Prof. J.A. Gareth Williams
(email at email@example.com)
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.
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 Germany.
Cyclometallated platinum(II) and iridium(III) complexesOrganic 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.1 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.5
Molecular LEGO: Cross-couplings in the synthesis of photoactive multimetallic assembliesWe are pioneering 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.6 We have also shown that boronic acid functionality can be introduced into such metal complexes.7 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.8 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.
Luminescent sensors for bioactive ions and molecules in solutionAlthough 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. We are investigating a number of systems for this purpose, including bis-terpyridyl iridium complexes and macrocyclic complexes of the luminescent lanthanide ions, terbium and europium,9 together with their interactions with a variety of biologically active ions and molecules such as chloride, zinc(II) ions and DNA. Ths group is also studying the use of brightly emissive metal complexes as novel imaging agents in live cells. 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.
- S.J. Farley, D.L. Rochester, A.L. Thompson, J.A.K. Howard and J.A.G. Williams, Inorg. Chem., 2005, 44, 9690-9703.
- A.J. Wilkinson, H. Puschmann, J.A.K. Howard, C.E. Foster and J.A.G. Williams, Inorg. Chem., 2006, 45, 8685-8699.
- M. Cocchi, D. Virgili, V. Fattori, D.L. Rochester and J.A.G. Williams, Adv. Funct. Mater., 2007, 17, 285-289; M. Cocchi, D. Virgili, V. Fattori, J.A.G. Williams and J. Kalinowski, Appl. Phys. Lett., 2007, 90, 023506.
- M. Cocchi, J. Kalinowski, D. Virgili, V. Fattori, S. Develay and J.A.G. Williams, Appl. Phys. Lett., 2007, 90, 163508.
- R.C. Evans, P. Douglas, J.A.G. Williams and D.L. Rochester, J. Fluorescence, 2006, 16, 201-206.
- W. Leslie, A.S. Batsanov, J.A.K. Howard and J.A.G. Williams, Dalton Trans., 2004, 623-631.
- C.J. Aspley and J.A.G. Williams, New J. Chem., 2001, 25, 1136-1147.
- K.J. Arm and J.A.G. Williams, Chem. Commun., 2005, 230-232; K.J. Arm and J.A.G. Williams, Dalton. Trans., 2006, 2172-2174.
- A.J. Wilkinson, D. Maffeo, A. Beeby, C.E. Foster and J.A.G. Williams, Inorg. Chem., 2007, 46, DOI: 10.1021/ic701113c.