Introduction

Molecular bistability controllable with external stimuli is one of the most important properties in the realization of molecule-based devices. Especially, photo-controllable systems are appreciated as i) no elaborate contact guide to molecules is required, ii) specific chromophores can be excited by the selection of the wavelength of light, and iii) repetitive operations free from chemical contamination are possible.

Bistable molecular systems based on coordination compounds and organometallics have been studied intensively. Especially, ruthenium complexes with ambidentate sulfoxide ligands, such as dimethylsulfoxide (dmso), have been reported to show both photochromism and electrochromism based on the linkage isomerization. Both oxidation from Ru(II) to Ru(III) and photoirradiation of the S-bound complex give rise to the O-bound isomer. On the other hand, both reduction from Ru(II)I to Ru(II) and irradiation with another wavelength of light open a channel to reproduction of the original S-isomer. Of noteworthy is that the isomerization behavior is accompanied by a large negative shift of the formal potential (E0') of the Ru(3+)/Ru(2+) couple. These complexes are, however, often unstable in the course of isomerization reactions due to dmso replacement by solvent molecules. To improve the shortcoming, those with polydentate sulfoxide ligands have been synthesized in this decade. However, most of them lack an O- to S- photochemical channel.

Molecular design

Under the circumstance noted above, we synthesized several new Ru(II)-sulfoxide complexes. Their isomerization activities are influenced by their electronic and steric factors, which are, for example, distortion of the ground or excited state, reorganization energy, trans effect among ligands, and solvent effect. In my research, I focused on the distortion and trans effects. The trans effect has been thought to strongly promote isomerization. I designed ruthenium complexes with bi- or tri-dentate sulfoxide ligand considering their electronic and steric effects in order to manage stable and effective isomerization reaction. Herein, I will report the results of photo- and electro-triggered isomerization of the designed complexes.

Next, I designed a functional Ru complex which responds to light and redox energy. I selected a functional moiety so that its redox potential exists between S- and O- isomer. It means that the excited electron will be caught to the functional moiety and the excited isomer expected to be stabilized. I will report the photo- electro- chemical reaction property of the complex with electron accepting moiety.

Photo- and electro-chromic properties of Ru-sulfoxide complexes with a bi- and tridentate sulfoxide ligand

A new photochromic complexes with a bidentate ligand

[Ru(bpy)2(pySO-Tol)(2+) (pySO- 2-((phenylsulfinyl)methyl)pyridine )

(Fig 1) was prepared. Its isomerization behaviors were compared

with [Ru(bpy)2(pySO-Me)](2+)(bpy 2,2'-bipyridine, pySO-Me 2-(isopropylsulfinylmethyl)pyridine)1 to understand the electronical effect of π-conjugating substrates on the sulfur to isomerization.

The ground state complexes (S-isomers) featured a low energy absorption maximum in the electronic spectrum at 360 nm and 380 nm for [Ru(bpy)2(pySO-Me)](2+) and [Ru(bpy)2(pySO-Tol)](2+), respectively. Irradiation of S-isomer by 365 nm light caused intramolecular S→O isomerization of the sulfoxide moiety. During the irradiation, the intensity of the absorption corresponding to S-isomer diminished, while a new peak corresponding to O-isomer at 470 nm increased. Both complexes showed thermal reversion (Fig. 2), but the one with distorting ligand seems to be unstable compared to [Ru(bpy)2(pySO-Me)](2+).

Fig. 3 shows a cyclic voltammogram of [Ru(bpy)2(pySO-Tol)](2+) in Bu4NPF6-acetone at a scan rate of 0.1 V s-1. The first scan shows one electron oxidation at around 1.30 V assignable to the Ru(3+/2+) couple of the each S-isomer. By reversing the polarity and scanning in the negative direction, no reduction wave was observed while a new peak at around 0.40 V was appeared after the irradiation. This new wave is ascribed to the O-isomer. Here I can conclude that both steric and electronic effects at the S atom have little influence, and the chelation effect disturbs the electrochemical isomerization.

Based on the results noted above, I prepared two new complexes with tridentate sulfoxide ligand [Ru(tpy)(bpySO)]+ (tpy terpyridine, bpySO 2-((methylsulfinyl)methyl)-6-(pyridine-2-yl)pyridine), [Ru(tpy)(picSO)](2+) (picSO ((methylsulfinyl)methyl)pyridine-2-carboxylic acid ). (Fig 4) They were designed with consideration of their steric structure. Here, I focus on the trans effect. The trans effect is the electronic effect between the ligands that are trans - directing. Ruthenium complexes form octahedral coordination structure and the sulfoxide moiety in the complexes are trans directed to nitrogen ([Ru(tpy)(bpySO)]+) or oxygen ([Ru(tpy)(picSO)](2+)). I compared spectroscopic and electrochemical properties using these rigid complexes with tridentate ligand to discuss steric effect for isomerization.

The initial absorption maximum in the electronic spectrum could be associated with the ground state complex (S-isomer) (420 nm ([Ru(tpy)(bpySO)](2+) (Fig 5), 425 nm ([Ru(tpy)(picSO)]+). Irradiation to the S-isomer with 405 nm light caused intramolecular S→O linkage isomerization of the sulfoxide moiety. (480 nm [Ru(tpy)(bpySO)](2+) Fig 5 ) , 525 nm [Ru(tpy)(picSO)]+).

Fig. 6 shows a cyclic voltammograms of [Ru(tpy)(bpySO)](2+) and [Ru(tpy)(picSO)]+ in Bu4NPF6-acetone solution at a scan rate of 0.1 V s-1. The first scan of both complexes showed a one electron oxidation assigned to the Ru(3+/2+)

couple of the S-bonded species (1.30 V [Ru(tpy)(bpySO)](2+) and 0.10 V [Ru(tpy)(bpySO)](2+)). By reversing the polarity and scanning in the negative direction, different reduction wave was observed. S-isomers of both complexes seems to be lost during isomerization, but O-isomer of [Ru(tpy)(bpySO)](2+) did not appeared. On the other hand, the case of [Ru(tpy)(picSO)]+ was different. One oxidation wave appeared near 1.00 V in the fast scan rate and obvious reductive wave at around 0.30 V which assigned to O-isomer appeared. The differences in the redox potential and electro-isomerization reaction should be derived from the trans effect.

Photo- and electro-chromic properties of Ru-sulfoxide complex with Electron accepting moiety

Next, I designed a functional Ru complex which responds to light and redox energy. Electron migration between the excited state O-isomer and the electron accepting moiety will be expected. N,N,N-triphenylamine (TPA) was selected as an electron accepting moiety. A new complex [Ru(tpyTPA)(bpySO)] was synthesized. TPA moiety will be activated to be amine radical by one electron oxidation.

The ground state S-isomer features absorption maximum in the electronic spectrum at 486 nm. Irradiation of S-isomer by 432 nm light caused S→O isomerization of the sulfoxide moiety. This absorption change for O-isomer is shown in Fig. 8. During irradiation, the intensity of the absorption assigned to S-isomer diminished, while absorption at 543 nm increased.

In Fig. 9 are shown voltammogram obtained in Bu4NPF6-acetone of [Ru(tpy-TPA)(bpySO)](2+) at a scan rate of 2.0 V s(-1). The first scan shows one-electron oxidation near 0.75 V assigned to TPA moiety and one-electron oxidation near 1.30 V assigned to the Ru(3+/2+), S-bonded couple. And an obvious reductive wave which assigned to O-isomer appeared instead of reductive wave of S-isomer. Linkage isomerization occurred upon electrical oxidation of the TPA moiety, which was proved by a scan at a range of 0.0-1.0 V in cyclic voltammetry. This behavior also suggests that an electron transfer took place from the S isomer to the activated acceptor.

Finally, we tried to observe the electron transfer triggered by light. Judging from the result of electrochemical measurement, there can be two types of electron transfers. (Fig. 10) Both of them produce the trivalent Ruthenium O-isomer, diminishing the TPA cation radical. I observed decreasing rates of the TPA cation radical in the dark and upon photoirradiation: If the photo-induced electron transfer occurs, the decreasing rate should be faster in the case of photoirradiation. This observation suggested the existence of photo-triggered electron transfer. (Fig. 11)

Conclusion

I discussed the structural and electronic effects of a series of bi- or tri-dentate sulfoxide ligands on the photo- and electrochemical linkage isomerization of their Ru complexes. Chelation of sulfoxide degrade the isomerization abilities, especially the electrochemical response. On the other hand, the trans-effect encourages isomerization. A tridentate ligand stabilizes the Ru complex, although its rigidity disturbs isomerization at the same time. This series of results indicates that it is possible to design photochromic Ru complexes with more stable and more efficient linkage isomerization.

I also designed a photochromic Ru complex with a more sophisticated function. The complex can manage electron migration induced by the linkage isomerization. Furthermore, electron transfer results in stabilizing the metastable state (O-isomer).

The present work will lead to higher functional molecules such as a molecular switch or a photonic sensor by taking advantage of the structural and electronic effects I stated formerly.

Fig 1. isomerization reaction of Ru complexwithbidentate SO ligand

Fig 2. Thermal reversion of [Ru(bpy)2(PySO-Me)](left) and [Ru(bpy)2(pySO-Tol)](2+)(right) in acetone

Fig 3. Cyclic voltammogram of [Ru(bpy)2(PySOTol)](2+) in Bu4NPF(6-acetone)

Fig 5. UV-Vis spectra of [Ru(tpy)(bpySO)]+ in acetone

Fig 6. Cyclic voltammogram of [Ru(tpy)(bpySO)](2+) (top), [Ru(tpy)(picSO)]+ (bottom) in Bu4NPF(6-acetone)

Fig 7. A proposed electron transfer system

Fig 8. UV-Vis spectra of [Ru(tpy-TPA)(bpySO)]+ in CH2Cl2

Fig 9. Cyclic voltammogram of [Ru(tpy-TPA)(bpySO)]+ in Bu4NPF(6-acetone)

Fig 10. Schematic illustration of the expected electron transfer

Fig 11. Decreasing ratio of activated acceptor was plotted with time

本論文は四章からなり、第一章は研究の背景と目的、第二章は結合異性化反応の効率と錯体の安定性向上を目的とした錯体の設計、第三章は機能性部位として電子のアクセプター部位を導入した錯体の光・電気化学特性と電子移動の観測について、第四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示す。

第一章では、第一章は研究背景として、本研究に用いたルテニウムースルホキシド錯体における光・電気化学的結合異性化反応について紹介している。ルテニウム錯体は電気化学的に安定であること、可視領域に強い吸収を持つことから、近年注目されている分子デバイス開発において効果的な光―電気変換素子としての利用が期待されている。また、その光応答特性からエネルギー状態が詳細に知ることができ、電子状態の解析と分子設計に有用な錯体である。中でも双安定性のフォトクロミック錯体は、光による情報記録媒体や、センサーとしての応用が期待される。本研究ではまず、双安定の光反応性錯体として知られるルテニウムースルホキシド錯体の結合異性化反応に着目し、光・電位応答性の向上と異性化反応の安定的駆動の実現を試みている。またルテニウムースルホキシド錯体では、ルテニウム中心の価数や配位子同士の電子的影響によりスルホキシド部位が配位形式の変換を引き起こし、その異性体は電位が大きく異なることが知られている。このことから、異性化に伴う電位変化を利用した分子設計を行うことで分子の構造変化による電子移動制御系の構築を目指した研究について述べている。

第二章では種々のピリジン系ルテニウム-スルホキシド錯体を合成し、スルホキシド部位に結合する置換基に受ける影響や、キレート効果、配位子のゆがみ、溶媒などの条件が錯体の異性化反応にもたらす効果について検証し、分子設計について考察している。スルホキシド部位の解離を防ぐためにキレート型配位子の導入は必須である一方で、刺激応答性や反応の繰り返し安定性低下が課題となる。そのため、分子内の立体的・電子的影響や外的影響を検討することで、異性化反応の制御を目指した。本論分においては、ルテニウム錯体の正八面体型配位構造において三座配位子がmer型配位をとることに着目しており、トランス位の配位子を考慮した分子設計を行うことで異性化効率の向上が可能であることを示している。

第三章ではさらに、異性化に伴う錯体の酸化還元電位変化を利用した電子の受容部位を導入することでめざした、光反応に伴う電子移動系の構築について述べている。両異性体の中間に電位を持つトリフェニルアミンを導入することで、光反応にともなう電子移動の実現を試みている。定電位電解により活性化されたアクセプターのアミンラジカルの失活を測定することにより、光による異性化反応で電子移動を誘起する系の構築に成功したことを示している。また、アクセプターの活性化により引き起こされる、熱平衡に基づく電子移動反応とそれに伴う異性化反応をも観測している。

第四章では、以上の結果を総括し、今後の研究展望を述べている。

以上、本論文は錯体の異性化に伴う電子状態変化を利用し、光結合異性化反応によって電子移動が誘起される系の構築を達成したことを記述している。本博士論文において達成された光・電位変化に伴う電子移動制御系の構築は、錯体化学・光化学の分野において基礎的な貢献をするだけでなく、新たな光―電子移動系分子デバイス開発に貢献すると期待される。なお、本論文は坂本 良太、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、解析を行ったものであり、論文提出者の寄与が十分であると判断する。

従って、博士(理学)の学位を授与できると認める。