14 R, S, BECKER 



difference in the two molecules, that is, the effect of the presence or 

 absence of magnesium in the molecules. 



EMISSION SPECTRA AND DEDUCTIONS 



It has been recently shown that tiie phosphorescence of chlorophyll 

 6, first reported by Calvin and Dorough (4), is a bona fide emission 

 occurring at 8650 A (2). An analog of chlorophyll, pheophorbide a, 

 has a strong fluorescence and no phosphorescence. However, in the 

 divalent copper complex of pheophorbide a, the fluorescence is com- 

 pletely quenched and only a strong phosphorescence is observed. 

 Moreover, the phosphorescence occurs at 8675 A. The complete 

 quenching of fluorescence in the metallophosphorbide and the like posi- 

 tions of the phosphorescent emissions indicate that the phosphores- 

 cences observed in the pheophorbide complex and in the chlorophyll 

 are the lowest triplet to singlet emissions. 



Although intrinsic phosphorescence was not unequivocally ob- 

 served from chlorophyll a, there was indication of this type of emission. 

 The difference between chlorophyll a and chlorophyll h is primarily 

 due to the greater sensitivity of chlorophyll a to photodecomposition 

 with accompanying spurious emissions. The difference in photosensi- 

 tivity of chlorophylls a and b was further borne out by Dr. Linschitz in 

 his report at the Conference on Photosynthesis in 1955. Chlorophyll a 

 is being subjected to further investigation. 



Although the quantum yield of phosphorescence was low (<0.1) 

 for chlorophyll b, the extrapolation to the in vivo system allows im- 

 portant details to alter the value of the quantum yield. Earlier con- 

 siderations by Becker and Kasha (1) indicated the very important 

 possibilities of strong intermolecular spin orbital perturbations which 

 may take place. These factors could substantially increase the quan- 

 tum yield of phosphorescence. 



The energy available in the lowest excited level corresponding to 

 the wavelength of the phosphorescent emission is about 33 kcal. per 

 mole. The energy available in the lowest singlet state is approxi- 

 mately 43 kcal. per mole. Although the energy available in the 

 singlet state is higher, a consideration of the amount of absorbed 

 energy reemitted from the singlet state deemphasizes this factor. 

 Approximately 90% of the absorbed energy is unaccounted for in 

 terms of reemission; 10% of the absorbed energy reappears as fluores- 



