Hydrogen-Bonding-Assisted Self-Doping in Tetrathiafulvalene (TTF) Conductor

The synthesis, characterization, and carrier generation mechanism of self-doping in a tetrathiafulvalene (TTF) conductor, ammonium tetrathiafulvalene-2-carboxylate (TTFCOO−NH4+), are described together with molecular orbital characteristics. Insulating TTFCOOH changes into a hole-doped conductor TTFCOO−NH4+ with a conductivity of σ = 2.0 × 10−4 S/cm (300 K), upon salt formation with NH3. A radical species, TTF•+COO−NH4+, is generated via protonation of the TTF moiety as demonstrated by UV−vis, ESR, and 1H NMR spectra. The X-ray crystallographic structure of TTFCOO−NH4+ reveals supramolecular arrays of TTFCOO− moieties with short S···S contact, assisted by the one-dimensional hydrogen-bonding network composed of the ammonium and carboxylate ions. Molecular orbital calculations of cluster models show that the singly occupied molecular orbital (SOMO) of TTF•+COO−NH4+ in the supramolecular array is not at the highest energy level, which is characterized as a quasi-closed-shell state. The ab initio periodic calculation with a one-dimensional boundary condition reveals that TTF•+COO−NH4+ behaves as a dopant leading to the semiconducting behavior of the stacked TTF moieties assembled by the hydrogen-bonding network. Namely, TTFCOO−NH4+ can be described as a “hydrogen-bonding-assisted self-doped conductor”. The contribution of the hydrogen-bonding interaction to the electron conduction is experimentally supported by a large isotope effect in the ac conductivity of TTFCOO−NH4+ at low temperature.