Electrostatic Fields Induce Accelerated Proton Coupled Electron Transfer Rates in Chlorophyll Model Compounds

Chlorophyll-based pigments are crucial mediators of redox processes in photosynthesis, serving as the primary electron donors in photosystems I and II. Despite their structural similarities, these pigments exhibit a wide range of redox potentials (0.5-1.3 V vs SHE), and little experimental insight into the origins of this variation is available. To address this deficit, we have synthesized two crown ether-appended Mg-porphyrin complexes as chlorophyll model compounds and demonstrated their ability to bind redox-inactive metal cations. Cation binding to the Mg-porphyrin complexes was found to increase their redox potentials in a manner that depends linearly on the total cationic charge felt by the complex, implicating a through-space electrostatic field effect. The corresponding 1-electron oxidized π-cation radical complexes were then prepared and characterized by UV-vis, FT-IR, and EPR spectroscopies and ESI-MS. The π-cation radical species were found to be competent for the PCET oxidation of a phenolic substrate, mimicking the reaction between photo-oxidized chlorophyll and tyrosine in photosystem II. Cation binding to the π-cation radical complexes was found to increase the rates of their PCET and ET reactions in a charge-dependent manner which could be rationalized using Marcus theory. This work provides direct experimental evidence that electrostatic fields can tune the redox potentials of chlorophyll model compounds, leading to an increase in their oxidative reactivity.