Multilayer porphyrins

Araki, K., M.J. Wagner, and M.S. Wrighton (1996). Layer-by-layer growth of electrostatically assembled multilayer porphyrin films. Langmuir 12,5393-5398. 

Jones R, Tredgold R FI and Floorfar A 1985 Effeots of thiokness on surfaoe-potential and surfaoe oonduotivity in non-insulating Langmuir-Blodgett multilayers of porphyrins Thin Solid Films 123 307-14... 

LB Films of Porphyrins and Phthalocyanines. The porphyrin is one of the most important among biomolecules. The most stable synthetic porphyrin is 5,10,15,20-tetraphenylporphyrin (TPP). Many porphyrin and phthalocyanine (PC) derivatives form good LB films. Both these molecules are important for appHcations such as hole-burning that may allow information storage using multiple frequency devices. In 1937 multilayers were built from chlorophyll (35). 

Collman JP, Hendricks NH, Leidner CR, Ngameni E, L Her M. 1988. Multilayer activity and implications of hydrogen peroxide in the catal3dic reduction of dioxygen by a dicobalt cofacial bis(porphyrin) (C02FTF4). Inorg Chem 27 387. 

Shiryaeva IM, Collman JP, Boulatov R, Sunderland CJ. 2003. Nonideal electrochemical behavior of biomimetic iron porphyrins Interfacial potential distribution across multilayer films. Anal Chem 79 494. 

As discussed in section 2.2, a mixture of AMP and AA showed two solid condensed phases above and below about 30 mN m- [5,10]. A loosely stacked structure of two porphyrins was proposed for LB films prepared at higher surface pressures than 30 mNmr1, which was caused by squeezing-out of a monomolecular structure formed at lower surface pressure [5,10]. In this section, photoelectric characteristics of LB films containing AMP and AA deposited at two solid condensed phases will be discussed in relation to multilayer structure and the anisotropic intermolecular tunneling rates [87]. Seven monolayers of 1 5 or 1 10 mixture of AMP and AA were deposited at 20 and 50 mN m-1 on an ITO plate at 18 °C to form stable Y-type LB films. Aluminum was vacuum evaporated onto LB films as sandwich-type electrodes at 10-6 Torr. Steady photocurrents were measured in a similar manner as mentioned above. 

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). 

Figure 5.56 Schematic illustrating the self-assembled multilayers of sodium 3-mercaptoethane-sulfonate (layer 1 ), hexacationic homooxacalix[3]arene-fullerene (2 1) complex (layer 2 ), with either the anionic porphyrin polymer ( 3 ) or its zinc porphyrin analogue ( 4 ) forming the third layer. Reprinted with permission from A. Ikeda, T. Hatano, S. Shinkai, T. Akiyama and S. Yamada, /. Am. Chem. Soc., 123,4855 (2001). Copyright (2001) American Chemical Society...

The self-assembly of these oppositely charged macrocycles can be extended to form multilayer and Langmuir-Blodgett (LB) films [81-83], As shown by absorption spectroscopy, the chromophores are aggregated within the layer. Upon excitation of the porphyrin subunit, long-lived radical ions are formed in the layers as a result of the high polarity experienced by the complexes in this medium. 

Most frequently, the formation of such supramolecular gels is observed upon cooling aqueous solutions [419—423]. In Figure 37a self-assembled thin fibres of a carbohydrate substituted porphyrin are depicted, while Figure 37b shows a multilayered fibre of a octylmannonamide [424]. 

Figure 37 Electron micrographs of self-assembled fibres, (a) Porphyrine-fibres with a diameter of 6nm. (b) A -octyl-o-mannonamide multilayered fibre, negatively stained with phosphotungstate, the central dark hole has a diameter of 50 nm. (a) and (b) were obtained from aqueous solution [424].

Addition of an inorganic sheet and a PVT-MV + layer results in efficient quenching of the porphyrin phosphorescence at 720 nm, due to electron transfer from excited porphyrin to MV +. Electron-transfer quantum yields were calculated as 0.58 and 0.76 for a-ZrP and HTiNbOs spacers, respectively. Figure 38 shows the steady-state emission spectrum of such a multilayer system. 

Figure 6.20 Lipid multilayers in cast films may be doped with water-soluble or amphiphilic porphyrins. The hydrophobic macrocycle of protoporphyrin derivatives (—) with one hydrophilic edge integrates into the hydrophobic membrane, but their orientation is not fixed. Water-soluble porphyrins with four sulfo-nated phenyl rings at the methine bridges are oriented parallel to the film surface. The ordering of the hydrophobic bilayer can also be controlled by azo-dyes which are covalently bound to the hydrophobic chains of the lipids, hereC,H,r-C6H4-N=N—C6H4-C,oH2o—N(CHs)sBr.

Another possibility is that the ring structure may have long hydrocarbon chains attached at the comers so that they stand up at one side. These chains provide the hydrophobic component and the polarizable ring stmcture provides the hydrophilic moiety. There are studies with porphyrins bearing four long hydrocarbon chains and whose hydrophilic moieties are associated with the ring stmcture [80, 81]. However, these materials did not lead to the formation of ordered multilayers. The same general principle applied to phthalocyanines led to stable films at the water-air interface and could produce multilayers by the LB technique [82. 83]. 

Figure 6.9.4 Assembly of a porphyrin multilayer on metal anodes using micelles with...

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