EXCITATION OF POLYENES AND PORPHYRINS 



113 



derlein had pointed out that the absorption of these compounds is hke 

 that in the corresponding chlorin, which has a 7-methyl group and a 

 6-carboxyl ester substituent. This approximation is shown in Fig. 2-23 

 for band I and is satisfactory. It is worthless for band III, which is 

 much more sensitive to the values and directions of the vector moments ; 

 consequently no predictions were made for the latter band. 



To account for the intensity of band I of the chlorophylls or the methyl 

 chlorophyllides, which are magnesium-phorbide complexes, we must 

 assume that the magnesium substitution on the central nitrogens con- 

 tributes a moment of value +90. Hemin, on the other hand, which is 

 the iron-protoporphyrin complex, has almost the same spectrum as proto- 



m 



100 



250 



7 

 8 

 9 

 lO 

 I I 

 12 

 13 

 14 

 15 



■ CHLORIN TYPE 

 RHODIN TYPE 



WITH ISOCYCLIC RINGS 



4S- 



BAND n 



CHLOROPHYLL 



BAND I 



-o) and calculated (x- 



x) total spectroscopic moments for 

 For the chloro- 



FiG. 2-2.3. Observed (o- 



chlorins and rhodins. (For meaning of negative values, see text. 



phyllide, 10, the points for band III have been displaced to the left as indicated.) 



porphyrin itself, indicating that iron has a moment of about and that it 

 rather surprisingly does not change the T>ih symmetry. 



The additional measurements of Stern et al. (Stern and Molvig, 1936; 

 Stern and Pruckner, 1937a, b, 1939; Pruckner and Stern, 1936; Pruckner, 

 1940, 19+1, 1942) still need to be examined to test this theory of the 

 intensities. 



The treatment given here derives the strong longest-wave-length tran- 

 sitions of the (jhlorins and bacteriochlorins by almost continuous stepwise 

 perturbations from the weak porphyrin bands, so that all these com- 

 pounds are encompassed by the same intensity formulas, Eqs. (2-15) and 

 (2-16). It should be noted that this is in contradiction to the theory of 

 Rabinowitch (1944). He proposed the introduction of new electronic 

 transitions in the chlorins and bacteriochlorins to account for their strong 

 red bands. 



Wave-length shifts have not been discussed here. The shifts obviously 

 go hand in hand with the intensity changes, but the theory of Forster 



