On the electronic states of the extended porphyrin family

The ground and lower excited electronic singlet and triplet states of eight macrocycles resulting from porphyrin by replacing pyrroles with pyridines or indoles and the meso methines by nitrogens (the extended porphyrin family) have been treated by Skala's version of the CNDO/S method in view of providing systematic reference computational data. The results are especially useful for comparisons capable of throwing light on the structure-property relations that make some of those macrocycles especially active for energy and electron transfer. A brief discussion is given. The uniqueness of porphyrin stands out clearly, but some of its features may be strictly related to the biochemical environment of heme groups, so that other macrocycles of the family might be better suited for technological applications simulating the in vivo behavior of porphyrin derivatives. Hints concerning the effect of coordination to metal atoms are given, pending the completion of a parallel study on some metal complexes of the family. A few novel experimental results from our laboratories are mentioned in the discussion.