ENERGY EXCHANGE IN PHOTOREACTIONS 45 



ation complexes that some pseudoisocyanine dyes form in solution (Jelley, 

 1936, 1937; Scheibe, 1937; Scheibe etal, 1939; Sheppard, 1942; Mattoon, 

 1944). West and Carroll (1947, 1951) have attributed the sensitization 

 action of dyes in photographic emulsions to exciton movements through 

 regular piles of these molecules on silver halide crystals. To be useful, 

 the exciton must be trapped in the halide. Apparently, nonplanar dye 

 impurities, which probably possess conditions favorable for internal con- 

 version, become supersensitizers when they trap excitons at the halide 

 surface. They act as antisensitizers when trapping takes place at a 

 position in the dye pile other than at the crystal surface. 



A third possible method for direct transfer of energy between electronic 

 degrees of freedom depends on charge transfer. The phenomenon is 

 represented on potential-energy surfaces in exactly the same way as any 

 process in which electronic quantum numbers change. During a cross- 

 ing process an electron moves from one chemical entity to another, and 

 if these levels are separated in energy, the difference is transported. 

 Theoretical treatments are similar to those for electronic-quanta transfer, 

 though the interaction functions used to calculate the resonance energy 

 are different. Application of Stueckelberg's theory (1932), which pre- 

 dicts long-range quanta transfer via field interaction, predicts consider- 

 ably shorter distances for maximum efficient electron migration. Elec- 

 tron exchange between atoms and ions moving at high relative velocities 

 has been well studied (Keene, 1949; Mott and Massey, 1949). At lower 

 velocities, in addition to Stueckelberg's treatment, that of Kallmann and 

 Rosen (1930) is appropriate, though both are unsatisfactory for poly- 

 atomic molecules (see also Mott and Gurney, 1948). 



Charge transfer unquestionably plays an important part in the mecha- 

 nism of many reactions. We do not propose to review the many publi- 

 cations dealing with such mechanism in nonphotochemical reactions. 

 For photoreactions Weiss (1935, 1939a,b, 1942, 1946) (see also Franck 

 and Levi, 1935) has proposed electron transfer as the basic mechanism in 

 photoreductions and has extended the theory to photooxidation reactions 

 and to quenching without net charge transfer. In photoreductions, 

 according to the theory, the excited molecule loses an electron to the 

 quencher. Consider, for instance, the self-quenching of anthracene, 

 which may proceed in the following way on the potential surfaces of 

 Fig. 1-15: One molecule of anthracene is excited to an upper state, per- 

 haps ultimately a triplet state, from which it readily loses an electron 

 to a second anthracene molecule. The two ions are attracted to each 

 other and form the metastable dimer, which is returned to the ground 

 state of two monomers via the left-hand crossing point with activation 

 energy £Jo (Bowen and Norton, 1939; Byk, 1908a,b; Weigert, 1908; Lauer 

 and Oda, 1936; Kautsky et al., 1933). Dimer or polymer formation is a 

 common result of quenching processes, but in numerous instances these 



