Overview Our research work spans the three-way interface between chemistry, physics and materials science. We design, synthesize and characterize the molecular and solid state properties of neutral radlcals, radical ions and biradicals, with the long term goal of generating processable molecular materials exhibiting unusual magnetic, conductive and/or optical properties.
Methodology Inorganic, organic and organometallic synthesis, NMR, EPR, UV-Vis and FTIR spectroscopy; elecrochemistry; single crystal and powder X-ray crystallography; variable temperature conductivity and magnetic susceptibilty measurements; high pressure effects on the structure and transport properties of molecular solids. Semi-empirical and density functional theory quantum calculations on molecules and molecular solids.
Positions Available Openings are available for new graduate students with a strong background in chemical synthesis and an interest in materials science who wish to pursue graduate research at the M.Sc. and/or Ph.D. level. Positions are also available for senior undergraduate researchers. For details on any of the research topics summarized below, contact Professor Richard Oakley. For information on how to apply for graduate school, contact the office of the Guelph Waterloo Centre for Graduate Work in Chemistry. For a printer friendly one-page summary of this research area, click here.
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Single component neutral radical conductors
Stable molecular radicals have found diverse applications in materials science, e.g., in EPR imaging, as spin probes and as initiators for free radical polymerizations. A more challenging goal, however, one which we have pursued for some time, is based on the concept of using heterocyclic thiazyl (-S=N-) and selenazyl (-Se=N-) radicals as building blocks for single component molecular conductors, materials in which the unpaired electron on the radical serves as a charge carrier. Pursuit of this target requires the development of radicals that possess a low molecular disproportionation energy and a large intermolecular overlap in the solid state. Optimization of overlap enhances the solid state electronic bandwidth W, while a low disproportionation energy heralds a low on-site Coulomb repulsion energy U, the criterion for a metallic state being that W > U (Figure 1). To date no metallic system has ever been isolated. Known systems are either paramagnetic Mott insulators, for which U > W, or diamagnetic semiconductors or insulators, the result of a Charge Density Wave (CDW) driven or Peierls distortion. |
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Resonance Stabilized Radicals
In recent years we have focused our efforts on the design of resonance stabilized radicals such as the bis-thiadiazinyls (bis-TDAs) and bis-thiadiazolyls (bis-DTAs) illustrated below. These compounds enjoy increased thermal stability, low gas phase disproportionation energies and solution based electrochemical cell potentials Ecell (Figure 2A), and crystallize as undimerized slipped pi-stack arrays (Figure 2B). Intermolecular overlap is, however, limited. As a result the electronic bandwidth (W < 0.5 eV) and conductivity (sigma(RT) < 10-6 S/cm) of these materials are limited.
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Figure 2A - Cyclic Voltammogram of the bis(DTA) with R1 = Me, R2 = Cl. |
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Figure 2B - Slipped pi-stacks of the bis(DTA) with R1 = Et, R2 = Cl. |
References
(1) L Beer, J. L. Brusso, A. W. Cordes, R. C. Haddon, M. E. Itkis, K. Kirschbaum, D. S. MacGregor, R. T. Oakley, A. A. Pinkerton and R. W. Reed. J. Am. Chem. Soc. 124, 9498-9509 (2002). (2) L. Beer, J. F. Britten, J. L. Brusso, A. W. Cordes, R. C. Haddon, M. E. Itkis, D. S. MacGregor, R. T. Oakley, R. W. Reed and C. M. Robertson. J. Am. Chem. Soc. 125, 14394-14403 (2003). (3) L. Beer, R. C. Haddon, M. E. Itkis, A. A. Leitch, R. T. Oakley, R. W. Reed, J. F. Richardson and D. G. VanderVeer. Chem. Commun. 1218-1220 (2005). |
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High Bandwidth Selenium-based Conductors
In order to address the bandwidth deficiency noted above we have sought to explore the effects on structure and transport properties of the replacement of sulfur by selenium, a design approach recognized
early on in the development of conductive charge transfer salts. Recently we have described the preparation and structural characterization of the first bis-1,2,3-thiaselenazolyls (bis-DTSAs). In the solid state these radicals can associate to produce sigma-bonded dimers (e.g. with R1 = Me, R2 = H), which behave as a small band gap semiconductors (Figure 3A). However, by judicious choice of R-groups, e.g., R1 = Me, R2 = Ph, dimerization can be prevented, and slipped radical pi-stacks can be generated (Figure 3B). From the analysis of the structural, electronic and transport properties of this latter material it is apparent that the replacement of sulfur by selenium,
has the desired effect of increasing bandwidth and hence reducing the thermal activation energy for electronic conduction.
Further improvement in performance can be achieved by the use
of external (physical) pressure. As shown in Figure 3C, the application
of 4 GPa pressure to both SSN and SSeN radicals increases in the room termparture conductivity by 3 orders
of magnitude, the selenium compound showing a larger response
at lower pressures. Variable temperature measurements (at 4 GPa for SSN) affords a thermal activation energy value of
0.25 eV, while that of SSeN radical (at 5 GPa) is reduced to 0.19 eV, i.e., about
one-half that observed at ambient pressure, and indicative of a
close approach to the metallic state. |
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Figure 3A - Cross braced slipped pi-stacks of bis(DTSA) dimers (R1 = Me, R2 = H). |
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Figure 3B - Slipped pi-stack arrays of bis(DTSA) radicals (R1 = Et, R2 = Ph). |
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Figure 3C - Pressure dependence of conductivity of a pair of isostructural bis (DTA)s and bis(DTSA). |
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References
(1) L. Beer, J. L. Brusso, R. C. Haddon, M. E. Itkis, R. T. Oakley, R. W. Reed, J. F. Richardson, R. A. Secco and X. Yu. Chem. Commun. 5745-5747 (2005).
(2) L. Beer, J. L. Brusso, R. C. Haddon, M. E. Itkis, H. Kleinke, A. A. Leitch, R. T. Oakley, R. W. Reed, J. F. Richardson, R. A. Secco and X. Yu. J. Am. Chem. Soc. 127, 18159-18170 (2005).
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| Magnetic Bistabilities |
Spin state crossovers in transition metal complexes are sometimes accompanied by magnetic hysteresis, and these so-called magnetically bistable materials have been actively pursued because of their potential applications in magneto-thermal and magneto-optical switching and information storage devices. However, while the magnetic signature of bistablity is well established, the underlying structural causes, i.e., the nature of the intermolecular interactions which generate the necessary cooperativity is not well understood. The potential applications of organic radicals in molecular spintronic devices has catalysed efforts to develop new stable and magnetically active molecules. Recently, magnetic hysteresis has been observed in a number of heterocyclic dithiazolyl radicals (see below). The bistability stems from the co-existence over a sometimes very wide temperature range of two solid state phases, one based on diamagnetic dimers (S = 0), the other on essentially paramagnetic radicals (S = ½).
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Figure 4 - EPR spectrum of PDTA (SW = 4.0 mT). |
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The key to the existence of bistability in thiazyl radicals is the network of strong intermolecular S---N interactions found in these systems; these S---N supramolecular synthons, acting cooperatively, are broken and then remade during the phase change between the paramagnetic and diamagnetic states. Two mechanisms for the structural inteconversion have been proposed. One involves a tectonic plate slippage of π-stacked layers, while the other requires a “domino cascade” of slipped radical π-stacks. An example of a radical (PDTA) which undergoes the "domino" phase interconversion is shown in Figures 5A,B,C.
Whether these dithiazolyl radical/dimer bistabilities will lead to useful device applications, e.g., in magneto-thermal or magneto-optical switching devices, remains to be seen. In the meantime, the understanding that current results provide of the molecular design features necessary to induce hysteresis augurs well for the development of new heterocyclic radicals displaying similar behaviour.
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Figure 5A - Packing of dimers in the low temperature
phase of PDTA.
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Figure 5B - Hysteretic behavior of
the magnetic susceptibility of PDTA. |
Figure 5C - Stacking of radicals in the
high temperature phase of PDTA. |
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References
(1) T. M. Barclay, A. W. Cordes, N. A. George, R. C. Haddon, M. E. Itkis, M. S. Mashuta, R. T. Oakley, G. W. Patenaude, R. W. Reed, J. F. Richardson and H. Zhang. J. Am. Chem. Soc. 120, 352-360 (1998). (2) W. Fujita and K. Awaga. Science 286, 261-262 (1999). (3) G. D. McManus, J. M. Rawson, N. Feeder, J. van Duijn, E. J. L. McInnes, J. J. Novoa, R. Burriel, F. Palacio, P. Oliete. J. Mater. Chem. 11, 1992-1998 (2001). (4) J. L. Brusso, O. P. Clements, R. C. Haddon, M. E. Itkis, A. A. Leitch, R. T. Oakley, R. W. Reed and J. F. Richardson. J. Am. Chem. Soc. 126, 8256-8265 (2004). 5) . J. L. Brusso, O. P. Clements, R. C. Haddon, M. E. Itkis, A. A. Leitch, R. T. Oakley, R. W. Reed and J. F. Richardson. J. Am. Chem. Soc. 126, 14692-14693 (2004).
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