Porphyrin amphiphile

As expected from the extremely low fluorescence of fibres made of alkyl-substituted porphyrin amphiphiles, flash photolysis is ineffective. Nevertheless, the formation of porphyrin anion radicals was detected on a millisecond time scale and was traced back to a charge separation within the porphyrin fibre. ... 

The first synthesis of amphiphilic porphyrin molecules involved replacement of the phenyl rings in TPP with pyridine rings, quaternized with C2QH 2Br to produce tetra(3-eicosylpyridinium)porphyrin bromide (3) (36). The pyridinium nitrogen is highly hydrophilic the long C2Q hydrocarbon serves as the hydrophobic part. Tetra[4-oxy(2-docosanoic acid)]phenyl-porphyrin (4) has also been used for films (37). 

It may seem curious to make specific mention of this matter, but in a biological context, it is crucial. The nuclei of porphyrins and related systems are hydrophobic it appears to be generally desirable that the photosensitizer has some degree of intermediate polarity (i.e., amphiphilic properties Section 9.22.3) and the incorporation of a metal, such as zinc(II), which can take on an axial ligand (e.g., H20 in aqueous media, or RNH2 in a biological fluid), or magnesium(II), which can take on two, is expected to enhance solubility in hydroxylic solvents, and shift the... 

For systemic administration, the photosensitizer usually has to be delivered into the bloodstream by intravenous injection. Since the photosensitizer is a solid, this means that a solution or a stable suspension has to be provided. Metal complexes of the basic porphyrin and phthalocyanine nuclei are insoluble in water, so that some effort has to be made to render the system water soluble, or at least amphiphilic, by placing various substituents (e.g., S03H, C02H, OH, NR3+, polyether, aminoacid, sugar) on the periphery of the molecule. The aromatic character of the ligand offers a suitable opportunity for such substitutions to be made. Examples will appear frequently in the following sections. 

Recently, the selforganization amphiphilic porphyrins with alkyl chains at the... 

The out-of-plane orientation of chromophores can be more easily controlled in LB films as compared with the in-plane orientation. Many chromophores are known to show anisotropic orientation in the surface normal direction. The molecular structure of chromophores and their position in amphiphile molecules, the surface pressure, the subphase conditions are among those affect their out-of-plane orientation. The out-of-plane orientation has been studied by dichroic ratio at 45° incidence, absorbance ratio at normal and 45° incidence, and incident angle dependence of p-polarized absorption [3,4,27,33-41]. The evaluation of the out-of-plane orientation in LB films is given below using amphipathic porphyrin (AMP) as an example [5,10,12]. 

With the above-mentioned variety of addition reactions based on the addition-elimination mechanism almost any functional group or molecule can be attached to CgQ. Some examples are acetylenes [43, 52], peptides [53], DNA-fragments [53], polymers [54], macrocycles [55, 56], porphyrins [56, 57], dendrimers [58-60] or ligands for complex formation [56], Cjq can be turned into hybrids that are biologically active, water soluble, amphiphilic or mixable with polymers [53-55, 58, 61-69],... 

Gayathri SS, Patnaik A (2006) Electrical rectification from a fullerene[60]-dyad based metal-organic-metal junction. Chem Commun (Cambridge, UK) 1977-1979 Matino F, Arima V, Piacenza M et al (2009) Rectification in supramolecular zinc porphyrin/ fulleropyrrolidine dyads self-organized on gold(lll). Chemphyschem 10 2633-2641 Acharya S, Song H, Lee J et al (2009) An amphiphilic Cgo penta-addition derivative as a new U-type molecular rectifier. Org Electron 10 85-94... 

Variations on the vertical dipping technique have been utilized to construct films containing divalent metal ions. For example, the quartz crystal microbalance (QCM) has been used to evaluate the horizontal lifting method of CdSt LB Film construction (26). In this method, the QCM quartz plate was touched to monolayers compressed on a subphase and lifted horizontally. Y-type transfer (transfer ratio of 1) was demonstrated with two centrosymmetric monolayers deposited for each cycle. A combination of the vertical and horizontal dipping techniques has been utilized to prepare multilayer films from an amphiphilic porphyrin compound (27). 

Fibers were also found to be formed, amongst others, by amphiphile s like 99 having a phospholipid nucleotide head group [50], by porphyrin derivatives like 100 [51], and by helicene derivative 101 [52]. 

Several additional studies were carried out to obtain information about the precise behavior of the various components in the model system. The interplay between the manganese porphyrin and the rhodium cofactor was found to be crucial for an efficient catalytic performance of the whole assembly and, hence, their properties were studied in detail at different pH values in vesicle bilayers composed of various types of amphiphiles, viz. cationic (DODAC), anionic (DHP), and zwitterionic (DPPC) [30]. At pH values where the reduced rhodium species is expected to be present as Rh only, the rate of the reduction of 13 by formate increased in the series DPPC < DHP < DODAC, which is in line with an expected higher concentration of formate ions at the surface of the cationic vesicles. The reduction rates of 12 incorporated in the vesicle bilayers catalyzed by 13-formate increased in the same order, because formation of the Rh-formate complex is the rate-determining step in this reduction. When the rates of epoxidation of styrene were studied at pH 7, however, the relative rates were found to be reversed DODAC DPPC < DHP. Apparently, for epoxidation to occur, an efficient supply of protons to the vesicle surface is essential, probably for the step in which the Mn -02 complex breaks down into the active epoxidizing Mn =0 species and water. Using a-pinene as the substrate in the DHP-based system, a turnover number of 360 was observed, which is comparable to the turnover numbers observed for cytochrome P450 itself. 

Other templates have been described in the literature. Three independent groups used a porphyrin template. Sasaki and Kaiser utilized a porphyrin bearing four carboxylic acids (39) to prepare a synthetic metalloprotein with a four a-helix topology (40) [34], The utilization of a porphyrin template allows not only the templation of amphiphilic peptide segments, but also the incorporation of a metal binding site, which could serve eventually in a catalytic process (Fig. 16). 

All these methods have been used successfully and examples will now be discussed. Films in which amphiphilic porphyrins alternate with some other molecule are discussed in the next chapter. 

It is an unfortunate fact that, whereas many hundreds of materials have been used to form LB films, in the majority of cases no serious effort has been to characterise the film structure or even to show that a regular layer structure has been achieved. The example of amphiphilic porphyrins discussed above is sufficient to emphasise the need for proper characterisation. It will be recalled that mesoporphyrin IX dimethyl ester and the equivalent diol can, when complexed with a large variety of divalent metals, form excellent apparent Y layers with a deposition ratio very near unity on both up and down strokes. However, it is quite impossible to demonstrate the existence of a regular layer structure using X-ray diffraction techniques. Analogous situations involving two kinds of molecule will be discussed in Chapter S. 

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