Abstract
Heterogeneous electron transfer (HET) between photoexcited molecules and
colloidal TiO
2
has been investigated for a set of Zn-porphyrin
chromophores attached to the semiconductor via linkers that allow to change
level alignment by 200 meV by reorientation of the dipole moment. These unique
dye molecules have been studied by femtosecond transient absorption spectroscopy
in solution and adsorbed on the TiO
2
colloidal film in vacuum. In
solution energy transfer from the excited chromophore to the dipole group has
been identified as a slow relaxation pathway competing with
S
2
-S
1
internal conversion. On the film heterogeneous
electron transfer occurred in 80 fs, much faster compared to all intramolecular
pathways. Despite a difference of 200 meV in level alignment of the excited
state with respect to the semiconductor conduction band, identical electron
transfer times were measured for different linkers. The measurements are
compared to a quantum-mechanical model that accounts for electronic-vibronic
coupling and finite band width for the acceptor states. We conclude that HET
occurs into a distribution of transition states that differs from regular
surface states or bridge mediated states.