Energy transfer in coumarin-sensitised lanthanide luminescence: investigation of the nature of the sensitiser and its distance to the lanthanide ion

A series of lanthanide complexes Ln(dpxCy)(3) have been synthesised. The ligands are composed of a coordinating dipicolinic acid backbone decorated with a polyoxyethylene arm fitted with a coumarin moiety at its extremity. The nature of the coumarin as well as the length of the linker have been varied. Upon excitation at 320 nm, the coumarin exclusively acts as an antenna while the dipicolinic acid core is not excited. Upon excitation below 300 nm, both parts are excited. With europium as a metal centre, the relaxation of the europium ion (intrinsic quantum yield Phi(Eu)(Eu) and radiative lifetime tau(r)) is constant for all the studied ligands. Therefore, the observed differences in overall quantum yield (Phi(Eu)(L)) in such systems come exclusively from the variation of the terminal coumarin. The overall quantum yields of the studied complexes are low (Phi(Eu)(L) < 2% in aqueous solution). In order to rationalise the mechanism of the energy transfer and to improve the sensitisation efficiency (eta(sens)), the distance between the coumarin sensitiser and the lanthanide centre was explored in solution and compared to the solid state. In the solid state, a dramatic effect was confirmed, with an improvement of 80% in the quantum yield Phi(Eu)(L) for short linkers ((-CH2CH2O-)(n) with n = 1 compared to n = 3). By monitoring the lifetime decay of the excited state of the lanthanide cation with nanosecond vs. microsecond time-resolved spectroscopy at low temperature, the sensitisation of the lanthanide ions by coumarin derivatives was demonstrated to mainly occur through the singlet excited state of the coumarin and not via the usual triplet pathway. No evidence of a different behaviour at room temperature was found by transient triplet-triplet absorption spectroscopy.

Europium Complexes of Tris(dipicolinato) Derivatives Coupled to Methylumbelliferone: A Double Sensitization

Julien Alexandre Andres, Anne-Sophie Chauvin

A para-poly(oxyethylene) dipicolinic acid derivative coupled to methylumbelliferone was synthesized and used as a ligand for coordination with europium ions. Characterization of the complex by mass spectrometry and spectrophotometry was performed in aqueous solution and points to the formation of the 1:3 Eu/L complex. Physicochemical properties are close to those of already reported dipicolinato complexes, confirming that the presence of a coumarin chromophore does not interfere with complex formation. Photophysical measurements result in observed lifetimes of 1.4 ms and intrinsic quantum yields, Phi(Eu)(Eu), ranging from 33 to 50% (depending on the solvent used). Selective excitation can be performed either mostly on the dipicolinate (DPA) backbone (at 280 nm) or exclusively on the coupled coumarin chromophore in the near visible range (320 nm). Despite the spatial distance between the antenna and the lanthanide ion, europium emission was observed upon 320 nm excitation together with the coumarin emission. The different sensitization pathways have been explored and rationalized. Finally, the complex has been used to probe the ratio of a binary mixture of solvent, establishing the potential interest of this new class of ligands in many fields of research, from photophysics to applied chemistry.

Wiley-Blackwell2010

Complexes luminescents de lanthanides avec des dérivés du cyclène comme briques pour l'ingénierie de sondes analytiques biomédicales

This work deals with the engineering of a new luminescent complex of lanthanide, able to be specifically grafted onto biological materials and to emit a characteristic light useful in quantifying the concentration of the target molecules. We have based our molecular design on TbL1, wich possesses a significant quantum yield in water pointing to an efficient intramolecular energy transfer. We have elaborated the synthesis of a ligand derived from L1 and named L3, by functionalizing one of the chromophoric groups into a moiety able to couple the luminescent sensor to biological materials. The synthesis of the complexes was then realized. The thermodynamic study of the ligands L1 and L3, and their terbium complexes in water was made to establish the distribution diagrams and to compare their behaviour, especially at neutral pH. In addition, EuL1 was also investigated and the complexation constants of TbL1, EuL1 and TbL3 in water showed that these molecules are thermodynamically stable in aqueous media. Characterization of EuL3 in the solid state by high resolution luminescence allowed us to determine the geometry of the coordination sphere of the Eu(III) ion, which possesses a distorded C4 symmetry. The detailed study of the Eu(III) ion transitions evidenced two different sites, possibly arising from an equilibrium induced by the competition between a water molecule and the carboxylate function. This interpretation is in line with the measured lifetimes of the Tb(III) and Eu(III) excited states and the calculation of the hydration number of the complexes in aqueous solution. The determination of the energy of the singlet state and triplet states of the ligand, as well as of the emitting levels of the Ln(III) ions in solution showed that their relative values are favorable for a good intramolecular energy transfer. The quantum yields of TbL3 and EuL3, which amount to 2.6 % and 1.4 %, respectively, are lower than those of the complexes with L1, denoting that the presence of the acid function increases losses in the energy transfer. However, these values remain sizeable enough for potential biomedical applications. The study of the intermolecular energy transfer from EuL1, EuL3 and TbL3 to the acceptor Cy5 has proved promising, and the resulting yields were 90 % for EuL1 and 80 % for the complexes with L3.

EPFL2004

Stable 8-hydroxyquinolinate-based podates as efficient sensitizers of lanthanide near-infrared luminescence

Jean-Claude Bünzli, Anne-Sophie Chauvin, Steve Comby

New polydentate ligands (e.g., Tsox and TsoxMe) have been synthesized to take advantage of the chelating effect of bidentate 8-hydroxyquinolinate subunits connected to a N,N,N¢,N¢-tetraaminopropyl-1,2-ethylenediamine framework and with the aim of sensitizing the NIR luminescence of NdIII and YbIII ions. Ten pKa’s have been determined and the interaction between the ligands and LnIII ions in dilute aqueous solution has been probed both by potentiometric and spectrophotometric titrations. These studies have been mostly performed with the EuIII ion, which is in the middle of the lanthanide series, and extended to other ions (LaIII, ErIII, LuIIII). Stable complexes with LnIII ions are formed (pLn in the range of 14-16), the four chromophoric units being coordinated to the metal center, exploiting the entropic effect generated by the anchor. The monometallic complexes [Ln(H2L)]3- exist as the major species at physiological pH regardless of the lanthanide used. Lifetime determinations of the Nd(4F3/2)and Yb(2F5/2) excited levels in both H2O and D2O at buffered pH point to the absence of water molecules bound in the inner coordination sphere of the LnIII. Photophysical properties of the free ligands and of their lanthanide complexes have been investigated in buffered aqueous solutions both at room temperature and 77 K. The lowenergy triplet state makes energy transfers from the ligand to the metal ions possible; this leads to a sizable sensitization of the NdIII- or YbIII-centered luminescence (QNd L ) 0.02% and QYb L ) 0.18%) for Tsox chelates. Methylation of the amide functions removes the quenching mechanism induced by the proximate N-H vibrations and increases both the lifetimes and quantum yields of the TsoxMe chelates (QNd L ) 0.04% and QYb L ) 0.37%). In fact, TsoxMe yields one of the most luminescent YbIII compounds known in water, and this ligand appears to be suitable for the development of NIR probes for bioanalyses.

2006

Complexes luminescents de lanthanides avec des dérivés du cyclène comme briques pour l'ingénierie de sondes analytiques biomédicales

EPFL2004

Stable 8-hydroxyquinolinate-based podates as efficient sensitizers of lanthanide near-infrared luminescence

Jean-Claude Bünzli, Anne-Sophie Chauvin, Steve Comby

2006