A thermochromic pigment composition including: (A) at least one electron-donor organic dye compound, (B) at least one electron-acceptor compound, and (C) at least one compound corresponding to the following formula (I): in which: X is 0 or a single bond, Z is COO or OCO, R.sub.1 is O-alkyl or (CH.sub.2).sub.mCH.sub.3—R.sub.2 is OCO(CH.sub.2).sub.nCH.sub.3 or H, —y=0-3, —m=12-18; —p=8-18, and on the condition that, when X is 0, y is other than 0. The thermochromic pigment composition are also useable in thermochromic pigment microcapsules ink compositions including the thermochromic pigment microcapsules, and writing instruments including the ink compositions.

An ink composition including at least one photochromic pigment or dye, at least one film-forming resin, and at least one solvent in which the at least one photochromic pigment or dye and the at least one film-forming resin are dispersible or dissolvable, wherein the components of the ink composition are present in respective amounts such that the ink composition is in the form of a liquid or gel until evaporation of the at least one solvent, and wherein, upon evaporation of the at least one solvent, the at least one film-forming resin forms a film that (a) inhibits the ink composition from degrading when covered with the sunscreen product or (b) inhibits the ink composition from washing off when the sunscreen is washed off or both, the film-forming resin including shellac, polyurethane, acrylic polymer, silicone polymer or a mixture thereof.

An ink composition including at least one photochromic pigment or dye, at least one film-forming resin, and at least one solvent in which the at least one photochromic pigment or dye and the at least one film-forming resin are dispersible or dissolvable, wherein the components of the ink composition are present in respective amounts such that the ink composition is in the form of a liquid or gel until evaporation of the at least one solvent, and wherein, upon evaporation of the at least one solvent, the at least one film-forming resin forms a film that (a) inhibits the ink composition from degrading when covered with the sunscreen product or (b) inhibits the ink composition from washing off when the sunscreen is washed off or both, the film-forming resin including shellac, polyurethane, acrylic polymer, silicone polymer or a mixture thereof.

A magnetic pigment flake includes a filtering film layer, with magnetic or magnetizable material, and a metal nanoparticles layer, formed on a surface of the filtering film layer. The metal nanoparticles layer is configured to generate scattered light enhanced by a local surface plasmon resonance under an irradiation of visible light exceeding a predetermined intensity. An optically variable ink includes an ink body and the above-mentioned magnetic pigment flakes. An anti-falsification article includes an article body and the above-mentioned optically variable ink. The magnetic pigment flake of the optically variable ink is magnetically oriented, such that a bright and dark areas are generated with a viewing angel changing under an irradiation of visible light below the predetermined intensity. Under an irradiation of visible light exceeding the predetermined intensity, light with a color different from that of the bright area is generated on a corresponding position of the dark area.

A magnetic pigment flake includes a filtering film layer, with magnetic or magnetizable material, and a metal nanoparticles layer, formed on a surface of the filtering film layer. The metal nanoparticles layer is configured to generate scattered light enhanced by a local surface plasmon resonance under an irradiation of visible light exceeding a predetermined intensity. An optically variable ink includes an ink body and the above-mentioned magnetic pigment flakes. An anti-falsification article includes an article body and the above-mentioned optically variable ink. The magnetic pigment flake of the optically variable ink is magnetically oriented, such that a bright and dark areas are generated with a viewing angel changing under an irradiation of visible light below the predetermined intensity. Under an irradiation of visible light exceeding the predetermined intensity, light with a color different from that of the bright area is generated on a corresponding position of the dark area.

A visible light activated ink that produces a color change when exposed to visible light is provided. The ink includes a visible light activated photochromic compound, one or more binders, additives including one or more surfactants, and a solvent. The visible light activated ink is substantially colorless in the as-deposited state and requires a visible light intensity of approximately 300 W/m.sup.2 or greater at a wavelength of approximately 400-700 nm to produce a color change.

Provided is an ink set for security image recording comprising an infrared-absorbing ink jet ink which contains a polymerizable compound and an infrared absorbing dye and in which a maximum value of an absorbance in a wavelength range of 750 nm to 1,000 nm is larger than a maximum value of an absorbance in a wavelength range of 400 nm to 750 nm, and color ink jet inks which each contain a polymerizable compound and a colored colorant and satisfy a ratio of a maximum value of an absorbance of each of the color ink jet inks in a wavelength range of 750 nm to 1,000 nm to the maximum value of the absorbance of the infrared-absorbing ink jet ink in a wavelength range of 750 nm to 1,000 nm of 0.10 or less. Also provided are applications of the ink set.

Provided is an ink set for security image recording comprising an infrared-absorbing ink jet ink which contains a polymerizable compound and an infrared absorbing dye and in which a maximum value of an absorbance in a wavelength range of 750 nm to 1,000 nm is larger than a maximum value of an absorbance in a wavelength range of 400 nm to 750 nm, and color ink jet inks which each contain a polymerizable compound and a colored colorant and satisfy a ratio of a maximum value of an absorbance of each of the color ink jet inks in a wavelength range of 750 nm to 1,000 nm to the maximum value of the absorbance of the infrared-absorbing ink jet ink in a wavelength range of 750 nm to 1,000 nm of 0.10 or less. Also provided are applications of the ink set.

Provided is a semiconductor nanoparticle complex dispersion liquid in which semiconductor nanoparticles are dispersed in a polar dispersion medium at a high mass fraction, and in which high fluorescence quantum efficiency (QY) is maintained. A semiconductor nanoparticle complex dispersion liquid according to an embodiment includes a semiconductor nanoparticle complex dispersed in an organic dispersion medium, wherein: the semiconductor nanoparticle complex is composed of two or more ligands including an aliphatic thiol ligand and a polar ligand, and a semiconductor nanoparticle with the ligands coordinated to the surface thereof; the ligands are composed of an organic group and a coordinating group; the organic group of the polar ligand includes a hydrophilic functional group; and an SP value of the organic dispersion medium is 8.5 or more.

Provided is a semiconductor nanoparticle complex dispersion liquid in which semiconductor nanoparticles are dispersed in a polar dispersion medium at a high mass fraction, and in which high fluorescence quantum efficiency (QY) is maintained. A semiconductor nanoparticle complex dispersion liquid according to an embodiment includes a semiconductor nanoparticle complex dispersed in an organic dispersion medium, wherein: the semiconductor nanoparticle complex is composed of two or more ligands including an aliphatic thiol ligand and a polar ligand, and a semiconductor nanoparticle with the ligands coordinated to the surface thereof; the ligands are composed of an organic group and a coordinating group; the organic group of the polar ligand includes a hydrophilic functional group; and an SP value of the organic dispersion medium is 8.5 or more.