Porphyrins electronic

L. Lee, N. H. Sabelli, and P. R. LeBreton, Theoretical characterization of phthalocyanine, tetraazaporphyrin, tetrabenzoporphyrin, and porphyrin electronic spectra, J. Phys. Chem, 86, 3926-3931 (1982). 

Diffuse reflectance spectroscopy (DRS) of VO-porphyrins on reduced and sulfided catalysts exhibit shifts in the porphyrinic electronic spectra (Soret, a, (3 bands) to higher frequencies. Adsorption results in modification of the delocalized electronic resonance structure not observed on the oxide form of the catalyst. X-ray photoelectron spectroscopy reveals shifts to higher Mo binding energies on reduced and sulfided catalysts following VO-porphyrin adsorption, consistent with transfer of electrons from Mo electron donor sites to the V02+ ion. Interaction at the electron donor sites is stronger than interaction at electron acceptor sites typical of the oxide catalyst. This gives rise to the possibility of lower VO-porphyrin diffusion rates on sulfided catalysts, but this effect has not been experimentally demonstrated. 

The polyimide-base PR system [79,80] was designed on the premise that porphyrin-electron acceptor (quinones or imide moieties) systems are well-known model compounds for photosynthetic processes and exhibit very interesting charge transfer properties [81], A high quantum yield of charge separation can be achieved in these systems. Polyimides are found to be photoconductive and allow charge transport [82], Furthermore, polyimides possess high Tg and therefore, the electric field-induced dipole orientation can be fixed after imidization [83],... 

Hoffman, B. M., T. Szymanski, T. G. Brown, and F. Basolo (1978), The Dioxygen Adducts of Several Manganese(II) Porphyrins, Electron Paramagnetic Resonance Studies. J. Am. Chem. Soc. 100, 7253-7259. 

Pb11. These metalloporphyrins are extremely acid-labile and produce the typical lead-porphyrin electronic spectrum with a Soret band close to 470 nm (see text relating to Fig. 1). 

Porphyrin Electronic centre of gravity (E )(eV) Oxidation I Reduction I (eV) ... 

Whitten and co-workers [230] have amply investigated stilbene isomerization induced by excited porphyrins. Electron-deficient nitrostilbenes quench excited Zn or Mg etioporphyrins through exciplex formation. Light absorbed by the porphyrin affects isomerization of 4-nitro- or 4,4 -dinitro-stilbene with low quantum yields ((%trans)sens = 99.5), possibly via a stilbene exchange chain mechanism [230]. Azastilbenes coordinate to me-talloporphyrins in both the ground and the excited state (but do not quench it), and isomerize efficiently upon selective excitation of the porphyrin [231]. Complexed 1,4 -NPE gives d , = 6.6, = 0.2, and (%trans)sens = 96,... 

Both mechanisms of oxidation of the manganese-oxo porphyrin, electron transfer and oxygen atom transfer were observed on the same DNA target. 

The systems studied by Schuster and Guldi are reported in Fig. 22 56 and 57" are analogous to rotaxanes 33 and 40 with two zinc(II) porphyrins as stoppers, a Cu(I)(dpp)2 as the spacer and a C6o as the electron acceptor are appended to the macrocycle or included in it. SS is a catenane derived from 56 upon axial binding by a bidentate ligand to the zinc ions of the porphyrin stoppers. Rotaxanes 59" and 60 differ from 56 and 57" in having two 50 electron acceptors as stoppers and a zinc porphyrin electron donor appended to the macrocycle and differ from each other in the distance between the electron donor and the copper complex, which has been increased by insertion of a phenylamido group. 

More recently, this group has incorporated these and related porphyrin-substituted nucleosides into tetranucleotides using standard solid support techniques to assemble chiral homo- and heteroporphyrinic arrays 38a-c. In this way, the composition and the physical properties of the array can easily be made to order simply by programming the DNA synthesis on an automated DNA synthesizer. The tetranucleotides were then able to form duplexes with the complementary strands (e.g. tetra-adenosine dA4 39). It was shown that the porphyrin electronic ground state environment is not particularly influenced by the presence or absence of the complementary strand, but the fluorescence intensity is decreased. CD measiuements confirm the helical nature of the duplexes. 

The discussed experimental results provide a basic insight into the physical mechanism of the porphyrin electronic structure alternation by protein in a pigment-protein complexes. ... 

This section introduces some examples where the delocalized electronic system of porphyrin has been extended synthetically. There are two now well-established methods for achieving the expansion of the porphyrin electronic system. The first, introduced by Crossley and coworkers, involves a sequence of reactions starting with nitration of pyrrole groups and oxidation to a 1,2-diketone. The latter can then be condensed with a 1,2-diamine or itself be converted to a 1,2-diamine. Using this strategy, large nanometric multichromophoric systems have been prepared, an example of which is shown in Figure 8(a). 

---