Fine Tuning the Performance of Multiorbital Radical Conductors by Substituent Effects

Abstract

A critical feature of the electronic structure of oxobenzene-bridged bisdithiazolyl radicals 2 is the presence of a low-lying LUMO which, in the solid state, improves charge transport by providing additional degrees of freedom for electron transfer. The magnitude of this multiorbital effect can be fine-tuned by variations in the π-electron releasing/accepting nature of the basal ligand. Here we demonstrate that incorporation of a nitro group significantly stabilizes the LUMO, and hence lowers U, the effective Coulombic barrier to charge transfer. The effect is echoed, at the molecular level, in the observed trend in E, the electrochemical cell potential for 2 with R = F, H and NO. The crystal structures of the MeCN and EtCN solvates of 2 with R = NO have been determined. In the EtCN solvate the radicals are dimerized, but in the MeCN solvate the radicals form superimposed and evenly spaced π-stacked arrays. This highly 1D material displays Pauli-like temperature independent paramagnetic behavior, with χ = 6 × 10 emu mol, but its charge transport behavior, with σ near 0.04 S cm and E = 0.05 eV, is more consistent with a Mott insulating ground state. High pressure crystallographic measurements confirm uniform compression of the π-stacked architecture with no phase change apparent up to 8 GPa. High pressure conductivity measurements indicate that the charge gap between the Mott insulator and metallic states can be closed near 6 GPa. These results are discussed in the light of DFT band structure calculations.

Molecular Semiconductors for Logic Operations: Dead‐End or Bright Future?

“…Two representative examples are dithiadiazolyl 31 and diselenadiazolyl 32 . A more recent one comes from bisdithiazolyl 33 , which exhibit a sharp dependence of electrical conductivity with pressure . Some neutral π‐radicals behave as semiconductors.…”

Mixed valence salts based on carbon-centered neutral radical crystals

“…However, most single component neutral π-radicals are insulators or poorly conductive semiconductors (σ RT < 10 -10 S cm -1 ) due to the conduction barrier originating from the on-site Coulomb repulsion (U, Mott insulator) 7,8 and the steric hindrance needed to impart kinetic stability inhibiting intermolecular interactions 9,10 . High conductivities in single component neutral radicals with σ RT > 10 -6 S cm -1 have been reported in several systems, such as bisdithiazolyl and related radicals [11][12][13][14][15][16][17] (σ RT = 4 × 10 -2 S cm -1 at most 17 , measured for single crystals), and betainic TCNQ 18 and N,N'-dicyanoquinodiiminide 19 radicals (σ RT = 3.2 × 10 -5 and 1.6 × 10 -5 S cm -1 and at most, measured for compressed pellets, respectively), betainic TTF radicals 20 (σ RT = 1.0 × 10 -1 S cm -1 at most, measured for compressed pellets), where the heteroatom effects and strong polarizability contribute to the reduction of U. It should be noted that carrier doping into neutral radical crystals generates the MV state and dramatically increases conductivity 21,22 .…”

Synthesis and Applications of 5‐Membered Chalcogen‐Nitrogen π‐Heterocycles with Three Heteroatoms

“…Variously fused 1,2,3‐dithiazolyls obtained by reduction of Herz cations and related 1,2,3‐dithiazoliums are more stable. Some of them were only observed by EPR, whereas some were isolated and structurally characterized by XRD ,,–,,. Especially interesting and important are bis(1,2,3‐dithiazolyls) (Scheme )–,, which revealed molecular conductor and magnetic properties in the solid state (see below).…”

“…Properties of 1,2,3‐dithiazole‐based radical conductors can be finely tuned by substitution effects . With DFT calculations on benzo‐ and azino‐fused bis(1,2,3‐dithiazoles), it was shown that their ground state multiplicity can be effectively controlled by substitution, namely, electron‐acceptor substituents in the central ring stabilize bipolar singlet states, whereas electron‐donor substituents favor a triplet ground state .…”