Macrocyclic Ligands as Antenna Chromophores

2 Macrocyclic Ligands as Antenna Chromophores 12.2.2.1 Cyclen Ligands 

Calixarenes are particularly useful as molecular scaffolds for the design of luminescent lanthanide complexes because they can be conveniently functionalized with various chromophores [83]. The introduction of an antenna chromophore can be accomplished by either use of the 

Porphyrin is one of the most widely studied macrocyclic systems suitable for complexation with lanthanide(III) ions. Porphyrins can absorb strongly in the UV-vis region so as to serve as efficient photo-sensitizers, making lanthanide(III)porphyrinate complexes ideal candidates for luminescence imaging agents. Indirect excitation of porphyrin antenna chromopheres in close proximity to lanthanide ions can make the energy in the triplet state of the porphyrin ligand transfer efficiently to the excited state of the lanthanide ion so as to sensitize the lanthanide luminescence, particularly NIR emission. 

Synthetic routes for the preparation of lanthanide monoporphyrinate complexes with non-diketonate anionic axial ligands have been developed in recent years. Wong and coworkers developed a convenient synthetic route for the preparation of cationic lanthanide(III) monoporphyrinate complexes [Ln(porphyrin)(H20)3]Cl via the protonolysis of lanthanide(III) amide 

The NIR emission intensity of the lanthanide porphyrinate complexes follows the trend Yb Nd Er. This agrees with observations on other luminescent lanthanide complexes and reflects the fact that the efficiency of nonradiative decay increases as the energy of the luminescent state decreases. The emission yields of the ternary lan-thanide(III) monoporphyrinate complexes with hydridotris(pyrazol-l-yl)borate or (cyclopen-tadienyl)tris(diethylphosphito)cobaltate as a co-ligand are generally higher than those of other Yb(III), Nd(III), and Er(III) complexes because the coordination environment provided by the porphyrinate in combination with the tripodal anion effectively shields the Ln + ion from interacting with solvent (C-H) vibrational modes that enhance the rate of nonradiative decay. 

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