Download Pigments The Visible Spectrum

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