Macrocyclic Aromaticity of Porphyrin Units in Fully Conjugated Oligoporphyrins

Abstract

Individual porphyrin macrocycles in fully conjugated oligoporphyrins exhibit different degrees of macrocyclic aromaticity. We proposed a theoretical method for estimating the degrees of macrocyclic aromaticity of these porphyrin units. This method, based on the concept of bond resonance energy, is applicable to all types of porphyrin oligomers. We then found that the degree of macrocyclic aromaticity is very sensitive to the way of oligomerization. In general, it is markedly suppressed by linkage or substitution at meso positions but is fairly insensitive to direct edge-to-edge fusion. These predictions are consistent with observed chemical shifts of protons attached to the porphyrin macrocycles. Antiaromatic circuits are created in doubly- and triply-linked porphyrin oligomers. These circuits must be the origin of paramagnetic currents induced around the naphthalene-like junction zones in triply-linked oligomers. The same approach may be applied to other fully conjugated systems consisting of two or more maclocyclic units.