Benzoquinones can undergo reversible reductions and are attractive candidates for use as active materials in green carbon-based batteries. Related compounds of potential utility include 4,4′-diphenoquinones, which have extended quinonoid structures with two carbonyl groups in different rings. Diphenoquinones are a poorly explored class of compounds, but a wide variety can be synthesized, isolated, crystallized, and fully characterized. Experimental and computational approaches have established that typical 4,4′-diphenoquinones have nearly planar cores in which two cyclohexadienone rings are joined by an unusually long interannular CC bond. Derivatives unsubstituted at the 3,3′,5,5′-positions react readily by hydration, dimerization, and other processes. Association of diphenoquinones in the solid state normally produces chains or sheets held together by multiple C–H···O interactions, giving structures that differ markedly from those of the corresponding 4,4′-dihydroxybiphenyls. Electrochemical studies in solution and in the solid state show that diphenoquinones are reduced rapidly and reversibly at potentials higher than those of analogous benzoquinones. Together, these results help bring diphenoquinones into the mainstream of modern chemistry and provide a foundation for developing redox-active derivatives for use in carbon-based electrochemical devices.

Quinonoid compounds play central roles as redox-active agents in photosynthesis and respiration and are also promising replacements for inorganic materials currently used in batteries. To design new quinonoid compounds and predict their state of protonation and redox behavior under various conditions, their pK a values must be known. Methods that can predict the pK a values of simple phenols cannot reliably handle complex analogues in which multiple OH groups are present and may form intramolecular hydrogen bonds. We have therefore developed a straightforward method based on a linear relationship between experimental pK a values and calculated differences in energy between quinols and their deprotonated forms. Simple adjustments allow reliable predictions of pK a values when intramolecular hydrogen bonds are present. Our approach has been validated by showing that predicted and experimental values for over 100 quinols and related compounds differ by an average of only 0.3 units. This accuracy makes it possible to select proper pK a values when experimental data vary, predict the acidity of quinols and related compounds before they are made, and determine the sites and orders of deprotonation in complex structures with multiple OH groups.

Benzoquinone and hydroquinone cocrystallize to form quinhydrone, a 1:1 complex with a characteristic structure in which the components are positioned by hydrogen bonds and charge-transfer interactions. We have found that analogous diphenoquinhydrones can be made by combining 4,4′-diphenoquinones with the corresponding 4,4′-dihydroxybiphenyls. In addition, mixed diphenoquinhydrones can be assembled from components with different substituents, and mismatched quinhydrones can be made from benzoquinones and dihydroxybiphenyls. In all cases, the components of the resulting structures are linked in alternation by O–H···O hydrogen bonds to form essentially planar chains, which stack to produce layers in which π-donors and π-acceptors are aligned by charge-transfer interactions. Geometric parameters, computational studies, and spectroscopic properties of diphenoquinhydrones show that the key intermolecular interactions are stronger than those in simple quinhydrone analogues. These findings demonstrate that the principles of modular construction underlying the formation of classical quinhydrones can be generalized to produce a broad range of hydrogen-bonded charge-transfer materials in which the components are positioned by design.