Introduction

Metal complexes bearing π-conjugated chelating ligands are valid for both application and novel properties. For example, photophysical properties of metal complexes are of much interest for dye-sensitized solar cell and light-emitting devices. For another example, metal complexes are often found to be redox-active molecule, where two oxidation states can be reversibly switched by electronic stimuli, and they are useful in nanotechnology applications such as in molecular electronics.

Our group has employed copper complex bearing a bidendate ligand that includes a coordinated pyrimidine moiety (Figure 1). The interconversion between pyrimidine ring rotational isomers in copper(I) state is described in Figure 1b, where the notation of inner (i-CuI) and outer (o-CuI) isomers indicates the direction of the pyrimidine ring. The steric effects cause a shift of the redox potential as well as photophysics, owing to well-established features of copper complexes. Our group has constructed single molecular systems exhibiting an electrochemical potential response from an artificial molecular rotor with a stimulus-convertible function (Figure la). In other words, function of our previous system is based on a collaboration of electrochemistry and rotational bistability.

The aim of studies in my Ph.D course is to develop new functions by photofunctionalization of this molecular system (Figure 1c). I describe herein the new classes of luminescence (Chapter 3) and photo responsibility (Chapter 4).

Detail of Molecular Bistability Based on Pyrimidine Ring Rotation in Copper(I) Complexes (Chapter 2)

The rational molecular design requires a detailed investigation for the equilibrium between i-CuI and o-CuI. 1・BBF4 (1+=[Cu(Mepypm)(DPEphos)]+, Mepypm =4-methyl-2-(2'-pyridyl)pyrimidine, DPEphos = bis[2-(diphenylphosphino)phenyl]ether), 1.B(C6F5)4, 2・1BF4 (2+=[Cu(Mepypm)(dppp)]+, dppp = 1,3-bis(diphenylphosphino)propane), and 2・BB(C6F5)4 were newly synthesized and characterized. The rotational bistability of these complexes in common organic solvents was characterized using 1H NMR analysis at several temperatures (Figure 2b). The interconversion between i-CuI and o-CuI is an intramolecular process, as confirmed by 1H NMR analysis of a mixed solution of 1・BBF4 and [Cu(bpy)(DPEphos)]BF4 (bpy = 2,2'-bipyridine). The isomer ratio of i-CuI and o-CuI was solvent- and counterion-sensitive. X-ray structural analysis revealed that two rotational isomers, i-CuI and o-CuI, of 2+ were separately obtained as single crystals (Figure 2a). The reduced contact of the counterion to the complex cation in polar solvent seems to contribute to the relative stability of i-CuI and o-CuI. The findings are valuable for the design of molecular mechanical units that can be readily tuned via weak electrostatic interactions.

Dual Emission Caused by Ring Rotational Isomerization of a Copper(I) Complex (Chapter 3)

Since i-CuI and o-CuI are expected to be different in the emission properties due to well-established relationship between coordination structure and photophysics in copper(I) complexes,2'3 dual luminescence caused by ring rotational isomerization can be expected. The complex exhibited characteristic charge transfer absorption and emission bands in solution (Figure 2c). I investigated photophysics of 1・BBF4 in acetone (i-CuI:o-CuI = 30:70) using time-resolved emission spectra. I conclude that the photoprocesses of the two isomers, i-CuI and o-CuI, are different in the identity of the excited state. Emission lifetime of i-CuI (τ= 40 ns) is much longer than that of o-CuI (τ= 2 ns) because of inhibition of both steric rearrangement and solvent-coordination quenching in the photoexcited state (Figure 2d). Emission wavelength of i-CuI is blue-shifted from that of o-CuI. Heat-sensitivity of emission of i-CuI is larger than that of o-CuI due to difference in photoexcited state properties between isomers. Both i-CuI and o-CuI coexist in solution, and emit around room temperature. This finding shows a novel way to handle photophysics of metal complexes bearing π-conjugated chelating ligands.

Repeatable Copper(II)/(I) Redox Potential Switching Driven by Visible Light-Induced Coordinated Ring Rotation (Chapter 4).

As I mentioned above, collaboration of photophysics and rotational bistability enable me to develop a new type of emission. Therefore, combination of photophysics, redox ability, and rotation must provide another new type of function. Since 1・BBF4 and 2・BBF4 did not exhibit reversible redox activities, I designed and prepared a novel copper(I) complex, 3・BBF4 (3+=[Cu(MepmMepy)(L(mes))]+, MepmMepy = 4-methyl-2-(6'-methyl-2' -pyridyl)pyrimidine, L(Mes) =2,9-dimesity1-1,10-phenanthroline). The rotational bistability of 3+ was characterized in a similar method to 1+ and 2+. Two rotational isomers, i-CuI and o-CuI, coexist and interconvert in the solution. The rotational interconversion between i-CuI and o-CuI is found to be frozen at 203 K and active at 250 K. The two redox reactions, i-Cu(II/I) and o-Cu(II/I), are different in potentials (ΔE°'= 0.14 V). The interconversion of oxidized rotational isomers, i-Cu(II) and o-CuII, is faster than that of copper(I) state, and o-Cu(II )is thermodynamically more preferred than i-Cu(II). Both i-CuI and o-CuI absorb visible light in solution, and the redox potential of the light excited state is sufficiently large to induce photoinduced electron transfer (PET) with a redox mediator, decamethylferrocenium ion (DMFc+). Difference in absorption between i-CuI and o-CuI is very small, considering the results of UV-vis spectra upon chemical oxidation at low temperature.

Two redox waves, one each for the i-Cu(II/I) and o-Cu(II/I) in a ratio of 30:70, were observed in a cyclic voltammogram at 203 K in dichloromethane electrolyte solution in the presence of 4 equiv. DMFc+ (Figure 3a top). The peak currents of the waves reflect the isomer ratio (i-CuI:o-CuI = 30:70). At 60 min, with photoirradiation with visible light (λ> 400 nm) at 203 K, the redox waves gradually converged to a wave corresponding to the o-Cu(II/I) (Figure 3a middle). The ratio of i-CuI to o-CuI in this metastable state is i-CuI:o-CuI = 12:88, estimated from a simulated model fit to the cyclic voltammograms. Subsequent heating for 2 min at 250 K recovered the initial voltammogram (i-CuI:o-CuI = 30:70) (Figure 3a bottom). Repeatable electrochemical potential switching based on photodriven rotation, i-CuI → o-CuI and heatdriven rotation, o-CuI→i-CuI was demonstrated. PET processes can take a bypass route in the rotation of the copper(I) complex (i-CuI + hv→ i-CuI*, i-CuI* + DMFc+→ i-Cu(II) + DMFc0, i-CuII→ o-CuII, o-CuII + DMFcO→ o-CuI + DMFc+). The system works not only with a redox mediator but also upon partial oxidation, in which the copper complex itself considerably assists the photo-driven rotation (i-CuI + hv→ i-CuI*, i-CuI* + o-Cu(II)→ i-Cu(II) + o-CuI, i-Cu(II)→ o-Cu(II)). Generally, photochromic molecules are accompanied by significant color changes. The photo- and heat-driven rotation system works without a significant color change, which is a representative feature for new type of photoresponsibility.

Conclusion I developed dual emission caused by ring rotational isomerization. This strategy is applicable not only for luminescence itself but also properties in photoexcited state related to photocatalysis and photoelectron conversion. I constructed a PET-induced rotational system which switches redox potential. This finding can provide electronic, magnetic, and other molecular signaling characteristics because of repeatable conversion of external stimuli into redox potential signals.

Figure 1. (a) Conceptual diagram showing function of coordinated pyrimidine ring rotation system. (b) Chemical equilibrium between two rotational isomers, i-CuI and o-CuI. (c) Studies in my Ph.D course.

Figure 2. (a) Crystal structures of o-2・BBF4・0.5MeOH (left) and i-2・B(C6F5)4 (right). (b) Aromatic 1H NMR signal of a Mepypm moiety in 1・BF4 in acetone-d6 at several temperatures. (c) UV-vis absorption spectrum (dotted line) and the steady-state emission spectrum using 400 nm excitation (solid line) of 1+ in acetone at room temperature.(d) Experimental 630 nm emission decay of 1+ in acetone at room temperature excited at 425 nm.

Figure 3. (Top) Conceptual diagram showing the photodriven and heatdriven pyrimidine ring rotational isomerization of 3・BBF4. (a) Photorotation experiments in the DMFc+ system with notes about the procedures. Experimental cyclic voltammograms at a scan rate of 50 mV s-1. Investigated DMFc+ systems comprise 3・BBF4 (0.45 mM) in 0.1 M Bu4NBF4-CH2Cl2 containing 1.8 mM DMFc・BBF4 at 203 K in the dark. Top: initial state. Middle: after 60 min visible light irradiation (λ> 400 nm) at 203 K. Bottom: after 2 min heating at 250 K in the dark. The reversible changes in the molar ratios of the isomers upon light irradiation and heating are illustrated in the right panels.

本論文は5章からなり、第1章は研究の概論と背景、第2章は銅錯体のピリミジン環反転平衡の詳細、第3章は環反転異性化に基づく二重発光性、第4章は光電子移動で駆動する環反転異性化による光応答性に基づく可視光電気化学ポテンシャル変換、第5章は研究成果の総括を述べている。以下に各章の概要を示す。

第1章では、本研究の概論と背景について述べている。まず金属錯体の光物性や電気化学特性は応用や新しい概念の創成に重要であることをまとめている。次にジイミン銅錯体のレドックス特性や光物性は銅周辺の配位構造に大きく依存する特徴について述べている。さらに、熱や化学試薬の外部刺激を銅錯体のピリミジン環反転を介して電極の自然電位などの電気信号へと変換できる分子スイッチの構築について述べている。ピリミジン環の反転により生じる二つの異性体の電気化学特性が異なっていることが鍵となり達成された機能である。そして、本論文の研究目的である環反転系の光機能化に基づく新機能の開発と、その戦略について説明している。

第2章では、種々の新規銅一価錯体を用いて、銅錯体のピリミジン環反転由来の平衡の詳細、特に溶媒和イオン対の影響について述べている。メチルピリジルピリミジンとかさ高いジホスフィンを配位子とする新規銅一価錯体を合成、同定し、その反転挙動を単結晶X線構造解析やNMRを用いて明らかとしている。また、ピリミジン環反転由来の二つの異性体は溶液中で共存し相互変換していること、反転異性体の割合に対する溶媒和イオン対の影響、相互変換が分子内プロセスであることを示す証拠、反転の速度論、単結晶中の挙動について述べている。目的の機能のために非配位性の有機溶媒中が適していることを示している。

第3章では、反転と光物性との融合に基づく新機能について述べている。アセトン溶液中において二つの反転異性体は共存し相互変換しており、同条件で電荷移動遷移由来の吸収及び発光を示すことを明らかとしている。一般的な銅錯体とは異なり二成分の発光減衰曲線を示したことから、室温アセトン溶液中において二つの反転異性体は両方とも発光するが発光寿命がそれぞれ異なることがわかった。ピリミジン環の反転による銅中心周辺の立体環境の変化が、光励起状態における構造緩和や溶媒分子による失活の性質を変えたためと解釈できる。ピリミジン環反転異性化由来の二重発光性を示しており、金属錯体の光励起過程を制御する新しい方法論と言える。

第4章では、反転、光物性に加え電気化学特性も駆使した機能について記述されている。新規銅一価錯体の合成、同定、電気化学測定や分光測定を駆使した反転挙動とその光物性及び電気化学特性の解明が述べられている。二つの反転異性体は溶液中で共存し相互変換しており、その相互変換速度のON/OFFを熱でスイッチすることが可能である。また二つの反転異性体の電気化学特性を反映する銅二価一価の酸化還元電位が0.14Vシフトしていることや、二つの反転異性体の色を反映する吸収スペクトルにほとんど差がないことを明らかとしている。さらにジクロロメタン電解質溶液中、レドックスメディエータ共存下における電気化学測定から反転挙動を詳細に検討し、可視光と熱による可逆な反転異性化に基づく分子スイッチになることが示されている。光電子移動により一時的に生じた銅二価の状態を経由することで、光異性化を引き起こす機構で駆動している。最もよく用いられる光応答性であるフォトクロミズムとは異なり、光電子移動でより効率良く駆動し、色変換は無く、代わりに電気化学ポテンシャルが変換する新しいタイプの光応答性である。

第5章では、今回の論文の総括が記されている。

以上、本論文では、銅錯体のピリミジン環反転に基づく分子スイッチの光機能化による新しい概念の発光、光応答性について述べられている。今回提案された戦略は、発光のみならず、金属錯体の光励起過程を用いると有利である光デバイス、例えば光触媒、光電変換、発光素子の性質を制御する新概念である。さらに光及び熱をピリミジン環反転を介して電気化学ポテンシャルへと変換できる機能を有する系の開発は、分子回路に組み込むことで、人間の五感から想像される機能を分子レベルで獲得しうることを示している。本論文第2章及び第4章は野元邦治、久米晶子、西原 寛との共同研究、第3章は野元邦治、久米晶子、井上圭一、酒井誠、藤井正明、西原 寛との共同研究であり、一部はすで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、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。