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Three molecular structures, each containing three different lanthanide(III) centres, have been prepared by coupling three kinetically inert lanthanide(III) complexes in an Ugi reaction. These 2 kDa molecules were purified by dialysis and characterised by NMR and luminescence techniques. The photophysical properties of these heterotrimetallic complexes were investigated and are discussed by comparison with simpler, but related, heterodimetallic compounds. It was found that an aminonaphthalene unit inhibits the sensitisation of terbium, and that the spatial arrangement of the chromophores and lanthanide(III) centres in these molecules inhibits the efficient sensitisation of europium. We conclude that the intramolecular collisions required for efficient Dexter energy transfer from the sensitiser to the lanthanide(III) centre can be prevented by steric congestion. [ABSTRACT FROM AUTHOR]

Fluoride binding by a series of europium and ytterbium complexes of DOTA-tetraamide ligands derived from primary, secondary and tertiary amides has been studied by NMR and luminescence spectroscopies. In all the systems studied, fluoride binding results in a change in the nature of the magnetic anisotropy at the metal centre from an easy axis, to an easy plane anisotropy. This results in reversal of the peaks in the NMR spectra, and in changes to the fine structure of the luminescence spectra. Furthermore, changes to the periphery of the binding cavity are implicated in determining the affinity constant for fluoride. There are clear differences in the entropic contribution to the free energy of activation between systems with benzylic amides and those with methylamides. [ABSTRACT FROM AUTHOR]

Sensitised luminescence from lanthanide complexes offers many potential advantages in imaging and assay, particularly when coupled with time-gating protocols that can be used to gate out background signal. In this perspective, we discuss the routes by which lanthanide arrays and polymetallic d-f hybrids can be prepared by conventional synthesis and self-assembly, and discuss and evaluate the possibilities for exploiting and evaluating the intermediates in the sensitisation process, with particular emphasis on the mechanisms of energy transfer. [ABSTRACT FROM AUTHOR]

The complexes RuL and OsL contain [M(bipy)3]2+ chromophores with a pendant aza-18-crown-6 macrocycle for binding of lanthanide(III) ions. The photophysical properties of the adducts RuL·Ln and OsL·Ln, prepared by addition of excess Ln(NO3)3 (Ln = Nd, Yb) to solutions of RuL and OsL in MeCN, were examined using time-resolved and steady-state luminescence methods. Whereas RuL does not act as an energy-donor to Yb(III), it will transfer energy to (and generate sensitised near-infrared luminescence from) Nd(III) with a Ru(II)→Nd(III) energy-transfer rate constant of 6.8 × 106 s-1. In contrast, OsL is quenched by both Yb(III) and Nd(III), but with faster energy-transfer to Yb(III) (2.6 × 107 s-1) than to Nd(III) (1.4 × 107 s-1). Thus d→f energy transfer is in both cases faster for Os(II) than for Ru(II), but the relative ability of Nd(III) andYb(III) to act as energy-acceptors is inverted from RuL·Ln to OsL·Ln. Reasons for this are discussed with reference to contributions from the Förster and Dexter mechanism for energy-transfer in RuL·Nd and OsL·Nd, using calculated spectroscopic overlap integrals coupled with molecular modelling to estimate inter-chromophore separations. The particular effectiveness of Os(II)→Yb(III) energy-transfer in OsL·Yb is explained in terms of the Horrocks redox mechanism involving an initial *Os(II)→Yb(III) photoinduced electron transfer step generating an Os(III)/Yb(II) state, which is shown to be marginally favourable for OsL·Yb, but not for RuL·Yb in which the [Ru(bipy)3]2+ unit is a poorer excited-state electron-donor by about 0.1 eV. [ABSTRACT FROM AUTHOR]

The complexes RuLand OsLcontain [M(bipy)3]2+chromophores with a pendant aza-18-crown-6 macrocycle for binding of lanthanide(iii) ions. The photophysical properties of the adducts RuL·Lnand OsL·Ln, prepared by addition of excess Ln(NO3)3(Ln = Nd, Yb) to solutions of RuLand OsLin MeCN, were examined using time-resolved and steady-state luminescence methods. Whereas RuLdoes not act as an energy-donor to Yb(iii), it will transfer energy to (and generate sensitised near-infrared luminescence from) Nd(iii) with a Ru(ii)→Nd(iii) energy-transfer rate constant of 6.8 × 106s−1. In contrast, OsLis quenched by both Yb(iii) and Nd(iii), but with faster energy-transfer to Yb(iii) (2.6 × 107s−1) than to Nd(iii) (1.4 × 107s−1). Thus d → f energy transfer is in both cases faster for Os(ii) than for Ru(ii), but the relative ability of Nd(iii) and Yb(iii) to act as energy-acceptors is inverted from RuL·Lnto OsL·Ln. Reasons for this are discussed with reference to contributions from the Förster and Dexter mechanism for energy-transfer in RuL·Ndand OsL·Nd, using calculated spectroscopic overlap integrals coupled with molecular modelling to estimate inter-chromophore separations. The particular effectiveness of Os(ii) → Yb(iii) energy-transfer in OsL·Ybis explained in terms of the Horrocks redox mechanism involving an initial *Os(ii) → Yb(iii) photoinduced electron transfer step generating an Os(iii)/Yb(ii) state, which is shown to be marginally favourable for OsL·Yb, but not for RuL·Ybin which the [Ru(bipy)3]2+unit is a poorer excited-state electron-donor by about 0.1 eV. [ABSTRACT FROM AUTHOR]

Strong bidentate ligation between a fluorinated isophthalate and binuclear lanthanide-DO3A species yields a new class of 19F NMR agent with very high nuclear relaxation rates at physiologically-relevant pH. [ABSTRACT FROM AUTHOR]

A heterotrimetallic tetranuclear lanthanide complex containing two dysprosium ions, a terbium ion and a europium ion has been prepared by coupling three kinetically stable complexes together using the Ugi reaction. The covalently linked trimetallic system exhibits luminescence from all the different lanthanide centres. [ABSTRACT FROM AUTHOR]

Alkyne appended lanthanide complexes derived from DO3A undergo copper catalysed cycloaddition reactions with azides to form triazole appended complexes: coordination of one of the triazole nitrogen atoms to the metal centre changes the local coordination environment and the spectroscopic properties of the complex. [ABSTRACT FROM AUTHOR]

Sensitized luminescence from trivalent lanthanide ions relies on an appropriate energy match between the sensitizer’s excited state (triplet or singlet) and the lanthanide excited state manifold, and also an efficient mechanism of energy transfer between the two. Here, the effect of remote substituents on the luminescence properties of a series of related lanthanide complexes has been investigated. The sensitized lanthanide centered emission is not invariably found to occur following excitation of all chromophores in all systems, and it is shown that only the most structurally congested of the systems investigated exhibited pronounced solvatochromism. [ABSTRACT FROM AUTHOR]

Abstract: We have observed direct two-photon excitation of samarium, europium and terbium ions in solution upon near IR excitation using a tuneable pulsed light source, and have also studied two-photon processes in a pair of related terbium complexes, namely [Tb.DOTA]− and Tb.propargylDO3A. Direct two-photon excitation of lanthanides is observed in simple systems in the absence of sensitizing chromophores. Where even simple chromophores such as a triple bond are present in the complex, then single and two-photon excitation of chromophore excited states competes with direct two-photon excitation of the ions and is the dominant pathway for sensitizing formation of the lanthanide excited state. [Copyright &y& Elsevier]

We report here a structural and photophysical study of lanthanide complexes with the di-deprotonated form of the bibracchial lariat ether N,N′-bis(2-salicylaldiminobenzyl)-4,10-diaza-12-crown-4 (L³) (Ln = Ho(III)-Lu(III)). The X-ray crystal structures of [Ho(L³-2H)](ClO4) (1) and [Er(L³-2H)](ClO4) (2) show the metal ion being eight-coordinate and deeply buried in the cavity of the dianionic receptor. Both sidearms of L-³ are on the same side of the crown moiety, resulting in a syn conformation. Likewise, the lone pair of both pivotal nitrogen atoms is directed inward of the receptor cavity in an endo-endo arrangement and the coordination polyhedron around the lanthanide ion may be described as a distorted square antiprism that shows a deformation toward a square prism by ca. 11°. Attempts to isolate complexes of the lightest members of the lanthanide series were unsuccessful, which suggests a certain degree of selectivity of L³ toward the heaviest Ln(III) ions. This was evaluated and rationalized on the basis of theoretical calculations performed in vacuo at the HF level, by using the 3-21G* basis set for the ligand atoms and a 46+4fn effective core potential for lanthanides. For the [Ln(L³-2H)]+ systems, the calculated bond distances between the metal ion and the coordinated donor atoms decrease along the lanthanide series, as usually observed for Ln(III) complexes. However, for the related [Ln(L¹-2H)]+ and [Ln(L²-2H)]+ systems our ab initio calculations provide geometries in which some of the bond distances of the metal coordination environment increase across the lanthanide series. Thus, thanks to the variation of the ionic radii of the lanthanide ions, receptors L¹ (N,N′- bis(2-salicylaldiminobenzyl)-4,13-diaza-18-crown-6) and L² (N,N′-bis(2-salicylaldiminobenzyl)-1,10-diaza-15-crown- 5) are specially adapted for the complexation of the lighter lanthanide ions. On the other hand, the erbium and ytterbium complexes of L³ have been shown to be emissive in the near-IR Time-resolved studies of complexes confirm that solvent is excluded from the inner coordination sphere in solution. The luminescence properties of the complexes make them ideally suited for use as luminescent tags and suggest that q = 0 complexes of erbium may, after all, be useful as luminescent tags in protic media. [ABSTRACT FROM AUTHOR]

The article presents a study which discusses the synthesis and properties of a bimetallic complex carrying a luminescent rhenium chromophore and a gadolinium (Gd)3+ ion used in magnetic resonance imaging (MRI). It outlines the preparative strategies used to access lanthanide (Ln) alkylation of triester with 4-bromomethylpyridine and sodium hydrogencarbonate. Results reveled that heterometallic complexes combing rhenium and Gd may serve as mutimodal imaging agents.