A possibly highly conducting state in an optically excited molecular crystal

Abstract

Unique electronic states and properties of the molecular charge transfer salts have been a center of interest for decades. A new molecular charge transfer salt, BPY[Ni(dmit)₂]₆·3CH₃CN (BPY²⁺ = N,N′-ethylene 2,2′-bipyridinium, dmit²⁻ = 2-thioxo-1,3-dithiole-4,5-dithiolato), exhibits semiconducting properties governed by its [Ni(dmit)₂]⁻ radical anions. This salt is an insulator but exhibits high conductance like a metallic substance. Structural, spectroscopic, and theoretical examinations reveal that the high conductance of the salt is based on a fluctuation in the charge distribution on the Ni(dmit)₂ anions, which are closely and isotropically packed in a two-dimensional sheet owing to its Ni(dmit)₂-rich stoichiometry (BPY : Ni(dmit)₂ = 1 : 6). The qualitative conduction behavior is hardly affected by temperature and pressure unlike other organic charge transfer salts. However, UV irradiation (375 ± 5 nm), corresponding to a charge transfer between Ni(dmit)₂ anions, perturbs the charge distribution to realize a highly conducting state based on our proposed analysis model.