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

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posted on 13.11.2020, 19:04 authored by Kazuhiro Uemura, Daiki Ito, Jenny Pirillo, Yuh Hijikata, Akinori Saeki
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.