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

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, EDTA^2- 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.