The bonding in lanthanide coordination complexes is often approximated as involving only non-directional ionic bonds between the ligands and the metal. In the following report, we take a critical view of this LnIII point charge approximation. We develop the idea that dπ/pπ covalent interactions between the LnIII metals and the ligands are essential to understanding the geometry found in solution. Since the luminescence of EuIII is found to depend strongly on the ligand geometry, we propose that any model neglecting the treatment of ligand-metal covalent interaction is necessarily incomplete.
Chapter 1. First, an overview of lanthanide coordination complexes and their application in homogenous time-resolved fluorescence (HTRF) assays is presented. Several examples of commercially relevant luminescent EuIII complexes are compared to the bright 1-hydroxypyridin-2-one (1,2-HOPO) complexes described in this report.
Chapter 2. Our group has previously reported an automated shape analysis for eight-coordinate systems using the angles between the normals of adjacent polytopal faces (δ dihedral angles). Herein we extend that automated analysis to coordination numbers four through nine, and we demonstrate its functional equivalence to the parent Muetterties and Guggenberger analysis. The shape parameters defined here are linearly correlated along intramolecular rearrangement paths between related pairs of idealized shapes. Derivations of the ideal shapes, particularly for the troublesome eight-coordinate C2v geometry, are also discussed.
Chapter 3. We report the novel 5LImXy-1,2-HOPO ligand, which forms one of the brightest EuIII 1,2-HOPO complexes (aqueous quantum yield of 22%). The DFT minimized coordinates provide a better structural model than the XRD coordinates for fitting the 1H-NMR isotropic shifts, confirming that the dodecahedral (Dod) ground state geometry predominates in solution. All of the lanthanide 5LImXy-1,2-HOPO complexes are found to be fluxional within the NMR timescale and solvent temperature limits, varying widely as a function of lanthanide size. Slow exchange 1H-NMR spectra were collected for all but La and Ce, representing the first opportunity to study M(bidentate)4-type twist-inversion rearrangements with trivalent metal ions. The slowness of the rearrangement barrier for Eu confirms that the Dod structure may be important for efficient energy transfer from ligand to metal.
Chapter 4. The racemization process characterized in chapter 3 is found to occur by either an intramolecular low temperature process (LTP) or a high temperature process (HTP) involving ligand dissociation. Two new groups of ligands are characterized, toward finding a system where the HTP can be conveniently measured by 1H-NMR selective inversion recovery (SIR) experiments. PEG linked versions of 5LImXy-1,2-HOPO are found to rearrange by an intramolecular mechanism, despite expectations that the PEG linker would shut down the LTP for these systems. The 2LImTHF-1,2-HOPO ligand allowed measurement of the HTP for most of the lanthanide series, and the associated barriers are 2.5-3 kcal/mol larger than those found for the LTP in chapter 3. DFT calculated structures of the 2LImTHF-1,2-HOPO complexes are used to model the 1H-NMR isotropic shifts, confirming that the Dod ground state geometry predominates in solution. For both groups of ligands, differences in geometry are found to affect the sensitization efficiency rather than the metal centered efficiency of EuIII photoluminescence.
Chapter 5. We demonstrate measureable stereochemical differences for 5LImXy-1,2-HOPO complexes of d0 and d10 metals, both by XRD and by DFT. In general, the d10 complexes tend toward the square antiprism (SA), while the d0 complexes tend toward Dod, once differences in normalized bites have been considered. The measured and calculated barriers for intramolecular racemization (LTP) are smaller for the d10 metal complexes, compared to d0 complexes. All of these effects can be rationalized by DFT, suggesting that the Dod stereochemistry uniquely maximizes dπ/pπ bonding between the ligands and the metal. Preference for Dod coordination and the associated strengthening of dπ/pπ bonding is proposed to maximize efficiency of energy transfer from ligand to metal in the photoluminescence of Eu.
Chapter 6. We report time-resolved X-ray absorption near edge structure (TR-XANES) measurements at the Eu L3 edge upon photoexcitation of several EuIII-based luminescent lanthanide complexes. We find an unambiguous signature of the 4f intrashell excitation that occurs upon energy transfer from the photoactive organic antennas to the lanthanide species. Phenomenologically, this observation provides the basis for direct investigation of a crucial step in the energy transfer pathways that lead to sensitized luminescence in lanthanide-based dyes. Interestingly, the details of the TR-XANES feature suggest that the degree of 4f−5d hybridization may itself vary depending on the excited state of the EuIII ion.
Chapter 7. We report two novel 1,2-HOPO ligands for the sensitization of EuIII photoluminescence. We find that the [Eu(2LIS-1,2-HOPO)2]- complex exhibits a remarkably high quantum yield of 38% in aqueous solution, much larger than the previous record of 23% for these systems. The 1H-NMR contact shifts and DFT structural parameters are used to rationalize the large differences in quantum yields among these structurally similar systems. These complexes challenge the luminescence model developed in previous chapters, which states that structures closer to Dod should exhibit brighter luminescence. Very small differences in ligand architecture for these extremely rigid ligands can have dramatic effects on the efficiency of EuIII photosensitization.
