CHLOROPHYLL-SENSITIZED REACTIONS m vitrO 25 



Rabinowitch {expanded in "proof) : As long as each //• has the chance of gcitting a 

 second quantum by sensitized fluorescence (resonance transfer) during its hfe- 

 time, i.e., as long as every triplet excitation becomes a double excitation, the pro- 

 portion of singlets which will succeed in emitting fluorescence will be independent 

 of light intensit}-. The fluorescence yield will, however, begin to increase when the 

 light intensity becomes so low that, during the lifetime of tr, no quantum will be 

 absorbed by a singlet molecule near enough for the excitation energy to be snatched 

 away by the tr. This intensity must depend on two things — the lifetime of tr, and 

 the range over which ir can grab quanta b\' resonance transfer. If //• lives 10~^ sec. 

 and can grab a second quantum from lU'' singlet neighbors, the critical light in- 

 tensity will be that at which each chlorophyll molecule absorbs a quantum once a 

 second — which corresponds to a quantum flux of the order of 3 X lO^^ photons 

 near the peak of chlorophyll absorption, or to several thousand lux of white light. 

 Experiments indicate no decrease of fluorescence yield with increasing in- 

 tensity in this region. If the phenomenon exists, it must be over-compensated by 

 an increase in yield due to other causes. 



Weigl : The theor,\' rec(uires that at the very lowest light intensity you would 

 get a ver>' sharp dropping of the quantum yield of whatever reaction evolves. 



Gaffron: One should perhaps point out that Franck's theory recreates, in a 

 sense, the concept of photosynthetic units. Each time this triplet state appears 

 it becomes momentarily a sink and the nearby excited molecules can deliver their 

 first singlet-state energy into it. 



Strehler : Does anyone have evidence for the appearance of a band in illumi- 

 nated chlorophyll solutions in the red region of sufficient intensity to absorb 

 fluorescent light from the normal chlorophyll singlet-singlet transition? 



Rabinowitch : Wasn't it reported todaj^ that the metastable state has an in- 

 creased absorption in the near infrared? 



Strehler : It was the solvation band that was out further. 



Linschitz: The absorption spectrum of the metastable state of chlorophyll, as 

 measured in our flash experiments, shows a band in the far red, just beyond the 

 main red band of chlorophyll itself. This new absorption band corresponds to a 

 transition from the ground level of the metastable molecule to what is probably 

 the first excited level, as follows (transition 2). 



'^ ^ 



-Ground level of metastable state of chlorophyll 



-Ground level of chlorophyll 



The evidence indicating the existence of this band is completely unambiguous. 

 (See Fig. 25 in our paper, for example. ) 



Weigl : It is in the position required by Franck's theory. 



Strehler : What is the cross section? Will it be an efficient trap for the energy? 

 It is the total optical cross-sectional overlap, as well as whether it is in the right 

 position, that is important. Can you estimate this? 



Linschitz : The quantitative measurements in the far red that we have to date 



