of Lowest Triplet States of Chlorophylls in 

 Energy Transfer 



In all the published discussions of excitation energy transfer in- 

 volving chlorophylls, the main emphasis has been on the study of 

 fluorescence. It is obvious that if ^f° is of the order of magnitude of 

 0.1, the use of fluorescence as a criterion for energy transfer focuses 

 attention on the minor part of the possible available energy. That the 

 remaining 90% of the absorbed energy must be accounted for to ex- 

 plain the efficiency of observed photosynthesis in plants has been 

 indicated previously, e.g., by Livingston (1949). 



When the quantum efficiency of chlorophyll phosphorescence has 

 been determined, we shall be in a position to evaluate whether the 

 lowest triplet state receives the missing 90% of the energy. If any 

 considerable amount of the excitation energy actually reaches the 

 lowest triplet state, then it may be entirely possible for this state to 

 be important in the energy transfer processes (preceding the photo- 

 synthetic primary step ) . In fact, it is entirely possible that chlorophyll 

 a may transfer its excitation energy to chlorophyll b as far as triplet 

 state energy is concerned. Of course, this would be just the reverse 

 of the commonly accepted course of energy transfer between chloro- 

 phylls, deduced from fluorescence studies. We emphasize that we 

 merely wish to point out the possibility of such a transfer, and not 

 to predict its necessity. 



We would be inclined to favor the idea of net energy transfer from 

 chlorophyll a to chlorophyll b on two points: first, chlorophyll b is 

 the minor component of chlorophylls in many living plants. It thus 

 would play a minor part in light absorption by the plant, and energy 

 transfer from chlorophyll b to chlorophyll a would have a minor role in 

 photosynthesis. On the other hand, the spectroscopic and structural dis- 

 tinctions between chlorophyll a and chlorophyll b (especially if the 



