Ionic channel structures in [(M+)x([18]crown-6)][Ni(dmit)2]2 molecular conductors

Area of Science:

• Materials Science
• Solid-State Chemistry
• Supramolecular Chemistry

Background:

• Molecular conductors based on [Ni(dmit)2] salts exhibit diverse electronic properties.
• Supramolecular cations (SC+), formed by alkali metal ions (M+) and [18]crown-6, can influence the crystal structures and conductivity of these materials.
• The self-assembly of [18]crown-6 and M+ ions dictates the formation of ionic channels or discrete units within the conductor matrix.

Purpose of the Study:

• To investigate the formation of supramolecular cations (SC+) using various alkali metal ions (Li+, Na+, Cs+, K+, Rb+) and [18]crown-6.
• To elucidate the relationship between SC+ structure, stoichiometry, and the resulting electronic properties of [Ni(dmit)2]-based molecular conductors.
• To understand how controlling the complex formation equilibrium influences the crystallization of different salt types (I, II, III) and their associated conductivity.

Main Methods:

• Electrocrystallization was employed to synthesize various [Ni(dmit)2] salts incorporating different alkali metal ions and [18]crown-6.
• X-ray crystallography was used to determine the isostructural relationships and detailed crystal structures of type I, II, and III salts.
• Electrical conductivity measurements at 300 K and variable temperatures were performed to characterize the electronic behavior of the synthesized materials.

Main Results:

• Three types of [Ni(dmit)2] salts (I, II, III) were identified, characterized by distinct supramolecular cation structures and stoichiometries.
• Type I salts, featuring ionic channels, exhibit quasi-one-dimensional metallic behavior (10-30 S cm(-1)) with temperature-independent conductivity at high temperatures.
• Type II and III salts, with dimerized or disk-shaped SC+ units, display semiconducting properties (0.01-0.1 S cm(-1)), with Li+ and Na+ salts showing transitions influenced by ion dynamics.

Conclusions:

• The formation of ionic channel structures in type I salts is favored by low complex formation equilibrium constants (Kc).
• The electronic properties of [Ni(dmit)2] conductors are strongly modulated by the structure and dynamics of the coexisting supramolecular cation assemblies.
• Controlling supramolecular assembly through Kc offers a pathway to tune the conductivity and electronic phases of molecular conductors.