Photochemical hydrogen production with molecular devices comprising a zinc porphyrin and a cobaloxime catalyst

Two new noble-metal-free molecular devices, {Co(dmgH)₂Cl}{Zn(PyTPP)}] (1, dmgH = dimethyloxime, PyTPP = 5-(4-pyridyl)-10,15,20-triphenylporphyrin) and {Co(dmgH)₂Cl}{Zn(apPyTPP)}] (2, apPyTPP = 5-4-(isonicotinamidyl)phenyl]-10,15,20-triphenylporphyrin), for light-driven hydrogen generation were prepared and spectroscopically characterized. The zinc porphyrin photosensitizer and the Co^III-based catalyst unit are linked by axial coordination of a pyridyl group in the periphery of zinc-porphyrin to the cobalt centre of catalyst with different lengths of bridges. The apparent fluorescence quenching and lifetime decays of 1 and 2 were observed in comparison with their reference chromophores, Zn(PyTPP) (3) and Zn(apPyTPP) (4), suggesting a possibility for an intramolecular electron transfer from the singlet excited state of zinc porphyrin unit to the cobalt centre in the molecular devices. Photochemical H₂-evolving studies show that complexes 1 and 2 are efficient molecular photocatalysts for visible light-driven H₂ generation from water with triethylamine as a sacrificial electron donor in THF/H₂O, with turnover numbers up to 46 and 35 for 1 and 2, respectively. In contrast to these molecular devices, the multicomponent catalyst of zinc porphyrin and Co(dmgH)₂PyCl] did not show any fluorescence quenching and as a consequence, no H₂ gas was detected by GC analysis in the presence of triethylamine with irradiation of visible light. The plausible mechanism for the photochemical H₂ generation with these molecular devices is discussed.