Conjugated systems porphyrins

Webster S, Odom SA, Padilha LA, Przhonska OV, Peceli D, Hu H, Nootz G, Kachkovski AD, Matichak J, Barlow S, Anderson HL, Marder SR, Hagan DJ, Van Stryland EW (2009) Linear and nonlinear spectroscopy of a porphyrin-squaraine-porphyrin conjugated system. J Phys Chem B 113 14854-14867... 

When the -C=C- unit is directly bonded to the zinc porphyrin unit (polymer of type B), the Soret and Q-bands are shifted to a longer wavelength, suggesting the formation of a highly 71-conjugated system along the main chain, due to the lack of steric hindrance around the zinc porphyrin ring. 

Tetrapyrroles contain an extended jt-conjugated system which is responsible for their use in a wide range of applications ranging from technical (pigments, catalysts, photoconductors) to medicinal (photodynamic therapy) uses. The electronic absorption spectra are governed by the aromatic 18 tt-electron system and typically consist of two main bands. In phthalocyanines the Q band around 660-680 nm is the most intense one accompanied by a weaker Soret band near 340 nm . In porphyrins the situation is reversed with an... 

The ring system of vitamin B12, like that of porphyrins (Fig. 16-5), is made up of four pyrrole rings whose biosynthetic relationship to the corresponding rings in porphyrins is obvious from the structures. In addition, a number of "extra" methyl groups are present. A less extensive conjugated system of double bonds is present in the corrin ring of vitamin B12 than in porphyrins, and as a result, many chiral centers are found around the periphery... 

Many of the above deficiencies were removed in further refinements of the Hiickel method, which was anyway made obsolete by the development of ab initio and DFT techniques. Still, organic chemists adhere to the original Hiickel description, which is often sufficient to make qualitative predictions about the nature of n-conjugated systems. In particular, the Hiickel model finds widespread use in porphyrinoid chemistry. The so-called annulene model, which will be used throughout this review, is outlined below for the parent porphyrin macrocycle. 

In comparison with MOEP and H2TPP, the increased planarity and tt conjugate system associated with the attached benzene rings on the pyrroles of porphyrin core render tetrabenzoporphyrins (TBPs) the most widely studied molecules among porphyrin derivatives for OFET devices. In 2004, Ono and co-workers... 

Porphyrin-(3.1.3.1) 4.68 was prepared by the HBr catalyzed reaction between the vinylogous dipyrrylmethane 4.66 and its formyl derivative 4.67. The presumed intermediate condensation product was not isolated. Instead, it was oxidized with air during the course of the reaction to produce the fully conjugated system directly. After workup and chromatographic purification over neutral alumina, the free-base macrocycle 4.68a was obtained in 19% yield (Scheme 4.2.1). 

The description and discussion of the oxidation states of metalloporphyrins is complicated by the fact that not only the central metal ion but also the -conjugated system of the porphyrin ligand is redox-... 

The extensive conjugated system of porphyrin gives blood its characteristic red color. Its high molar absorptivity (about 160,000) allows concentrations as low as 1 X 10 M to be detected by UV spectroscopy (Section 8.10). 

Similar to quinones and NAD+ the flavins are one- and two-electron redox systems (Fig. 7.2.10a). Upon half-reduction a quinhydrone-type dimer appears, producing a charge transfer band at 820 nm (Fig. 7.2.10b). The noncovalent interactions of flavins arise from electrostatic attraction between electron-rich donor atoms, in particular the basic oxygen atoms of amide groups, and electron-deficient aromatic systems, the inner conjugation system of oxidized flavins (Breinlinger et al., 1998). This is reminescent of the interactions between porphyrin macrocycles (see Fig. 6.2.15). 

Porphyrins form a planar macrocycle that contains a conjugated, cyclic delocalized system of 22 n-electrons [17]. 18 Electrons can be assigned to the perimeter of a l,16-diaza[18]annulene, an arrangement which is also found in the chlorin and bacteriochlorin systems. Porphyrins and chlorins are intensely coloured (1 red to violet, Amax 500-700 nm 2-4 green, 600-700 nm). 

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