Prof. Kazuo Akagi has made it possible to dynamically control the helical structure and functions of optoelectronic conjugated polymers using an external force-responsive liquid crystal field and has cultivated and developed a novel research field named "Interdisciplinary Chemistry Based on Integration of Liquid Crystals and Conjugated Polymers". His research achievements are summarized below.
1. Construction of external stimuli-responsive liquid crystal field and control of higher-order helical structure
He developed for the first time a chiral reaction field that shows reversible chirality inversion upon a change in temperature. He synthesized a novel chiral compound that bears both an axially chiral binaphthyl moiety, which exhibits a decrease in chirality with increasing temperature, and an asymmetric center-containing chiral moiety that exhibits an increase in chirality with increasing temperature, and then added this compound into a nematic liquid crystal (N-LC) to construct a chiral liquid crystal (N*-LC) that provides a chirality inversion upon temperature change. Through acetylene polymerization in the N*-LC, he succeeded in the synthesis of a helical polyacetylene film, which showed an opposite helicity in helical structure and spiral morphology at low and high temperature.
He also synthesized a photoresponsive chiral compound by introducing an axially chiral binaphthyl group into both terminal sites of a photoresponsive dithienylethene moiety. Through adding the photoresponsive chiral compound as a chiral dopant into N-LC, he developed the photoresponsive N*-LC, which showed reversible chirality inversion via photochemical ring closing and opening of the dithienylethene moiety upon irradiation with UV and visible light, respectively. He synthesized helical polyethylenedioxythiophene (H-PEDOT) using the N*-LC as an asymmetric solvent for the electrochemical polymerization of EDOT, hence making it possible to control the helicity of the helically π-stacked structure and spiral morphology of H-PEDOT through irradiation of an electrochemical polymerization cell with UV and visible light.
Next, he synthesized a poly-para-phenylene derivative with cationic moieties in both side chains and an axially chiral binaphthyl derivative with anionic substituents. The mixed system of the cationic polymer and the anionic chiral compound was found to form a helically π-stacked aggregate due to electrostatic ionic interactions and π-π interactions, and the helically π-stacked aggregate was found to further form polymer spherulites through hierarchical self-assembly and even exhibit a blue circularly polarized luminescence.
2. Amplification of circularly polarized luminescence and dynamic control of luminescent functions.
He synthesized a polyacetylene derivative with substituents in both side chains and found that the polyacetylene derivative shows not only thermotropic but also lyotropic liquid crystallinity and exhibits circularly polarized luminescence (CPL) with a dissymmetry factor on the order of 10-1, which is the highest value among aliphatic conjugated polymers. Furthermore, he achieved an enhancement of CPL intensity, leading to a high dissymmetry factor very close to an ideal value of 2, by introducing CPL of the polyacetylene derivative into an N*-LC-containing cell that enabled the selective reflection and transmission of CPL.
Next, he synthesized novel aromatic conjugated polymers bearing photoresponsive moieties at the side chains and succeeded in photoswitching control between luminescence and quenching of the conjugated polymers via ring closing and opening photoisomerizations of the dithienylethene moiety at the side chain. He synthesized a photoresponsive aromatic conjugated polymer with red, blue, and green (RGB) fluorescence and prepared nanoparticles of these polymers. He provided white illuminating polymer solutions by mixing the RGB polymer nanoparticles and achieved a photoswitching of white luminescence and quenching through irradiation of the polymer solution with UV and visible light. He also prepared polymer systems that enabled control of the photoswitching between white luminescence and luminescence of each RGB color. Thus, he developed polymer illuminants whose luminescence and quenching are reversibly photoswitched by irradiation with UV and visible light.
3. Control of hierarchical structure of conjugated polymers and development of morphology-retaining carbonization
He found that acetylene polymerization under an ultimately twisted N*-LC field gives a helical polyacetylene with fibril bundle-free (single fibril) morphology but not ordinary fibril bundle morphology. The result is explained by a peculiar mechanism in which the helical pitch of the highly twisted N*-LC field is much smaller than the diameter of the fibril bundle, so that the N*-LC cannot provide enough of the spatial field to enable the formation of the fibril bundle, resulting in a single fibril of helical polyacetylene. This situation indicates that it is possible to control the breadth of the spatial field for polymerization by tuning the twisting extent of N*-LC. Furthermore, it is elucidated that the N*-LC field is useful for the syntheses of not only helical polyacetylene but also a variety of helical aromatic conjugated polymers and copolymers bearing helical π-stacked structures.
He discovered that the carbonization at 800 °C and even the graphitization at 2600 °C of chemically iodine-doped polyacetylene film used as a precursor can produce carbon and graphite films whose morphologies resemble the precursor, and furthermore, the yields are higher than 90 %. Using the novel procedure named "morphology-retaining carbonization method", he succeeded in preparing a helical graphite film with spiral morphology. He clarified that while iodine-doping has been regarded as essential for the evolution of the electrical conductivity of conjugated polymers, it also plays an indispensable role for morphology retention in the carbonization and graphitization of conjugated polymers. Actually, he confirmed that electrochemically oxidized (doped) helical PEDOT provides a helical graphite film bearing spiral morphology resembling that of the precursor.
As stated above, Prof. Akagi has promoted research on conjugated polymers and developed the research field named "Interdisciplinary Chemistry Based on Integration of Liquid Crystals and Conjugated Polymers", which is founded on an original concept and the consideration of electron theory. His research achievements are highly valued at home and overseas as distinguished ones leading to new concepts and progress in the field of optoelectronic materials chemistry, and he has indeed made outstanding contributions in chemistry and interdisciplinary research fields. Therefore, his achievements are recognized as deserving The Chemical Society of Japan Award.