Molecular Orientation of −PO₃H₂ and −COOH Functionalized Dyes on TiO₂, Al₂O₃, ZrO₂, and ITO: A Comparative Study

Modifying metal oxides (MOx) with organic monolayers is widely employed to tailor interfacial properties in organic electronic devices and dye-sensitized solar cells. The effects of modification are frequently assessed by performing experiments on model monolayer|MOx interfaces, where an “inert” MOx (e.g., Al2O3) is used as a control for an “active” MOx (e.g., TiO2). An underlying assumption in these studies is that the inert and active monolayer|MOx structures are similar. This assumption was examined here. Using UV–vis attenuated total reflection spectroscopy, we measured the mean tilt angle of 4,4′-(anthracene-9,10-diyl)bis(4,1-phenylene)diphosphonic acid (A1P) adsorbed on indium tin oxide (ITO), TiO2, ZrO2, and Al2O3. When the surface roughness of the MOx substrate and the surface coverage (Γ) of the A1P film were constant, the orientation of A1P was the same. 4,4′-(Anthracene-9,10-diyl)bis(4,1-phenylene)dicarboxylic acid (A1C) was adsorbed on the same set of MOx substrates. The orientation of A1C and A1P on ITO was the same, which is likely due to the intermolecular interactions resulting from the high and approximately equal Γ of both films. Comparing A1C films at equal Γ on TiO2 and Al2O3 with equal surface roughness, again, the orientation was the same. MD simulations of A1C and A1P on TiO2 produced nearly identical tilt angle distributions, supporting the experimental findings. This study provides the first experimental validation of the assumption that the monolayer|MOx structure is the same regardless of the nature of the metal oxide substrate.