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Pigments Pigment: Coloring matter used to make paint. Pigments work by selectively absorbing a portion of the visible light while the remaining visible light is reflected. For more info see http://webexhibits.org/pigments/ The Visible Spectrum UV Violet Blue Green Yellow Orange Red Near IR 100-400 nm 400-425 nm 425-492 nm 492-575 nm 575-585 nm 585-647 nm 647-700 nm 10,000-700 nm 12.4 - 3.10 eV 3.10 - 2.92 eV 2.92 - 2.52 eV 2.52 - 2.15 eV 2.15 - 2.12 eV 2.12 - 1.92 eV 1.92 - 1.77 eV 1.77 - 0.12 eV If absorbance occurs in one region of the color wheel the material appears with the opposite (complimentary color). For example: – a material absorbs violet light → Color = Yellow – a material absorbs green light → Color = Red – a material absorbs red, orange & yellow → Color = Blue E = hc/λ = {(6.626 x 10-34 J-s)(2.998 x 108 m/s)}/λ 1 eV = 1.602 × 10-19 J 1 Absorption spectrum for [Ti(H2O)6]3+ Absorbs: Blue, Green Partially Absorbs: Violet, Yellow Transmits: Orange, Red Color = ReddishReddish-Purple UVUV-Visible Absorption Spectroscopy Light Absorption & Electronic Transitions • • • • • • Intra-atomic (localized) excitations – – Transition metal ions, complex ions & compounds (d-orbitals) Lanthanide ions and compounds (f-orbitals) – – cation to cation (i.e. sapphire, prussian blue) Anion (ligand) to cation (i.e. chromate, permanganate) – Conjugated (alternating double and single bonds) organic molecules (i.e. chlorophyll, dyes) Interatomic (charge transfer) excitations Molecular Orbital (HOMO to LUMO) excitations Band to Band transitions – Semiconductors (i.e. vermillion, cadmium yellow) – Metals (i.e. gold, copper) – Minerals and Ionic Salts (i.e. amethyst, topaz) Intraband excitations Defects and Color Centers For more info see http://webexhibits.org/causesofcolor/ 2 Intra-atomic (localized) excitations – Transition metal ions, complex ions & compounds (dorbitals) – Lanthanide ions and compounds (f-orbitals) [Ni(NH3)6]2+ NiSO4 Cu3(CO3)2(OH)2 CuSO4 Malachite In these complexes the color comes from absorption of light that leads to excition of an electron from an occupied d-orbital to an empty or ½-filled dorbital. The energy separation between d-orbitals depends upon the interaction between the d-orbitals and the ligands. Cr3+ Gemstones Corundum - Al2O3 Beryl - Be3Al2Si6O18 Ruby Al2O3:Cr3+ In both gemstones Cr3+ substitutes for Al3+, which is surrounded by 6 oxygen ions in an octahedron. The color comes from a d-to-d excitation on the Cr3+ center. Emerald Be2Al2Si6O18:Cr3+ 3 Interatomic (charge transfer) excitations – cation to cation (i.e. sapphire, prussian blue) – anion (ligand) to cation (i.e. chromate, permanganate) Cr PbCrO4 In these complexes the color comes from absorption of light that leads to excitation of an electron from one atom to another. Charge Transfer in Sapphire • The deep blue color the gemstone sapphire is also based on impurity doping into Al2O3. The color arises from the following charge transfer excitation: Fe2+ + Ti4+ → Fe3+ + Ti3+ (λmax ~ 2.2 eV, 570 nm) • The transition is facilitated by the geometry of the corundum structure where the two ions share an octahedral face, which allows for favorable overlap of the dz2 orbitals. • Unlike the d-d transition in Ruby, the chargetransfer excitation in sapphire is fully allowed. Therefore, the color in sapphire requires only ~ 0.01% impurities, while ~ 1% impurity level is needed in ruby. 4 Molecular Orbital (HOMO-LUMO) excitations – Conjugated (alternating double and single bonds) organic molecules (i.e. chlorophyll, dyes) Chlorophyll See also the following discussions in your text: The Chemistry of Vision (p.342, BLB) & Organic Dyes (p.353, BLB). Indian Yellow Euxanthic acid (Mg salt) - C19H16O11Mg·5 H2O) Synthesis Procedure Derived from urine of cows that had been fed mango leaves. The cow urine is then evaporated and the resultant dry matter formed into balls by hand. Finally the crude pigment is washed and refined. 5 Band to Band Transitions – Wide band gap semiconductors HgS (Vermillion) CdS (Cadmium Yellow) As2S3 (Realgar) In these complexes the color comes from absorption of light that leads to excitation of an electron from a filled valence band to an empty conduction band. 6