Synthetic and Spectroscopic Investigations of Ligand Field Effects in Molecular Lanthanide Ion Complexes

Abstract

This thesis focuses on defining the electronic structure of molecular lanthanide complexes. Dy(III)/Ga(III) metallacrowns in three general coordination geometries (C4v, D4d, and C1) were isolated to examine the effects on Dy(III) emission. The ligand energy levels were tuned by making chemical substitutions onto the salicylhydroxamate or isophthalate backbone ligands. The spectral profile corresponded with the geometry in each case, despite ligand singlet and triplet energy levels ranging in the series from 25870-28860 cm-1 and 21550-22680 cm-1, respectively. Blue ligand fluorescence was also found to modify the compounds’ emission profile because variation in ligand energy levels led to shifts in the peak of blue ligand emissions (ranging 360-403 nm), as well as different ligand-Dy(III) interactions leading to a variety of ligand/Dy(III) quantum emission ratios (ranging 0.0035-0.455). Incorporation of ligand emissions leads to a wide tunability of Commission Internationale de l'éclairage coordinates ranging from x=0.29-0.40 and y=0.28-0.43, with coordination geometry and ligand emission contribution being the most important effects. The effect of shifting ligand energy levels on the thermal dependence of lanthanide emissions was examined to develop new molecular nanothermometers. Working with Ga8Ln2L8L’4 compounds (where Ln=Sm(III) or Tb(III), L’=isophthalate, L=salicylhydroxamate, 5-methylsalicylhydroxamate, 5-methoxysalicylhydroxamate, or 3-hydroxy-2-naphthohydroxamate), ligand-centric singlet energy levels ranged from 23300-27800, while triplet levels ranged from 18150-21980 cm-1. Comparison with relevant excited Sm(III)* and Tb(III)* energy levels (17800 and 20400 cm-1, respectively), showed that the excited Ln(III)*-ligand triplet gap was most important in dictating thermal dependence of emission intensity via back energy transfer, however, when the singlet-triplet ligand energy gap was especially small (3760 cm-1), energy transfer across this gap is also important. This also applies to designing imaging probes, with room-temperature, visible emission quantum yields ranging 2.07(6)-31.2(2)% for Tb(III) and 0.0267(7)-2.27(5)% for Sm(III). Maximal thermal dependence occurred over a wide thermal range (ca. 150-350 K), based on the ligand-lanthanide energy gaps. By mixing two of the Sm(III) and Tb(III) compounds, an optical thermometer was created based on the emission ratio of these two ions. For (1:1 Sm2L8:Tb2L8, [L=5-methoxysalicylhydroxamate]) in the solid state, peak temperature sensitivity greater than 3 %/K at 225 K was found. When (1:1 Sm2L8:Tb2L8, [L=salicylhydroxamate acid]) was placed in polystyrene nanobeads and examined as an aqueous suspension, maximum sensitivity of 1.9 %/K was found at 328 K, thus these materials may be useful in biological applications such as cellular thermal imaging. Finally, magnetic field-dependent luminescence was used to extract g-factors from Zeeman splitting patterns for ground and excited states of Yb(III). Extraction of the relevant parameters was performed by fitting the Zeeman response of resonant energies via luminescence. For the four states of the ground manifold for the crystal in the B/parallel/C3 orientation it was found g/par/1=4.47(0.27), g/par/2=3.69(0.39), g/par/3=2.97(0.43), and g/par/4 = 0.98(0.28). In the B/perpendicular/C3 orientation it was found g/perp/1=2.50(0.19) and g/perp/2=1.33(0.23). Meanwhile the emitting state’s g-factor could also be extracted, with g/par/E=2.70(0.27) and g/perp/E=0.52(0.19). These properties are difficult to measure by other experimental techniques and provide direct experimental insight into wavefunction mixing in the complex. In summary, this thesis used synthesis and spectroscopy to evaluate the impact of the ligand field on lanthanide physical properties while also developing technologies to probe the electronic structure more fully. These studies should assist in the design of future lanthanide based materials that exploit ligand field effects.