Conformational and Binding Effects on Interfacial Electron Transfer from Dual-Linker Sensitizers

Interfacial
electron transfer (IET) between novel dual-linker perylenes
and TiO2 was investigated by ultrafast spectroscopy and
nonadiabatic quantum mechanics/molecular mechanics (QM/MM) simulations.
The influence of the number of linkers (one vs two) and their substitution
position (ortho vs peri) on IET was investigated. Perylene sensitizers
derivatized with two acrylic acid linkers in the peri position, pDtBuPe­(C2H2COOH)2 (bis-peri), and in the ortho
position, oDtBuPe­(C2H2COOH)2 (bis-ortho),
were compared to their single-linker counterparts: pDtBuPeC2H2COOH (peri) and oDtBuPeC2H2COOH
(ortho). MM simulations of the bis-peri and bis-ortho sensitizers
predicted that, in both cases, only one linker binds covalently to
the {101} surface of anatase TiO2, while the second one
binds weakly via vdW or hydrogen-bonding interactions. Results from
QM/MM simulations corroborated experimental injection times for all
compounds, thereby supporting single-linker binding to the surface
for the dual-linker sensitizers. The study showed that, surprisingly,
IET from bis-peri was significantly slower than that from peri. Theory
attributes this behavior to small conformational changes caused by
steric interactions. The degrees of freedom that are most strongly
correlated with variations in IET times were identified by principal
component analysis. Comparison between bis-ortho and ortho showed,
as expected, that the dual-linker sensitizer injects faster than the
single-linker variant. However, the influence of the substitution
position on IET was much smaller than expected based on the difference
in the static electronic structure. Finally, the strong effect of
conformational changes on IET was also observed experimentally in
modulations of IET due to coupling to vibrational modes, in excellent
agreement with the QM/MM simulations. This study shows that small
conformational changes and vibrational coupling can have a significant
influence on IET even on the femtosecond time scale. The addition
of linkers that differ in bonding strength and serve different functions
opens up new avenues for controlling IET dynamics.