Molecular self-assembly is a promising approach for developing new functional soft materials. Liquid crystals are soft materials that form fluidic and ordered states of molecules. For development of new dynamically functional liquid-crystalline (LC) materials, unconventional design of the molecules and self-assembled structures from nano to macro scale is important. These new LC assemblies can be built by using a combination of specific intermolecular interactions, nanosegregation behavior, and the molecular shape of liquid crystals. On the other hand, LC assemblies that can change their LC nanostructures and molecular orientations in response to stimuli such as photoirradiation have attracted attention because these stimuli-responsive liquid crystals have great potential as ordered materials exhibiting anisotropic and dynamic function. In this thesis, the use of spiropyran compounds for development of new LC materials is described. Spiropyrans are unique compounds that show photo-, thermo-, chemo-, and solvatochromic properties. Spiropyrans have been widely used for various applications such as molecular switches, optical devices, metal sensor, and probes due to their reversible spiro-merocyanine (SP-MC) isomerization. The molecular isomerization of spiropyrans from non-planar and non-ionic spiro form to planar and ionic merocyanine form is expected to promote aggregation and formation of the ordered nanostructures. However, spiropyran-based liquid crystals are very limited. In fact, to the best of our knowledge, there is no any report on the columnar LC assemblies of spiropyran compounds. Furthermore, the potential applications of spiropyran-based LC materials have not been explored until now.

In this thesis, the author is interested in using acid- and photoinduced SP-MC isomerization of spiropyrans for the induction of new LC assemblies. For this purpose, four different approaches have been explored, there are: a) addition of an acid, b) photoirradiation, c) a combination of addition of acid and photoirradiation, and d) a combination of photoirradiation and addition of ionic liquids. The author also expected that if columnar LC assembly can be induced in the ionic merocyanine form of spiropyran derivatives, the resulting LC materials may show interesting ion conduction properties.

In chapter 1, an overview of liquid crystals and a basic introduction of spiropyran compounds are presented. In addition, the objectives and the outline of this thesis are stated.

In chapter 2, the development of the first example of spiropyran-based columnar liquid crystals is described. Since spiropyrans are acid-responsive, the author aimed at manipulating acid-induced SP-MC isomerization for: i) the formation of columnar phases through self-assembly of merocyanine isomers of a spiropyran fan-shaped compound and ii) the use of the columnar nanostructures formed by the merocyanine isomers as the conduction path for an ion transportation. A spiropyran-based compound having a fan-shaped trialkoxy benzene group has been designed and prepared. The induction of columnar LC phases for spiropyran derivatives is achieved by the addition of 4-methylbenzenesulfonic acid. The induction and stabilization of these columnar assemblies are ascribed to the acid-induced SP-MC isomerization and the presence of protonated merocyanine isomers. Several key factors for the induction of columnar LC assemblies such as acidities, the sizes of anions of the added acids, molar fraction of the added acids, and miscibilities of the added acids with the spiropyran derivatives have been examined. In addition, the anisotropic ionic conductivities of the LC mixtures of a fan-shaped spiropyran derivative with 4-methylbenzenesulfonic acid have been measured. One-dimensional ionic conductivities of these LC mixtures are measured by an alternating current impedance method with comb-shaped gold electrodes as reported previously. The self-assembled columnar polydomains are aligned in the direction perpendicular and parallel to the gold electrode via mechanical shearing. The ionic conductivities parallel to the columnar axis (σ||) for the LC mixtures are higher than those perpendicular to the axis (σ⊥) in the ordered LC state. The highest σ|| value in the columnar state is 8.5 × 10-6 S cm-1. Anisotropy (σ||/σ⊥) of ionic conductivities in the columnar state is ca. 10. The measured anisotropic ionic conductivities are due to ion conduction and possibly including proton hopping in phenolic moieties of the merocyanine stacks in the columnar nanostructures.

In chapter 3, the development of spiropyran-based ionic liquid crystals is stated. Imidazolium-based ionic liquids having a sulfonic acid group have been used to induce LC assemblies of a fan-shaped spiropyran derivative. The binary mixtures of this spiropyran derivative with the acidic ionic liquids exhibit columnar phases with wider temperature ranges. The ionic interactions formed by the imidazolium ionic moiety should contribute to the stabilization of the columnar phases. On the contrary, the addition of an ionic liquid without a sulfonic group to the same spiropyran derivative does not lead to formation of columnar nanostructure although a miscible mixture is obtained.

In chapter 4, the induction of LC phases of nitrospiropyran derivatives is studied by four different approaches: a) addition of an acid, b) UV irradiation, c) combined effects of the addition of an acid and UV irradiation, and d) combined effects of UV irradiation and the addition of an imidazolium-based ionic liquid. The addition of an acid successfully induced LC phases for several nitrospiropyran derivatives. The induction of the mesomorphism is due to: a) ionic interactions of these nitrospiropyran derivatives in the protonated merocyanine form, b) nanosegregation of the ionic and non-ionic moieties, and c) the filling of space by anion of the added acid. Moreover, the formation of hydrogen bonds between the phenolic OH group and the anion of the acid might also contribute to the stabilization of LC nanostructure. On the other hand, an induction of LC assemblies by UV irradiation for nitrospiropyran derivatives was expected because of the formation of ionic MC isomers. The MC isomers can form ordered nanostructures through ionic interactions. However, the results indicate that UV irradiation alone fails to induce mesomorphism. It is assumed that the formation of ionic interactions for the single component of MC isomers is not suitable for the induction of LC nanostructure. The combined effects involving UV irradiation and the addition of an acid or imidazolium-based ionic liquid also fail to induce mesomorphism for nitrospiropyran derivatives.

In chapter 5, a conclusion and perspectives of this thesis are summarized.

In this thesis, acid-induced LC assemblies of spiropyran compounds have been developed. The results clearly indicate that the complementary effects of ionic interaction, nanosegregation, hydrogen bonding, and space-filling effect of the anion of the added acid are important for the induction and stabilization of LC assemblies. The new LC materials exhibit anisotropic ionic conductivities. They are potentially to be used as one-dimensional ion conductors. In future, these materials could be further developed as media for gas sensing and gas entrapment.

液晶は、有機分子が動的な状態を保ったまま秩序化している。このような特徴は他の材料には見られないユニークなものであり、これを生かすことにより、表示機能にとどまらない機能材料としての新しい発展が大いに期待できる。本論文は、スピロピラン誘導体の液晶組織化に着目し、酸添加によるスピロピラン構造からメロシアニン構造への構造異性化を利用した新しい液晶性分子集合体の構築とイオン伝導機能の発現に関する研究について述べている。本学位論文は以下の5章から構成されている。

第1章は序論であり、液晶材料一般およびスピロピラン誘導体の機能性について紹介している。さらに液晶性スピロピラン誘導体の開発の経緯について述べ、これらの研究背景を基に、本学位論文の研究目的と意義について述べている。

第2章では、スピロピラン誘導体のカラムナー液晶化について述べている。例えば、扇型分子構造のスピロピラン誘導体を設計・合成しており、4-メチルベンゼンスルホン酸の添加によりカラムナー液晶性を発現させることに成功している。ここでは、スピロピランへの酸添加により、スピロ-メロシアニン異性化が起こり、メロシアニン構造がイオン相互作用および水素結合を駆動力として自己組織化し、カラムナー液晶構造が形成されると結論づけている。また、添加する酸の酸性度・分子サイズ・添加の量がカラムナー液晶相の発現において重要な役割を果たすことを明らかにしている。さらに、カラムナー液晶相において、異方的なイオン伝導機能が発現することを見出している。

第3章では、スピロピラン誘導体の液晶性発現に及ぼす添加する酸性分子の構造についてより深い議論をしている。特に、スルホン酸基を有するイミダゾリウム型イオン液体を添加することで、室温を含む幅広い温度範囲で液晶性を発現させることに成功している。ここでは、イオン液体の特性に由来したナノ相分離構造の形成により、カラムナー液晶構造の熱的安定化が達成されている。

第4章では、ニトロ基を導入したスピロピラン誘導体を設計・合成し、液晶性発現に及ぼす酸添加の効果および紫外線照射の効果について詳細に述べている。酸添加により、プロトン化メロシアニン構造に異性化することで、液晶性を発現させることに成功している。一方、紫外線照射により、スピロピランをメロシアニン構造に異性化させても、液晶性が誘起されないことを報告している。酸添加と光照射の結果より、液晶性ナノ構造の形成には、プロトン化メロシアニン状態におけるイオン相互作用、フェノール性水酸基とアニオンとの水素結合、イオン部位と非イオン部位のナノ相分離が協調的に働くことが重要であると結論づけている。

第5章は本論文の結論であり、本研究を通して得られた新しい知見および新しいイオン機能性液晶材料の開発指針について述べている。

以上のように、本論文はスピロピラン誘導体と酸からなる複合系における液晶組織化とイオン機能発現に関して述べたものであり、今後の自己組織性機能材料の展開に大きく貢献するものである。

よって本論文は博士(工学)の学位請求論文として合格と認められる。