The fascinating optical properties of lanthanide ions have promoted the use of their complexes in an increasing number of technological applications ranging from biomedical analysis (fluoroimmunoassays, and cellular imaging) to materials science (lasers, optical fibers, light emitting diodes, optical displays). Lanthanide ions feature long luminescence lifetimes, large Stoke shift and characteristic sharp line emissions which span from infrared to blue. Efficient luminescence requires usually sensitization by suitable coordinated ligands ("antennas"). In our group we use polydentate ligands with finely tuned geometry and electronics to introduce lanthanide(III) ions in functional edifices. The potential application of these architectures in the development of lighting devices, sensors and tags for biomedical and material imaging are also explored.
The direct excitation of the lanthanide ions into the parity-forbidden and therefore weak f-f transition is difficult and requires high energy sources. Significantly enhanced luminescence emission with low energy sources can be achieved by incorporating lanthanide ions in multidentate linkers of various geometries and denticity which contain organic chromophores that can efficiently absorb energy and transfer it to the metal ion (often referred as "antenna effect"). Such high denticity linkers also allow for a good protection of the metal center from O-H oscillators of coordinated or closely diffusing solvent molecules which would lead to non-radiative deactivation of the lanthanide excited states. Moreover the incorporation of Ln(III) in carefully designed polydentate ligands provides the stability of the Ln(III) chelates in physiological media required for biomedical applications. However, incorporating Ln(III) ions into molecular edifices with specific properties is very difficult in view of their poor steric requirements and essentially electrostatic bonds.
In our group, predisposed flexible multidentate ligands in which an anchor allows to ensure the geometry and orientation of dangling chelating units have been successfully used to carefully tune the coordination and the photophysical properties of lanthanide ions in the rational design of monometallic complexes. Moreover original chromophores (such as pyridinetetrazolates) capable of providing stability and optimize the luminescent properties have been designed. Several monometallic compounds have been isolated that present luminescence quantum yields which are among the highest reported in the literature with red (Eu, 35%), green (Tb, 60%) and Near-IR emitters (Nd, 0.3%).
Some of these compounds have been dissolved in PVK to form homogeneous films suitable for wet preparation of OLED'S (Figure below).The high water stability of these chelates in water at pH ranging from 4 to 8 let envisage their application in biomedical analysis and has allowed their encapsulation in silica nanoparticles of controlled size and composition providing new versatile magnetic and luminescent imaging tools. Careful ligand design and original synthetic strategies have also been developed for the selective self-assembly of Ln(III) ions in multimetallic nanosized architectures and for the rational design of highly luminescent lanthanide-based coordination polymers. In particular we have selectively introduced two different lanthanide ions in large luminescent polymetallic wheels. Selective assembly of different Ln emitters in large molecular architectures, although very challenging due to the similar properties of Ln ions, is of high interest for the obtention of precursors of doped materials, for the implementation of energy transfer and for the development of dual colour devices. The use of highly flexible ligands led in our group to the supramolecular assembly of highly luminescent dynamic 1D, 2D or 3D lanthanide polymers which can selectively and reversibly absorb small molecules. The luminescence properties are affected by structural modification suggesting the possibility of developing luminescent sensors. The use of polymeric systems is also particularly attractive for OLED's applications because allows the use of larger concentrations of lanthanides compounds and the preparation of multicolour devices.
Selected publication(s) N. Chatterton at al. Angew. Chem., Intl. Ed. Engl., 44 (2005) 7595; M. Giraud et al. Inorg. Chem. 46 (2007) 625 ; M. Giraud et al. Inorg. Chem. 47 (2008) 3952. X. Y. Chen et al. Chem. Commun. (2008) 3378 ; C. Marchal et al. Chem. Eur. J. 15, (2009) 5273
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