Lanthanide carboxylates, luminescence

Due to the presence of hard anionic oxygen atoms, phenolate and carboxylate groups are often employed as donors in lanthanide coordination chemistry. Ligand [L18]4- is reported as an excellent triplet sensitizer for lanthanide luminescence (61). Indeed aqueous lifetimes of 0.57 and 1.61 ms are reported for europium and terbium, respectively quantum yields of 0.20 and 0.95 respectively refer to the efficiency of the energy transfer process alone. 

The importance of the carboxylate donors is underlined by a study of the lanthanide coordination chemistry of the similar terdentate ligand 2,6 -bis( 1 -pyrazol-3 -yl)pyridine, L24 (63). The complex structure of [Tb(L24)3][PF6]3, shown in Fig. 11, appears to be fairly robust in methanolic solution, with Horrocks analysis (q = 0.6) suggesting the 9-coordinate structure is retained the small quenching effect of outer sphere coordination explains the q-value. However, in aqueous solution, the lability of the ligands dramatically changes the luminescence. Whilst the emission decays are not exactly single exponential, approximate lifetimes in H20 and DoO suggest a solvation value of 4-5. 

The detection of aromatic carboxylates via the formation of ternary complexes using lanthanide ion complexes of functionalised diaza-crown ethers 30 and 31 has been demonstrated [134]. Like the previous examples, these complexes contained vacant coordination sites but the use of carboxylic acid arms resulted in overall cationic 2+ or 1+ complexes. Furthermore, the formation of luminescent ternary complexes was possible with both Tb(III) and Eu(III). A number of antennae were tested including picolinate, phthalate benzoate and dibenzoylmethide. The formations of these ternary complexes were studied by both luminescence and mass spectroscopy. In the case of Eu-30 and Tb-30, the 1 1 ternary complexes were identified. When the Tb(III) and Eu(III) complexes of 30 were titrated with picolinic acid, luminescent enhancements of 250- and 170-fold, respectively, were recorded. The higher values obtained for Tb(III) was explained because there was a better match between the triplet energy of the antenna and a charge transfer deactivation pathway compared to the Eu(III) complex. 

The Eu(III) and Tb(III) complexes of 36, developed by Parker et al., showed modulation of the lanthanide emission caused by changes in pH [139]. The coordinating antenna was covalently attached to the complex but only coordinates to the metal centre under specific pH conditions. This was demonstrated on a cyclen complex where three carboxylate pendant arms were attached, allowing coordination to take place, and resulted in the formation of a charge-neutral complex. The fourth position contained an aromatic sulfonamide tethered via an ethyl carbon chain, 36a-36c. Studies showed that luminescence was switched on when the complexes were in alkaline conditions because the two metal-bound waters (q = 2) were displaced. This led to a situation where the pendant aryl sulfonamide groups were coordinated... 

In a first step, intermediate species form rapidly upon mixing the lanthanide ion with EUdota. Both excited state luminescence lifetime determinations for the Eum complex (Chang et al., 2001) and molecular mechanics calculations are consistent with a structure in which the lanthanide ion is coordinated to four carboxylate groups, well away from the nitrogen atoms of the macrocycle, two of which are protonated. This intermediate may react with a hydroxide group to form monoprotonated neutral species in a rapid equilibrium. 

Carboxylates With the aim of developing highly luminescent Er111 complexes for advanced photonic applications, some of the research concentrates on the synthesis of new ligands, based on simple molecular frameworks such as alkyl chains or benzene rings fitted with carboxylate groups for coordination to the lanthanide ions. For instance, the photophys-... 

Pope, S.J.A., Burton-Pye, B.P., Berridge, R., et al. (2006) Self-assembly of luminescent ternary complexes between seven-coordinate lanthanide(lll) complexes and chromophore bearing carboxylates and phosphonates. Dalton Transactions, 2907. 

Gunnlaugsson, T., Harte, A.J., Leonard, J.R, and Nieuwenhuyzen, M. (2002) Delayed lanthanide luminescence sensing of aromatic carboxylates using heptadentate triamide Tb(III) cyclen complexes the recognition of salicylic acid in water. Chemical Communications, 2134-2135. 

Elhabiri, M., Scopelliti, R., Biinzli, J.C.G, and Piguet, C. (1999) Lanthanide helicates self-assembled in water a new class of highly stable and luminescent dimetallic carboxylates. Journal of the American Chemical Society, 121, 10747-10762. 

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