Significant Effect of Electronic Coupling on Electron Transfer between Surface-Bound Porphyrins and Co^2+/3+ Complex Electrolytes

Abstract

The effect of electronic coupling on electron transfer (ET) rates has been considered minor and often ignored compared to the effect of driving force (ΔG°) between similarly structured donor and acceptor molecules. In this work, ET rates are determined between surface-bound Zn or free-base porphyrin electron acceptors and five similarly structured Co2+/3+ complex electron donors covering a ΔG° range of 0.78 eV. Unexpectedly, [Co­(bpy)3]2+/3+ showed the fastest ET rates that were attributed to the highest coupling among the Co2+/3+ complexes. The difference in coupling is explained by the increased tunneling distance due to the additional size of substituted Co2+/3+ complexes. To elucidate if factors other than the distance affect the coupling, we propose a protocol using normalization of the measured ET rates with the square of the ratio in coupling and fitted to Marcus theory. In acetonitrile, no significant difference in electronic coupling between free-base and Zn porphyrins was found. Measured ET rates in the five times lower dielectric constant solvent 1,2-dimethoxyethane resulted in a 7-fold enhancement in ET rates involving free-base porphyrins compared to their Zn analogues, implying that the partial charge could be used to enhance the coupling. The key finding here is that electronic coupling can be as a significant contributor to ET rates as the ΔG° even among similarly structured molecules. In view of design, to control ET rates the structure should be designed not only to decrease ΔG° but also to enhance the electronic coupling.

Probing Interfacial Halogen-Bonding Effects with Halogenated Organic Dyes and a Lewis Base-Decorated Transition Metal-Based Redox Shuttle at a Metal Oxide Interface in Dye-Sensitized Solar Cells

“…We note that accurately quantifying regeneration efficiencies is challenging. 62,63 We have used a simplified equation set common in the literature which likely overestimates the regeneration efficiencies as others have noted. 64 A modified approach to these equations is presented in the Supporting Information which attempts to account for competitive electron-transfer reactions in the presence of an RS; however, we note that the values presented are only estimates with the goal of showing trends in dye behavior for this study.…”

“…Future studies are focused on exploring other noncovalent RS−dye interactions within these systems such as those recently highlighted in the literature. 63 ■ ASSOCIATED CONTENT * sı Supporting Information…”

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

“… Electron transfer influenced by H DA values due to different electronic tunnelling distance of a series of alkyl‐substituted Co complexes [40] . I. Cho et al., J. Phys.…”

“…However, it was not explicitly demonstrated whether the origin of the faster rate is electrostatic interaction or any other factors such as blocking effects of bulkier group or lower reorganisation energy. A systematic study on the electrostatic effect on the charge transfer kinetics was reported by us in 2020, [40] taking account the effects of H DA and λ. In the study, density functional theory (DFT) calculations revealed that free‐base porphyrin dyes with lone‐pair electrons have higher electron density as compared to the structurally similar Zn‐substituted analogues.…”

“…Our group recently reported that even small substitutions of Co complexes such as methyl, methoxy, and halogen groups used to tune the redox potential of can significantly decrease H DA hence slow down regeneration rates when porphyrin dyes were used as electron acceptors. [40] The simple porphyrin dyes employed in the study had geometrically open ring structure with 18 π-conjugated electrons providing strong H DA . One of the fastest regeneration rates was (3.6 × 10 7 s À 1 ) reported in the literature.…”