Spectroscopic Results

Molecular Lanthanide Tetra-Carbonates 85 unknown parameters are the Raman coefficients CRaman, the attempt times 0 and the energy barriers E, which correspond to the energies of real intermediate crystal field states. Due to the strong curvature in the Arrhenius plot for the slow process in 1-Er, the magnetic data are not sufficient to unequivocally determine the energy of the crystal field state involved in the Orbach process. For 1-Dy, a preliminary linear fit yielded an effective energy barrier of Ueff = 30 cm^-1 but since a reasonable fit can also be obtained by not including the Orbach process at all, this value might be wrong. Spectroscopic measurements are thus mandatory for determining the crystal field states of 1-Er and 1-Dy. As will be shown below, the first crystal field excited doublets are located at 52 cm^-1 (1-Er) and 29 cm^-1 (1-Dy). Using them as fixed values for E, the best-fit parameter values given in Table 4 were obtained. The corresponding simulations are shown in Figure 44.

Altogether, the derived parameter values indicate the dominance of the Raman mechanism and the direct relaxation for the slow process in 1-Er in the studied temperature range. In contrast, the contribution of the Orbach mechanism for 1-Dy is much more pronounced although the energy barrier, i.e. the energy of the first excited Kramers doublet, seems to be lower. These results clearly demonstrate that SIM behavior cannot be solely explained by large crystal field splittings.

Table 4: Best-fit parameters describing the thermally assisted magnetic relaxation in 1-Er and 1-Dy.

1-Er 1-Dy

fast process slow process

E / cm^-1 - 52 29

0 / s^-1 - 1.2 ∙ 10^-12 1.8 ∙ 10^-7

CRaman / K^-9 s^-1 0.57 0.02 0.001

between 0 T and 6 T were recorded. Figure 46 shows the obtained transmission spectra as well as the normalized spectra obtained by dividing by the spectra at 6 T. For 1-Er, three crystal field excitations were observed, namely at 52, 84 and 105 cm^-1. The splitting of the middle feature is attributed to the coupling of crystal field and vibrational transitions. Similar splittings were observed in the far-infrared spectra of the four-coordinate Co(II) complexes studied in the further course of this work (section 4.3.3), for which theoretical calculations confirmed the presence of spin-phonon couplings.¹⁶⁷ Crystal field analysis for 1-Er (see below) confirmed that the level at 52 cm^-1 corresponds to the first excited Kramers doublet and this value was therefore used as E for the simulation of the Arrhenius plot (Figure 44).

The normalized FIR-spectrum of 1-Dy shows an intense feature at around 100 cm^-1 that can be attributed to a crystal field excitation. Interestingly, there is no clear signal close to 30 cm^-1 as expected from the magnetic data. Further spectroscopic data are required to find out the reason for this discrepancy as well as the correct energetic position of the first excited crystal field state.

Figure 46: Far-infrared spectra of 1-Er (left) and 1-Dy (right) recorded at 9 K and 10 K, respectively. Asterisks indicate signals that arise from crystal field excitations. The spectra were recorded with the help of Raphael Marx and Dr. María Dörfel.

Molecular Lanthanide Tetra-Carbonates 87 Another useful method for gaining information about the Kramers doublets within the electronic ground term is luminescence spectroscopy. Thus, solid state luminescence spectra of 1-Er and 1-Dy at low temperatures were recorded at the University of Copenhagen with the help of Maren Gysler (Institute of Physical Chemistry, University of Stuttgart), Dr. Stergios Piligkos and Theis Brock-Nannestad (both Department of Chemistry, University of Copenhagen).

Er(III) is mainly known for its NIR emission^102,168 but some Er(III) compounds display luminescence in the visible range as well.^169-171 Especially the transition from the excited ⁴S3/2

multiplet to the ⁴I15/2 ground state has been shown to be very useful for the determination of the crystal field level structure of the electronic ground term.¹⁷⁰ However, no Er(III) emission was observed in the luminescence spectra of 1-Er, neither in the visible nor in the NIR range.

Instead of the expected Er(III)-based sharp luminescence signals a very broad feature was observed, exhibiting negative dips located at 355, 364, 379, 403, 442, 449, 485, 520 and 541 nm. An example of a spectrum recorded at 20 K using an excitation wavelength of 290 nm is shown in the appendix, section 8.3.6. The energies of the negative dips match the optical absorption bands (see below) and therefore might be attributed to resonant reabsorption of the ligand emission by the Er(III) center. Similar reabsorption phenomena have been already observed by others.^172-174 According to the Dieke diagram⁶², the observed dips can be assigned to the following f-f-transitions of the Er(III) ion: ⁴I15/2  ²G7/2, ²K15/2,

4G9/2 (335 and 364 nm), ⁴I15/2  ⁴G11/2 (379 nm), ⁴I15/2  ²H9/2 (403 nm), ⁴I15/2  ⁴F3/2

(442 nm), ⁴I15/2  ⁴F5/2 (449 nm), ⁴I15/2  ⁴F7/2 (485 nm), ⁴I15/2  ²H11/2 (520 nm) and

4I15/2  ⁴S3/2 (541 nm). Increasing the excitation wavelengths in order to avoid ligand excitation led to a weakening of the negative dips, but still no Er(III) luminescence was observed, indicating efficient quenching mechanisms, e.g. due to the surrounding water molecules. Luminescence spectroscopy thus turned out to be unsuitable for determining the ground state crystal field splittings of 1-Er. Instead, excited state splittings probed by electronic absorption and MCD-spectroscopy had to be used for indirectly obtaining more information about the ground state levels.

In contrast, usable luminescence data were obtained for 1-Dy and signals arising from the transitions ⁴F9/2  ⁶H15/2 and ⁴F9/2  ⁶H13/2 were observed in the recorded low temperature luminescence spectra. As shown in Figure 47, the emission bands show splitting patterns due to the crystal field splitting of the respective final states and the ⁴F9/2  ⁶H15/2

emission thus yields information about the ground state level structure while the ⁴F9/2  ⁶H13/2

transition complements the absorption and MCD data. However, Figure 47 clearly shows that

the resolution is not sufficient to unequivocally determine the energies of all the crystal field levels involved and the observed patterns thus only allow their rough estimation. The low resolution might be due to overlapping vibronic transitions, which is a common problem in optical lanthanide spectra, or due to distributions in the crystal field parameters.⁶⁰ Only the better resolved emission lines were thus initially included in the crystal field analysis for 1-Dy (section 4.2.4). Interestingly, the high-energy peak in the ⁴F9/2  ⁶H15/2 emission spectrum shows a shoulder, for which Gaussian deconvolution yielded an energy separation of 29 cm^-1. It is not fully clear at this stage if this energy separation corresponds to the energy of an excited Kramers doublet or if it is due to a vibronic transition. Strikingly, the value of 29 cm^-1 coincides well with the effective energy barrier derived from the ac susceptibility measurements and should thus be considered at least as an option for the energy of the first excited doublet in the crystal field analysis. If so, the luminescence spectrum hints at the second excited doublet lying at 94 cm^-1, in reasonably good agreement with the observed signal in the FIR-spectrum.

All in all, FIR and luminescence spectroscopy provided information about the energies of some single Kramers doublets but they did not allow the full determination of the ground state level structures, neither for 1-Er nor for 1-Dy. Even the observation of all eight expected transitions would not be sufficient for the unambiguous determination of the nine crystal field parameters required in C2v symmetry.

Figure 47: Low temperature luminescence spectra of 1-Dy for the transitions from ⁴F9/2 to the ground multiplet

6H15/2 (left) and to the first excited multiplet ⁶H13/2 (right). Blue solid lines show the experimentally obtained spectra while green lines show the deconvolution into individual Gaussian bands (dashed lines) and their sums (solid lines). Black bars show the calculated transition energies based on the crystal field analysis. The spectra were recorded with the help of Maren Gysler, Dr. Stergios Piligkos and Theis Brock-Nannestad.

Molecular Lanthanide Tetra-Carbonates 89 That means that spectroscopic methods which solely probe the ground multiplet of low-symmetry compounds at best allow the determination of energies but not the determination of crystal field parameters that correctly describe the nature of the states. However, it is mainly the nature of the states that determines the dynamic magnetic properties.

Electronic absorption and MCD-spectroscopy at low temperatures were thus used for determining as many energy levels as possible, including those that are most sensitive to the variation of certain crystal field parameters Bkq. For Er(III), these are the levels arising from the free ion terms ⁴S3/2, ⁴F3/2 (k = 2), ⁴F5/2, (k = 4) and ⁴I9/2 (k = 6) while for Dy(III) the levels arising from ⁶F3/2 (k = 2), ⁶F5/2 (k = 4) and ⁶F7/2, ⁴F9/2 (k = 6) are most sensitive to changes of the parameters with the k-values given in brackets.⁶⁰ A wealth of high resolution UV/Vis/NIR-absorption and MCD-spectra of 1-Er and 1-Dy dispersed in transparent silicone grease were recorded and the observed signals were assigned to the corresponding free ion terms according to the Dieke diagram⁶². Some selected examples are shown in Figure 48 and Figure 49 while further spectra are shown in the appendix, section 8.3.7.

Figure 48: Selected examples of electronic absorption and MCD-spectra of 1-Er recorded at 2 K and 3 T.

Experimentally observed spectra are shown in blue while red lines show the deconvolution into individual Gaussian lines (dotted) and their sums (solid). Black bars depict the calculated transition energies based on the parameters obtained from crystal field analysis.

Figure 49: Selected examples of electronic absorption and MCD-spectra of 1-Dy recorded at 2 K and 3 T.

Experimentally observed spectra are shown in blue while green lines show the deconvolution into individual Gaussian lines (dotted) and their sums (solid). Black bars depict the calculated transition energies based on the parameters obtained from crystal field analysis.

The signals are clearly split due to the excited state crystal field splittings and the individual energetic positions were determined by careful deconvolution into sums of Gaussian lines. In some cases, the signal shapes were better reproduced by adding more Gaussian lines than expected according to the multiplicity of the final states. Similarly to the luminescence spectra, the additional peaks can be attributed to vibronic excitations since most f-f transitions are induced electric dipole transitions and might gain intensity by vibronic coupling to ungerade vibrational modes.⁶⁰ The FIR-spectra already confirmed the existence of vibrational transitions in the same energy range as the crystal field splittings. This aspect was kept in mind during the subsequent crystal field analysis and in most cases the lower-energy component was used. However, structural imperfections, e.g. caused by the lattice water molecules in 1-Er and 1-Dy, might also have led to the observed satellite bands. From the FIR and optical spectra together, the energetic positions of no fewer than 48 crystal field levels of 1-Er and 55 levels of 1-Dy were determined, with the main contribution provided by

Molecular Lanthanide Tetra-Carbonates 91 electronic absorption and MCD-spectroscopy. These energy levels provided the foundation for the subsequent crystal field analysis.

The above-mentioned spectroscopic measurements were complemented by EPR-spectroscopy since this method is exquisitely sensitive to the composition of the lowest Kramers doublet and can therefore be applied as a tool for verifying the correct description of the ground state by a set of empirically determined crystal field parameters. Low-temperature EPR-spectra of mulls of 1-Er and 1-Dy in fluorolube® were recorded at conventional X-band frequency (9.5 GHz) and at higher frequencies (90 – 400 GHz). The high-frequency EPR (HFEPR) spectra were recorded with the help of Raphael Marx and Dr.-Ing. Petr Neugebauer (both Institute of Physical Chemistry, University of Stuttgart). As shown in Figure 50, two clear and one weaker resonance line with effective g-values of g1 = 7.64, g2 = 4.85 and g3 = 1.94 are observed in the HFEPR-spectra of 1-Er.

Figure 50: Low temperature multi-frequency EPR-spectra of 1-Er (left) and 1-Dy (right). Blue lines correspond to experimental spectra while dashed red (1-Er) or green (1-Dy) lines show the simulations based on the effective g-tensors obtained from the crystal field analysis. The HFEPR-spectra were recorded with the help of Raphael Marx and Dr.-Ing. Petr Neugebauer.

For 1-Dy, the situation looks more complicated: The EPR signals show complex structures and the determination of the principal g-values from the experimental spectra is not straightforward. Within the Seff = ½ model and neglecting the structure observed in the spectra, the best simulation is obtained using g1 = 12.5, g2 = 6.0 and g3 = 2.5 but the uncertainties are rather high. The reason for the observed structure is not fully clear. One possible explanation is the presence of very low-lying excited Kramers doublets that are populated at low temperatures and contribute to the observed signals. However, according to the results of the crystal field analysis (see below), this explanation can almost certainly be ruled out. Another, more probable explanation is the effect of structural distortions like impurities or disordered water molecules that lead to slight variations of the environment of the individual Dy(III) centers.³⁸ Indeed, as evidenced by the crystallographic R-indices of 4.73 % for 1-Er and 7.42 % for 1-Dy, the crystal quality of 1-Dy was worse compared to that of 1-Er, which further supports this explanation.