Site-selective Eu(III) spectroscopy of highly efficient luminescent mixed-metal Pb(II)/Eu(III) coordination polymers

Abstract

Novel mixed-metal coordination polymers (CPs) based on Pb_1−xEu_x–BDC (x = 0.05, 0.10, 0.25, 0.50 and BDC = 1,4-benzenedicarboxylic acid) are reported. The materials display different coordination structures as the Pb : Eu metal ratio is modified. Whereas for x < 0.25 the Eu³⁺ ions essentially replace the Pb²⁺ ones in the [Pb(BDC)_n] structure, for x > 0.25 the samples are isotypical with [Eu₂(BDC)₃(H₂O)₄], as shown by powder (including Rietveld analysis) and single-crystal X-ray diffraction. The unique Pb_0.75Eu_0.25–BDC compound displays a mixture of both crystalline phases with the highest emission quantum yield value reported for luminescent CPs based on Pb(II), 69 ± 7%. Site-selective excitation using a common Xe lamp permits the detailed study of the two Eu³⁺ local sites, illustrating how powerful the Eu³⁺ luminescence is as a local probe spectroscopic tool.

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Introduction

Coordination polymers (CPs) and Metal–Organic Frameworks (MOFs) constitute ionic classes of hybrid materials having received a wide scientific attention.^1–4 The hybrid character of these materials arising from the coordination between metal ions (or their clusters) and multifunctional organic ligands grant them a wide range of applications.^4–6 Mixed-ligand and mixed-metal CPs have been then synthesized through distinct synthetic protocols based in mixtures of different types of organic ligands and metal ions.^7–10 The presence of more than one metal ion or organic ligand in CP structures permits a fine tuning of the material's properties, e.g., optical, catalytic, electronic, host–guest, magnetic, shape specificity and ion exchange.^11–15

In particular, Pb-containing CPs present interesting luminescent properties,^16–18 and mixed-metal materials based on Pb²⁺ have been previously reported.^19–21 Nevertheless, materials based on Pb²⁺ and Ln³⁺ ions are scarce. The first example of a 6p–4f heterometallic MOF was reported by Wang et al.²² and, more recently, Pan et al. showed white-light emission in two series of heteronuclear Pb²⁺–Ln³⁺ complexes.²³

Here, we report new photoluminescent 6p–4f heterometallic CPs, based on distinct Pb²⁺/Eu³⁺ ratios and 1,4-benzenedicarboxylic acid (BDC). The Pb_1−xEu_x–BDC (x = 0.05, 0.10, 0.25, 0.50 and 1.00) materials were characterized by X-ray powder diffraction, single crystal X-ray diffraction and luminescence spectroscopy. Whereas for x = 0.05 and 0.10 the Pb_1−xEu_x–BDC compounds are structurally similar to [Pb(BDC)]_n,^24–27 for x = 0.50 the CP exhibits the structure of the [Eu₂(BDC)₃(H₂O)₄] hydrated phase.^28,29 The Pb_0.75Eu_0.25–BDC compound displays a mixture of both crystalline phases with a intriguing high emission quantum yield (69 ± 7%). Moreover, site-selective and time-resolved spectroscopy enabled the detailed study of the two Eu³⁺ local sites in that CP illustrating how powerful the Eu³⁺ luminescence is as a local probe spectroscopic tool. Although Eu³⁺ luminescence was often used as a local structural probe in nanoparticles,^30,31 sol–gel derived hybrid materials^32,33 and ceramics,^34,35 this is, as far as we know, the first example of the use of this powerful tool in CPs and MOFs.

Experimental

Synthesis

Disodium terephthalate, C₆H₄(COONa)₂ (Na₂BDC, lç 0 Alfa Aesar, 99.0%), Pb(NO₃)₂ (VETEC, 99.0%) and Eu(NO₃)₃·5H₂O (Aldrich, 99.9%) were analytical grade and used as purchased. All solutions were prepared with ultrapure water. With the overall aim to include in the Pb²⁺–terephthalic acid optically-active as Eu³⁺ ions in order to design new photoluminescent materials, Pb_1−xEu_x–BDC (x = 0.05, 0.10, 0.25, 0.50) CPs were prepared using different Pb²⁺/Eu³⁺ (mmol mmol⁻¹) molar ratios (0.95 : 0.05, 0.90 : 0.10, 0.75 : 0.25, 0.50 : 0.50) and 0.50 mmol of Na₂BDC, with 14 mL of H₂O. The mixture was transferred to Teflon® lined autoclaves (25 mL) and the reaction took place under static hydrothermal conditions in ovens preheated at 393 K over a period of 3 days. The obtained materials were filtered, washed at room temperature with ultrapure water and dried at 313 K. For comparative purposes, the monometallic [Pb(BDC)]_n and [Eu₂(BDC)₃(H₂O)₄] CPs were synthesized following the same procedure described above.

Scanning electron microscopy (SEM)

SEM images of Pb_1−xEu_x–BDC (x = 0.05, 0.10, 0.25 and 0.50) were acquired using a Shimadzu SS550 microscope with a tungsten filament operated at 15–20 kV. SEM images of the [Pb(BDC)]_n and [Eu₂(BDC)₃(H₂O)₄] CPs were collected using a Hitachi TM3000 microscope operated at 20 kV.

Powder X-ray diffraction (PXRD)

The PXRD data were collected at room temperature (300 K) on a Bruker D8 Advance powder X-ray diffractometer using Bragg–Brentano geometry with Cu K_α radiation. The diffractograms were acquired in the angular range 2θ between 5° and 50° in steps of 0.01°. The crystallinity and phase mixtures have been determined by Rietveld refinements to the XRD patterns.³⁶ The Rietveld refinement for Pb_1−xEu_x–BDC (x = 0.05, 0.10, 0.25, 0.50) were performed with the GSAS/EXPGUI³⁷ software using as starting premise the atomic coordinates of the previously reported [Pb(BDC)]_n (ref. 24–27) and [Eu₂(BDC)₃(H₂O)₄]^28,29 structures. The preferential orientation was corrected using the spherical harmonic model (tenth order) proposed by Jarvinen,³⁸ the peak profile was adjusted by the Thompson–Cox–Hastings function modified by Young and Desai (pV-TCHZ),^39,40 the surface roughness correction was refined by the Pitschke function and the background was fitted by an eighth-degree shifted Chebyshev polynomial function.⁴¹ In the final run, the following parameters were refined: scale factor, background and absorption coefficients, spherical harmonic, unit-cell parameters and pV-TCHZ correction. The quantitative phase analysis was performed in accordance with the method proposed by Howard et al.⁴²

Single-crystal XRD of Pb_0.90Eu_0.10–BDC

Single crystals of the mixed-metal Pb_0.90Eu_0.10–BDC CPs were manually harvested from dried samples and immersed in highly viscous FOMBLIN Y perfluoropolyether vacuum oil (LVAC 140/13, Sigma-Aldrich).⁴³ Crystals were mounted on a Hampton Research CryoLoop with the help of a Stemi 2000 stereomicroscope equipped with Carl Zeiss lenses. XRD data were collected at 180(2) K on a Bruker X8 Kappa APEX II charge-coupled device (CCD) area-detector diffractometer (Mo K_α graphite-monochromated radiation, λ = 0.7107 Å) controlled by the APEX2 software package,⁴⁴ and an Oxford Instruments Cryostrem 700 Series low temperature device monitored remotely by the software interface Cryopad.⁴⁵ Diffraction images were processed using the software package SAINT+,⁴⁶ and data were corrected for absorption by the multiscan semi-empirical method implemented in SADABS.⁴⁷ The structure was solved using the algorithm implemented in SHELXT-2014,⁴⁸ which allowed the immediate location of almost all of the heaviest atoms composing the asymmetric unit. The remaining missing and misplaced non-hydrogen atoms were located from difference Fourier maps calculated from successive full-matrix least-squares refinement cycles on F² using the latest SHELXL from the 2014 release.^48,49 All structural refinements were performed using the graphical interface ShelXle.⁵⁰

The inclusion of both Pb²⁺ and Eu³⁺ metallic centers in the crystal structure was achieved by restraining the atomic coordinates and the thermal parameters to common values. In later stages of the overall refinement, the site occupancies of the two metal centers were allowed to refine freely, ultimately converging to values close to 0.85 and 0.10, respectively. These values were fixed for the last refinement of the crystal structure, ultimately accounting for the inclusion of 5% of site vacancies and explaining the reason why the unit cell contains a non-integer number of atoms. Hydrogen atoms bound to carbon were placed at their idealized positions using the HFIX 43 instruction in SHELXL. These hydrogen atoms were included in subsequent refinement cycles with isotropic thermal displacements parameters (U_iso) fixed at 1.2 × U_eq of the parent carbon atoms.

Crystal data for Pb_0.90Eu_0.10–BDC: C₈H₄Eu_0.10O₄Pb_0.85, M = 355.42, orthorhombic, space group Pbca, Z = 8, a = 7.6820(12) Å, b = 10.2672(15) Å, c = 18.256(3) Å, V = 1439.9(4) ų, μ(Mo-Kα) = 20.768 mm⁻¹, D_c = 3.279 g cm⁻³, colorless prism with crystal size of 0.08 × 0.06 × 0.02 mm³. Of a total of 11 484 reflections collected, 1922 were independent (R_int = 0.0452). Final R₁ = 0.0253[I > 2σ(I)] and wR₂ = 0.0543 (all data). Data completeness to theta = 25.24°, 99.8%. The last difference Fourier map synthesis showed the highest peak (1.476 e Å⁻³) and the deepest hole (−1.406 e Å⁻³) located at 0.69 and 0.78 Å from the mixed-metal site Pb1/Eu1, respectively. Structural drawings have been created using the software package Crystal Impact Diamond.⁵¹

Crystallographic data (including structure factors) for the crystal structure of compound Pb_0.90Eu_0.10–BDC have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC 1490068.†

Photoluminescence spectroscopy

Photoluminescence spectra were recorded on a Fluorolog-3® (Horiba scientific) with double-excitation spectrometer and a single-emission spectrometer (TRIAX 320) coupled to a R928 photomultiplier, using the front-face acquisition mode. The excitation source was a 450 W xenon lamp. The emission spectra were corrected for the spectral response of the excitation monochromator using a correction file provided by manufacture and the excitation spectra were weighed for the spectral distribution of the lamp intensity using a photodiode reference detector. Time-resolved emission spectra and emission decay curves were acquired using the same equipment with a pulsed Xe–Hg lamp (6 × 10⁻³ s pulse at half-width and 20–30 × 10⁻⁶ s tail) as the excitation source. The emission quantum yields were measured using the C9920-02 from Hamamatsu system with a 150 W Xe lamp coupled to a monochromator for wavelength discrimination, an integration sphere as sample chamber and a multichannel analyzer for signal detection. Three measurements were made for each sample, and the average values obtained are reported with accuracy within 10% accorded to the manufacturer.

Results and discussion

XRD and PXRD studies

Fig. 1 shows the PXRD data collected for [Pb(BDC)]_n, Pb_1−xEu_x–BDC, (x = 0.05, 0.10, 0.25, 0.50) and [Eu₂(BDC)₃(H₂O)₄]. The simulated patterns of the [Pb(BDC)]_n and [Eu₂(BDC)₃(H₂O)₄] compounds are also displayed. SEM images in Fig. 2 show that the size and morphology of the microcrystals are greatly influenced by the relative amount of incorporated Eu³⁺ ions. Similarly to [Pb(BDC)]_n, the crystal habit of Pb_1−xEu_x–BDC, with x = 0.05, 0.10, is based on block-like microcrystals. For x = 0.10, some crystals show the presence of a minor amount of smaller particles with a well-defined plate-like shape. These particles correspond to the [Eu₂(BDC)₃(H₂O)₄] phase (see below). However, the x = 10 microcrystals chosen for single-crystal XRD studies do not shown evidence of any plate-like particles, as confirmed by the corresponding structural elucidation: a single phase mixed-metal Pb_0.90Eu_0.10–BDC CP (see below). In Pb_1−xEu_x–BDC (x = 0.25 and 0.50) the block-like microcrystals are stacked together with well-defined platelets, analogous to those seen in [Eu₂(BDC)₃(H₂O)₄], Fig. 2. This mixture of phases is also discerned in PXRD and photoluminescence data.

Fig. 1 Simulated patterns of [Pb(BDC)]_n and [Eu₂(BDC)₃(H₂O)₄] and PXRD patterns of [Pb(BDC)]_n, Pb_1−xEu_x–BDC, with x = 0.05, 0.10, 0.25, 0.50, and [Eu₂(BDC)₃(H₂O)₄].

Fig. 2 SEM images of (A) [Pb(BDC)]_n, Pb_1−xEu_x–BDC CPs (B) x = 0.05, (C) x = 0.10, (D–F) x = 0.25, (G) x = 0.50 and (H) [Eu₂(BDC)₃(H₂O)₄].

The PXRD patterns show clearly that the mixed-metal Pb_1−xEu_x–BDC (x = 0.05 and 0.10) compounds are structurally similar to [Pb(BDC)]_n, whose structure was originally reported by Tan et al.²⁴ and later re-determined a handful of times.^25–27 This was unequivocally confirmed by Rietveld refinement, Fig. S1 and S2 (ESI†). Thus, we report the reader to these previous publications for additional structural details which are not described in this manuscript.

For approximately an 0.25% inclusion of Eu³⁺ a structural change occurs and Pb_0.75Eu_0.25–BDC is composed by a mixture of the [Pb(BDC)]_n phase and another known CP structure solely based on Eu³⁺:[Eu₂(BDC)₃(H₂O)₄].^28,29 For this phase the Eu³⁺ eight-coordination sphere involves six oxygen of the BDC ligand and two water molecules in a C₄ local pseudo-symmetry group.²⁹ Rietveld quantitative phase analysis reveals that Pb_0.75Eu_0.25–BDC has 0.90 and 0.10 of [Pb(BDC)]_n and [Eu₂(BDC)₃(H₂O)₄], respectively (Fig. S3, ESI†). These structures should be, somehow, related and to further investigate this we decided to employ a typical topological analysis in which each network is mathematically reduced into central nodes and connecting rods. Their selection needs, however, to be careful and structurally meaningful. Following the description by Alexandrov et al.,⁵² who suggested that any moiety (ligand or atoms) connecting more than two metallic centers (μ_n) should be considered as a network node, the center of gravity of the ligand itself was considered as a node, alongside with the metallic center. In this way, as revealed by the software package TOPOS, while Pb_0.90Eu_0.10–BDC is a 6,6-connected binodal network of the 6,6T23 topological type, [Eu₂(BDC)₃(H₂O)₄] is instead a 4,4,6-connected trinodal framework belonging to the 4,4,6T24 family. We note that for both structures the metallic centers are 6-connected. Because of the smaller ionic radius of Eu³⁺ and the absence of the lone pair of electrons of Pb²⁺, in the latter framework the overall connectivity of the organic linkers is reduced from six to four.

Pb_1−xEu_x–BDC, with x = 0.50 and 1.00, are structurally similar to [Eu₂(BDC)₃(H₂O)₄] (the Rietveld refinement for x = 0.50 is shown in Fig. S4, ESI†).

Single-crystal XRD studies of Pb_0.90Eu_0.10–BDC

For approximately an 10% inclusion of Eu³⁺ (Pb_0.90Eu_0.10–BDC), we were able to obtain single-crystals large enough which allowed an unequivocally structural elucidation using single-crystal XRD at low temperature. As expected, the unit cell parameters and space group match those previously reported for the pure Pb²⁺ material,^24–27 showing that the three-dimensional network remain essentially unaltered. However, during the refinement stages, a charge problem arose: because of the inclusion of +3 charges at the sites of the +2 charges, and in the absence of voids capable of accommodate partially-occupied counter-ions to balance the crystal charge, the metallic center site has mandatorily to also be occupied by vacancies. Indeed, during the last stage of the refinement stage the site occupancies for Pb²⁺ and Eu³⁺ were allowed to refine freely, concerning to values of ca. 0.85 and 0.10, respectively. This agrees well with a presence of ca. 5% of vacancies while the crystal charge remains neutral.

The mixed-metal Pb_0.90Eu_0.10–BDC CP is based on a single metallic center (M), statistically occupied by ca. 85% of Pb²⁺, 10% of Eu³⁺ plus 5% of vacancies (Fig. 3).

Fig. 3 Schematic representation of the highly distorted pentagonal bipyramidal coordination environment of the mixed-metal site present in compound Pb_0.90Eu_0.10–BDC. Atoms are represented as thermal ellipsoids drawn at the 80% probability level. Bond distances (in Å; M stands for Pb²⁺ or Eu³⁺): M1–O1 2.560(3); M1–O1^v 2.775(4); M1–O2ⁱⁱ 2.623(3); M1–O2^iv 2.652(3); M1–O3ⁱ 2.537(4); M1–O3ⁱⁱⁱ 2.646(3); M1–O4ⁱ 2.483(4). Symmetry transformations used to generate equivalent atoms: (i) x, −y + 3/2, z − 1/2; (ii) −x + 3/2, y + 1/2, z; (iii) −x + 2, y + 1/2, −z + 1/2; (iv) −x + 2, −y + 1, −z; (v) x + 1/2, −y + 3/2, −z.

As previously described for the other structural determinations,^24–27 the M center appear 7-coordinated, {MO₇}, with the overall coordination polyhedron being significantly distorted due to the presence of a stereoactive lone pair of electrons belonging to the Pb²⁺ metallic center. Indeed, considering the position of this lone pair of electrons as a possible coordination site, the overall coordination sphere resembles very much a highly distorted pentagonal bipyramid, with the M–O distances ranging between 2.483(4) and 2.775(4) Å. It is important to notice that there is a nearby oxygen atom from a carboxylate group which could be, in theory, interacting with the metallic center [at a distance of 3.037(4) Å]. A search in the Cambridge Structural Database (Version 5.37 with two updates – February 2016)⁵³ reveals that even though the known range of Pb–O interactions shows up distances up to 3.33 Å, the number of structures in which this appears is extremely small with the vast majority showing values around the mean value of 2.61 Å.

This coordination environment resembles a polyhedron with a C_2v pseudo-symmetry. Taking into account the fact that when the Pb²⁺ metallic center is replaced by Eu³⁺ the lone pair is removed from the overall coordination polyhedron, it is thus feasible to assume that the Eu³⁺ environment will readjust itself so to better accommodate the organic ligands. Ultimately, the Eu³⁺ local symmetry is also changed, as we will discuss below.

The crystal packing of Pb_0.90Eu_0.10–BDC is rather featureless, being as depicted in Fig. 4 described as the parallel packing of inorganic metal oxide layers (in this compound with also ca. 5% of vacancies) placed in the ab plane of the unit cell, pillared along the [001] direction by the organic linkers. Though the structure is rich in oxygen atoms capable of accepting hydrogen atoms in supramolecular interactions, the structure only possesses such atoms attached to carbon which are also not located in a position to establish such interactions. Thus, no structurally-relevant supramolecular interactions exist in the present compound, being the packing solely driven by the coordination features to the metallic centers.

Fig. 4 Schematic representation of the crystal packing of compound Pb_0.90Eu_0.10–BDC viewed in perspective along the (top) [100] and (bottom) [010] directions of the unit cell.

For higher concentration of Eu³⁺ in the reactive gel (≥25%) the structure does not seem to be robust enough to accommodate the total inclusion of these optically-active metallic centers in the framework. This is most likely due to the fact that the amount of vacancies needed to maintain the crystal charge neutral is so large that the structure would collapse (or, in this case, it is not even formed).

Eu³⁺ luminescence as a structural local probe

Fig. 5 shows the emission spectra of the Pb_1−xEu_x–BDC (x = 0.05, 0.10, 0.25, 0.50) and [Eu₂(BDC)₃(H₂O)₄] CPs. The relative intensity and number of Stark components of the ⁵D₀ → ⁷F_0–4 transitions strongly depend on the Eu³⁺ concentration, due to the presence of the [Pb(BDC)]_n and [Eu₂(BDC)₃(H₂O)₄] phases.

Fig. 5 (A) Emission spectra (300 K) of Pb_1−xEu_x–BDC (x = 0.05, 0.10, 0.25, 0.50) and [Eu₂(BDC)₃(H₂O)₄] excited at 315 nm. (B) Magnification of the ⁵D₀ → ⁷F₀ transition.

The emission spectra of Pb_1−xEu_x–BDC (x = 0.05, 0.10) display a single ⁵D₀ → ⁷F₀ line (17 295.1 ± 3.0 cm⁻¹, 578.2 nm) and a local-field splitting of the ⁷F_1,2 levels in 3 and 6 components (at least), respectively. For Pb_0.5Eu_0.5–BDC, however, the ⁵D₀ → ⁷F₀ transition red-shifts (17 268.2 ± 3.0 cm⁻¹, 579.1 nm) and the fewer number of ⁷F_1,2 Stark components detected (clearly 2 and 3, respectively) confirms the structural change from [Pb(BDC)]_n to [Eu₂(BDC)₃(H₂O)₄] pointed out by XRD. It is interesting to note that for x = 0.25 two ⁵D₀ → ⁷F₀ lines are unequivocally observed, being characterized by the same energy values as those found for the lower and more concentrated CPs, indicating the presence of a phase mixture of [Pb(BDC)]_n and [Eu₂(BDC)₃(H₂O)₄], in accord with the PXRD data. To render easier the following discussion, the local environment of the Eu³⁺ ions belonging to the [Pb(BDC)]_n and [Eu₂(BDC)₃(H₂O)₄] compounds will be hereafter termed as site 1 and site 2, respectively.

The number of ⁷F₂ Stark levels discerned for the Pb_1−xEu_x–BDC (x = 0.05 and 0.10) low-concentrated samples, 6 (arrows in Fig. 5A), is higher than the maximum 2J + 1 allowed splitting, suggesting the presence of more than one Eu³⁺ local environment. This is unequivocally confirmed by the dependence of the emission spectra of those compounds with the excitation wavelength, Fig. S5 (ESI†). In fact, for the ⁵D₀ → ⁷F_1,2 transitions besides the Stark components ascribed to Eu³⁺ ions in the [Pb(BDC)]_n phase (site 1), it is possible to discern the presence of components from [Eu₂(BDC)₃(H₂O)₄] (site 2), as highlighted by the vertical lines in Fig. 5A. The fact that the [Eu₂(BDC)₃(H₂O)₄] structure is not detected by PXRD readily indicates that for the lower concentrated samples this phase appears in a very reduced relative amount, and thus the dominant local environment of the Eu³⁺ ions belongs to the [Pb(BDC)]_n phase (site 1). The emission spectra of Pb_0.5Eu_0.5–BDC are independent of the excitation wavelength, Fig. S5 (ESI†), indicating that the sample is formed only by the [Eu₂(BDC)₃(H₂O)₄] phase.

Whereas for site 2 each Eu³⁺ ion occupies the center of a slightly distorted square plane, yielding a metal ion environment with pseudo-C₄ symmetry,²⁹ the discussion about the Eu³⁺-local coordination in the [Pb(BDC)]_n phase is, as far as we know, performed for the first time here. As mentioned above, the Eu³⁺ ions in site 1 replace the structural position of the Pb²⁺ ions in a C_2v local symmetry group. When Eu³⁺ replace Pb²⁺ its local environment will readjust itself so to better accommodate the organic ligands and it is expectable a distortion from the C_2v local symmetry group. Actually, the presence of the ⁵D₀ → ⁷F₀ transition indicates a local point symmetry of the type C_nv or C_n,^54,55 and the ligand field-splitting observed for the ⁵D₀ → ⁷F_1,2 transitions are in agreement with this, even taken into account that the mentioned overlapping between the Stark components of the two phases for Pb_1−xEu_x–BDC (x = 0.05, 0.10 and 0.25). It is plausible that in this seven-fold first coordination sphere, the oxygen-ligating atom leading to a distorted C_2v point symmetry contributes to decrease the rank 1 and 3 summations over spherical harmonics in the first coordination sphere (the main contributions to the Ω₂ Judd–Ofelt intensity parameter), which would explain the noticeable decrease of the relative intensity of the ⁵D₀ → ⁷F₂ transition, relatively to site 2 local symmetry.

Fig. 6 shows the excitation spectra of Pb_1−xEu_x–BDC (0.05 ≤ x ≤ 0.50) and [Eu₂(BDC)₃(H₂O)₄] monitored around the Eu^{3+ 5}D₀ → ⁷F₀ transition revealing the presence of a broad band between 240 and 350 nm and a series of low-relative intensity intra-4f⁶ transitions between the F₀ ground state and the ⁵D_4–1, ⁵G_2–6 and ⁵L₆ excited levels. The broad band is formed by one high-energy component centered at ∼280 nm and by a low-energy one, whose peak position depends on the Eu³⁺ amount, it peaks at ∼312 nm for 0.05 ≤ x ≤ 0.25 and at ∼322 nm for 0.25 ≤ x ≤ 0.50. A similar broad band with two components around 285 nm and 321 nm was also detected for the Na₂BDC, being ascribed to BDC excited states (Fig. S6, ESI†). Although the presence of ligand-to-metal charge transfer (LMCT) states which are expected in the range 300–350 nm cannot be ruled out,^56,57 we note that a similar broad band with two components around 285 nm and 321 nm was also detected for the Na₂BDC. Therefore, we assign the components in the excitation spectra to the preferential contribution of the BDC excited states (Fig. S6, ESI†), as we will detail below. The variation of the energy peak position with the Eu³⁺ content can be ascribed to the dependence of the local coordination site on the Eu³⁺ concentration, as it will be detailed below. Independently of the Eu³⁺ amount the higher relative intensity of the ligand-related components points out that the Eu³⁺ excited states are mainly sensitized by the ligands rather than by direct intra-4f⁶ excitation.

Fig. 6 Excitation spectra (300 K) of Pb_1−xEu_x–BDC (x = 0.05, 0.10, 0.25, 0.50) and [Eu₂(BDC)₃(H₂O)₄]. The spectra were monitored at 578.2 nm for x = 0.05 and 0.10 and at 579.3 nm for x = 0.50 and [Eu₂(BDC)₃(H₂O)₄]. For x = 0.25 the excitation spectra were monitored at 578.2 nm (solid blue line) and at 579.1 nm (magenta line with circles).

We note that for x = 0.25, the excitation spectra depend on the monitoring wavelength being observed a red-shift of the low-energy component from 312 nm to 322 nm, as the monitoring wavelength increases from 578.2 nm (site 1) to 579.1 nm (site 2), readily indicating that the Eu³⁺ excited states in each local coordination are distinctly populated. Thus, the Pb_0.75Eu_0.25–BDC CP can be used to demonstrate the Eu³⁺ ability as a spectroscopic local probe. As above described, there are unequivocal experimental evidences supporting the presence of site 1 and site 2 at x = 0.25 with energy values for the ⁵D₀ → ⁷F₀ transition of 17 268.2 ± 3.0 cm⁻¹ (579.1 nm) and 17 295.1 ± 3.0 cm⁻¹ (578.2 nm), respectively. Usually, site-selective excitation in Eu³⁺ compounds was performed using a tunable dye laser.^55,58,59 The Pb_0.75Eu_0.25–BDC CP is one of the rare examples in which two Eu³⁺ sites can be selectively excited through a common Xe lamp, as evidenced below.

The high-resolution excitation spectra were monitored at the ⁵D₀ → ⁷F₀ energy values of site 1 and site 2 for Pb_0.75Eu_0.25–BDC CP, Fig. 7. The spectra show the BDC ligand-related bands at ∼288 nm and ∼312 nm for site 1, being observed an enlargement and a shift of these components to ∼275 nm and ∼322 nm, respectively, when site 2 is monitored.

Fig. 7 Excitation spectra (300 K) of Pb_0.75Eu_0.25–BDC CP monitored within the ⁵D₀ → ⁷F₀ transitions at 578.2 nm (site 1, black line) and 579.3 nm (site 2, red line). The vertical lines assign the BDC ligand-related components.

While monitoring site 1, the component at 366 nm shows an increase in its relative intensity, indicating that the Eu³⁺ ions at site 1 are preferentially excited at this wavelength. We note that while monitoring site 2, it is observed a significant enhancement of the relative intensity of the higher-energetic component, thus, indicating that under excitation around 288 nm, site 2 is favored with respect to the emission arising from site 1. Moreover, site 2 is also favored under direct-intra-4f⁶ excitation, as the higher relative intensity of the Eu³⁺ transitions (in comparison with that in the excitation spectrum monitored within site 1) points out.

Based on the information provided by the selective monitoring of the excitation paths that preferentially populate the ⁵D₀ excited states of the Eu³⁺ at site 1 and site 2, Fig. 8 shows the emission spectra dependence on the excitation wavelength for Pb_0.75Eu_0.25–BDC. Fig. 8 shows that while exciting site 2 at 366 nm the emission spectrum resembles that of [Eu₂(BDC)₃(H₂O)₄], whereas at 280 nm, the spectrum drastically changes being similar to that of the Pb_0.95Eu_0.05–BDC CP.

Fig. 8 Emission spectra (300 K) of Pb_1−xEu_x–BDC excited at 280 mm and at 366 nm for x = 0.25 and excited at 315 nm for x = 0.05 and [Eu₂(BDC)₃(H₂O)₄].

The ⁵D₀ lifetime values were determined by selectively monitoring the emission at the ⁵D₀ → ⁷F₀ transitions at 578.0 nm (site 1) and 579.0 nm (site 2) and under the same excitation wavelength (315 nm). All the emission decay curves are well reproduced by a single-exponential function (Fig. S7–S12, ESI†), yielding the lifetime values listed in Table 1. The ⁵D₀ lifetime values are higher for site 1 compared with that found for site 2, which can be ascribed to the presence of water molecules coordinated to the Eu³⁺ placed in site 2, which favors the non-radiative transition probability through OH oscillators. We also note that the increase of Eu³⁺ concentration causes a decrease in the ⁵D₀ lifetime values of site 1, suggesting the presence of concentration quenching effects. For site 2, the ⁵D₀ lifetime values are almost independent of the Eu³⁺ concentration.

Table 1 ⁵D₀ lifetime values at 300 K (ms) for Pb_1−xEu_x–BDC (x = 0.05, 0.10, 0.25, 0.50) (site 1 and site 2)

x	Site 1	Site 2
0.05	1.21 ± 0.07
0.10	1.00 ± 0.10
0.25	0.90 ± 0.03	0.40 ± 0.01
0.50		0.42 ± 0.01
1.00		0.44 ± 0.01

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The distinct ⁵D₀ lifetime values of site 1 and site 2 were rationalized in terms of the calculation of the ⁵D₀ radiative (A_r) and nonradiative (A_nr) transition probabilities and of the ⁵D₀ quantum efficiency (η) following a methodology described elsewhere.⁶⁰ These calculations were performed for Pb_0.95Eu_0.05–BDC, site 1, and for [Eu₂(BDC)₃(H₂O)₄], site 2. We note that the very close η values (0.27 and 0.21, respectively), result from distinct contributions from A_r and A_nr. Despite higher A_r values (0.479 ms⁻¹) found for [Eu₂(BDC)₃(H₂O)₄] compared with that (0.224 ms⁻¹) of Pb_0.95Eu_0.05–BDC, we note the increase of A_nr for site 2 ([Eu₂(BDC)₃(H₂O)₄], 1.794 ms⁻¹) relatively to that in site 1 (Pb_0.95Eu_0.05–BDC, 0.602 ms⁻¹). The higher value of the A_nr value for the [Eu₂(BDC)₃(H₂O)₄] can be ascribed to the presence of 4 water molecules in the coordination sphere which are known as very efficient quenchers from the ⁵D₀ emission.⁶⁰

The different ⁵D₀ lifetime values of site 1 and site 2 allow the use of time-resolved spectroscopy to distinguish the Stark components that arise from each site, as illustrated for the most intense ⁵D₀ → ⁷F₂ transition. Fig. 9 shows the time-resolved emission excited at 315 nm acquired under distinct starting-delay (SD) values in the spectral regions of the ⁵D₀ → ⁷F_0,2 transitions. At shorter SD values (0.05 ms), the emission spectrum reveals two lines for the ⁵D₀ → ⁷F₀ transition region (already detected under steady-state mode, Fig. 5) ascribed to site 1 (578 nm) and site 2 (579 nm). At longer SD values (2.50 ms), only the long-lived ⁵D₀ → ⁷F₀ transition from site 1 remains, corroborating the longer ⁵D₀ lifetime values listed in Table 1. At shorter SD values (0.05 ms) the ⁵D₀ → ⁷F₂ spectral region is dominated by the short-lived line at 616 nm ascribed to site 2, whereas for longer SD values (2.50 ms) the emission is governed by the long-lived Stark components at 612.0 nm (16 340 ± 3 cm⁻¹) and 621.0 nm (16 103 ± 3 cm⁻¹) characteristic of site 1, Fig. 5B.

Fig. 9 Time-resolved emission spectra of Pb_0.75Eu_0.25–BDC (300 K) in the (A) ⁵D₀ → ⁷F₀ and (B) ⁵D₀ → ⁷F₂ spectral regions, excited at 315 nm and acquired at SD = 0.05 ms (dashed line) and SD = 2.50 ms (solid line) with an integration window of 0.05 and 20 ms, respectively.

The emission properties of the CPs were further quantified by measuring the emission quantum yield as a function of the excitation wavelength, namely through the ligands excited states (280–370 nm) and via direct intra-4f⁶ excitation (⁵L₆, 393 nm and ⁵D₂, 465 nm), Fig. 10. The higher emission quantum yield was found for the Pb_0.75Eu_0.25–BDC, 69 ± 7%, under excitation at 315 nm, which is well above the values found for similar materials based on Pb(II).^21,23 Analyzing the dependence of the emission quantum yield on the Eu³⁺ amount it is observed that the maximum values (independent of the excitation wavelengths) are found for x = 0.25 which may be ascribed to the simultaneous contribution of site 1 and site 2 for the radiative emission. Focusing our attention into the low-concentrated samples (site 1), it is observed that under the ligands-preferential excitation at 315 nm (Fig. 6), the emission quantum yield increases as x increases from 0.05 to 0.25 which may be attributed to the fact that as the amount of Eu³⁺ increases there is a larger number of ligands with Eu³⁺ ions in the neighborhoods, thus, the ligand-to-Eu³⁺ energy transfer is favored. Another interesting aspect is that when the intra-4f⁶ levels are excited (393 nm, 465 nm), the quantum yield is larger for the more concentrated samples, Pb_1−xEu_x–BDC, x = 0.25, 0.50 and [Eu₂(BDC)₃(H₂O)₄], in good agreement with the preferential excitation of site 2 under direct intra-4f⁶ excitation.

Fig. 10 Dependence of the emission quantum yield on the excitation wavelengths for [Pb(BDC)]_n, Pb_1−xEu_x–BDC (x = 0.05, 0.10, 0.25, 0.50) and [Eu₂(BDC)₃(H₂O)₄].

The quantum yield values for Pb_0.95Eu_0.05(BDC) and [Eu₂(BDC)₃(H₂O)₄] are analogous the values found for the ⁵D₀ quantum efficiency. It is known that the quantum efficiency η is the superior limit for q (q ≤ η)⁶⁰ and that the overall absolute emission quantum yield q is the product of the sensitization efficiency of the ligand (η_tr) and η, q = η_tr × η.⁶¹ The similarity between q and η points out that an efficient ligand-to-Eu³⁺ energy transfer (η_tr ∼ 1) occurs for those CPs. This conclusion is in good agreement with the absence of LMCT bands in the excitation spectra (Fig. 6) which are known to play an important role in quenching the Eu³⁺ luminescence.⁵⁷

Conclusions

We provide an intriguing example of a series of luminescent mixed-metal Pb_1−xEu_x–BDC (x = 0.05, 0.10, 0.25, 0.50) CPs, including, for comparison purposes, the mononuclear [Pb(BDC)]_n and [Eu₂(BDC)₃(H₂O)₄] phases. Increasing the amount of Eu³⁺, the mixed-metal CPs display a gradual change in their structure from the [Pb(BDC)]_n phase to the [Eu₂(BDC)₃(H₂O)₄] one, well-monitored through luminescence spectroscopy. In fact, the discussion about the Eu³⁺-local coordination in the [Pb(BDC)]_n phase is, as far as we know, performed for the first time here. The Pb_0.75Eu_0.25–BDC material display a mixture of both crystalline phases with a high emission quantum yield of 69 ± 7%, one the highest value measured for luminescent CPs based on Pb(II). Moreover, the Pb_0.75Eu_0.25–BDC CP is one of the rare examples in which two Eu³⁺ sites can be selectively excited through a common Xe lamp illustrating how powerful the Eu³⁺ luminescence is as a local probe spectroscopic tool.

Acknowledgements

The authors gratefully acknowledge CNPq (RH-INCT/INAMI), DPP-UNB, FAP-DF, and CAPES for financial support. This work was partially developed in the scope of the project CICECO – Aveiro Institute of Materials (Ref. FCT UID/CTM/50011/2013), financed by Portuguese funds through the FCT/MEC and when applicable co-financed by FEDER under the PT2020 Partnership Agreement. LDC acknowledges CAPES and CNPq (Brazil) for a Pesquisador Visitante Especial grant (313778/2013-2) within the Science without Borders program.

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Footnote

† Electronic supplementary information (ESI) available: Crystallographic data, excitation spectrum and lifetime decay curves. CCDC 1490068. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra27850g

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