Electronic Excitations and Types of Pigments

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

2. 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.

3. Photon of light 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.

4. 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.

5. n = ∞ 4p 0 n = 3 3d 4s 3s 3p 3d n = 2 3p 2s 2p 3s Energy Energy 2p 2s n = 1 1s 1s Orbital Energies in Multielectron Atoms Single Electron Atom Multi-Electron Atom

6. The Influence of Surrounding Atoms The s and p orbitals are larger than the d orbitals. Therefore, the interaction with the ligands raises their energy to a greater extent 4px 4py 4pz 4s The interaction with the ligands splits the d-orbitals into two groups (for an octahedron) 4p 3dz2 3dx2-y2 3d Energy 3dxz 3dxy 3dyz 4s Transition Metal surrounded by an octahedron of ligands Isolated Transition Metal Atom

7. z z z z z y y y y y dx2−y2 x x x x x Energy dz2 dyz dxz dxy Intra-atomic (localized) excitations [Ni(NH3)6]2+ CuSO4∙5H2O Cu3(CO3)2(OH)2 Malachite Al2−xCrxO3 Ruby 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). This is the main cause of color in most compounds containing transition metal ions (provided the d-orbitals are partially filled).

8. Cr oxygen orbitals Interatomic (charge transfer) excitations CrO42− ion PbCrO4 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. 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 d0 electron configuration (i.e. MnO4−, CrO42−, VO43−)

9. Antibonding Molecular Orbital Antibonding Molecular Orbital Photon of light H 1s orbital H 1s orbital H 1s orbital H 1s orbital Bonding Molecular Orbital Bonding Molecular Orbital Excited State (High Energy) Excitations involving Molecular Orbitals Lowest (energy) unoccupied molecular orbital - LUMO Highest (energy) occupied molecular orbital - HOMO Ground State (Low Energy)

10. 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 See also the following discussions in your text: The Chemistry of Vision (p.342, BLB) & Organic Dyes (p.353, BLB). Chlorophyll This is the main cause of color in organic molecules containing alternating single and double bonds (conjugated molecules).

11. Energy Band to Band Transitions Empty Conduction Band “Cation band” • Wide band gap semiconductors Eg 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. Filled Valence Band “Anion band” This is the main cause of color in metal sulphides, selenides and iodides.

12. Only visible light with energy less than Eg is reflected, the remaining visible light is absorbed Conduction Band Absorbance Eg Wavelength 400 nm 700 nm Energy Energy Eg UV IR 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 Valence Band

13. Pigments Transition metal complexes & salts Excitations: Intra-atomic d-to-d transitions Examples: Malachite – Cu3(CO3)2(OH)2 Cobalt Blue – ZnAl2−xCoxO4 Charge Transfer Salts Excitations: Interatomic charge transfer transitions Examples: Chrome Yellow – PbCrO4 Prussian Blue – Fe(Fe3+Fe2+(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 – C19H16O11Mg·5 H2O Chlorophyll Azo Dyes

14. 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+)

15. 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.

16. Synthetic Pigments and Art “Wheatfield with Crows” 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. “Christ in a Storm” by Rembrant van Rijn

17. 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. 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 Claude Monet The Japanese Bridge 1899