CHLOROPHYLL-SENSITIZED REACTIONS in VltrO 21 



phyll and the absorption of its lowest triplet state. This condition 

 strongly favors a radiationless transition of the excitation energy 

 from the first-excited singlet to the lowest triplet level of chlorophyll. 



If I understand Franck's suggestion, it is that in grana, where the 

 molecules are close together and where they are probably ordered, 

 there is a very high probability of transfer of the energy of excitation 

 from one molecule to another, that eventually this energj^ will end up 

 in a sink and that sink can be a chloroph}^! molecule already in its 

 triplet state. 



This doubly excited molecule should have a short life, comparable 

 with that of the fluorescent state. It is, therefore, most improbable 

 that the molecule in its excited triplet state could take part in an 

 efficient chemical reaction which is controlled by diffusion. In other 

 words, if this short-li\'ed molecule is to use its energy efficiently in a 

 chemical reaction, it must have come in contact with its reaction 

 partner before it has received its second quantum of energy. 



Even at the low dye concentrations usually chosen in experiments 

 on photochemical processes in vitro, the probability of exciting the 

 higher triplet state is not too small to allow the low rates of endo- 

 thermal photochemical reactions. If this hypothesis is correct, the 

 fjuantum efficiency of such processes should become much higher 

 whenever chlorophyll is adsorbed on suitable surfaces in such a way 

 that the average distance between the chlorophyll molecules is very 

 small. This should raise the probability of Step 3 without interference 

 from quenching impacts between the chlorophyll molecules. 



Discussion 



Weigl : There is a rather stringent restriction on the lifetime of this doubly ex- 

 cited state, unless it is a ver}^ unusual species (for instance, one stabilized by 

 some prior chemical attachment). Internal conversion to the lowest member of 

 any series of states of a given multiplicity is usually very fast — it proceeds in 

 about 10""' to 10"'^ second. Therefore, a doubly excited triplet state would have 

 to engage in chemical reaction within roughly lO'"'^ second, before the molecule 

 drops to its lowest triplet state and the energy of the second quantum can be 

 lost to molecular vibration and heat. 



Rabinowitch (remarks in proof) : If this were so, no molecules could have a 

 fluorescence yield 0.01%. 



Brugger (remarks in proof): Prof. Franck believes that the transition tr* —*■ tr, 

 first excited triplet to ground triplet state, resembles s* ^^ s, first excited singlet 

 to ground singlet state. In each case, the transition is to the ground state of a 

 given multiplicity series. One expects the lifetimes of tr* and s*, as well as the 

 fluorescence yields for tr* -^ tr and s* — »- s, to be similar. In addition, tr* can pass 

 non-radiatively to s* just as s* passes to tr. 



Limiry: In other words: the conditions should be as stringent for the triplet- 

 triplet transition as they are for a singlet-singlet transition. 



Brugger : Figure 1 may aid in clarifjang some points. 



