Electronic Excitations and Types of Pigments

Electronic Excitations and
Types of Pigments

Chemistry 123
Spring 2008
Dr. Woodward

Electronic excitations and Absorbed Light

• Intra-atomic excitations

– Transition metal ions, complexes and compounds (d-orbitals)
– Lanthanide ions, complexes and compounds (f-orbitals)

• 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

– 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.

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Hydrogen Line Spectrum

n=6 n=5 n=4 n=3
to to to to
n=2 n=2 n=2 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 3d 4s
3p
n=2
3s
2s 2p

Energy 2p
Energy 2s

n=1 1s

1s Multi-Electron Atom

Single Electron Atom

The Influence of Surrounding Atoms

Energy 4p 4px 4py 4pz The s and p orbitals
3d are larger than the d
4s orbitals. Therefore,
3dz2 3dx2-y2 the interaction with the
3dxz 3dxy 3dyz ligands raises their
energy to a greater

extent

The interaction with
the ligands splits the

d-orbitals into two
groups (for an
octahedron)

4s

Isolated Transition Transition Metal
Metal Atom surrounded by an
octahedron of ligands

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Intra-atomic (localized) excitations

zz

x y x y
dx2−y2 dz2
[Ni(NH3)6]2+ CuSO4·5H2O

Energy

Cu3(CO3)2(OH)2 Al2−xCrxO3 z z z
Malachite Ruby dyz
yx yx
The color comes from absorption of x dxz dxy y
light that leads to excitation of an
electron from an occupied d-orbital
to an empty (or ½-filled d-orbital).

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 oxygen orbitals
from absorption of light that leads to
Charge transfer excitations absorb
excitation of an electron from one light much more strongly than intra-
atom to another. The charge
atomic excitations. This is very
transfer in the CrO42− ion is from the attractive for pigment applications.
filled oxygen 2p orbitals to the
empty chromium 3d orbitals.

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 Antibonding
Molecular Orbital Molecular Orbital

Photon
of light

H 1s H 1s H 1s H 1s
orbital orbital orbital orbital

Bonding Highest (energy) occupied Bonding
Molecular Orbital molecular orbital - HOMO Molecular Orbital

Ground State Excited State
(Low Energy) (High Energy)

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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

– Wide band gap semiconductors Empty Conduction
Band “Cation band”

Energy Eg

HgS (Vermillion) CdS (Cadmium Yellow)

In these complexes the color comes from absorption of Filled Valence Band
light that leads to excitation of an electron from a filled “Anion band”

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.

This is the main cause of color in metal sulphides, selenides and iodides.

Energy Conduction Only visible light
Band with energy less
Absorbance than Eg is reflected,
Eg the remaining visible

Eg light is absorbed

400 nm Wavelength 700 nm
Energy

UV IR

Valence Band Gap (eV) Color Example
Band > 3.0 White ZnO
3.0-2.4 Yellow CdS
2.3-2.4 Orange GaP
1.8-2.3 HgS
< 1.8 Red CdSe
Black

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Pigments

Transition metal complexes & salts Charge Transfer Salts
Excitations:
Excitations:
Intra-atomic d-to-d transitions Interatomic charge transfer
Examples:
transitions
Malachite – Cu3(CO3)2(OH)2
Cobalt Blue – ZnAl2−xCoxO4 Examples:
Chrome Yellow – PbCrO4
Semiconductors Prussian Blue – Fe(Fe3+Fe2+(CN)6)
Excitations:
Conjugated Organic Molecules
Valence to conduction band
transitions Excitations:
Examples: HOMO (pi bonding) to LUMO (pi
Cadmium Yellow – CdS antibonding) transitions
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.

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Synthetic Pigments and Art

“Christ in a Storm” by “Wheatfield with Crows”
Rembrant van Rijn by Vincent van Gogh

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 Claude Monet
toxic. It is also called Paris The Japanese Bridge

Green because it was used to kill 1899
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.

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