The Electronic Structures of Porphyrins and Metalloporphyrins

Chlorophylls, heme, and bile pigments are made of four pyrrole units connected by methine bridges. We first introduce the electron-rich pyrrole unit and then dis- 

Further condensation reactions of a-substituted pyrroles lead to macrocychc 

The overall D2J, symmetry in free base porphyrins and the symmetry in metal complexes produce an aromatic character in the macrocycle. All bonds have about 50% double bond and 50% single bond nature. This does not, however, mean that all bonds have the same length. Within the inner 16-member macrocycle C-C bonds typically measure 1.41 A, and C-N bonds are only slightly shorter, namely 1.37 A. The pyrrolic C-C bonds are, however, different the Ca-CP bond is 1.46 A, the Ca-Cp bond only 1.38 A long (Fig. 6.2.3) Lauher and Ibers, 1973 Scheldt and Lee, 1987). 

The fact that the inner conjugation pathway dominates the electronic spectra of symmetrical porphyrins and metalloporphyrins does not mean that P-pyrrolic substituents or central metal ions have no influence on the UV/vis 

Hydrogenation of a 3,4-pyrrolic double bond produces chlorins, the mother chromophores of chlorophyll, and the whole aromatic porphyrin spectrum changes to a polyene-type chlorin spectrum. It consists mainly of two bands of comparable intensity at 400 and 650 nm and several smaller bands in between (see Fig. 6.2.9) (Cox et al., 1974 Smith, 1975 Gouterman, 1978). 

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