Development of Molecular Switches Based on Molecular Recognition of Organic Dianions

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Charge-transfer (CT) complex formation based on molecular recognition of organic π-dianions has been investigated by electrochemistry and spectroelectrochemistry combined with ab initio MO calculations. Detailed analyses of the cyclic voltammetric and spectral behaviors for the hydrogen-bonding systems reveal that para-quinone dianions (PQ^<2->) form the 1 : 2 complexes at low concentrations of MeOH and the 1 : 4 complexes at high concentrations by the hydrogen bonding involving strong n-σ type CT interaction. The HF/6-31G (d) calculation results show that the structure of PQ^<2-> is characterized by a lengthening of the C=O bonds and a benzenoid ring. In conclusion, the differing functions and properties of biological quinones are conferred by the n-σ CT interaction through hydrogen bonding of the dianions with their protein environment. The bistable complex formation systems consisting of biphenylene (BP) and redox-active organic molecules such as chloranil (CL) and TCNE have been experimentally and theoretically investigated, based on an intermolecular interaction which characteristically occurs in the electrogenerated dianions forming a π-π type charge-transfer complex. It has been found that BP forms the redox-mediated bistable complexes with TCNE and CL, characterized by the geometrical alteration and the chromatic change. The interconversion in the systems is modulated through redox control of the intermolecular HOMO-LUMO interaction, with trichromic change arising from the neutral complex formation, the anion radical generation, and the dianion complex formation. This is an original approach to the redox-mediated bistable complex formation in terms of use of the properties of organic dianions. The present conclusion is important for extended discussion on generation of highly designed, redox-mediated recognition systems involving the electrogenerated π-dianions and the design of molecular devices utilizing the recognition.