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Anisotropic Chemical Pressure Effects in Single-Component Molecular Metals Based on Radical Dithiolene and Diselenolene Gold Complexes
On the basis of the reported radical neutral complex
[AuÂ(Et-thiazdt)<sub>2</sub>] (Et-thiazdt = <i>N</i>-ethyl-1,3-thiazoline-2-thione-4,5-dithiolate),
a series of single-component conductors derived from [AuÂ(Et-thiazdt)<sub>2</sub>], also noted as [AuS<sub>4</sub>(î»S)<sub>2</sub>],
has been developed, by replacing the outer sulfur atoms of the thiazoline-2-thione
rings by oxygen atoms and/or by replacing the coordinating sulfur
atoms by selenium atoms toward the corresponding diselenolene complexes.
Comparison of the X-ray crystal structures and transport properties
of the four isostructural complexes, noted as [AuS<sub>4</sub>(î»S)<sub>2</sub>], [AuS<sub>4</sub>(î»O)<sub>2</sub>], [AuSe<sub>4</sub>(î»S)<sub>2</sub>], and [AuSe<sub>4</sub>(î»O)<sub>2</sub>], shows that the oxygen substitution on the outer thiazoline ring
actually decreases the conductivity by a factor of 100, despite a
contracted unit cell volume reflecting a positive chemical pressure
effect. On the other hand, the S/Se substitution increases the conductivity
by a factor of 100, and the pressure needed to transform these semiconductors
into the metallic state is shifted from 13 kbar in [AuS<sub>4</sub>(î»S)<sub>2</sub>] to only â6 kbar in [AuSe<sub>4</sub>(î»S)<sub>2</sub>]. Analysis of unit cell evolutions and ab
initio band structure calculations demonstrates the strongly anisotropic
nature of this chemical pressure effect and provides an explanation
for the observed changes in conductivity. The greater sensitivity
of these neutral single-component conductors to external pressure,
as compared with âclassicalâ radical salts, is also
highlighted