Syntheses, photophysical properties, and reactivities of novel bichromophoric Pd complexes composed of Ru(II)-polypyridyl and naphthyl moieties

A series of novel bichromophoric Ru(II) complexes bearing a naphthyl chromophore linked with 2,2'-bipyridyl (bpy) and 2,2'-bipyrimidine (bpm) ligands were synthesized. Complexation with a Pd-alkyl unit led to the formation of bichromophoric Ru-Pd catalysts. The photochemical and electrochemical properties of the newly synthesized compounds were studied. It has been shown that the UV-light energy absorbed at the pendant naphthyl moiety is effectively transferred to the Ru(II) center; this is supported by the observation of metal-to-ligand charge-transfer luminescence through excitation of the naphthyl unit, which is identical to the luminescence observed through visible light excitation. The luminescence lifetime of the complex bearing a naphthyl moiety directly connected to the bpy ligands was lengthened to ca. 1000 ns. When a Pd complex containing the bichromophoric unit with an extended lifetime was used as a catalyst, styrene polymerization under visible or UV light irradiation was observed, in clear contrast to dimerization using other catalysts. These results indicate the formation of an excited-state with a prolonged excited-state lifetime, which promotes an insertion step in preference to the competing beta-H elimination step.