Metalloporphyrins self-assembly

SERS Studies on Metalloporphyrin Self-Assembled Films on Organic-Monolayer-Modified Substrates... 

Most studies of ORR catalysis by metalloporphyrins have been carried out using water-insoluble catalysts absorbed on a graphite electrode in contact with aqueous solution. In a limited number of cases, four other approaches have been used catalysts imbedded in an inert film (i.e., Nafion or lipid) on the electrode surface self-assembled monolayers of catalysts catalysts in aqueous or mixed organic/aqueous solutions in contact with an electrode and catalysis in mixed aqueous/organic medium using... 

Relatively little work has been done on ORR catalysis by self-assembled mono-layers (SAMs) of metalloporphyrins. The advantages of this approach include a much better defined morphology, structure, and composition of the catalytic film, and the surface coverage, and the capacity to control the rate at which the electrons ate transferred from the electrode to the catalysts [CoUman et al., 2007b Hutchison et al., 1993]. These attributes are important for deriving the catal5d ic mechatfism. The use of optically transparent electrodes aUows characterization of the chemical... 

Porphyrin-based self-assembled molecular squares 389 can form mesoporous thin films in which the edge of a square, thus the size of the cavity, can be adjusted by appropriate choice of substituents [8]. Fibers that form coil-coiled aggregates with distinct, tunable helicity are built from crown ethers bearing porphyrins 390 [9]. In addition to the porphyrin applications discussed in Sections 6.3.2.2 and 6.4, dendrimer metalloporphyrins 391 to be applied in catalysis [10] and the water-soluble dendritic iron porphyrin 319 modelling globular heme proteins [11] can be mentioned. 

ANION RECOGNITION AND SENSING BY AMIDOFERROCENE SELF-ASSEMBLED MONOLAYERS ON GOLD ELECTRODES AND ZINC METALLOPORPHYRIN-FUNCTIONALISED GOLD NANOPARTICLES... 

Metal nanoparticles have extraordinary size-dependent optical properties, not present in the bulk metal and have, consequently, been the subject of intense research during the past decade or so.27 Attention has recently focused on functionalising colloidal nanoparticles with molecular recognition components for potential sensing applications.28,29,30 We have prepared a new amido-disulfide functionalised zinc metalloporphyrin (8) which was self-assembled on to gold nanoparticles to produce a novel anion-selective optical sensing system (9) (Scheme 3).4... 

Figure 4.15 Schematic representation of the sequential steps taken for the formation of multilayers based on electrostatic self-assembly using cationic polymers and anionic a-ZrP sheets (see text for further details). Reprinted from Coord. Chem. Rev., 185-186, D.M. Kaschak, S.A. Johnson, C.C. Waraksa,J. Pogue and T.E. Mallouk, Artificial photosynthesis in lamellar assemblies of metal poly(pyridyl) complexes and metalloporphyrins, 403-416, Copyright (1999), with permission from Elsevier Science...

Porphyrins and metalloporphyrins exhibit planar, relatively rigid structures suitable for the preparation of materials displaying cavities or that can self-assemble by electrostatic and n-n interactions. They also provide very interesting functional building blocks because of their rich coordination, catalytic, electro-catalytic, photochemical, and redox properties, as described in many papers (100-102) and review articles by Toma and co-workers (31,38,39), Latos-Grazynski et al. (103), Imamura and Fukushima (104), Sanders (105), Chambron et al. (106), Baldini and Hunter (107), and Prodi et al. (108). 

Figure 7 Two stereo side view ofNMR structure of a self assembled metalloporphyrin-peptides model. (Reprinted with permission from M.M. Rosenblatt, J. Wang, and K.S. Suslick, Proc. Natl. Acad. Sci. U.S.A., 2003, 100, 13140. 2003 National Academy of Sciences, USA)...

Self-assembled monolayers have recently attracted much attention as a new methodology for molecular assembly [249, 342]. They enable highly organized chemical binding of molecules of interest to the surfaces of, e.g., metals, semiconductors, and insulators. The well-ordered structure of self-assembled monolayers is in sharp contrast with conventional Langmuir-Blodgett films and lipid bilayer membranes in terms of stability, uniformity, and manipulation. Functional molecules can be arranged unidirectionally at the molecular level on substrates when substituents which will self-assemble on the substrates are attached to a terminal of the molecules. The wide variety of examples reported to date include porphyrins and metalloporphyrins in self-assembled monolayers [299-339]. 

Self-assembled monolayers of metalloporphyrins 11-16, similar to 5 and 7, have also been constructed on gold surfaces [307, 335]. The F value of surface coverage was estimated to be 3.8 x lO mol cm (44A molecule ), suggesting closely packed structures of the porphyrin with perpendicular orientation to the gold surface [335]. The insertion of the metal ion into a self-assembled monolayer of free base porphyrin 17 on the gold surface has been performed by refluxing the metal ion solution in which the modified gold electrode was immersed [335]. 

Photoinduced Electron Transfer of Metalloporphyrins in Self-assembled Monolayers... 

The self-assembling principle has begun to be exploited by chemists in the design of synthetic molecular receptors. Hunter and co-workers [61] describe the use of metalloporphyrin coordination chemistry to build up the self-association of a very stable cyclic porphyrin dimer 45 which recognizes derivatives of terephthalic acid (46, 47) through hydrogen-bonding interactions (Scheme 15). 

Kalyuzhny G, Vaskevick A, Ashkenasy G, Shanzer A (2000) UV/Vis spectroscopy of metalloporphyrin and metallophthalocyanine monolayers self-assembled on ultrathin gold films. J Phys Chem B 104 8238-8344... 

In a different reaction scheme, one can take advantage of the fimctional porphyrin macrocycle to create metalloporphyrin compounds and nanoarchitectures in 2D. Upon exposure of regular TPyP arrays self-assembled on Ag(lll) to iron monomers supplied by an atomic beam, selective com-plexation occurs whereby the template structure is strictly preserved [156]. This expands the diversity of metalloporphyrin layers conventionally realized by evaporation of integral species, because in-situ metalation provides a route towards novel metalloporphyrin nano architectures and patterned surfaces [156-158]. In a related reaction pathway, evidence could be obtained for in-situ complexation and metal center-induced switching of phenanthroline-based catenane units deposited the Ag(lll) surface [182]. 

Finally, a DEER study on models for molecular wires made from butadiyne-linked zinc porphyrin oligomers, end-labeled with nitroxide radicals, was performed by Lovett, Anderson, and coworkers [107]. Unlike in [104—106], one can control the conformations of these metalloporphyrin-based strucmres by self-assembly with multidentate amine ligands, which bend the rigid oligomeric structure. The experimentally found end-to-end distances in these complexes match the predictions from molecular dynamics calculations. This study thus presents a proof-of-principle that DEER spectroscopy is also well suited for understanding more complex supramolecular stmctures. 

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