46 



RADIATION BIOLOGY 



products are due to subsequent reactions of atoms or radicals produced 

 in the primary process (Lind and Livingston, 1930, 1933; Norrish and 

 Griffiths, 1928; Thompson, 1939; Pringsheim, 1939). 



Quenching of fluorescence of polycycHc hydrocarbons, chlorophyll, 

 and similar molecules by molecular oxygen or nitric oxide (Bowen and 

 Williams, 1939; Weil-Malherbe and Weiss, 1942, 1943, 1944; West and 

 Miller, 1940) is an example of reactions probably explained by Weiss's 

 theory. Excitation of conjugated molecules in the visible can reduce the 



REACTION COORDINATES 



Fig. 1-15. Possible potential surfaces, based on Weiss's theory, for the mechanism of 

 self-quenching of anthracene. 



energy required to remove an electron from an average ionization poten- 

 tial in the ground state of 9 to 6 ev (Sugden et al., 1941), so that these 

 molecules on excitation can become very strong reducing agents, thus 

 suggesting the validity of Weiss's ideas in these cases. Similarly the 

 partially vacant ground state of the excited molecule can make it a strong 

 oxidizing agent for photooxidations. Quenching without net charge loss 

 to either participant can result from double electron migration, which 

 restores electrical neutrality but transfers electronic excitation energy 

 from one molecule to another. Evidence pertinent to the substantiation 

 of the theory is considerable but not yet completely confirmatory. One 

 striking observation bearing on the problem is that electrical conductivity 

 increases during illumination of many quenching substances (Weiss, 

 1946). Rowell and LaMer (1951) have provided correlations between 

 the relative oxidizability of excited and quenching molecules and the 

 efficiency of quenching reactions. 



