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A model is presented for the excited states of porphin, which are viewed as arising from two configurations which can be mixed to varying degrees. The effects of substituents are then discussed, first the limitations imposed by symmetry requirements and then the requirements imposed if they act as one electron perturbations. The theory is applied to certain experimental problems. First it is shown how the metal porphins can be viewed as a series showing increasing mixing of the two fundamental configurations. Next the effect of free base formation is explained as a cyclic polyene perturbation. This theory predicts that the lowest porphin transition is polarized parallel to the H–H axis. Formulas are derived for intensity changes caused by various external substituents; their effect on transition energies are also discussed. Finally formulas are developed to account for the effects of aza substitution on the methine bridges.

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