Introduction

Molecular machines have attracted much attention in nanoscale research, and their development has been accompanied by introduction of new technologies for handling and assembling single molecules. The construction of molecular machines via combination and synchronization of molecular modules with well-characterized responses constitutes an efficient design strategy.

For example, a photoelectric conversion system has previously been developed based on the ligand-exchange reaction between a copper complex containing azobenzene-appended bipyridine ligands and free bipyridines. The ligand exchange was modulated by the reversible photoisomerization of the azobenzene moieties.

The next synthetic goal was to expand the functionalities of this system in novel ways by introducing appropriate substituents to this copper complex system. One purpose of this study is to construct a single molecular machine by introducing the azo-substituted bipyridine complex and bipyridine into a single molecule. The other purpose of this study is to toggle the photoelectric response of the copper complex system using solution pH changes. For these purposes, 4-hydroxyazobenzene-appended bipyridine, oAB-2OH, was synthesized. The hydroxyl groups act as proton sensors.

Construction of a Single Molecular Machine for Photoelectric Conversion― Synchronization of Isomerization and Ligand Exchange

In this chapter, construction of a single molecule was tried. For this purpose, a cyclic ligand, oAB-bpy, was synthesized from oAB-2OH. This cyclic ligand had azobenzene units for photoisomerization and bipyridine units for the coordination site. A cyclic compound, oAB-O13, was synthesized as a model compound.

The photoisomerization behavior of oAB-O13 and oAB-bpy in CD2Cl2 was monitored using NMR measurements. Before irradiation, they were in the trans2 form. New signals were observed under 365-nm light irradiation, indicating a trans-to-cis isomerization. Both the cis2 form and the cis-trans form were observed. The isomerization in the reverse direction was observed under 436-nm light irradiation, confirming the reversibility of the isomerization.

Next, the coordination behavior of oAB-O13 to copper was examined. When oAB-O13 and [Cu(CH3CN)4]BF4 were mixed in CD2Cl2, 1H NMR signals were broadened (Figure 1(a)) because the copper was coordinated by not only bipyridine units but also by polyether chains and was not fixed. Then, oAB-2OH was added and the signals became sharp (Figure 1(b)), indicating stabilization by formation of [Cu(oAB-O13)(oAB-2OH)]BF4, which had the interligand π-π stacking. This means that oAB-2OH could penetrate the cyclic ligand and coordinated to copper.

Next, synchronization of isomerization and ligand exchange was tried. [Cu(CH3CN)4]BF4 was added to the cis form of oAB-bpy and oAB in CD2Cl2. 1H NMR signals of [Cu(oAB-bpy)]BF4 and oAB were observed (Figure 2(a)). When these were converted to the trans form, the occurrence of penetrating oAB-bpy of oAB was indicated (Figure 2(b)). These results indicate that the construction of a single molecular machine which can convert photo energy into electric energy through synchronization of the photoisomerization and the ligand exchange is feasible.

A Hydroxyazobenzene-appended Bipyridine Complex of Copper ― Acid-base Response and its Application to Photoelectric Response

In this chapter, switching of the photoelectric conversion was tried. For this purpose, 4-hydroxyazobenzene-appended bipyridine complex of copper, [Cu(oAB-2OH)2]BF4, was synthesized from oAB-2OH. The [Cu(oAB-2OH)2]BF4 system, shown in Chart 1, contains interligand π-π stacking interactions that stabilize [Cu(oAB-2OH)2]BF4. Theπ-stacking stabilization is lost by the trans-to-cis isomerization upon UV irradiation, and ligand exchange with bipyridine derivatives is favored. This chemical process modulates the reduction potential of the CuII/CuI redox couple. Because the original state can be regenerated by visible light irradiation, this process can be cycled repeatedly. The addition of acid or base respectively lowers or raises the barrier to photoisomerization, modulating the photoelectric response.

The UV-Vis absorption spectrum of [Cu(oAB-2OH)2]BF4 in THF showed an intense band at 370 nm (Figure 3), ascribed to theπ-π* transition of the azobenzene moieties. The n-π* and d-π* transition (MLCT) bands overlapped in the visible region from 450 nm to 550 nm. The intensity of theπ-π* transition band decreased and the n-π* transition band increased under 365-nm light irradiation, indicating a trans-to-cis isomerization. The isomerization in the reverse direction was observed under 436-nm light irradiation (Figure 3(a)). The solution reached a photostationary state by irradiation with 365 nm or 436 nm light for 5 min. It was reversible between these two photostationary states. This confirmed the reversibility of the isomerization. When 0.2 eq. of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was added to this solution, the photoisomerization reaction did not take place (Figure 3(b)). Suppression of the photoresponse was caused by the deprotonation of the hydroxyl group, which accelerated the thermally driven cis-to-trans isomerization of the azo groups. A small quantity of base was sufficient to suppress the photoisomerization of [Cu(oAB-2OH)2]BF4 because the thermally driven cis-to-trans isomerization of the deprotonated form is much more rapid than the photochemically driven trans-to-cis isomerization of the protonated form. The photoresponse was recovered, and reversible photoisomerization was observed again after further addition of trifluoroacetic acid (TFA) (Figure 3(c)). In summary, photoisomerization could be toggled using acid or base addition.

An acetone-d6 solution containing [Cu(oAB-2OH)2]BF4 and bpy-2COOEt presented 1H NMR signals of [Cu(oAB-2OH)2]BF4, oAB-2OH, and bpy-2COOEt. The signals from [Cu(bpy-2COOEt)2]BF4 were not observed due to broadening attributed to ligand self-exchange. In contrast, the signals of [Cu(oAB-OH)2]BF4 were sharp due to stabilization by interligandπ-π stacking. These results indicate the presence of an equilibrium between the copper coordination of oAB-2OH and bpy-2COOEt.

Figure 4 shows the cyclic voltammograms of [Cu(oAB-2OH)2]BF4. The CuII/CuI couple showed a reversible redox wave at 0.31 V vs. ferrocenium (Fc+)/ferrocene (Fc), a typical potential for 6,6'-disubstituted bipyridine complexes. The redox potential was not significantly affected by the addition of acid or base (see Figure 4). A cyclic voltammogram of [Cu(bpy-2COOEt)2]BF4 is also shown in Figure 4. A reversible redox wave for the CuII/CuI couple was observed at-0.06 V vs. Fc+/Fc, a typical potential for bipyridine derivative complexes. The large difference between the redox potentials of [Cu(oAB-2OH)2]BF4 and [Cu(bpy-2COOEt)2]BF4 can be harnessed to achieve a photoelectric response via the ligand exchange reaction.

The oAB-2OH/[CuI(bpy-2COOEt)2]BF4/[CuII(bpy-2COOEt)2](BF4)2 system yielded a reversible rest potential response under alternating photoirradiation at 365 nm (UV) and 436 nm (visible) (Figure 5(a)). The response first exhibited a negative potential shift accompanying the trans-to-cis isomerization under 365 nm irradiation. This shift was caused by formation of [CuI(bpy-2COOEt)2]+, with a less positive CuII/CuI redox potential, through the ligand exchange coupled to photoisomerization. After addition of 0.2 eq. DBU, the rest potential response to irradiation was silenced (Figure 5(b)) due to the suppressed photoisomerization, as noted above. Addition of TFA recovered the reversible rest potential response to UV and visible light (Figure 5(c)), because the barrier to photoisomerization was lowered by protonation. These results indicate that the photoelectric response switched between OFF and ON states by the addition of small amounts of DBU and TFA, respectively. In conclusion, the ON/OFF switching of photoelectric response using acid and base was achieved.

Figure 1. 1H NMR spectral changes of oAB-O13 (1.0 × 10-3 M) and [Cu(CH3CN)4]BF4 (1.0 × 10-3 M) in CD2Cl2 (a) initially and (b) after addition of oAB-2OH (1.0 × 10-3 M).

Figure 2. 1H NMR spectral changes of oAB-bpy (4.6 × 10-3 M), oAB (4.6 × 10-3 M), and [Cu(CH3CN)4]BF4 (4.6 × 10-3 M) in CD2Cl2 (a) in the cis form and (b) in the trans form.

Chart 1. ON/OFF toggling of the photoelectric response.

Figure 3. Absorption spectral changes of [Cu(oAB-2OH)2]BF4 (6.4 × 10-6 M) in THF (a) initially, (b) after addition of 0.2 eq. DBU, and (c) after further addition of 0.2 eq. TFA. Blue line: before irradiation; red line: upon irradiation at 365 nm for 5 min; green line: upon irradiation at 436 nm for 5 min.

Figure 4. Cyclic voltammograms of [Cu(bpy-2COOEt)2]BF4 (2.4 × 10-4 M) in acetone with 0.1 M Bu4NClO4 (black line) and [Cu(oAB-2OH)2]BF4 (2.8×10-4 M) in acetone with 0.1 M Bu4NClO4. Blue line: initially; red line: after addition of 0.2 eq. DBU; green line: after further addition of 0.2 eq. TFA.

Figure 5. Rest potential changes of oAB-2OH (2.0 × 10-5 M), [Cu(bpy-2COOEt)2]BF4 (9.0 × 10-6 M), and [Cu(bpy-2COOEt)2](BF4)2 (1.0 × 10-6 M) in acetone with 0.1 M Bu4NClO4 (a) initially, (b) after addition of 0.2 eq. DBU, and (c) after further addition of 0.2 eq. TFA.

本論文は4章からなり、第1章は研究の背景と目的、第2章はアゾベンゼン部位とビピリジン部位を有する環状配位子の合成とその物性、第3章は水酸基を導入したアゾ置換ビピリジン銅錯体を用いた光電応答のスイッチ、第4章は研究成果のまとめと展望について述べられている。以下に各章の概要を記す。

第1章では研究の背景について述べている。人工分子機械において、既知の応答性を持つ分子モジュールを組み合わせる事で、目的とする応答性を持つ分子機械を作り上げることができる。そして、近年単一分子を扱う技術と分子設計技術の進歩により分子機械に対する注目が高まっている。そこで本研究では、新たな光電変換系を既知の分子モジュールを組み合わせることによる構築を行った。

第2章では、アゾベンゼン部位とビピリジン部位を有する環状配位子について行った研究について述べている。具体的にはまず、ビピリジン部位を2種類有する環状配位子と1個だけ有する環状配位子を合成した。そして、これらの環状配位子が可逆に光異性化することを示した。また、これらの環状配位子の銅に対する配位挙動を観測し、別の配位子がその内部を貫通して銅に配位できる柔軟性と大きさをこれらの環状配位子が有していることを示した。さらに、ビピリジン部位を2種類有する環状配位子を用いて異性化と配位子交換を連動させることが可能であることを示し、その結果、光電変換機能を持つ単一分子機械の構築が可能であることを示した。

第3章においては、水酸基を導入したアゾ置換ビピリジン銅錯体を用いて光電応答を酸塩基によりスイッチした結果について述べている。この銅錯体の水酸基の可逆な脱プロトン化とプロトン化、及びアゾベンゼン部位の可逆な光異性化が観測された。さらに、水酸基をプロトンセンサーとして働かせることでその光異性化を酸塩基によりオンオフすることに成功した。また、水酸基を導入したアゾ置換ビピリジン配位子とエトキシカルボニル基を導入したビピリジン配位子の銅に対する配位において平衡が成立した。この平衡は外部刺激により動かすことで光電応答に利用できる。また、この銅錯体は可逆な酸化還元を示し、その酸化還元電位は酸塩基に影響されなかった。さらに、その酸化還元電位はエトキシカルボニルビピリジン銅錯体のものと差があり、その差は光電応答に利用できる。そして、光電応答を酸塩基によりオンオフできる系をこの銅錯体を用いて実現し、その機構について考察した。

第4章では、以上の結果を総括し、今後の研究展望を述べている。

以上、本論文では、光電変換機能を持つ単一分子機械の構築が可能であること、また光電応答の酸塩基によるスイッチに成功したことを記述している。本博士論文において示された新たな光電変換系の既知の分子モジュールの組み合わせによる構築は、錯体化学、電気化学、光化学の分野に基礎的な貢献をするのみならず電子構造が関与する機能分子化学の分野を大きく進展させると期待される。なお、本論文第2章は久米晶子、西原 寛との共同研究、3章は久米晶子、西原 寛との共同研究であり、一部は既に学術雑誌として出版されたものであるが、論文提出者が主体となって実験および解析を行ったもので、論文提出者の寄与が十分であると判断する。

したがって、博士(理学)の学位を授与できると認める。