Intramolecular Hydrogen-Bonded Networks for Tuning Anthraquinone Redox Properties

Intramolecular hydrogen-bonded networks have been designed and synthesized to control quinone redox thermodynamics in a family of anthraquinone (AQ) derivatives. The networks contain imidazole fused to AQ, which is hydrogen-bonded to the AQ carbonyl, and the extended motifs include phenol (P) and benzimidazole (BI). Cyclic voltammetry reveals a systematic anodic displacement of the AQ/AQ•- couple as the hydrogen-bonded network is extended, resulting in a cumulative +440 mV shift. Visible spectroelectrochemistry provides direct reduced-state markers: AQ exhibits AQ•- bands at ∼410 and ∼539 nm, whereas the intramolecular hydrogen-bonded constructs display only blue-shifted AQ•- signatures at ∼490 nm (BI-AQ), ∼505 nm (BIP-AQ), and ∼494 nm (BI2P-AQ), consistent with hydrogen-bond-dependent perturbation of the reduced quinone electronic structure. Infrared spectroelectrochemistry further resolves the underlying hydrogen-bonding interactions, revealing depletion of the neutral AQ ν(C═O) band (∼1667 cm-1) under reductive polarization with the emergence of hydrogen-bonded AQ•- band at ∼1480 cm-1 in the AQ derivatives. Oxidative polarization, yielding the phenoxyl radical, induces benzimidazolium formation and a blue-shifted AQ ν(C═O) band (∼1678 cm-1), highlighting the directional dependence of the perturbation and the role of the AQ carbonyl as a reporter of the interaction with the intramolecular hydrogen-bonded network. Treating the hydrogen-bonding protons quantum mechanically, nuclear-electronic orbital density functional theory calculations show strengthening of the hydrogen bonds cooperatively across the network upon one-electron reduction and oxidation, correctly predicting the observed stabilization of the AQ•- state. These fundamental insights will assist in the design of more precisely tuned catalysts.