Ultraviolet photoelectron spectroscopy of poly(pyridine-2,5-diyl), poly(2,2′-bipyridine-5,5′-diyl), and their K-doped states

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  • Takayuki Miyamae
    Department of Chemistry, Faculty of Science, Nagoya University, Chikusa-ku, Nagoya 464, Japan
  • Daisuke Yoshimura
    Department of Chemistry, Faculty of Science, Nagoya University, Chikusa-ku, Nagoya 464, Japan
  • Hisao Ishii
    Department of Chemistry, Faculty of Science, Nagoya University, Chikusa-ku, Nagoya 464, Japan
  • Yukio Ouchi
    Department of Chemistry, Faculty of Science, Nagoya University, Chikusa-ku, Nagoya 464, Japan
  • Kazuhiko Seki
    Department of Chemistry, Faculty of Science, Nagoya University, Chikusa-ku, Nagoya 464, Japan
  • Takafumi Miyazaki
    Institute for Molecular Science, Okazaki 444, Japan
  • Tsuneaki Koike
    Chemical Synthesis Laboratories, Mitsui Petrochemical Industries, LTD., Nagaura, Sodegaura-shi, Chiba 299-02, Japan
  • Takakazu Yamamoto
    Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227, Japan

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<jats:p>Ultraviolet photoelectron spectra were measured using synchrotron radiation for two kinds of π-conjugated polymers, poly(pyridine-2,5-diyl) (PPy) and poly(2,2′-bipyridine-5,5′-diyl) (PBPy) which exhibit n-type electrically conducting properties. The two compounds show similar spectra and they were analyzed with MO calculations and the comparison with the data of related molecules. The ionization threshold energies of PPy and PBPy were found to be 6.3 and 6.35 eV, respectively. These values are higher than those of π-conjugated conducting polymers capable of p doping. Upon potassium doping of PBPy, two new states appeared in the originally empty energy gap and the intensity of the state at 0.65 eV from EF grows as the doping proceeds. This finding and the change of optical absorption spectra upon doping indicate that bipolaron bands are formed in K-doped PBPy. While K-doped PPy also shows similar gap states, it requires higher dopant concentration to create bipolaron bands than in the case of K-doped PBPy. The difference of the dependence on dopant concentration between K-doped PPy and K-doped PBPy is discussed based on the conformational difference between these polymers.</jats:p>

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