Smart Optical Materials by Sol-Gel Method
by R. Reisfeld
Dept. of Inorganic and Analytical Chemistry , The Hebrew University
Jerusalem 91904, Israel

Tunable Laser samples prepared by the sol-gel method

Rare earths complexes in sol gel glasses Luminescent materials based on heteroaromatic lanthanide cryptates are attractive as labels for advanced time-resolved fluoroimmunoassays, and molecular markers, their potential use is also conceivable in the field of luminescent displays, molecular photonics and highly luminescent materials in hybrid organic/inorganic glasses.  The recent findings of lanthanide complexes trapped in sol-gel inorganic glasses based on silica and zirconia networks are discussed and the theoretical basis of their spectroscopy allow preparation of new family of such complexes in sol-gel based materials.

Browse Article Introduction Solar materials Electrochromics Gasochromics Sensors Laser materials Nanoparticles Rare earth complexes References

Much effort has been devoted to the study of complexes which contain ligands that have high absorbance in the U.V. followed by efficient energy transfer to excited f states of R.E. The required structure of such complex are for instance 3,3’-bi isoquinaline –‘2,2’ dioxide (biq O₂) and biq O₂-cryptate protect also from quenching the R.E. ions from water vibration.

The dominant characteristics which determine the luminescence quantum yield of these complexes are the energy gap law corresponding to the

difference in energy between the excited emitting state and the highest state of the ground ^2J+1L term, the location and influence of ligand metal charge transfer (LMCT) states, and the competition with non radiative decay processes. Inter- and intramolecular dynamics also affect the luminescence properties of lanthanide(III) complexes. These are severely reduced in solid samples, and, more recently, in experiments in which the luminescent species have been incorporated into transparent sol-gel.

Incorporation of R.E. ions into zirconia matrix allowed to obtain better luminescence than in silica matrix however this luminescence could be greatly increased when Eu³⁺ was incorporated into complexes such as dibenzoylmethane Eu(DBM)₃ or into 3,3’biisoquinaline ,2,2’dioxide cryptate.

The complexes Eu(DBM)₃ and Eu cryptate were prepared by Prof. M. Pietraszkiewicz and incorporated into films of zirconia and zirconia glymo in our laboratory. The emission spectra of the complexes when compared to non complexed europium show a dramatic increase in emission intensity of the europium. When normalized to the absorption an increase of factor of five is observed in Eu³⁺ luminescence in the glymo films as compared with zirconia glass. For preparation of cryptate in zirconia films the following procedure was used.

Two zirconia-doped films were prepared: one with Eu(III) cryptate, and a hybrid material obtained by cross-condensation of zirconia tetrapropoxide and 3-glycidoxypropyltrimethoxysilane (abbreviated as "glymo"). The hybrid matrix incorporating silica and organic part was expected to bring two advantages: to bind water via oxirane ring opening, and to provide organic hydrophobic environment advantageous to repel the remaining water molecules from the proximity of the Eu(III) cryptate. The expectation was to enhance the luminescence performance. The Eu ion was also prepared in zirconia film for comparison.

The incorporation of the cryptate complexes into zirconia films results in dramatic increase of the emission intensity as well as in the increase of the absorption intensity of Eu. The former results from the shielding of OH –vibrations responsible for the non-radiative relaxation and lowering of the symmetry site in which Eu is situated . The most dramatic effect must arise from energy transfer from the organic ligand to the Eu.