Electronic Energy Levels of Dysprosium(III) Ions in Solution

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Electronic Energy Levels of Dysprosium(III) Ions in Solution – Assigning the Emitting State, the Intraconfigurational 4f-4f Transitions in the vis-NIR, and Photophysical Characterization of Dy(III) in Water, Methanol and Dimethylsulfoxide Nicolaj Kofod, Riikka Arppe-Tabbara, and Thomas Just Sørensen J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b12034 • Publication Date (Web): 05 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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The Journal of Physical Chemistry

Electronic Energy Levels of Dysprosium(III) ions in Solution – Assigning the Emitting State, the Intraconfigurational 4f-4f Transitions in the vis-NIR, and Photophysical Characterization of Dy(III) in Water, Methanol and Dimethylsulfoxide Nicolaj Kofod, Riikka Arppe-Tabbara & Thomas Just Sørensen* Department of Chemistry & Nano-Science Center, University of Copenhagen, Universitetsparken 5, 2100 København Ø, Denmark ABSTRACT: Dysprosium(III) ions are the third most luminescent lanthanide(III) ions. Dy(III) is used as dopant in optical fibers and as shift reagent in NMR imaging, and is the element at the forefront of research in single molecule magnets. Nonetheless, the excited state manifold of the dysprosium(III) ion is not fully mapped and the nature of the emitting state has not been unequivocally assigned. In the work reported here, the photophysical properties of dysprosium triflate dissolved in H2O, MeOH, and DMSO have been studied in great detail. The solvates are symmetric, all oxygen donor atom complexes where the coordination number is 8 or 9. By comparing protonated and deuterated solvents, performing variable temperature spectroscopy, and by determining the excited state lifetimes and luminescence quantum yields, the solution structure can be inferred. For the three complexes, the observed electronic energy levels were determined using absorption and emission spectroscopy. The Dy(III) excited state manifold of the three solvates differ from that reported by Carnall, in particular for the low lying 6F-states. It is shown that dysprosium(III) complexes primarily luminesce from the 4F9/2 state, although thermal population of, and subsequent luminescence from the 4I15/2 state is observed. The intrinsic luminescence quantum yield is moderate (~10 %) in DMSO-d6, and is significantly reduced in protonated solvent as both C-H and O-H oscillators act as efficient quenchers of the 4F9/2 state. We are able to conclude that emitting state in dysprosium(III) is 4F9/2, that the mJ levels must be considered when determining electronic energy levels of dysprosium(III), and that scrutiny of the transition probabilities may reveal the structure of dysprosium(III) ions in solution.

INTRODUCTION Lanthanide luminescence manifests in unique narrow emission lines with a luminescence lifetime in the micro- or millisecond range. These features have been extensively exploited in bioimaging and bioassays.1-8 Lanthanide luminescence arise from intraconfigurational 4f-4f transitions in the massive excited state manifold of the lanthanide(III) ions. In the visible region of the spectrum, the most luminescent lanthanide(III) ions are terbium(III) (4f 8) and europium(III) (4f 6). The reason why these are the most luminescent is simple: the energy gap between the lowest excited state and the ground state manifold is ~13.000 cm-1 and ~12.100 cm-1 respectively, the biggest of the series (discounting Gd(III) that luminesce in the UV).9-11 In dysprosium(III) (4f 9) the energy gap is ~6.000 cm-1, which results in dysprosium(III) ions being significantly less luminescent than europium(III) and terbium(III) due to the pronounced effect of nonradiative relaxation. Even so, dysprosium(III) has found use in optical bioimaging, although the high

paramagnetism of the ion makes it more useful in MRI based techniques.12-16 Lanthanide coordination chemistry is dominated by the hard Lewis acid character of the predominately trivalent lanthanide ions. Without directional interactions with donor atoms, the structure of lanthanide(III) complexes is dominated by the size of the central ion and the need to maximize the number of donor atoms while minimizing steric repulsion between ligands. For small ligands that allow for coordination numbers of nine (CN=9) the result is a Thompson problem, where the solution is a tricapped trigonal prism (TTP). Although it only takes a small perturbation to make the transformation from TTP to capped square antiprism (cSAP), the former dominates with small identical ligands.17-20 Differences in ligand size, or steric demands from the ligand backbone,18 can enforce a specific complex geometry or reduce the donor number e.g. from a capped square antiprism to a pure square antiprism (SAP, CN=8), see chart 1.

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Dysprosium(III) ions exhibit coordination chemistry similar to that of other lanthanide(III) ions of similar size, yet is significantly less explored.21-28 The solid-state luminescence properties of dysprosium(III) are already relatively well-understood, and the NIR transition of Dy(III) has been exploited in telecommunication.29-31 The pale blue visible luminescence has found use in lighting, where single component white-light emitters need blue to add to green and red pigment.32-35 Chart 1. Structures of dysprosium solvates. O

O

O

O O O

Dy O

O O O

cSAP CN = 9

O

O

O

Dy O O

O O O

TTP CN = 9

O

O

O

O O O

Dy

O O

SAP CN = 8

The electronic states of dysprosium(III) have been investigated in the solid state and in solution previously,11, 36-37 but not at the level of detail described here. Recently, the 6H15/2 ground state of dysprosium(III) has come under close scrutiny,38-40 as the best single molecule magnets are based on dysprosium.41-45 Here, we investigate the full excited state manifold up to 40,000 cm-1 (250 nm). We revisit the assignment by Carnall,11, 36-37 and collate the first complete account of the photophysical properties of dysprosium(III) in solution. Since Carnall’s initial work the theoretical methods to treat 4f-electrons have improved, although the accuracy is still not such that experimental observations can be described.46-47 While the excited states prove elusive, the mJ levels of the ground states in the lanthanide(III) ions can be reproduced using computational chemistry and properties arising from the ground state can be predicted.48-50 Thus experimental benchmarks remain important. Here, we provide such a benchmark in the form of high resolution optical spectra of Dy(CF3SO3)3 in water, methanol and dimethylsulfoxide. The full dissociation of the triflate salt in competitive solvent allows us to report on hitherto unassigned states in the three solvates. Variable-temperature studies reveal the single emissive 4F9/2 state, and determination of luminescence lifetimes and absolute quantum yields show structural differences between the three solvent complexes formed in water, methanol and dimethylsulfoxide. We show that mJ levels must be considered and demonstrate that using the experimental data to calculate the oscillator strength is a powerful tool to analyze intraconfigurational 4f-4f transitions.

EXPERIMENTAL SECTION Sample preparation. All chemicals were used as received. 305±2 mg of Dy(CF3SO3)3 (98% AB Chem Inc.) was dissolved in 5.00 mL of DMSO-d6 (Eurisotop), D2O (Cambridge Isotope Laboratories, Inc.) and MeOH-d4 (Sigma-Aldrich) NMR grade solvents as well as MeOH and DMSO (SigmaAldrich) HPLC grade and milliQ grade H2O. We detected no optical signals from other lanthanide ions in any spectra. All samples were at 0.100±0.001 molar concentrations. Optical Spectroscopy. Absorption measurements were carried out on a Lambda 1050 double-beam spectrophotometer from PerkinElmer using a deuterium lamp and a halogen lamp for ultraviolet and visible/NIR radiation, respectively, at room temperature. Pure solvent was used as reference. The absorption was measured on absolute scale zeroed using solvent and blocked beam. Slits were kept at 2 nm. Steady state emission

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spectra were recorded on a PTI QuantaMaster8075 from Horiba Scientific using a xenon arc lamp as excitation source. For excitation spectra, emission was detected at 575 nm and excitation and emission slits were 2 nm and 8 nm, respectively. For emission spectra, the samples were excited at 350 nm and excitation and emission slits were set at 8 nm and 2 nm, respectively, in the visible region and at 8 nm and 16 nm, respectively, for the NIR region. For samples cooled with liquid nitrogen, slits were kept at 0.5 nm (excitation slits) and 8 nm (emission slits) for excitation spectra, 8 nm (excitation slits) and 0.5 (emission slits) for emission spectra in the visible region and 8 nm (excitation and emission slits) for emission spectra in the NIR region. Emission and excitation wavelengths were the same as for non-frozen spectra. Emission intensities in the visible region were corrected for wavelength dependence of the detector sensibility using a factory provided correction file. Lamp intensity fluctuations were corrected by a reference detector. A constant nitrogen flow in the sample chamber was used to avoid fogging of cooled samples. Absorption was kept below 0.1 at excitation wavelengths to avoid inner filter effects. When necessary, neutral density filters from Edmunds Optics was used to avoid detector saturation. Starna Scientific 10mm quartz cuvettes were used for all samples at ambient temperatures. For frozen samples quartz tubes from Mirit Glas were used. All spectra are shown in the SI. Determination of molar absorption coefficients. The molar absorption coefficients ε were determined from the absorbance and known concentration of the solutions. 305±2 mg of Dy(CF3SO3)3 was dissolved in 5.00 mL of pure solvent giving concentrations of 0.100±0.001 molar. No changes in absorbance was observed over time. Luminescence lifetime determination. Time-resolved measurements were performed using either a PTI QuantaMaster8075 from Horiba Scientific with a xenon flash lamp as excitation source (355 nm) or a TCSPC FluoTime300 from PicoQuant with a 355 nm laser as excitation source. Emission was detected at 575 nm. Excitation and emission slits were set at 4 nm and 8 nm, respectively. Lifetime traces were fitted to a monoexponential decay using the FelixGX (Horiba Scientific) and Origin 2017 (OriginLab) softwares. All lifetime traces are shown in the SI. Quantum yield determination. Quantum yields were determined using single point procedures following IUPAC recommendations. For the purpose of determining the electronic energy levels this precision is sufficient. Quinine sulfate was used as reference (ϕf l= 0.546).51-56 𝜙𝜙Lm = 𝜙𝜙𝑓𝑓𝑓𝑓 (𝑟𝑟𝑟𝑟𝑟𝑟) ∙

𝜂𝜂 2

𝜂𝜂 2 (𝑟𝑟𝑟𝑟𝑟𝑟)

∙

𝜀𝜀

𝜀𝜀(𝑟𝑟𝑟𝑟𝑟𝑟)

∙

∫ 𝐼𝐼𝐿𝐿𝐿𝐿

∫ 𝐼𝐼𝑓𝑓𝑓𝑓 (𝑟𝑟𝑟𝑟𝑟𝑟)

(1)

where ΦLm is the luminescence quantum yield, Φfl(ref) is the fluorescence quantum yield of the reference, η is the refractive index, ε is the molar absorption coefficient at the excitation wavelength and ∫I is the integrated emission intensity. All spectra used for the determination are included in the SI. Determination of excited state energies. The emission band edges were determined as the intersection between the tangent line of the slope of the peak and a 1 sigma increase above the noise level, see SI. Sigma was defined as √(number of counts) of the most intense peak. Absorption peaks were fitted to single Gaussian functions and the band edges were defined as the center ± the FWHM, see SI. The energy levels were determined as described in the text.

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Radiative rate constant determination. The radiative rate constants were determined using the quantum yields and observed lifetimes or following the procedure of Strickler-Berg and Hirayama & Phillips.57-58 𝑘𝑘𝑟𝑟𝑟𝑟𝑟𝑟 =

1

𝜏𝜏0

= 2.88 ∙ 10−9 ∙

𝑔𝑔𝑙𝑙

𝑔𝑔𝑢𝑢

3 ∙ 𝜂𝜂2 ∙ 𝜈𝜈𝑒𝑒𝑒𝑒 ∙∫

𝜀𝜀(𝑣𝑣) 𝜈𝜈

𝑑𝑑 ln 𝜈𝜈

(2)

where krad is the radiative rate constant of the transition, τ0 is the radiative lifetime, gu and gl is the degeneracy of the upper and lower state respectively, η is the refractive index, υem is the emission wavenumber and ∫ε is the integrated molar absorption coefficient 𝜙𝜙Lm =

𝜏𝜏0

𝜏𝜏𝑜𝑜𝑜𝑜𝑜𝑜

=

𝑘𝑘𝑟𝑟𝑟𝑟𝑟𝑟 𝑘𝑘𝑜𝑜𝑜𝑜𝑜𝑜

(3)

where ΦLm is the luminescence quantum yield, τ0 and τobs is the radiative lifetime of the transition and observed lifetime respectively and krad and kobs are the radiative rate constant and observed rate constant respectively

RESULTS The 6H15/2 ground state of dysprosium(III) has been scrutinized as it is the origin of the unique properties of dysprosium(III) based single molecule magnets.38-39, 41-44 The ligand field splitting of the state has been maximized in order to make “hightemperature” single molecule magnets.59 In the recent reports, the mJ levels in 6H15/2 are shown to span as much as ~2000 cm1 in the best single molecule magnets,41-42, 44 while other reports estimate a ground state mJ manifold spanning as little as ~400 cm-1.27, 39 However, the exact order of the remaining energy states in Dy(III) is not well established. The differences in the ground state should result in dramatic broadening/narrowing of absorption and emission bands involving 6 H15/2, but to exploit this fact and other structure related features of the intraconfigurational 4f-4f transitions the excited state manifold must be mapped and the basic photophysical properties of dysprosium(III) must be established. We start from the seminal work of Dieke and Carnall, supported by recent theoretical work on dysprosium(III).11, 36-37, 46

Optical spectroscopy The excited state manifold of Dy(III) can be probed directly using optical spectroscopy. Absorption spectroscopy shows the transitions originating in the 6H15/2 ground state, while emission spectra show the transitions from one (or more) emitting state(s) to the ground state manifold. Each is between two states, and provide the relative energy difference between the two and information on the mJ manifold in each state. In the initial state at room temperature, the splitting of the mJ levels determines the number of mJ states that have to be considered. A separation of 100 cm-1 gives a Boltzmann distribution of ~1:2 between the upper and lower state. At 300 cm-1 it is ~1:5, at 600 cm-1 it is ~1:20, and at 1200 cm-1 the distribution is ~1:323. Thus, at room temperature we have to consider mJ levels within a 300-400 cm-1 envelope. At 77 K this reduces to 150 cm-1, where the Boltzmann distribution is ~1:17. Considering that the mJ envelope of the 6H15/2 ground state has been shown to be as large as 1800 cm-1, 41-44 we have to consider that we will only see transitions from a subset of mJ levels in the absorption spectrum, while transitions to all mJ levels should be observed in the emission spectrum. With this in mind, we examined the spectra of dysprosium(III) triflate in heavy water. Figure 1 shows the absorption spectrum from 40,000 cm-1 (250 nm) to 20,000 cm-1 (500 nm), the excitation spectrum from 44,444 cm-1 (225 nm) to 17,860 cm-1 (560 nm), and emission spectra in the visible from 27,470 cm-1 (365 nm) to 11,111 cm-1 (900 nm) and at the near-IR from 44,444 cm-1 (800 nm) to 5700 cm-1 (1750 nm). It has been shown that the triflate salts of lanthanide(III) ions are fully dissociated in solution,19, 60-61 thus the bands in Figure 1 arise from [Dy(D2O)9]3+. To identify the excited states in lanthanide(III) ions, Russel-Saunders nomenclature is used.62-63 We base the assignment of each band on the work of Carnall, for the aqua ion [Dy(D2O)9]3+ and for Dy(III) doped into LaF3.11, 3637

To identify the emitting states in dysprosium(III) we start by

Figure 1. Emission (exc at 355nm), excitation (em at 575nm) and absorption spectra of Dy(CF3SO3)3 in D2O.



examining the emission spectrum in water, methanol and dimethylsulfoxide. Figure 2 shows the room temperature emission spectra from the perdeuterated solvents normalized so that the band at 840 nm has a peak height of 1. The method of normalization was chosen because this band is clearly resolved in the spectra recorded in both the PMT detector used in the visible and the InGaAs detector used in the near-IR region. This ensures that any differences in peak intensity due to the use of different slit widths are ignored.

Figure 2. Emission spectra (exc at 355 nm) of Dy(CF3SO3)3 in MeOH-d4, D2O and DMSO-d6 at 22 °C. Electronic energy levels from Carnall are indicated.11, 36-37

It is commonly accepted that the emitting state of dysprosium(III) is 4F9/2 and the large bands in Figure 2 all originate from 4F9/2. The two largest peaks at 480 nm and 575 nm are from the transitions 4F9/2 6H15/2 and 4F9/2 6H13/2, respectively. A large band in the NIR at 1160 nm is assigned to 4F9/2  6 F7/2. The wavelength of each band is compiled in Table 1. The energy of each transition is included and the transition is assigned as described below. There are only small differences in band intensities between the three solvents, that is the emission spectra of [Dy(D2O)9]3+, [Dy(CD3OD)9]3+, and [Dy(DMSO)8]3+ are fairly similar. Note that the emission spectra in the protonated solvents are of identical shape, yet the increased quenching in these solvents significantly reduce the intensity of all bands, see supporting information for details.

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Figure 3. Emission spectra (Exc at 355 nm) of Dy(CF3SO3)3 in DMSO-d6 at 77K, 281K, 295K, 319K and 333K. Note that 77K and 281K samples are frozen solutions.

Closer inspection of Figure 2 shows a series of blue-shifted satellite bands. These arise from the 4I15/2 state that is 1200 cm1 higher in energy than the 4F9/2 state. The Boltzmann distributions above show that 4I15/2 state can be thermally populated from the 4F9/2 state. This is known from solid state systems, but this is, to the best our knowledge, the first time this is observed in solution.64-65 This is convenient as it allows for a second measure for the energies of the 6H15/2 and 6H13/2 states. To validate that the small bands indeed originate from thermal population of the 4I15/2 state, low temperature spectra were recorded in DMSO-d6. Figure 3 shows the spectra recorded in solution at temperatures from 333K to 281K, and flash frozen at 77K. The bands originating in the 4I15/2 state are clearly temperature dependent, see insert in Figure 3, and vanish at 77K confirming the assignment and thermal population of 4 I15/2 from 4F9/2. Cursory inspection of Figure 3 shows that all bands become narrow at 77K. As thermal fluctuations in structure are assumed to have a very limited influence on the lineshape, the spectral change as a function of temperature must be due to changes in the population of the mJ levels. At 77 K an energy gap of 150 cm-1 is enough to ensure that only the lower mJ set will give rise to significant emission. For the 4F9/2 state, several mJ levels are emitting at room temperature, while a single mJ level is emitting at 77 K. This gives rise to the sharper bands seen in the 77K emission spectrum in Figure 3. This creates the opportunity to investigate the mJ manifold of the ground states directly in the emission spectrum. For example, the 6 H15/2 ground state consists of two groups of mJ levels separated by 520 cm-1, and at least three mJ levels with a separation greater than 100 cm-1 can be identified in the 6H13/2 state. Table 1 – Transitions and their energies from luminescence spectra at room temperature Band

Transition energies – cm-1

Transition

nm

MeOH

DMSO

H2O

F9/2  6H15/2

4

480

21357

21104

21061

4

6

575

17913

17922

17907

4

6

F9/2  H11/2

665

15468

15482

15445

4

F9/2  H13/2 F9/2  6H9/2

750

13619

13650

13637

4

6

835

12241

12299

12268

4

6

930

10912

10995

10925

4

6

1000

10326

10335

10255

1150

8846

8826

8829

F9/2  H7/2 F9/2  H5/2 F9/2  F9/2

4

F9/2  6F7/2

4

F9/2  6F5/2

1325

7789

7789

7808

4

6

1500

7073

7057

7137

4

6

F9/2  F1/2

1700

5944

6108

6102

I15/2  6H15/2

455

22120

22101

22092

540

18546

18579

18540

F9/2  F3/2

4 4

6

I15/2  H13/2


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Table 2 – Transition energies and band widths from absorption spectra at room temperature Transition energies and (widths)a – cm-1 MeOH

DMSO

H2O

6

Transition 6

H15/2  H11/2

5559 (505)

5472 (588)

5542 (713)

6

6

7376 (770)

7338 (756)

7317 (822)

8708 (744)

8688 (698)

8716 (694)

10675 (637)

10679 (570)

10683 (607)

H15/2  H9/2

6

H15/2  6H7/2

6

H15/2  6H5/2 + 6F9/2

6

H15/2  6F7/2

12199 (363)

12215 (292)

12230 (307)

6

6

12933 (484)

12984 (358)

12999 (392)

6

4

H15/2  F9/2

20609 (918)

20650 (806)

20615 (909)

H15/2  4I15/2

21814 (657)

21805 (610)

21821 (659)

23142 (500)

23124 (472)

23104 (594)

H15/2  F7/2

24652 (1219)

24633 (1088)

24667 (1116)

H15/2  4I13/2

25507 (618)

25448 (640)

25479 (661)

H15/2  4K17/2

25996 (664)

26008 (585)

26031 (600)

6

6

27097 (615)

27078 (564)

27134 (587)

6

6

28117 (782)

28093 (709)

28163 (725)

6

4

29331 (515)

29275 (569)

29367 (472)

H15/2  6P3/2

30440 (595)

30403 (573)

30469 (588)

??

30499 (1280)

H15/2  F5/2

Figure 4. UV to NIR absorption spectra of Dy(CF3SO3)3 in MeOH-d4, D2O and DMSO-d6 at room temperature. Electronic energy levels from Carnall are indicated.11, 36-37

Assigning state energies The emission spectra in Figure 2 are plotted along with the state energies reported by Carnall. These confirm the assignment of the emitting states, and allow the origin of each band to be assigned as done in Table 1. Similarly, Figure 4 shows the absorption spectra of [Dy(D2O)9]3+, [Dy(CD3OD)9]3+, and [Dy(DMSO)8]3+ plotted together with the state energies determined by Carnall. The absorption spectra recorded in the protonated solvents are identical to those plotted in Figure 4, see supporting information. Each band was resolved using Gaussian functions, and the state energies and bandwidths are compiled in Table 2. All bands except for one in the UV could be assigned using data from Carnall.11, 36-37 The data from the absorption and emission spectra can be used to determine the state energies. This has to be done carefully, as the mJ envelopes of both the initial and final states influence the observed energy. If an emission band arises from a single electronic transition it has a vibrionic envelope and the red edge will correspond to the energy of the transition in absorption spectrum. Similarly, the blue edge of a band in the emission spectrum corresponds to state energy. In a lanthanide(III) ion all the possible transitions from every single occupied mJ level are individual electronic transitions. So, the interpretation of the bands in the optical spectra of dysprosium has to take the mJ levels of the initial and final states into account. Consider Figure 5. The 6F1/2 state is a single Kramer’s doublet, and the 4 F9/2  6F1/2 transition reveals the mJ splitting in the 4F9/2 state. In the room temperature emission spectrum two lines are observed, and in combination with the blue edge of the 6H15/2  4F9/2 transition in the absorption spectrum the five mJ levels in the 4F9/2 state can be assigned, as done in Figure 5. Similarly, the splitting of the 6H15/2 state was determined from the 77K emission spectrum, where only the lowest mJ levels of the 4 F9/2 state are populated, see Figure 5.

6 6

4

H15/2  G11/2

6

4

6 6

H15/2  P5/2 H15/2  P7/2 H15/2  F5/2

6

30511 (1254)

30693 (939)

6

4

32814 (551)

32748 (747)

6

4

33123 (658)

33134 (581)

H15/2  4K13/2

33608 (593)

33573 (612)

6

33860 (763)

33945 (572)

6

H15/2  4F5/2

34507 (574)

-

-

6

H15/2  4L17/2

35630 (569)

-

-

H15/2  4K11/2

36053 (828)

-

35958 (1029)

6

4

37442 (804)

-

37350 (1073)

6

4

38519 (924)

-

38524 (955)

6

4

39372 (715)

-

-

H15/2  L19/2

H15/2  H13/2

6

4

H15/2  F3/2

6

H15/2  F3/2 H15/2  P5/2 H15/2  P3/2

32879 (1106) 33449 (1276)

a The error on energy determination is 25 cm-1 for solitary bands, and 50 cm-1 for bands in groups.



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Table 3. Electronic energy levels in the dysprosium(III) (4f9) ion determined in this work and by Carnall. Transition energies – cm-1 Term 6

H15/2 a

MeOHd

DMSOd

H2Od

Carnall Obs

Carnall calc

-248

74

54

0

0

6

a

3196

3256

3208

3503

3502

6

c

5641

5696

5670

5883

5875

6

c

7490

7528

7478

7632

7630

H7/2 c

H13/2 H11/2 H9/2

6

8868

8879

8847

8992

8996

6

a

10197

10183

10190

10222

10220

6

c

10783

10843

10860

9074

9085

6

c

12263

12352

12286

11037

11038

F5/2 c

13320

13389

13307

12456

12466

H5/2 F9/2 F7/2

6 6

a

14036

14121

13978

13271

13288

6

a

15165

15070

15013

-

13839

4

c

20609

20650

20615

21057

21058

I15/2 b

21814

21805

21821

22022

21996

b

F3/2 F1/2 F9/2

4 4

23142

23124

23104

23468

23460

4

G11/2 F7/2

b

24652

24633

24667

25661

25674

4

I13/2

b

25507

25448

25479

25918

25929

K17/2 b

25996

26008

26031

25940

25952

6

b

27097

27078

27134

27574

27580

Figure 5. Energy level diagram for the H15/2, F1/2, and F9/2 states of Dy(III) including the experimentally determined grouping and energy splitting of mJ levels of each state. The thermal energy corresponding to a 1:9 Boltzmann distribution at 295K (red) and 77K (blue) is indicated, and the arrows show the initial and final mJ levels of the high and low energy transitions.

6

b

28117

28093

28163

28536

28523

4

b

29331

29275

29367

29527

29517

P3/2 b

30440

30403

30469

30879

30862

??

30511

30693

30499

With the mJ levels of initial and final states in place, we can consider their implications on the band structure in the absorption and emission spectra. Only the lowest mJ grouping of the 6 H15/2 state is populated at room temperature, and the low energy absorption—the red-edge of the absorption band—is from the top of the lower mJ grouping in 6H15/2 to the lowest mJ level in 4F9/2. The blue edge of the absorption spectrum is from the lowest mJ level in 6H15/2 to the highest mJ grouping in 4 F9/2. These two transitions are shown with vertical (upward) arrows in Figure 5. The initial state for the bands observed in the emission spectrum is the 4F9/2 state. The low energy transition—the red-edge of the emission band—is from the lowest mJ level in 4F9/2 to the highest mJ level in the final state, in Figure 5 illustrated using 6H15/2. The blue edge—the emission line with the highest energy—is from the middle grouping of mJ levels in 4F9/2 to the lowest mJ level in the final state. The highest mJ level in 4F9/2 is not significantly populated (1:190) at room temperature. The two transitions defining the edges of the emission bands are shown with vertical (downward) arrows in Figure 5.

4

L19/2

b

32814

32748

-

31369

31370

4

H13/2

b

33123

33134

32879

33500

33500

K13/2 b

33608

33573

33687

33185

33185

4

b

33860

33945

-

33628

33632

4

b

34507

-

-

-

34260

4

b

35630

-

-

34406

34400

K11/2 b

36053

-

35958

-

35776

4

b

37442

-

37350

37933

37921

4

b

38519

-

38524

38926

28911

4

b

39372

-

-

39159

39163

4

P5/2

6

6

4

P7/2 F5/2

6

4

F3/2 F5/2

L17/2

4

F3/2 P5/2 P3/2

a c

From

Luminescence

From

both

spectrum. luminescence

b

From and

absorption absorption

spectrum. spectroscopy.

d The error on energy determination estimated at 25 cm-1.

Thus, the red edge of the absorption band and the blue edge of the emission band of a transition reveals the state energy. As the value from the emission spectrum is from higher lying mJ


Page 7 of 21

levels, the energy must be corrected by 500 cm-1 to give the state energy. The transition energies were determined and are compiled in Tables 1 and 2. Table 1 lists the blue edge onset energy for each band in the emission spectrum, and Table 2 lists the rededge energy for each band in the absorption spectra. From these values, the electronic energy levels can be determined as described above. This was done and the result is compiled in Table 3, where the state energies determined here are listed along with the values determined and calculated by Carnall. The result is visualized in Figure 6, where the absorption spectrum and the emission spectra are plotted on an appropriate energy scale. The energy of the J states as determined here and by Carnall are plotted on the same energy scale. Cursory inspection of Figure 6 shows that several of the experimentally determined energy levels do not match the energies assigned by Carnall. These energies are indicated in bold font in Table 3, and originate in a confusion of the 6H and 6F manifolds. We propose a re-ordering based on the assumption that the intensity of the absorption bands within the 6H manifold is higher than between the 6H and the 6F manifold. Furthermore, the energy of the 6F manifold predicted by Carnall is significantly lower (1000-2000 cm-1) than what we observe. For all the other electronic energy levels the values determined and predicted by Carnall are in good agreement with what is found here for dysprosium(III) in water, methanol, and dimethylsulfoxide. The bands in the near-IR and the visible region of the spectrum are readily assigned. In the ultraviolet the presence of many and overlapping transitions make the determination of electronic energy levels more difficult. We suggest that only

energies up to ~33000 cm-1 are trusted. With the width of the mJ envelope of the 6H15/2 state established at 150 cm-1, all bands in the absorption spectrum can inform on the mJ splitting in the accepting state. Apart from a few states in the 6F manifold that have a narrow distribution of mJ levels (300-400 cm-1), the majority of states has a distribution of mJ levels that spans 500-700 cm-1. A few states in the 4 F manifold have a wider distribution exceeding 800 cm-1, but the overall conclusion must be that all electronic energy levels in dysprosium(III) ions in the [Dy(D2O)9]3+, [Dy(CD3OD)9]3+, and [Dy(DMSO)8]3+ solvates have mJ envelopes that are significantly different from what has been reported for dysprosium(III) complexes investigated as single molecule magnets.27, 39, 41-42, 44 It is worth noting that the mJ envelope of 6H15/2 can be directly, experimentally determined from three spectra.

Photophysical properties Having established the electronic energy levels and mJ splitting of Dy(CF3SO3)3 in water, methanol, and dimethylsulfoxide, the photophysical properties arising from the 4F9/2 state can be investigated. As the triflates are fully dissociated in solution, the species under scrutiny are the [Dy(D2O)9]3+, [Dy(CD3OD)9]3+, and [Dy(DMSO)8]3+ solvates. The luminescence lifetimes were measured, the molar absorption coefficient was determined, single point quantum yields were calculated, and the oscillator strength of each band found from both absorption and emission spectra were determined. The results are compiled in Tables 4 and 5, and all calculated values are given with a relative error of 50%. Based on the purity of the Dy(CF3SO3)3 used this level of precision was deemed appropriate.

45000 40000 35000

Wavenumber (cm-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30000 25000 20000 15000 10000

4

4

P5/2 K11/2

4

4

4

F3/2

K13/2 H13/2 6 P3/2

6

P7/2 4 K17/2 4 F7/2 4

I15/2

6

F3/2 6 F7/2 6 F9/2

6

H9/2 H11/2

5000

6

0

6

4

F3/2

Carnall This Work

4 F5/2 6 P 4 5/2 I13/2 4 G11/2 4 F9/2

6

F1/2 F5/2

6 6

H5/2 H7/2

6

6

H13/2 H15/2

-5000 Figure 6. Energy level diagram for Dy(III) predicted by Carnall et al. and determined in this work. Emission (right) and absorption (left) spectra of Dy(CF3SO3)3 in D2O (from Figure 2 and 4).



Table 4. Intrinsic quantum yields following direct excitation and excited state lifetimes of the 4I15/2 and 4F9/2 states of Dy(CF3SO3)3 dissolved in deuterated methanol, dimethylsulfoxide and water. MeOH-d4

4

4

ϕLm ( I15/2, F9/2)

a

DMSO-d6

D2O

2.02 %

11.8 %

1.7 %

τobs (4I15/2,4F9/2) (µs)

47.09 ± 0.022

184.9 ± 0.017

33.47 ± 0.039

MeOH

DMSO

H2O

τobs (4I15/2,4F9/2) (µs)

2.4 ± 0.16

7.1 ± 1.9

1.6 ± 0.07

a

Excitation wavelength 350 nm and excitation and emission slits at 2 nm and 2 nm.

First it should be noted that the excited state lifetimes are identical across the emission spectrum, including in the bands originating in 4I15/2. This further supports the conclusion that the emission from 4I15/2 arises from thermal population of the 4 I15/2 state from 4F9/2. This is true for all three solvents in both protonated and deuterated forms, see the SI for details. The excited state lifetime varies greatly as a function of solvent, and appears to follow the established mechanism of nonradiative deactivation by high energy oscillators.60-61, 66-67 The lifetimes in the protonated solvents are significantly shorter than the lifetimes in the deuterated counterparts. While the presence of C-H oscillators reduces the excited state lifetime, the effect of O-H oscillators is significantly larger. In the data in Table 4, six C-H oscillators reduces the lifetime by a factor of ~25 compared to six C-D oscillators, while replacing the six C-D oscillators with two O-H oscillators reduces the lifetime by a factor ~100. The luminescence quantum yield (ϕLm) of the dysprosium(III) ion is low in methanol-d4 and deuterated water, which shows that O-D oscillators—although less efficient than O-H—still facilitate fast non-radiative deactivation of 4 I15/2 and 4F9/2 states. In stark contrast to terbium(III) and europium(III) which have quantum yields of luminescence up to unity in methanol-d4 and deuterated water, the lower quantum yield is readily explained by the much closer spacing of electronic energy levels in dysprosium(III) compared to the highly luminescent lanthanide(III) ions. The effect of the close lying electronic energy levels is also observed in dimethylsulfoxided6, a solvent free of high energy intramolecular vibration. Despite the lack of high energy oscillators, the quantum yield of luminescence in dimethylsulfoxide-d6 is only 11.8 %, which is a direct consequence of the low, 6000 cm-1 gap between the emitting 4F9/2 state and the next state in the excited state manifold.

Transition probabilities Our theoretical description of ‘forbidden’ electronic transitions remains poor, and our ability to treat the lanthanide centered excited states using quantum chemical methods is imprecise at best.68 Thus, we revert to the basic laws of physics and interpret our results based on energy conservation. Molecular photophysics started with the Einstein coefficients for stimulated absorption and spontaneous emission as interpreted in the Strickler-Berg equation.58, 69 The relationship between molar

Page 8 of 21

absorption coefficient of a given electronic transition and rate constants of spontaneous emission occurring through the same transmission is conveniently linked to the quantum mechanical transition dipole operator through the experimentally determined oscillator strength f.70 The oscillator strength f of a given emitting transition in the electronic energy level manifold thus describes the branching ratio using a property of said transition. This property can be related directly to the molar absorption coefficient of this transition. Thus two independent measurements can be used to determine the oscillator strength of an electronic transition. In dysprosium(III) this is only possible for the transitions between 6H15/2 and 4F9/2, as this is the only transition where both absorption and emission is observed. The oscillator strengths (f) were calculated from the integrated absorption spectra and from the quantum yield, excited state lifetime and branching ratios using equations 4 and 5 and the results were compiled in Table 5. 𝑘𝑘𝑟𝑟𝑟𝑟𝑟𝑟 = 2.88 ∙ 10−8 ∙ (𝜐𝜐0 )2 ∫ 𝜀𝜀 𝑑𝑑𝑣𝑣̅ 𝑓𝑓 = 4.3 ∙ 10

−9

𝑘𝑘𝑟𝑟𝑟𝑟𝑟𝑟

∫ 𝜀𝜀 𝑑𝑑𝑣𝑣̅ ≅ (𝜐𝜐

0)

2

(4) (5)

where krad is the radiative rate constant, υ0 is the wavenumber of emission, ∫ε is the integrated molar absorptivity, f is the oscillator strength, and Note that this treatment completely ignores the number of mJ levels in each band and the effect of the refractive index when converting radiative lifetimes to oscillator strengths.58, 69 The oscillator strength is a convenient measure not only because it puts a physical constant on the branching ratio, but also as it can be compared to oscillator strengths of classical transitions from molecular photophysics.71 All the calculated values of f are in the order of 10-6 to 10-7, which is comparable to allowed singlet-triplet transitions in organic chromophores. Traditionally, the intensity of the 4F9/2  6H15/2, assigned as a magnetic dipole transition, is much lower than the intensity of the 4F9/2  6H13/2 transition, an induced electric dipole transition.72 The transition type is used to label the assumed nature of dominating operator in the transition dipole moment integral, i.e. electric dipole operator μ, magnetic dipole operator m, or an induced electric dipole operator. The polarization and intensity of a transition is mJ specific when the electric dipole or magnetic dipole operator is the dominate. For transitions dominated by the induced electric dipole operator, the polarization of the incident light determines the intensity and polarization of the transition. Thus the relationship between the μ and m can be probed using circular polarized spectroscopy,73 to our knowledge the nature of the transitions have not been determined experimentally. The relative band intensities in between bands arising from (groups of) transitions of predominately magnetic dipole and electric dipole character have been used to indicate symmetry. This is based on the assumption that the selection rules determining the size of the transition dipole moment integral are stricter for the electronic dipole operator than for the magnetic dipole operator.72 Therefore, it can be deduced that the electric dipole operator dominated transitions are more sensitive to changes in symmetry than magnetic dipole operator dominated transitions.


Page 9 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Table 5. Branching ratios in emission spectrum, integrated molar absorption coefficients, and calculated oscillator strengths f of the transitions observed in Dy(CF3SO3)3 in water, methanol, and dimethylsulfoxide MeOH

DMSO

H2O

6

F9/2  H15/2

4

F9/2  6H13/2

1.00

1.00

DMSO

ΔS

ΔL

ΔJ

Type

4.5

1

2

3

forbidden

2

2

forbidden

H2O 7

Oscillator strength ·10 - f

Branching ratio 4

MeOH

1.00

3.6

4.8

0.84

1.02

0.77

4.3

6.9

4.9

1

4

6

F9/2  H11/2

0.05

0.06

0.04

0.4

0.6

0.4

1

2

1

forbidden

4

6

0.07

0.11

0.07

0.6

1.4

0.8

1

2

0

forbidden

4

6

0.03

0.05

0.03

0.3

0.7

0.4

1

2

1

forbidden

1

2

2

forbidden

1

0

0

forbidden

F9/2  H9/2 F9/2  H7/2

4

F9/2  6H5/2

4

F9/2  6F9/2

4

F9/2  6F7/2

0.03

0.07

0.05

0.4

1.1

0.7

0.08

0.15

0.10

1.2

2.9

1.8

0.28

0.60

0.37

6.0

17

10

1

0

1

forbidden

0.03

0.06

0.03

0.9

2.2

1.2

1

0

2

forbidden

0

3

forbidden

4

6

4

6

0.06

0.09

0.08

1.9

4.3

3.3

1

4

6

0.02

0.05

0.03

1.1

3.3

1.9

1

0

4

forbidden

ΔS

ΔL

ΔJ

Type

F9/2  F5/2 F9/2  F3/2 F9/2  F1/2

Integrated Molar Absorption Coefficient

Oscillator strength ·107 - f

6

368

489

395

16

21

17

0

4

6

forbidden

6

52

87

46

2.2

3.7

2.0

1

2

5

forbidden

6

850

1440

813

37

62

35

0

4

4

forbidden

6

H15/2  6P5/2

498

607

513

21

26

22

0

4

5

forbidden

H15/2  4K17/2

94

115

73

4.1

4.9

3.1

1

2

1

forbidden

255

115

262

11

17

11

1

1

1

forbidden

H15/2  4F7/2

186

393

261

8.0

8.3

6.2

1

2

4

forbidden

H15/2  4G11/2

24

42

28

1.0

1.8

1.2

1

1

2

forbidden

6

98

42

93

4.2

4.8

4.0

1

1

0

forbidden

6

47

51

48

2.0

2.2

2.1

1

2

3

forbidden

6

78

78

80

3.3

3.4

3.4

0

2

5

EDa

6

297

307

293

13

13

13

0

2

4

EDa

-

-

-

EDa

H15/2  6P3/2 H15/2  4F5/2 H15/2  6P7/2

6

6

H15/2  4I13/2

6 6

H15/2  4I15/2 H15/2  4F9/2 H15/2  6F5/2 H15/2  6F7/2

6

H15/2  6H5/2 , 6 F9/2

526

566

535

-

-

-

609

711

584

26

33

25

0

0

4

EDa

6

H15/2  6H9/2

652

967

550

28

42

24

0

0

3

EDa

6

H15/2  6H11/2

559

326

737

29

14

32

0

0

2

EDa

6

6

H15/2  H7/2

a

Electric Dipole/Induced Electric Dipole

It is worth noticing that the experimental variation in oscillator strengths for dysprosium(III) is small (10-6 to 10-8), when considering that experimentally determined f for optical transitions takes values from 1 to 10-9. Relative variations between bands in [Dy(D2O)9]3+, [Dy(CD3OD)9]3+, and [Dy(DMSO)8]3+ solvates are small, but well within the experimental uncertainty. It is certain that the variations are due to changes in the ligand field, but the experimental data does not allow us to

distinguish between the effect of changes in the two mJ envelopes and changes in the oscillator strength of the transition between individual mJ levels. Note that bringing an additional mJ level down in energy can increase the apparent degeneracy and thereby the observed oscillator strength. Three factors may change the oscillator strength of a transition: a reduced overlap of initial and final states, the overall symmetry of the transition, and the demand for conservation



of spin and momentum. As most transitions in dysprosium(III) are forbidden due to symmetry, poor overlap and momentum conservation (spin), the relative changes between [Dy(D2O)9]3+, [Dy(CD3OD)9]3+, and [Dy(DMSO)8]3+ are hard to quantify, in particular when considering that the oscillator strengths of the nominally allowed transitions are only marginally higher than that of the forbidden transitions. There are four things to note in the data shown in Table 5. The first is that the allowed transitions in the 6H manifold have higher f than the allowed transition to the 6F manifold. The second is that the values of f determined for the 6H15/2 to 4F9/2 and the 4F9/2 to 6H15/2 transition are of similar magnitude. The third is that the oscillator strength compensates for the energy of the transition thus giving a true, rate constant based branching ratio. Finally, the values of f determined for [Dy(D2O)9]3+ and [Dy(CD3OD)9]3+ are fairly similar while those determined for [Dy(DMSO)8]3+ are different. We propose that this is due to a difference in structure, where [Dy(D2O)9]3+ and [Dy(CD3OD)9]3+ are nine-coordinated in the form of trigonal tricapped prisms and [Dy(DMSO)8]3+ is eight-coordinated square antiprismatic. The lowering of symmetry from D3h to C4 allows a non-vanishings dipole operator in [Dy(DMSO)8]3+. Thus, the origin of the higher oscillator strengths of the optical transitions in [Dy(DMSO)8]3+ can be that they are less symmetry forbidden.

CONCLUSION We have assigned the electronic energy levels of Dy(CF3SO3)3 dissolved in methanol, dimethylsulfoxide and water. We conclude that the salt is fully dissociated and that the properties are those of [Dy(D2O)9]3+, [Dy(CD3OD)9]3+, and [Dy(DMSO)8]3+. By considering the population of each mJ level the energy of the lowest mJ state of each electronic energy level was determined. We found minor discrepancies with the values reported by Carnall and Dieke, and conclude that the energy levels reported here should be used. We also observe the first example of emission from the thermally populated 4I15/2 state in solution. The electronic energy levels do not change significantly (±100 cm-1) between the three solvates. By determining the oscillator strength we show that there is a difference in structure between [Dy(D2O)9]3+, [Dy(CD3OD)9]3+, and [Dy(DMSO)8]3+. The most significant difference is with [Dy(DMSO)8]3+ which is more luminescent, not only due to less solvent quenching but also due to a higher rate of spontaneous emission. We conclude that the difference is due to difference in coordination number and the following lower symmetry. By analyzing the widths of the bands in the absorption spectra, it was shown that also the mJ manifolds are different in [Dy(D2O)9]3+, [Dy(CD3OD)9]3+, and [Dy(DMSO)8]3+. It is well-documented by the molecular magnetism community that the mJ manifold strongly depends on the ligand field, and as the mJ levels are specific to each solvates we conclude that there is also a difference in structure between [Dy(D2O)9]3+ and [Dy(CD3OD)9]3+. Thus, the mJ envelopes must be determined for each complex and we conclude that this is readily done using a combination of absorption and emission spectroscopy.

Page 10 of 21

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. ORCID Nicolaj Kofod: 0000-0003-2905-8938 Riikka Arppe-Tabbara: 0000-0002-9930-1676 Thomas Just Sørensen: 0000-0003-1491-5116 Notes There are no competing financial interests to declare.

ACKNOWLEDGMENTS The authors thank Carlsbergfondet, Villum Fonden (grant #14922), and the University of Copenhagen for support.

Supporting information Supporting Information. Assitioal absorption, excitation and emission spectra, time-resolved emission decay profiles and fits, data from resolving the absorption and emission spectra, and branching ratios are supplied as Supporting Information.

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30. Tsang, Y. H.; El-Taher, A. E.; King, T. A.; Jackson, S. D., Efficient 2.96 μm dysprosium-doped fluoride fibre laser pumped with a Nd:YAG laser operating at 1.3 μm. Opt. Express 2006, 14 (2), 678-685. 31. Zhu, X.; Peyghambarian, N., High-Power ZBLAN Glass Fiber Lasers: Review and Prospect. Advances in OptoElectronics 2010, 2010. 32. Bünzli, J.-C. G., Chapter 287 - Lanthanide Luminescence: From a Mystery to Rationalization, Understanding, and Applications. In Handbook on the Physics and Chemistry of Rare Earths, Bünzli, J.-C. G.; Pecharsky, V. K., Eds. Elsevier: 2016; Vol. 50, pp 141176. 33. Wahsner, J.; Gale, E. M.; Rodríguez-Rodríguez, A.; Caravan, P., Chemistry of MRI Contrast Agents: Current Challenges and New Frontiers. Chemical Reviews 2018. 34. Shang, M.; Li, C.; Lin, J., How to produce white light in a singlephase host? Chemical Society Reviews 2014, 43 (5), 1372-1386. 35. Shrivastava, R.; Kaur, J.; Dubey, V., White Light Emission by Dy3+ Doped Phosphor Matrices: A Short Review. Journal of Fluorescence 2016, 26 (1), 105-111. 36. Carnall, W. T.; Fields, P. R., Lanthanide and actinide absorption spectra in solution. Argonne National Lab., Ill. Advan. Chem. Ser. 1967, 71: 86-101(1967). 37. Carnall, W. T.; Fields, P. R.; Rajnak, K., Electronic Energy Levels in the Trivalent Lanthanide Aquo Ions. I. Pr3+, Nd3+, Pm3+, Sm3+, Dy3+, Ho3+, Er3+, and Tm3+. The Journal of Chemical Physics 1968, 49 (10), 4424-4442. 38. Klahn, E. A.; Gao, C.; Gillon, B.; Gukasov, A.; Fabrèges, X.; Piltz, R. O.; Jiang, S.-D.; Overgaard, J., Mapping the Magnetic Anisotropy at the Atomic Scale in Dysprosium Single-Molecule Magnets. Chemistry – A European Journal 2018, 24 (62), 1657616581. 39. Vieru, V.; Ungur, L.; Cemortan, V.; Sukhanov, A.; Baniodeh, A.; Anson, C. E.; Powell, A. K.; Voronkova, V.; Chibotaru, L. F., Magnetization Blocking in Fe2(III)Dy2(III) Molecular Magnets: Ab Initio Calculations and EPR Spectroscopy. Chemistry – A European Journal 2018, 24 (62), 16652-16661. 40. Chorazy, S.; Charytanowicz, T.; Majcher, A. M.; Reczyński, M.; Sieklucka, B., Connecting Visible Photoluminescence and Slow Magnetic Relaxation in Dysprosium(III) Octacyanidorhenate(V) Helices. Inorganic chemistry 2018, 57 (22), 14039-14043. 41. Goodwin, C. A. P.; Reta, D.; Ortu, F.; Chilton, N. F.; Mills, D. P., Synthesis and Electronic Structures of Heavy Lanthanide Metallocenium Cations. Journal of the American Chemical Society 2017, 139 (51), 18714-18724. 42. Goodwin, C. A. P.; Ortu, F.; Reta, D.; Chilton, N. F.; Mills, D. P., Molecular magnetic hysteresis at 60 kelvin in dysprosocenium. Nature 2017, 548, 439. 43. Guo, F.-S.; Day, B. M.; Chen, Y.-C.; Tong, M.-L.; Mansikkamäki, A.; Layfield, R. A., A Dysprosium Metallocene Single-Molecule Magnet Functioning at the Axial Limit. Angewandte Chemie 2017, 56 (38), 11445-11449. 44. Guo, F.-S.; Day, B. M.; Chen, Y.-C.; Tong, M.-L.; Mansikkamäki, A.; Layfield, R. A., Magnetic hysteresis up to 80 kelvin in a dysprosium metallocene single-molecule magnet. Science 2018, 362 (6421), 1400-1403. 45. Sørensen, M. A.; Hansen, U. B.; Perfetti, M.; Pedersen, K. S.; Bartolomé, E.; Simeoni, G. G.; Mutka, H.; Rols, S.; Jeong, M.; Zivkovic, I. et al, Chemical tunnel-splitting-engineering in a dysprosium-based molecular nanomagnet. Nature Communications 2018, 9 (1), 1292. 46. Freidzon, A. Y.; Kurbatov, I. A.; Vovna, V. I., Ab initio calculation of energy levels of trivalent lanthanide ions. Physical Chemistry Chemical Physics 2018, 20 (21), 14564-14577. 47. Seijo, L.; Barandiarán, Z., Chapter 285 - Ab Initio Calculations on Excited States of Lanthanide Containing Materials. In Handbook on the Physics and Chemistry of Rare Earths, Bünzli, J.-C. G.; Pecharsky, V. K., Eds. Elsevier: 2016; Vol. 50, pp 6589. 48. Chilton, N. F.; Goodwin, C. A. P.; Mills, D. P.; Winpenny, R. E. P., The first near-linear bis(amide) f-block complex: a blueprint



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Associate Professor Thomas Just Sørensen obtained his PhD from the University of Copenhagen in 2010. After working with Prof Stephen Faulkner at Oxford, Sir J. Fraser Stoddard at UCLA, Prof Jerome Lacour in Geneva, and Profs Ignacy and Karol Gryczynski at UNT Thomas returned to take a permanent position at the University of Copenhagen in 2014. Thomas is an entrepreneurial scientist and with three spin-out companies under his belt, his research focuses on both fundamental and applied aspects of lanthanide chemistry in solution.


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Figure 1 188x126mm (300 x 300 DPI)


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Figure 2 188x147mm (300 x 300 DPI)



Figure 3 188x145mm (300 x 300 DPI)


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Figure 4 188x148mm (300 x 300 DPI)



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ToC graphic 39x19mm (300 x 300 DPI)


Electronic Energy Levels of Dysprosium(III) Ions in Solution

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