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Nanostructured Films of Supramolecular Metal-Cluster Porphyrins

Henrique E. Toma*, Andre L. B. Formiga, Marcelo Nakamura, Herbert Winnischofer, Ana F. Nogueira, and K. Araki

Instituto De Quimica, Universidade De São Paulo,
São Paulo 05513-970 Brazil

This is an abstract for a presentation given at the
11th Foresight Conference on Molecular Nanotechnology

 

Supramolecular assemblies of porphyrins and (m3-oxo)hexakis(m-acetato)triruthenium(III) clusters have been recently investigated in our laboratory [1,2]. They are particularly interesting, because the porphyrins are important prosthetic groups in metalloproteins and enzymes, and their combination with ancillary transition metal ions gives rise to a wide range of supramolecular, catalytic, photochemical and self-assembling properties. In special, the trinuclear ruthenium clusters provide unique porphyrin modifiers, exhibiting a peculiar triangular structure held by strong metal-metal bonds, in addition to a central m3-oxo-ruthenium and six m2-carboxylate-ruthenium bonds. The close proximity of the ruthenium ions gives rise to strong electronic and magnetic interactions, stabilizing a great number of redox states. In our previous investigations we have shown that the supramolecular species obtained by the coordination of those (m3-oxo)triruthenium(III) clusters to meso-tetrapyridylporphyrins, M(TCP), are rather efficient catalysts and electrocatalysts [3,4]. Their enhanced activity has been ascribed to the electronic effects induced by the peripheral cluster units, in particular, to their ability to self-adjust their oxidation states to the redox potential of the environment, thus enhancing the oxidizing/reducing properties of the porphyrin center [1,2].

An important characteristic of such polymetallic systems is their facility to form rather homogeneous films, allowing also the assembly of organized multi-layers by dip-coating with suitable anionic sulphonated porphyrins and phtalocyanines (MTSP) species. AFM features and molecular simulations can be seen in Fig. 1.


Fig. 1. MAC mode AFM image and molecular simulation for cluster-porphyrin films.

The molecular films exhibit electrical rectifying response, in addition to enhanced conductivity and photoaction response in the presence of light. The conductivity is strongly enhanced in the presence of reducing substrates, allowing applications as amperometric sensors. In the case of the Co-porphyrin tetracluster films, rapid 4-electron reduction of dioxygen has been demonstrated, revealing one of the most efficient tetraelectronic reduction molecular catalysts ever reported in the literature. The versatile electronic, optical and electrochemical properties of these supramolecular films are being currently exploited in our laboratory, pursuing the development of molecular and integrated devices based on thin film technology, and of a new generation of photoelectrochemical dye cell. In this presentation, typical applications will be described, including field emission and AFM characterization of the nanostructured, functional films.

Acknowledgements: FAPESP, CNPq, RENAMI, Inst. MilÍnio Materiais Complexos

References

[1] H. E. Toma and K. Araki, Coord. Chem. Rev., 196 (2000) 307.
[2] H. E. Toma, K. Araki, A. D. P. Alexiou, S. Nikolaou, S. Dovidauskas, Coord. Chem. Rev., 219 (2001) 187.
[3] K. Araki, S. Doovidauskas, H. Winnischofer, A. D. P. Alexiou and H. E. Toma J. Electroanalytical Chem. 498 (2001) 152.
[4] S. Dovidauskas, H. E. Toma, K. Araki, H. C. Sacco, Y. Iamamoto, Inorg. Chimica Acta, 305 (2000) 206.


*Corresponding Address:
Henrique E. Toma
Instituto De Quimica, Universidade De São Paulo
Caixa Postal 26077
São Paulo 05513-970 Brazil
Phone: 55 (11) 3091 3887 Fax: 55 (11) 3815 5579
Email: henetoma@iq.usp.br



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