Half band, dimer model

Three-quarters localization may be achieved by changing every second carbon atom in PA to a nitrogen atom. Optimization by the one-electron, unrestricted PM3 model leads either to an SDW or a CDW solution. The result is a deformed molecule with a repetition period of four atoms. It is clear that there are two possibilities for the ground state, and that correlation effects have to be included to obtain a correct result. The conclusion is that in the one-dimensional systems, one-half band occupation leads to a trivial Peierls dimerization, while the cases of one-quarter or three-quarters occupation may lead to interesting dynamics. 

Model meso-tetraphenylporphyrins are monomeric in solution and produce a narrow Soret band with a half-width of 15 nm. -Octaalkylporphyrins, e.g., oc-taethylporphyrin or protoporphyrin derivatives, dimerize in solution down to a concentration of about 10" M and show a Soret band with a half-width of 30 nm. 

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