Molecular Magnetic Architectures: From Mononuclear to Polynuclear complexes

Abstract

The broad field of molecular magnetic materials has potential applications in the storage and processing of the information. Despite of the enormous applications of these materials, till now, there are no suitable candidates ready for the real application. So, over the decades, plenty of research has been progressing to develop suitable candidates in the field of Spin-crossover (SCO) and single molecule magnetism (SMM). In SCO, there are several reports with large hysteresis but found to be not suitable for applications since they do not satisfy many other criteria, which include the stability of the hysteresis that should be around room temperature. At the same time, for the development of the molecular devices, the bottom-up fabrication of these functional molecules is needed. Even though most of the SCO complexes are mononuclear, significant progress has been seen towards polynuclear complexes due to their tunable properties. Molecular chirality is an additional concept that plays a key role in magnetism, particularly in spintronic applications. So introducing the concept of chirality into these polynuclear SCO complexes may result in novel magneto-optical hybrid complexes. On the other hand, in the field of SMMs, lanthanide complexes have risen as attractive materials due to extremely large anisotropy in lanthanide ions. However, most of these complexes show characteristic SMM behaviour at liquid helium temperatures; there is a need for designing newer SMMs with higher blocking temperatures. \nIn the present thesis, in the first chapter, a brief introduction to fields - SCO and SMM, and their importance to the present technological world is provided. Moreover, the methods for characterization used for these complexes are presented. \nThe second chapter deals with the studies of bis(pyrazolyl)pyridine (bpp) derivatives in bulk and surface. The ligand (bpp-COOH) was deposited on Ag(111) surface and was found to form a Kagome lattice structure on annealing, and the ligand showing two types of coordination modes with Fe on Ag(111). STM and XPS were used to study the self-assembled structures formed on the surface. On the other hand, the bulk SCO Fe(II) complexes were prepared using a derivative of bpp ligand by varying the counter anions. The structural and SCO properties of the complexes were investigated by various techniques like X-ray diffraction (XRD), Superconducting Quantum Interference Device (SQUID) magnetometry, and differential scanning calorimetry (DSC). Interestingly, our magnetic studies on the complex synthesized with perchlorate anion showed a stable hysteresis of 60 K around room temperature. \nThe third chapter deals with the chiral resolution of tetra-nuclear Fe(II) SCO grid complexes. For this deconvolution of grid complexes, we designed and synthesized a novel chiral ligand. The complexation of these ligands with Fe(II) resulted in enantiomerically pure grid complexes which were elucidated by XRD and Circular Dichroism (CD) studies. These enantiomeric complexes showed gradual SCO and photo-induced SCO properties. The CD spectra calculated using TDDFT showed good agreement with experimental results obtained from Mossbauer spectroscopy and SQUID magnetometry. \nThe fourth chapter deals with mononuclear, binuclear and tri-nuclear Tb-sandwich SMM complexes. We explored and characterized the series of complexes of mixed porphyrin and phthalocyanine mono Tb SMM sandwich complexes by tuning the periphery of porphyrin ligand and redox properties. Moreover, a series of binuclear complexes were synthesized by varying the length of the linker, and their magnetic properties were characterized. Besides, preliminary electron paramagnetic resonance (EPR) studies were performed in the neutral complexes to study the radical. \nIn summary, various kinds of molecular magnetic architectures were synthesized and characterized in this thesis as below - (i) surface and bulk studies of bpp based Fe(II) complexes, (ii) tetranuclear enantiomeric Fe(II) SCO grid complexes, and (iii) mononuclear, binuclear and trinuclear Tb- SMM complexes based on porphyrins and phthalocyanine. Such design and studies of these magnetic materials are key for future devices based on a bottom-up approach.