Photophysical and electrochemical properties of platinum(ii) complexes bearing a chromophore–acceptor dyad and their photocatalytic hydrogen evolution

Abstract

A series of platinum(II) complexes bearing a chromophore–acceptor dyad obtained by reacting 4-(p-bromomethylphenyl)-6-phenyl-2,2′-bipyridine or 4′-(p-bromomethylphenyl)-2,2′:6′,2′′-terpyridine with pyridine, 4-phenylpyridine, 4,4′-bipyridine, 1-methyl-4-(pyridin-4′-yl)pyridinium hexafluorophosphate respectively, were synthesized. Their photophysical properties, emission quenching studies by Pt nanoparticles and methyl viologen, electrochemical properties and photoinduced electron-transfer reactions in a photocatalytic hydrogen-generating system containing triethanolamine and colloidal Pt without an extra electron relay, were investigated. A comparison of the rates of hydrogen production for the two photocatalytic systems, one containing a metal–organic dyad and the other comprising a 1 : 1 mixture of the parental platinum(II) complexes and the corresponding electron relay, showed that intramolecular electron transfer improves the photocatalytic efficiency. Compared with cyclometalated platinum(II) complexes, the related platinum(II) terpyridyl complexes exhibited poor performance for photocatalytic hydrogen evolution. An investigation into the amount of hydrogen generated by three platinum(II) complexes containing cyclometalated ligands with methyl groups located on different phenyl rings revealed that the efficiency of hydrogen evolution was affected by a subtle change of functional group on ligand, and the hydrogen-generating efficiency in the presence or absence of methyl viologen is comparable, indicating electron transfer from the excited [Pt(C^N^N)] chromophore to colloidal Pt. ¹H NMR spectroscopy of the metal–organic dyads in an aqueous solution in the presence of excess triethanolamine revealed that the dyad with a viologen unit was unstable, and a chemical reaction in the compound occurred prior to irradiation by visible light under basic conditions.