Neutral-Type One-Dimensional Mixed-Valence Halogen-Bridged Platinum Chain Complexes with Large Charge-Transfer Band Gaps

One-dimensional (1D) electronic systems
have attracted significant attention for a long time because of their
various physical properties. Among 1D electronic systems, 1D halogen-bridged
mixed-valence transition-metal complexes (the so-called MX chains)
have been thoroughly studied owing to designable structures and electronic
states. Here, we report the syntheses, structures, and electronic
properties of three kinds of novel neutral MX-chain complexes. The
crystal structures consist of 1D chains of Pt–X repeating units
with (1R,2R)-(−)-diaminocychlohexane
and CN– in-plane ligands. Because of the absence
of a counteranion, the neutral MX chains have short interchain distances,
so that strong interchain electronic interaction is expected. Resonance
Raman spectra and diffuse-reflectance UV–vis spectra indicate
that their electronic states are mixed-valence states (charge-density-wave
state: Pt2+···X–Pt4+–X···Pt2+···X–Pt4+–X···).
In addition, the relationship between the intervalence charge-transfer
(IVCT) band gap and the degree of distortion of the 1D chain shows
that the neutral MX chains have a larger IVCT band gap than that of
cationic MX-chain complexes. These results provide new insight into
the physical and electronic properties of 1D chain compounds.