Different orientations of large rigid organic chromophores at the rutile $\mathrm{Ti}{\mathrm{O}}_2$ surface controlled by different binding geometries of specific anchor groups

Polarization and angle-resolved two-photon photoelectron spectroscopy was employed to determine the adsorption geometry of di-tert-butyl-perylene when anchored via two different acid groups on rutile $\mathrm{Ti}{\mathrm{O}}_2\left(110\right)$. With the carboxylic acid group as anchor and a rigid bridge group the binding geometry of the chromophore was found with the long molecular axis perpendicular to the surface. In contrast, with the phosphonic acid as anchor group the long axis of perylene showed a tilt angle of around 66° with respect to the surface normal and an alignment in the direction perpendicular to [001]. Our experimental results agree with adsorption geometries recently predicted from DFT calculations by Persson’s group.

Figure 1

Chemical structure of DTB-Pe-acrylic acid (1) and DTB-Pe-rod (2).

Figure 2

Surface structure and crystal directions of rutile $\mathrm{Ti}{\mathrm{O}}_2$ (110).

Figure 3

2PPE kinetic energy spectra of DTB-Pe-acrylic acid with the pump pulse $p$-polarized (filled circle) and $s$-polarized (open circle). The background signal stemming from monochromatic two-photon processes was measured separately, and subtracted from both spectra.

Figure 4

Diagram of the experimental setup for angle-resolved 2PPE measurements.

Figure 5

Schematic representation of a plane, linear polarized wave incident at an angle $\Theta$.

Figure 6

Peak height of the 2PPE difference signal between $p$- and $s$-polarized pump pulse as a function of the angle of incidence (black) for DTB-Pe-acrylic acid on rutile measured with the plane of incidence perpendicular (a) and parallel (b) to [001]. Plot of Eq. (6) (solid).

Figure 7

Coordinate systems used for Eq. (10). $[x,y,z]$ is fixed with respect to the $\vec{E}$ field and $[\tilde{x},\tilde{y},\tilde{z}]$ is fixed with respect to the long molecular axis.

Figure 8

Peak height of the 2PPE difference signal between $p$- and $s$-polarized pump pulse as a function of the angle of incidence for DTB-Pe rod on rutile (110) (black), measured with the plane of incidence perpendicular (a) and parallel (b) to [001]. Plot of equation (10) (solid).