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Summary

In recent years, our program has been focused on the synthesis and characterization of lanthanide molecules and clusters with extraordinary NIR emission properties. Chalcogen based anions (i.e. SC₆F_5-, SeSe^2-, Te^2-) are particularly useful for the preparation of highly NIR emissive materials, because the relatively long, weak Ln-E bond effectively decreases energy transfer and vibrational quenching processes. Our molecular efforts currently focus on the design and synthesis of new types of chalcogenolate ligands intended to produce lanthanide compounds with superior chemical/electronic properties. Our cluster program is devoted to the preparation of increasingly large/complex cluster compounds, using chalcogen based anions to encapsulate and ‘stabilize’ cluster cores with a surprising variety of ligand types. The utility of these molecules and clusters in emerging polymer based optical fiber technologies has been demonstrated in collaboration with Ajith Kumar and Richard Riman.

Lanthanide Oxide Clusters

The persistent request for ‘air stable’ emissive clusters led us to investigate the synthesis of oxide clusters with chalcogen encapsulants. Santanu Banerjee's first product in this area was from a reaction with SeO₂ that gave (THF)₈Ln₈Se₂O₂(SePh)₁₆. The Nd compound was particularly significant as it was the most efficient molecular source of 1.34 µm emission currently available (up until the fluoride cluster mentioned below), and this energy is important to the telecommunications industry. The compound was also notable as the first molecular Nd compound to emit at 1.81 µm. Santanu's latest oxocluster result is a highly emissive Nd₁₂O₆ compound shown below. The structural characteristics of the oxo core of this material are distinctly similar to those of solid state Nd₂O₃, but the compound differs in that it emits NIR radiation at 1352 nm, while vibations of the Nd₂O₃ lattice quench this emission. Note that in the Nd₁₂ cluster both phonon coupling and concentration quenching lead to elimination of the 1840 nm emission found in the previously described octanuclear cluster.

Lanthanide Fluorides

The apparent stability of these oxo materials with respect to disproportionation led Mike Romanelli to attempt the synthesis of even more stable cluster cores, and his first success in this area came with the isolation of the nanoscale product
(py)₂₄Ln₂₈F₆₈(SePh)₁₆ (Ln = Pr, Nd). The 44% quantum efficiency of the Nd cluster is more than twice that of our oxide clusters.

Chalcogenolate complexes

Our lanthanide work has led, indirectly, to the synthesis and characterization of novel main group and transition metal chalcogenolate complexes with extraordinary physical properties, i.e. the II-VI precursors of Yifeng Cheng. We are still pursuing a number of chemically interesting synthetic targets with significant CVD applications. These chelating ligands are important in Ln chemistry because they form relatively air stable products, as found in the homoleptic Ln(III) pyridinethiolates of Mike Berardini.

Virtually air stable thiolates can also be prepared with fluorinated thiolate ligands. Jonathan Melman has described the synthesis of Ln(SC₆F₅)_n (n = 2, 3) and Kieran Norton has just completed a structural and spectroscopic investigation of the analogous fluorinated phenoxides. These fluorinated compounds are considerably more stable than their C₆H₅ counterparts and they are soluble in hydrocarbon solvents. Using these ligands Mark Fitzgerald prepared the first toluene soluble lanthanide sulfido and selenido clusters.

• M. Romanelli, G. A. Kumar, R. E. Riman, T. J. Emge, and J. G. Brennan, "Highly Emissive Nanoscale Lanthanide Fluoride Clusters", Angew. Chemie. 2008, xxxx.
• S. Banerjee, A. Kumar, T. Emge, R. Riman, and J. G. Brennan, "Thiolate-Bound Thulium Molecules: Synthesis, Structure, and NIR Emission", Chem. Mater. 2008, 20, xxxx.
• R. Riman, G. A. Kumar, V. Atakan, J. G. Brennan, J. Ballato, "Engineered solution synthesis of rare-earth nanomaterials and their optical properties.", Proc. SPIE 2007, 670707/1.
• K. Norton, T. Emge, and J. Brennan, "Lanthanide Compounds with Fluorinated OC₆F₅ Ligands: Homo- and Hetero-valent Complexes of Eu(II) and Eu(III)", Inorg. Chem. 2007, 46, 4060.
• S. Banerjee, G. A. Kumar, R. Riman, T. J. Emge, and J. Brennan, "Monodisperse Oxoclusters of the Lanthanides Begin to Resemble Solid State Materials at Very Small Cluster Sizes: Structure and Near-IR Emission from Nd(III)", J. Amer. Chem. Soc. 2007, 129, 5926.
• G. A. Kumar R. E. Riman, L.A. Diaz Torres, S. Banerjee, M. D. Romanelli, T. J. Emge, J. G. Brennan, "Near Infrared Optical characteristics of Chalcogenide-Bound Nd Organometallic Complexes", Chem. Mater. 2007, 19, 2937.
• L. A. Diaz-Torres, G. A, Kumar, R. E. Riman, S. Banerjee, A. Kornienko, J. G. Brennan, "Photoluminescence of bound rare earth nanoscale complexes.", Optical Materials 2006, 29, 12.
• R. Riman, G. A. Kumar, S. Bannerjee, J. G. Brennan, "Molecular Minerals: Lyophilic Colloids for Ceramicists", J. Am. Ceram. Soc. 2006, 89, 1809.
• G. A. Kumar, R. Riman, S. Chen, D. Smith and J. Ballato, S. Banerjee, A. Kornienko, J. G. Brennan, "Infrared Fluorescence and Optical Gain Characteristics of new Er Nanocluster Polymer Composites", Appl. Phys. Lett. 2006, 88, 91902.
• S. Banerjee, L. Huebner., M. D. Romanelli, G.A. Kumar, R.E. Riman, T. J. Emge, and J. G. Brennan, "Oxoselenido Clusters of the Lanthanides: Rational Introduction of Oxo Ligands and Near-IR Emission from Nd(III)", J. Am. Chem. Soc. 2005, 127, 15900.
• A. Kornienko, S. Banerjee, A. Kumar, R. Riman, T. Emge, and J. Brennan, "Heterometallic Lanthanide Main Group Metal Chalcogenido Clusters; Highly Emissive Precursors to Ternary Solid State Materials ", J. Am. Chem. Soc. 2005, 127, 14008.
• G. A. Kumar, R. Riman, S. Banerjee, A. Kornienko, J. G. Brennan, "Chalcogenide-Bound Erbium Complexes: Paradigm Molecules for Infrared Fluorescence Emission", Chem. Mater. 2005, 17, 5130.
• L. Huebner, A. Kornienko, T. J. Emge, and J. G. Brennan, "Lanthanide Clusters with Internal Ln: Fragmentation and the Formation of Dimers with bridging Se^2- and SeSe^2- Ligands", Inorg. Chem. 2005, 44, 5118.
• A. Kornienko, T. Emge, A. Kumar, R. Riman, and J. Brennan, "Lanthanide Clusters with Internal Ln: Highly Emissive Molecules with Solid State Cores", J. Amer. Chem. Soc. 2005, 127, 3501.
• S. Banerjee, T. Emge, and J. Brennan, "Heterometallic LnHg Compounds with Fluorinated Thiolate Ligands", Inorg. Chem. 2004, 43, 6307.
• L. Huebner, A. Kornienko, T. Emge, and J. Brennan, "Heterometallic Lanthanide Group 12 Metal Iodides", Inorg. Chem. 2004, 43, 5659.
• A. Kornienko, L. Huebner, D. Freedman, T. Emge, and J. Brennan, "Lanthanide-Transition Metal Chalcogenido Cluster Materials", Inorg. Chem. 2003, 42, 8476.
• M. Fitzgerald, T. Emge, and J. Brennan, "Chalcogen Rich Lanthanide Clusters with Fluorinated Thiolate Ligands", Inorg. Chem. 2002, 41, 3528.
• D. Freedman, T. Emge, and J. Brennan, "Chalcogen Rich Clusters of the Lanthanides with Te^2-, (TeTe)^2-, TePh, TeTePh, (TeTeTe(Ph)TeTe)^5- and [(TeTe)₄TePh]^9- Ligands", Inorg. Chem. 2002, 41, 492.
• A. Kornienko, T. Emge, G. Hall and J. Brennan, "Chalcogen Rich Clusters of the Lanthanides from Halogenated Starting Materials (II): Selenido Compounds and the Synthesis of LnSe_x", Inorg. Chem. 2002, 41, 121.
• J. Melman, C. Rhode, T. Emge, and J. Brennan, "Fluorinated Thiolates of the Lanthanides: the Influence of Ionic Radii on the Ln-F Bond", Inorg. Chem. 2002, 41, 28.
• A. Kornienko, T. Emge, and J. Brennan, "Chalcogen Rich Lanthanide Clusters: Cluster Reactivity the Control of Structure by Ancillary Ligands", J. Am. Chem. Soc. 2001, 123, 11933.
• J. Melman, T. Emge, and J. Brennan, "Fluorinated Thiolates of Divalent and Trivalent Lanthanides; Ln-F Bonds and the Synthesis of LnF₃", Inorg. Chem. 2001, 40, 1078.
• A. Kornienko, D. Freedman, T. Emge, and J. Brennan, "Heteroleptic Lanthanide Compounds with Chalcogenolate Ligands: Reduction of PhNNPh/PhEEPh (E = Se or Te) Mixtures with Ln (Ln = Ho, Er, Tm, Yb); Thermolytic Decomposition can give LnN or Ln", Inorg.Chem. 2001, 40, 140.

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