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

Abstract

Modifying metal oxides (MO x ) 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|MO x interfaces, where an "inert" MO x (e.g., Al 2 O 3 ) is used as a control for an "active" MO x (e.g., TiO 2 ). An underlying assumption in these studies is that the inert and active monolayer|MO x 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), TiO 2 , ZrO 2 , and Al 2 O 3 . When the surface roughness of the MO x 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 MO x 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 TiO 2 and Al 2 O 3 with equal surface roughness, again, the orientation was the same. MD simulations of A1C and A1P on TiO 2 produced nearly identical tilt angle distributions, supporting the experimental findings. This study provides the first experimental validation of the assumption that the monolayer|MO x structure is the same regardless of the nature of the metal oxide substrate.

Molecular Orientation and Energy Transfer Dynamics of a Metal Oxide Bound Self-Assembled Trilayer

“…The dichroic ratio of the adsorbed A film was calculated by integrating the TE- and TM-polarized ATR spectra over the wavelength ranges listed in Table S1, and the mean dipole tilt angle, θ A , was obtained from the dichroic ratio, as described previously. ,, θ A is the angle between the linear transition dipole of A (aligned along the anthracene short axis through the 9,10-carbon atoms) and the surface normal to ITO substrate. , The result, 31° (±3°), agrees with our prior measurement of 29° (±5°) as well as studies of other aromatic, phosphonic acid-modified molecules adsorbed on ITO, and other metal oxide substrates. , The estimated surface coverage of A was 3.6 (±0.55) × 10 –10 mol/cm –2 , which is approximately one monolayer (ML) on an atomically flat substrate . However, since the rms surface roughness of the ITO is about 0.5 nm, the actual surface coverage is slightly less than one ML.…”

“…1,52 The estimated surface coverage of A was 3.6 (±0.55) × 10 −10 mol/cm −2 , which is approximately one monolayer (ML) on an atomically flat substrate. 11 However, since the rms surface roughness of the ITO is about 0.5 nm, 53 the actual surface coverage is slightly less than one ML. ATR spectra were then acquired during the incubation of the A film with 100 μM zinc acetate in ethanol (Figure 2D,E).…”

“…Furthermore, although Figure 5 depicts the ITO as atomically flat, it is not�the rms surface roughness obtained from atomic force microscopy is 0.5 ± 0.1 nm (500 nm × 500 nm scan area). 53,59 Prior studies have shown that as the surface roughness of a substrate increases, the measured tilt angle distribution of an adsorbed molecular film becomes broader and is shifted toward larger angles. 53,59−61 This means that the measured tilt angles listed in Table 1 are somewhat larger than the "true" mean tilt angles.…”

“…Note that these measurements were performed on ZrO 2 , rather than ITO, because its relatively high conduction band (E cb = −2.5 V vs NHE) inhibits excited state electron injection into the metal oxide substrate from F or P. 64 However, our recent study suggests that at high surface coverage, the general structure of the assembly is similar regardless of the nature of the metal oxide. 53 The TA spectra for ZrO 2 -P-Zn-F and ZrO 2 -3 P* were acquired in MeCN to mimic previously used device conditions. 12 The solution spectrum for F was acquired in MeOH due to solubility limitations.…”

Metal Ion-Linked Molecular Multilayers on Inorganic Substrates: Structure and Applications

“…67,78 Interestingly, recent polarized visible attenuated total reflection measurements on −COOH and −PO 3 H 2 functionalized anthracene dyes indicate that regardless of binding group or metal oxide identity (i.e., Al 2 O 3 , ZrO 2 , TiO 2 , ITO), the orientation of the molecules relative to surface normal was the same. 79 This suggests that at least in some cases the coordination mode at the surface has less impact on molecular orientation than intermolecular interactions in the monolayer.…”