20 J. FKANCK 



state to the next higher triplet state by sensitized fluorescence. The 

 energy needed for this process comes from a neighboring chlorophyll 

 molecule in the first ex(;ited singlet state. (4) Utilization of the energy 

 difference between the excited triplet and the singlet ground state for 

 photochemistry. 



I am not competent to discuss the theory of what Franck calls 

 sensitized fluorescence, but let me at least outline the experimental 

 evidence for its existence and the usual criteria for a probable transi- 

 tion of this sort. 



When chlorophyll absorbs red light it is raised to its first excited (or 

 fluorescent) state, usually with some extra vibrational energy. This 

 excess vibrational energy is quickly lost, bringing the electronically 

 excited molecule into thermal equilibrium with its surroundings. 

 There is a large probability (between V4 and 1 ) that the excited mole- 

 cule will not emit a photon of fluorescence light but will be transferred 

 by a process of internal conversion into its lowest triplet level. In this 

 state, the life of the molecule will be long compared to its life in the 

 fluorescent state. When two similar molecules are close together but 

 not in actual contact, there is a certain probability that, if one is elec- 

 tronically excited, it can lose that energy and the energy of excitation 

 will appear in the second molecule. This can also occur when the 

 molecules are unlike, if certain requirements are satisfied. An empiri- 

 cal requirement is that there should be a strong overlap between the 

 emission band of the first (primarily excited) molecule and the absorp- 

 tion band of the second molecule. The probability of such a transi- 

 tion caused by sensitized fluorescence is great when there is a large 

 overlap between the emission band and the absorption band. For this 

 reason, one might expect a greater probability of such radiationless 

 transfer between unlike molecules than between like molecules. We 

 find that there is a higher probability for energy transfer from chloro- 

 phyll & to o than for a transfer from a to a or h to h because the over- 

 lapping of the fluorescence band of h and the absorption band of a is 

 great. 



Because the visible absorption spectra of the triplet state and of 

 the Molisch phase test intermediate of chlorophyll are similar, Franck 

 predicted that the triplet state, like the phase-test intermediate, 

 would have an appreciable absorption in the near infrared close to 

 the red maximum of the singlet absorption. This prediction was 

 verified by Linschitz and more recently by Fujimori. There is, there- 

 fore, a strong overlap between the fluorescence of singlet-state chloro- 



