22 



J. FRANCK 



2) 

 H- 



tr 



-OH -^^^ H- 



R OH 



-OH -I- H- 



R OH 



-OH 



R OH 



•OH 



R OH 



tr 



tr 



■OH 



R OH 



-OH -t-H- 



R OH 



•OH 



R OH 



3A) 



Ox-HH- 



3B) 



Ox-H- 



tr 



-OH Ox-'-H- 



R OH 



tr 



■OH + H- 



R OH 



tr 



-OH 



R OH 



-OH — *■ OX---H- 



R OH 



tr 



•OH -I- H- 



■OH 



R OH 



R OH 



4) 



Ox+H- 



tr 



(OxH)- 



•OH 



R OH 



•OH — »► (OxH)- -I- 



R OH 



Ox= oxidant 



-OH 



R OH 



Fig. 1 



The grouping 



H — Cio — Cg — OH 



I I 



R OH 



represents a hydrated form of chlorophyll and specifically shows hydration of the 

 carbonyl group in ring V. In reaction sequence ( 1 ) of Fig. 1 , chlorophyll is excited 

 to the first excited singlet state s*. Then, by a nonradiative transition, the singlet 

 state crosses over into the ground triplet state tr. By a process of sensitized 

 fluorescence, as shown in reaction sequence (2), this same molecule in its triplet 

 state tr absorbs a quantum of energy liberated when some nearby chlorophyll 

 molecule in the first excited singlet state s* reverts to its ground singlet state s. 

 This method of raising chlorophyll molecules to the excited triplet state tr* is a 

 unique feature of Dr. Franck's theory. The triplet state has an absorption far 

 enough in the red to make this process feasible. Dr. Franck believes that ad- 

 sorption (reaction (3A)) of some molecule, oxidant and/or enzyme, on the chloro- 

 phyll in the triplet state tr would make it an even better sink for light energy, viz., 

 increase the overlap between singlet fluorescence emission and ground triplet ab- 

 sorption. As a second consequence of such adsorption, one could have the mole- 



