EXCITATION OF POLYENES AND PORPHYRINS 



79 



transition can now be at higher frequencies than was predicted by the 

 simple square-well potential, and it fits the observations better. The 

 bottom part of Fig. 2-6 shows schematically the orbital energies of several 

 polyenes, as observed and as calculated by the simple free-electron treat- 



d}e^fg 



50,000 





5,000 15,000 



dVf'A 





d^e^fg 



''''^'^^"^^^7^ 



OR POLYENES 



10 12 



N 



14 



16 18 



20 



22 



— \ a e f A 



>- 



X 



[n-c-c-c-c-c-n] [n-cvc-<:>n] 

 I I I I I 



m 



2 



< 



>- 2 



2 y 



ui o 



o 



IT 

 < 

 U 

 I 



59. 



WIDEt^JING 

 FROM KUHN 

 PERIODICITY 



Fig. 2-6. States and orbitals of conjugated chains. The dotted energy states are for 

 the indicated cyanine dyes. Figures over the states indicate peak extinction vakxes. 



ment, and shows how the Kuhn (1950) periodicity accounts qualitatively 

 for the wider observed gap. 



Removal of Alternation: Cyanine Dyes. This explanation of the sur- 

 prisingly high frequencies in polyenes becomes more convincing when we 



