One of our studies in this field is to synthesize and study the characteristics of single-molecule magnets (SMMs). These molecules have a non-zero spin and magnetic anisotropy, preferably along the z-axis. At low temperatures, SMMs behave similarly to traditional bulk magnets, exhibiting magnetic bi-stability and hysteresis; in other words, they preserve magnetization in the same way as a 3-D ordered magnet does, but for fundamentally different reasons. The origin of SMM behavior is a heat barrier (Ueff) that prevents magnetization reversal. Such magnets have the potential to revolutionize the world of electronics, with applications ranging from data storage to quantum computing and spintronics. However, the best SMMs only show these magnetic properties at temperatures below 80 K (-193° C). The popularity of alternate relaxation paths, such as quantum tunneling of magnetization (QTM), which tunnels through the barrier rather than over it, bypasses the thermal relaxation barrier, limiting the viability of using these molecules in advanced technology. To make SMMs functional at higher temperatures, we are developing techniques to increase the thermal barrier and minimize QTM by synthesizing new mononuclear SMMs also known as Single-Ion Magnets, Lanthanide-based SMMs, mixed 3d-4f-based SMMs, and radical-bridged SMMs.

1) Single-Ion Magnets (SIMs): Mononuclear SMM research has become more and more prominent in the field of molecular magnetism in recent years. Our goal is to create transition metal and lanthanide molecules with highly symmetric discrete geometries, which should lead to SMMs with significant spin reversal barriers because of their inherent electrical properties. Unquenched spin-orbit coupling and orbitally degenerate ground states are anticipated to induce significant negative axial zero-field splitting in some scenarios, making these geometries unique. They also result in strong axial symmetry, reducing the quantum tunneling probability. To address these compounds, we use a variety of large ligands to enforce the desired geometry.

References

1.  K. Schulte, K. R. Vignesh, K. R. Dunbar, Chem. Sci., 2018, 9, 9018–9026 

2. D. I. Alexandropoulos, K. Schulte, K. R. Vignesh, K. R. Dunbar, Chem. Commun., 2018, 54, 10136–10139.

3. K. R. Vignesh, D. I. Alexandropoulos, H. Xie, K. R. Dunbar, Dalton Trans., 2020, 49, 4694–4698. 

2) Di- and Polynuclear Lanthanide-based SMMs: The inherent magnetic anisotropy and the number of unpaired f-electrons are responsible for the high energy barrier for magnetization reversal in Ln-SMMs. Importantly, Dy(III) is the most promising candidate, with Dy(III) organometallic molecules exhibiting blocking temperatures of 60 K and 80 K having been reported. Although there have been considerable efforts to elucidate the mechanisms operative for mononuclear 4f SMMs, much less attention has been devoted to understanding the relaxation mechanisms in polynuclear 4f SMMs. The primary hurdle is to assess the impact of the Ln⋯Ln exchange interaction on the SMM property where the individual single-ion anisotropies are assumed to be the dominant phenomenon. We address the SMM property of Ln complexes by carefully choosing the bridging group,  the interplay between the ligand field effect, the geometry, and the strength of the magnetic exchange interaction between the lanthanide ions.

References

1.  K. R. Vignesh, D. I. Alexandropoulos, B. S. Dolinar, K. R. Dunbar, Dalton Trans., 2019, 48, 2872–2876 

2. C. P. Burns, B. Wilkins, C. M. Dickie, T. P. Latendresse, L. Vernier, K. R. Vignesh, N. S. Bhuvanesh, M. Nippe, Chem. Commun., 2017, 53, 8419–8422. 

3) Mixed 3d/4d/5d-4f SMMs: Besides lanthanide SMMs, heterometallic 3d–4f and 4d/5d-4f complexes are crucial in improving SMM behavior due to their combination of a large spin of the 3d/4d/5d ions with the spin/anisotropy of lanthanide ions. Understanding these interactions is highly desirable if such interactions are ferromagnetic exchanges which can also reduce QTM in lanthanide-based SMMs. We are developing this topic with hybrid transition metals such as Cr(III), Mn(III), Fe(II), Co(II)/(III), Ni(II), Ru(III), Re(IV), and lanthanide ions. We determine these interactions using experimental instruments; moreover, DFT and ab initio calculations aid in understanding the electronic structure and nature of magnetic interactions in the systems. 

References

1. A. S. Armenis, V. Vipanchi, K.N. Pantelis, L. Cunha-Silva, K. R. Vignesh, D. I. Alexandropoulos, T. C. Stamatatos, Chem. Eur. J., 2023, 29, e202302337.  

2. K. R. Vignesh, S. K. Langley, K.S. Murray, G. Rajaraman, Inorg. Chem., 2017, 56, 2518−2532. 

3. K. R. Vignesh, S. K. Langley, K.S. Murray, G. Rajaraman, Chem. Eur. J, 2017, 21, 1654−1666. 

4. K. R. Vignesh, S. K. Langley, B. Moubaraki, K.S. Murray, G. Rajaraman, Chem. Eur. J, 2015, 21, 16364–16369. 

5. A. Swain, R. Martin, K. R. Vignesh, G. Rajaraman, K. S. Murray, S. K. Langley, Dalton Trans., 2021, 50, 12265–12274.

4) Radical-bridged SMMs: Recently, there has been interest in using radical-bridged single-molecule magnets to enhance the magnetic characteristics of polynuclear SMMs. A high degree of magnetic coupling can result from direct exchange interactions between the unpaired electron on the radical organic bridging ligand and the metal center. SMM behavior is usually enhanced by strong coupling interactions, which help to separate the excited states from the magnetic ground state. We investigate this concept in our laboratory by synthesizing and characterizing radical- and neutral-bridged SMMs. 

References

1. D. I. Alexandropoulos, B.S. Dolinar, K. R. Vignesh and K.R. Dunbar, J. Am. Chem. Soc., 2017, 139, 11040-11043.

2. T. J. Woods, H. D. Stout, B. S. Dolinar, K. R. Vignesh, M. F. Ballesteros-Rivas, C. Achim, and K. R. Dunbar, Inorg. Chem., 2017, 20, 12094–12097.

3. B. S. Dolinar, D. I. Alexandropoulos, K. R. Vignesh, T. James, K. R. Dunbar, J. Am. Chem. Soc., 2018, 140, 908–911.

Single-molecule Toroics (SMTs) are bi-stable molecules similar to Single-Molecule Magnets (SMMs) but show toroidal moments and a non-magnetic ground state, and they are insensitive to a homogeneous magnetic field. Toroidal moments can be effectively manipulated by optical signals and hence have an edge over conventional SMMs. The toroidal moment is the third kind of electromagnetic moment, apart from conventional polarization and magnetization. It consists of a multi-spin object, which breaks both space-inversion and time-reversal symmetry and can be generated by persistent orbital current or a certain spin ordering. The non-collinear arrangement of the magnetic moments of each metal center, especially with Ln(III) ions in SMTs can lead to interesting magnetic behavior. These SMTs have a variety of possible applications in quantum computing, memory storage devices, and the development of magnetoelectric coupling for multiferroic materials. Notably, the magnetic field produced by toroidal moments decays more rapidly than the field produced by a magnetic dipole, thus the qubits designed utilizing toroidal moments will be more densely packed than SMM or spin qubits, respectively. Moreover, these SMTs are very stable at room temperature and can be subjected to magnetic/electrical manipulation. We study the toroidal magnetism in planar rings such as {Dy3} triangles, {Dy4} squares, {Dy6} hexagons, and ‘double triangular’ {Dy3MDy3} clusters that show the rare phenomenon of ferrotoroidal behavior. 

References

1. K. R. Vignesh, A. Soncini, S. K. Langley, K. S. Murray, W. Wernsdorfer, G. Rajaraman, Nat. Comm., 2017, 8, 1023.

2. K. R. Vignesh, S. K. Langley, A. Swain, B. Moubaraki, M. Damjanovic, W. Wernsdorfer, G. Rajaraman, K. S. Murray, Angew. Chem. Int. Ed., 2018, 57, 779–784.

3. K. R. Vignesh, G. Rajaraman, ACS Omega, 2021, 6, 32349–32364.  

4. K. R. Vignesh, S. K. Langley, B. Moubaraki, G. Rajaraman, K.S. Murray, Chem. Eur. J., 2019, 25, 4156-4165.

The change in spin state exhibited by certain metal complexes under the application of external factors like temperature, pressure, irradiation of light, and the magnetic field is referred to as "Spin crossover (SCO)" or "Spin transition" or "Spin equilibrium". This phenomenon is commonly observed with first-row transition metal complexes having a d4 – d7 electron configuration in an octahedral ligand field and can be observed in both the solid state and solution. SCO leads to both an electronic (change in the d-electron orbital configuration) and structural change and is often accompanied by a color and/or a magnetic moment change.

         The most significant consequences of SCO are the changes in metal-to-ligand bond distances due to the population or depopulation of the eg orbitals which have a slight antibonding character and the changes in magnetic properties of the complex. The occurrence of thermal hysteresis in the HS→LS→HS cycle, observed for some remarkable SCO compounds, is a requirement for molecular bistability. Such bistability is a key property for some technical applications. The SCO phenomenon has been tested in many applications such as switches, data storage devices, and optical displays due to the basic bistability (HS and LS) which leads to changes in the color of the material and major magnetic changes. However, molecular switches, like electrical switches, have the requirement for a mechanism that will turn the switch ON, and OFF and this is not easy to achieve at an applied level.

We investigate the SCO behavior of 3d4 – 3d7 ions such Fe(II), Co(II), and Mn(II) in an octahedral ligand geometry with N4O2, N3O3 type of moderate ligands.

References

1. S. Sundaresan, J. Eppelsheimer, E. Gera, L. Wiener, L. M. Carrella, K. R. Vignesh, Eva Rentschler, Dalton Trans., 24, 2024, 3786–3797.  

2. W. Phonsri, D. S. Macedo, K. R. Vignesh, G. Rajaraman, C. G. Davies, G. N. L. Jameson, B. Moubaraki, J. S. Ward, P. E. Kruger, G. Chastanet, K. S. Murray, Chem. Eur. J, 2017, 23, 7052–7065. 

The main areas of catalysis are focusing on minimizing the environmental impacts while combusting fuels from automobile engines. Our group is searching for the three-way catalyst (TWC) of a car that exploits three reactions which include NO reduction, CO oxidation, and oxidation of unburnt hydrocarbons. We are adopting the computational approach to understand CO and NO adsorption and to intensively seek a suitable metal oxide surface for the investigation of the NO-CO reaction mechanism. We perform slab model calculations by spin-polarized DFT methods using the VASP program.

References

1. Roy, et al., Chem. Rev. 2009, 109, 4054.  

2. Tan, et al., RSC Adv. 2018, 8, 26448.