88 



RADIATION BIOLOGY 



These observations then offer some support to the theoretical picture 



given in Fig. 2-9. 



EFFECT OF SUBSTITUTION 



Saturated Hetero Substitution. The effects of substitution on a pure 

 hydrocarbon conjugated system depend on the nature of the substituent. 

 A summary of the effects was given by Piatt (1951a). 



log € 



'mox 

 24242424242424 



a 4 



-1 — I — r 



15,000 



T — I — r 



1 — I — r 



7000 



"1 — I — r 



50,000 



K| 



c=c 



C = C-CH, 



C=C-CI 



10,000 



C=C-NH, 



C=C-SH 



15,000 



C 



20,000 



c=c-c=c 



'B V-- 



'=-=- 



c=c+c=c 



c=c-c-c=c 



H2 



"T — I FT 



y 



c--c-o-c=c 



1 — I — r 



'B 



c^c-c=c-c-c 



Fig. 2-10. Ethylene substitution by saturated auxochromes (variconjugate sequences). 



Formally saturated substituents — "auxochromes" — added to the con- 

 jugated system produce shifts to longer wave lengths and general increases 

 in intensity, with especially large relative increases in forbidden tran- 

 sitions if symmetry is destroyed by the substitution. An exception is 

 fluorine, which produces shifts to shorter wave lengths. The magnitude 

 of these effects follows the approximate sequence F, CH3, CI, Br, OH, I, 



conjugated substituents that can twist, such as nitro, amine, acid, or aldehyde groups. 

 Rigid heterocyclics often have sharp structure in nonpolar solvents but lose it in 

 polar solvents, possibly because loosely coupled solvent molecules are free to twist 

 (and to execute other motions) and so to vary thermally the energy of the excited 

 state of the whole system. See Merkel and Wiegand (1947). 



Kasha (personal communication) has suggested that the well-known loss of struc- 

 ture in the higher transitions of all conjugated systems has a different and more general 

 explanation, namely, the great speed of the higher radiationless transitions. This 

 would be a sort of "internal predissociation " rather than the external or normal pre- 

 dissociation traditionally invoked to account for diffuseness in spectra. If these 

 transitions are as fast as 0.3 X 10~^^ sec (three C — H vibration periods), they will 

 produce an uncertainty of 3 X 10^^ cycles per second or a diffuseness of 100 A near 

 3000 A, comparable to that shown for jS-carotene in Fig. 2-1. In this connection, see 

 especially I^ewis and Calvin (1939). 



