18 RADIATION HIOLOGY 



synthesis, such a course would divert the energy from useful function. 

 Conjugated molecules fulfill the requirement because they absorb low- 

 energy radiation inadecjuate for dissociation reactions. 



2. Organic molecules that fluoresce in the visible region generally 

 contain conjugated systems of bonds. Photosensitization efficiency for 

 photoreactions of other molecules is associated with the ability to fluo- 

 resce, as would be expected, since fluorescence indicates the absence of 

 internal conversion (Norrish, 1939; Hurd and Livingston, 1940). The 

 probability of crossing to the ground surface from at least the first excited 

 state is low, especially for planar, symmetrical, conjugated molecules that 

 have widely spaced electronic states. The efficiency of crossing among 



H2C=CH CHj 



-=\,/=>.,/S 



C-C?H, 



2"5 



y- \ ^c/ 



CH, / \n N-^ \ 



\^ II I \-CH3 



h/\ 



'C^ ^c 



«^\ I I 



H CH? HC C. 



/ I \ 



CH, = C 



I I 



= C 



I I 



CH3 



CzoHsa 



("■) (b) 



Fig. 1-8. (a) Chlorophyll a and {h) bacteriochloiophyll. {Fischer, 1940.) 



upper levels will vary somewhat but is generally good, as witnessed by the 

 common occurrence of long-wave-length fluorescence when excitation is at 

 short wave lengths (benzene and chlorophyll are well-studied examples). 

 Poor crossing to the ground surface can be roughly explained by the exist- 

 ence of "molecular" electrons rather than "atomic" electrons. The 

 readjustment of electric fields which occurs in the crossing process is 

 associated with changes in the equilibrium positions of most nuclei in the 

 molecule rather than with the few nuclear position changes that occur in 

 the crossing process for unconjugated molecules. In order for the con- 

 figuration to reach the crossing point, a large number of internuclear 

 distances must be simultaneously extended or shortened. The actual 

 equilibrium C — C distance in benzene in its first symmetrical excited 

 state is 0.037 A larger than the ground-state distance (Craig, 1950). 

 If a similar compression is required for the crossing between — C=C — 

 states of an olefin and this occurs with probability p, all six conjugated 



