Ligand-metal energy transfer efficiency, lanthanide complexes

Mameri et al. developed acyclic ligand 51 containing bipyridine carboxylic moieties, which gave high stabihty and hydrophihcity of the lanthanide complex [131]. This formed luminescent 1 1 complexes with Eu " and Tb cations, where two water molecules located in the first coordination sphere of the lanthanide centers. The efficient ligand-to-metal energy transfer was ensured by the bipyridine photoantenna. Upon addition of ATP anion, the Eu luminescence intensity decreased to 20% of its initial value. Since the luminescence lifetime increased from 0.28 to 0.58 and 0.65 ms with the addition of 10 and 20 equivalents of ATP anion, the two boimd water molecules were replaced by the external ATP anion. In contrast, ADP, AMP, and NOs an-... 

Ligand labeling with fluorescent metal chelates has created a versatile class of fluorescent probes. The chelates of rare earth metals have unique emission characteristics in that, upon excitation of aromatic portions of the ligands of the lanthanide complex, the energy of excitation is efficiently transferred to the lanthanide ion. This causes f-f transitions that produce very narrow almost line-like emission bands that permit all of the emitted light to be collected by the detector with narrow emission slits. In addition, the rare earth... 

Lanthanide p-diketonates are amongst the best smdied rare-earth luminescent complexes [58]. They are brightly luminescent and volatile so that incorporation into various electroluminescent materials is simple. Moreover their photophysical properties are easily tuned by a judicious choice of ancillary ligands. Indeed, conventional synthesis usually yields bis(hydrated) lanthanide tris(P-diketonates), but the two solvent molecules can be substituted by either a fourth diketonate anion or a donor ligand with adequate functionalisation as to provide convenient light harvesting and subsequent energy transfer onto the metal ion. It is noteworthy that not only visible but also near-infrared luminescence [59,60] is efficiently sensitised in lanthanide p-diketonates. In the case of Eu , some ternary complexes have quantum yields up to 85% [8] and the main asset of their luminescent properties is an emission essentially concentrated in the hypersensitive Dq transition... 

As described earlier, the lifetimes and quantum yields of emissive Ln complexes vary dramatically due to the extremely sensitive nature of the 4/-centred excited states to 0-H, N-H and C-H vibrational manifolds, which can provide efficient, non-radiative deactivation pathways the efficiency of energy transfer between the antenna and lanthanide ion also determines overall quantum yields. A classical approach to maximising the emissivity of Ln complexes is to therefore inhibit the approach of water solvent to the inner coordination sphere (and where q denotes the number of coordinated solvent molecules) high denticity, metal ion encapsulating ligands with hydrophobic peripheries can achieve this very effectively, reducing q to zero [4]. 

Sabbatini et al. 1993). An efficient antenna is expected to lead to metal luminescence much more intense than that obtained upon metal excitation since lanthanide ions are characterized by very low molar absorption coefficients. These complexes can be considered light-conversion molecular devices because they are able to transform light absorbed by the ligand into light emitted by the ions via an intramolecular energy transfer (Balzani and Scandola 1991). 

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