Photochemical Hydrogen Evolution via Singlet-State Electron-Transfer Quenching of Zinc Tetra(N-methyl-4-pyridyl)porphyrin Cations in a Zeolite L Based System

Lalchan Persaud, Allen J. Bard, Alan Campion, Marye Anne Fox, Thomas E. Mallouk, Stephen E. Webber, J. M. White

Research output: Contribution to journalArticle

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Abstract

Molecular electron transport chains composed of EDTA, zinc tetra(/V-methyl-4-pyridyl)porphyrin (ZnTMPyP4+), and methylviologen (MV2+), spatially organized by 1 Am diameter zeolite L particles, were studied. MV2+ ion exchanges into zeolite L to a maximum loading of 2.5-3.0 X 10“4mol/g of zeolite, while the bulkier ZnTMPyP4+ adsorbs only onto the outer surface in approximately monolayer (8 X 10–6mol/g) quantities. At pH 4.0, EDTA2- is strongly adsorbed onto the ZnTMPy4+-coated surface. When the composite is prepared from internally platinized zeolite L particles, hydrogen is evolved photochemically from water in pH 4.0, 2 X 10–3M EDTA solution. The rate of hydrogen evolution depends on the MV2+ loading, no H2 being evolved below 1.5 X 1CT4mol MV2+/g (ca. 0.4 MV2+ ion per large cavity). ZnTMPyP4+ shows a biphasic fluorescence decay when adsorbed on the zeolite L surface. The rapidly decaying component has a lifetime varying from <20 ps to 150 ps; the inverse lifetime (fluorescence decay rate) shows the same dependence on MV2+ loading as the hydrogen evolution rate. The slowly decaying fluorescence component and the time-resolved triplet-triplet absorbance are invariant with MV2+ loading. These observations are explained in terms of singlet-state electron-transfer quenching of ZnTMPyP4+ by MV2+. The triplet excited state reactivity of ZnTMPyP4+ is suppressed by a 200-mV positive shift of its redox potentials caused by adsorption onto the zeolite surface.

Original languageEnglish (US)
Pages (from-to)7309-7314
Number of pages6
JournalJournal of the American Chemical Society
Volume109
Issue number24
DOIs
StatePublished - Nov 1 1987

All Science Journal Classification (ASJC) codes

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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