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Lanthanides & Actinides
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 LnIII 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 LnIII)

    [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 
  • CeIII and TbIII have high intensity bands in the UV

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

    why? n-1 =0 (empty sub-shell) for CeIII = 7 (half-filled sub-shell) for TbIII

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, H 2O...)
    • 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+
    • 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

--Info & DownloadsBibliography  [textbook & online resources]

Source: Dr. S.J. Heyes; University of Oxford
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