Proton-Coupled Intervalence Charge-Transfer in a Mixed-Valent Phenol-Benzimidazole Dimer

<jats:p>A phenol-benzimidazole dimer, t-d(PI)B, was designed to examine proton-coupled intervalence charge transfer (PC-IVCT) in a symmetric mixed-valence mixed-protonation state. Electrochemical measurements reveal two reversible single-electron oxidation processes separated by &gt;200 mV. Infrared spectroelectrochemistry and density functional theory modeling confirm that the singly oxidized form exists in a mixed-protonation state, in which the phenoxide proton has transferred to the adjacent benzimidazole. An IVCT transition is observed in the near-infrared region, the intensity of which decreases upon deuteration of the phenolic protons, yielding an isotope effect of ~1.8. Electron-vibrational calculations show that an imidazolium NH stretching mode is strongly coupled to the optical IVCT process. Together, the electrochemical and spectroscopic properties are consistent with a class II mixed-valence species whose behavior is governed by protonation changes at the redox-active phenols.</jats:p>