Control and Function of Hydrogen Bond of Bioactive Quinones on the Charge Separation

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It is well known that self-assembled monolayer (SAM) electrodes are applied to electroorganic syntheses, biosensors, and investigation on the electron transfer at an electrode interface. In this study the SAM-anthraquinone (SAM-AQ) and SAM-naphthoquinone (SAM-NQ) gold electrodes were prepared, and the function of hydrogen bonds for electron transfer to the hydrogen-donating guests was investigated with the aid of electrochemical measurements with the SAM electrodes, combined with molecular orbital calculations.1. The SAM-AQ and SAM-NQ electrodes gave reversible waves on the cyclic voltammograms, corresponding to the generation of the AQ and NQ radical anions, respectively. The presence of the guest molecule of 4-substituted phenols allowed the waves to be irreversible, showing the mediation current. The mediation depended upon the hydrogen donating ability of the guests. Spectroelectrochemistry of the phenols showed that their radical anions generated on the electrodes at the reduction potential of quinones.2. The function of hydrogen bonds for the electron transfer has been discussed on the basis of MO calculations. It has been found that the apparently up-hill charge separation form the quinone on the electrode to the guests gets energy balance caused by large stabilization and potential shift arising from hydrogen bonding between the quinone radical and the neutral phenol.3. Mechanism on the control of the redox potentials with restrict of the structure of the hydrogen-bonding complexes has been demonstrated in the o-quinone hydrogen-bonding system with dimethylurea. Electrochemical measurements and molecular orbital calculations have revealed that the redox potentials in the system are controlled by the entropy driven stabilization for the hydrogen-bond formation.The results obtained here are important for the extended discussion on the function of biological quinones as a charge separator.