Professor Ikeda has been working on photofunctional materials for more than 30 years. As a new working principle, he became interested in 'cooperative effects'. In the cooperative effects, if the changes of the properties of a small number of molecules are brought about by external stimuli such as light, these changes are transferred to a whole system, and the changes in the whole system are induced. This means that only a small amount of energy is needed to induce the change of the whole system, which will lead to an energy-saving process. Liquid crystals are a typical organic material that shows this cooperative effect in relation to alignment; if the alignment of a small number of liquid-crystalline molecules is changed by external stimuli, the surrounding liquid-crystalline molecules will change their alignment. Basic strategy of Prof. Ikeda for the development of novel photofunctional materials is to provide polymer materials with high photoresponsiveness and liquid-crystalline properties for the resultant polymer materials to exert cooperative actions. Some of his outstanding achievements are summarized below.
His major contributions to the progress in the chemistry of molecular catalysis include the following three points:
1. Photochromic liquid-crystalline polymers as high-performance photofunctional materials
Prof. Ikeda and coworkers found that if an azobenzene derivative is dispersed in a nematic liquid crystal and the resultant mixture is irradiated with UV light so as to bring about trans ? cis isomerization of the azobenzene guest molecules, the mixture undergoes a nematic ? isotropic phase transition isothermally (photochemical phase transition). This process is reversible and an isotropic ? nematic phase transition is also induced isothermally if the mixture in the isotropic state is exposed to visible light to cause cis ? trans back isomerization. This photochemical phase transition can be induced not only in mixtures of azobenzene and low molecular-weight liquid crystals but also in those of azobenzene and liquid-crystalline polymers. However, in the low molecular-weight liquid crystals the fluidity is so high that induced isotropic phases are unstable, which leads Prof. Ikeda to focus his attention on photochromic liquid-crystalline polymers, in which photochromic moieties are incorporated in the side chains of liquid-crystalline polymers. Prof. Ikeda and coworkers demonstrated that two-dimensional images can be recorded in thin films of these photochromic liquid-crystalline polymers by photoirradiation with high resolution, and these images can be kept very stable below Tg of the polymers. One of the very important achievements by Prof. Ikeda is the discovery of azobenzene liquid-crystalline polymers, in which an azobenzene moiety plays both roles of a photoresponsive chromophore and a mesogen with its trans form. In these polymers, the photochemical phase transition can be induced very fast, in 200 ns.
2. High-performance holographic materials by means of photochromic liquid-crystalline polymers
Three-dimensional displays are a dream of human beings. Among a variety of methods for 3D displays, holography is known as an effective way with the least stress on human eyes. Prof. Ikeda and coworkers have succeeded in fabricating photochromic liquid-crystalline polymers as high-performance holographic materials with ultrafast response (200 ns) and large modulation of refractive index (0.1). Upon exposure to writing beams, a nematic - isotropic phase transition is brought about in bright areas of the interference pattern and the original liquid-crystalline phase remains unchanged in the dark areas, which results in transcription of information in a form of light intensity into a periodic change of phases (nematic/isotropic). Prof. Ikeda and coworkers observed a diffraction efficiency of 32%, which is close to the theoretically anticipated maximum value of 34% of thin holograms in the Raman-Nath regime.
3. Photomobile polymer materials - from nano to macro
Solar energy is a valuable energy source that we can utilize with no cost. At present, solar energy is converted to electricity and mechanical work is produced with the electric energy. Prof. Ikeda considered that if light energy can be converted directly to mechanical work, both high conversion efficiency and miniaturization of devices could be achieved simultaneously. He believed that if he can drive polymer materials precisely by light, he can fabricate photomobile polymer materials that show various three-dimensional movements upon exposure to actinic light. Prof. Ikeda and coworkers succeeded for the first time in inducing bending of a crosslinked azobenzene liquid-crystalline polymer film by irradiation with UV light and unbending upon exposure to visible light. Furthermore, it was demonstrated that irradiation of crosslinked polymer films in a polydomain state with linearly polarized light leads to bending along the polarization direction of the linearly polarized actinic light, which means that bending of the polydomain films can be induced in any direction just by choosing the polarization direction of the actinic light. This finding has made a profound impact on the community of materials chemistry in the world and has stimulated great progress in this field: a number of research groups in the world have started studies on this topic. Prof. Ikeda and coworkers fabricated laminated films of the crosslinked azobenzene liquid-crystalline polymer layer and polymer substrates such as low-density polyethylene and found that these laminated films show much better performance as photomobile materials. They successfully demonstrated a first light-driven plastic motor and other three-dimensional motions such as an inchworm walk and a robotic arm motion driven only by light.
In conclusion, Prof. Ikeda has created novel photofunctional polymer materials based on cooperative effects by liquid crystallinity and photochromic properties. With these materials he successfully demonstrated two- and three-dimensional image storage with high resolution and high stability of the stored images, and various three-dimensional movements. In his systems, a photochemical reaction such as trans - cis photoisomerization of an azobenzene moiety, which is a change at a molecular level, is converted and/or amplified to a macroscopic change of materials through cooperative motion of liquid crystals and strong correlation between a polymer chain conformation and an alignment of mesogens in crosslinked liquid-crystalline polymers. These polymers can act successfully as smart photomobile polymer materials that show various three-dimensional movements upon exposure to actinic light. His research has opened a new field in materials science and has acquired high reputation in the relevant community in the world, by which Prof. Ikeda deserves "The Chemical Society of Japan Award".