ENERGY TRANSFER IN ORDERED AND 

 UNORDERED PHOTOCHEMICAL SYSTEMS ' 



GORDON TOLLIN, POWER B. SOGO, and MELVIN CALVIN 



Radiation Laboratory and Department of Chemistry, University of 

 California, Berkeley, California 



The phenomenon of energy transfer has been receiving an ever increas- 

 ing amount of attention from physicists, physical chemists, and bio- 

 chemists ahke since the pioneering work of Franck (Cairo and Franck, 

 1922, 1923; Franck and Teller, 1938) and Vavilov (1925). This 

 concept has proved to be of fundamental importance for an under- 

 standing of many of the photoinduced phenomena of molecules, both 

 in solution and in the solid state, and is proving to be of increasing 

 significance to biology. 



Our concern here will be mainly with a qualitative discussion of the 

 theoretical aspects of energy migration, with some of the experimental 

 criteria of this phenomenon, and, finally, with its possible role in the 

 primary quantum conversion act in photosynthesis. 



GENERAL CONSIDERATIONS OF ENERGY TRANSFER 

 IN UNORDERED SYSTEMS 



Spectroscopic Properties of Molecules in Solution 



Some of the main qualitative features of the effects of visible and 

 UV radiation on molecules in solution may be understood by a con- 

 sideration of the diagram in Fig. 1 . Process a represents the absorption 

 of a quantum of light by the molecule resulting in a change in its 

 electronic state. Molecules in the lowest excited singlet state may then 

 undergo one of four processes: they may emit a quantum of light as 

 fluorescence (process b); the electronic excitation energy may be de- 

 graded into heat (process c) ; a small portion of the electronic energy 



^ The work described in this paper was sponsored by the U. S. Atomic Energy 

 Commission. 



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