The Chemical Society of Japan

Prof. Masahiro Irie
Department of Chemistry and Biochemistry
Graduate School of Engineering
Kyushu University

Development of Photochromic Molecular Systems with Outstanding Performance

Prof. Masahiro Irie has been conducting the research of photochromic molecular systems for the past 30 years. In the middle of '70s he introduced the concept of "Photoresponsive Polymers" and created various types of photoactive polymers, which reversibly change the physical and chemical properties by light, such as viscosity, wettability, phase transition and even bending of a polymer gel. In the middle of '80s he invented a new class of photochromic molecules, named "diarylethene" , which undergo fatigue resistant and thermally irreversible photochromic reactions. Now, the derivatives are widely used as useful photoswitching elements of molecular switches. In the following his main achievement will be briefly introduced.   Established families of organic photochromic molecules suffer from photochemical degradation. The fatigue is a major drawback and precludes many practical applications. In 1985 Prof. Irie invented a new class of photochromic molecules, named "diarylethene" , which exhibit fatigue resistant and thermally irreversible properties. The thermal irreversibility is an indispensable property for the application to photonics devices, such as optical memory media and switching devices. "Diarylethene" is a derivative of cis-stilbene, in which phenyl groups of cis-stilbene are replaced with heterocyclic aryl groups, such as thiophene, benzothiophene or furan groups. Although the photogenerated closed-ring isomer of cis-stilbene disappears in a few minutes at room temperature, both the open- and closed-ring isomers of diarylethenes, such as bis(2,4-dimethyl-5-phenylthiophen-3-yl)perfluorocyclopentene, are stable for more than 1000 years at room temperature. Many diarylethene derivatives undergo coloration/decoloration cycles upon irradiation with UV and visible light more than 10,000 times. The cyclization quantum yields are close to 1 and the response times of both cyclization/cycloreversion reactions are less than 10 ps. Molecular orbital theory has revealed the detailed mechanism of the diarylethene photochromism. Prof. Irie has made an epoch in the research of photochromism.   So far photochromic reactions are mainly studied in solution. For practical use the reactions should take place in solid state. Although single crystals are an ideal solid state for the reactions, there was no molecule which undergoes thermally stable photochromism in crystals. Prof. Irie found that some diarylethene derivatives exhibit thermally irreversible photochromic reactions even in the single-crystalline phase. The reaction mechanism in the crystalline phase was studied in detail by using X-ray crystallographic analysis, solid state NMR, absorption anisotropy and AFM. The photogenerated isomers were found to be in the constrained form and the molecular distortion affected the absorption spectra as well as the crystal-surface morphology.   One of the dreams of organic photochemists is to directly observe photochemical reactions at the single-molecule level. This was demonstrated by Prof. Irie using diarylethene derivatives linked with a fluorescent unit. Although uncontrollable fluorescence switching of single molecules, such as short-time blinking and spectral diffusion, has been extensively studied, on/off digital switching of synthetic molecules by photoirradiation in a controllable fashion has not yet been accomplished for the lack of sufficiently durable photochromic molecules. As described above, diarylethenes are exceptionally fatigue resistant and can be used for the single-molecule experiment. Several new fluorescent diarylethene derivatives were synthesized and on/off digital photoswitching of the derivatives on a polymer film was observed at the single-molecule level. The digital fluorescence switching of the single molecules is the first step to achieve the ultimate ultra-high density "Single-Molecule Optical Mmeory" .   Photoresponsive polymers can be constructed by introducing photochromic molecules into polymer backbone or side groups. Upon photoirradiation photoresponsive polymers change reversibly their physical and chemical properties. Polyamides with azobenzene chromophores in the backbone, for example, reversibly change the conformation or viscosity in solution by photoirradiation. A polymer gel containing triphenylmethane leucocyanide groups changed the shape upon UV irradiation. The wettability of the polymer surface also changed upon photoirradiation. The dynamics of the conformational changes could be detected with a time-resolved light scattering measuring system combined with a short laser pulse. The photoinduced reversible physical and chemical property changes may find applications in photoactive biomedical devices.   In conclusion, Prof. Irie has accomplished an exceptional breakthrough in designing and developing novel smart photochromic molecular systems able to reversibly modulate physical and chemical properties by light. He is highly regarded in Japan and all over the world for his contribution to the field of photochemistry.

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