Ligand-enhanced lanthanide emission

By redesigning the above acyclic podand-type ligand 3 into a cyclic cryptate, the issue of stability can be resolved resulting in kinetically stable complexes (Scheme 4) [102]. The Tb(III) and Eu(III) complexes of cryptate 5 show an increase in lanthanide emission lifetimes of 0.72 ms and 0.41 ms, respectively, upon excitation at 310 nm. Similar results are found with the phenanthroline analogue 6 with Eu(III). A large number of modifications of these cryptates have been reported, all showing enhancements in the lanthanide ion emission [103-106]. 

Another related example is 52 [157], which is a tetra-substituted cyclen ligand with four quinoline receptor moieties capable of sensing protons in a similar manner to 50. The lanthanide emission was found to be switched on in highly acidic conditions, with a luminescent enhancement of over 300-fold. Luminescent enhancement was attributed to an enhancement in the population of the Si and subsequent Ti excited states of the quinoline chromophore when in acidic media. A bell-shaped pH profile was found to exist at pH 1.8-3.5, whereas at more acidic pH the emission was switched off. 

The excited triplet level of the ligand in the enhancement system must lie above the emissive level of the lanthanide ion for efficient transfer of energy. The emission levels of Eu3+ (613 nm) and Sm3+ (643 nm) are at lower energy than Tb3+ (545 nm) and Dy3+ (573 nm) and hence it is difficult to have one optimum universal ligand for energy transfer in the case of all four lanthanides. Emission levels of lanthanides along with the triplet levels of aliphatic and aromatic /3-diketones are shown in Fig. 12.34. 

Many lanthanides in the form of complexes emit fluorescence when excited with light absorbed by ligands. Lanthanide ions in crystals or in solution also exhibit fluorescence. The fluorescence observed due to energy transfer from the triplet state of the ligand to excited state of Ln3+ is known as enhanced fluorescence. The fluorescence emission spectra of Tb3+ aquo ion in the absence and presence of porcine trypsin at pH 6.3 is shown in Fig. 11.3. 

Some lanthanide ions when complexed with UV-absorbing ligands, can efficiently accept energy from the excited state of the ligand and produce highly enhanced emission characteristics of the metal ion. Rare earth complexes have some advantages over organic fluorescent probes such as fluorescein, rhodamines, umbelliferones such as... 

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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