Perfluoroalkyl Chain Length Effect on Crystal Packing and [LnO₈] Coordination Geometry in Lanthanide-Lithium β-Diketonates: Luminescence and Single-Ion Magnet Behavior

Abstract

Functionalized perfluoroalkyl lithium β-diketonates (LiL) react with lanthanide(III) salts (Ln = Eu, Gd, Tb, Dy) in methanol to give heterobimetallic Ln-Li complexes of general formula [(LnL₃)(LiL)(MeOH)]. The length of fluoroalkyl substituent in ligand was found to affect the crystal packing of complexes. Photoluminescent and magnetic properties of heterobimetallic β-diketonates in the solid state are reported. The effect of the geometry of the [LnO₈] coordination environment of heterometallic β-diketonates on the luminescent properties (quantum yields, phosphorescence lifetimes for Eu, Tb, Dy complexes) and single-ion magnet behavior (U_eff for Dy complexes) is revealed.

1. Introduction

Advances in lanthanide coordination chemistry continues to be the cornerstone in the development of promising sensitizers, agents for theranostics, catalysts, and optical and magnetic materials [1,2,3,4,5,6,7,8]. Exploring the luminescent and magnetic properties of Ln^III compounds allows the structure–property correlations to be determined for further optimization of the ligand environment.

One of the bright areas of β-diketones application as ligands is the sensitization of Ln^III ions [9,10,11,12]. A number of lanthanide complexes (mainly Ln = Eu^III and Tb^III) containing fluorinated β-diketonates were found to exhibit both long luminescence lifetimes and high photoluminescence quantum yields (PLQY) [13,14,15,16,17,18,19]. The main strategy to improve the luminescent properties of Ln^III complexes based on β-diketonates includes using the heterocyclic coligands with an extended π-system, phosphine oxides, sulfoxides [13,14,15,16,17,18,19,20,21,22]. Increasing the length of fluoroalkyl groups in β-diketonate anions is another important tool in fine-tuning the optical properties of compounds [17,18,23]. However, less attention has been paid to the ligand structure and cation nature variation when designing Ln^III tetrakis-diketonates [24,25,26,27,28,29,30,31]. Despite this, the directed synthesis of compounds with a [EuO₈] environment is highly relevant, which is confirmed by the high quantum yield value (equal to 85%) reported for Eu(dik)₃(Ph₂SO)₂ [32].

Over the past two decades, the magnetic properties of lanthanide complexes have been extensively studied. In particular, the introduction of the Dy^III ion into a suitable ligand environment remains among the most efficient strategies in the design of single-ion magnets (SIMs) for advanced memory storage technologies and spintronic devices [33,34,35]. The ligand structure manipulation has been regarded to increase the energy barrier for spin reversal and/or blocking the temperature of magnetization. While a considerable effort has been made to enhance the SIMs performance, more detailed approaches are still required to elucidate the origin of slow relaxation, as well as the impact of ligand structure and crystal packing to obtain the improved SIMs. According to previous studies on high-performance SIMs, highly symmetrical Dy^III-based SIMs, such as D_4d, D_5h and D_6h, provide reduced electron repulsion around the lanthanide ion and the ^mJ = ±15/2 state stabilization, thereby preventing the quantum tunnelling magnetization (QTM) [36,37,38,39,40]. In this context, the structures having approximately square-antiprismatic or dodecahedral coordination polyhedra are of much interest because of their high potential as SIMs. The magnetic relaxation behavior of these SIMs has been shown to be affected by distortion of the coordination sphere, typically caused by subtle modification of the lattice solvent, auxiliary ligand, and intermolecular interaction [41,42,43,44,45,46,47,48]. Therefore, the anisotropy of the lanthanide ions and their magnetic dynamics are strongly influenced by both the ligand field and coordination geometry [36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51]. However, it is still a big challenge to control the coordination geometry around the lanthanide ions in order to understand how it affects the relaxation mechanism. For this reason, shaping the geometries with different organic ligands has become the main strategy in most studies.

One of the first examples of a single-molecular magnet was tris-diketonate [Dy(acac)₃·2H₂O], in which the replacement of water molecules by diazaaromatic ligands increased the energy barrier [52,53,54]. Since then, β-diketonates have remained an essential tool in the field of molecular magnetism [55,56,57,58,59,60,61,62,63,64,65]. Varying different substituents in the diketonates can modify the lanthanide crystal field, including by changing the geometry of [LnO₈] or [LnO₇] polyhedra [51,61,64]. Furthermore, the electrostatic potential (ESP) around the central Dy^III ions indirectly depends on the electron-donating/withdrawing properties of the auxiliary ligands. This opens the way to the regulation of magnetic dynamics, especially in the suppression of fast relaxation [62]. In particular, the electron-withdrawing effect of fluorinated β-diketone was demonstrated to increase the magnetic relaxation barriers [62].

However, in heterometallic lanthanide-alkali metal β-diketonates, the Ln^III ion is surrounded by four identical β-diketones to form the [LnO₈] environment [64,65,66]. In this case, we can observe how minor changes in geometry of the coordination polyhedron [LnO₈] induced by the different fluoroalkyl substituents affect the properties of the complexes. Therefore, this class of molecules provides an ideal model system for studying the crystal field effect on magnetic properties.

Because there are very few examples of such compounds, their detailed magnetic studies are scarce to date. As far as we know, slow magnetization relaxation has been reported for only two complexes of this type: [Cs{Dy(Ph₂acac)₄}]_n (Ph₂acac = dibenzoylmethanide) and [M{Ln(hfac)₄}] (M = K, Cs; Ln = Dy, Er) [64,65].

Previously, we have synthesized a series of heterometallic complexes based on asymmetric fluorinated β-diketonates (L¹, L²) (Figure 1) [66,67,68,69,70,71,72]. To the best of our knowledge, these structures are the first described examples of [Ln-Li] β-diketonates. Their luminescent and magnetic properties have been studied in detail [66]. Particularly, the single-molecular magnet properties were found for [(DyL¹₃)(LiL¹)(MeOH)] (1) with trifluoromethylated β-diketonate as L¹ (Figure 1). Its energy barrier separating two opposite magnetic states was 37.7 cm⁻¹ [66].

The current work aims to investigate the correlations between the structure and magneto-optical properties of Ln^III complexes with a similar [LnO₈] coordination environment. By varying the fluoroalkyl substituent in the β-diketonates (from L¹ (R^F = CF₃) to L⁴ (R^F = C₄F₉)), we have demonstrated how the crystal packing, intermolecular interactions, and minor changes of [LnO₈] geometry influence the properties of luminescent and magnetoactive molecules.

2. Results and Discussion

2.1. Synthesis of Dinuclear [Dy-Li] β-Diketonates

Ln^III-Li^I diketonates 1–16 were synthesized through the reaction of functionalized fluorinated diketonates with corresponding lanthanide(III) chloride in methanol or ethanol as a solvent (Scheme 1 and Scheme 2),

Table 1. A list of complexes 1–16 with CCDC numbers.

Compound	Ln	RF	Ln	CCDC	Ref.
1	L1	CF3	Eu	1855391	[66]
2	L1	CF3	Gd	2031096	[68]
3	L1	CF3	Tb	1855392	[66]
4	L1	CF3	Dy	1855393	[66]
5	L2	C2F5	Eu	2258787	this work
6	L2	C2F5	Gd	2258788	this work
7	L2	C2F5	Tb	2011088	[70]
8	L2	C2F5	Dy	2255120	this work
9	L3	C3F7	Eu	2258789	this work
10	L3	C3F7	Gd	2258790	this work
11	L3	C3F7	Tb	2258791	this work
12	L4	C4F9	Eu	2258792	this work
13	L4	C4F9	Gd	2258793	this work
14	L4	C4F9	Tb	2258794	this work
15	L4	C4F9	Dy	2255119	this work
16	L1	CF3	Dy	1855396	[66]

) according to the previous reports [66,68,70]. Changing the solvent from methanol to ethanol leads to the formation of heterometallic diketonate 16, in which the geometry of the [LnO₈] coordination polyhedron and crystal packing differ from those of complex 4 [66].

2.2. Structure of Dinuclear [Ln-Li] β-Diketonates

The structures of CF₃-containing [Ln-Li] β-diketonates 1–4, 7 and 16 have been described before [66,68,70]. In this work, we have synthesized and characterized heterometallic complexes 5, 6, 8–15.

Based on the XRD data, the structures of novel discrete heterometallic complexes 5, 6, 8–15 are similar to that of trifluoromethylated analogue 1 and correspond to a composition of [(LnL₃)(LiL)(MeOH)] (Figure 2, Figure 3 and Figure 4, Table 1 and Table S1). XRD powder patterns of complexes 4, 7, 10, 14 with different L coincide with the theoretical ones calculated for the dysprosium(III), terbium(III) and gadolinium(III) complexes (Figures S2–S5).

In the series of complexes 1–16, the coordination sphere of Ln atoms includes eight oxygen atoms O(1) → O(8) from four β-diketonate molecules. Three ligand molecules form three six-membered 4f-metallocycles due to the β-diketonate-anions and result in a tris-diketonate lanthanide fragment (Figure 5a). The fourth β-diketonate molecule is an initial LiL, which coordinates the lanthanide ion via one methoxy group (O(7)) and oxygen atom of the β-diketonate-anion (O(8)) to give a heterometallic complex structure (Figure 5b). Similarly, one of the molecules from the tris-diketonate fragment coordinates with the Li atom. Methanol molecule completes the coordination sphere of penta-coordinated Li(I) ion. The values of the O-Ln-O angles in the five- and six-membered metallocycles of complexes 1–16 and the bond lengths between the Ln and O(1)–O(8) atoms are given in SI (Tables S2–S9).

In complexes 1–16, the bond lengths of the Ln atom with the bridging oxygen atoms O(2) and O(8) are reasonably longer than the corresponding Ln-O bonds of the tris-diketonate fragment (Tables S6–S9). Varying the length of the fluoroalkyl substituent from the CF₃ to C₃F₇ group in diketonates leads to obvious changes in both the Li···Ln distance (~3.49 vs. 3.53) and the Li-O(7)-Ln angle (~104° vs. 106°) (Tables S2–S9).

The shortest distances between the Ln centers in complexes indicate some specific features of their crystal packing (Figures S6–S17). For example, the Ln···Ln distance increases only when the L¹ diketonate is replaced by L² one. Trifluoromethyl complexes [LnL¹₃)(LiL¹)(MeOH)] 1–4 with the shortest Ln···Ln distance form a zigzag chain with ∠Ln(1)-Ln(2)-Ln(3)~131° (Figures S3–S5). However, in the case of L²-L⁴ diketonates, we observe linear 1D chains, which lead to layered crystal structures of complexes (Figures S7–S9). The shortest distance between the two adjacent molecules of complex 16 is determined by the presence of intermolecular hydrogen bonds of the coordinated water without molecule chain motif.

Using the SHAPE program [73,74], we have calculated the geometry of coordination [LnO₈] polyhedra of synthesized heterometallic β-diketonates 1–16 (

Table 2. Continuous shape measures (CShM) calculations values for Ln-Li complexes using SHAPE.

	RF = CF3	RF = C2F5	RF = C3F7	RF = C4F9
[EuL3)(LiL)(MeOH)]	1.315 (TDD-8), D2d 1.983 (BTPR-8), C2v	1.055 (TDD-8), D2d	2.052/1.877 1 (BTPR-8), C2v	1.932 (BTPR-8), C2v 2.015 (TDD-8), D2d
[GdL3)(LiL)(MeOH)]	1.253 (TDD-8), D2d 1.941 (BTPR-8), C2v	1.003 (TDD-8), D2d	1.728 (BTPR-8), C2v	1.868 (BTPR-8), C2v 1.963 (TDD-8), D2d
[TbL3)(LiL)(MeOH)]	1.134 (TDD-8), D2d	0.986 (TDD-8), D2d	1.966/1.764 1 (BTPR-8), C2v	0.947 (TDD-8), D2d
[DyL3)(LiL)(MeOH)]	1.144 (TDD-8), D2d	0.894 (TDD-8), D2d	-	0.911 (TDD-8), D2d
[DyL3)(LiL)(H2O)]	1.777 (TDD-8), D2d 1.990 (BTPR-8), C2v	-	-	-

¹ Two S_Q(BTRP-8) values are due to the disordering of oxygen atoms in [LnO₈].

, also see Tables S10–S13). Table 2 shows that increasing the length of fluoroalkyl group in diketonates strongly distorts the triangular dodecahedron (TDD-8) geometry of [LnO₈] polyhedron in Eu and Gd complexes. Calculated values of coordination polyhedra of Tb and Dy complexes are closer to those of the ideal TDD-8 geometry, except for C₃F₇ containing diketonate. In compound 16, the calculated values of the distorted geometry of the Dy coordination environment correspond better to TDD-8 and biaugmented trigonal prism (BTPR-8).

To gain more insight into the solid structure analysis of obtained complexes, we have compared the dihedral angles between the LnOO and β-diketonate planes (Table S14). The length of the fluoroalkyl group in diketonates greatly affects the planarity of the five- and six-membered chelate cycles. As follows from Table S14, the sum of the dihedral angles between the LnOO and β-diketonate planes increases from CF₃ (L¹) to C₄F₉ (L⁴) complexes. The exception is C₂F₅ β-diketonates (L²), for which the deviations from the planarity of the chelate cycles are the smallest in this series. For the five-membered chelate cycles of the complexes, the values of dihedral angles vary irregularly with increasing fluorine atoms in β-diketonates (Table S14).

The IR spectra of 1–16 are similar and display strong absorption bands in the range of 1645–1633 and 1541–1468 cm⁻¹, which is typical for β-diketonates, that can be attributed to enolate C=O and C=C vibrations, respectively (Figure S18) [16]. Typical C–F vibrations are observed in the range of 1198–1122 cm⁻¹ (the most intensive peaks are at 1137–1122 cm⁻¹), and C–H vibrations can be seen between 3000 and 2840 cm⁻¹.

2.3. Mechanoluminescence

Previously, we have observed the mechanoluminescence (ML) of polycrystalline trifluoromethyl Ln-Li diketonates 1, 3 and 4 [66]. However, complexes based on ligands with R^F ≥ C₂F₅ are ML-inactive 5, 7–9, 11, 12, 14, and 15. This indicates the crucial role of structural differences in the crystal packing for ML activity of the complexes.

The crystal packing of trifluoromethyl diketonates 1, 3, 4 consists of zigzag 1D chains of isolated molecules of complexes with the shortest distance between Ln atoms (Figures S7–S9). In complexes based on ligands with R^F ≥ C₂F₅, molecules are arranged lengthwise to form linear 1D chains. The coordinated water molecule in complex 16 forms hydrogen bonds to give dimers as the structural unit with the shortest Dy···Dy distance (Figure S17). Therefore, the molecular herringbone arrangement is a feature of ML activity that is consistent with the reported examples of Ln^III tris-diketonates [13].

2.4. Photoluminescence

The photoluminescence properties of Eu³⁺, Tb³⁺ and Dy³+ complexes are studied in the solid state. Complexes under illumination by a standard laboratory UV lamp at 365 nm clearly showed visible red, green and yellow luminescence depending on the nature of the lanthanide ion. Broad excitation band in the UV region of the spectrum with the maximum at 320–330 nm indicates the ligand-centered absorbance resulted from π* ← π absorption of β-diketonate fragment L (Figure 6). Upon excitation at 340 nm, complexes showed the characteristic emission bands of the corresponding Ln^III ion originating from the following transitions: ⁵D₀ → ⁷F_J for 1, 5, 9, 12 (Figure 7a); ⁵D₄→ ⁷F_J for 3, 7, 11, 14 (Figure 7b); and ⁴F_9/2 → ⁶H_J for 4, 8, 15, 16 (Figure 8).

We have calculated the radiative ($A_{rad}$) and non-radiative ($A_{nrad}$) decay rates, as well as the ⁵D₀ intrinsic quantum yield ($Q_{Ln}^{Ln}$) of the complexes using the following Equations:

$$\begin{gathered} A_{rad}=A_{MD,0}{n}^3\frac{I_{total}}{I_{MD}} \\ {A}_{nrad}=\frac{1}{{\tau }^{obs}}-A_{rad} \\ {Q}_{Ln}^{Ln}={\tau }^{obs}·A_{rad} \end{gathered}$$

where A_MD,0 is the spontaneous emission probability of the magnetic dipole ⁵D₀ → ⁷F₁ transition, which is equal to 14.65 s⁻¹; n is the refractive index (considered to be 1.5 for solids); (I_total/I_MD) is the ratio of the total integrated area of the corrected Eu³⁺ emission spectrum to the area of the ⁵D₀ → ⁷F₁ band [13,75]; and ${\tau }^{obs}$ is measured ⁵D₀ luminescence lifetime of [EuL₃)(LiL)(MeOH)] 1, 5, 9, 12 (

Table 3. The radiative ($A_{rad}$) and non-radiative ($A_{nrad}$) decay rates, lifetimes (${\tau }^{obs}$), intrinsic ($Q_{Ln}^{Ln}$) and overall ($Q_{Ln}^L$) quantum yields and sensitization effeciency (${ŋ}_{sens}$) of the [Ln-Li] complexes.

Complex	Ln	RF	${\mathit{\tau }}^{\mathit{o}\mathit{b}\mathit{s}}$, μs	${\mathit{A}}_{\mathit{r}\mathit{a}\mathit{d}}$, s−1	${\mathit{A}}_{\mathit{n}\mathit{r}\mathit{a}\mathit{d}}$, s−1	${\mathit{Q}}_{\mathit{L}\mathit{n}}^{\mathit{L}}$(%)	${\mathit{Q}}_{\mathit{L}\mathit{n}}^{\mathit{L}\mathit{n}}$ (%)	${\mathbf{ŋ}}_{\mathit{s}\mathit{e}\mathit{n}\mathit{s}}$ (%)
1	Eu	CF3	690	836	613	53 ± 1	58	91
5		C2F5	701	969	458	39 ± 0.8	68	57
9		C3F7	655	949	578	52 ± 1	62	84
12		C4F9	703	900	522	55 ± 1	63	87
3	Tb	CF3	608	-	-	35 ± 0.7	-	-
7		C2F5	523	-	-	64 ± 1.3	-	-
11		C3F7	405	-	-	47 ± 0.9	-	-
14		C4F9	541	-	-	60 ± 1.2	-	-
4	Dy	CF3	6	-	-	1 ± 0.02	-	-
16		CF3	8	-	-	3 ± 0.06	-	-
8		C2F5	4	-	-	1 ± 0.02	-	-
15		C4F9	4	-	-	1 ± 0.02	-	-

, Figures S24, S27, S30 and S32).

Sensitization efficiency (${ŋ}_{sens}$) for the Eu^III complex was estimated as the ratio of the measured ⁵D₀ luminescence quantum yield ($Q_{Ln}^L$) to the calculated intrinsic quantum yield (Table 3):

$${ŋ}_{sens}=Q_{Ln}^L /Q_{Ln}^{Ln}$$

To elucidate the energy transfer process of the lanthanide(III) complexes, the triplet energy levels of the ligands were estimated. Since the lowest lying excited level (⁶P_7/2 → ⁸S_7/2) of Gd³⁺ is located at 32,150 cm⁻¹, we can determine the ³ππ* energy levels of ligand L¹-L⁴ anions based on the phosphorescence spectra of [GdL₃)(LiL)(MeOH)] (Figures S20–S23). The phosphorescence spectra of compounds 2 (L¹), 6 (L²), 10 (L³) and 13 (L⁴) were recorded at 77 K and their zero-phonon transition energies of triplet states (3ππ*) corresponded to 443 nm (22,540 cm⁻¹), 446 nm (22,420 cm⁻¹), 445 nm (22,480 cm⁻¹) and 450 nm (22,200 cm⁻¹), respectively. The obtained values of L¹-L⁴ triplet levels are lower than that of acac (24,800 cm⁻¹) [62], but close to those of trifluoromethyl non-functionalized diketones, e.g., tfac (22,700 cm⁻¹) and hfac (22,200 cm⁻¹) [62].

Because of the large difference of energy levels in Eu^III complexes, the energy transfer from E_0-0(3ππ*) to ⁵D₂ is the most probable process for them [75]. In Tb^III complexes, the energy levels E_0-0(3ππ*) and ⁵D₄ are close, thus quenching of the luminescence intensity is possible due to the reverse energy transfer from the excited level of Tb^III to ligand.

Since the differences in the triplet levels of the ligands are insignificant, we consider the geometry of the Ln coordination environment as the key factor influencing the luminescence properties. It was previously reported that a low-symmetry ligand environment around Ln³⁺ ion (from D_4d, D_2d to C_3v, C_2v) improves the luminescence efficiency [15]. This correlation is observed in the case of Eu^III and Dy^III complexes. In C₂F₅-diketonate-based complexes, the geometry of the coordination environment [EuO₈] is closest to that of TDD-8 (D_4d), which explains the difference of luminescence characteristics in this series of complexes. The largest deviation of the coordination polyhedron [DyO₈] from the ideal TDD-8 geometry in the case of CF₃-diketonate leads to a significant increase in the quantum yield. In contrast, a number of diketonates with C₂F₅/C₄F₉ substituents have a high quantum yield due to the [TbO₈] geometry close to C_2v symmetry.

2.5. Magnetic Properties

The effective magnetic moment μ_eff temperature dependences for polycrystalline samples of complexes 8, 15, 16 are shown in Figure 9. The room temperature values of μ_eff are 10.74–10.85 μ_B, in accordance with the theoretical value of 10.64 μ_B for Dy^III free ion (⁶H_15/2 ground state with g_J = 4/3). Under cooling, μ_eff gradually decreases for 15, 16 and increases for 8 to 10 K below, and the moment drops to a value of 8–8.9 μ_B by 2 K. Field dependencies of magnetization are nonlinear at low temperatures (Figures S38 and S39). The decrease of μ_eff at low temperatures suggests the mixture of anisotropy and possible intermolecular exchange interactions.

AC magnetic measurements were performed for Dy^III complexes 8, 15 and 16 (Figure 10, Figures S36 and S37). Fast magnetic relaxation is observed under zero dc field due to quantum tunneling magnetization. Applying dc field of 1000 Oe quenches QTM processes, and complexes 8, 15 and 16 exhibit slow relaxation of the magnetization. Frequency dependences of the in-phase (χ′) and out-of-phase (χ″) AC magnetic susceptibility were analyzed using the generalized Debye model:

$$\begin{gathered} {\chi }^{\prime }\left(\omega \right)={\chi }_S+\left({\chi }_T-{\chi }_S\right)\frac{1+{\left(\omega \tau \right)}^{1-\alpha }\sin \left(\frac{\pi \alpha }{2}\right)}{1+2{\left(\omega \tau \right)}^{1-\alpha }\sin \left(\frac{\pi \alpha }{2}\right)+{\left(\omega \tau \right)}^{2-2\alpha }}, {\chi }^{″}\left(\omega \right)={\chi }_T- \\ {\chi }_S\frac{{\left(\omega \tau \right)}^{1-\alpha }\cos \left(\frac{\pi \alpha }{2}\right)}{1+2{\left(\omega \tau \right)}^{1-\alpha }\sin \left(\frac{\pi \alpha }{2}\right)+{\left(\omega \tau \right)}^{2-2\alpha }}, \end{gathered}$$

where χ_T—adiabatic susceptibility, χ_S—isothermal susceptibility, τ—relaxation time and α—relaxation times distribution width parameter. The best fit values of χ_T, χ_S, τ and α are listed in SI (Tables S15–S17).

Arrhenius plots of relaxation time for complexes 8, 15 and 16 are nonlinear (Figure 11), which implies more than just the Orbach relaxation mechanism (${\tau }^{-1}={\tau }_0^{-1}{e}^{-U_{\mathrm{eff}}/{\mathrm{k}}_{\mathrm{B}}T}$). Raman (${\tau }^{-1}=C{T}^n$) and Direct (${\tau }^{-1}=B{H}^{2}T$) processes are possible as alternative relaxation mechanisms, and the best fit of the experimental data for 8, 15 and 16 was achieved by taking into account Orbach and Raman relaxation mechanisms [34,35,37]. Optimal parameter values τ₀ and U_eff for Orbach and C and n for Raman processes are listed in

Table 4. Best fit values of relaxation parameter values for Orbach and Raman processes.

Complex	Ln	RF	Ueff, cm−1	τ0, 10−6 s	C, s−1K−n	n	τ at 4 K, ms
8	L2	C2F5	21 ± 1	11 ± 1	0.029 ± 0.008	5.5 ± 0.2	10.4 ± 0.1
15	L4	C4F9	22 ± 1	9 ± 3	0.09 ± 0.01	5.23 ± 0.07	5.4 ± 0.1
16	L1	CF3	19 ± 1	14 ± 1	0.0011 ± 0.0001	7 (fixed)	12.6 ± 0.3

. Dashed lines in Figure 11 are simulated curves with the obtained best fit parameters for individual Orbach and Raman processes. Fitting the linear region of ln(τ) vs. T dependencies, considering only the Orbach relaxation mechanism, allows us to estimate energy barrier values U_eff, which are close to those listed in Table 4. The obtained U_eff values for 8, 15 and 16 are significantly lower than those for 4 (37.7 cm⁻¹). In the case of 16, the strong linear dependence of ln(τ) at a low temperature implies a domination of the Orbach relaxation mechanism. However, its curvature at a higher temperature indicates the competing relaxation processes, which makes it difficult to obtain Raman process parameters properly. Therefore, n = 7 was fixed to avoid overparameterization for 16. The obtained values of n are expected for Kramer Ln^III ions, for which n may be in the range of 5–9; the deviation may be caused by differences in the crystal field strength [76].

Magnetic anisotropy and crystal field strength are strongly related to the environment of the lanthanide metal ion. The coordination geometry of the Dy atom in the complexes 4, 8, 15 and 16 is close to the triangular dodecahedron (TDD-8). Comparison of distortion of the [DyO₈] polyhedron from the TDD-8 shape with thermal barrier energy did not reveal correlations between S_Q(TDD-8) and U_eff values (Table 4 and Table S13 and Figure 12). Considering that the coordination geometry of the Dy atom as a square antiprism (SAPR-8) shows that decreasing S_Q(SAPR-8) values correlates with increasing energy barriers, the highest energy barrier is found for 4 with R^F = CF₃ with the lowest S_Q(SAPR-8) value. An increase in fluorinated substituent size (R^F ≥ C₂F₅) in 8 and 15 or change of Li substituent to H₂O in 16 leads to distortion of the [DyO₈] polyhedron from the SAPR-8 shape, and thereby decreases the thermal barrier U_eff. Therefore, both variations of Li and diketonate substituents affect the [DyO₈] local symmetry as a result of modified crystal packing, and the greater correspondence to the ideal TDD-8 shape of the Dy^III polyhedron does not promote SIM behavior of the complex, whereas the square antiprismatic coordination environment is more favorable for higher thermal barrier U_eff.

3. Experimental

3.1. Materials and Methods

All reactions were carried out in air. Lanthanide salts TbCl₃·6H₂O (99.99%), DyCl₃·4H₂O (99.99%) were obtained from Alfa Aesar (Lancashire, UK), EuCl₃·6H₂O (99.99%) and Gd(NO₃)₃·5H₂O (99.9%), which were obtained from Merck and used without further purification.

The lithium β-diketonates (L¹, L², L³, L⁴) were synthesized according to the previously reported procedures [77,78,79,80].

IR diffuse-reflectance spectra were recorded with a Perkin-Elmer Spectrum One FTIR instrument in the range 400–4000 cm⁻¹. Fluorescence and phosphorescence spectra were recorded in the solid state on a Varian Cary Eclipse fluorescence spectrophotometer with mutually perpendicular beams. The emission lifetimes (${\tau }^{obs}$) and quantum yields ($Q_{Ln}^L$) have been measured using FS5 Edinburgh Instruments spectrofluorometer at room temperature with absolute error ±2%; excitation was performed through a ligand, and the absolute method in the integration sphere was used. Elemental analysis was performed using a Perkin Elmer (Waltham, MA, USA) PE 2400 Series II analyzer.

The single-crystal X-ray diffraction studies of 5, 6, 9–14 were carried out on a Bruker D8 Venture diffractometer (Mo-K_α, λ = 0.71073 Å, graphite monochromator, ω/2θ-scanning technique). The intensity data for the single crystals of 8 and 15 were collected by the standard procedure on SMART APEX II CCD (Bruker AXS, Billerica, MA, USA) automated diffractometers (Mo Kα radiation, graphite monochromator, T = 240 K) [81]. Semiempirical absorption correction for 5, 6, 9–14 was applied for all compounds [82]. The structures were solved by direct methods and refined by the full-matrix least squares in the anisotropic approximation for non-hydrogen atoms. The calculations were carried out by the SHELX-2014/2018 program package [83] using Olex2 1.2/1.3 [84]. The hydrogen atoms of the ligands were positioned geometrically and refined using the riding model. Some restrictions were applied when solving the structure (DFIX (for 5, 6, 9–14), DELU (for 5, 6, 10, 12, 13), ISOR (for 5, 6, 10, 12, 13), SADI (for 6, 9–11, 13), EADP (for 9, 11, 14), DANG (for 6, 9–14)). The crystal structure of 5 was solved taking into account the disordering of the methyl and methoxy groups at the C6 (here and below, their population (determined from the Fourier synthesis, except for 8) corresponds to 0.66(2):0.34(2)), C24 (0.771(19):0.229(19)) and C33 (0.76(2):0.24(2)) atoms). The crystal structure of 6 was solved by taking into account the disordering of the methyl and methoxy groups at the C15 (0.68(3):0.32(3)) and C24 (0.76(2):0.24(2)) atoms, and 1,1-dimethoxyethyl group at the C3 atom (0.62(3):0.38(3)). The crystal structure of 8 was solved by taking into account the disordering of fluorine atoms in the CF₂- and CF₃-groups at the C3C, C3D, C6A, C6C, and C6D atoms (0.5:0.5). The crystal structure of 9 was solved by taking into account the disordering of the 1,1-dimethoxyethyl group at the C3 atom (0.847(6):0.153(6)), the 1-methoxyethyl groups at the C13 (0.874(6):0.126(6)) and C33 (0.561(4):0.439(4)) atoms, and part of the diketonate group (O13, C31 and C32 atoms) with the perfluoropenthyl group (0.657(2):0.343(2)). The crystal structure of 10 was solved by taking into account the disordering of the methyl and methoxy groups at the C7 (0.874(7):0.126(7)), C17 (0.932(9):0.068(9)) and C27 (0.874(7):0.126(7)) atoms, the 1-methoxyethyl group at the C32 atom (0.545(10):0.455(10)), and the perfluoropenthyl group at the C30 atom (0.591(4):0.409(4)). The crystal structure of 11 was solved by taking into account the disordering of the 1,1-dimethoxyethyl group at the C3 atom (0.828(9):0.172(9)), the 1-methoxyethyl groups at the C13 (0.859(9):0.141(9)) and C33 (0.515(5):0.485(5)) atoms, and part of the diketonate group (O13, C31 and C32 atoms) with the perfluoropenthyl group (0.518(3):0.482(3)). The crystal structure of 12 was solved by taking into account the disordering of the methyl and methoxy groups at the C8 (0.864(8):0.136(8)), C19 (0.834(9):0.166(9)) and C41 (0.945(8):0.055(8)) atoms. The crystal structure of 13 was solved by taking into account the disordering of the methyl and methoxy groups at the C8 (0.849(7):0.151(7)) and C41 (0.827(8):0.173(8)) atoms, and two fluorine atoms in the CF₃-group at the C7 atom (0.41(2):0.59(2)). The crystal structure of 14 was solved by taking into account the disordering of the 1-methoxyethyl group at the C34 atom (0.583(8):0.417(8)), the methyl group at the O15 atom (0.583(8):0.417(8)), and fluorine atom in the CF₂-group at the C20 atom (0.583(8):0.417(8)).

Powder X-ray diffraction data were collected using a Bruker D8 Advance diffractometer (Bruker, Billerica, MA, USA) (CuKα, λ = 1.54 Å, Ni-filter, LYNXEYE detector, geometry reflection).

The magnetic susceptibility of the polycrystalline samples was measured with a Quantum Design MPMSXL SQUID magnetometer in the temperature range 2–300 K in the magnetic field of 5 kOe. Diamagnetic corrections were made using the Pascal constants. The effective magnetic moment was calculated as µ_eff(T) = [(3k/NAµB2)χT]1/2 ≈ (8χT)^1/2, where k is Boltzman constant, N_A—Avogadro’s number and µ_B—Bohr magneton. Check of the field dependence ac-susceptibility in the range 200–2000 Oe revealed that optimal dc magnetic field is 1 kOe. Therefore, frequency-dependent ac susceptibilities for complexes 8, 15 and 16 were measured under 1 kOe dc field at various temperatures.

3.2. Synthesis of the Compounds 1–16 (General Method)

For a solution of LiL (1 mmol) in 15 mL of methanol (for 1–15) or ethanol (for 16), the Ln^III salt (0.25 mmol) was added and the mixture was stirred at room temperature for 1 h. The resulting solution was slowly evaporated, and solids were washed with water and cold methanol. The polycrystalline products were recrystallized from the corresponding alcohol (MeOH or EtOH) and filtered off through Celite^® 545 to afford a clear solution. Its slow evaporation at 5–10 °C gave colorless or slightly colored crystals suitable for single-crystal X-ray diffraction structure analysis.

[LiEu(L²)₄(MeOH)] (5). Yield 45%. Found: C, 33.97; H, 3.23. Calc. for C₃₇H₄₄EuF₂₀LiO₁₇: C, 34.19; H, 3.41.

[LiGd(L²)₄(MeOH)] (6). Yield 53%. Found: C, 33.88; H, 3.21. Calc. for C₃₇H₄₄F₂₀GdLiO₁₇: C, 34.06; H, 3.40.

[LiDy(L²)₄(MeOH)] (8). Yield 58%. Found: C, 33.73; H, 3.15. Calc. for C₃₇H₄₄DyF₂₀LiO₁₇: C, 33.92; H, 3.39.

[LiEu(L³)₄(MeOH)] (9). Yield 51%. Found: C, 32.63; H, 2.75. Calc. for C₄₁H₄₄EuF₂₈LiO₁₇: C, 32.84; H, 2.96.

[LiGd(L³)₄(MeOH)] (10). Yield 49%. Found: C, 32.56; H, 3.05. Calc. for C₄₁H₄₄F₂₈GdLiO₁₇: C, 32.72; H, 2.95.

[LiTb(L³)₄(MeOH)] (11). Yield 61%. Found: C, 32.49; H, 2.78. Calc. for C₄₁H₄₄TbF₂₈LiO₁₇: C, 32.69; H, 2.94.

[LiEu(L³)₄(MeOH)] (12). Yield 57%. Found: C, 31.64; H, 2.39. Calc. for C₄₅H₄₄EuF₃₆LiO₁₇: C, 31.80; H, 2.61.

[LiGd(L⁴)₄(MeOH)] (13). Yield 50%. Found: C, 31.46; H, 3.21. Calc. for C₄₅H₄₄F₃₆GdLiO₁₇: C, 31.70; H, 3.35.

[LiTb(L⁴)₄(MeOH)] (14). Yield 54%. Found: C, 31.45; H, 2.44. Calc. for C₄₅H₄₄TbF₃₆LiO₁₇: C, 31.67; H, 2.60.

[LiDy(L⁴)₄(MeOH)] (15). Yield 44%. Found: C, 31.47; H, 2.35. Calc. for C₄₅H₄₄DyF₃₆LiO₁₇: C, 31.60; H, 2.59.

4. Conclusions

Based on acetal-containing β-diketonates of variable perfluoroalkyl chain length, the discrete heterometallic [Ln-Li] complexes were synthesized. The fluoroalkyl group affects both the crystal packing structure and the distortion of the coordination geometry in complexes. Increasing the number of fluorine atoms in the β-diketonates has no significant impact on the triplet-state energy level of the ligand. By increasing the length of fluoroalkyl substituent in the ligand, the distortion of the coordination geometry [LnO₈] from ideal symmetry in [Ln-Li] complexes is irregular and depends on the nature of the Ln^III ion. In the series of Eu^III and Dy^III complexes, PLQY increases when the coordination polyhedron [LnO₈] changes from TDD-8 to BTRP-8 geometry. However, the opposite correlation was observed in case of Tb^III complexes 7, 14: the closer the [TbO₈] environment is to TDD-8 symmetry, the higher the PLQY (60–64%). It has been shown that decreasing the S_Q(SAPR-8) value of coordination polyhedron [DyO₈] correlates with increasing U_eff values in Dy^III complex 4.