Electronic States of Quinones for Organic Energy Devices: The Effect of Molecular Structure on Electrochemical Characteristics

The molecular design of organic energy-storage devices relies on correlations between the electrochemical properties of organic materials and their molecular structures. Here we report a systematic study of the fundamental electronic states of the quinone family of redox-active materials. Poly(ethylene oxide) coatings, as elution inhibitors, facilitated the evaluation of the electrochemical properties of single quinone molecules. Moreover, we confirmed experimentally how LUMO energies and their corresponding redox potentials depend on molecular structure, including the number of aromatic rings, the positions of functional groups, and coordination structures; this was achieved by elemental and chemical-state-selective X-ray absorption spectroscopy, and DFT calculations. We introduce an energy diagram depicting a segmentalized reduction process; this diagram considers the intermediate states during redox reactions to discuss processes that dominate changes in electrochemical properties as molecular structures are altered. Our results and analysis strategy are widely applicable to the material design of future organic molecular-based devices.