The strategies to enhance electron transfer rates between redox-active, light-harvesting molecules attached to semiconductor surfaces and redox mediators in solution by modifying molecular structure are not fully investigated yet. Therefore, the design of molecules with controlled electron transfer rates remains a challenge. The aims of this work are to quantify the effect of long alkyl chain substitution on the electron transfer from cobalt(II/III) tris(2,2′-bipyridine) to organic molecules containing carbazole and thiophene and to demonstrate that alkyl chains can be used to enhance electron transfer between donor-acceptor pairs. To this end, we study the effect of using a combination of donor and acceptor molecules with and without alkyl chains on electron transfer kinetics. Using transient absorption spectroscopy, we show that when only the molecules or the mediators have long alkyl chains, electron transfer is slightly blocked as expected. Counterintuitively, electron transfer is up to 13 times faster when long alkyl chains are attached to both the redox-active molecules and the redox mediators. The faster electron transfer is explained by an alkyl-alkyl chain interaction between the donor/acceptor, leading to the proximity (trapping) of the redox mediators close to the π-conjugated backbone of the molecules. These results suggest that intermolecular interactions can be used to enhance the electron transfer rates significantly even with well-established insulating alkyl chains attached to molecules without changing the electrochemical driving force.
Electron
transfer kinetics between donor and acceptor molecules
in electrolytes has been described by Marcus theory using reorganization
energy (λ), electronic coupling (H), and free
energy difference (ΔG°). In solution,
the molecules can collide freely, while collision occurs only at the
exposed area of the molecules when the donors or the acceptors are
anchored onto an electrode, altering the values of λ and H. To date, these structural effects of electrode-bound
molecules have not been considered in detail. To study geometrical
effects, we fabricate TiO2 electrodes with nine different
donor-(π-bridge)-acceptor type molecules and measure the kinetics
of electron transfer from five different Co complexes in electrolytes.
For densely adsorbed electrodes, the molecules with larger donor moieties
have faster reduction kinetics and the kinetics are independent of
the length of the π-bridge. When the amount of the adsorbed
molecules is reduced, the kinetics become faster and the kinetics
depend on the π-bridge length. These phenomena can be partially
correlated to the increased exposed area of the molecules to the electrolyte.
By fitting the data, we obtain lower λ values for lower dye-loading
conditions, which is not expected if only the effect of solvent molecules
is considered. Obtained H values with various geometries
suggest that it is important not only to increase the exposed area
but also to expose the point giving high H values
to increase the kinetics. One example found is designing molecules
with small molecular orbitals to increase H values,
though this would also give large λ values.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.