Electronic Excitations and Types of Pigments Chemistry 123 Spring 2008 Dr. Woodward

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 photon’s energy corresponds to the energy difference between two orbitals.

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

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.

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

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

Intra-atomic (localized) excitations x y z d yz x y z d xz x y z d xy x y z d x2−y2 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 2−x 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 ½-filled d-orbital).

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

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

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).

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.

Energy Conduction Band Valence Band EgEgEgEg Band Gap (eV) Color Example > 3.0 White ZnO Yellow CdS Orange GaP 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

Pigments Transition metal complexes & salts Excitations: Intra-atomic d-to-d transitions Examples: Malachite – Cu 3 (CO 3 ) 2 (OH) 2 Cobalt Blue – ZnAl 2−x 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 Mg·5 H 2 O Chlorophyll Azo Dyes

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 Mg·5 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+ )

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 Mg·5 H 2 O “The Milkmaid” by Johannes Vermeer

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.

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 Gogh’s mental illness, Monet’s blindness, Cezanne’s diabetes) have been linked to the use of toxic pigments.