Molecular rectification by a bimetallic Ru-Os compound anchored to nanocrystalline TiO2

The coordination compound [Ru(dcb)(2)(Cl)-bpa-Os(bpy)(2)(Cl)](PF6)(2), abbreviated Ru-bpa-Os, where dcb is 4,4'-(COOH)(2)-2,2'-bipyridine and bpa is 1,2-bis(4-pyridyl)ethane, was anchored to nanocrystalline (anatase) TiO2 films for interfacial electron transfer studies. Blue or green light excitation results in the sequence of electron transfer events shown schematically where the Ru chromopihore injects an electron into the semiconductor, step 1, and the Os(II) center reduces Ru(III), step 2, to farm an interfacial charge-separated state abbreviated TiO2(e(-))\Ru-bpa-Os(III). This same charge-separated state, TiO2(e(-))\Ru-bpa-Os(III), can be formed by a novel remote electron injection process from the Os chromophore to the semiconductor solid. The electron injection rates into the semiconductor are at least 5 orders of magnitude faster than charge recombination. This is the first example of a bimetallic coordination compound that rectifies charge at a semiconductor interface.