Proton Transfer and a Dielectric Phase Transition in the Molecular Conductor (HDABCO⁺)₂(TCNQ)₃

One-dimensional dielectric (N···H···N)∞ hydrogen-bonding chains of monoprotonated 1,4-diazabicyclo[2.2.2]octane (HDABCO+) were introduced into an electrically conducting 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) salt as the countercation structure. Room-temperature electrical conductivity was ∼10-3 S cm-1, with a semiconductive behavior. The temperature-dependent dielectric constants of (HDABCO+)2(TCNQ)3 indicated a dielectric transition at 306 K. A large deuterium isotope effect for the dielectric transition (ΔT = 70 K) was observed for the deuterated salt, (DDABCO+)2(TCNQ)3. Thermally activated order/disorder of the protons or deuteriums within the one-dimensional hydrogen-bonding chains of (HDABCO+)∞ and (DDABCO+)∞ affected the dielectric responses in the TCNQ-based semiconductors.