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-imiclazole 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 15 salt, alternating donor-acceptor chain in chloranilic acid complex, and D-A-D-A cyclic tetramer in nitranilic acid complex. Furthermore, TTF-imiclazole 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.