Hydrogen-Bonded Networks in Organic Conductors: Crystal Structures and Electronic Properties of Charge-Transfer Salts of Tetracyanoquinodimethane with 4,4‘-Biimidazolium Having Multiprotonated States

Novel hydrogen-bonded charge-transfer salts of TCNQ with mono- and diprotonated 4,4'-biimidazolium were synthesized in order to demonstrate the high potential of the 4,4'-biimidazole
system in a molecular conductor from the viewpoint of crystal engineering and electronic modulation.
Crystal structure analyses of neutral 4,4'-biimidazole and TCNQ salts revealed the formation of
two types of hydrogen-bonding modes of the 4,4'-biimidazole moiety depending on the protonated
states. Neutral 4,4'-biimidazole possessed a linear chain mode of hydrogen-bonding to construct
two-dimensional network. In the TCNQ salt of monoprotonated 4,4'-biimidazolium, the 4,4'-biimidazole moiety formed a dimer by a complementary mode of hydrogen-bonding. In contrast,
the salt of diprotonated 4,4'-biimidazolium showed a double linear chain mode of hydrogen-bonding
to construct a three-dimensional network. The formation of two types of hydrogen-bonding modes
made the difference in the stacking patterns of TCNQ columns and in their transport properties.
The TCNQ salt of diprotonated 4,4'-biimidazolium exhibited high electrical conductivity (σrt = 1.1
× 10-1 S cm-1).