Electronic Structure and Properties of Lanthanide Nitrotrispyrazolylborates
Open Access DepositedSeveral generations of technologies incorporate the lanthanides (Ln), from the cathode-ray tube TVs of the not so distant past to the LEDs in the screens of modern smartphones and potential future utilizations, including the formation of molecular qubits in quantum devices. All these applications stem from the desirable optical and magnetic properties intrinsic to the lanthanides and their 4f valence electron configuration. However, despite all of the progress that has been achieved with lanthanide-based technologies, the current prevailing theories that govern our foundational understanding of their chemistry and electronic behavior are adapted from theories developed for transition metals. As such, the theoretical framework for the lanthanides is rife with oversimplifications, inaccurate approximations and have seen minimal development since their inceptions. In light of this, there is a need for systematic studies that rigorously characterize the electronic structure of Ln compounds and identify trends that link optical and magnetic properties to 4f electron count, to help develop predictive models for 4f-based electronic properties. My approach is to identify a well-studied family of Ln compounds, introduce a single functional group and quantify the impact of that substituent on the Ln-ligand interaction (or crystal field) and the resultant electronic (optical and magnetic) properties; thereby assessing how the change in the chemical environment (induced by the functional group change) affects 4f electronic behavior. The materials presented herein detail my contribution to this herculean effort of understanding f-electron behavior, by way of the synthesis and structural, photophysical and magnetic characterization of several related families of lanthanide nitrotrispyrazolylborates. The molecular and electronic structures (crystal field + electronic properties) of two families of lanthanide n-nitrotrispyrazolylborates (n = 3, 4) are studied by utilizing a wide range of spectroscopic, magnetic and computational tools. The molecular structures were determined using single crystal X-ray diffraction, while the photophysical properties were assessed using several optical spectroscopies (absorption/diffuse reflectance and steady-state and time resolved luminescence) with supplemental time-dependent density functional theory calculations. The magnetic properties were characterized via SQUID magnetometry and electron paramagnetic resonance; subsequent data modelling and fitting with CASSCF calculations were employed to study the crystal field interaction between the lanthanide ions and trispyrazolylborate ligands. Beginning with the structural and photophysical characterization of a family of structural diverse lanthanide 3 nitrotrispyrazolylborates (Chapter 3), followed up by the structural, photophysical and magnetic characterization of an isostructural series of lanthanide 4 nitrotrispyrazolylborates (Chapter 4). Lastly, the culmination of research efforts coalesces in Chapter 5 with a rigorous, multi-technique analysis of the crystal field of the two families of Ln trispyrazolylborates, to assess the impact of a single functional group on the electronic structure and properties of lanthanides as a function of f orbital occupancy.
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