Exploiting Intermolecular Interactions between Alkyl-Functionalized Redox-Active Molecule Pairs to Enhance Interfacial Electron Transfer

Abstract

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.

Effect of Molecular Structure on Interfacial Electron Transfer Kinetics in the Framework of Classical Marcus Theory

“…Attractive intermolecular interactions between electron donor and acceptor have been a recently emerging factor to enhance interfacial electron transfer. In 2018, we reported a way to enhance electron transfer kinetics when a densely packed dye layer is present [33] . Unexpectedly, when both a dye (MK2) and a redox mediator had long alkyl chains, electron transfer was enhanced due to alkyl‐alkyl intermolecular interactions (see Figure 17).…”

“…In 2018, we reported recombination rates in DSCs using a series of alkyl‐substituted Co complexes paired with dye molecules decorated with hexyl chains [33] . The recombination rates were slowed down by a factor of two using nonyl‐substituted Co complexes as compared to that of the methyl‐substituted Co complex at the same Δ G ° and electron density.…”

“…In 2018, we reported recombination rates in DSCs using a series of alkyl-substituted Co complexes paired with dye molecules decorated with hexyl chains. [33] The recombination Figure 11. Chemical structures of hexyl-substituted organic dyes MK-1, MK-2, and a non-substituted dye MK-3.…”

Heterogenization of Molecular Water Oxidation Catalysts in Electrodes for (Photo)Electrochemical Water Oxidation

“…Therefore, to address these issues, several alternatives have been used. On the one hand, functional groups that are used as anchors have been introduced further from the active side by using long alkyl chains as spacers in the linkers [75,83,84]. This approach causes fewer electronic changes in the active side of WOCs as well as minimizes direct interactions with the (photo)electrode surface, preserving their catalytic activity and minimizing electron-hole recombination, respectively.…”

Tailored Assembly of Molecular Water Oxidation Catalysts on Photoelectrodes for Artificial Photosynthesis

“…It is well‐known that the introduction of electron withdrawing and donating groups has a significant effect on their redox potentials and catalytic activity , , . To address the former issue, Wang et al suggested the utilization of anchoring groups with long alkyl chains , , . They found that long‐chain anchors are more effective than shorter ones to preserve catalytic activity of molecular electrocatalysts.…”