94 RADIATION BIOLOGY 



^-orbital as in the /-orbital at every atom. Aza substitution, or any 

 other substitution, at any position therefore lowers both the /- and 

 ^-energies by equal amounts, and the transition frequency remains con- 

 stant in first approximation. Actually a blue shift of 100 or 200 A is 

 characteristic of aza substitution in these molecules and seems to be inde- 

 pendent of the position of substitution. The shifts in Kuhn's ions, on 

 the other hand, should be strongly sensitive to the position of substitution. 

 The fact that, for stable neutral even-ring hydrocarbons, the fluo- 

 rescent and phosphorescent states will have electron densities approxi- 

 mately unchanged from those in the ground state means that their 

 photochemistry will be fundamentally different from the photochemistry 

 of odd-ring molecules or molecules with ''zero-energy orbitals," such as 

 those of Fig. 2-13, where optical excitation produces large electron trans- 

 fers from point to point in the system. It would seem that photo- 

 excitation where the electron density is unchanged would be especially 

 likely to lead to simple intramolecular rearrangement, and that chemical 

 reactions would be favored in cases where the electron density changes. 



RING-CHAIN SYSTEMS 



Figure 2-13 also illustrates the resemblance between polyenes and 

 phenyl chain systems, including phenyl polyenes, diphenyl polyenes, and 

 p-polyphenyls (Piatt, 1951a). For every ring in such systems there is 

 one filled orbital v and one empty orbital w of a special type. They are 

 called "vinyl type" in Fig. 2-13 because they have about the same energy 

 as the ethylene /- and ^-orbitals, respectively, and this energy is almost 

 independent of the number of rings or of the lengths of the polyene parts 

 of the chain. 



The remaining orbitals are independent of these orbitals and are called 

 "polyene type," since they are just the same in number as the orbitals 

 of a simple polyene of the same length, and each orbital resembles closely 

 its polyene counterpart in its symmetry and number of nodes, electron 

 distribution, and energy. In computing the length of the equivalent 

 polyene, each phenyl counts as four atoms. The orbital similarity 

 accounts for the similarity of the long-wave-length spectra of the poly- 

 enes to those of phenyl chains, which was one of the earliest important 

 results of spectral comparisons. 



Substitution of a vinyl group on the side of a polyene to make a phenyl 

 ring then does not much affect the long-wave-length spectrum of the 

 polyene. Such spectral stability is frecjuently observed in large conju- 

 gated systems, provided the smaller systems that are added are at the 

 side of the large one and not at the end. 



Transitions among the polyene orbitals, f-g, e-g, etc., will be polarized 

 approximately along the length of the molecule if it is in the all-trans 

 configuration or is as extended as possible. Transitions between the 



