Rules for Absorption Spectra of Porphins

The neutral porphins all have four absorption bands in the visible between 500-700 m t, and a strong ultraviolet Soret band at around 400 m/i. The position of the band maxima vaiy depending on the solvent in 

Following the classification of Stem, three types of visible spectra may be distinguished (Fig. 2). Etio type ( stepladder ) has a regularly ascending series of bands the rhodo type has a fourth band lower than the third—the third band is too strong and the phyllo type has a third band that is too weak.  

In the fully substituted porphins, changing the relative positions of the side chains (with the exception of the C=0 groups) to form different isomers has no effect on the absorption spectrum. Substitution of one 

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