Electrons transfer from plasmonic nanoparticles to semiconductors by exploiting the energy of light, and this effect is applied to photovoltaics, photocatalysis, sensing, photochromisms, photoswitchable functionalities and nanofabrications.

Let's do the twist! Molecular crystals of a diarylethene derivative exhibit photoreversible twisting upon irradiation with UV and visible light. The left‐handed and right‐handed helix formation is ascribed to the contraction of the crystal in a diagonal direction, depending on which face is irradiated with UV light.

The photocycloreversion reaction of a diarylethene upon irradiation with visible light was found to be accelerated in proximity to a gold nanoparticle covered with diarylethene polymers (Au-poly(DE)) dispersed in a solution. Enhancement of the reaction rate by the gold nanoparticle was quantitatively evaluated by kinetic analysis on the basis of absorption spectroscopy. Multicomponent bleaching of the closed-ring form of poly(DE) was observed around the gold nanoparticle upon irradiation with visible light; the irradiation time dependence of the absorbance could be explained using a reaction model with two decay components rather than one. The results indicate that the cycloreversion reaction was enhanced only in the vicinity of the gold nanoparticle. The faster reaction inside the polymer shell was confirmed by numerical spectral simulation of Au-poly(DE) using a double shell model based on Mie theory. The enhancement factor determined as the ratio of the enhanced reaction rate to the nonenhanced rate was estimated to be 2−5, and the enhanced region was evaluated to be 9−12 nm from the surface of the gold nanoparticle. The enhancement factor tended to increase with irradiation at longer incident wavelength, which did not correlate with the spectral shape of the local surface plasmon resonance (LSPR) band of the gold nanoparticle.

Various sizes of gold nanoparticles covered with photochromic diarylethene polymers (Au-poly(DE)s) were prepared by Brust’s method, citrate reduction, or a seeding growth method. The diarylethene polymers on the particles exhibited reversible photochromism upon alternating irradiation with ultraviolet and visible light, both in solution and in the solid state. Au-poly(DE) of larger size, prepared by the seeding growth method, showed a significant shift in the local surface plasmon resonance (LSPR) band photoreversibly as a result of photochromic reaction of the diarylethene polymers around the gold nanoparticles. This change is due to a change in the refractive index of the photochromic chromophore and is remarkably enhanced in the solid state. However, the smaller sized Au-poly(DE) prepared by Brust’s method hardly showed any such spectral shift. These results are qualitatively reproduced by a theoretical simulation, which indicates that the sensitivity of the LSPR band strongly depends on the particle size of the gold nanoparticle. The Au-poly(DE)s can be potentially used as new types of plasmonic materials with a light-controllable LSPR band.

Gold nanoparticles covered with photochromic diarylethene polymers or block copolymers with styrene were prepared according to Brust's method. These nanoparticles expressed a bright red color because of their surface plasmon resonance absorption, and exhibited reversible photochromism upon alternate irradiation with ultraviolet and visible light in the solid state as well as in solution. The photocyclization reactivity of the diarylethenes around the gold nanoparticles decreased by shortening the distance between the surface of the gold nanoparticles and the diarylethene chromophore. Some effects of the gold nanoparticles on the optical properties of the diarylethene chromophores were evaluated by comparison of the difference spectra between the UV-visible absorption spectra in the photostationary state and in the initial state under various conditions. The effect was found to appear as a bathochromic shift in the absorption maximum of the diarylethene closed-ring form, which is related to the local electric field generated by the surface plasmon resonance of the gold nanoparticles.

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Photochromic polymers with various diarylethene derivatives were synthesized by a conventional radical polymerization of styrene derivatives having diarylethene chromophores as the pendant. All the polymers exhibited reversible photochromism in the film as well as in solution, while the photocyclization conversion in the film decreased in comparison with that in solution because of a restriction of the conformational structure in the solid state. Although the photocyclization and photocycloreversion quantum yields at the initial stage of the reactions in the film were comparable to those in solution, the apparent cycloreversion quantum yield decreased along with the reaction, which is derived from a distribution of the quantum yields in the solid state. Finally, the photochromic terpolymers consisting of three diarylethene monomers which show photochromism changing color to cyan, magenta, and yellow were synthesized. The terpolymers were demonstrated to show bright colors including black, green, red, and blue both in solution and in the solid state by selective bleaching processes. Such multicolor photochromic polymers composed of diarylethene derivatives have a potential for rewritable photochromic display devices.