Significant Effect of Electronic Coupling on Electron Transfer between Surface-Bound Porphyrins and Co2+/3+ Complex Electrolytes
Copyright © 2020 American Chemical Society. 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.