52 PHOTOCHEMICAL PRINCIPLES 



(Terenin and Ermolaev, 1952, 1956; Ermolaev, 1955) has demon- 

 strated sensitized phosphorescence in rigid solutions at — 180°C. These 

 workers studied various combinations of naphthalene, benzaldehyde, 

 biphenyl, and benzophenone. The donor molecule was selected to have 

 its lowest excited singlet state below that of the acceptor and its lowest 

 triplet above that of the acceptor. When such mixtures were illumi- 

 nated with light absorbed only by the donor, the phosphorescence of 

 the acceptor was sensitized and that of the donor was quenched. They 

 interpreted these results in terms of an energy transfer between the 

 triplet states of the molecules involved. 



Although the application to biology of the concepts outlined above 

 must at present be reserved mainly for the future, there are a number 

 of examples which one might cite in which energy transfer is of im- 

 portance. The first, of somewhat trivial significance, is scintillation 

 counting (Birks, 1953), in which solutions of hydrocarbons are used 

 to detect and measure high-energy radiations in tracer work. The 

 second is the well-known demonstration of the transfer of energy from 

 various plant pigments such as phycocyanin and phycoerythrin to 

 chlorophyll in plant material (Wassink, 1948; French and Young, 

 1952). Finally, we might mention the experiments of Arnold and 

 Meek (1956), who have demonstrated the transfer of energy among 

 chlorophyll molecules in the grana through a study of the polarization 

 of the fluorescence. Rabinowitch ( 1957) has suggested that this energy 

 migration is a result of resonance transfer and has estimated that, for 

 excited state lifetimes of about 10"-' sec and intermolecular distances 

 between chlorophyll molecules of about 10 A, energy transfer chains 

 of the order of 100 or 1000 molecules could easily occur. 



GENERAL CONSIDERATIONS OF ENERGY TRANSFER 

 IN ORDERED SYSTEMS 



Theoretical Aspects 



There are three mechanisms by which energy transfer may proceed 

 in ordered systems. These are: 



1 . The emission of a quantum of radiation followed by its reabsorp- 

 tion by an unexcited molecule. This is analogous to the mechanism 

 proposed for fluid solutions. The main proponent of this theory has 



