Intermolecular Charge-Transfer Interaction and Molecular Complex Formation of the Electrogenerated Organic Dianions
Noriko OKUMURA and Bunji UNO*
Gifu Pharmaceutical University; Mitahora-higashi, Gifu-shi 502-8585 Japan
Recent development in the research on charge-transfer (CT) complex formation based on molecular recognition of organic π-dianions is described. Detailed analyses of the cyclic voltammetric and spectro-electrochemical behaviors of the hydrogen-bonding systems reveal that p-quinone dianions (PQ2−) form 1 : 2 complexes with MeOH at low concentration of MeOH and 1 : 4 complexes at high concentration by the hydrogen bonding involving strong n-σ type charge-transfer (CT) interaction characterized by the geometrical and spectral properties. The results suggest that the differing functions and properties of biological quinones are imparted by the n-σ CT interaction through hydrogen bonding of the dianions with their protein environment. On the other hand, it has been demonstrated that π-dianions of redox-active organic molecules such as chloranil (CL) and TCNE form π-π type CT complexes with 4nπ biphenylene (BP). The complex formations are due to molecular recognition based on the favorable intermolecular HOMO-LUMO interaction of the dianions with BP, and the geometries of the dianion complexes differ from those of the neutral complexes. This background led to the development of the redox-mediated bistable complex formation systems characterized by the geometrical alteration and the chromatic change. The interconversion of the bistable complex formation 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. These results are important for extended discussion on the function of biological quinones as a charge separator and the development of highly designed, redox- mediated recognition systems involving the electrogenerated π-dianions.