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.
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