Electrocatalytic Activity of an Immobilized Cofacial Diporphyrin Depends on the Electrode Material

Abstract

A thiolated bis(cobalt) cofacial diporphyrin chemisorbed on an edge plane pyrolytic graphite electrode has the electrocatalytic activity of a four-electron (n g 3.6) dioxygen reduction catalyst. When chemisorbed on a gold electrode surface, the same catalyst exhibits the activity of only a two-electron catalyst, producing hydrogen peroxide (n ) 2.1). The edge plane graphite surface thus plays a crucial, but not understood, role in designed dioxygen reduction catalysis. Analysis of X-ray photoelectron spectroscopy and UV-vis results is consistent with the rings of the thiolated porphyrins being coplanar to the Au electrode plane. A structurally modified catalyst exhibits greater coplanarity and a slight increase in activity (n ≈ 2.4). The present results set the stage for a strategy of cochemisorbing functionalities onto the thiolated diporphyrin-coated Au surface, seeking those functionalities which will chemically mimic the graphite surface and elevate the catalytic reactivity to a four-electron dioxygen reduction. Such functionalities could include host-guest cochemisorption of putative carbon surface ligands within the porphyrin electrode cavity. † Present address: Collman, J. P.; Hutchison, J. E.; Lopez, M. A.; Tabard, A.; Guilard, R.; Seok, W. K.; Ibers, J. A.; L'Her, M. J. Am. Chem. Soc. 1992, 114, 9869-9877.(3) (a) Collman, J. P.; Hendricks, N. H.; Leidner, C. R.; Ngameni, E.; L'Her, M. Inorg. Chem. 1988, 27, 387. (b) Collman, J. P.; Wagenknecht, P. S.; Hutchison, J. E.

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“…They deduced the red-shifted Soret band as a result of off-edge configuration, which was later ascribed to the head-totail dipolar interactions between the neighboring porphyrin molecules. 15 Similar red shifts were additionally noted in other covalently attached porphyrins in the form of SAMs, [16][17][18] as well as in a solidstate (i.e., evaporated) porphyrin film. 19 With respect to the excitonic coupling theory of the electronic transitions in porphyrin π systems, the face-to-face π-π stacking interaction responsible for a perpendicular (i.e., stack-of-cards) orientation has been well-known to produce a blue-shifted Soret band with or without band spitting.…”

“…By applying the rule to the observed SEIRA spectra with a combination of the UV-vis results, the molecular orientation is more consistent with an inclined configuration-the adsorbed porphyrin molecules aligned the rings parallel to each other, but the molecular plane is likely tilted, at least to some extent, with respect to the gold island surface. 17 However, a clearer analysis of the ring alignment must await some additional in-depth spectroscopic studies. in a KBr matrix and Fig.…”

“…12,13 Enormous efforts have been made to study porphyrins in the form of a chemisorbed (i.e., self-assembled) monolayer. [14][15][16][17][18][19][20] Considering the chemical structures shown in Fig. 1, the porphyrin molecules used in this study, on the other hand, tend to physisorb on a metal island surface by van der Waals forces-no chemical interaction produced.…”

Proton Pump for O2 Reduction Catalyzed by 5,10,15,20‐Tetraphenylporphyrinatocobalt(II)

“…[13][14][15] In most cases, monomeric cobalt porphyrins catalyze the electroreduction of O 2 to hydrogen peroxide (H 2 O 2 ), whereas dimeric cofacial cobalt porphyrins demonstrate the catalysis of four-electron reduction of O 2 to water. [3,7,8,10,12,15,16,[24][25][26][27] Employing ferrocene derivatives as electron donors, O 2 reduction catalyzed by various cobalt porphyrins has been investigated by Fukuzumi et al in organic media in the presence of HClO 4 . [13][14][15] In the reaction scheme, the steps of electron coordination to form a superoxide adduct and its reduction by ferrocene derivatives to produce H 2 O 2 /H 2 O and Co III are fast, and that of reducing Co III by ferrocene derivatives is slow and rate limiting.…”

Química de sistemas supramoleculares constituídos por porfirinas e complexos metálicos

“…Apesar dos inúmeros estudos realizados até o momento, ainda não existe uma explicação definitiva para os fatores que, de fato, levam à ativação das Co(P) como catalisadores para a redução do O 2 à H 2 O. Por exemplo, foi mostrado em estudos recentes que simplesmente trocando-se o material do eletrodo, de carbono pirolítico para ouro 91 , pode-se provocar a mudança no mecanismo de tetraeletrônico para bieletrônico, mesmo no caso da cobalto porfirina cofacial de Collman e da [Co(4-TPyP){Ru(NH) 3 } 4 ]. Além disso, foi verificado que a presença de um íon Cu(II) nas proximidades da metaloporfirina não altera significativamente a atividade de compostos-modelo de citocromo-c oxidase 92 .…”