Metalloporphyrins and Related Compounds

The simplest porphyrin, porphin (R, R = H and R = H), forms complexes with a variety of metal ions. It has 105(3 x 37-6) normal vibrations which are classified as shown in Table 111-9. Ogoshi et al. prepared porphin complexes with Zn, Zn, Cu, and Ni, and carried out normal coordinate calculations 

As stated in Sec. 1-21. metalloporphyrins are ideal for resonance Raman (RR) studies since they exhibit strong absorption bands in the visible and near-UV regions. It is well established that excitation at the a- and j3-bands (see Fig. 1-18) causes resonance enhancements of the Ai, and A,  

In going from the low- to high-spin state, the porphyrin cSre lends to expand or be domed, and this results in weakening of the C -C , bond. Thus, P3, Pm, 

Kincaid and Nakamoto observed the /(Fe-F) of Fe(OEP)F at 595 cm with the 514.5 nm excitation. Kitagawa et al. also observed the (Fe-X) of Fe(OEP)X at 364 and 279 cm for X = CI and Br, respectively and the y,(L-Fe-L) of [Fe(OEP)L2] (L= ImH) at 29 J cm using the 488 nm excitation. These results show that the axial vibrations can be enhanced via resonance with in-plane tt-tt transitions (a and /3 bands). According to the latter workers, vibrational coupling between these axial vibrations and totally symmetric in-plane porphyrin-core vibrations is responsible for their resonance enhancement. On the other hand, Spiro prefers electronic coupling, namely, the TT-TT transition induces the changes In the Fe-X (or L) distance, thus activating the axial vibration. 

Complexes of tetrapheny I porphyrin (TPP, R, Rs H, R = C6H5) have been studied most extensively due to their relatively high-yield synthesis and convenient purification. Thus, many biomimetic compounds such as picket-fence and capped porphyrins (Sec. V-l) have been derived from the basic TPP structure. However, IR spectra of TPP complexes are extremely difficult to assign because vibrations of phenyl groups are mixed with or overlapped by those of the porphyrin-core. Recently, Oshto et al. prepared 20 Fe(TPP)LL - (L, L axial ligands) type complexes, and located two spin-state 

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Metalloporphyrins and related compounds continue to attract attention as photosensitizers and as building blocks for construction of artificial lightharvesting arrays. The photophysical properties of several structurally modified porphyrins have been measured with a view to identifying new sensitizers for use in photodynamic therapy. The triplet state properties of porphyrins adsorbed onto the outer surface of vesicles have been describedand the fluorescence spectral properties of some amphiphilic porphyrins have been recorded. Similar studies have been carried out with halogenated tetraar-... 

Vibrational spectra of metalloporphyrins and related compounds have been studied extensively because of their biological importance as prosthetic groups of heme proteins (Sec. V-1 and V-2). Figure 111-13 shows the planar D4/, structure of a metalloporphyiin. 

Dioxygen Adducts of Base-free Metalloporphyrins and Related Compounds... 

Because the four nitrogens form an optimal cavity, porphyrins can bind a wide variety of metal ions to form metalloporphyrins (MP). Iron porphyrins are found in many biological systems and act as electron transfer mediators (cytochromes) or oxygen transfer agents (hemoglobin). Modified porphyrins are found in nature also in the form of chlorophylls and vitamin B,2 and related compounds. 

The porphyrin macrocycle contains conjugated double bonds that form an 18-membered Tr-electron system. This Tr-system can accept several electrons in a stepwise manner. The addition of one electron forms a porphyrin Tr-radical anion. Such Tr-radical anions have been prepared from metalloporphyrins and from metal free porphyrins and related compounds by irradiation in various environments. 

Recent results in the field of catalytic oxidations with hydrogen peroxide are reviewed. Most effective catalysts fall into three categories metallorganic compounds, phase-transfer catalysts, redox zeolites. Metalloporphyrins and Pt-phosphine complexes are representative of first category. Mo and W polyoxome-talates and related systems, in association with phase transfer agents, belong to the second one. Titanium silicalite (TS-1) is the most studied redox zeolite. The oxidation of nitrogen and sulphur compounds and Fenton-like reactions are not reviewed. 

This present review will not attempt to provide a comprehensive description of all known porphyrin electrochemistry in non-aqueous media, but will concentrate in part on specific types of porphyrin macrocycles, in part on specific groups of metalloporphyrins, and in part on guiding the reader through the vast array of elechon-transfer mechanisms that can exist for a related series of compounds under a given set of experimental conditions. It is hoped that this approach will answer the majority of the reader s questions as to what has been done in the past, while at the same time enabling the reader to utilize the data in the literature to predict what might be observed in future studies involving the electrochemistry of yet-to-be synthesized metalloporphyrin complexes. 

Photocontrol of chemical and physical functions could be used in this system (Fig. 6.21). Stilbazole, a compound related to natural photoresponsive molecules, was used as the photoresponsive switch [136]. Because stilbazole undergoes isomerization from fhe trans form to fhe cis form on UV irradiation, and fhe reverse in visible hght, via complexation of the pyridine group to fhe metal center of metalloporphyrins, fhe (TPP)Al-OMe-stilbazole system on irradiation wifh UV light serves as an on-switch to speed up the reaction whereas in visible hght fhe system worked as an off-switch to reduce the rate of the once accelerated reaction. 

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