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Comparative Study on the Structural, Optical, and Electrochemical Properties of Bithiophene-Fused Benzo[c]phospholes

Authors

  • Yoshihiro Matano Prof. Dr.,

    1. Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615–8510 (Japan), Fax: (+81) 75-383-2571
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  • Tooru Miyajima,

    1. Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615–8510 (Japan), Fax: (+81) 75-383-2571
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  • Tatsuya Fukushima,

    1. Institute for Chemical Research, Kyoto University, Uji, Kyoto 611–0011 (Japan)
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  • Hironori Kaji Prof. Dr.,

    1. Institute for Chemical Research, Kyoto University, Uji, Kyoto 611–0011 (Japan)
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  • Yoshifumi Kimura Prof. Dr.,

    1. Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606–8502 (Japan)
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  • Hiroshi Imahori Prof. Dr.

    1. Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615–8510 (Japan), Fax: (+81) 75-383-2571
    2. Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Nishikyo-ku, Kyoto 615–8510 (Japan)
    3. Fukui Institute for Fundamental Chemistry, Kyoto University, Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606–8103 (Japan)
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Abstract

Three types of bithiophene-fused benzo[c]phospholes were successfully prepared by TiII-mediated cyclization of the corresponding dialkynylated bithiophene derivatives as a key step. Each σ3-phosphorus center of the benzo[c]phosphole subunits was readily transformed into σ4-phosphorus center by Au coordination or oxygenation. In addition, the bithiophene subunit was functionalized at the α,α′-carbon atoms by Pd-catalyzed cross-coupling reactions with heteroarylmetals and by an SNAr reaction with hexafluorobenzene. The experimentally observed results (NMR spectroscopy, X-ray analysis, UV/Vis absorption/fluorescence spectroscopy, and cyclic/differential-pulse voltammetry) have revealed that the structural, optical, and electrochemical properties of the bithiophene-fused benzo[c]phospholes vary considerably depending on the π-conjugation modes at the bithiophene subunits and the substituents of the heterocyclopentadiene components. The appropriately ring-annulated σ3-P derivatives and σ4-P-AuCl complexes were found to emit fluorescence in the orange–red region, and the σ4-P-oxo derivatives proved to undergo reversible one-electron reduction at −1.4 to −1.8 V (vs ferrocene/ferrocenium). These results indicate that the bithiophene-fused benzo[c]phospholes possess narrow HOMO–LUMO gaps and low-lying LUMOs, which was confirmed by density functional theory calculations of their model compounds. The time-of-flight measurement of an ITO/benzo[c]phosphole/Al device showed that the electron mobility in the P-oxo derivative is one-order higher than that in Alq3 at low electric fields. The present study demonstrates that the arene-fused benzo[c]phosphole skeleton could be a highly promising platform for the construction of a new class of phosphole-based optoelectrochemical materials.

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