SPECTRUM AND STRUCTURE OF PORPHIN DERIVATIVES 



02 1 



The conjugated double bond system of porphin (which is the basic 

 structure of all chlorophyll pigments as well as of the porphyrins) is a 

 chromophore, capable of pi'txlucing sti'ong absorption bands in the visible 

 and near ultraviolet. This is iUustrated b}' the absorption spectrum of the 

 parent substance of the group, porphin (c/. fig. 21.9). This spectrum has a 

 typical pattern of four bands between 480 and 700 mn, generally increasing 

 in intensity' toward the violet, but with the third band from the red weaker 

 than the second one. Stern and Wenderlein (1936) called this pattern the 

 "phjdlo type" (fig. 21.10, 3); it is exhibited by many porphyrins. Other 



Electronic 

 States 



B 



Vibrational 

 States 



Fig. 21.11. Interpretation of porphin 

 spectrum in terms of transitions from the 

 ground state, X, to two excited electronic 

 states A (vibrational states Ao, Ai, A2, 

 A3...) and B. 



5.0x10 



4.2 



D 



■5 2.6 



E 



— Chloroporphyrin-e4 



dimethyl ester 



— Chlorin-e4 dimettiyi ester 



480 520 560 600 640 670 

 WAVE LENGTH, m/i 



Fig. 21.12. Spectroscopic effect 

 of transition from the porphin to 

 the dihydroporphin (chlorin) sys- 

 tem (in dioxane) (after Stern and 

 Wenderlein 1935). 



porphyrins have similar foiu'-band spectra with a somewhat different dis- 

 tribution of intensities; these arc called by Stern the "etio t^^pe" and the 

 "rhodo type" (fig. 21.10, 1 and 2). Compounds of these three types trans- 

 mit freel}^ in the red; their color is red or purplish, hence the name "por- 

 phyrin." 



In addition to the series of bands shown in figiue 21.9 and 21.10, all 

 porphin derivatives have the so-called "Soi'et"band in the blue- violet (simi- 

 lar to the blue- violet band of chlorophyll ). The distances between the foiu' 

 l)ands in the green, yellow and red are such as to make plausible their in- 

 terpretation as vil)rational l^ands, corresponding to a common electronic 

 transition; while the blue-violet band stands apart and probably corre- 

 sponds to a different electronic excitation (cf. Table 21. V and fig. 21.11). 



The porphin spectrum vmdergoes a far-reaching change (cf. spectra 

 4. and 5 in fig. 21.10) upon the hydrogenation of one pj^role nucleus, i.e., 



