Cooperative loading of multisite receptors with lanthanide containers: an approach for organized luminescent metallopolymers

Abstract

Metal-containing (bio)organic polymers are materials of continuously increasing importance for applications in energy storage and conversion, drug delivery, shape-memory items, supported catalysts, organic conductors and smart photonic devices. The embodiment of luminescent components provides a revolution in lighting and signaling with the ever-increasing development of polymeric light-emitting devices. Despite the unique properties expected from the introduction of optically and magnetically active lanthanides into organic polymers, the deficient control of the metal loading currently limits their design to empirical and poorly reproducible materials. We show here that the synthetic efforts required for producing soluble multi-site host systems Lk are largely overcome by the virtue of reversible thermodynamics for mastering the metal loading with the help of only two parameters: (1) the affinity of the luminescent lanthanide container for a single binding site and (2) the cooperative effect which modulates the successive fixation of metallic units to adjacent sites. When unsymmetrical perfluorobenzene-trifluoroacetylacetonate co-ligands (pbta⁻) are selected for balancing the charge of the trivalent lanthanide cations, Ln³⁺, in six-coordinate [Ln(pbta)₃] containers, the explored anti-cooperative complexation processes induce nearest-neighbor intermetallic interactions twice as large as thermal energy at room temperature (RT = 2.5 kJ mol⁻¹). These values have no precedent when using standard symmetrical containers and they pave the way for programming metal alternation in luminescent lanthanidopolymers.

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Introduction

Metal-containing (bio)organic polymers are of the utmost importance in actual daily life and have found a wealth of applications in tailor-made materials for energy storage and energy conversion,¹ drug delivery,² shape-memory systems,³ supported catalysts and organic conductors,⁴ smart photonic devices,⁵ and modern light-emitting diodes.⁶ In the absence of metal, rationalization of the organization of (bio)organic (co)polymers relies on micro-segregation events described by the Huggins–Flory theory of phase demixing,⁷ a model which has been recently extended for the programming of thermotropic liquid crystals.⁸ The introduction of open-shell transition metals within the polymeric backbone results in novel optical and magnetic properties that are not available in pure organic polymers.^1–5 However, the multifaceted (supra)molecular interactions operating between the polymeric backbone and the metal are at the origin of major difficulties in mastering the formation of metallopolymers with respect to stability, organization, processability and reproducibility. These limitations are even amplified for lanthanidopolymers (i.e. lanthanide-containing polymers) because of the versatile coordination behavior of these metallic centers.⁵ In order to avoid this pitfall, coordination chemists have restricted the concept of metallopolymers to infinite crystalline assemblies made of anionic ligand strands and cationic nodes forming extended 3D-networks, originally referred to as Wolf-type III metallopolymers⁴ or coordination polymers,⁹ and nowadays often named metal–organic frameworks (MOFs).¹⁰ These infinite assemblies resemble ionic solids in their essence and lead to robust and inert scaffolds possessing variable porosities. When trivalent emissive lanthanides occupy some nodes of the network, the associated luminescence can be tuned by metal doping¹¹ or by external stimuli such as temperature jumps or mechanical stress.¹² However, the production of solvent-resistant lanthanidopolymers compatible with partial metal occupancy, specific organization, flexible shaping and adaptable sensing properties is not yet available. Current efforts exploit the empirical mixing of polymeric organic receptors with metallic salts, from which non-soluble adducts are isolated. Their limited reproducibility and difficult characterization¹³ severely restrict pertinent developments along this line. An alternative approach proposes to take advantage of the powerful toolkit of reversible thermodynamics that has been developed to unravel multiple protonations in linear polyelectrolytes,¹⁴ for the in-depth deciphering of the loading of linear polymeric multi-terdentate receptors P1^N with neutral luminescent [Ln(hfa)₃] complexes (Ln = trivalent lanthanide, hfa = hexafluoroacetylacetonate) in dichloromethane (Fig. 1a).¹⁵ In solution, lanthanide tris-betadiketonate complexes exist as Lewis base adducts [Ln(hfa)₃X] where X is a solvent molecule or an additional donor atom borne by co-ligands. Upon reaction with the target binding site of a polymer, for instance P1^N, only the core [Ln(hfa)₃] is transferred (Fig. 1a). In order to use a single concept for the source [Ln(hfa)₃X] and the transferable core [Ln(hfa)₃], we use the term ‘neutral lanthanide container’ for describing these beta-diketonate coordination complexes. Using the site-binding model,^14,16 complexation processes can be modeled using two simple parameters: (1) the free energy of the intermolecular affinity of the entering metal for one terdentate N₃ site , which is the unique parameter to be considered for a monomeric unit and (2) the inter-site interaction energy which accounts for modulations of the intrinsic affinity when multiple binding events operate in binuclear (or higher-order) linear receptors (i.e. cooperativity).¹⁷ Whereas major attention is usually focused on the maximization of host-guest affinities (i.e. minimizing ), an instructed programming of is more tricky since both negative (cooperative) or positive (anti-cooperative) issues are attractive for specific applications.

Fig. 1 (a) Thermodynamic model showing successive intermolecular connections of [Ln(hfa)₃] to one-dimensional multi-terdentate receptors P1^N (N = 10–30).¹⁵ is the intrinsic intermolecular affinity and is the nearest-neighbor interaction (color code: C = grey, N = blue, O = red, F = green, Ln = orange). (b) Pictorial representation of the effect of intermetallic interactions on the microstates of a half-filled metallopolymer (orange disc = occupied site, white disc = empty site).

For instance, metal clustering compatible with efficient short-range intermetallic communication and energy migration¹⁸ results when (cooperative process, top of Fig. 1b). In contrast, metal alternation produced by anti-cooperative processes (, bottom of Fig. 1b) in half-filled linear metallopolymers is appealing for applications in drug sensing,¹⁹ light-upconversion²⁰ and metal segregation.²¹ Finally, (middle of Fig. 1b) produces uncontrolled statistical metallic dispersion, a behavior found in most doped MOFs, and which is encountered for metallopolymers possessing large inter-site separations.^14c The experimental magnitude of the inter-site interaction parameters observed for the loading of polymers P1^N with symmetrical [Ln(hfa)₃] containers (Ln = Y, La, Eu and Lu, Fig. 1a) in dichloromethane did not exceed the average thermal energy (RT = 2.5 kJ mol⁻¹), and therefore no organization could be induced on the macroscopic scale.¹⁵

A recent focus¹⁷ on a family of monomeric L1 and dimeric model ligands L2–L4 (Fig. 2) demonstrated that (i) the intrinsic affinities exponentially decrease with the number of binding sites, (ii) the inter-site interactions are affected by the relative geometries of the binding sites and (iii) minor changes in solvation processes, well described by the Onsager equation,²² are responsible for the tuning of these thermodynamic parameters. With these theoretical toolkits in hand, we suspect that the successive fixation of unsymmetrical lanthanide containers [Ln(tfa)₃], [Ln(pta)₃], [Ln(tta)₃] and [Ln(pbta)₃], instead of symmetrical [Ln(hfa)₃] (Fig. 3), could provide magnified anti-cooperative processes.¹⁷ Taking the same set of ligands L1–L4 as a proof-of-concept, we report here that unsymmetrical [Ln(pbta)₃] follows the predicted trend and exhibits, to the best of our knowledge, the largest intermetallic inter-site interactions ever quantified for neutral polynuclear lanthanide complexes.

Fig. 2 Chemical structures of ligands L1–L4 gathering the thermodynamic intermolecular affinities and nearest-neighbor interactions for their loading with [Eu(hfa)₃] (CD₂Cl₂, 298 K).

Fig. 3 Reaction pathways for the syntheses of unsymmetrical lanthanide containers.

Results and discussion

Synthesis and characterization of unsymmetrical neutral lanthanide containers [Ln(tfa)₃dig], [Ln(pta)₃dig], [Ln(tta)₃dig] and [Ln(pbta)₃dig]

The desired unsymmetrical lanthanide containers were synthesised following the strategy developed for [Ln(hfa)₃dig].²³ Bis(2-methoxyethyl)ether (diglyme = dig) was added to a suspension of either lanthanide oxide Ln₂O₃, hydroxide Ln(OH)₃·xH₂O or chloride LnCl₃·xH₂O (combined with 3 eq. NaOH) in aprotic solvent (CH₂Cl₂, hexane or toluene) followed by the addition of the pertinent β-diketonate ligand. Purification by crystallisation or sublimation provided the target products [Ln(X)₃dig] (X = pta, tta and pbta) in various yields (Fig. 3 and Appendix 1 in the ESI†). The europium container [Eu(tfa)₃dig] could not be obtained and the reaction stopped with the formation of Eu(tfa)₃·xH₂O, probably because the bound anionic diketonate ligands are too strong as donors for releasing sufficient residual positive charge on the lanthanide for further complexation with neutral diglyme. A similar limitation was encountered with pivaloyl-trifluoroacetone (pta) ligands, for which tricky sublimations of intricate mixtures provided only 9% of the target product [Eu(pta)₃dig]. The remaining electron-deficient β-diketonate ligands, tta and pbta, gave [Ln(tta)₃dig] and [Ln(pbta)₃dig] (Ln = La, Eu, and Y) in moderate (Ln = Y) to good (Ln = Eu and La) yields.

The ¹H and ¹⁹F-NMR spectra confirm the formation of pseudo-C₃ species in solution, which are dynamic on the NMR time scale (Fig. 4 and S1–S4 in the ESI†).²⁴ Elemental analyses of the microcrystalline powders point to the formation of unsolvated [Ln(β-diketonate)₃dig] adducts (Appendix 1), a trend confirmed by the X-ray crystal structures solved for [Eu(tta)₃dig] (1), [Y(tta)₃dig] (2) and [Eu(pbta)₃dig] (3) (Fig. 5, S4–S6 and Tables S1–S13†). All bond lengths in the europium complexes [Eu(tta)₃dig] (1) and [Eu(pbta)₃dig] (3) are within the standard range and are comparable to those reported for symmetrical [Eu(hfa)₃dig] (Table S13†),²⁵ a trend in line with similar affinities displayed by any of the fluorinated β-diketonate ligands (hfa⁻, tta⁻ and pbta⁻) for the central trivalent cation.

Fig. 4 ¹H NMR spectra of europium-containing lanthanide containers (a) [Eu(hfa)₃dig], (b) [Eu(pta)₃dig], (c) [Eu(tta)₃dig] and (d) [Eu(pbta)₃dig] with numbering scheme (CDCl₃, 298 K).

Fig. 5 Molecular structures of (a) [Eu(tta)₃dig] (1), (b) [Y(tta)₃dig] (2) and (c) [Eu(pbta)₃dig] (3) as observed in their crystal structures. Color codes: C = grey, O = red, S = yellow, F = green, Eu = magenta, and Y = light blue. The hydrogen atoms have been omitted for clarity.

Subject to SHAPE²⁶ analysis, the EuO₉ coordination spheres correspond best to monocapped square antiprisms with the central oxygen donor of the diglyme ligand occupying the capping position (Fig. S7a†). The smaller size of trivalent yttrium leads to eight-coordination in [Y(tta)₃dig] (2) where the diglyme acts as a bidentate ligand (Fig. 5b). The eight bound oxygen atoms occupy the vertices of an approximate square antiprism,²⁶ a trend in line with quantum chemical calculations performed on [Y(tta)₃(H₂O)_x] in the gas-phase.²⁷ Compared with [Eu(tta)₃dig] (Fig. 5a), two tta diketonate ligands are significantly twisted in [Y(tta)₃dig] (Fig. S8†). All bond valences significantly increase as a result of the larger residual charge carried by the eight-coordinate Y^III (Table S13†). The electric dipole moments computed for the gas-phase optimised geometries (Fig. S7b†) roughly point along the Ln–O_central(diglyme) bond direction and decrease in the order hfa (7.7 D) > pbta (6.7 D) > tta (4.6 D), a tendency which contrasts with pioneer calculations performed for six-coordinate [Ln(β-diketonate)₃] systems²⁸ and with chemical intuition, which naively associates larger dipole moments with unsymmetrical β-diketonate ligands. We conclude that the magnitudes of the dipole moments in [LnX₃dig] are indeed controlled by the residual positive charge borne by the trivalent lanthanide, which increases with the electron-withdrawing effect of the bound β-diketonate ligands (hfa > pbta ≫ tta).

Thermodynamic loading of the terdentate ligand L1 with the unsymmetrical neutral lanthanide containers [Ln(pta)₃dig], [Ln(tta)₃dig] and [Ln(pbta)₃dig]: a strategy for addressing intrinsic intermolecular affinities

The ¹H NMR titrations of 10 mm of the monomeric terdentate ligand L1 with [LnX₃dig] (Ln = La, Eu, and Y; X = pta, tta and pbta; dig = diglyme) conducted in dichloromethane containing an excess of diglyme ([dig]_tot = 0.14 m) show the formation of a single adduct [LnX₃L1] complex according to eqn (1) (Fig. 6 and Tables S14 and S15†).^15a

Fig. 6 ¹H NMR titration of L1 (10 mM) with [La(pbta)₃dig] in CD₂Cl₂ + 0.14 m diglyme at 298 K.

Since the total concentration of diglyme does not significantly vary, the stability constant associated with the ligand exchange process depicted in eqn (1) reduces to the conditional association constant summarized in eqn (2), where is the conditional affinity factor of a terdentate site for a [LnX₃] container (Fig. 1a).^15a

Experimentally, the integrations of the ¹H NMR signals recorded for a given proton connected either to the free ligand L1 (I_L1^H) or to the coordinate ligand in [LnX₃L1] can be combined to determine the occupancy factor θ_Ln^L1 (left part of eqn (3)), which is related to the association constant via the Langmuir binding isotherm (right part of eqn (3)).^15a

Repeating this procedure for different 〈[L1]_tot;[Ln]_tot〉 ratios (Fig. 6) yields occupancy factors θ_Ln^L1 (eqn (3), left) for various free concentrations of the lanthanide container [LnX₃dig] (eqn (3), middle). Plots of θ_Ln^L1 related to log([LnX₃dig]) (Fig. 7), often referred to as binding isotherms, are fit to eqn (3) (right part) using non-linear least-square techniques to give the conditional association constant collected in Table S16† and the intrinsic exchange affinities depicted in Fig. 8.

Fig. 7 Binding isotherms built from ¹H NMR titrations of L1 with [LaX₃dig] (X = hfa (red), tta (green) or pbta (blue)) in CD₂Cl₂ + 0.14 m dig at 293 K. The dotted traces correspond to the fitted curves computed with eqn (3) and using collected in Table S16.†

Fig. 8 Intrinsic exchange affinities for the loading of monomeric ligand L1 with [Ln(hfa)₃dig] (red), [Ln(tta)₃dig] (green) and [Ln(pbta)₃dig] (blue) in CD₂Cl₂ at 298 K. 1/R^Ln_CN=9 is the inverse of the nine-coordinate lanthanide radius. The dotted trend lines are provided as guides.

For the unsymmetrical perfluorobenzene-trifluoroacetylacetonate lanthanide containers [Ln(pbta)₃dig] (Ln = La, Eu, and Y, the blue traces in Fig. 7 and 8), the intrinsic exchange affinities (eqn (1)) increase by 8–14% with respect to those found for the famous symmetrical [Ln(hfa)₃dig] analogues (red traces in Fig. 7 and 8). For the unsymmetrical, but less electron-deficient thienyl-trifluoroacetylacetonato lanthanide containers [Ln(tta)₃dig] (Ln = La, Eu, and Y, the green traces in Fig. 7 and 8), the affinity for L1 is still detectable, but low enough when Ln = Y for providing fast exchange on the NMR scale. In these conditions, the monitoring of a single broadened ¹H NMR signal δ^H_obs for each proton in solution during the titration prevents the use of eqn (3), which is replaced by eqn (4) for estimating the occupancy factor θ_Y^L1 (δ_L1^H and stand for the chemical shifts of the same proton in pure L1 and [Y(tta)₃L1], respectively).

For these [Ln(tta)₃] containers, occupancy factors larger than 20% are only obtained in a large excess of [Ln(tta)₃dig], the total concentrations of which may reach 0.1 M in solution. The express requisite for using an invariant total concentration of diglyme in eqn (2) is no more obeyed and deviations from simple Langmuir isotherms are observed (green trace in Fig. 7). The conditional association constants reported for [Ln(tta)₃L1] complexes are therefore only mere estimations. Finally, the pyvaloyl-trifluoroacetylacetonate europium container [Eu(pta)₃dig], which bears the lowest residual positive charge on the metal,²⁹ displayed no measurable affinity for L1 in CD₂Cl₂ at millimolar concentrations, a result in line with the lack (to the best of our knowledge) of crystal structures reporting a nine-coordinate [Ln(pta)₃L] adduct, where L is a terdentate chelate nitrogen donor ligand.³⁰ Leaving pta-based complexes aside, pure [Ln(pbta)₃L1] (Ln = La, Eu, and Y) and [Ln(tta)₃L1] (Ln = La and Eu) complexes could be isolated as crystalline materials from concentrated solutions in 41–92% yield (Appendix 1, Tables S13, S14 and S17†). The crystal structure of [Eu(pbta)₃L1]·CH₂Cl₂ (4) confirms nine-coordination around the europium cation by the three nitrogen atoms of the bound aromatic ligand and the six oxygen atoms of the three bidentate pbta anions (Fig. 9, Tables S18–S21 and Fig. S9†). The [EuN₃O₆] chromophore adopts a distorted monocapped square antiprism geometry,²⁶ very similar to that previously described for [La(hfa)₃L1] (Table S22 and Fig. S10†).^15a

Fig. 9 Molecular structure of [Eu(pbta)₃L1] found in the crystal structure of [Eu(pbta)₃L1]·CH₂Cl₂ (4). Color codes: C = grey, O = red, F = green, and Eu = magenta. The hydrogen atoms have been omitted for clarity.

In conclusion, two strong electron-withdrawing groups per β-diketonate in hfa (two CF₃ groups) or in pbta (CF₃ and C₆F₅) are required in order to get intrinsic affinities large enough to form significant amounts of [LnX₃L1] at millimolar concentrations (Fig. 8, X represents the β-diketonate anion). The connections of less electron-withdrawing thenoyl (X = tta) or pivaloyl groups (X = pta) are detrimental in terms of the global stabilities of the adducts. This trend is correlated with the dipole moments computed for the gas-phase optimized [YX₃L1] complexes, which reveal that the global asymmetry of the charge distribution increases along the series [Y(pbta)₃L1] (μ = 13.1 D) ≈ [Y(hfa)₃L1] (μ = 13.8 D) < [Y(tta)₃L1] (μ = 14.9 D, Fig. S11†).

Thermodynamic loading of multi-terdentate ligands L2–L4 with unsymmetrical neutral lanthanide containers [Eu(tta)₃dig] and [Eu(pbta)₃dig]: a strategy for addressing nearest-neighbor interactions

A reliable analysis of the inter-site interactions , which control the successive binding of two adjacent [LnX₃] containers in linear multi-site receptors, requires the investigation of the dimeric receptors L2–L4 (eqn (5) and (6)), for which the cumulative conditional association constants are given in eqn (7) and (8).

According to the site binding model,¹⁶ (eqn (7)) reflects the intrinsic binding affinity of the lanthanide container for a single terdentate site, but modulated by a statistical factor of 2 in a dimer. With this in mind, (eqn (8)) easily provides an estimation of the inter-site interactions measured as their Boltzmann factors , a parameter often referred to as the allosteric cooperativity factor.³¹ As the intrinsic affinity of [Ln(tta)₃dig] for the terdentate binding unit is too weak to produce significant quantities of the binuclear [Ln₂(tta)₆Lk] complexes at millimolar concentrations, the thermodynamic studies performed on the symmetrical [Eu₂(hfa)₆Lk] dimers¹⁷ are completed here with the recording of ¹H and ¹⁹F NMR spectra for the complexation of L2–L4 (5 mm) with [Eu(pbta)₃dig] in CD₂Cl₂ + 0.14 m diglyme (Fig. S12†). The integrations of the various signals (I^H and I^F) gave occupancy factors (eqn (9) and Fig. 10), which could be fitted to eqn (10) to get the intrinsic affinities and cooperativity factors collected in Fig. 11 (Table S23†).

Fig. 10 Occupancy factors and binding isotherms built from NMR titrations of L3 with [Eu(hfa)₃dig] (red disks, ¹H NMR)¹⁷ and [Eu(pbta)₃dig] (blue triangles for ¹H NMR, blue crosses for ¹⁹F NMR) in CD₂Cl₂ + 0.14 m dig at 298 K. The dotted traces correspond to the fitted curves computed with eqn 10 and using and collected in Table S23.†

Fig. 11 (a) Intrinsic exchange affinities and (b) intermetallic interactions for the loading of dimeric ligands L2–L4 (geometries of the spacers are highlighted) with [Eu(hfa)₃dig] (red) and [Eu(pbta)₃dig] (blue) (CD₂Cl₂, 298 K).

In line with the trend observed for the monomeric ligand L1 (Fig. 8), the intrinsic affinities of a terdentate unit in the dimeric ligands L2–L4 are roughly 10% larger for [Eu(pbta)₃dig] than for [Eu(hfa)₃dig] (Fig. 11a). The observed intermetallic interactions are systematically shifted toward more anti-cooperative processes for [Eu₂(pbta)₆Lk] complexes (Fig. 11b).

This drift culminates for [Eu₂(pbta)₆L3], in which the anti-cooperative interaction between the two unsymmetrical [Eu(pbta)₃] containers amounts to almost +5 kJ mol⁻¹, a value which should prevent a statistical redistribution due to thermal energy in half-filled linear polymers integrating this bis-terdentate structural motif. Considering that (i) the intermetallic distances are similar in the structures of [Eu₂(hfa)₃L3] (Eu⋯Eu = 12.61 Å in its crystal structure)¹⁷ and [Eu₂(pbta)₃L3] and (ii) the dipole moment calculated for [Y(pbta)₃L1] is only slightly smaller than that found for [Eu(hfa)₃L1] (Fig. S11†), we assign the gain in intermetallic repulsion by a factor of 3.7(6), going from [Eu₂(hfa)₃L3] (symmetrical β-diketonate) to [Eu₂(pbta)₃L3] (unsymmetrical β-diketonate), to specific solvation effects which are known to largely dominate over the minor contributions produced by intramolecular dipole–dipole interactions.¹⁷

Photophysical properties and luminescence quantum yields of the europium containers [EuX₃(dig)] (X = hfa, tta, pbta) and of their mononuclear [EuX₃L1] and binuclear [Eu₂(hfa)₆Lk] complexes (Lk = L2–L4)

The electronic absorption spectra of the europium containers [EuX₃(dig)] (X = hfa, tta, and pbta) are dominated by β-diketonate-centred π* ← π transitions occurring in the near-UV range (dotted traces in Fig. 12 and S13†). The maximum of the absorption envelope is shifted stepwise toward lower energy upon the replacement of the trifluoromethyl substituents found in [Eu(hfa)₃dig] (33 100 cm⁻¹, Fig. S13†) with aromatic groups in [Eu(pbta)₃dig] (32 000 cm⁻¹ for one pentafluorobenzene unit, Fig. 12b) and [Eu(tta)₃dig] (30 000 cm⁻¹ for one thenoyl unit, Fig. 12a). Exchanging the saturated diglyme ligand with the aromatic terdentate ligand L1 in [EuX₃L1] provides absorption spectra (full traces in Fig. 12) which combine the contributions of each individual chromophore, i.e. the β-diketonate-centred π* ← π transitions (dotted traces in Fig. 12) together with L1-centered and transitions (dashed traces in Fig. 12).³² Room-temperature excitations ([small nu, Greek, tilde]_exc = 26 000–30 000 cm⁻¹) of the [EuX₃dig] or [EuX₃L1] complexes (X = hfa, tta, and pbta) in solution (Fig. 13 and S14†) or in the solid state (Fig. S15 and S16†) produce typical europium-centred red luminescence, which results from indirect sensitization according to the antenna mechanism.³⁴ The initial ligand-centred ¹π* ← ¹π excitation is followed by a (probable)³⁵ intersystem crossing process, then L1 → Eu and/or X → Eu energy transfers occur, thus leading to Eu(⁵D₁) and Eu(⁵D₀)-centred emissions.^34,35 The europium-centred emission bands recorded for [EuX₃dig] and [EuX₃L1] are diagnostic for the existence of low-symmetry tris-β-diketonates Eu(III) complexes with (i) Eu(⁵D₁ → ⁷F_J) emissions being far less intense than for Eu(⁵D₀ → ⁷F_J)^15a,33 and (ii) Eu(⁵D₀ → ⁷F_J) emissions being dominated by the 2J + 1 = 5 split components of the hypersensitive forced electric dipolar ⁵D₀ → ⁷F₂ transition monitored around 16 320 cm⁻¹ (Fig. 13 and S14†).³⁶ Only minor changes can be detected in terms of the relative intensities, splitting and energies for the Eu(⁵D₀ → ⁷F_J) transitions when the diglyme in [EuX₃dig] is replaced with L1 in [EuX₃L1]. However, the global quantum yields Φ^L_Eu, recorded upon UV-irradiation of the ligands while measuring the Eu-centered emission, rise by factors of 2 to 10 going from [EuX₃dig] (3% ≤ Φ^L_Eu ≤ 10%) to [EuX₃L1] (22% ≤ Φ^L_Eu ≤ 45%, Table S24†). These considerable global quantum yields are typical among tris-β-diketonate complexes, for which a systematic optimization may lead to 40–70% quantum yield values.^24,37

Fig. 12 Electronic absorption spectra recorded for (a) L1 (dashed trace), [Eu(tta)₃dig] (dotted trace) and [Eu(tta)₃L1] (full trace), and (b) L1 (dashed trace), [Eu(pbta)₃dig] (dotted trace) and [Eu(pbta)₃L1] (full trace) in CH₂Cl₂ + 10⁻⁶ m diglyme at 293 K. The absorption spectra were recorded for [Eu]_tot = 10⁻⁵ m and corrected for partial dissociation in solution (Appendix 2).

Fig. 13 Emission spectra recorded for (a) [EuX₃dig] with X = hfa ([small nu, Greek, tilde]_exc = 29 850 cm⁻¹, [Eu]_tot = 8.6 × 10⁻⁵ m, red full trace), X = tta ([small nu, Greek, tilde]_exc = 28 570 cm⁻¹, [Eu]_tot = 1.8 × 10⁻⁵ m, green dashed trace) and X = pbta ([small nu, Greek, tilde]_exc = 29 850 cm⁻¹, [Eu]_tot = 3.3 × 10⁻⁵ m, blue dotted trace) and for (b) [EuX₃L1] with X = hfa ([small nu, Greek, tilde]_exc = 25 840 cm⁻¹, [Eu]_tot = 4.0 × 10⁻⁵ m, red full trace), X = tta ([small nu, Greek, tilde]_exc = 25 000 cm⁻¹, [Eu]_tot = 5.5 × 10⁻⁴ m, green dashed trace) and X = pbta ([small nu, Greek, tilde]_exc = 25 970 cm⁻¹, [Eu]_tot = 6.1 × 10⁻⁵ m, blue dotted trace) in CH₂Cl₂ + 10⁻⁶ m diglyme at 293 K.

More precisely, we note that the quantum yield Φ^L_Eu = 5(1)% obtained here for [Eu(hfa)₃dig] in dichloromethane is much lower than Φ^L_Eu = 35% reported for the same complex in ether : isopentane : ethanol (5 : 5 : 2).³⁸ In contrast, those obtained for [EuX₃L1] in the solid state (15–40%) are in line with Φ^L_Eu = 20–30% previously reported for closely related [Eu(hfa)₃L] complexes where L is a terdentate N-donor ligand.³³ Although no detailed separation of radiative and non-radiative contributions to the various rate constants implied by the antenna mechanism are currently available, the observed mono-exponential millisecond range Eu(⁵D0) lifetimes (Table S24†) reveal a reasonable protection of the Eu(III) emissive center against the harmonics of high-energy oscillators in these unsymmetrical complexes. Finally, the introduction of bis(hexyloxy)phenyl spacers in the binuclear complexes [Eu₂(hfa)₆Lk] (Lk = L2–L4) has a deleterious effect on the global quantum yields, which are reduced by one order of magnitude (2% ≤ Φ^L_Eu ≤ 5%) regarding those collected for the mononuclear analogues [Eu(hfa)₃Lk]. This specific effect was previously documented for related binuclear europium complexes^35a and unambiguously assigned to a drastic reduction of the Lk → Eu(III) energy transfer rate constants when electron-donor groups are connected to the phenyl spacers. Our overview of the luminescence properties of [EuX₃dig], [Eu(hfa)₃L1] and [Eu₂(hfa)₆Lk] confirms that the replacement of symmetrical X = hfa ligands with unsymmetrical β-diketonate analogues (X = tta and pbta) does not dramatically impact the global photophysical properties of these complexes, which remain highly luminescent and therefore compatible with the preparation of strong red emissive lanthanidopolymers assuming that the thermodynamic metal loading is under control.

Conclusion

In line with theoretical predictions,¹⁷ the replacement of symmetrical X = hfa with unsymmetrical X = tta or X = pbta β-diketonates in nine-coordinate neutral [LnX₃dig] lanthanide containers induces major changes in the thermodynamic loading of terdentate N₃ binding units. For both monomeric [LnX₃L1] and dimeric [Eu₂X₆Lk] complexes (Lk = L2–L4), the intrinsic affinity depends on the residual positive charge borne by the metal. It increases in the order , which matches the electron-withdrawing character of the substituent groups located in the β-diketonate ligand (Fig. 8 and 11a). The supplementary inter-site repulsion operating in the dinuclear [Eu₂X₆Lk] complexes (Lk = L2–L4) is greatest for [Eu₂(pbta)₆L3] (Fig. 11b) with a remarkable anti-cooperative factor of , which translates into a repulsive free energy of . The latter value is almost twice as large as the room temperature thermal energy.

Assuming a thought experiment in which (i) a linear infinite polymer P2^N (N → ∞) based on the meta-substituted phenyl spacers, as found in L3, is at hand (Fig. 14 top), and (ii) the anti-cooperative factors and found for L3 similarly operate in P2^N, the cluster expansion techniques¹⁴ predict the appearance of an unprecedented identifiable ‘plateau’ in the binding isotherm of the potential unsymmetrical linear {[Eu(pbta)₃]_mP2^N} lanthanidopolymer (N → ∞, full red trace in Fig. 14), while its symmetrical analogue {[Eu(hfa)₃]_mP2^N} would exhibit only the standard stochastic distributions of metal ions (dashed black trace in Fig. 14). In other words, a predictable |P2^N|_tot/|Eu(pbta)₃|_tot stoichiometric ratio should provide an inflection point at θ_Eu = 0.5, where the half-filled polymer {[Eu(pbta)₃]_N/2P2^N} is dominated by the formation of a microspecies, in which metal-occupied sites and metal-free sites strictly alternate (Fig. 1b bottom). The exact physical origin of this beneficial anti-cooperative effect remains elusive in the absence of quantitative access to solvation energies³⁹ and Born–Haber cycles,^17,22,40 but computational efforts are underway for unravelling the conditions required for exploiting this unprecedented effect for the preparation of highly organized metallopolymers.

Fig. 14 Occupancy factors and binding isotherms computed for the loading of polymer P2^N (N → ∞) in CH₂Cl₂ + 0.14 m diglyme at 293 K with [Eu(hfa)₃dig] (dotted black trace, , Table S23†) and [Eu(pbta)₃dig] (full red trace, , Table S23†).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The Swiss National Science Foundation is gratefully acknowledged for financial support. The work performed in France was supported by La Ligue contre le Cancer, Cancéropôle Grand Ouest and INSERM.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1551909–1551912. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7sc03710d

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