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Faculty Research

Brennan, John

JohnBrennan v1aProfessor and Chair

Research Synopsis: Molecular and solid-state inorganic chemistry, thin films and nanometer-sized clusters

Phone: (848) 445-5677

E-mail: brennan@chem.rutgers.edu

 

 

 

Research 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. SC6F5-, SeSe2-, Te2-) 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. Most recently, we started to explore the related chemistry of the actinides Th and U.

Lanthanide Oxide Clusters

jbrennanresearch01 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.

jbrennanresearch02 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

jbrennanresearch03The 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

jbrennanresearch04Our 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.

Actinides

Actinides v2We’ve recently started investigating the related chemistry of the actinides. Using fluorinated chalcogenolates, we’ve prepared hydrocarbon soluble molecular Th compounds (i.e. the thiolate and selenolate pictured here) and are using these as entry points for the preparation of nanoscale actinide materials.

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