The Lanthanides and Their Complexes

Here, the lanthanides and their complexes are introduced. We show you where you can find the lanthanides in the periodic table, how the lanthanide ions get their interesting photophysical properties and how this can lead to luminescent lanthanide complexes.

More information on the subject can be found in the references mentioned in the Luminescent Lanthanide Bibliography.

1. Lanthanides in The Periodic Table

Rare Earth Elements: series of chemical elements of the periodic table. The rare earth elements (or rare earth metals) include the elements with atomic numbers 57 through 71, and, named in order, are lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Yttrium (Y, atomic no. 39) and scandium (Sc, atomic no. 21) are sometimes included in the group of rare earth elements. The elements cerium (Ce, atomic no. 58) through lutetium (Lu, atomic no. 71) are commonly known as the lanthanide series.

Remark: This description of 'the lanthanides' is based on the IUPAC (International Union of Pure and Applied Chemistry) definition. Dr. Binnemans pointed out that according to the Rare Earth Information Center (Ames, USA) also Lanthanum (no. 57) itself is a lanthanide.

Rare earth elements are separated from other elements in a mineral by precipitation with a suitable reagent. Separation of the rare earth elements from each other by ordinary chemical means is difficult because their chemical properties are similar, and the isolation of an individual element may involve hundreds of fractional crystallizations. With the use of ion-exchange methods the separation of an individual rare earth element can be accomplished with greater ease and precision. Oxides of the rare earth elements are called rare earths, and are found in minerals that are actually more abundant than those of some other metals, such as those in the platinum group. The principal source of rare earths is the mineral monazite. Some other rare minerals that also contain small amounts of rare earths include cerite, gadolinite, and samarskite 

2. Photophysical Properties of Trivalent Lanthanide Ions

The lanthanides usually exist as trivalent cations, in which case their electronic configuration is (Xe) 4fn, with n varying from 1 (Ce3+) to 14 (Lu3+). The transitions of the f-electrons are responsible for the interesting photophysical properties of the lanthanide ions, such as long-lived luminescence and sharp absorption and emission lines. The f-electrons are shielded from external perturbations by filled 5s and 5p orbitals, thus giving rise to line-like spectra.

The f-f electronic transitions are forbidden, leading to long excited state lifetimes, in the micro- to millisecond range. The forbidden nature of the f-f transitions is also reflected in low extinction coefficients, making direct photoexcitation of lantahide ions difficult. This can be overcome by using energy transfer from organic chromophores to lanthanide ions. See also: "Luminescent Lanthanide Ions: Making Them Shine Brightly" (Sensitization of lanthanide luminescence).

3. Luminescent Lanthanide Complexes

Lanthanide ions can form complexes with various organic molecules such as beta-diketones, polyaminopolycarboxylic acids (EDTA and the like), (poly)pyridines and calixarenes. When the ligands contain organic chromophores with suitable photophysical properties, highly luminescent lanthanide complexes can be obtained.

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