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Electronic Excitations and
Types of Pigments
Chemistry 123
Spring 2008
Dr. Woodward
Electronic excitations and Absorbed Light
•
Intra-atomic excitations
–
–
•
Interatomic (charge transfer) excitations
–
–
•
Ligand to metal (i.e. O2−  Cr6+ in SrCrO4)
Metal-to-Metal (i.e. Fe2+  Ti4+ in sapphire)
Molecular Orbital Excitations
–
•
Transition metal ions, complexes and compounds (d-orbitals)
Lanthanide ions, complexes and compounds (f-orbitals)
Conjugated organic molecules
Band to Band Transitions in Semiconductors
–
Metal sulfides, metal selenides, metal iodides, etc.
When a molecule absorbs a photon of ultraviolet (UV) or visible radiation, the
energy of the photon is transferred to an electron. The transferred energy excites
the electron to a higher energy atomic or molecular orbital. Because atoms and
molecules have quantized (discrete) energy levels light is only absorbed when the
photon’s energy corresponds to the energy difference between two orbitals.
Absorption of Light by Atoms
Photon
of light
When atoms absorb light the energy of a photon is transferred to an electron
exciting it to a higher energy atomic orbital. This is illustrated above for a the
excitation of an electron from a 1s orbital to a 2s orbital in a hydrogen atom.
Hydrogen Line Spectrum
n=6
to
n=2
n=5
to
n=2
n=4
to
n=2
n=3
to
n=2
Recall from Chem 121 the line spectrum of a
hydrogen atom (shown above). The light is
produced due to emission, where the electron falls
down to a lower energy level and gives of a photon
of light whose energy corresponds to the energy
difference between orbitals. Emission is simply
the opposite of absorption. To get electrons into
higher energy orbitals electrical energy is used.
Neon lights work on the same principle.
Orbital Energies in Multielectron Atoms
n=∞
4p
0
n=3
3d
3s
3p
4s
3d
n=2
3p
2s
2p
Energy
Energy
3s
2p
2s
n=1
1s
Single Electron Atom
1s
Multi-Electron Atom
The Influence of Surrounding Atoms
4px 4py 4pz
The s and p orbitals
are larger than the d
orbitals. Therefore,
the interaction with the
ligands raises their
energy to a greater
extent
Energy
4s
4p
3d
3dz2 3dx2-y2
3dxz 3dxy 3dyz
4s
Isolated Transition
Metal Atom
Transition Metal
surrounded by an
octahedron of ligands
The interaction with
the ligands splits the
d-orbitals into two
groups (for an
octahedron)
Intra-atomic (localized) excitations
z
z
y
x
CuSO4∙5H2O
Cu3(CO3)2(OH)2
Malachite
Al2−xCrxO3
Ruby
dx2−y2
dz2
Energy
[Ni(NH3)6]2+
y
x
The color comes from absorption of
light that leads to excitation of an
electron from an occupied d-orbital
to an empty (or ½-filled d-orbital).
z
z
y x
x
dyz
z
y x
dxz
y
dxy
This is the main cause of color in most compounds containing transition
metal ions (provided the d-orbitals are partially filled).
Interatomic (charge transfer) excitations
Cr
PbCrO4
CrO42− ion
In these complexes the color comes
from absorption of light that leads to
excitation of an electron from one
atom to another. The charge
transfer in the CrO42− ion is from the
filled oxygen 2p orbitals to the
empty chromium 3d orbitals.
oxygen orbitals
Charge transfer excitations absorb
light much more strongly than intraatomic excitations. This is very
attractive for pigment applications.
This is the main cause of color in compounds containing oxoanions where the
transition metal ion has a d0 electron configuration (i.e. MnO4−, CrO42−, VO43−)
Excitations involving Molecular Orbitals
Lowest (energy) unoccupied
molecular orbital - LUMO
Antibonding
Molecular Orbital
Antibonding
Molecular Orbital
Photon
of light
H 1s
orbital
H 1s
orbital
Bonding
Molecular Orbital
Ground State
(Low Energy)
Highest (energy) occupied
molecular orbital - HOMO
H 1s
orbital
H 1s
orbital
Bonding
Molecular Orbital
Excited State
(High Energy)
Molecular Orbital (HOMO-LUMO) excitations
In these complexes the color comes from absorption of light that leads to
excitation of an electron from an occupied molecular orbital to an empty
molecular orbital. The HOMO orbital(s) is generally a pi-bonding orbital, while
the LUMO orbital(s) is generally a pi-antibonding orbital
Chlorophyll
See also the following discussions in your text:
The Chemistry of Vision (p.342, BLB) & Organic
Dyes (p.353, BLB).
This is the main cause of color in organic molecules containing alternating
single and double bonds (conjugated molecules).
Band to Band Transitions
Empty Conduction
Band “Cation band”
HgS (Vermillion)
CdS (Cadmium Yellow)
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. These
excitations can be considered a subset of charge transfer
excitations because the filled valence band has more anion
character while the empty conduction band has more
“cation” character.
Energy
– Wide band gap semiconductors
Eg
Filled Valence Band
“Anion band”
This is the main cause of color in metal sulphides, selenides and iodides.
Absorbance
Conduction
Band
Eg
400 nm
Energy
Only visible light
with energy less
than Eg is reflected,
the remaining visible
light is absorbed
Wavelength
Energy
700 nm
Eg
UV
Valence
Band
IR
Band Gap (eV) Color
> 3.0
White
3.0-2.4
Yellow
2.3-2.4
Orange
1.8-2.3
Red
< 1.8
Black
Example
ZnO
CdS
GaP
HgS
CdSe
Pigments
Transition metal complexes & salts
Charge Transfer Salts
Excitations:
Intra-atomic d-to-d transitions
Excitations:
Interatomic charge transfer
transitions
Examples:
Malachite – Cu3(CO3)2(OH)2
Cobalt Blue – ZnAl2−xCoxO4
Examples:
Chrome Yellow – PbCrO4
Prussian Blue – Fe(Fe3+Fe2+(CN)6)
Semiconductors
Conjugated Organic Molecules
Excitations:
Valence to conduction band
transitions
Excitations:
HOMO (pi bonding) to LUMO (pi
antibonding) transitions
Examples:
Cadmium Yellow – CdS
Vermillion – HgS
Examples:
Indian Yellow – C19H16O11Mg·5 H2O
Chlorophyll
Azo Dyes
History of Yellow and Red Pigments
• Ancient Pigments
–
–
–
–
–
–
Red Ochre: Fe2O3 (O2− to Fe3+ charge transfer)
Yellow Ochre: Fe2O3∙H2O (O2− to Fe3+ charge transfer)
Red Lead: Pb3O4 (O2− to Pb4+ charge transfer)
Lead-Tin Yellow: Pb2SnO4 (O2− to Sn4+ charge transfer)
Vermillion: HgS (band to band transition, S2− to Hg2+)
Orpiment: As2S3 (band to band transition, S2− to As3+)
• Synthetic pigments
–
–
–
–
1797, Chrome yellow: PbCrO4 (O2− to Cr6+ charge transfer)
1800, Indian yellow: C19H16O11Mg·5 H2O (Mol. Orb. Transition)
1807, Lemon yellow: SrCrO4 (O2− to Cr6+ charge transfer)
1818, Cadmium Yellow: CdS (band to band transition, S2− to Cd2+)
Indian Yellow
Euxanthic acid (Mg salt)
C19H16O11Mg·5 H2O
“The Milkmaid” by
Johannes Vermeer
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.
Synthetic Pigments and Art
“Wheatfield with Crows”
by Vincent van Gogh
“Christ in a Storm” by
Rembrant van Rijn
The traditional yellow and red ochres are earthy
hues which tend to make the paintings darker.
Note the difference between Rembrant who
painted before synthetic pigments were
discovered and van Gogh who in his later years
extensively used CdS and PbCrO4.
Pigments & Toxicity
Emerald Green was one of the favorite pigments
of many impressionist painters (van Gogh,
Cezanne, Monet) the chemical formula of this
pigment is
Cu(CH3COO)2 · 3 Cu(AsO2)2
However, Emerald green is quite
toxic. It is also called Paris
Green because it was used to kill
rats in the sewers of Paris. It has
also been used as an insecticide.
The health problems of some of
the impressionist painters (van
Gogh’s mental illness, Monet’s
blindness, Cezanne’s diabetes)
have been linked to the use of
toxic pigments.
Claude Monet
The Japanese Bridge
1899