108 



James Franck and J. L. Rosenberg 



"b) Why does the stronger Davydov absorption at 680 ni|i give a 

 good quantum yield while at the TOO m^i component the quantum 

 yield is reduced to one-fourth or less? 



c ) Why can simultaneous or almost simultaneous absorption by 

 the amorphous chlorophyll enhance the effectiveness of absorp- 

 tion by the crystals? 



d) What is the role of the reversible photo- or oxidative 

 bleaching and why is this bleaching observed mainly in the 

 700 m^l absorption? 



e) In view of the energy migration through the super-unit 

 which we introduced in the previous section, what prevents the 

 degradation of excitation in the amorphous layers to the lowest 

 excitation level of the crystalline layers, 7OO m\x, by migration 

 throiigh the exposed chlorophylls? 



To answer the first three questions we use the idea previously 

 advanced by us that excitation provided by 7OO mn absorption is 

 not sufficient to excite the singlet level of the exposed 

 cMLorophyll, which is an amorphous type molecule having its 

 absorption at about 675 m[x whether it is attached to an all- 

 amorphous -unit or to a unit containing a crystalline patch. As 

 a result excitation moving quickly in the crystal to the contact 

 point with the exposed chlorophyll will suffer there a trans- 

 ition into a metastable state. A re-interpretation of Becker's 

 observations (18) shows that the order of the metastable levels 

 of crystalline and amorphous chlorophyll is the reverse of that 

 of the singlet levels, and that a transfer of metastable energy 

 to the amorphous exposed molecule will occur. The higher energy 

 Davydov crystal component, 68O m|j. is energetically so close to 

 the singlet level of the exposed chlorophyll that by thermal 

 fluctuation the 68O excitation may easily excite the singlet 

 level of the exposed chlorophyll. In competition with this 

 process, degradation of the 68O to 7OO or even further to the 

 metastable level is an inherently slower event. 



If a reason can be found why the metastable state of an 

 exposed chlorophyll is inefficient in sensitizing Reaction (2), 

 we could understand the long wave-length decline and the 

 enhancement. We considered several possible mechanisms, as 

 follows : 



a) Back-reaction between AH and CytQx 



