72 



RADIATION BIOLOGY 



dozens of papers; of the other two methods, to hundreds; and of experi- 

 mental measurements, to hundreds more; therefore only key references 

 will be cited. For brevity, also, the basic principles of interpretation 

 will be given somewhat ex cathedra, without elaborate empirical or theo- 

 retical justification. They have been discussed by Piatt (1953a). 



Electronic Origin of Color. The only organic compounds that absorb 

 hght strongly between 2000 and 10,000 A are unsaturated. The strong- 

 est absorbers contain long conjugated chains, ring-chain systems, or ring 



ETHYLENE 



BUTADIENE 



150,000 

 100,000 

 50,000 



2000 



3000 4 000 



WAVE LENGTH, A 



5000 



6000 



Fig. 2- 



1948), 

 1943). 



1. Spectra of polyenes: ethylene {Piatt et al., 1949), butadiene (Jacobs and Piatt, 

 diphenyl butadiene {Pinckard et al., 1948), /3-carotene (Zechmeister and Polgdr, 



systems of alternating single and double bonds. The spectra are due 

 principally to excitation of the loosely bound " unsaturation electrons" 

 or "TT-electrons" of such systems. In a conjugated pure hydrocarbon, 

 each carbon atom brings one such electron to the system. These elec- 

 trons are largely responsible for the chemical reactivity as well as for the 

 light absorption of such systems, since they may be excited or removed 

 from the molecule comparatively easily. The conjugated system is held 

 planar by the 7r-electrons ; their wave functions have a node in the plane. 

 If it is twisted out of the plane, as by steric hindrance, the conjugation is 

 broken, and the spectra and chemical properties are greatly altered. 



We shall see later how another class of loosely bound electrons — the 

 nonbonding electrons of conjugated hetero atoms — also contributes to 

 the spectra, just as it does to the chemical reactivity. 



POLYENES 



We begin with conjugated chains. The spectra of such systems as 

 those shown in Figs. 2-1 and 2 have at the longest wave lengths a strong 

 first transition whose molar extinction coefficient e is greater than 15,000. 



