PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 233 



other solids, although the connections are as yet imperfectly appreciated. 

 There is no possibility, in the present chapter, of dealing with this 

 aspect of biological systems, but it will be touched upon in the follow- 

 ing subsection. In Sect. 5, where biological processes are treated, a 

 biological system will be considered simply as a "large polyatomic 

 molecule." 



3-4d. I Titer molecular Transfer and Migration of Excitation Energy. ^'^ In 

 Sect. 3-lc, an example of transfer of excitation energy from one atom to 

 another, namely, the process of sensitized fluorescence, was discussed. 

 There is no doubt that the same process must occur with diatomic or 

 polyatomic molecules in the gaseous phase, although no case has been 

 demonstrated thus far. In the case of polyatomic molecules in liquid or 

 solid solution, however, sensitized fluorescence as well as other kinds of 

 migration of excitation energy have been observed. Such processes are 

 important for an understanding of certain actions of light or of high- 

 energy radiation on biological systems. The occurrence of sensitized 

 fluorescence has been especially carefully studied in the case of complex 

 dyestuffs (cf., for example, Forster, 1951 and Pringsheim, 1949). In a 

 solution containing two or more of them, dyestulT A may absorb light and 

 B (or subsequently C or D) may emit most of the fluorescence light. A 

 necessary condition for the occurrence of this effect is that the fluorescence 

 emission spectrum of A overlap the absorption spectrum of B (and in turn 

 the same for transfer between C and D, etc.). If the overlapping is 

 strong, the energy transfer between the molecules may occur over dis- 

 tances of 20 A or more in single acts. Clearly, such an energy transfer 

 cannot be understood in terms of simple "collisions" between B and A* 

 (cf. Franck and Livingston, 1949). An important example of this process 

 is provided by the luminescence of certain dilute solutions of one aromatic 

 hydrocarbon in another. For instance, energy incident on a dilute solu- 

 tion of terphenyl in liquid toluene, either as light or as high-energy radia- 

 tion, is absorbed for the most part by the solvent but emitted by the 

 solute with high yield. (The system cited is an important liquid scintilla- 

 tion counter.) 



Energy migration observed in certain condensed systems, especially in 

 crystals, may be caused by a different process called exciton migration, or 

 by still another one called electron migration. In the former an exceed- 

 ingly quick transfer of energy between the electronic systems of identical 

 molecules occurs. If these are arrayed in contact with each other and 

 in a regular fashion, as in an ideal crystal, their electronic systems are in 

 perfect resonance and the excitation energy may migrate far before being 

 emitted as light or used for photochemical purposes, or dissipated into 

 heat. However, crystal lattice vibrations will interfere with exciton 



1* Cf. Franck and Livingston (1949). 



