# Magneto-Optical Correlations in a High Symmetry Dysprosium Complex Reveal an Almost “Ideal Non-SMM” and Orange-To-Red DyIII-Based Emitter

Nimisha Jain, Grégoire David, Marie Cordier, Yan Suffren, Boris Le Guennic, Yann Sarazin, Kevin Bernot

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Nimisha Jain, Grégoire David, Marie Cordier, Yan Suffren, Boris Le Guennic, et al., Magneto-Optical Correlations in a High Symmetry Dysprosium Complex Reveal an Almost “Ideal Non-SMM” and Orange-To-Red DyIII-Based Emitter. ChemistryEurope, 2025, pp.202500193. 10.1002/ceur.202500193. hal-05229991

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# Magneto-Optical Correlations in a High Symmetry Dysprosium Complex Reveal an Almost “Ideal Non-SMM” and Orange-to-Red Dy<sup>III</sup>-Based Emitter

Nimisha Jain, Gregoire David, Marie Cordier, Yan Suffren, Boris Le Guennic, Yann Sarazin,* and Kevin Bernot*

The importance of investigating high-symmetry systems in order to reveal properties that would otherwise be concealed in less symmetrical compounds is demonstrated. This is particularly true for discrete coordination complexes that tend to adopt low symmetries. The magnetic, optical, and theoretical investigation of [Dy{N(SiMe₃)₂}₃], an amido complex that features an almost trigonal electrostatic environment around the Dy<sup>III</sup> ion, is reported here. While this specific coordination is ideally suited to the stabilization of high magnetic ground state for "prolate" Yb<sup>III</sup>-based single-molecule magnet (SMM), it is the opposite for oblate Dy<sup>III</sup>.

Hence, [Dy{N(SiMe₃)₂}₃] is expected to display the lowest conceivable magnetic anisotropy. This is confirmed herein by magneto-optical and theoretical correlations. An extremely pure Kramers doublet composition is observed on a totally reversed energetic profile compared to the standard Dy<sup>III</sup>-SMM. This results in a paramagnetic but dynamically silent compound, even when an external dc field is applied. Additionally, this unusual energetic profile induces an exceptional optical signature, with orange (300 K) to red (4 K) emission, far from the traditional white-to-yellow emission expected for a Dy<sup>III</sup> ion.

## 1. Introduction

Synthetic and physico-chemical studies in highly symmetrical molecules represent fundamental topics in molecular design, since such molecules provide ideal experimental models to delineate structure–properties relationships. Their structural features make it possible to isolate and study the effect of symmetry on various properties, avoiding the added complexity introduced by distortions of lower symmetries. In particular, these well-defined systems provide optimal keys to investigate the direct consequences of molecular symmetry on electronic configurations, selection rules, and energy level degeneracy that are otherwise blurred in lower-symmetry systems.

For lanthanide (Ln) complexes, high symmetry often comes with low coordination numbers that can be tricky to obtain and handle. Some remarkable examples are the two-coordinate divalent Yb<sup>II</sup>-complexes [Yb{C(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>],<sup>[1]</sup> [Yb{N(SiPh<sub>3</sub>)<sub>2</sub>}<sub>2</sub>],<sup>[2]</sup>

[Yb{N(SiPh<sub>2</sub>Me)<sub>2</sub>}<sub>2</sub>],<sup>[3]</sup> or three-coordinate trivalent lanthanide compounds [Ln{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>3</sub>], where Ln can be any lanthanide except Pm<sup>III</sup>.<sup>[4]</sup> These discrete complexes were designed long ago, mainly to probe the boundaries of organometallic chemistry. Yet, when they were synthesized, little was known about their magnetic and optical properties.

The interest in low-coordinate Ln-based molecules was rejuvenated in the field of magnetism because of the discovery of the extraordinary magnetic performance of lanthanide-based molecules.<sup>[5]</sup> Indeed, in lanthanide-based single-molecule magnets (SMM),<sup>[6]</sup> the effects of the symmetry of the coordination polyhedron<sup>[7]</sup> and that of the associated electrostatic potential<sup>[8]</sup> have been extensively investigated, both from the experimental<sup>[9]</sup> and theoretical point-of-views.<sup>[10]</sup> The oblate/prolate empirical approach for the stabilization of the largest magnetic ground state is now widely used to design high-performing SMM.<sup>[10a]</sup> Yet, this entails that the notion of ideal symmetries cannot be applied in a similar way for all trivalent and paramagnetic lanthanides. Two categories can be distinguished, depending on the shape of the most stable electronic density of the free-ion ground state: a sphere flattened along one axis, known as the "oblate" shape for Ce<sup>III</sup>, Pr<sup>III</sup>, Nd<sup>III</sup>, Tb<sup>III</sup>, Dy<sup>III</sup>, and Ho<sup>III</sup> ions, while Pm<sup>III</sup>, Sm<sup>III</sup>, Er<sup>III</sup>, Tm<sup>III</sup>, and Yb<sup>III</sup> ions prefer an elongated "prolate" shape along the same axis. This property combines with the splitting of the low-lying multiplet by the crystal field and the relative strength of its ground-state magnetic moment. Accordingly, the archetypal representative for optimal SMM behavior of "oblate" ions is Dy<sup>III</sup>, while Yb<sup>III</sup> and Er<sup>III</sup> are the best "prolate" ones. The terms "oblate"/"prolate" for lanthanide ions are thus a misnomer, but they are now widely implanted.

Many Dy<sup>III</sup>-based SMMs are known, because ideal axial coordination is reasonably accessible. For instance, hexagonal (D<sub>6h</sub>),<sup>[11]</sup>

N. Jain, G. David, M. Cordier, Y. Suffren, B. L. Guennic, Y. Sarazin, K. Bernot
INSA Rennes
CNRS
ISCR (Institut des Sciences Chimiques de Rennes)
UMR 6226
Univ Rennes
F 35000 Rennes, France
E-mail: yann.sarazin@univ-rennes.fr
kevin.bernot@insa-rennes.fr

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Supporting information for this article is available on the WWW under https://doi.org/10.1002/ceur.202500193

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© 2025 The Author(s). ChemistryEurope published by Chemistry Europe and Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
<figure><img src="image_5.png" /></figure>

pentagonal bipyramidal geometry (D₅h),[¹²] or compressed octa-
hedral geometry (D₄)[¹³] have been largely investigated. The most
striking example is undoubtedly the high-performing SMM family
of dysprosocenium cations [Dy(Cp')₂]⁺ (Cp' = substituted cyclo-
pentadienyl)[¹⁴] that aims for Dₐh symmetry with a linear Cp'-Dy-
Cp' axis. Spectacular results have been achieved, even if structural
distortions (bent Cp'-Dy-Cp' angles, nonuniform Dy-C distances)
lower the effective site symmetry.[¹⁵] These compounds have
made it possible to record SMM behavior up to 80 K,[¹⁵b] whereas
the ceiling temperature was previously of only a few dozen
Kelvins.

On the contrary, examples with “prolate” Yb<sup>III</sup> and Er<sup>III</sup> are less common,[¹⁶] because strict equatorial coordination of the negatively-charged ligands is difficult to achieve. It requires the use of ligands sufficiently bulky to warrant a coordination number of three only, but small enough to fit in the equatorial plane of a given lanthanide. The sterically encumbered [Ln{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>3</sub>] trisamides are prime candidates to observe SMM behavior with such ‘prolate’ ions.

Accordingly, [Er{N(SiMe₃)₂}₃] was the first SMM obtained with an ion with prolate electronic density,[¹⁷] owing to its almost perfect equatorial coordination only marginally tweaked by Er···H—C agostic interactions. More recently, we reported the study of its Yb analog, [Yb{N(SiMe₃)₂}₃], which indeed featured the targeted structural and electronic features.[¹⁸] Yet, against all expectations, it did not behave as a good SMM. As a matter of fact, the oblate/prolate guideline is only one among several others to design SMMs. In this specific case, we surmised the vibronic features inherent to the relatively small N(SiMe₃)₂ disilazides induced fast relaxation in this molecule, and as a result, poor SMM performance.

However, the [Yb{N(SiMe₃)₂}₃] molecule offers a major advan-
tage in order to rationalize its magnetic properties: it is highly
luminescent, which allows for precise magneto-optical correla-
tions to be drawn. This recent approach[¹⁹] makes use of a prop-
erty of the excited state, the luminescence emission, to probe a
property of the ground state, the magnetic behavior.[²⁰] By care-
fully monitoring the radiative transition between the first excited
state and the ground state of a lanthanide at the lowest achiev-
able temperature, one can correlate the energy splitting of the
ground-state multiplet with the energy splitting of the corre-
sponding emission peak. This is particularly straightforward with
Yb derivatives, where only four transitions are targeted (²F<sub>5/2</sub> →
²F<sub>7/2</sub>, ²F<sub>5/2</sub>, ²F<sub>3/2</sub>, and ²F<sub>1/2</sub>) in a low-energy part of the spectrum,
thus allowing for smooth peak deconvolution (Figure 1).

In [Yb{N(SiMe₃)₂}₃], the optimal repartition of the three negative charge in a near-trigonal symmetry induces an exceptional ground-state energy splitting of 1321 cm⁻¹, the closest reported to the value predicted theoretically for an ideal C₃ environment,[¹⁰b] while common eight-coordinate Yb³⁺ complexes provide total ground-state splitting of, at best, 400 cm⁻¹. [Yb{N(SiMe₃)₂}₃] hence provided a textbook example of near-perfect ground-state stabilization in the case of prolate free-ion electronic density.

With these [Ln{N(SiMe₃)₂}₃] amides of near-ideal trigonal planar geometry in hands, we thought the investigation of the magnetic and optical properties of the lanthanide ion least suited to an all-equatorial coordination, Dy<sup>III</sup>, would be valuable in the context of a more general study of SMMs designed from oblate/prolate considerations. Beyond its sheer magnetic properties,<sup>[17]</sup> we are presenting herein a comprehensive magneto-optical study of [Dy{N(SiMe₃)₂}₃] that also reveals an unexpected and exceptional

<figure><img src="image_6.png" /><figcaption>**Figure 1.** (Double column): representation of ideal coordination environments and magneto-luminescent correlations on typical Dy<sup>III</sup> “oblate” (top) and Yb<sup>III</sup> “prolate” (bottom) molecules. The purple ellipse represents the most stable electronic density of the free-ion ground state, the red arrows are the orientation of the magnetic anisotropy axis, and the gray spheres are the electrostatic charges of hypothetical monoanionic ligands. Emission spectra associated with the transition toward the ground states with peak indexation compared with the electronic levels of the ideal ground state multiplet (labeled as KD). Adapted from refs. [18,19b,21]</figcaption></figure>
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orange-to-red emission, arising from the high-symmetry environment around the metal ion.

## 2. Results and Discussion

### 2.1. Molecule Design and Crystal Structure Description

The known [Dy{N(SiMe₃)₂}₃] was prepared by adapting a protocol from Bradley et al.[⁴] Salt metathesis between DyCl₃ and Li{N(SiMe₃)₂} in THF, followed by extraction in petroleum ether, reproducibly afforded colorless crystals suitable for Single Crystal-X-Ray Diffraction (SC-XRD) analyses in ca. 60% yield. The compound crystallizes in a trigonal P̅-3 space group. The Dyill ion sits 0.565 Å above the average plane defined by the three nitrogen atoms, resulting in a slight pyramidalization with C₃v symmetry (Figure 2). The Dy1–N1 bond length of 2.218(7) Å is unremarkable. The complex features a pattern of agostic interactions involving several, but not all, methyl substituents. Hence, the Dy1–C5 interatomic distance of 2.920(7) Å with the interacting C5 atom is substantially shorter than the pertaining Dy1–C1 one (3.778(106) Å) to the noninteracting C1 atom. There is also a large discrepancy in the corresponding Dy1–Si1 (3.181(2) Å) and Dy1–Si2 (3.554(33) Å) distances due to the presence of Dy···H···C···Si agostic contacts involving Si1 but not Si2. Finally, the agostic interaction also generates perturbation of the pertinent angles; for instance, Dy–N1–Si1 (107.35(3)°) is noticeably less obtuse than Dy–N1–Si2 (128.24(4)°). Beyond the crystalline structure, continuous shape measurements indicates that the coordination of the Dyill in [Dy{N(SiMe₃)₂}₃] is best described by a slightly distorted C3v site symmetry (Table S1, Supporting Information).

### 2.2. Theoretical Approach

The energetic scheme of the molecule [Dy{N(SiMe₃)₂}₃] was obtained using ab initio SA-CASSCF/RASSI-SO calculations (see Supporting Information for computational details).

Based on our experience of the Yb derivative, [Yb {[(N(SiMe₃)₂)₃],[¹⁸] full optimization of the atomic positions from

crystallographic determined positions could not be performed on [Dy{N(SiMe₃)₂}₃]. Indeed, such an approach is not able to properly model the Dy···H···C agostic interactions, and by underestimating them, induces a drastic displacement of the Dyill in the center of the trigonal plane defined by the three coordinating N atoms. Since this deviation from the ideal trigonal symmetry is crucial for the reliable determination of the energetic scheme of the molecule, only the positions of the hydrogen atoms were optimized. Pseudospin Hamiltonian tensors and wavefunction composition of the Dyill multiplet are gathered in Table S2 and represented in Figure 3 and S1, Supporting Information.

As shown in Figure 3a, [Dy{N(SiMe₃)₂}₃] molecule presents a very peculiar energetic profile as a consequence of having an oblate ion surrounded by an almost perfect equatorial electrostatic environment: 1) all Kramers doublets (KDs) are almost pure Mⱼ states, with their main component representing at least 96% of the wavefunction, a finding that is very rare on molecular compound; 2) the KD order is reversed compared to conventional Dy-SMM with, the lowest KD as Mⱼ = ±1/2, and the highest as Mⱼ = ±15/2; 3) a very weak splitting is observed between KD1 and KD2 (44.8 cm⁻¹), and 4) the ground state (KD1) magnetic anisotropy is strongly affected with remarkably large contributions over the transversal plane, with gₓ = 11.1, gᵧ = 10.0, gz = 1.4 (Table S2). The opposite is observed on KD8, that is strongly axial, with gₓ = 0.0, gᵧ = 0.0, gz = 19.7). This results in a striking representation of the ellipsoids of the g tensors (Figure 3b). The comparison with the Yb derivative, [Yb{N(SiMe₃)₂}₃],[¹⁸] is particularly telling. Indeed, the Ybill that is ideally adapted for trigonal electrostatic environment showed almost perfect axial (KD1: gₓ = 0.26, gᵧ = 0.26, gz = 7.91) and in-plane (KD4: gₓ = 4.56, gᵧ = 4.53, gz = 1.13) anisotropies for the lowest (KD1) and highest (KD4) states, respectively. All these findings highlight that [Dy{N(SiMe₃)₂}₃] is a perfect textbook example of ill-designed SMM and a further proof of the pertinence of the oblate/prolate approach.

### 2.3. Magnetic Investigation

Magnetic measurements were performed on [Dy{N(SiMe₃)₂}₃] to evaluate the impact of such a near-perfect trigonal ligand field

<figure><img src="image_8.png" /><figcaption>**Figure 2.** Representation of the molecular solid-state structure of [Dy{N(SiMe₃)₂}₃] along (left) and perpendicular (right) to the C3 axis of the molecule. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Dy1–C5 = 2.920(7), Dy1–N1 = 2.218(7), Si1–C4 = 1.851(9), Si1–C5 = 1.896(9), Si1–C6 = 1.894(10), Si2–C1 = 1.857(9); Si2–C2 = 1.863(10); Si2–C3 = 1.865(10); N1–Dy1–N1 = 113.76(15), Si2–N1–Si1 = 123.91(4).</figcaption></figure>
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<figure><img src="image_10.png" /><figcaption>**Figure 3.** (double column) a) Energies (in cm⁻¹), wavefunction composition, and projected µz (in µB) values along the ground magnetic axis for the eight KD of [Dy[N(SiMe₃)₂]₃] (left) and 4 KDs of [Yb[N(SiMe₃)₂]₃]¹⁸ (right). The values of the magnetic (i.e., isotropic Zeeman) transition moments between the states are given for comparison. The values in red correspond to quantum tunneling of the magnetization (for the ground state) and thermally-assisted quantum tunneling of the magnetization (for the excited states) mechanisms of the magnetization relaxation, whereas blue and green values correspond to Orbach mechanisms. b) Representation of the calculated magnetic anisotropy tensors for the lowest and highest KD of [Dy[N(SiMe₃)₂]₃] (left) and [Yb[N(SiMe₃)₂]₃] (right). Representation of all KD tensors available in Supporting Information.</figcaption></figure>

around the Dy''' ion. Static magnetic measurement shows χ<sub>M</sub>T<sub>(220K)</sub> value of 14.03 emu. K. mol<sup>-1</sup> in line with the one expected at room temperature for an isolated Dy''' ion (14.17 emu. K. mol<sup>-1</sup>, *J* = 15/2, *g*<sub>J</sub> = 4/3). As expected, when the temperature is lowered, χ<sub>M</sub>T shows a monotonous decrease as the ground state multiplet levels are depopulated. No open hysteresis is observed at 2 K. Remarkable agreement is found between experimental and theoretical χ<sub>M</sub>T versus T and M versus H curves, validating the theoretical approach previously described (Figure S2–S4, Supporting Information).

Dynamic magnetic measurements were performed and as
expected no out-of-phase signal is observed either without or
with external dc field, in line with theoretical predictions. This
is a very rare feature on Dy'''-based compounds and is further
proof of the purity of the compound. Indeed, if the Dy''' ion were
in another environment than the trigonal one provided in

[Dy{N(SiMe₃)₂}₃], a small but measurable out-of-phase contribu-
tion should be observed, at least under the application of an
external dc field. The null contribution observed here confirms
that the compound is pure from any by-product that would
be less symmetrical and somehow slowly relaxing. Incidentally,
this could be an explanation of the unexpected out-of-phase
signal observed in previous studies.[17a]

**2.4. Luminescence and Magneto-Optical Correlation**

In order to elaborate magneto-optical correlation on
[Dy{N(SiMe₃)₂}₃] and to validate the energetic scheme obtained
via theoretical simulations, luminescence measurements were
performed at 4 K. Detailed spectra are gathered in Supporting
Information (Figure S5 –S9).
<figure><img src="image_11.png" /></figure>

No antenna effect was observed, and the radiative de-
excitation toward the ground state multiplet was best detected
for λ<sub>exc</sub> = 299 nm. Emission peaks associated with the <sup>4</sup>F<sub>9/2</sub> → <sup>6</sup>H<sub>15/2</sub>
transition are visible in the 473–507 nm range. A detailed analysis
shows that seven of the eight expected peaks corresponding to
the splitting of the <sup>6</sup>H<sub>15/2</sub> ground state multiplet are observed
(Figure 4), the remaining one being possibly hidden in a peak
shoulder (black circle in Figure 4). As expected, the total energy
splitting of these peaks is small and corresponds to ΔE<sub>(exp)</sub> =
1354 cm<sup>-1</sup> (ΔE<sub>(theo)</sub> = 1212.5 cm<sup>-1</sup>) with a very small KD1–KD2
separation of ΔE<sub>1,2(exp)</sub> = 44.6 cm<sup>-1</sup> (ΔE<sub>1,2(theo)</sub> = 44.8 cm<sup>-1</sup>).
These values agree with the very small energy barrier for magnetic
relaxation (Effective energy barrier = U<sub>eff</sub>) expected for such sym-
metry.<sup>[10b]</sup> Overall, when comparing the relative peak positions and
the values of the energetic levels obtained by ab initio calculations,
a remarkable agreement is observed (see Figure 4 and Table S3,
Supporting Information) and KD separations are well reproduced.

It is worth noting that the levels resulting from the splitting of
the ground state by the crystal field are those of interest for
magneto-structural correlations. They are often called via their
*Mj* number mainly because of spectroscopists' habits. However,
this can be largely misleading as in most cases these levels do not
have a pure *Mj* composition. Accordingly, they could be better
called 'crystal-field levels' or even KD when dealing with lanthanide
Kramers ions (Ce<sup>III</sup>, Nd<sup>III</sup>, Sm<sup>III</sup>, Gd<sup>III</sup>, Dy<sup>III</sup>, Er<sup>III</sup>, Yb<sup>III</sup>). Each of the KD,
being then described by a wavefunction composition that mixes
different *Mj* states, depending on the symmetry provided by the
crystal field that can be very far from ideal. As a result,
[Dy{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>3</sub>] is (together with [Yb{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>3</sub>]<sup>[18]</sup> one of the very
few molecular examples where sublevels could be reliably called
using *Mj* notation, reconciling spectroscopic and magnetic
notations.

Beyond the magneto-structural correlations made on the ground state, the other emissive levels, and therefore, the radiative transitions arising from them, are also affected by [Dy{N(SiMe₃)₂}₃]¹'s particular symmetry. Indeed, the total visible emission spectrum of this molecule is remarkable. At 4 K, the emission is dominated by a main peak at extremely high wavelength for a ⁴F₉/₂ → ⁶H₁₃/₂ transition of a Dyill ion, that is, 604.5 nm. This, combined with other emissions around 672.5 nm (⁴F₉/₂ → ⁶H₁₁/₂) and 740 nm (⁴F₉/₂ → ⁶H₉/₂), affords a total emission spectrum that is remarkably red-shifted with compared with standard Dyill-compounds (Figure S5–S9, Supporting Information). The lifetime collected for the emission wavelength around 604.5 nm at 4 K is 3.4(1) µs (λexc = 299 nm, see Table S4, and Figure S10, Supporting Information), while the quantum yield measured under ff excitation at 346.5 nm led to a value of 1.0(1)%.

In fact, Dyill is scarcely used as phosphor compared with Ceill
(blue), Tbill (green), Eu (blue as EuII, red as EuIII), and Smill (orange).
It presents a dual blue and yellow emission with main peaks at
≈480 nm (⁴F₉/₂ → ⁶H₁₅/₂ transition) and ≈575 nm (⁴F₉/₂ → ⁶H₁₃/₂
transition) able to produce white light with CIE coordinates[22] of
(0.33, 0.34),[23] (0.322, 0.339),[24] (0.309, 0.334),[25] and even white-
light emitting SMMs (≈0.33, ≈0.33).[26] In solution, common Dyill
solvates that are found 8 or 9-coordinated, show mainly yellow
emission from the ⁴F₉/₂ → ⁶H₁₃/₂ transition,[21] as already also
observed on other Dy-SMM (CIE of 0.46, 0.46).[27] However, on
[Dy{N(SiMe₃)₂}₃], the anomalous position of the main ⁴F₉/₂ →
⁶H₁₃/₂ transition at 4 K imposes CIE coordinates of (0.469, 0.328)
and so a remarkable deep-red emission (Figure 5). As the tem-
perature is raised toward 300 K, the main emission peak is
progressively even more red-shifted, reaching 608 nm, but is
also associated with the appearance of a rich and blue shifted
peak substructure in the 560–608 nm range. The combination

<figure><img src="image_12.png" /><figcaption>**Figure 4.** Solid-state excitation (λ<sub>em</sub> = 604.5 nm), and emission spectra (λ<sub>exc</sub> = 299 nm) measured at 4 K for [Dy{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>3</sub>] with main transitions evidenced; in inset a zoom on the peak resulting from the ⁴F<sub>9/2</sub> → ⁶H<sub>15/2</sub> transition (dark disks indicating the peaks used for the determination of the KD splitting) and comparison with the ground state splitting extracted from calculations (red lines) from lowest (KD1) to highest (KD8).</figcaption></figure>
<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 5.** Solid-state emission (λ<sub>exc</sub> = 299 nm) measured between 4 and 300 K for [Dy{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>3</sub>] with main transitions evidenced; in inset CIE coordinates obtained at 4 K (0.469, 0.328, red) and 300 K (0.548, 0.396, orange) and representative sample under UV light (312 nm).</figcaption></figure>

of both phenomena induces an orange emission at room temper-
ature (CIE 0.548, 0.396). Such substructures in the emission peaks
are also visible but to a lesser extent on the other transitions. This
intriguing feature is likely to be a vibronic signature, that is usually
hardly visible on less symmetrical and more rigid molecules.
Similar features have been observed on [Yb{N(SiMe<sub>3</sub>)<sub>2</sub>}<sub>3</sub>] and were
confirmed by its dynamic magnetic behavior where, despite a
large multiplet splitting, only vibronic Raman magnetic relaxation
was observed.[18]

To the best of our knowledge, this is the first time that such
optical behavior combining red (4 K) to orange (300 K) emission
and large peak substructures is observed on pure Dy" molecules
acting as emissive phosphors.

**3. Conclusion**

In conclusion, we have performed a complete magnetic and opti-
cal analysis of [Dy{N(SiMe₃)₂}₃], a Dy"'-based complex featuring an
almost ideal trigonal electrostatic environment. This high symme-
try provides a remarkably pure electronic scheme, and, as con-
firmed through theoretical analysis and magneto-optical
correlations, the equatorial electrostatic environment around
the Dy" ion makes it the worst possible SMM owing to a totally
reversed energetical scheme compared with conventional SMMs.
The comparison with its Yb derivative provides a textbook exam-
ple of the efficiency of the prolate/oblate theory often applied to
stabilize highly magnetic ground states of paramagnetic mole-
cules. This original energetic profile and crystal-field level splitting
also impact on the optical properties, resulting in a spectacular
emission color change of the Dy" ion, usually yellow and

sometimes white, but that here shifts from orange (300 K) to
red (4 K). These findings demonstrate that highly symmetrical
molecules are particularly potent and convenient objects for
the rationalization of physico-chemical behavior and also reveal
unprecedented physico-chemical molecular properties otherwise
hidden on less symmetrical compounds. We are continuing our
efforts along these lines, and with the hope that our findings will
stimulate similar endeavors in the community.

Acknowledgements

The authors acknowledge Région Bretagne for the provision of
SAD 2022 research grant (MOL4DATA), and ANR NANOCHAINS
(grant), CPER Mat&Trans for low-temperature optical cryostat
funding, Rennes Metropole, and INSA Rennes. Thierry
Guizouarn from ISCR "Centre de mesures physiques" is thanked
for his assistance in collecting magnetic data. Remi Marchal is
acknowledged for the development of aniso_to cube tool.
Dr. O. Maury, Prof. O. Cador, and Dr. F. Pointillart are acknowl-
edged for discussions.

**Conflict of Interest**

The authors declare no conflict of interest.

**Data Availability Statement**

The data that support the findings of this study are available from
the corresponding author upon reasonable request.
<figure><img src="image_15.png" /></figure>

**Keywords:** dysprosium · lanthanides · luminescence · magneto-optical correlations · prolate · single-molecule magnets

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