R. S. BECKER AND M. KASHA 33 



while the former undergoes a strong bhie shift. Thus, we accept the 

 discussion by Livingston et al. (1949) concerning the stabihzation of 

 a keto form by hydrogen-bonding solvents. However, we add to this 

 the spectroscopic point that only the hydrogen-bonded keto form 

 would be capable of fluorescence, if our energy scheme is correct. 



For chlorophyll h, we observe that an additional conjugated car- 

 bonyl group is present (the fonnyl group). With two ;i,7r-transitions 

 possible for the b species, we anticipate a larger net 7r,7r-/i,7r-separation. 

 In Fig. 4 of the paper by Livingston et al. (1949), a considerable 

 shoulder on the long-wavelength side of the lowest absorption band 

 for hydrocarbon solutions of chlorophyll b is found, which is absent for 

 chlorophyll b in hydrogen-bonding solvents. We attribute the disap- 

 pearance of this shoulder to a blue shift of the additional n,7r-transition 

 which we ascribed to the formyl group, as well as the blue shift of 

 the n,7r-transition, which is also present in chlorophyll a. 



The above interpretation thus is capable of accounting for the 

 differences between chlorophyll a and chlorophyll b spectra observed 

 by Livingston et al. (1949), as well as for the appearance of fluores- 

 cence in the hydrogen-bonded molecules. Nevertheless, we note that 

 the results of the Russian workers (Evstigneev, Gavrilova, and Kras- 

 novski, 1950) emphasize the role of magnesium in the activation 

 phenomena. The reconciliation of their results with our understand- 

 ing of the spectroscopic aspects of this problem we have not yet 

 achieved. However, we can state that we can picture no mechanism 

 based on the hydration of the magnesium which would account for 

 both spectroscopic observations, i.e., the activation of the fluores- 

 cence as well as the observed changes in the absorption spectrum. 



Piatt (1951) noted the possibility which we have discussed above, 

 but he did not carry out the interpretation properly; in particular his 

 idea of a negligible blue shift is not supported by subsequent work. 



V. Lowest Triplet State of Chlorophyll 



We have in progress a comprehensive study of the luminescence 

 emission properties of porphyrin-like molecules including chlorophyll 

 a and chlorophyll b. In the following discussion we shall outline the 

 theory and approach which we are using as a guide to this problem 

 and shall indicate the positive results obtained thus far. At the outset 



