102 



James Franck and J, L. Rosenberg 



Actually chlorophyll exists in the xonits in two different 

 modifications^ as the occurrence of a minor red-shifted set of 

 absorption "bands indicates. One of these modifications is 

 usually present to a small extent and is contained in only a few 

 units, so that it can exert only a minor influence on photo- 

 synthesis . (At this point we must state that in our previous 

 publications we ascribed the weak spectrum to a so-called n-jt 

 transition which woiild be present in all units. We used special 

 assumptions to explain why the energy would not always flow to 

 this level. Our reasons for abandoning this interpretation will 

 be presented in the forthcoming extended discussion). Under these 

 conditions intensity measurements of the red fluorescence d\iring 

 and in the absence of photosynthesis are an uneq-uivocal 

 criterion of the utilization of the arriving singlet excitation 

 energy for photochemistry. If only singlet energy would be used 

 with optimal quantum yield there should be no fluorescence during 

 photosynthesis. If^ on the other hand, photochemistry always 

 occurred by the action of metastable states formed from the 

 excited singlet state in competition with fluorescence emission, 

 the intensity of fluorescence would be independent of photo- 

 chemical utilization. Observations showed that under the 

 conditions mentioned above the ratio of the intensity of fluo- 

 rescence in the presence or absence of photochemical utilization 

 of excitation turned out to be very nearly one-half. This result 

 was the same for three different methods of suppressing and 

 permitting optimal photosynthetic activity. The obvious conclu- 

 sion is that singlet and metastable excitation energy are \ised 

 equally often for photosynthesis . That both singlet and triplet 

 states are used for photosynthesis equally often is an indication 

 that two photochemical reactions, one at the cost of each of 

 these two states, must be coupled in such a way that they auto- 

 matically occur at the same rate. That this coupling can be 

 explained without contradicting any of the new important obser- 

 vations of the second Emerson effect and of the occurrence, 

 spectroscopy, and photochemistry of the chlorophyll modification 

 absorbing at 7OO mjj., together with earlier and more recent 

 studies of light emission of chlorophyll a is the theme of this 

 paper. No attempt has been published so far to explain the 

 emission and absorption phenomena of chlorophyll a together with 

 the Emerson effect and reversible bleaching on the basis of a 

 two-center model. We have attempted to adapt niomerous published 

 proposals to the above criteria but have not succeeded. 



