Electronic Excitations and Types of Pigments Chemistry 123
Spring 2008 Dr. Woodward Slide 2 Electronic excitations and
Absorbed Light Intra-atomic excitationsIntra-atomic excitations
Transition metal ions, complexes and compounds (d-orbitals)
Lanthanide ions, complexes and compounds (f-orbitals) Interatomic
(charge transfer) excitationsInteratomic (charge transfer)
excitations Ligand to metal (i.e. O 2 Cr 6+ in SrCrO 4 )
Metal-to-Metal (i.e. Fe 2+ Ti 4+ in sapphire) Molecular Orbital
ExcitationsMolecular Orbital Excitations Conjugated organic
molecules Band to Band Transitions in SemiconductorsBand 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 photons energy corresponds to the energy
difference between two orbitals. Slide 3 Absorption of Light by
Atoms 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. Photon of light Slide 4
Hydrogen Line Spectrum n=3 to n=2 n=4 to n=2 n=5 to n=2 n=6 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. Slide 5 Orbital Energies in
Multielectron Atoms Energy 1s1s 2s2s2p2p 3s3s3p3p3d3d n = 1 n = 2 n
= 3 n = 0 Energy 1s1s 2p2p 3d3d 2s2s 3p3p 3s3s 4p4p 4s4s Single
Electron AtomMulti-Electron Atom Slide 6 The Influence of
Surrounding Atoms Energy 3d3d 4s4s Isolated Transition Metal Atom
4p4p 4p x 4s 4p y 4p z 3d x2-y2 3d z2 3d xz 3d xy 3d yz Transition
Metal surrounded by an octahedron of ligands The interaction with
the ligands splits the d-orbitals into two groups (for an
octahedron) The s and p orbitals are larger than the d orbitals.
Therefore, the interaction with the ligands raises their energy to
a greater extent Slide 7 Intra-atomic (localized) excitations x y z
d yz x y z d xz x y z d xy x y z d x2y2 x y z d z2 Energy [Ni(NH 3
) 6 ] 2+ Cu 3 (CO 3 ) 2 (OH) 2 Malachite CuSO 4 5H 2 O Al 2x Cr x O
3 Ruby This is the main cause of color in most compounds containing
transition metal ions (provided the d-orbitals are partially
filled). The color comes from absorption of light that leads to
excitation of an electron from an occupied d-orbital to an empty
(or -filled d-orbital). Slide 8 Interatomic (charge transfer)
excitations 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 CrO 4 2 ion is from the filled
oxygen 2p orbitals to the empty chromium 3d orbitals. PbCrO 4
Charge transfer excitations absorb light much more strongly than
intra- atomic 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 d 0
electron configuration (i.e. MnO 4 , CrO 4 2, VO 4 3 ) Cr oxygen
orbitals CrO 4 2 ion Slide 9 Excitations involving Molecular
Orbitals Photon of light Antibonding Molecular Orbital H 1s orbital
Bonding Molecular Orbital H 1s orbital Ground State (Low Energy)
Antibonding Molecular Orbital H 1s orbital Bonding Molecular
Orbital H 1s orbital Excited State (High Energy) Highest (energy)
occupied molecular orbital - HOMO Lowest (energy) unoccupied
molecular orbital - LUMO Slide 10 Molecular Orbital ( HOMO-LUMO )
excitations Chlorophyll See also the following discussions in your
text: The Chemistry of Vision (p.342, BLB) & Organic Dyes
(p.353, BLB). 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 This is the main
cause of color in organic molecules containing alternating single
and double bonds (conjugated molecules). Slide 11 Band to Band
Transitions Wide band gap semiconductors 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. CdS (Cadmium Yellow) HgS (Vermillion) Energy Filled
Valence Band Anion band EgEgEgEg Empty Conduction Band Cation band
This is the main cause of color in metal sulphides, selenides and
iodides. Slide 12 Energy Conduction Band Valence Band EgEgEgEg Band
Gap (eV) Color Example > 3.0 White ZnO 3.0-2.4 Yellow CdS
2.3-2.4 Orange GaP 1.8-2.3 Red HgS < 1.8 Black CdSe UVIR
Absorbance Wavelength Energy 700 nm400 nm EgEgEgEg Only visible
light with energy less than E g is reflected, the remaining visible
light is absorbed Slide 13 Pigments Transition metal complexes
& salts Excitations: Intra-atomic d-to-d transitions Examples:
Malachite Cu 3 (CO 3 ) 2 (OH) 2 Cobalt Blue ZnAl 2x Co x O 4 Charge
Transfer Salts Excitations: Interatomic charge transfer transitions
Examples: Chrome Yellow PbCrO 4 Prussian Blue Fe(Fe 3+ Fe 2+ (CN) 6
) Semiconductors Excitations: Valence to conduction band
transitions Examples: Cadmium Yellow CdS Vermillion HgS Conjugated
Organic Molecules Excitations: HOMO (pi bonding) to LUMO (pi
antibonding) transitions Examples: Indian Yellow C 19 H 16 O 11 Mg5
H 2 O Chlorophyll Azo Dyes Slide 14 History of Yellow and Red
Pigments Ancient PigmentsAncient Pigments Red Ochre: Fe 2 O 3 (O 2
to Fe 3+ charge transfer) Yellow Ochre: Fe 2 O 3 H 2 O (O 2 to Fe
3+ charge transfer) Red Lead: Pb 3 O 4 (O 2 to Pb 4+ charge
transfer) Lead-Tin Yellow: Pb 2 SnO 4 (O 2 to Sn 4+ charge
transfer) Vermillion: HgS (band to band transition, S 2 to Hg 2+ )
Orpiment: As 2 S 3 (band to band transition, S 2 to As 3+ )
Synthetic pigmentsSynthetic pigments 1797, Chrome yellow: PbCrO 4
(O 2 to Cr 6+ charge transfer) 1800, Indian yellow: C 19 H 16 O 11
Mg5 H 2 O (Mol. Orb. Transition) 1807, Lemon yellow: SrCrO 4 (O 2
to Cr 6+ charge transfer) 1818, Cadmium Yellow: CdS (band to band
transition, S 2 to Cd 2+ ) Slide 15 Indian Yellow 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. Euxanthic acid (Mg salt) C 19 H 16 O 11 Mg5 H 2
O The Milkmaid by Johannes Vermeer Slide 16 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 PbCrO 4. Slide 17 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(CH 3
COO) 2 3 Cu(AsO 2 ) 2 Claude Monet The Japanese Bridge 1899
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 Goghs mental illness, Monets
blindness, Cezannes diabetes) have been linked to the use of toxic
pigments.
