Photochemical hydrogen evolution via singlet-state electron-transfer quenching of zinc tetra(N-methyl-4-pyridyl)porphyrin cations in a zeolite L based system

Abstract

Molecular electron transport chains composed EDTA, zinc tetra (N-methyl-4-pyridyl)porphyrin(ZnTMPyP/sup 4 +/), and methylviologen (MV/sup 2 +/), spatially organized by 1 ..mu.. diameter zeolite L particles, were studied. MV/sup 2 +/ ion exchanges into zeolite L to a maximum loading of 2.5-3.0 x 10/sup -4/ mol/g of zeolite, while the bulkier ZnTMPyP/sup 4 +/ adsorbs only onto the outer surface in approximately monolayer (8 x 10/sup -6/ mol/g) quantities. At pH 4.0, EDTA/sup 2 -/ is strongly adsorbed onto the ZnTMPy/sup 4 +/-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/sup -3/ M EDTA solution. The rate of hydrogen evolution depends on the MV/sup 2 +/ loading, no H/sub 2/ being evolved below 1.5 x 10/sup -4/ mol MV/sup 2 +//g (ca. 0.4 MV/sup 2 +/ ion per large cavity). ZnTMPyP/sup 4 +/ 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 MV/sup 2 +/ loading as the hydrogen evolution rate. The slowly decaying fluorescence component andmore » the time-resolved triplet-triplet absorbance are invariant with MV/sup 2 +/ loading. These observations are explained in terms of single-state electron-transfer quenching of ZnTMPyP/sup 4 +/ by MV/sup 2 +/. The triplet excited state reactivity of ZnTMPyP/sup 4 +/ is suppressed by a 200-mV positive shift of its redox potentials caused by adsorption onto the zeolite surface.« less