Abstract
We have developed a new class of lanthanide nano-clusters that self-assemble using flexible Schiff base ligands. Cd–Ln and Ni–Ln clusters, [Ln8Cd24(L1)12(OAc)39Cl7(OH)2] (Ln = Nd, Eu), [Eu8Cd24(L1)12(OAc)44], [Ln8Cd24(L2)12(OAc)44] (Ln = Nd, Yb, Sm) and [Nd2Ni4(L3)2(acac)6(NO3)2(OH)2], were constructed using different types of flexible Schiff base ligands. These molecular nano-clusters exhibit anisotropic architectures that differ considerably depending upon the presence of Cd (nano-drum) or Ni (square-like nano-cluster). Structural characterization of the self-assembled particles has been undertaken using crystallography, transmission electron microscopy and small-angle X-ray scattering. Comparison of the metric dimensions of the nano-drums shows a consistency of size using these techniques, suggesting that these molecules may share similar structural features in both solid and solution states. Photophysical properties were studied by excitation of the ligand-centered absorption bands in the solid state and in solution, and using confocal microscopy of microspheres loaded with the compounds. The emissive properties of these compounds vary depending upon the combination of lanthanide and Cd or Ni present in these clusters. The results provide new insights into the construction of novel high-nuclearity nano-clusters and offer a promising foundation for the development of new functional nanomaterials.
Highlights
PaperLanthanide nanomaterials are being increasingly investigated for applications in bio-imaging and molecular recognition,[1] due in part to their advantageous photophysical properties, which include a very large pseudo-Stokes shi between the excitation and emission wavelengths, the absence of photo-bleaching, long lived excited states and narrow emission bands.[1,2] Since emissions via 4f–4f transitions are parity forbidden,[3] Ln3+ ions usually have a relatively long photoluminescence lifetime of the excited state
High-nuclearity heterometallic nano-clusters composed of d-block transition metals and lanthanide ions (Ln) have received extensive attention due to their aesthetically stunning molecular structures and their versatile applications in optoelectronics, magnetism, and as probes in biological systems.[4]
Many reports have so far been focused on high-nuclearity 3d–4f clusters, such as Cu/Ln,7a Mn/Ln7b,c and Ni/Ln7d,e cluster systems, in order to study their magnetic properties as singlemolecule magnets
Summary
PaperLanthanide nanomaterials are being increasingly investigated for applications in bio-imaging and molecular recognition,[1] due in part to their advantageous photophysical properties, which include a very large pseudo-Stokes shi between the excitation and emission wavelengths, the absence of photo-bleaching, long lived excited states and narrow emission bands.[1,2] Since emissions via 4f–4f transitions are parity forbidden,[3] Ln3+ ions usually have a relatively long photoluminescence lifetime of the excited state. The exible Schiff-base ligands H2L1, H2L2 and H2L3 exhibit “stretched” coordination modes with metal ions (Scheme 1b), resulting in the large sizes of 1–7. To further explore the self-assembly properties and imaging of these molecules, we report preliminary small angle X-ray scattering measurements and confocal imaging of Cd–Ln complexes containing the H2L2 Schiff base ligand.
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