84 III. PORPHYRIN CHEMISTRY 



nylporphin, obtained by condensation of pyrrole with benzaldehyde, could 

 also be separated into fractions with different HCl numbers and absorption 

 spectra. Again, the absorption spectra of tlie complexes also differed, which 

 excludes prototropic isomerism. Pruckner [2188) has shown that one of the 

 presumed isomerides of tetraphenylporpliin had an absorption spectrum with 

 a much higher band in the red, which indicates its nature as a dihydroporphin 

 (chlorin) rather than a porphin.* She has also drawn attention to the fact 

 that a comparison of the absorption spectra of Rothemund's porphin and 

 isoporphin with that of the porphin of Fischer and Gleim {SIS) as measured 

 by Stern and Molvig (2G36) indicated that neither of Rothemund's substances 

 was uniform. 



The evidence against the resonance structure of the porphin nucleus is 

 thus unconvincing. 



6.4. Absorption Spectra and Fine Structure of the Nucleus 



A number of attempts have been made to draw conclusions as to the fine 

 structure of the nucleus from data on the absorption spectra of porphyrins. 

 In numerous papers Stern and collaborators {2()3 1,2032,2037, 2642,2643) 

 have tried to deduce the position of the pyrrole or pyrrolene nuclei and of the 

 double bonds in relation to the sul)stituting side chains in porphyrins and 

 related compounds. In view of the lesonance character of the porphin nucleus 

 (c/. G."2. and 6.3.) such conclusions must be accepted with reserve. The influ- 

 ence of the substituting side chains ought to be considered dynamically 

 rather than statically as Stern does. Some of his correlations are rather 

 arbitrary; one may doubt, e.ij., whether azaporphyrins are so similar to 

 chlorophyll derivatives as to allow conclusions to be drawn with regard to 

 the constitution of the latter. These conclusions hav'e, indeed, been later 

 abandoned by Pruckner {21SS), one of Stern's co-workers, who now stresses 

 the influence of the substituents on the symmetry of the whole molecule 

 rather than on the position of double bonds in the porphin nucleus as a cause 

 of the variations in the absorption spectra. The absorption bands I and III 

 of the neutral porphyrin spectrum (rf. Table VI) appear to be increased by a 

 greater degree of symmetry, the bands II and IV by unsymmetrical sub- 

 stitution. 



The attempts of Clar and Haurowitz (444) to deduce diradical character 

 for porphyrins from their absorption Spectra are not convincing (r/. 4S2,10S5, 

 2642). Attempts have been made to fit the position of porphyrin absorp- 

 tion and fluorescence bands into energy level schemes {llSo,1234,210Sa). 

 Hellstrom assumes five different electron excitations with two oscillation 

 frequencies due to oscillation in phase of several atoms of the ring. Hausser, 

 Kuhn, and Seitz assume only one electron excitation with superimposed 

 oscillations. They explain the mirror image-like arrangement of fluorescence 

 and absorption bands by assuming that in absorption the changes are from 

 ground level without oscillation to excited levels with oscillations, while in 

 fluorescence the changes are from excited levels without oscillation to oscil- 



*This has been proved to be so by Calvin and co-workers {12oa). 



