EXCITATION OF POLYENES AND POKPHYRINS 



95 



vinyl orbitals, v-w, ^A-^H, etc., will be polarized in approximately the 

 same direction — along the 1,4 axes of the rings. 



Transitions between the two groups of orbitals, v-g, ^A~^G, etc., will be 

 polarized in the plane of the rings but perpendicular to the 1,4 axes. 

 They will be w^eak, since the 2^-orbi4al is localized, whereas the polyene 

 orbitals are spread out. They will move to the red only about half as 

 fast with increasing chain length as the f-g orbitals. This is the reason 

 that the first transition in benzene and styrene, which is presumed to be 

 of the v-g type,^ becomes hidden by the stronger f-g transition in the 



PORPHIN TETRAHYDROPORPHIN 



Fig. 2-14. Structure and numbering of positions of porphin and tetrahydroporphin. 



longer molecules phenyl butadiene and biphenyl. No v-g transitions 

 have been identified in longer ring-chain systems; low-temperature studies 

 are needed. 



Some strong absorptions that did not move much with increasing chain 

 length were found in phenyl polyenes by Smakula (1934) near 2000 A. 

 These may be the v-w transitions, ^A-^H. 



The conclusions reached here about the classification and sequence of 

 transitions in ring chains do not depend on the location of the rings along 

 the chain. Thus the 7r-electron transitions in quinone are almost identi- 

 cal with those in styrene, which has the same chain length; quinone, of 

 course, has additional weak n-g transitions at long wave lengths, from 

 the oxygen nonbonding electrons. 



Analogues of phenyl chains, in which the benzene rings are replaced by 

 thiophene or pyrrole rings, will have similar spectra, except where forbid- 

 den transitions in a symmetrical phenyl compound may be enhanced by 

 the loss of symmetry [see Sease and Zechmeister (1947) on polythienyls]. 



Naphthyl chain systems seem not to have been examined theoretically, 

 but it seems likely that the result will be similar: When the chain becomes 



■* It would not be the first transition except that it is pushed to long wave lengths 

 (and made weak, in benzene) by electron interaction (Piatt, 1950, 1951a) between the 

 configurations we are here calling /</ and vw (which would be degenerate in benzene). 

 A similar interaction seems to occur in porphin, described later in this chapter. 



