Oral Presentation The 35th Biennial Conference of the Society of Crystallographers in Australia and New Zealand 2024 (Crystal 35)

High-pressure crystal structures of 6-oxo-verdazyl radicals for applications in molecular electronics (109035)

James R Brookes 1 , Varshini J Kumar 1 , Isabelle M Jones 1 , Aston M Summers 1 , Stephanie A Boer 2 , Paul J Low 1 , Stephen A Moggach 1 , Dino Spagnoli 1
  1. School of Molecular Sciences, The University of Western Australia, Crawley, Western Australia, Australia
  2. Macromolecular Crystallography Beamline, Australian Synchrotron, Clayton, Victoria, Australia

The emergence of organic radicals in molecular electronics and materials science draws attention to their properties in the bulk crystalline phase and the potential to manipulate these in response to external influences such as pressure or temperature.[1] Pressure-induced modification of intermolecular contacts in the organic solid state can lead to extensive phase diversity through interactions such as hydrogen bonds and van der Waals contacts. Several interesting physical phenomena have been observed in small radical molecules, including semiconductor–insulator phase transitions and magnetic bistability,[2] with applications spanning modern microelectronics. The synthetic pathways and single molecule properties of 6-oxo-verdazyl analogue compounds have been previously studied,[3,4] yet a structure-property relationship with respect to pressure has not been observed for this class of organic radical. Making this area of materials chemistry discovery ripe for exploration.


In this work, high-pressure crystal structures of the 1,5-tolyl-3-phenyl-6-oxo-verdazyl radical have been obtained at the MX1 beamline of the Australian Synchrotron at pressures from ambient to 3.3 GPa. Upon compression to 2.73 GPa, the radical crystal undergoes a relatively uncommon P21/n to C2/c phase transition, driven by a ~90Ëš rotation of the tolyl moiety and a pseudo-enantiomeric change in the asymmetric unit. This rotation causes an increase in the planar intermolecular separation of verdazyl radical centres. Gas-phase single molecule calculations revealed a barrier to tolyl group rotation of approximately 30 kJ/mol. Such changes in radical interactions are desirable for electronic and magnetic bistability. We discuss a comprehensive analysis on the resulting effect of this uncommon subgroup-to-supergroup transition on the crystal packing and structure. This was complemented with an investigation into changes in electronic band structure and density of states through density functional theory calculations. Post phase transition, there was a change in band gap from direct to indirect. Analysis of electrical and magnetic properties provide an insight into the viability of these organic radical crystals as components in future electronics.

  1. Richardson, J. G.; Mizuno, A.; Shuku, Y.; Awaga, K.; Robertson, N.; Morrison, C. A.; Warren, M. R.; Allan, D. R.; Moggach, S. A. CrystEngComm 2021, 23 (25).
  2. Itkis, M. E.; Chi, X.; Cordes, A. W.; Haddon, R. C. Science. 2002, 296 (5572)
  3. Kumar, V. J.; Wu, J. Z.; Judd, M.; Rousset, E.; Korb, M.; Moggach, S. A.; Cox, N.; Low, P. J. J. Mater. Chem. C. 2022, 10 (5).
  4. Naghibi, S.; Sangtarash, S.; Kumar, V. J.; Wu, J. Z.; Judd, M. M.; Qiao, X.; Gorenskaia, E.; Higgins, S. J.; Cox, N.; Nichols, R. J.; Sadeghi, H.; Low, P. J.; Vezzoli, A. Angew. Chem. Int. Ed. 2022, 61 (23).