R. S. BECKER AND M. KASHA 39 



Z7-isomer has a readily available triplet state) may make chlorophyll 

 b the important intermediate acceptor of the excitation energy. Thus, 

 a net transfer of energy from chlorophyll a to chlorophyll b may be 

 important. Moreover, the fact that chlorophyll a is a major compo- 

 nent of plant chlorophylls would make its dominant light absorption 

 the chief role of this isomer in photosynthesis. 



In the next section we shall discuss one additional spectroscopic 

 phenomenon which may dominate the properties of the chlorophylls 

 in energy utilization. This involves the electronic interaction of the 

 vinyl group present in chlorophylls and the possibility that it behaves 

 as an energy dissipator in isolated chlorophylls in vitro. This may 

 account for possible low quantum yield of luminescence of chloro- 

 phyll as isolated molecules. 



VI. Interaction of Low Excited States of Chlorophyll with 



Torsional Potential 



The theory of the electronic-vibrational interactions in the ethylene 

 molecule has been developed by Mulliken (1933) and Mulliken and 

 Roothaan ( 1947 ) , with particular reference to the effect of twisting 

 or torsion about the double bond. This t>^pe of distortion is unique, 

 among possible intramolecular distortions, in lowering vastly the en- 

 ergy of the excited singlet and triplet states ( cf . Fig. 3 ) of ethylene. 



The effects of this torsional potential as it is called are profound in 

 determining the spectroscopic behavior of the molecule. For example, 

 the absorption spectra change shape upon change of temperature and 

 viscosity, and no fluorescence is observed (Potts, 1954). The reasons 

 for this are clear from the diagram. Absorption is mainly to a molecule 

 distorted far from its most stable configuration, quite the opposite of 

 normal one-electron excitation in molecules. Upon cascading through 

 torsional levels under the influence of collisions, the molecule reaches 

 a minimum from which no emission could be observed at accessible 

 wavelengths. Even in methylethylenes in rigid glasses (Potts, 1954) 

 no fluorescence can be observed. However, in ^rans-stilbene (trans- 

 diphenylethylene), fluorescence can be observed in rigid glasses or 

 crystals, though not in fluid solutions (Kasha, 1950). Such behavior 

 is general in complex ethylenic molecules. 



Apparently, the phenyl groups are sufiBciently large to inhibit the 



