Energy Alignment, Molecular Packing, and Electronic Pathways: Zinc(II) Tetraphenylporphyrin Derivatives Adsorbed on TiO2(110) and ZnO(11–20) Surfaces

The relation between energy alignment, adsorption geometry, and electron transfer between a chromophore and an oxide surface has been explored for a series of Zn(II) tetraphenylporphyrin derivatives adsorbed on TiO2(110) and ZnO(112̅0) surfaces. The electronic occupied and unoccupied structure has been obtained using UV-photoemission and inverse photoemission spectroscopies. From these results, a full picture of the energetics at the chromophore–oxide interface was established. The alignment of the molecular levels relevant for optical transition was found independent of the functionalization of the meso-phenyl groups. However, to explain the observation of different optical properties and electron transfer efficiencies of these different dyes, the adsorption geometry of two of these dyes was determined using scanning tunnel microscopy and near edge absorption fine structure spectroscopy. Functionalization of the meso-phenyls with COOH groups in the meta-position results in the ZnP macrocycle adsorbed parallel to the surface. Functionalization of the meso-phenyl groups with COOH groups in the para position results in a bounding geometry where the ZnP macrocycle makes an angle of ∼50° from the surface normal. This geometry, which allows face-to-face stacking of the porphyrin rings, opens a new electronic channel for exciton delocalization that competes with direct electron injection into the substrate conduction band.