The Significant Effect of Electronic Coupling on Electron Transfer between Surface-Bound Porphyrins and Co2+/3+ Complex Electrolytes

Abstract

The coordination of metal ions such as Zn2+ with the π-conjugated porphyrin ring is one of the simplest and the most often used structural modification to alter the electronic nature of a molecule without significantly affecting its size or geometry. The question answered in this work is to what extent electron transfer rates between a surface bound Zn2+porphyrin and a cationic electron donor (Co2+ polypyridyl complexes) is affected by the altered electrostatic interaction between the two molecules at the same electrochemical driving force (DG°). To date, very few studies addressed the effect of electrostatic intermolecular interactions on electron transfers explicitly.We measured electron transfer rates between 4 porphyrin (2 freebase and 2 Zn porphyrin) molecules attached to TiO2 electrodes and 5 Co2+/3+ complexes dissolved in electrolytes covering a DG° range of close to 1 V. The porphyrin molecules were photo-excited at 532 nm leading to an oxidized porphyrin and an electron injected into the conduction band of the TiO2 electrode. The rate of electron transfer was been determined using transient absorption spectroscopy (TAS) by monitoring the absorption change of the oxidized porphyrin molecules in the presence of the redox mediators in various concentrations. The measured rate constants were fitted to Marcus theory. We have found that even the simplest dimethyl substitution on the bipyridine ligands of the Co2+/3+ complexes resulted in a significant decrease in electron coupling. To account for the variation in electron coupling among the Co2+/3+ complexes, the electron rate constants were normalized by the square of the ratio of the electronic coupling factors. The normalized rate constants showed a 30% increase in electron coupling of the free base porphyrins with Co2+/3+ compared to the electronic coupling between Zn porphyrins and Co2+/3+. Increased coupling suggests shorter donor – acceptor distance or better wavefunction overlap, which could be due to electrostatic interaction of the Co2+ with the lone pairs of the free base porphyrin, which interaction is blocked by Zn2+ coordination. The measurements were also performed in 1,2-dimethoxyethane instead of acetonitrile with a five times lower dielectric constant, resulting in even larger difference in electron transfer rates between free base and Zn porphyrins. This suggests that the origin of the different coupling is indeed electrostatic in nature.The key finding of this work is that electronic coupling can be as significant contributor to electron transfer rates as the driving force even between similar molecular structures, hence redox active molecules should not be selected based on their free energy difference alone.