Improvement of stability and luminescence properties in water of Eu(III) and Tb(III) complexes photosensitized by a bipyridine antenna

Bimodal System (Luminophore and Paramagnetic Contrastophore) Derived from Ln(III) Complexes Based on a Bipyridine-Containing Macrocyclic Ligand

The synthesis of a new 15-membered polyaza-macrocyclic ligand L3H3, which is based on a 2,2'-bipyridine moiety and a diethylenetriaminetriacetic acid core, is reported. The lanthanide chelates of this octadentate ligand were programmed for bimodal probes, luminescent agents (Sm, Eu, Tb, Dy), and magnetic resonance imaging agents (Gd3+). The neutral 1:1 complexes with these Ln3+ ions were prepared and studied in aqueous solution by luminescence and NMR techniques. The main photophysical characteristics of these complexes (i.e., the absorption and luminescence spectra, the metal-centered lifetimes, and the overall luminescence yields, Phi) were measured. In addition, the role played by nonradiative pathways (vibrational energy transfer involving coordinated water molecules, involvement of ligand-to-metal charge-transfer excited states, or metal --> ligand back transfer) is discussed. The L3.Eu and L3.Tb complexes show very bright luminescence when photoexcited from the lowest-energy absorption band of the bipyridine chromophore. The luminescence quantum yields in an air-equilibrated water solution at room temperature are 0.10 and 0.21, respectively, despite the presence of one water molecule in the first coordination sphere of the metal ion. NMR data show that L3.Gd contains also one H2O molecule in the inner sphere. The proton longitudinal relaxivity, r1, of this complex is 3.4 s(-1) mM(-1) (0.47 T, 310 K) and the rotational correlation time, tau(R), is 57 ps (310 K). These values are comparable to those of the clinically used Gd-DTPA. Interestingly, the water exchange rate between the coordination site and the bulk solvent is slow (tau(M) = 3.5 micros at 310 K). The presence of water molecules in the second sphere and in rapid exchange with the solvent is discussed. Finally, it was found by luminescence and NMR experiments that these lanthanide complexes are stable versus transmetalation by several cations (especially Ca2+ and Zn2+) at physiological pH and have no interaction with blood proteins.

Direct Access to Terpyridine-Containing Polyazamacrocycles as Photosensitizing Ligands for Eu(III) Luminescence in Aqueous Media

[reaction: see text] The synthesis of new 18-membered hexaazamacrocycles containing a functionalized 2,2':6',2' '-terpyridine moiety as part of the cyclic backbone and three acetate pendant arms is described. The reported synthetic procedure is based on the use of an efficient metal template ion effect which controls the macrocyclization step. This procedure is compatible with some functional groups present in the macrocyclic structure. The photophysical properties of the Eu(III) complexes derived from these ligands were examined in aqueous solutions. Their luminescence lifetimes (tau approximately 1 ms) and quantum yields (13% < Phi < 18%) on one hand, their high kinetic inertness on the other hand, and the presence of additional functionality allowing their covalent conjugation to biomolecules seriously nominate these complexes as very promising candidates for luminescent labeling of biological materials.

Synthesis and metal complexation properties of Ph-DTPA and Ph-TTHA: novel radionuclide chelating agents for use in nuclear medicine

We wish to report the synthesis and metal complexation properties of new radionuclide chelating agents for use in nuclear medicine. The strategy includes the facile preparation of rigid analogues of DTPA and TTHA possessing an aromatic ring. The aromatic structure used increased the stability of the complexes formed (pre-organization concept) and they are easily functionalised for attaching to any support. The poly(amino)poly(carboxylic) acids, Ph-DTPA (5a) and Ph-TTHA (5b) were obtained in five steps from phenylenediamine as the starting material with overall yields of 42 and 20%, respectively. The key step in this synthetic process is the preparation of tri- and tetra-amino compounds, 3a and 3b, respectively. In order to assess the ability of both ligands to complex with different metals ((111)In, (153)Sm, (90)Y, (177)Lu, (213)Bi, (225)Ac), along with their suitability for use in nuclear medicine, we used a number of complementary tests. We were able to demonstrate the high complexation capacity of Ph-DTPA (5a) with a broad range of radionuclides in a slightly acidic medium. In vitro stability studies show the high stability of Ph-DTPA with (111)In in human serum, a necessary condition for all medical applications. The protonation constant (log K(H)(i)) of Ph-DTPA (5a) was determined by potentiometric methods.

Influence of Anionic Functions on the Coordination and Photophysical Properties of Lanthanide(III) Complexes with Tridentate Bipyridines

A series of four ligands based on a 5'-methyl-2,2'-bipyridyl framework substituted in the 6 position by a carboxylic acid, a phosphonic acid, a monoethyl ester phosphonic acid, or a diethyl ester phosphonic acid are described. The pK(a) values of all ligands and their assignments are determined by a combination of UV-vis absorption spectroscopy and (1)H and (31)P NMR spectroscopy. The ability of the tridentate ligands to form complexes with trivalent lanthanide cations (Ln = La, Nd, Eu, and Lu) in buffered water solutions (Tris-HCl, pH = 7.4) is studied by UV-vis absorption spectroscopy and (1)H NMR. While the two ester ligands display a weak coordination ability toward lanthanide cations, the acid ligands form stable complexes with 1:1, 1:2, and 1:3 Ln/L ratios. A weak selectivity is observed for the middle of the lanthanide series, and the complexes of the phosphonic acid derivative are up to 2 orders of magnitude more stable than those of the carboxylic acid ligand. Photophysical properties of the free phosphonic and carboxylic acid ligands and of their complexes with La, Eu, Gd, Tb, and Lu are investigated in buffered aqueous solutions both at room temperature and 77 K. An efficient ligand-to-metal energy transfer is observed for both the Eu and Tb complexes. Despite a relatively large energy gap between the ligand-centered (3)pipi* and the Eu((5)D(0)) or Tb((5)D(4)) emitting states, the metal-centered luminescence is well sensitized with quantum yields reaching up to 45.5 and 42.2% for the Tb 1:3 complexes with carboxylic and phosphonic acid ligands, respectively.

Lanthanide(iii)/actinide(iii) differentiation in coordination of azine molecules to tris(cyclopentadienyl) complexes of cerium and uranium

Reaction of azine molecules L with the trivalent metallocenes [M(C5H4R)3](M = Ce, U; R = But, SiMe3) in toluene gave the Lewis base adducts [M(C5H4R)3(L)](L = pyridine, 3-picoline, 3,5-lutidine, 3-chloropyridine, pyridazine, pyrimidine, pyrazine, 3,5-dimethylpyrazine and s-triazine), except in the cases of M = U and L = 3-chloropyridine, pyridazine, pyrazine and s-triazine where oxidation of U(III) was found to occur. In the pairs of analogous compounds of Ce(III) and U(III), i.e.[M(C5H4But)3(L)](L = pyridine, picoline) and [M(C5H4SiMe3)3(L)](L = pyridine, lutidine, pyrimidine and dimethylpyrazine), the M-N and average M-C distances are longer for M = Ce than for M = U; however, within a series of azine adducts of the same metallocene, no significant variation is noted in the M-N and average M-C distances. The equilibria between [M(C5H4R)3], L and [M(C5H4R)3(L)] were studied by 1H NMR spectroscopy. The stability constants of the uranium complexes, KUL, are greater than those of the cerium counterparts, KCeL. The values of KML are much greater for R = SiMe3 than for R = But and a linear correlation is found between the logarithms of KML and the hydrogen-bond basicity pKHB scale of the azines. Thermodynamic parameters indicate that the enthalpy-entropy compensation effect holds for these complexation reactions. Competition reactions of [Ce(C5H4R)3] and [U(C5H4R)3] with L show that the selectivity of L in favour of U(III) increases with the [small pi] donor character of the metallocene and is proportional to the pi accepting ability of the azine molecule, measured by its reduction potential.