Chapter 1 : General introduction

In these days, much attention has been collected on molecules featuring photofunctionalities, conversion of light to signals such as luminescence and isomerization, because of their potential use in molecular optoelectronics, and ultimately, in molecular devices. One of well-known strategy of molecular design to harvest visible light is connecting electron donor and acceptor through π-conjugation in order to express intramolecular charge transfer (ICT) transition. Our laboratory has researched such molecules like donor-acceptor-donor (D-A-D) types, and one of good examples is those containing dimethyl diethynylfumarate (DDF) as an acceptor scaffold. For example, E-1, comprising DDF and triarylamine (TAA) as an electron donor site (Fig. 1) shows curious photochemical activities; strong absorption in the visible region and visible-light E-Z photoisomerization accompanied by switching of fluorescence and electronic communication in the mixed-valent (MV) state [1]. These properties would be suitable for molecule-based photofunctional devices; however, the efficiency of fluorescence and photoisomerization reaction of E-1 is slightly low (ΦF = 0.15 and Φ(total) = 0.075), so that some efforts are required to improve these properties.

My Ph.D work focuses on the creation of new photofunctional molecules based on E-1. In chapter 2, I focus on the improvement of photoisomerization property and adopt N-phenylcarbazole (NPC) group to new terminal donor moieties [2]. In chapter 3, I focus on the improvement of luminescent property, and attempt to convert the DDF framework into the Pechmann dye motifs as a central acceptor moiety by means of my original intramolecular double lactonization reaction. As a result, I found the missing structural isomer of Pechmann dye [3]. In chapter 4, I achieved the effective synthesis of the Pechmann dye's missing framework (P(56)-framework) and successfully introduced it into a new D-A-D system. As a result, I revealed that the P(56)-framework with two anisyl moieties shows efficient fluorescence (Φf = 0.91); in contrast, that with two TAA moieties displays slightly low luminescent property (Φf = 0.20).

Chapter 2 : Introduction of NPC as the donor site focusing on improvement of photoisomerization property

NPC, a fused analogue of triarylamine, collects much attention because of its utility for the hole transport and luminescent materials in organic electronics. Their efficient fluorescent property mainly stems from the radiative process from locally excited (LE) state, but in case of the D-A shaped molecules consisting of NPC and acceptor moieties, they also show fluorescence from the intramolecular charge transfer (ICT) excited state stabilized with the mesomeric effect [4]. Thus, I introduced NPC and dimetoxy substituted NPC moieties as two donors into D-A-D shaped molecules containing DDF framework, and synthesized E-2 and E-3. As a result, they show strong absorption in visible region assignable to the ICT transition from NPC to DDF. In particular, E-3 shows reversible photoisomerization reaction by excitation of the ICT band with high efficiency (Φ(total) = 0.40) (Figs. 2 and 3). In addition, E-3 is redox active because of preventing the electropolymerization reaction derived from carbazole moiety, so that its photochromism is accompanied by the modulation of electronic communication in the one-electron oxidized MV state; E-3 features more intense interaction than Z-3. This behavior is a suitable property for photofunctional material of molecule-based devices.

Chapter 3 : Conversion of DDF framework into Pechmann dyes focusing on improvement of luminescent property and discovery of a new structural isomer and an application in organic electronics

Pechmann dye (P(55)-framework) and its known structural isomer (P(66)-framework) are good pigments and acceptors, and feature rigid planar structures that stem from the cross-conjugated electron-withdrawing lactone rings [5]. These properties are optimal for the construction of D-A-D quadrupolar molecules with intense D-A interaction, and applications in organic electronics; however, the Pechmann dye frameworks have not been sufficiently appreciated in such studies. I regarded the DDF framework as an open form of Pechmann dyes protected by the methyl ester groups, and expected that with an appropriate ring closure reaction, it could open up a new route to the Pechmann dye family, and could provide access to their missing structural isomers (Fig. 4). Treatment of E-1 in boiling acetic acid containing a catalytic amount of hydrochloric acid gave rise to P(55)-1, P(66)-1, and surprisingly, P(56)-1 comprising an unreported structural isomer of Pechmann dye (Fig. 5). The three compounds feature more intense and redshifted ICT bands in the visible and NIR region, and higher fluorescence quantum yields than those of E-1 (Table 1 and Fig. 6). In particular, the value of fluorescent quantum yield of P(66)-1 is distinguished (Φf = 0.81) as displayed in Table 1. Moreover, the thin film of P(66)-1 fabricated by means of vacuum deposition on a silicon substrate works as stable P-type semiconductor with a hole mobility of 5.6´10-5 cm2 V-1 s-1.

Chapter 4 : The synthesis of a new molecule comprising P(56)-framework and its photochemical property

In Chapter 3, P(56)-1 does not show efficient fluorescence as seen in P(66)-1 although they consist of similar condensed lactone frameworks. To clarify this phenomenon, the creation of another molecules comprising P(56)-framework is required, so that the next challenge is to achieve effective synthesis. A plausible reaction mechanism for P(55), P(66), and P(56)-frameworks is shown in Scheme 1. In case of E-1 as a starting material, contiguous intramolecular double lactonization yields the products, where three types of cyclization, 5-endo, 6-endo and 5-exo take part (Fig. 7). As seen in Scheme 1, there are two synthetic routes of P(56)-framework in which the inversion of cyclization modes between 6-endo and 5-exo is required. I expected that this inversion could be achieved by varying the activator of an ethynyl moiety as a stepwise fashion. In particular, the following order, 5-exo and 6-endo is more attractive way because the P5'-framework is a precursor to not only P(56)-framework but also P(55)'-framework which is another missing structural isomer of Pechmann dye as displayed in Scheme 1; in other words, the synthesis of the P5'-framework will be a breakthrough to expand this research. Hereafter, I adopt this strategy to a new starting material, E-4 which has DDF framework and two anisyl moieties as terminal donors.

I focused on three kinds of activator, proton, triphenylphosphinegold(I) chloride and silver powder. As a result, I found two ways to produce the P5'-framework; one is the reaction of E-4 with triphenylphosphinegold(I) chloride and tetrabutylammmonium hydroxide (Bu4NOH), and the other is de-esterification reaction of E-4 and following process to disengage the dicarboxylic acid moiety as seen in Scheme 2. Especially the latter structure is revealed to possess both the P5'-framework and a carboxylic acid moiety by several measurement. This result indicates that its reaction with silver powder is expected to produce both P(56)-and P(55)'-frameworks because silver powder is a catalyst to mainly promote the cyclization in 5-exo mode in case of a single lactonization reaction between carboxyl acid and ethynyl moieties. Unexpectedly, this reaction gave rise to only P(56)-framework and I selectively synthesized P(56)-4 in the yield of 32%.

The crystal structure of P(56)-4 is shown in Fig. 8, and optical data for P(55)-4, P(56)-4, and P(66)-4 are assembled in Table 2. P(56)-4 shows the highest fluorescence quantum yield (Φf = 0.91), which assures the usability of the P(56) framework. From this result, the low Φf of P(56)-1 would stem from the interaction between TAA moiety and P(56)-framework in the ICT excited state; in other word, P(66)-1 would be a special case to show efficient fluorescence derived from the planar configuration of the ICT excited state [6].

Conclusions

By substituting the TAA moiety of E-1 with NPC, I achieved the improvement of the photoisomerization quantum yield. Also, I for the first time observed MV communication in a NPC-based compound, the intensity of which is switched upon the photoisomerization. The DDF framework can be converted into the Pechmann dye families via intramolecular double lactonization reaction, including one of the missing structural isomers, P(56)-framework. Donor-appended Pechmann dyes constitute D-A-D quadrupolar molecules, which show good photochemical properties and usability in organic electronics. The stepwise approach of this reaction leads the effective synthesis of a new molecule comprising P(56)-framework. The comparison of luminescent properties with molecules consisting of P(56)-or P(66)-framework reveals that the nonradiative decay would be enhanced by the introduction of large donor moieties except for P(66)-1 derived from the high planarity in the excited state.

Fig. 1. E-1

Fig. 2. Reversible E/Z photoisom erization in E/Z-3

Fig. 3. Time-course UV/Vis spectral change of E-3 in toluene upon irradiation with λ=500nm light. The inset shows a time-course 1HNMR spectral change in [D8] toluene under the same condition.

Fig. 4. My insight of the relationship between DDF framework and the frameworks of Pechmann dye family including a missing structural isomer.

Fig. 5. ORTEP drawing of P(56)-1 in the crystal with thermal ellipsoids set at 50 % probability. Hydrogen atoms are omitted for clarity. • = carbon, • = oxygen and • = nitrogen. Disordered atoms are remained in the center part.

Table 1. Photochemical properties of all D-A-D shaped molecules

Scheme 1. Plausible reaction mechanism of the intramolecular double lactonization

Fig. 7. The three modes of cyclization

Fig. 8. ORTEP drawing of P(56)-4 in the crystal with thermal ellipsoids set at 50 % probability. Hydrogen atoms are omitted for clarity. • = carbon and • = oxygen.

Scheme 2.Two way of reaction condition to produce P5'-framework

Table 2. Luminescent properties of P(55)-4, P(56)-4, and P(66)-4

本論文は全五章からなる。各章の内容に関して、第一章は研究の背景と目的、第二章はジエチニルジメチルフマレート(DDF)骨格の効率的光異性化挙動、第三章はDDF骨格を利用したペックマン色素骨格群の新規合成法の開発、未知構造異性体の発見、OFETへの応用、第四章は未知構造異性体を有する分子の合成と光化学物性、第五章は研究成果のまとめと展望で構成されている。以下に概要を記す。

第一章は研究の背景と目的について述べている。光機能を持つデバイスの開発はエネルギー問題解決への有用な手段であり、特に電子供与基(ドナー:D)と電子受容基(アクセプター:A)を連結した有機分子における光機能性の導出と、それらの有機エレクトロニクスデバイスへの応用は広く研究されている。本研究ではレドックス活性なD-A-D連結型有機分子の発光挙動に着目し、その効率的なスイッチング機能と強発光機能の導出を目的とした。その手段として、前者ではE/Zの構造異性を示すフォトクロミック骨格として知られるDDF骨格を、後者ではペックマン色素骨格群をアクセプター部位として利用した。

第二章はDDF骨格の可視光による可逆な光異性化挙動に焦点を当て、3,6ジメトキシフェニルカルバゾール(PCZ)をドナー部位としての導入したDDF骨格を有するD-A-D型有機分子における光異性化機能の高効率化について述べている。その結果、比較的高い光異性化量子収率(Φ(total) = 0.40(400nm)、Φ(total) = 0.31(500nm))の導出を達成し、PCZの剛直性に由来する無輻射放射の軽減、分子内電荷移動(ICT)遷移に由来する励起状態から光異性化反応を起こす二重結合のねじれた遷移状態への早い内部変換が示唆された。DDF骨格を含むD-A-D連結系ではE/Z異性体間での発光挙動、一電子酸化混合原子価状態でのレドックス核間電子的相互作用の変化を示す。よって本研究から、DDF骨格を含むD-A-D連結系において二種の可視光照射による光化学物性と電気化学物性の効率の良いスイッチング機能の導出が可能である事を示した。

第三章、第四章はペックマン色素骨格のラクトン環骨格に由来するアクセプター性と剛直性に由来する強発光に焦点を当て、D-A-D連結系へのペックマン色素骨格の導入について述べている。第三章ではペックマン色素骨格の新規合成法としてDDF骨格と両末端ドナー部位にトリアリールアミン部位を有すD-A-D型分子を出発原料とした分子内二重環化反応を提案し、既知合成法では得られない六員環と五員環の縮環した未知構造異性体(P(56)骨格)を含む三種のペックマン色素骨群の導入に成功した。光化学、電気化学測定よりペックマン色素骨格群がD-A-D連結系において強いアクセプター性を発現することを示した。特に六員環同士が縮環したP(66)骨格はトルエン溶液中での強発光(Φf = 0.82)、その薄膜がP型のOFETとして動作する事を明らかにした。一方でP(56)骨格のトルエン溶液中での発光量子収率はΦf = 0.20程度である事を明らかとした。

第四章では未知構造異性体(P(56)骨格)の効率的な合成とその光化学物性について述べている。DDF骨格と両末端ドナー部位としてアニシル基を有す出発原料を利用し、分子内二重環化反応の段階的な制御によるP(56)骨格の効率的な合成手法を見出した。アニシル基を両末端ドナー部位としたペックマン色素骨格を持つ分子群の発光挙動を比較し(Φf = 0.80(P(66))、Φf = 0.91(P(56)))、ICT励起状態でのペックマン色素骨格とドナー部位のトリアリールアミノ基との相互作用により生じる共鳴構造の剛直性の違いが、第三章で示した発光挙動の差異として現れる事を明らかとした。

第五章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D-A-D連結型有機分子における光機能性機能の導出に着目し、発光のスイッチング、強発光性の高効率化に成功した事を述べている。特に本博士論文において明らかとされた、レドックス活性なD-A-D連結系における未知構造異性体を含むペックマン色素骨格群の新規合成手法、アクセプター部位としての有用性と有機デバイス材料への応用は、有機化学、光化学、電気化学の分野における基礎的な貢献に加え、有機エレクトロニクスに適した新規骨格の提案という観点から、発展的な寄与をもたらすと期待される。なお、本論文第二章は坂本良太、西原 寛との共同研究、第三章は利光史行、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、博士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。

よって、博士(理学)の学位を授与できるものと認める。