Ordered arrays of copper nanostructures were fabricated and modified with porphyrin molecules in order to evaluate fluorescence enhancement due to the localized surface plasmon resonance. The nanostructures were prepared by thermally depositing copper on the upper hemispheres of two-dimensional silica colloidal crystals. The wavelength at which the surface plasmon resonance of the nanostructures was generated was tuned to a longer wavelength than the interband transition region of copper (>590 nm) by controlling the diameter of the underlying silica particles. Immobilization of porphyrin monolayers onto the nanostructures was achieved via self-assembly of 16-mercaptohexadecanoic acid, which also suppressed the oxidation of the copper surface. The maximum fluorescence enhancement of porphyrin by a factor of 89.2 was achieved as compared with that on a planar Cu plate (CuP) due to the generation of the surface plasmon resonance. Furthermore, it was found that while the fluorescence from the porphyrin was quenched within the interband transition region, it was efficiently enhanced at longer wavelengths. It was demonstrated that the enhancement induced by the proximity of the fluorophore to the nanostructures was enough to overcome the highly efficient quenching effects of the metal. From these results, it is speculated that the surface plasmon resonance of copper has tremendous potential for practical use as high functional plasmonic sensor and devices.

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We demonstrate that Pd nanospheres exhibit much higher susceptibility of the localized surface plasmon resonance (LSPR) peak to medium refractive index changes than commonly used plasmonic sensing materials such as Au and Ag. The susceptibility of spherical Au nanoparticle-core/Pd-shell nanospheres (Au/PdNSs, ca. 73 nm in diameter) was found to be 4.9 and 2.5 times higher, respectively, than those of Au (AuNSs) and Ag nanospheres (AgNSs) having similar diameters. The experimental finding was theoretically substantiated using the Mie exact solution. We also showed from a quasi-static (QS) approximation framework that the high susceptibility of Pd LSPR originates from the smaller dispersion of the real part of its dielectric function than those of Au and Ag LSPR around the resonant wavelength. We conclude that the Pd nanoparticle is a promising candidate of "the third plasmonic sensing material" following Au and Ag to be used in ultrahigh-sensitive LSPR sensors.

Self-assembling porphyrins and chlorophylls, either natural or artificial, harvest light to effect energy conversion reactions.

The isomerization behaviors of 2-(phenylazo)imidazole (Pai-H) and 1-N-methyl-2-(phenylazo)imidazole (Pai-Me) have been investigated. The crystal structure of trans-Pai-Me was determined, revealing that key structures around the azo group are nearly identical among azobenzene, Pai-H, and Pai-Me. Pai-Me undergoes reversible cis/trans photoisomerization, whereas Pai-H responds poorly to irradiation. The quantum yields of trans-to-cis isomerization of Pai-Me on 454 and 355 nm excitation are 0.35 +/- 0.03 and 0.25 +/- 0.03, respectively, in toluene. The wavelength-dependent isomerization quantum yield is well-known for azobenzene, but these values are substantially higher than those of azobenzene. The activation energy of thermal cis-to-trans isomerization of Pai-Me in toluene is 79.0 +/- 3.5 kJ mol(-1), which is lower than that of azobenzene by 15 kJ mol(-1). The thermal cis-to-trans isomerization of Pai-H is even faster. Density functional theory calculations were performed, revealing that the energy gaps between the azo n-orbital and the highest pi-orbital of azoimidazoles are much narrower than that of azobenzene. Finally, a preliminary study suggested that metal ions can modulate the absorption spectrum of Pai-Me without a loss of the gross photochromic behavior.