ANU Chemistry-Prof Mark Humphrey-Molecular Materials and Organometallic Chemistry

Molecular Materials and Organometallic Chemistry

Professor Mark Humphrey

Our current research involves two areas of interest, both related to materials with potential use in optical technology:

Nanotechnology has been recognized as a priority area of research world-wide, and has been defined as "the creation and utilization of functional materials, devices and systems with novel properties and functions that are achieved through the control of matter atom by atom, molecule by molecule or at the macromolecular level." (National Science Foundation, USA)

Dendrimers are hyperbranched nanomolecules prepared by sequential addition of simple branched monomer building blocks to a central core. Their step-wise synthesis allows the preparation of precise chemical structures which can be easily and systematically varied to manipulate their physical properties. Dendritic molecules have been shown to mimic the bioactivity of enzymes and proteins, or to produce previously unknown or significantly improved physical and chemical properties, compared to traditional linear polymers. As a consequence, dendrimers are considered to be one of the prime building blocks for the construction of nanoscale objects, molecular devices, advanced drug-delivery systems, etc.

We are looking at oligomeric and dendritic assemblies with potential in nonlinear optical applications for emerging photonics technologies.

Materials based on incorporating metal complexes into processable organic-based polymeric backbones i.e. metallic polymers. These materials have potential as "optical limiters", affording optical device protection which is of potential use in both laboratory and military applications.

Materials for Nonlinear Optical Applications

Current trends suggest that light, rather than electricity, will increasingly be used in the area of information technology, with potential in optical communications, data storage and computer systems. Although materials being studied are diverse, they are usually "off-the-shelf" compounds selected by the physicists and engineers active in this area, so that there are many areas in which chemists can make important contributions by rational synthesis of designed candidate molecules for the optical technology industry.

Materials whose optical properties depend on the intensity of the incident light are termed non-linear optical (NLO) materials. Organic compounds that are asymmetrically polarizable e.g. through conjugated p systems, have been shown to produce large NLO responses. We are interested in both organic and organometallic materials. Organometallic complexes combine the advantage of organic materials (fast NLO responses) with the design flexibility of inorganic complexes (variation in oxidation state, coordination number, coordination geometry, and co-ligands, and intense MLCT transitions). Initial work has focussed on metal acetylide complexes. These are usually thermally robust and oxidatively stable, and accessible in high yields by well-established synthetic methodologies. We have systematically varied molecular components in order to derive structure-NLO property relationships to facilitate organometallic NLO materials design and produced complexes have the largest quadratic and cubic nonlinearities for organometallic complexes thus far.

These studies have utilized hyper-Rayleigh scattering and electric field-induced second harmonic generation (experimental molecular quadratic nonlinearities), semi-empirical ZINDO (computational molecular quadratic nonlinearities), Kurtz powder (bulk second-order susceptibilities), and degenerate four-wave mixing and Z-scan (molecular cubic nonlinearities) measurements.

Our ongoing studies are involved with extending these small molecule-based studies into the macromolecular realm to afford useful and processable materials. There has been an explosion of interest in dendrimers (hyperbranched oligomers) recently, with the current move to prepare functional dendritic materials. We are currently investigating the NLO properties of arylalkynyl metal-based dendrimers with a variety of core units, branching groups and spacers. For example, our Ru₉ species pictured has been shown to have the largest cubic nonlinearity for an organometallic complex thus far.

Materials for Optical Limiting

Complexes containing metal-metal bonds are of interest for both their chemical reactivity and their physical properties. We are currently focussing on the physical properties of mixed-metal Group 6-Group 9 and high-nuclearity ruthenium clusters, specifically in their potential for use in optical limiting applications. Results from our clusters are competitive with those of the other "hot" optical limiting materials (phthalocyanines and fullerenes), but our mixed-metal clusters possess the additional virtue of easy tailoring of response by molecular structure modification. To fully exploit such materials, processability needs to be built in. We are consequently now working on incorporating our tailored polymetallic clusters into polymeric supports via the polymer backbone to afford processable polymetallic polymers.

The information on this page was updated in July 2007.
The page has been authorised by Mark Humphrey, Department of Chemistry.
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