Colloquium: Smart Magnetic Materials: Bistability in Metallo-Supramolecular Assemblies and Transition Metal-Based Coordination Architectures

In recent times, molecule-based magnets illustrate a diverse and rapidly advancing category of magnetically bistable materials that have received considerable attention for their exciting physical properties and potential applications in information storage devices,¹ molecular qubits,² sensors,³ spintronics⁴etc. Molecular Bistability is defined as the ability of a molecular system, having two easily accessible electronic states, to undergo a conversion from one state to another state by the application of an external stimulus such as temperature, light, pressure, magnetic field, electric field, etc. Over the past several decades, there has been a notable rise in the significance of switchable magnetic materials derived from 3d transition metals, especially those that reveal magnetic, photomagnetic, electric bistabilities such as Spin Crossover (SCO), Metal-to-Metal electron transfer (MMET), Single-Molecule Magnets (SMMs) and Single Chain Magnets (SCMs) etc.^{5, 6, 7}

            My thesis is mainly focused on the design, synthesis and characterisation of novel Metallo-supramolecular architectures and transition metal-based coordination architectures which exhibit spin-state switching as well as single-molecule magnet behaviour. The SCO phenomenon is mostly observed for mononuclear iron(II) and iron(III) complexes using a suitable ligand field strength, but a very small number of studies have been reported for the SCO in metallosupramolecular cages. Of this relatively lesser-known field of SCO in metallosupramolecular cages, most of the reports are based on iron(II) metal centres and are scarcely known for cobalt(II) cages. Hence, efforts have been put to find out a suitable ligand field strength for exhibiting spin-state switching phenomenon for iron(II) and cobalt(II) metallosupramolecular tetrahedral cages in solid-state and/or solution-state.^{8, 9}Also, these architectures have been extended from discrete to three dimensions (3D) using suitable tripodal linkers and their distinct magnetic properties have been investigated.¹⁰Besides, I have rationally designed and synthesize discrete mononuclear cobalt(II) and manganese(III) complexes that exhibit SCO using different stimuli such as temperature, electrochemical potential as well as mechanical strain which is quite unique and very rare in the literature.^{11, 12}Apart from that, I have incorporated a luminescent-active moiety into the ligand system of SCO-active manganese(III) complexes to establish a synergistic corelation among SCO and luminescence in the complexes.¹³ Detailed understanding of the SCO and SMM behaviours of these complexes has been carried out using various physical techniques such as magnetic, optical, spectroscopic, electrochemical, single crystal X-ray diffraction structures, and also supported by theoretical studies. In conclusion, this work is presented with a thorough summary that provides valuable insights for future innovations in the field of multifunctional bistable magnetic materials, thereby contributing to the enhancement of this research area.