Hydrogen-Bond Interaction in Organic Conductors: Redox Activation, Molecular Recognition, Structural Regulation, and Proton Transfer in Donor−Acceptor Charge-Transfer Complexes of TTF-Imidazole

Hydrogen-bond interaction in donor−acceptor charge-transfer complexes of TTF-imidazole
demonstrated the electronic effects in terms of control of component ratio and redox activation. These
unprecedented effects of hydrogen bonds renewed the criteria giving “a high probability of being organic
metals” and produced a number of highly conductive complexes with various acceptors having a wide
range of electron-accepting ability. In p-chloranil complex, both molecules were linked by hydrogen bonds
and formed a D−A−D triad, regulating the donor−acceptor composition to be 2:1. Theoretical calculations
have revealed that the polarizability of hydrogen bonds controls the redox ability of the donor and
p-benzoquinone-type acceptors and afforded different ionicity in complexes from those expected by the
difference of redox potentials between donor and acceptors. In the p-chloranil complex, this electronic and
structural regulation by hydrogen bond realized the first metallic donor−acceptor charge-transfer complex
based on hydrogen bond functionalized TTF. Hydrogen bonds controlled also molecular arrangements in
charge-transfer complexes, giving diverse and highly ordered assembled structures, D−A−D triad in the
p-chloranil complex, one-dimensional zigzag chain in I5 salt, alternating donor−acceptor chain in chloranilic
acid complex, and D−A−D−A cyclic tetramer in nitranilic acid complex. Furthermore, TTF-imidazole acted
as electron donor as well as proton acceptor in anilic acid complexes and realized the simultaneous charge-
and proton-transfer complexes. These investigations demonstrated the new and intriguing potentials of
the hydrogen bond in the development of organic conductors and multifunctional molecular materials.