Modulation of Band Gaps toward Varying Conductivities in Heterometallic One-Dimensional Chains by Ligand Alteration and Third Metal Insertion

A heterometallic one-dimensional
(1-D) chain consisting of multiple
kinds of metals, Rh, Pt, and Pd, with direct metal–metal bonds
was successfully obtained by mixing a Rh dinuclear complex and Pt–Pd–Pt
trinuclear complex. The Pt–Pd–Pt trinuclear complex
can be reversibly one-electron-oxidized or -reduced, where the electron
paramagnetic resonance spectrum of the one-electron-oxidized one shows
an axially symmetric signal with hyperfine splitting by two Pt and
Pd, indicating that an unpaired electron is delocalized to the dz2 orbital of Pt–Pd–Pt.
Utilized with the highest occupied molecular orbital and lowest unoccupied
molecular orbital interaction at the dz2 orbital, simple mixing of the Pt–Pd–Pt
trinuclear complex and Rh dinuclear complex in adequate solvents afforded
heterometallic 1-D chains, which are aligned as −Rh–Rh–Pt–Pd–Pt–.
Several physical measurements revealed that the metal oxidation state
is +2. Diffuse reflectance spectra and theoretical calculations show
that heterometallic 1-D chains have σ-type conduction and valence
bands where π*­(Rh2) are lying between them, whose
gaps become narrower than the prototype chains aligned as −Rh–Rh–Pt–Pt–Pt–Pt–.
The narrower band gaps are induced by destabilization of the σ-type
valence bands and accompanied by insertion of Pd ions because the
d-orbital energy level of Pd is closer in value to Rh compared with
Pt. Flash-photolysis time-resolved microwave conductivity measurements
exhibited an increase in the product of charge carrier mobility and
its generation efficiency (8.1 × 10–5 to 4.6
× 10–4 cm2 V–1 s–1) with narrowing the band gaps, suggesting
that the better conductivity is attributed to shorter metal–metal
distances in 1-D chains. These results imply the possibilities of
controlling band gap with ligand modification and third metal insertion
in heterometallic 1-D chains to show various conductivities.