LANTHANIDES

Electronic Spectroscopy

• Transitions which involve only a redistribution of electrons within the 4f orbitals (f ´ f transitions) are orbitally-forbidden by the Selection Rules

  Þ pale colours of Ln^III compounds are usually not very intense

• Crystal/Ligand field effects in lanthanide 4f orbitals are virtually insignificant

  4f electrons are well shielded from external charge by 5s² & 5p⁶ shells

  Þ f ´ f absorption bands are very sharp (useful fingerprinting and quantitation of Ln^III)

  [d<->d transitions in transition metal compounds are also orbitally forbidden, but gain intensity from and are broadened by the effects of molecular vibrations in distorting the crystal field]

  Þ optical spectra are virtually independent of environment

  - similar spectra in gas/solution/solid (sharp lines like typical gas atom spectra)

• Insensitivity of f ´ f transitions Þ of limited use in study of lanthanide materials 
• Ce^III and Tb^III have high intensity bands in the UV

  due to 4fⁿ ´ 4f^n-15d¹ transitions i.e. f ´ d and therefore not orbitally forbidden

  why? n-1 =0 (empty sub-shell) for Ce^III = 7 (half-filled sub-shell) for Tb^III

Fluorescence / Luminescence

of certain lanthanides e.g. Tb, Ho & Eu [see West, Solid State Chemistry Chapter 17]

Luminescence:emission of light by material as a consequence of its absorbing energy

• Photoluminescence:use of photons for excitation

  Photoluminescent materials generally require a host [H] crystal structure, doped with an activator [A] {sometimes a second dopant is added to act as a sensitizer [S]}

• Fluorescence: short time lapse (~ 10^-8 s) between excitation & emission
• Phosphorescence: long decay times Þ luminescence continues long after excitation source is removed

Applications for Fluorescence/Phosphorescence

• e.g. red phosphor in TV tubes from Eu³⁺ doped into Y₂O₃
• other luminescence colours:-
  • green: Ce³⁺ in CaS Eu³⁺ in SrGa₂S₄
  • blue: Eu²⁺ in (Sr,Mg)₂P₂O₇
• mixed red/green/blue Æ effectively white luminescence

  used in fluorescent lamp coatings to convert blue/UV discharge to white light

• Anti-Stokes Phosphors have the remarkable property of emitting photons of higher energy than the incident exciting radiation!

  e.g. YF₃ host doped with Yb³⁺ as a sensitizer and Er³⁺ as an activator can convert incident IR radiation into green luminescence 

• Luminescence of Eu³⁺ is used to probe its environment

  ligand charges / binding constants / ligand exchange rates / site symmetry

  Ln3+ as a Probe for Ca2+ Sites in Bioinorganic Chemistry Ln3+ may replace Ca2+ in its binding sites in proteins Similar ionic radii Coordination number of Ln3+ (7-9) close to Ca2+ (6-8) Hard metal ions / Prefer oxygen ligation (Ca2+ bound by O of Glu, Asp, Thr, Ser, H2O...) Ln3+ binds ligands ca. 105x more strongly than Ca2+ Ligand exchange rates on Ln3+ are ca. 102x slower than on Ca2+ When Ln3+ replaces Ca2+ at a catalytic site reaction rates decrease (explains mild toxicity of rare earth ions!) Use of Luminescence Spectra Eu3+ (green) & Tb3+ (red) luminesce strongly at ca. 296 K after laser excitation because excitation may occur strongly to excited ligand states which are just above the Ln3+ excited states involved in the luminescence for these ions, which are therefore easily populated Determine Number of H2O molecules bound to the active-site metal ion e.g. for the protein thermolysin Eu3+ luminescence Þ 1H2O for Ca2+ site 1 and 3H2O at sites 3 & 4 The different sites may be replaced independently with different Ln3+ Energy transfer expts. (e.g. Eu3+ÆTb3+) Þ intersite distances Use of Paramagnetism Ln3+ bound in a metallo- site acts as NMR shift/relaxation agent Æ active site protein geometry from 1H NMR spectra Use of Electron Density Ln3+ Æ Heavy atom derivatives to assist solving X-ray diffraction structures

• Lasers

  One of the most common high power lasers is the Neodymium YAG laser

  • Host material is Yttrium Aluminium Garnet (YAG), Y₃Al₅O₁₂, doped with Nd³⁺
  • Examine the garnet structure (also considered for its magnetic properties in e.g. YIG)
  • a '4'-Level Laser System

• Change of host material makes small differences in laser radiation frequency
• Change of dopant ion makes large changes in laser radiation frequency