PH-Dependence of Binding Constants and Desorption Rates of Phosphonate- and Hydroxamate-Anchored [Ru(bpy)3]2+ on TiO2 and WO3.

Abstract

The binding constants and rate constants for desorption of the modified molecular dye [Ru(bpy)3]2+ anchored by either phosphonate or hydroxamate on the bipyridine ligand to anatase TiO2 and WO3 have been measured. In aqueous media at pH 1-10, repulsive electrostatic interactions between the negatively charged anchor and the negatively charged surface govern phosphonate desorption under neutral and basic conditions for TiO2 anatase due to the high acidity of phosphonic acid (p Ka,4 = 5.1). In contrast, the lower acidity of hydroxamate (p Ka,1 = 6.5, p Ka,2 = 9.1) leads to little change in adsorption/desorption properties as a function of pH from 1 to 7. The binding constant for hydroxamate is 103 in water, independent of pH in this range. These results are true for WO3 as well, but are not reported at pH > 4 due to its Arrhenius acidity. Kinetics for desorption as a function of pH are reported, with a proposed mechanism for phosphonate desorption at high pH being the electrostatic repulsion of negative charges between the surface and the anionic anchor. Further, the hydroxamic acid anchor itself is likely the site of quasi-reversible redox activity in [Ru(bpy)2(2,2'-bpy-4,4'-(C(O)N(OH))2)]2+, which does not lead to any measurable deterioration of the complex within 2 h of dark cyclic voltammogram scans in aqueous media. These results posit phosphonate as the preferred anchoring group under acidic conditions and hydroxamate for neutral/basic conditions.