Solid-phase synthesis and photoactivity of Ru-polypyridyl visible light chromophores bonded through carbon to semiconductor surfaces.

1,10-Phenanthroline (phen) was grafted to either indium tin oxide (ITO), fluorine-doped tin oxide (FTO), or titanium dioxide (TiO2) semiconductors (SC's) by electrochemical reduction of 5-diazo-phen. The phen ligand is bonded to the semiconductor at C5, and it can be handled in air. The semiconductor-phen (SC-phen) complexes displace both CH3CN ligands from either cis-[Ru(Mebipy)2(CH3CN)2]2+ (Mebipy = 4,4'-methyl-2,2'-bipyridine), cis-[Ru(tBubipy)2(CH3CN)2]2+ (tBubipy = 4,4'-tert-butyl-2,2'-bipyridine), or cis-[Ru(pheno)(bipy)(CH3CN)2]2+ (bipy = 2,2'-bipyridine; pheno = 1,10-phenanthroline-5,6-dione) dissolved in DCM/THF (4 h, 70 °C) to form the corresponding surface-bound SC-[(phen)Ru(bipyridyl)2]2+ chromophores. The identities of the SC-[(phen)Ru(Mebipy)2]2+, SC-[(phen)Ru(tBubipy)2]2+, and SC-[(phen)Ru(pheno)(bipy)]2+ (SC = ITO, FTO or TiO2) chromophores were confirmed by X-ray photoelectron spectroscopy (XPS); inductively coupled plasma mass spectrometry (ICP-MS); UV-vis and reflectance infrared spectroscopies; and cyclic voltammetry (CV). The data were compared to analogous Ru-polypyridyl control compounds dissolved in solution. A facile ketone-amine condensation solid-phase synthesis reaction between SC-[(phen)Ru(pheno)(bipy)]2+ and [Ru(1,10-phenthroline-5,6-diamine)(bipy)2]2+ in ethanol (80 °C, 1 h) formed the dinuclear, bound chromophore SC-[(phen)(bipy)Ru(tpphz)Ru(bipy)2]4+ (tpphz = tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine). Photoelectrochemical oxidation of hydroquinone and triethylamine under acidic, neutral, or basic conditions showed that the SC-chromophore photoanodes are active, and that TiO2-[(phen)Ru(Mebipy)2]2+ is the most active and stable under basic- and neutral conditions. The dinuclear chromophore SC-[(phen)(bipy)Ru(tpphz)Ru(bipy)2]4+ was most active and stable under potentiostatic conditions in acid.

Modular Construction of Photoanodes with Covalently Bonded Ru- and Ir-Polypyridyl Visible Light Chromophores.

1,10-phenanthroline is grafted to indium tin oxide (ITO) and titanium dioxide nanoparticle (TiO2) semiconductors by electroreduction of 5-diazo-1,10-phenanthroline in 0.1 M H2SO4. The lower and upper potential limits (-0.20 and 0.15 VSCE, respectively) were set to avoid reduction and oxidation of the 1,10-phenanthroline (phen) covalently grafted at C5 to the semiconductor. The resulting semiconductor-phen ligand (ITO-phen or TiO2-phen) was air stable, and was bonded to Ru- or Ir- by reaction with cis-[Ru(bpy)2(CH3CN)2]2+ (bpy = 2,2'-bipyridine) or cis-[Ir(ppy)2(CH3CN)2]+ (ppy = ortho-Cphenyl metalated 2-phenylpyridine) in CH2Cl2 and THF solvent at 50 °C. Cyclic voltammetry, X-ray photoelectron spectroscopy, solid-state UV-vis, and inductively coupled plasma-mass spectrometry all confirmed that the chromophores SC-[(phen)Ru(bpy)2]2+ and SC-[(phen)Ir(ppy)2]+ (SC = ITO or TiO2) formed in near quantitative yields by these reactions. The resulting photoanodes were active and relatively stable to photoelectrochemical oxidation of hydroquinone and triethylamine under neutral and basic conditions.