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Department of Chemistry

Prof. J.A. Gareth Williams

Personal web page

Professor in the Department of Chemistry
Telephone: +44 (0) 191 33 42124

(email at j.a.g.williams@durham.ac.uk)

Research Interests

Research interests are centred around the synthesis and properties of light-emitting molecules. Applications include: (i) organic light-emitting diodes (OLEDs) for new flat-screen display technology, (ii) luminescent probes for bioimaging and as sensors for bioactive molecules in solution, and (iii) photosensitisers of energy- and electron-transfer for solar energy conversion.

We are also interested in the bacteristatic effects of ligands related to EDTA - how such chelants can interfere with the ability of bacteria to acquire the metal ions they need to survive. Such research has huge implications for the shelf-life of many consumer products, ranging from mayonnaise to face cream! 

Our synthetic work includes both organic synthesis and the coordination chemistry of transition metal and lanthanide ions. We also study the luminescence and photophysical properties of conjugated organic molecules and new metal complexes. The work is multi-disciplinary in nature, and embraces all three of the main branches of chemistry. We have close links with Universities in France, Italy and North America, and industrial laboratories in the UK and USA.

Cyclometallated platinum(II) and iridium(III) complexes

Organic light emitting devices (OLEDs) are set to be at the forefront of future display screen technology. Luminescent, charge-neutral complexes of third-row transition metal ions will be important as "triplet-harvesting agents" in OLEDs. The high spin-orbit coupling constant of these heavy metal ions promotes emission from the triplet excited states that are normally non-emissive and wasted in such devices, allowing huge gains in efficiency and lower power consumption. Charge-neutral complexes can be obtained by cyclometallation: formation of a metal-carbon bond within a chelate ring. We are investigating N^C^N-coordinating ligands for this purpose, bound to Pt(II) and Ir(III).1, 2

For example, the platinum complexes of these ligands are amongst the most emissive ever reported, with quantum yields in excess of 60%, and the colour of emission can be tuned from green to red according to the substituents on the ligand.2 These compounds function well in OLEDs giving very efficient performance.3 At high concentrations, intense excimer emission is also observed and, by choosing an appropriate concentration, the combination of blue-green emission from isolated molecules with red excimer emission leads to the production of white light, a feature that is attractive for lighting applications. The figure shows four OLEDs prepared using different concentrations of platinum and the different colours that result, superimposed on the CIE coordinates. The asterisk indicates the ideal position for ambient room lighting – we’re not far off! 4

We’re also exploring the utility of these compounds as oxygen sensors. By immobilising them in an ethyl cellulose film also containing platinum octaethylporphyrin, a wide-range O2 sensor is obtained that responds as a molecular traffic light.2

Molecular LEGO: Cross-couplings in the synthesis of photoactive multimetallic assemblies

We have pioneered the use of palladium-catalysed cross-coupling reactions for building on the back of metal-bound ligands. For example, ruthenium(II) and iridium(III) complexes with bipyridyl and terpyridyl ligands, incorporating a bromo substituent, can react with aryl boronic acids, offering a reliable route to larger systems. We have also shown that boronic acid functionality can be introduced into such metal complexes. But most significant is the fact that these two mutually complementary types of complex can be cross-coupled with one another, leading to a controlled synthesis of multimetallic assemblies. An example is shown in the figure. This "building block" approach offers major advantages of control and diversity over conventional methods which rely on pre-formed bridging ligands. Photosensitised energy and electron transfer processes between the metal centres are being investigated using time-resolved spectroscopy. See our recently published book chapter on Multinuclear Iridium Complexes for more details of this area of our research.5

Luminescent sensors for bioactive ions and molecules in solution

Although a large number of fluorescent sensors for a variety of species are commercially available, most rely on changes in the wavelength or intensity of the short-lived (nanosecond) emission. We are seeking to develop new light-emitting components for sensors, in which the emission is long-lived, in the microsecond-to-millisecond range. This allows time-resolved detection methods of analysis to be employed, which gets round the problem of background interference from other fluorescent material, and also offers the potential for lifetime-based sensing. See our recent book chapter for more about the background to time-resolved imaging.6

We are investigating a number of systems for this purpose, including cyclometallated platinum(II) and iridium(III) complexes, together with their interactions with a variety of biologically active ions and molecules. The group is studying the use of such brightly emissive metal complexes as novel imaging agents in live cells.7,8 The figure shows a culture of CHO cells under a fluorescent microscope, which have been incubated with a new iridium complex that accumulates and emits brightly in the nucleoli. Some of the iridium complexes under study also show potential for photodynamic therapy - for the light-activated destruction of cancer cells.9 

References

  1. L. F. Gildea and J. A. G. Williams, Iridium and platinum complexes for OLEDs, in "Organic light-emitting diodes: Materials, devices and applications", ed. A. Buckley, Woodhead, 2013.
  2. J. A. G. Williams, Chem. Soc. Rev., 2009, 38, 1783-1801.
  3. J. Kalinowski, V. Fattori, M. Cocchi and J. A. G. Williams, Coord. Chem. Rev., 2011, 255, 2401-2425.
  4. L. Murphy, P. Brulatti, V. Fattori, M. Cocchi and J. A. G. Williams, Chem. Commun., 2012, 48, 5817-5819.
  5. J. A. G. Williams, Multinuclear Iridium Complexes in "Iridium(III) In Optoelectronics and Photonics Applications" ed. E. Z. Colman, Wiley, 2017. 
  6. E. Baggaley, J. A. Weinstein and J. A. G. Williams, "Time-resolved emission imaging microscopy using phosphorescent metal complexes: taking FLIM and PLIM to new lengths", Struct. Bond., 2015, 165, 205-256.
  7. E. Baggaley, S. W. Botchway, J. W. Haycock, H. Morris, I. V. Sazanovich, J. A. G. Williams and J. A. Weinstein, Chem. Sci., 2014, 5, 879-886.
  8. E. Baggaley, I. V. Sazanovich, J. A. G. Williams, J. W. Haycock, S. W. Botchway and J. A. Weinstein, RSC Advances, 2014, 4, 35003-35008.
  9. L. K. McKenzie, I. V. Sazanovich, E. Baggaley, M. Bonneau, V. Guerchais, J. A. G. Williams, J. A. Weinstein and H. E. Bryant, Chem. Eur. J., 2017, 23, 234-238.

Publications

Chapter in book

Journal Article