Role of Inner- and Outer-Sphere Bonding in the Sensitization of EuIII-Luminescence Deciphered by Combined Analysis of Experimental Electron Density Distribution Function and Photophysical Data

Jean-Claude Bünzli
2008
Journal paper

Abstract

A series of lanthanide adducts with different amounts of 1,10-phenanthroline, chloride ions, and water molecules in the inner and outer coordination spheres are investigated with the aim of relating the chemical bonding pattern in the crystals to the luminescence properties of the Eu ion: [LnCl1Phen2(H2O)3]Cl2(H2O) (Ln ) Eu, 1Eu; Gd, 1Gd; Tb, 1Tb), [EuCl2Phen2(H2O)2]Cl1(H2O) (2), and [EuCl2Phen1(H2O)4]Cl1(H2O) (3). The influence of inner- versus outersphere ligands on the Ln-X bond lengths and angles in the structure is examined. A detailed topological analysis of the electron density function derived from the X-ray diffraction data for 1Gd is performed within the frame of the “atoms in molecule” theory for the first time for a lanthanide complex. The chemical bonding pattern is interpreted in terms of net atomic charges, bond energies, and electron transfers from the ligands to the metal ion. A noteworthy finding is that the energy of extended noncovalent interactions occurring in the second coordination sphere (Hbonding and π-stacking interactions) is comparable to that of Ln-ligand bonds. The luminescence properties of the three Eu adducts are interpreted with the results of electron density distribution function topology. An intraligand charge transfer state is identified, and its contribution in the ligand-to-europium energy transfer process is analyzed. The outcome of this study is that specific interionic interactions which are usually not considered in theoretical calculations or in the interpretation of luminescence properties play an important role in the sensitization of the Eu luminescence.

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Jean-Claude Bünzli
2008
Journal paper

Abstract

Official source

https://infoscience.epfl.ch/record/129617?ln=en

About this result

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Thermodynamic Parameters Governing the Self-Assembly of Head–Head–Head Lanthanide Bimetallic Helicates

Jean-Claude Bünzli, Thomas Binderup Jensen, Rosario Scopelliti

Abstract: The heterobitopic ligands LABX (X=1,2, 3,4 or 5),diff ering only by a Cl or NEt2 substituent,have been designed to complex with a pair of lanthanide ions to form triple-stranded bimetallic helicates of overall composition [Ln2ACHTUNGTRENUNG(LABX)3]6+. The percentage of HHH (head–head–head) isomer,in which each of the three ligand strands coordinates to the same lanthanide ion with the same coordination unit,is deciding the ability of the ligands to selectively form heterobimetallic complexes containing one luminescent and one magnetic or two different luminescent ions. It deviates significantly from the statistical value of 25% and ranges from 6–20% for LAB2 complexes to 93–96% for LAB4 complexes. The equilibrium between HHT (head–head–tail) and HHH isomers has been investigated in detail for homobimetallic helicates (Ln=Y,La,Ce , Pr, Nd,Sm,Eu,Lu) by means of variable temperature NMR and thermodynamic parameters have been determined. The equilibrium is characterized by small values of DH and DS,whic h vary in opposite direction along the lanthanide series for complexes with the same ligand in a way that keeps the value of DG almost constant. The results are interpreted in terms of differences in interstrand stacking,ion–dip ole interactions and metal–metal repulsion.

2007

, ,

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

Intermolecular Interactions as Actors in Energy-Transfer Processes in Lanthanide Complexes with 2,2′-Bipyridine

Jean-Claude Bünzli

A series of lanthanide complexes [LnClx(bpy)y(H2O)z]Cl3-x(H2O)n(EtOH)m (Ln ) Eu, Gd, Tb; x ) 1, 2; y ) 1, 2; z ) 2-4) with different numbers of 2,2′-bipyridine (bpy), chloride ions, and water molecules in the inner coordination sphere were synthesized and investigated with the aim of relating their molecular geometry and crystal packing to the efficiency of ligand-to-metal energy transfer. In conjunction to the rotation of the pyridine rings upon coordination to the Ln ion, the high flexibility displayed by bpy ligands leads to rather unexpected bending of these rings with respect to the central bond, owing to intermolecular interactions such as Cl · · ·π and π-stacking ones. Deciphering the luminescence properties of the Eu and Tb complexes needs to take into account both the composition of the inner coordination sphere and the peculiarities of the crystal packing. For instance, in addition to the classical ligand f Eu charge-transfer state (LMCT), another chargetransfer state induced by π-stacking interactions (SICT) could be identified. These two states, located between the singlet and triplet states of the bpy ligand(s), provide relays facilitating the energy migration from the singlet to the triplet states and eventually to the excited Eu states, improving the overall ligand-to-Eu energy transfer. Another point is the involvement of the inner-sphere water molecules in H-bonding with chloride ions, which considerably lowers their luminescence quenching ability, so that the adducts remain highly luminescent. For instance, the terbium chloride with two bpy ligands is an efficient near-UV to green light converter, with an overall quantum yield equal to 37% despite the coordinated water molecules. The interpretations given are substantiated by DFT and TD-DFT theoretical calculations of the complexes and ligand assemblies.

2009