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Unoccupied electronic states in polythiophene as probed by XAS and RAS

XASとRASで明らかにしたポリチオフェンにおける非占有電子状態

池浦 広美*; 関口 哲弘  

Ikeura, Hiromi*; Sekiguchi, Tetsuhiro

内殻電子励起吸収端領域におけるX線吸収(XAS)測定法は一般に空軌道バンドの状態密度やエネルギー幅に関する情報を提供するため、放射光を利用して数多くの化合物について測定されてきた。本研究において有機ポリマーの一つであるポリチオフェンについてXASスペクトル、及び共鳴オージェ電子スペクトルを測定し空状態のエネルギー的な広がりを調べた。分子軌道計算等により行われ報告されている基底状態における伝導帯との比較を行った。結果、X線励起の場合、内殻正孔の存在による空状態の局在化が起こるため、基底状態よりもかなり狭いバンド幅を与えることが明らかになった。

Polythiophene is one of the typical conducting polymers. Unoccupied electronic states near the Fermi level of polythiophene polycrystalline powder were probed by S 1s X-ray absorption spectroscopy (XAS) and S$$KL_{2,3}$$, $$L_{2,3}$$ resonant Auger spectroscopy (RAS). Its overall XAS spectral shape is similar to those of 2,-2'-bithiophene and alpha-terthiophene polycrystalline powder. Any hybridization of the energy levels of thiophene unit was not observed in the $$pi^{*}$$-LUMO (the lowest unoccupied molecular orbital). It is found that the position of $$sigma^{*}$$(C-C) resonance shifts to lower energy as increasing thiophene repeating units. This finding reflects the different bond lengths of C-C. Although $$pi^{*}$$ and $$sigma^{*}$$(S-C) states in XAS are not resolved due to their overlap of electronic transitions, those were clearly separated by plotting spectator Auger yields as a function of excitation energy. If the Auger spectator shifts of the corresponding states are different each other, such unresolved states in XAS can be distinguished by RAS technique. RAS also indicates that the $$pi^{*}$$-LUMO of polythiophene has localized character. The present study demonstrated that excitation energy-dependent RAS can overcome the limitation of conventional XAS. The obtained results suggest that polythiophene is poorly conjugated, where the $$pi^{*}$$ delocalization is interrupted by the conformational disorder.

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パーセンタイル:36.83

分野:Chemistry, Physical

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