PHOTOCHEMISTRY 13 



At moderately high concentrations (10~* M or greater), the fluorescence 

 of most compounds decreases as the concentration is increased. In some 

 cases this self-quenching is due to the reversible formation of nonfluores- 

 cent dimers (Rabinowitch and Epstein, 1941; Lewschin, 1935). This is 

 by no means invariably true. More generally, self-quenching appears to 

 be related to the "migration of excitation energy" (Vavilov, 1943; For- 

 ster, 1948, 1950; Franck and Livingston, 1949). As a consequence of this 

 effect, photochemical reactions which do not involve direct optical dis- 

 sociation or predissociation should be expected to become corresponding!}- 

 inefficient at high concentrations of the light absorber (cf. Gaffron, 1927). 



TRANSFER OF EXCITATION 



Transfer of energy of excitation between like or unlike molecules may 

 well play an important role in photochemistry, particularly in heterogene- 

 ous sj^stems and in solutions in which the concentration of the nonabsorb- 

 ing reactant is relatively small. In crystals such a transfer of excitation 

 may be caused by migration of electrons in a conductivity band of the 

 crystal (Franck, 1948), by "exciton migration" (Frenkel, 1931), or by 

 a radiationless transfer which may be called "classical resonance" 

 (Franck and Livingston, 1949). In a solution such a transfer of excita- 

 tion can occur only on collision or as the result of classical resonance. 

 This latter possibility has been carefully analyzed by Forster (1948), who 

 has shown that for certain dyes and pigments such a transfer can occur 

 efficiently at distances as great as 50 or 100 A. Sensitized fluorescence, 

 in which the photon is absorbed by one molecule and the energy trans- 

 mitted to an unlike molecule which emits its characteristic fluorescence, is 

 one consequence of classical resonance. It is a well-established phenom- 

 enon in gases at low pressure, such as mixtures of mercury and thallium 

 (Carlo and Franck, 1923), and has been reported for at least one case in 

 liquid solutions (Watson and Livingston, 1950). Transfer of excitation 

 between like molecules can be most readily detected by the depolarization 

 (Vavilov, 1943; Forster, 1948) of fluorescence which is excited with plane 

 polarized light. In some cases, self-quenching (Watson and Livingston, 

 1950) and quenching by added substances (Forster, 1950) appear to be the 

 consequence of classical resonance. 



Energy of excitation may be exchanged between different groups within 

 a complex molecule by a similar mechanism (Franck and Livingston, 

 1949). Weissmann (1942) made the interesting observation that light 

 which is absorbed by the organic part of the europium salicylaldehyde 

 molecule excites fluorescence characteristic of the europium ion ; this effect 

 is very probably the result of such a radiationless transition between the 

 separate parts of the molecule. When the carbon monoxide-myoglobin 

 complex is illuminated with light of either 5400 or 2800 A, carbon monox- 

 ide is split off, and the quantum yield of the process is about unity 



