ENERGY MIGRATION AND THE PHOTOSYNTHETIC UNIT 



1293 



about 1500 cm.-^ from the position of the monomer band. This shift 

 corresponds to a frequency of the order of 5 X 10^^ sec.-^, or to a sweep of 

 the excitation over 10'^ pigment molecules during the life-time (if the latter 

 remains of the order of 10 ~^ second). 



The polymeric molecules of this type show strong resonance fluorescence, 

 in agreement with the theoretical prediction; a striking result, because 

 resonance fluorescence is usually associated with vapors of low pressure, 

 consisting of free atoms or simple molecules, and not with complex mole- 

 cules or condensed systems. 



A, + A, 



DISTANCE 



Fig. 32 8. Potential energy curves of two atoms with no bonding in the ground 

 state and exchange bonding in the excited state (example: Hej). 



Obviously, nothing similar to Scheibe's. polymers exists in chloroplasts : 

 the red absorption band of chlorophyll is about as broad, if not broader 

 in vivo than in vitro, as if no uncoupling of intramolecular vibrations from 

 electronic excitation has occurred. Furthermore, the fluorescence of green 

 cells is weak and not of the resonance type. 



These facts seem to preclude the interpretation of the red band shift 

 in vivo as due to a "fast" resonance migration of excitation energy through 

 a closely packed chlorophyll layer. They caused Franck and Teller (1938) 

 to reject the "fast" energy migration mechanism as a possible basis of the 

 "photosynthetic unit" concept; it was mentioned above that they have 

 also rejected the "slow" mechanism because in their estimate it did not 

 provide a sufficiently wide range of energy migration. 



