Antenna ligands

An important measure of the luminescence is the quantum yield. In effect, this is the probability that a photon will be emitted by the lanthanide given that one photon has been absorbed by the antenna ligand. Since measurement of absolute quantum yields is particularly difficult, the overall quantum yield ( ) is normally measured with reference to certain standards (26) these are routinely [Ru(bpy)3]2+ in water or SulfoRhodamine 101 in methanol for Eu3 +, and quinoline sulfate in 0.1 M HC1 or fluorescein in 1 N NaOH for Tb3+ (27,28). A method has been developed that measures energy transfer from the lanthanide complex to an acceptor of known quantum yield (28). 

Related complexes where two ligands encapsulate the lanthanide center have been reported (38,39). Coordination of a second antenna ligand may increase the luminescence by excluding deactivating solvent molecules to date, however, photophysical studies for these complexes have not been published. A recent paper details ternary complexes with an additional chromophore, antipyrene (antipy)... 

A number of aromatic groups are known to intercalate DNA. Among these are tetraazatriphenylenes, which have been shown to bind to the minor groove of DNA. These functional groups have been incorporated into a luminescent lanthanide complex (71), where it serves not only to bind to DNA, but also acts as the antenna ligand. As a consequence of this, intercalation prevents luminescence by quenching the ligand excited state (194). 

Antenna ligand A ligand containing a chromophore capable of strong light absorption and of then transferring the energy of excitation to a bound metal ion. 

Petushkov, V. N., Gibson, B. G., Visser, A. J. W. G., and Lee, J. (2000). Purification and ligand exchange protocols for antenna proteins from bioluminescent bacteria. Method. Enzymol. 305 164-180. 

Square-planar zinc compounds predominate with these ligand types as would be predicted. This is in contrast to the prevalence of tetrahedral or distorted tetrahedral geometries for four-coordinate species that have been discussed thus far. Zinc porphyrin complexes are frequently used as building blocks in the formation of supramolecular structures. Zinc porphyrins can also act as electron donors and antenna in the formation of photoexcited states. Although the coordination of zinc to the porphyrin shows little variation, the properties of the zinc-coordinated compounds are extremely important and form the most extensively structurally characterized multidentate ligand class in the CSD. The examples presented here reflect only a fraction of these compounds but have been selected as recent and representative examples. Expanded ring porphyrins have also... 

Earlier experiments based on EAG and SSR highlighted the inordinate specificity and sensitivity of the insect olfactory system. While minimal structural modifications to pheromone molecules render them inactive [12], a single molecule of the native ligand is estimated to be sufficient to activate an olfactory neuron in male antennae [14]. The large number of detectors certainly contributes to the sensitivity of the olfactory system, but selectivity is a matter of... 

Nowadays, most reaction center models carry suitable antenna pigments and acceptor groups and in effect are photosystem models. A typical example for a state-of-the-art system that incorporates many aspects of a photosystem consisted of a boron dipyrrin covalently linked to a zinc(II) porphyrin, which carried a suitably modified C60 derivative as axial ligand. Selective excitation of the boron dipyrrin as antenna pigment resulted in energy transfer to a zinc(II) porphyrin followed by electron transfer to the acceptor109. 

Various approaches have been taken to the synthesis of effective luminescent materials, using a variety of large encapsulating antenna-containing ligands, including podands, calixarenes, macrocycles, and macrobicycles (cryptands). These have been divided into acyclic (sub-section A) and cyclic (sub-section B). Representative ligands and complexes will be presented and discussed. 

Galaup, C. Azema, J. Tisnes, P. Picard, C. Ramos, P. Juanes, O. Brunet, E. Rodrfguez-Ubis, J.-C. Luminescence of Eu3 + and Tb3 + complexes of two macrobicyclic ligands derived from a tetralactam ring and a chromophoric antenna. Helv. Chim. Acta 2002, 85(6), 1613-1625. 

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