758 FLUORESCENCE OF PIGMENTS IN VITRO CHAP. 23 



or weaker) "sensitized" fluorescence of the quencher, or (if the quencher 

 is nonfluorescent) by complete conversion of the excitation energy into 

 heat. 



Three types (or rather three Hmiting cases) of electronic energy trans- 

 fer mechanisms are known. The first, which is the only one possible when 

 the distance between the excited molecule and the quencher is >10~^ cm. 

 (for visible light quanta), is trivial — the emission of a light quantum by the 

 primarily excited molecule and its reabsorption by a molecule of the 

 ciuencher, a process similar to the "self-absorption" of fluorescence (page 

 745). The second mechanism is energy transfer by kinetic collisions 

 (so-called "colhsions of the second kind"), or "encounters," to use a term 

 more appropriate for molecules in solution. It is associated with the 

 mutual disturbance of the electronic structures of the two molecules in 

 contact, and requires approach to within the kinetic collision diameter 

 (10~^ to 10"'' cm.). In this case, the energy exchange is not contingent 

 on "resonance" between the electronic excitation states of the two part- 

 ners, since a considerable fraction of electronic energy can be converted into 

 vibrational or kinetic energy in the collision. A third and perhaps most 

 interesting possibility is the "resonance transfer" of electronic excitation 

 energy between two practically undisturbed molecules, which can occur 

 when these molecules are within a distance smaller than the wave length 

 of the exchanged quantum (^^10~-' cm. for visible light), and does not 

 require an actual "contact" between them. The probability of this kind 

 of transfer depends decisively on resonance between the energy-exchanging 

 molecules (i. e., on the mutual overlapping of the fluorescence band of the 

 donor and the absorption band of the acceptor). The phenomenon was 

 first discussed by Kallman and London in application to sensitized fluores- 

 cence in gases. Similar considerations were afterward applied to solutions 

 by J. Perrin (1926, 1927), who used classical electrodynamics, and by F. 

 Perrin (1929, 1932), who first attempted a quantum-mechanical treatment. 

 F. Perrin used this energy transfer mechanism to interpret so-called 

 "concentration depolarization" of fluorescence in solution (decrease in the 

 degree of polarization with increasing concentration). Subsequently, 

 several other phenomena in fluorescence and photochemistry have been 

 ascribed to energy exchanges of this type, and improved theoretical treat- 

 ments were evolved by Vavilov and co-workers (1942, 1943, 1944), Forster 

 (1946, 1947, 1948) and Arnold and Oppenheimer (1950). Because of 

 the importance of the resonance transfer concept for the photochemical 

 mechanism of photosynthesis (in particular, for the possible participation 

 of phycobilins and carotenoids in it), these papers will be discussed in 

 greater detail in chapters 30 and 32. Here, we are concerned only with 

 the possibility of quenching (or excitation) of chlorophyll fluorescence being 



