Luminescent lanthanide complexes supported by ditopic Schiff-base/calix[4]arene macrocycles: synthesis, structure, and luminescence properties of [Ln₂(H₂L²)(H₂O)₂] (Ln = La, Eu, Tb, Yb)

Abstract

The ditopic macrocycle H₈L² comprising two 3,3′-dihydroxy-4,4′-bis(iminomethyl)-biphenyl and two calix[4]arene units was synthesized and its ability to bind lanthanide ions has been studied. H₈L² reacts readily with Ln(III) nitrate salts in the presence of NEt₃ to support dinuclear neutral complexes of composition [Ln₂(H₂L²)(H₂O)₂] (Ln = La³⁺ (1), Eu³⁺ (2), Tb³⁺ (3), and Yb³⁺ (4)). The crystal structure of the ditopic calix[4]arene ligand reveals a centrosymmetric molecule with an overall “S-shape” structure and a cone-conformation. The molecular structure of the Yb complex 4 is also “S-shape” indicative of some degree of preorganization imposed by the ligand constraints. The Yb³⁺ ions are seven-coordinated (distorted monocapped trigonal prismatic coordination environment (NO₆ donor set)). DFT calculations for the La complex 1 (at the r²SCAN-3c level of computation) implies an isostructural series of compounds. The ligand and complexes exhibit strong absorptions (ε > 10⁴ M⁻¹ cm⁻¹) in the 250–440 nm range attributed to ligand-based π–π* and n–π* transitions. The bis(iminomethyl)biphenyl linkers were found to sensitize Eu^III and Tb^III emission (λ_ex = 370 nm and 405 nm) in the solid state at 295 K. A material containing statistically distributed Tb^III and Eu^III ions reveals upon 370 nm excitation only Eu³⁺ emission lines indicative of intermolecular energy transfer processes between the Ln³⁺ ions.

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Introduction

Luminescent materials based on Ln³⁺ ions are of interest due to their unique photophysical properties that make them suitable for a range of potential applications such as optical imaging, bioprobes, sensors, visible emitters for display, solar energy conversion, and lighting technology, near-infrared (NIR) emitters for lasers, optical fibers and amplifiers to name but a few.^1–6 Much attention is currently focused on the development and study of well-defined lanthanide compounds containing two or more different Ln(III) ions.^7–14 A number of classes of heterolanthanide assemblies have been described, such as multi-dimensional coordination frameworks, discrete molecules, and flexible large molecules formed by two or more coordinating units connected by a linker.^15,16 The combination of different lanthanide complexes in polymeric structures, for example, enables the simultaneous emission of different lanthanide ions upon single-wavelength excitation.^16,17 There is a wide range of color shade accessible from red to green^18–22 and from blue to green²⁰ in Tb/Eu mixed complexes. Besides these shifts it is also possible to force an energy transfer between two or more lanthanide ions as observed for e.g. in Y/Tb,²³ Sm/Tb,²⁴ and Tb/Eu^22,25–29 complexes. The understanding and control of the luminescent properties by effective sensitization is extremely important particularly in sensory applications or in light-emitting diodes (LED) technology.³⁰

The synthesis of discrete, bimetallic lanthanide complexes can be accomplished by using multidentate ligand systems that allow juxtaposition of the Ln³⁺ ions in a stable and rigid coordination environment.³¹ Functionalized calix[4]arenes^32–34 have turned out to be a class of versatile supporting ligands for a number of stable lanthanide complexes.^35–42 Several calixarenes have been functionalized with organic chromophore units in order to sensitize Ln-based luminescence.^43–48 Bimetallic lanthanoid complexes based on (Thia-)calix[4]arenes are rather rare.

Recently we have reported a ditopic macrocycle H₆L¹ comprising two calixarene and two bis(iminomethyl)phenyl head units (Fig. 1). This ligand supports dinuclear mixed-ligand complexes of the composition [Ln₂L¹(H₂O)₂] (Ln = La³⁺, Eu³⁺, Tb³⁺, Yb³⁺)⁴⁹ comprising two seven-coordinated Ln³⁺ ions.^50,51 The Schiff-base units act as antennae thus enabling sensitization of Eu^III and Tb^III luminescence emission under irradiation with UV light in the solid state (λ_ex = 370 nm).

Fig. 1 Chemical formulae of the ditopic macrocycles H₆L¹ and H₈L² (with Schiff-base head units highlighted in magenta). H₆L¹ has been reported previously.⁴⁹

We have now investigated the synthesis of related Ln³⁺ complexes derived from the macrocycle H₈L² in which the bis(iminomethyl)phenyl linkers have been replaced by bis(iminomethyl)biphenyl units, which changes the rigidity and size of the system. We found that H₈L² is an effective ditopic ligand stabilizing dinuclear macrocyclic lanthanide complexes of composition [Ln₂(H₂L²)(H₂O)₂] (Ln = La³⁺ (1), Eu³⁺ (2), Tb³⁺ (3), and Yb³⁺ (4)). Herein, their structures and photophysical properties are reported and compared with the previously published [Ln₂(L¹)(H₂O)₂] complexes.⁴⁹ We also briefly examined the possibility of synthesizing the heterodinuclear Eu^III/Tb^III complex (5) in order to study a potential energy transfer process between the two Ln ions.

Results and discussion

Synthesis and characterization of H₈L²

The macrocyclic ligand H₈L² was prepared by a [2+2] Schiff-base condensation reaction according to literature procedures as illustrated in Scheme 1.^52,53 The two precursors 25,27-bis(aminoethoxy)-26,28-dihydroxy-calix[4]arene and dihydroxy-4,4′-biphenyldicarbaldehyde reacted readily in a 1 : 1 stoichiometric ratio in a mixed CH₂Cl₂/EtOH solution at room temperature and provided the macrocycle as a yellow product. The low solubility of H₈L² drives the reaction to near completion with an almost quantitative yield (98%).

Scheme 1 Synthesis of the ligand H₈L².

The new ligand gave satisfactory elemental analysis and was further identified by ESI mass spectrometry, FTIR, UV-vis, ¹H and ¹³C NMR spectroscopy, and by X-ray crystallography. Fig. S2 (ESI†) displays the ¹H-NMR spectrum for H₈L² in CD₂Cl₂ along with the assignments of the NMR signals. The two intense peaks at 1.15 and 1.24 ppm are attributed to the tert-butyl groups. The calix[4]arene methylene groups give rise to two AB spin systems centered at 3.41 ppm and 4.35 ppm. This spectral feature is consistent with the presence of a cone conformation for the calix[4]arene⁵⁴ as also observed for H₆L¹.⁴⁹ The CH₂ groups of the ethylene linkers give rise to two triplets at 4.09 and 4.27 ppm, respectively. The four signals at 6.96, 7.05, 7.07 and 7.20 ppm are due to the aromatic protons. Two broad signals are seen for phenolic hydroxyl groups at 8.06 and 13.09 ppm indicating that they are involved in hydrogen bonding interactions. The four imine protons give rise to one singlet at 8.74 ppm. The ¹³C NMR spectrum for H₈L² in CD₂Cl₂ reveals only 22 signals for the 124 C atoms.

Synthesis and characterization of lanthanide complexes (1–4)

The reaction of H₈L² with La(NO₃)₃·6H₂O and triethylamine as a base in a 1 : 2 : 8 molar ratio in MeOH/CH₂Cl₂ results in the formation of a yellow precipitate of the composition [La₂(H₂L²)(H₂O)₂] (1, where (H₂L²)⁶⁻ represents the sixfold deprotonated form of the ligand) in good yield (65%) (Scheme 2). The analogously synthesized Eu³⁺ (2), Tb³⁺ (3), and Yb³⁺ (4) complexes were obtained in a similar good yield.

Scheme 2 Synthesis of the complexes [Ln₂(H₂L²)(H₂O)₂] (Ln = La (1), Eu (2), Tb (3), Yb (4).

The complexes are air stable solids, which are moderately soluble in cyclohexane and insoluble in other common organic solvents. All new compounds were further characterized by ESI mass spectrometry, ATR-FTIR, UV/Vis spectroscopy, and luminescence spectroscopy. The Yb³⁺ complex 4 was additionally identified by X-ray crystallography. Selected analytical data are listed in Table 1.

Table 1 Selected FTIR and diffuse reflectance UV/Vis data for the ligand H₈L² and the lanthanide complexes 1–4 and their assignments

Compound	[small nu, Greek, tilde] /cm^−1	λ max /nm
a ATR-mode. b Solid state UV/Vis diluted in BaSO4, sh = shoulder.
H8L^2	3425 ν(H2O)/ν(R–OH)	294 nm (π–π* calixarene)
	1631 (ν(C=N))	306 nm (sh, π–π* calixarene)
	1362, 1484 (δ(–N–CH2–))	340 nm (π–π*, PhCH=NR)
		428 nm (sh, n–π*, PhCH=NR)
1	3424 ν(H2O)/ν(R–OH)	294 nm (sh, π–π* calixarene)
	1631, 1602 (C=N)	306 nm (π–π* calixarene)
	1385, 1481 (δ(–N–CH2–))	340 nm (sh, π–π*, PhCH=NR)
		406 nm (sh, n–π*, PhCH=NR)
2	3440 ν(H2O)/ν(R–OH)	292 nm (sh, π–π* calixarene)
	1653, 1607 (C=N)	308 nm (π–π* calixarene)
	1385, 1477 (δ(–N–CH2–))	342 nm (sh, π–π*, PhCH=NR)
		406 nm (sh, n–π*, PhCH=NR)
3	3417 ν(H2O)/ν(R–OH)	294 nm (sh, π–π* calixarene)
	1647, 1608 (C=N)	308 nm (π–π* calixarene)
	1385, 1481 (δ(–N–CH2–))	342 nm (sh, π–π*, PhCH=NR)
		406 nm (sh, n–π*, PhCH=NR)
4	3414 ν(H2O)/ν(R-OH)	294 nm (sh, π–π* calixarene)
	1647, 1608 (C=N)	308 nm (π–π* calixarene)
	1385, 1482 (δ(–N–CH2–))	342 nm (sh, π–π*, PhCH=NR)
		406 nm (sh, n–π*, PhCH=NR)

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FTIR spectra of 1–4 (powder spectra) were recorded in the ATR mode. Fig. 2 shows a comparison of the ATR-FTIR spectra of H₈L² and the complexes 1–4 in the 1800–1500 cm⁻¹ range. The spectrum of H₈L² reveals one intense absorption band at 1631 cm⁻¹, typical for the imine (ν(C=N)) stretching vibration.⁵⁵ In contrast to the free ligand, all complexes display two ν(C=N) absorption bands between 1650 and 1600 cm⁻¹, indicative of the presence of two kinds of imine groups (i.e. coordinating and free imine groups). Likewise, one of the deformation vibrations (wagging or twisting)⁵⁶ of the CH₂ group next to the imine observed for H₈L² at 1362 cm⁻¹ are shifted to higher frequencies in the complexes (1385 cm⁻¹), indicating the coordination of Ln³⁺ ions to the corresponding imine.⁵⁵ The broad feature at 3400 cm⁻¹ does not disappear after coordination of lanthanide ions (Fig. S10–S14, ESI†). The vibrations are tentatively assigned to the O–H stretching vibrations of the Ln³⁺ bound water molecules.

Fig. 2 Section of the ATR-FTIR Spectra of H₈L² and the complexes 1–4 in the 1800–1500 cm⁻¹ region.

Given the low solubility of the complexes 1–4, all UV/Vis spectra were recorded in diffuse reflectance mode. The reflectance data were converted to absorption utilizing the Kubelka–Munk function.⁵⁷ The spectrum of the free ligand H₈L² (Fig. 3) shows strong absorption bands at 294 nm and 340 nm. The former maximum exhibits a shoulder at ∼306 nm. There is also another shoulder around 428 nm with lower intensity. The two low-energy absorptions are most likely associated with π–π* or n–π* transitions centered on the hydroxy-iminomethyl-functionalized biphenyl linkers. In agreement to the literature, π–π* transitions centered on the phenol ether and phenol groups of the calix[4]arene backbone absorb at the peak at 294 nm⁵⁸ and the parent biphenyl system absorbs at 306 nm.⁵⁹

Fig. 3 Diffuse reflectance UV/Vis Spectra of H₈L² and the complexes 1–4 measured as neat solids inside an integration sphere.

The diffuse reflectance UV/Vis spectra of the Ln complexes 1–4 differ only slightly from that of the free ligand. All complexes exhibit strong asymmetric absorption maxima at 306 nm with a shoulder at 294 nm. Absorption maxima at 340 nm and 406 nm also appear as shoulders in the spectrum. A slight hypsochromic shift of the n–π* transition centered on the hydroxy-iminomethyl-functionalized biphenyl linkers can be observed relative to the free ligand H₈L². This shift can be traced to the coordination of the lanthanide ions. A similar behavior was reported for related Ln³⁺ complexes of other Schiff-base/calix[4]arene complexes.⁶⁰

A comparison of the electronic absorption spectra of H₈L² with those of H₆L¹ shows many similarities. Differences can be seen for the π−π* transitions of the calixarene units as well as the n–π* transition of the iminophenol units of H₈L², which are shifted bathochromically (by 18 nm) relative to those of H₆L¹.⁴⁹ The π−π* transitions of the bis(iminomethyl)biphenyl linkers in H₈L², on the other hand, are not shifted relative to those of the iminophenol units in H₆L¹ which is not surprising in view of the twisting of the biphenyl linkers (that does not lead to a more extended π-system in H₈L²). The UV/Vis spectral data for a corresponding set of two complexes [Ln₂(L¹)(MeOH)₂] and [Ln₂(H₂L²)(H₂O)₂] are also similar, as far as the calix[4]arene chromophores are concerned.⁴⁹ There are, however, significant differences in the positions of the n–π* transitions of the imino phenolate units in the two complexes. These differences may be due to the different types of second-sphere hydrogen-bonding interactions. The Ln bound solvate molecules in the [Ln₂(H₂L²)(H₂O)₂] complexes, for example, are involved in hydrogen bonding interactions with the calix[4]arene O atoms, whereas they are involved in direct H-bonds with the non-coordinating imino-phenolate N atoms in the [Ln₂(L¹)(H₂O)₂] complexes (see below), which will exert an effect on the energy of the orbitals involved in the electronic transitions.

Crystallographic characterization

Description of the crystal structure of H₈L²·4CH₂Cl₂. Single crystals of H₈L²·4CH₂Cl₂ picked from the reaction mixture were subjected to X-ray crystallographic analysis. H₈L²·4CH₂Cl₂ crystallizes triclinic, space group P1̄. The crystal structure is composed of discrete H₈L² molecules (shown in Fig. 4) and co-crystallized CH₂Cl₂ solvate molecules. Each calix[4]arene group accommodates one CH₂Cl₂ molecule, which interacts with the electron-rich aryl rings via CH⋯π interactions (Fig. 5). The other CH₂Cl₂ molecules are enclathrated in the voids generated by the specific packing of the H₈L² molecules. The individual H₈L² molecules exhibit crystallographically imposed inversion symmetry. The molecules exhibit an overall “S-shape” structure, with the calix[4]arenes each bent away from an imaginary axis passing along the direction of the biphenyl moieties.

Fig. 4 Ball and stick representation of the molecular structure of compound H₈L² as determined by single-crystal X-ray diffraction. H atoms except those involved in hydrogen bonding interactions and CH₂Cl₂ molecules situated in the cavities of the calix[4]arenes have been omitted for clarity.

Fig. 5 Representation of the intermolecular CH⋯π interactions between the CH₂Cl₂ solvate molecules and the calix[4]arene units of H₈L². Parts of the ligands have been omitted for clarity. Only one orientation of the disordered CH₂Cl₂ molecule is displayed. Symmetry code used to generate equivalent atoms: 1 − x, 1 − y, 1 − z (′). Selected distances/Å: C63a⋯X2 (center of aryl ring) 3.422, C63a⋯X3 3.671, C64′⋯X1 3.375, C64′⋯O6 3.249.

The calix[4]arene units adopt the cone conformation in agreement with the ¹H NMR data. The aryl rings of the biphenyl moieties are twisted by 37.16° which is a typical value for the biphenyl system.⁶¹ This twisting prevents intramolecular π–π stacking interactions between the two biphenyl linkers, despite the fact that they are held in close proximity by the calix[4]arene groups (closest intramolecular C⋯C distance between biphenyl groups at 3.557 Å (C31⋯C33′)). All Schiff-base groups are nearly planar, stabilized by intramolecular OH⋯N hydrogen bonding interactions typical for O-hydroxyaryl Schiff bases. Intramolecular OH⋯O hydrogen bonding interactions are also present between the phenol and phenolether units of the calix[4]arene (e.g. O2⋯O5′ = 2.710 Å).

Self-assembly of the “S-shape” molecules within the crystal structure shows a bilayer arrangement with alternating calix[4]arene and biphenyl segments. The thicknesses of the two layers were found to have similar values of approx. 5 Å and 8 Å, respectively (Fig. 6). Hierarchical self-assembly of calix[4]arene molecules (resembling that of mineral clays) has precedence in the literature.^62,63

Fig. 6 Self-assembled structure of the ligand H₈L², depicting segregation into alternating layers of biphenyl and calix[4]arene moieties (view is normal to (010)).

Description of the crystal structure of [Yb₂(H₂L²)(H₂O)₂]·2H₂O·1.5MeCN (4·2H₂O·1.5MeCN)

Single crystals of [Yb₂(H₂L²)(H₂O)₂]·2H₂O·1.5MeCN (4·2H₂O·1.5MeCN) obtained from a mixed solvent system (MeOH/MeCN) are monoclinic, space group P2₁/c. Although the quality of the X-ray data is quite low, it can confirm the atom connectivity. The crystal structure is composed of discrete, dinuclear [Yb₂(H₂L²)(H₂O)₂] complexes and co-crystallized H₂O and CH₃CN solvate molecules. Fig. 7 shows the structure of the dinuclear ytterbium complex. The [Yb₂(H₂L²)(H₂O)₂] complexes exhibit crystallographically imposed inversion symmetry. Each Yb ion is seven-coordinated by the four calix[4]arene O atoms, the imine N and O atoms of one of the two Schiff base linkers, and one H₂O molecule. The coordination geometry is intermediate between distorted mono-capped trigonal prismatic (shape^64,65 symmetry factor: 1.669) and mono-capped octahedral (symmetry factor: 0.879), which are common coordination geometries for seven coordinate lanthanide complexes.^50,51

Fig. 7 Single-crystal X-ray diffraction structure of [Yb₂(H₂L²)(H₂O)₂]·2H₂O·1.5MeCN (4·2H₂O·1.5MeCN) (ball-and-stick representation). H₂O and MeCN solvate molecules as well as H atoms (except those involved in H bonding interactions) are omitted for clarity. Dashed lines refer to intramolecular hydrogen bonding interactions (O7⋯O1 = 3.060 Å, O5⋯N1 = 2.551 Å, O5⋯O5′ = 3.153 Å). Symmetry code used to generate equivalent atoms 1 − x, −y, −z (′).

The Yb–O bond lengths in 4 span a rather broad range from 2.08 to 2.50 Å. The Yb–O bond lengths appear to depend on the type of O donor. They increase in the following order: Yb–O^phenolate < Yb–O^{iminophenolate} < Yb–O^H₂O < Yb–O^arylether (O1, O3 phenolether). This ranking of Yb–O bonds reflects the different donor abilities of the four types of O donors, with the two extremes represented by the hard, charged phenolate (O2, O4) and the neutral phenolether functions (O1, O3) of the calixarenes. The weaker donor ability of the iminophenolate functions can be traced to electron withdrawing effects of the imine functions, which reduce the electron density of the phenolato donors. The stronger electron delocalization in the iminophenolate group is also manifested by a shorter Ar^C–O bond length (1.32 Å for the iminophenolate group vs. 1.35 Å for the phenolate of the calixarene). The H₂O donor is comparable in donor ability to the phenol ether O atoms. The Yb–N^imine bonds are only slightly larger than the Yb–O^H₂O bonds (2.406 vs. 2.421 Å). The intramolecular Yb⋯Yb distance in 4 amounts to 15.9 Å, and is thus much longer than the shortest intermolecular Yb⋯Yb distance of 9.1 Å.

The mutual arrangement of the two biphenyl linkers in the Yb complex is quite different from that observed for the free ligand, although the overall “S-shape” structure is maintained. Thus, unlike in H₈L², the two non-coordinating Schiff-base units in 4 are co-planar towards each other and are more shifted relative to each other than in the free ligand (distance of the center of the biphenyl moiety = 8.5 Å (4) versus 4.3 Å (H₈L²)). Nevertheless, the biphenyl moieties are still twisted as in the free ligand. The Yb complexes pack in a similar fashion as the H₈L² ligand, with the biphenyl and Yb-calix[4]arene entities arranged in 2D sheets (Fig. S25, ESI†).

We have also been able to grow single-crystals of the La complex 1. Although the quality of the data set is not as good as desirable for publications, it can prove the atom connectivity. According to this data set, the La complex 1 is isostructural with the Yb complex 4. Therefore, the La atom in 1 exhibits the same coordination environment and coordination number as the Yb atom in complex 4. Thus, we believe that all the Ln complexes presented in this series form an isostructural set of complexes. This behavior may be related to the rigid constraints imposed by the macrocyclic calix[4]arene ligand. We have observed a similar behavior for a set of bimetallic Ln complexes supported by a calix[4]arene ligand bearing a pendant picolyl-imine arm.⁶⁶ For other complexes with less constrained ligands, it is not uncommon that the coordination number decreases by one unit when the Ln series is traversed from the left to the right.⁶⁷

It should be mentioned, that the coordination mode of the ligand H₈L² in 4 is very similar to that of the ligand H₂L¹ in the europium complex [Eu₂(L¹)(MeOH)₂] (Fig. 8).⁴⁹ Thus, each macrocycle utilizes all four calix[4]arene O atoms and the N and O donors of just one Schiff base unit in the coordination of a Ln³⁺ ion. The other Schiff base moiety remains free as in 4 or in the [Ln₂(H₂L₁)(MeOH)₂] complexes. The ligands are thus bis-hexadentate ligands comprising of two O₅N binding pockets. Thus, as observed before,⁴⁹ one additional co-ligand is coordinated to the lanthanide ions to saturate its coordination demands. The similarity in the coordination environments is also illustrated by the shape factors, which provide a measure for the deviation from the ideal coordination environments. Thus, irrespective of the type of Schiff base linkers only seven-coordinated Ln complexes can be accommodated, most likely due to the steric demand of the macrocyclic structures.

Fig. 8 Comparison of the immediate coordination environments in [Yb₂(H₂L²)(H₂O)₂] (4) (left) and in [Eu₂(L¹)(MeOH)₂] (right). Parts of the structures were omitted for clarity.

DFT calculations

So far, we have not been able to grow single crystals of the other lanthanide complexes suitable for structure determination. Therefore, density functional theory (DFT) calculations were carried out for the complex [La₂(H₂L²)(H₂O)₂] (1) (r²SCAN, def2-mTZVPP, 3c (D4+gcp) level of computation). The calculated and experimental structures of 1 and 4 compare reasonably well with each other (Fig. 9). The main difference between the calculated and experimental structures is, as expected, an increase of the average M–O and M–N distances of 0.22 Å (Table S2, ESI†) in the calculation of 1 compared to the experimental structure of 4. This is in agreement with the larger atomic radius of the lanthanum ion in 1 compared to the ytterbium ion in 4.⁶⁸ Compensating for the difference in the ion radius results in an average distance difference of 0.07 Å for the M–O and M–N bonds. The average M–O bond lengths are ca. 0.078 Å different (absolute differences) and the M–N bond is on average 0.01 Å longer in the calculated structure of 1 (Table S2, ESI†). The bond angles around the lanthanum atom in the optimized structure of 1 deviate from the experimental structure of 4 by a maximum of 19.9° with an average deviation of 6.5° (e.g. O2–La–O6 137.6 vs. 157.6(5)°) (Tables S3 and S4, ESI†). Based on these calculations, we conclude that all the Ln complexes supported by H₈L² adopt the same structures.

Fig. 9 Structural differences between XRD derived and DFT optimized structure. Blue: XRD derived structure of 4, red: DFT optimized structure of 1.

We also optimized a MeOH solvated complex using the same methodology. Again, the calculated structure for this compound agreed well with the experimental structure for 4. It can be concluded that MeOH ligands can be accommodated in the coordination sphere of the Yb complexes as well. The difference of the reaction free energy for complexation reaction: (2 La³⁺ + (H₂L²)⁶⁻ + 2 L′ → [La₂(H₂L²)(L′)₂]) between water and methanol was calculated to be ΔΔG = 4.7 kJ mol⁻¹, indicating that water binds only slightly stronger than methanol. In other words, the co-ligand L′ in the [La₂(H₂L²)(L′)₂] complex is exchangeable from a thermodynamic perspective. This behavior has also been observed for dinuclear lanthanide complexes supported by H₂L¹.⁴⁹

Spectroscopic and photophysical properties

H₈L² and complexes 1–4 were characterized by photoluminescence spectroscopy to probe a potential sensitization of the emission of the coordinated lanthanide ions (antenna effect). Photoluminescence spectra were recorded for the neat solids at room temperature. Fig. 10 displays an overlay of the photoluminescence spectra for the free ligand and the Eu (2) and Tb (3) complexes. The spectrum for H₈L² displays a very broad emission band at 556 nm under excitation with 370 nm UV-radiation, which can be attributed to emission from a π–π* state located on the bis(iminomethyl)biphenyl system. The spectrum of the Eu complex also displays a similar broad emission band with a maximum at 560 nm, however, the emission intensity is reduced, relative to that of the free ligand. The spectrum of the Eu complex 2 displays also several line-like emission bands at 579 nm, 591 nm, 614 nm, 649 nm, and 685–701 nm attributed to the characteristic ⁵D₀ → ⁷F_J transitions (J = 0–4) from the excited ⁵D₀ state of the Eu³⁺ ion.⁶⁹ The appearance of the emission maximum at 579 nm attributed to the ⁵D₀ → ⁷F₀ transition is in good agreement with the low local C₁ symmetry of the coordination environment about the Eu³⁺ ions.⁶⁹ However, a potential splitting of the ⁵D₀ → ⁷F₁ and the ⁵D₀ → ⁷F₂ transitions due to crystal field effects, which is typically observed for seven-coordinated Eu Schiff-base complexes,⁷⁰ is not observed under our conditions.

Fig. 10 Photoluminescence spectra of H₈L², [Eu₂(H₂L²)(H₂O)₂] (2) and [Tb₂(H₂L²)(H₂O)₂] (3) at 295 K (neat solids). The excitation wavelength is 370 nm. The Ln transitions start from the ⁵D₀ state of the Eu³⁺ and the ⁵D₄ state of the Tb³⁺ ion.

The photoluminescence spectrum for the Tb complex also shows a broad emission band with a maximum at 560 nm. The reduction of the intensity relative to the free ligand is less pronounced than in the Eu complex. The weak, line like emission maxima centered at 545 nm, 585 nm, and 618 nm, can be attributed to the ⁵D₄ → ⁷F_J (J = 5, 4, 3) transitions from the excited ⁵D₄ state of the Tb³⁺ ion. The ⁵D₄ → ⁷F₆ transition (expected at 480 nm) is not observed.

The excitation spectra of the Eu and Tb complexes monitored at 620 nm and 540 nm, respectively, show broad absorption bands in the 300 to 450 nm range (Fig. S26, ESI†). These are attributed to the transitions into the ¹(π–π*) state of the bis(iminomethyl)phenolato ligand, clearly showing that they can function as an antenna for sensitizing the Eu³⁺ and Tb³⁺ ions. This is also in line with previous work, where we determined triplet state energies of several Schiff-base substituents appended to calix[4]arene-ligands. All were found to lie well above the ⁵D₄ and ⁵D₀ resonance levels of the Tb³⁺ and Eu³⁺ ions (∼20.500 cm⁻¹ and ∼17.500 cm⁻¹, respectively).⁷¹ The triplet state energy in the parent 2-(methylphenol)imino-ethoxy substituent (which is also part of the present ligand system), for example, is found at 21 882 cm⁻¹.⁴⁹ The fact that the emission of the ligand is also observed can be traced to the fact that one of the Schiff-base units in the linking biphenyl groups is not coordinated to the lanthanide ions.

Photophysical properties of a heterodinuclear Eu/Tb complex

It was of interest to synthesize a heterodinuclear [TbEu(H₂L²)(H₂O)₂] (5) complex and to investigate its photoluminescence properties, in view of literature reports that such complexes give rise to energy transfer between the Tb³⁺ and Eu³⁺ ions.¹⁴ Thus, in an orienting experiment the ligand H₈L² was reacted with a 1 : 1 mixture of Eu(NO₃)₃·6H₂O and Tb(NO₃)₃·6H₂O to afford a material containing the heterodinuclear complex [EuTb(H₂L²)(H₂O)₂] (5). The presence and thus successful synthesis of 5 was validated by ESI-MS spectrometry (Fig. S8, ESI†). Fig. 11 displays the photoluminescence spectrum of this material. Only the characteristic emission maxima for the Eu(III) complexes luminescence are discernible. The characteristic emission band for the Tb(III) centers, as for instance the maximum centered at 545 nm, is no longer present (or greatly reduced in intensity). Moreover, the luminescence lifetime for 5 is significantly reduced (178 μs in 2 to 57 μs in 5) than observed for compound 2 under the same experimental conditions (Table S7, ESI†). The obtained luminescence excitation spectra show a decrease in energy from 445 nm in 2 (and a 1 : 1 mixture of 2 and 3) to 465 nm in 5 (Table S6, ESI†). These results may be traced to intermolecular energy transfer processes between the Tb(III) and Eu(III) centers. Intermolecular energy transfer processes have been observed in other heterometallic Tb/Eu complexes.¹⁶ For efficient intermetallic communication such as an energy transfer distances between interacting Ln ions must be reduced below 3.5–4 Å for an orbital overlap (Dexter energy transfer) or below 10 Å for efficient dipolar through-space interactions (Förster resonance energy transfer).¹⁶ Based on the results of X-ray diffraction and DFT calculations we can safely assume that intramolecular distances between Eu and Tb centers are in the range of 16 Å and intermolecular distances between metal centers are in the range of 9 Å. Based on these distances an intermolecular energy transfer is presumably more likely than an intramolecular one.

Fig. 11 Photoluminescence spectra of [EuTb(H₂L²)(H₂O)₂] (5) at 295 K (neat solids). The excitation wavelength is 370 nm.

Conclusion

A new ditopic calix[4]arene-Schiff base ligand, comprising two calix[4]arene head units linked by two 3,3′-dihydroxy-4,4′-biphenyldicarbimin units, has been synthesized and its coordination chemistry towards selected lanthanide ions (La³⁺, Eu³⁺, Tb³⁺, Yb³⁺) investigated in solution and solid state. H₈L² was found to act as a bis-hexadentate ligands comprising two O₅N binding pockets. Since it cannot saturate the coordination demands of the Ln³⁺ ions, one additional co-ligand is coordinated to the lanthanide ions to produce mixed-ligand complexes of composition [Ln₂(H₂L²)(H₂O)₂]. The Yb³⁺ ions are seven-coordinated in a distorted monocapped trigonal prismatic coordination environment (NO₆ donor set). DFT calculations for the La complex 1 (at the r²SCAN-3c level of computation) implies an isostructural series of compounds. The electronic absorption spectra of the [Ln₂(H₂L²)(H₂O)₂] complexes reveal strong absorption maxima in the 250–440 nm range (ε > 10⁴ M⁻¹ cm⁻¹) attributed to ligand-based π–π* and n–π* transitions. The enlarged bis(iminomethyl)biphenyl linkers in comparison to H₆L¹ were found to sensitize Eu^III and Tb^III emission (λ_ex = 370 nm and 405 nm) in the solid state at 295 K. A material containing statistically distributed Tb^III and Eu^III ions reveals upon 370 nm excitation only Eu³⁺ emission lines indicative of intermolecular energy transfer processes between the Ln³⁺ ions. The ability of the ligand H₈L² to form heterodinuclear complexes may be further exploited in regard to understanding and applying tunable luminescent lanthanide complexes.

Experimental section

Materials and methods

All reagents and solvents were commercial grade and used without further purification. The precursors 25,27-di(aminoethoxy)-4-tert-butylcalix[4]arene and 3,3′-dihydroxy-[1,1′-biphenyl]-4,4′-dicarbaldehyde were synthesized according to the literature.^53,72,73 Melting points were determined with an Electrothermal IA9000 series instrument using open glass capillaries and are uncorrected. Mass spectra were obtained using the positive ion electrospray ionization modus (ESI) on a Bruker Daltronics ESQUIRE 3000 Plus ITMS or Impact II UHR Qq-TOF instrument. Infrared spectra (4500–600 cm⁻¹) were recorded at 2 cm⁻¹ resolution on a Bruker-Vertex 80V FT-IR spectrometer using a diamond-ATR unit. The diffuse reflectance spectra were collected at room temperature on a Jasco V-670 UV-vis-NIR spectrophotometer with an ARN-914 absolute reflectance measurement unit, equipped with a photomultiplier and a PbS photoconductive cell. The complexes were mixed with BaSO₄ powder in a mortar to a content of 5% (by weight). The spectrum was collected in the 200–1600 nm range with a resolution of 2 nm. Diffuse reflectance UV-visible data were analyzed using a normalized absorbance, which is referred to as the KUBELKA–MUNK function,⁵⁷f(R_∞), as shown in the following equation:
where R – diffuse reflectance of a sample is related to the absorption coefficient (K) and its scattering coefficient (S). Assuming the scattering coefficient being independent from the wavelengths, f(R_∞) is directly proportional to the absorption of the sample.

Synthesis and analysis of compounds

Calix[4]arene ligand H₈L². This compound was prepared by a modified literature procedure.⁵² Thus, to a solution of 25,27-di(aminoethoxy)-4-tert-butylcalix[4]arene (200 mg, 272 μmol) in a mixed solvent system (EtOH/CH₂Cl₂ 1 : 1, 30 mL) was added a solution of 3,3′-dihydroxy-[1,1′-biphenyl]-4,4′-dicarbaldehyde (66.0 mg, 272 μmol). The reaction mixture was stirred for 12 h at room temperature. The resulting yellow precipitate was filtrated, washed with ethanol, and dried in a vacuum. Yield: 251 mg (133 mmol, 49%). M.p.: > 300 °C (decomp). ATR-IR (diamond): ν/cm⁻¹ = 3420, 2960, 2904, 2867, 1631, 1485, 1362, 1194, 1124, 1047, 872, 797. m/z (ESI+, MeCN): 1883.484 ([M + H⁺], calc.: 1883.09). ¹H-NMR (400 MHz, CD₂Cl₂) δ = 13.09 (s, 4 H, ipsoHO–Ar c[4]a); 8.74 (s, 4 H, N = CH), 8.06 (s, 4 H, orthoHO–Ar biphenyl), 7.20 (d, 4 H, ³J = 8.06 Hz, metaHAr biphenyl), 7.07 (d, ³J = 1.3 Hz, 16 H, metaHAr biphenyl), 7.05 (d, ³J = 1.7 Hz, 4 H, orthoHAr biphenyl), 6.96 (dd, ³J = 8.0, 1.8 Hz, 4 H, metaHAr (relative to imine) orthoHAr (rel. hydroxy) biphenyl), 4.35 (d, ²J = 12.8 Hz, 8 H, Ar–CHH_ax–Ar), 4.27 (t, ³J = 6.1 Hz, 8 H, C=N–CH₂–CH₂–O), 4.09 (t, ³J = 5.9 Hz, 8 H, C=N–CH₂–CH₂–O), 3.41 (d, ²J = 12.9 Hz, 8 H, Ar–CHH_eq–Ar), 1.24 (s, 32 H, HO–Ar–C(CH₃)₃), 1.15 (s, 32 H, CH₂–O–Ar–C(CH₃)₃). ¹³C NMR (75 MHz, CD₂Cl₂): δ = 166.92 (Ar–CH=N), 161.37 (C^Ar–OH(biphenyl)), 151.11 (C^Ar–Ar(biphenyl)), 150.39 (C^Ar–CH₂–Ar), 148.01 (C^Ar–CH₂–Ar), 143.75 (C^Ar–O–CH₂), 142.25 (C^Ar–OH(calixarene)), 133.04 (C^Ar–C–CH₃), 132.30 (C^Ar–H(biphenyl)), 127.45 ((C^Ar–C–CH₃)), 126.53 (C^Ar–H(calixarene)), 125.98 (C^Ar–H(calixarene)), 118.81 (C^Ar–CH=N), 117.70 (C^Ar–H(biphenyl)), 115.44 (C^Ar–H(biphenyl)), 76.01 (O–CH₂–CH₂–N), 59.23 (O–CH₂–CH₂–N), 34.49 (C(CH₃)₃), 34.08 (C(CH₃)₃), 32.65 (Ar–CH₂–Ar), 31.77 (C(CH₃)₃), 31.36 (C(CH₃)₃). Elemental analysis calc. (%) for C₁₂₄H₁₄₄N₄O₁₂ 2H₂O (1882, 53 + 36.04): C 77.63, H 7.78, N 2.92; found: C 77.69, H 7,73, N 2.72. UV/Vis (solid): λ_max = 362 nm, 434 nm, emission (λ_ex = 370 nm): λ_max = 556 nm.

Complex 1. To a solution of H₈L² (25 mg, 13 μmol) and NEt₃ (16 μL, 115 μmol) in CH₂Cl₂ (5 mL) was added a solution La(NO₃)₃·6H₂O (12 mg, 26 μmol) in methanol (5 mL) at room temperature. The reaction mixture was stirred for 24 h. [La₂(H₂L²)(H₂O)₂] precipitated as a pale-yellow solid from the reaction mixture. The product was washed with ethanol and dried in high vacuum to constant weight to a yield of 19 mg (66%). M.p.: >300 °C (decomp), ESI-MS+ (m/z): 2360.144 ([M + H⁺ + 2 CH₂Cl₂], calc.: 2360.78), ATR-IR (diamond): ν/cm⁻¹ = 3420, 2955, 2904, 2869, 1631, 1602, 1482, 1385, 1193, 1123, 1044, 871, 793 cm⁻¹. Elemental analysis calc. (%) for C₁₂₄H₁₄₂La₂N₄O₁₄·3CH₂Cl₂ 3H₂O: C 61.04, H 6.21, N 2.24; found: C 61.93, H 6.153, N 2.36. UV/Vis: λ_max = 386 nm, emission (λ_ex = 370 nm): λ_max = 556 nm.

Complex 2. H₈L² (50 mg, 26 μmol), NEt₃ (31 μL, 230 μmol), and Eu(NO₃)₃·6H₂O (33 mg, 53 μmol) were reacted in analogy to the procedure detailed above for the lanthanum complex to give 38 mg (65%) of [Eu₂(H₂L²)(H₂O)₂] as a yellow powder. M.p.: >300 °C (decomp.). ESI-MS (m/z): 2490.829 ([M–H⁺ + 2 CH₂Cl₂ + 2 CH₃OH + CH₃CN], calc.: 2490.87). ATR-IR (Diamond): ν/cm⁻¹ = 3420, 2954, 2927, 2678, 1647, 1653, 1636, 1607, 1472, 1385, 1313 cm⁻¹. ESI-MS+ (m/z): 2180.907 ([M + H⁺], calc.: 2180.41). Elemental analysis calc. (%) for C₁₂₄H₁₄₂Eu₂N₄O₁₄·CH₂Cl₂·5H₂O: C 62.78, H 6.49, N 2.34; found: C 63.22, H 6.08, N 2.24. UV/Vis (diffuse reflectance): λ_max = 400 nm. Emission (λ_ex = 370 nm): λ_max = 579 nm, 591 nm, 614 nm, 649 nm, 685–701 nm.

Complex 3. H₈L² (20 mg, 10 μmol), NEt₃ (12 μL 86 μmol), and Tb(NO₃)₃·6H₂O (10 mg, 21 μmol) were reacted in analogy to the procedure detailed above for the lanthanum complex to give 15 mg (65%) of [Tb₂(H₂L²)(H₂O)₂] as a yellow powder. M.p.: >300 °C (decomp.); ESI-MS+ (m/z): 2236.588 ([M + H⁺ + CH₃CN–2CH₃OH], calc.: 2235.92). ATR-IR (Diamond): ν/cm⁻¹ = 3420, 2926, 2954, 2854, 2678, 2492, 1647, 1608, 1480, 1385, 1303, 1202, 1123, 872, 825, 804 cm⁻¹. ESI-MS+ (m/z): 2194.894 ([M + H⁺], calc.: 2194.33). Elemental analysis calc. (%) for C₁₂₄H₁₄₂Tb₂N₄O₁₄·3CH₂Cl₂·3H₂O: C 60.07, H 6.11, N 2.21; found: C 60.93, H 5.96, N 2.15. UV/Vis (diffuse reflectance): λ_max = 416 nm. Emission (λ_ex = 370 nm): λ_max = 545 nm, 585 nm, 618 nm.

Complex 4. H₈L² (65 mg, 47.45 μmol), NEt₃ (29 μL, 393.83 μmol), and Yb(NO₃)₃·5H₂O (23 mg, 104.39 μmol) were reacted in analogy to the procedure detailed above for the Lanthanum complex to give 15 mg (14%) of [Yb₂(H₂L²)(H₂O)₂] as a yellow powder. M.p.: >300 °C (decomp.). ESI-MS+ (m/z): 2223.919 ([M + H⁺], calc.: 2223.92). ATR-IR (diamond): ν/cm⁻¹ = 3420, 2940, 2738, 2678, 2492, 1647, 1476, 1385, 1195, 1123, 1099, 1037, 825 cm⁻¹. elemental analysis calc. (%) for C₁₂₄H₁₄₂Yb₂N₄O₁₄·6H₂O: C 62.93, H 6.56, N 2.37; found: C 63.29, H 5.94, N 2.26. UV/Vis (diffuse reflectance): λ_max = 412 nm. Emission (λ_ex = 370 nm): λ_max = 556 nm.

Complex 5. H₈L² (40 mg, 21.25 μmol) and NEt₃ (24 μL, 170 μmol) were dissolved in CH₂Cl₂, EtOH (1 : 1, 20 mL) and a solution of Eu(NO₃)₃·6H₂O (10 mg, 21 μmol) and Tb(NO₃)₃·6H₂O (9 mg, 21 μmol) in CH₂Cl₂, EtOH (1 : 1, 10 mL) was added. The solution was stirred at reflux for 48 hours to give 18 mg (40%) of [EuTb(H₂L²)(H₂O)₂] as yellow-orange powder. ESI-MS+ (m/z): 2187.92 ([M + H⁺], calc.: 2187.90).

Crystallography

Suitable specimens of H₈L²·4CH₂Cl₂, [La₂(H₂L²)(H₂O)₂]·2H₂O·(1·2H₂O) and [Yb₂(H₂L²)(H₂O)₂]·2H₂O·1.5MeCN (4·2H₂O·1.5MeCN) were selected and mounted on a cryoloop using perfluoropolyether oil. The data sets were collected at 180(2) K on a Stoe Stadivari X-ray diffractometer equipped with a GeniX 3D Cu-HF (Xenocx) microfocus X-ray source with a graded multilayer mirror (Cu-K_α, λ = 1.54186 Å) and a hybrid pixel detector Pilatus3 300K (Dectris). Data processing was carried out with the Stoe X-Area software including a spherical absorption correction and scaling routine.⁷⁴ The structures were solved with SHELXT 2018/2[X2]⁷⁵ using dual methods and refined by full-matrix least-squares techniques based on all data against F² using version 2018/3 SHELXL⁷⁵ and Olex2.⁷⁶ The coordinates of all non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were included in idealized positions. Due to the low quality of the [Yb₂(H₂L²)(H₂O)₂]·2H₂O·1.5MeCN measurement, it was not possible to refine the position of the hydrogen atoms connected to the disordered solvent water molecule.

Crystallographic data for H₈L²·4CH₂Cl₂

Crystallographic data for H₈L²·4CH₂Cl₂. C₁₂₈H₁₅₂Cl₈N₄O₁₂, M_r = 2222.13 g mol⁻¹, triclinic, space group P1̄, a = 12.1498(2) Å, b = 12.5940(3) Å, c = 20.1633(4) Å, α = 85.048(2)°, β = 84.743(2)°, γ = 83.090(2)°, V = 3041.13(11) ų, Z = 1, ρ_calcd = 1.213 g cm⁻³, T = 180(2) K, μ(Cu-Kα) = 2.166 mm⁻¹ (λ = 1.54186 Å), crystal size 0.249 × 0.215 × 0.186 mm³, 52 723 reflections measured, 11 421 unique, 7253 with I > 2σ(I). Final R₁ = 0.0614 (I > 2σ(I)), wR₂ = 0.1942 (all data), 729 parameters and 44 restraints, min./max. residual electron density = −0.54/0.26 e⁻ Å⁻³.

Crystallographic data for [Yb₂(H₂L²)(H₂O)₂]·2H₂O·1.5MeCN (4·2H₂O·1.5MeCN)

Crystallographic data for [Yb₂(H₂L²)(H₂O)₂]·2H₂O·1.5MeCN. C₁₂₇H_150.5N_5.5O₁₆Yb₂, M_r = 2356.10 g mol⁻¹, monoclinic, space group P2₁/c, a = 21.2650(12) Å, b = 13.1176(12) Å, c = 21.2994(11) Å, β = 114.929(4)°, V = 5894.6(7) ų, Z = 2, ρ_calcd = 1.327 g cm⁻³, T = 180(2) K, μ(Cu-Kα) = 3.370 mm⁻¹ (λ = 1.54186 Å), crystal size 0.270 × 0.213 × 0.140 mm³, 25639 reflections measured, 6041 unique, 3089 with I > 2σ(I). Final R₁ = 0.0885 (I > 2σ(I)), wR₂ = 0.2425 (all data), 676 parameters and 105 restraints, min./max. residual electron density = −0.40/1.59 e⁻ Å⁻³.

Crystallographic data for [La₂(H₂L²)(H₂O)₂]·2H₂O (1·2H₂O)

Crystallographic data for [Yb₂(H₂L²)(H₂O)₂]·2H₂O. C₁₂₄H₁₄₆ La₂N₄O₁₆, M_r = 2226.26 g mol⁻¹, monoclinic, space group P2₁/c, a = 23.283(2) Å, b = 13.1277(7) Å, c = 21.396(2) Å, β = 115.459(7)°, V = 5904.7(9) ų, Z = 2, ρ_calcd = 1.252 g cm⁻³, T = 180(2) K, μ(Cu-K_α) = 6.009 mm⁻¹ (λ = 1.54186 Å), crystal size 0.057 × 0.029 × 0.024 mm³, 35 888 reflections measured, 7414 unique, 2209 with I > 2σ(I). Final R₁ = 0.1317 (I > 2σ(I)), wR₂ = 0.4352 (all data), 345 parameters and 12 restraints, min./max. residual electron density = −0.44/0.70 e⁻ Å⁻³.

The quality of the data set is not as good as desirable for publication. However, it can prove the atom connectivity. The cif file for 1 has been deposited in the CCDC.

Photophysical characterization

Photoluminescence spectra were recorded with an inverted confocal microscope (Olympus IX71) fiber coupled to a spectrometer (iHR320, synapse CCD, HORIBA JobinYvon). The light was focused and collected in a backscattering geometry by a 10× objective (Olympus LUCPlan FLN 10× NA 0.3). The excitation wavelength was 370 nm and 405 nm, respectively (equipped with a Becker & Hickl Diode Laser operating in CW mode).

The luminescence excitation spectra were determined using a Tecan microplate reader (Infinite 200 Pro, Tecan Trading AG) and UV transparent microplates. Observed emission wavelength was chosen according to the luminescence (λ_em = 520 nm for the ligand, λ_em = 540 nm for the terbium complex and λ_em = 620 nm for the europium complex). An integration time of 500 μs and number flashes of 100 were use for the measurement.

The luminescence lifetimes of the ligand and the complex were determined using a fluorescence lifetime imaging microscope (FLIM) and phosphorescence lifetime imaging microscope (PLIM) approach. The microscope is based on a DCS-120 FLIM system (Becker and Hickl) combined with an inverted confocal microscope (IX71, Olympus) equipped with 10× objective (Olympus MPlan 10× NA = 0.25) and a 435 long pass filter. For FLIM the excitation a 405 nm, 80 ps laserdiode (repetition rate 80 MHz) was applied. For the PLIM measurements of the complexes a burst of multiple laser pulses (LB) was applied for the excitation.

Density functional theory calculations

Pre- and postprocessing employed functionality from the PLAMS⁷⁷ and ASE⁷⁸ python packages. Plots have been produced using the matplotlib library,⁷⁹ and VMD⁸⁰ was used for structural representations. The full workflow including all scripts, plots, inputs, and outputs generated in this project are available on Zenodo.⁸¹ General calculation settings are described below.

ORCA version 5.0.4^82,83 was used to perform restricted (spin-unpolarized) Kohn–Sham density-functional theory (DFT) calculations. The finest grid available in ORCA (input keyword “DEFGrid3”) was used for all calculations. The SCF convergence criterion was set to “TightSCF” with the default convergence check mode and geometries were converged using the “TightOpt” keyword (see the ORCA manual for a list of all tolerance values set by these keywords). The meta-GGA r²SCAN functional by Furness et al.^84,85 was used in the r²SCAN-3c methodology⁸⁶ with a def2-mTZVPP⁸⁷ basis set. The geometrical counterpoise correction (gCP) by Kruse and Grimme⁸⁸ is part of this methodology as well kruseas Grimme's D4 dispersion^89,90 correction scheme. Calculations including La employ the Def2-ECP (effective core potential)⁹¹ for it and an auxiliary coulomb fitting basis def2/J.⁹² Thermochemical quantities were calculated from the analytical frequencies with the default ORCA settings (temperature of 298.15 K and pressure of 1.0 atm). ORCA uses the Quasi-RRHO approach by Grimme⁹³ to correct issues related to the problematic harmonic approximation for low frequencies. To reduce computational cost and complexity, we limit DFT calculations of these large molecules to Ln = La, as the La³⁺ ion does not have f-electrons and results in a closed shell system, reducing the computational cost by a factor of two compared to open shell systems.

Data availability

The data supporting this article have been included as part of the ESI.† Crystallographic data for compound H₈L² and Yb complex 6 has been deposited at the CCDC under CCDC-2384275 and CCDC-2384276.† DFT: the full workflow including all scripts, plots, inputs, and outputs generated in this project are available on Zenodo, https://zenodo.org/. P. Melix, Luminescent Lathanide Complexes supported by Macrocyclic Schiff-base/calix[4]arene-ligands: synthesis, Structures, and Luminescence properties, 2023.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful to Prof. Dr H. Krautscheid for providing facilities for X-ray crystallographic measurements. We would like to thank Christopher Fischer for assistance in the synthesis of compounds. This work was supported by the University of Leipzig and by the Federal Ministry for Education and Research (BMBF) under grant number 02NUK014C. This work was funded in part by the German Research Foundation (DFG) within the CRC Transregio TRR 386 – B1 (514664767). We also thank the DFG for an FTIR spectrometer (Project number 448298270). Computations for this work were done (in part) using resources of the Leipzig University Computing Center.

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Footnotes

† CCDC 2384275, 2384276 and 2410730. For crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4tc04407j

‡ These authors contributed equally to this work.

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