Porphyrin units

The synthesis of cyclic compounds is a battle for the yield. The general synthetic strategies of CPOs are illustrated in Fig. 2. The first one involves one-pot synthesis using a porphyrin unit (method A), and the second one is the ringclosing reaction of the chain-shaped PO (method B) [11]. 

To select the metal to be incorporated into the substrate porphyrin unit, the following basic properties of metalloporphyrins should be considered. The stability constant of MgPor is too small to achieve the usual oligomeric reactions and purification by silica gel chromatography. The starting material (Ru3(CO)i2) for Ru (CO)Por is expensive and the yield of the corresponding metalation reaction is low. Furthermore, the removal of rutheniirm is difficult, and it is likewise difficult to remove the template from the obtained ruthenium CPOs. Therefore, ZnPor is frequently used as a substrate in this template reaction, because of the low prices of zinc sources (zinc acetate and/or zinc chloride), the high yield in the metalation reaction, the sufficient chemical stability of the ZnPor under con-... 

Although we choose neither method A nor B, difficulties in the purification and/or isolation of CPOs are unavoidable. The component porphyrin unit with polar substituents such as an ester group produces further polar products in oligomerization reactions. The polarities of the products are proportional to the degrees of oligomerization caused by the increase in the number of func-... 

A poly(L-lysine) dendrimer 23 which carries 16 free-base porphyrins in one hemisphere and 16 Zn porphyrins in the other has been synthesized and studied in dimethylformamide solution [54]. In such a dendrimer, energy transfer from the Zn porphyrins to the free-base units can occur with 43% efficiency. When the 32 free base and zinc porphyrins were placed in a scrambled fashion, the efficiency of energy transfer was estimated to be 83% [55]. Very efficient (98%) energy transfer from Zn to free-base porphyrins was also observed in a rigid, snowflake-shaped structure in which three Zn porphyrin units alternate with three free-base porphyrin units [56]. 

The X-ray structures are known of a number of ketone ligands, including hexanedione and 4-methylcyclohexanone, that have bound as monodentate ligands in the fifth coordination site of zinc porphyrin complexes.347,348 Oxaphlorin dimeric compounds are also known where the oxygen from a neighboring porphyrin ring coordinates in the fifth coordination site of a neighboring zinc porphyrin unit.349... 

Other supramolecular structures such as dendrimers have also been synthesized with zinc-containing porphyrins. Sixteen free base and sixteen zinc porphyrin units were added at the fifth generation of dendritic poly(L-lysine) and intramolecular fluorescence energy transfer was observed.823 Assembly of supramolecular arrays in the solid state has been achieved with the incorporation of an amide group for hydrogen bonding. Zinc meso-tetra(4-amidophenyl)porphyrin... 

Metalloproteins fall into three main structure categories depending on whether the active site consists of a single coordinated metal atom, a metal-porphyrin unit, or metal atoms in a cluster arrangement. In the context of electron-transfer metalloproteins, the blue Cu proteins, cytochromes, and ferre-doxins respectively are examples of these different structure types. Attention will be confined here mainly to a discussion of the reactivity of the blue Cu protein plastocyanin. Reactions of cytochrome c are also considered, with brief mention of the [2Fe-2S] ferredoxin, and high potential Fe/S protein [HIPIP]. 

Porphyrine chromophore units have also been introduced to the PPV backbone but the PLQY of such materials decreased rapidly with increasing ratio of the porphyrine units and no EL devices have been reported [177,178]. 

Fluorescence lifetime measurements on the aggregate have shown that the rate constant of the intermolecular energy transfer from the zinc porphyrin unit to the free-base porphyrin unit has been evaluated to be 3.0 x 109 s-1. This value is reasonable from a model in which dendritic donor 6b and acceptor 5a contact each other directly at their exterior surfaces (Scheme 2). Therefore, electrostatic assembly of positively and negatively charged dendrimers provides a promising supramolecular approach to construct photofunctional materials with nanometric precision. 

Recently a porphyrin unit has been incorporated into an anion receptor (Fig. 46) (Beer et al., 1995g). H nmr titration experiments with this compound demonstrated the formation of 1 1 stoichiometric complexes with tetrabutylammonium halides, nitrate, hydrogensulfate and dihydrogenphosphate. 

Recently, Kimura et al. have reported the encapsulation of electro chemically and photo chemically active groups such as a porphyrin unit (51) or a ruthe-nium(II) bis(terpyridyl) (52) unit in the interior of a 1,3,5-polyphenylene-based dendritic structure (Scheme 21). The dendritic porphyrins have been synthesiz-... 

Other examples involve the immobilization of ruthenium porphyrin catalysts [74]. While Severin et al. generated insoluble polymer-embedded catalysts 16 by co-polymerizing porphyrin derivatives with ethylene glycol dimethacrylate (EGD-MA) [74 a], Che et al. linked the ruthenium-porphyrin unit to soluble polyethylene glycol (PEG) 17 [74b]. Both immobilized catalysts were employed in a variety of olefin epoxidations with 2,6-dichloropyridine N-oxide (Gl2pyNO), providing similar conversions of up to 99% and high selectivities (Scheme 4.9). 

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. 

The thin films are electrochemically active in both the oxidation and reduction regions, due to their having the electrochemically active zinc porphyrin units. The electrochemical processes are accompanied by color changes of the film, between black and yellowish-green in the oxidation region, and between red and yellowish-green in the reduction region. 

The heterometallic system [(bpy)2Ru(234)] exhibits several intramolecular energy-transfer processes (i) ultrafast singlet-to-singlet transfer, (ii) fast triplet-to-singlet transfer and (iii) singlet-to-triplet transfer. Excitation into the Ru(bpy)3 " " domain is followed by rapid energy transfer to the triplet state of the Zn(porphyrin) fragment. There is no evidence for intramolecular electron transfer between the Ru(bpy)3 and Zn(porphyrin) units. 

Reaction of 175 with Cgg yields a hydroxy-functionalized fullerene that can be further derivatized. This hydroxy-fullerene was coupled with a porphyrine unit via a polyethyleneglycol-Hnker. This linker can be arranged similarly to a crown-ether to complex metal cations. Complexation is used to tune the distance between the porphyrin imit and the Cgg-moiety and thus tune the donor-acceptor properties of this porphyrin-fuUerene hybrid [177]. 

A simple method has been developed to construct a variety of molecular architectures containing free base-magnesium or magnesium-metalloporphyrin systems consisting of two to nine porphyrin units. Compound 25 is a typical example for such a compound that are important for studying the electronic communication in multichromophoric... 

In the present study we investigated energy transfer between the Zn-porphyrin units in a sequence of dendrimers varying in size from 4 to 64 porphyrin units (Fig. 1). Reference measurements were performed on the monomer, P1D1. In order to follow energy transfer within the dendrimers, the fluorescence anisotropy decay were analysed. To determine the lifetime of the dendrimers, additional analysis of the kinetics measured at magic angle was performed. The fluorescence anisotropy is defined by... 

The steady-state absorption spectra of the compounds are characterized by a strongly absorbing Soret band at 429 nm (So — S2) and two minor Q-bands at 558 and 598 nm (So —> Si) (Fig. 2). Increasing size of the dendrimers does not produce any significant changes in the absorption spectra. This indicates weak interactions between the porphyrin units, i.e. the energy transfer can be described by FSrster theory. Excitation of the dendrimers in the Soret band results in two fluorescence bands (not shown here) with maxima at 600 and 655 nm (Si —> So for both transitions). Again, only minor differences can be observed dependent on size. 

Respiratory pigments similar to the vertebrate haemoglobins have also been identified in many invertebrates. These vary from small proteins with two Fe-porphyrin units to large molecules containing up to 190 Fe-porphyrin units. Myoglobin, the 02 storage protein in muscle tissue, is also a small iron-protoporphyrin protein. The crystal structures of this and a number of other porphyrin proteins are now known (Chapter 20.2, Table 11). 

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