Takumi’s Paper is pubished in Chem. Commun. and selected for a Front Cover / 塩川くんの論文が掲載され、表紙絵に選ばれました

• 2025-07-04
• News, Publication

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キノイド型オリゴチオフェンの還元種に関する論文を発表しました。

キノイド型オリゴチオフェン（チエノキノイド）は，機能性色素や有機半導体の汎用的な基本骨格として注目されてきました。これらの分子骨格に共通する特徴は LUMO の低さであり，そのためチエノキノイド類は優れた電子受容性を示すほか，狭い HOMO–LUMO ギャップに起因した長波長域での光吸収など魅力的な性質を示します。また，チエノキノイド類の多くは，単に電子親和性が高いだけでなく，多段階の酸化還元（特に還元）に対して安定であるという特徴を有しています。こうした電子受容性は，定性的に「還元状態でチオフェン環が芳香族性を獲得するため」と定性的に理解されてきました。しかし，私たちは様々な含チオフェンπ電子系に関する研究を通して，ベンゼン環をもつ類縁体とは異なる性質をもつことを実感してきました。炭素のみからなるベンゼン環を，硫黄を含む複素芳香環であるチオフェンに置き換えたときにみられる様々な物性や反応性の違いの多くは，「ベンゼンと比較した芳香族性の弱さ」で合理的に説明できることを見出しています[1–4]。このような経験をもとに，「チエノキノイドの還元種は本当にそれほどの芳香族性をもつのだろうか？」という疑問を抱きました。ここ10-20年の間に，芳香族性を定量的に評価する手法は急速な進歩を遂げており，これらの方法を駆使すれば，チエノキノイドの電子受容体としての特徴の本質を明らかにできると考えました。

そこで題材に選んだのが，チエノキノイドの一種である S-ペックマン色素です。S-ペックマン色素は当グループで以前から興味をもって研究してきた電子受容性π電子系であり[5]，近年では狭バンドギャップポリマーの電子受容性ビルディングブロックとしての有用性も示されつつあります [6]。3,3′-ビチオフェンから数段階で合成することができ，安定性にも優れ，扱いやすいのが特徴です。今回，この S-ペックマン色素の還元種（ジアニオン）を化学還元により合成・単離することに成功し，実験結果に基づく S-ペックマン色素ジアニオンにおける芳香族性の議論が可能になりました。

ここで問題になるのは，何と比較するかです。芳香族性の絶対評価では得られる知見は限定的ですので，構造の近い関連化合物との比較が必要です。私達は，構造有機化学において古くから用いられてきた「等電子性」という考え方に基づき，S-ペックマン色素がペンタフルバレンと等電子構造をもつことに着目しました。すべて炭素原子からなるペンタフルバレンのジアニオンは，顕著な芳香族性をもつことがすでによく知られていますので，これとS-ペックマン色素ジアニオンの芳香族性の度合いを多角的・定量的に比較することにしました。その結果，(1) S-ペックマン色素ジアニオンはある程度の芳香族性をもつが，その程度はペンタフルバレンと比較してかなり弱いこと，(2) S-ペックマン色素ジアニオンでは，負電荷は五員環で環状に非局在化するよりも，むしろ C₄O という鎖状構造に沿って非局在化することがわかりました。この特徴的な非局在化の様式では，硫黄原子はほとんど電子の非局在化に関与していません。これは，第3周期元素である硫黄と炭素骨格との軌道相互作用の弱さによるものと考えられ，C–S結合の長さや，軌道サイズ，硫黄の 3p 軌道と炭素の 2p 軌道のエネルギー準位のミスマッチによりもたらされていると考えられます。今回の研究は S-ペックマン色素に限って行われたものですが，得られた知見は，チエノキノイド類全般の理解に資するものであると考えられます。

A paper on the reduced species of quinoid-type oligothiophenes has been published.

Quinoid-type oligothiophenes (thienoquinoids) have attracted considerable attention as versatile structural motifs for functional dyes and organic semiconductors. A key feature of these π-conjugated systems is their inherently low LUMO levels, which endow thienoquinoids with strong electron-accepting properties and narrow HOMO–LUMO gaps, leading to absorption in the long-wavelength region. Furthermore, many thienoquinoids exhibit not only high electron affinity but also remarkable stability across multiple redox states, particularly in their reduced forms. Traditionally, this electron-accepting ability has been qualitatively interpreted as a gain in aromatic character of the thiophene rings upon reduction.

However, through our continued investigations into various thiophene-containing π-systems, we have become increasingly aware that their properties differ significantly from those of related benzenoid systems. We have found that many such differences can be rationalized by the markedly lower aromaticity of thiophenes compared to benzene rings composed solely of carbon atoms [1–4]. This observation led us to question a common assumption: Are reduced thienoquinoids truly as aromatic as generally believed? Over the past two decades, methods for quantifying aromaticity have become increasingly sophisticated and diverse. We hypothesized that applying these tools to thienoquinoids would allow us to probe the fundamental basis of their electron-accepting behavior.

In this study, we focused on a particular thienoquinoid, the S-Peckmann dye, a π-electron-deficient compound of longstanding interest to our group [5]. In recent years, the S-Peckmann dye has also gained attention as a promising electron-accepting building block for narrow-bandgap polymers [6]. It can be synthesized in a few steps from 3,3′-bithiophene, exhibits excellent stability, and is relatively easy to handle. In this work, we successfully synthesized and isolated the chemically reduced form of the S-Peckmann dye (its dianion), enabling us to experimentally assess its aromaticity.

A key question then emerged: What should we compare it to? Evaluations of aromaticity of one specific series of compounds often provide limited insight; instead, comparisons with structurally related compounds are essential. Drawing on the classical concept of isoelectronic structure in organic chemistry, we recognized that the S-Peckmann dye is isoelectronic with pentafulvalene, a well-known cross-conjugated hydrocarbon composed entirely of carbon atoms. This provided a logical and meaningful benchmark for aromaticity.

Our comparative analysis yielded two major findings: (1) While the S-Peckmann dye dianion does exhibit a certain degree of aromatic character, it is significantly weaker than that of its all-carbon isoelectronic counterpart, the pentafulvalene dianion. (2) In the S-Peckmann dianion, the negative charge does not primarily delocalize around the five-membered ring. Instead, it preferentially delocalizes along a C₄O substructure. Notably, the sulfur atoms contribute very little to this delocalization. This is likely due to weak orbital interactions between sulfur atoms in the third row of the periodic table and the carbon framework, arising from bond-length discrepancies and poor orbital size/energy matching. Although this study focused specifically on the S-Peckmann dye, the insights gained are broadly applicable to the fundamental understanding of thienoquinoid systems.

This work was inspired by a realization regarding the isoelectronic relationship between the S-Peckmann dye and pentafluorocyclopentadienyl dianion, and further developed through in-depth discussions with Takumi. He carried out the synthesis, structural analysis, quantum chemical calculations, and drafted the initial manuscript. Great work, Takumi!

The paper has been published online in Chemical Communications, and was selected for the Outside Front Cover. The cover art uses a sushi conveyor belt as a metaphor for π-electron movement, illustrating in a friendly and accessible way how delocalization patterns differ between the dianions of the S-Peckmann dye and the pentafulvalene. In the S-Peckmann dianion, electrons do not circulate around the five-membered ring, but instead move along a path involving four carbon atoms and one oxygen atom. (We are also grateful to YAP Co., Ltd. for their kind assistance in producing the delicious-looking cover illustration.)

参考文献 / References

1. J. Usuba, M. Hayakawa, S. Yamaguchi, A. Fukazawa, Dithieno[a,e]pentalenes: Highly Antiaromatic Yet Stable π-Electron Systems without Bulky Substituents, Chem. Eur. J. 2021, 27, 1638–1647. [DOI: 10.1002/chem.202004244]
2. J. Usuba, A. Fukazawa, Thiophene-fused 1,4-Diazapentalene: A Stable C=N-Containing π-Conjugated System with Restored Antiaromaticity, Chem. Eur. J. 2021, 27, 16127–16134. [DOI: 10.1002/chem.202103122]
3. A. Fukazawa, H. Oshima, Y. Shiota, S. Takahashi, K. Yoshizawa, S. Yamaguchi, Thiophene-Fused Bisdehydro[12]annulene That Undergoes Transannular Alkyne Cycloaddition by Either Light or Heat, J. Am. Chem. Soc. 2013, 135, 1731–1734. [DOI: 10.1021/ja3126849]
4. A. Fukazawa, D. Kishi, Y. Tanaka, S. Seki, S. Yamaguchi, Diarylated Bi(thieno[2,3-c]thiophene)s: A Ring-fusing Strategy for Controlling the Molecular Alignment of Oligoarenes, Angew. Chem. Int. Ed. 2013, 52, 12091–12095. [DOI: 10.1002/anie.201306323]
5. A. Fukazawa, M. Adachi, K. Nakakura, S. Saito, S. Yamaguchi, S-Pechmann Dye: A Thiolactone-Containing Organic Dye with Pronounced Electron-Accepting Character and Its Solid-State Photophysical Properties, Chem. Commun. 2013, 49, 7117–7119. [DOI: 10.1039/C3CC41007B]
6. T. Mikie, M. Hayakawa, K. Okamoto, K. Iguchi, S. Yashiro, T. Koganezawa, M. Sumiya, H. Ishii, S. Yamaguchi, A. Fukazawa, I. Osaka, Extended π-Electron Delocalization in Quinoid-Based Conjugated Polymers Boosts Intrachain Charge Carrier Transport, Chem. Mater. 2021, 33, 8183–8193. [DOI: 10.1021/acs.chemmater.1c02072]