Structural Features of Eu³⁺ and Tb³⁺-Bipyridinedicarboxamide Complexes

Abstract

Photoluminescent lanthanide complexes of Eu³⁺ and Tb³⁺ as central atoms and N⁶,N⁶’-diisopropyl-[2,2′-bipyridine]-6,6′-dicarboxamide as ligand were synthesized. The structure of these complexes was established by single-crystal X-ray diffraction, mass spectrometry, ¹H and ¹³C nuclear magnetic resonance, ultraviolet-visible, infrared spectroscopy, and thermogravimetry. Bipyridinic ligands provide formation of coordinatively saturated complexes of lanthanide ions and strong photoluminescence (PL). The Eu³⁺- and Tb³⁺-complexes exhibit PL emission in the red and green regions observed at a 340 nm excitation. The quantum yield for the complexes was revealed to be 36.5 and 12.6% for Tb³⁺- and Eu³⁺-complexes, respectively. These lanthanide compounds could be employed as photoluminescent solid-state compounds and as emitting fillers in polymer (for example, polyethylene glycol) photoluminescent materials.

1. Introduction

Luminescent lanthanide complexes are receiving a strong interest in their application in optoelectronics [1], photonics [2], amplifiers [3], cell dyes fabrication [4], photocatalysis [5], UV light-sensing or dosimeter materials [6], theranostics [7], and development of fluorescent probes for bioimaging [8]. The mentioned fields associated with lanthanide complexes are due to their narrow emission lines, large Stokes displacement, high quantum yield (QY), and long-lived luminescence lifetimes, as opposed to lanthanide ions directly, which possess the poor light absorption because of Laporte forbidden f-f transitions [9].

Among a great range of photoluminescent lanthanide complexes, europium(III) and terbium(III) organic luminophores are broadly investigated, since Eu³⁺ and Tb³⁺ cations have intense pure red and green emission in the visible region [10]. These emission colors are the components of the RGB system [11] that could be used as a light converter for light-emitting diodes and colored displays fabrication.

As mentioned in the ref. [12], 2,2′-bipyridine-6,6′-dicarboxylate ligand in complexes is a highly efficient sensitizer for the “antenna effect”. Thus, the use of 2,2′-bipyridine-6,6′-dicarboxylate ligands, especially amides—N⁶,N⁶’-diisopropyl-[2,2′-bipyridine]-6,6′-dicarboxamide (BDCA), in europium and terbium complexes, can lead to a relatively higher QY compared to several europium and terbium complexes with some ligands, including phenanthroline derivatives, thenoyltrifluoroacetone, β-diketonates, substituted pyridine, and 2,2′-bipyridine derivatives [13,14,15,16,17,18,19].

To the best of our knowledge, the photoluminescent properties of lanthanide(III) coordinatively saturated complexes with BDCA have never been studied. The only known similar lanthanide(III)-incorporating polymer-metal complexes (PMCs) are based on polydimethylsiloxanes functionalized by tetradentate bipyridine, functioning as ligands toward Eu³⁺ and Tb³⁺ [11]. Previously, we designed PMCs of Eu³⁺ and Tb³⁺-bipyridinedicarboxamide-co-polydimethylsiloxanes, which exhibit a high photoresponse and are applied as flexible, self-healing, and color-tunable photoluminophores in flexible device applications [11]. The main advantage of these PMCs is that they can be mechanically stacked one above another to achieve the desired emission color in the spectral range from green to yellow and red. The external quantum efficiency of these PMCs was good enough, however, it did not exceed 10.5% and 18.3% at a 340 nm excitation in the case of Eu and Tb-PMCs, respectively. These QY values are related to luminescence quenching due to the use of a polymer ligand [20]. Therefore, the use of low-molecular-weight BDCA ligand instead of polysiloxane bipyridine-containing ligand can increase QY compared to ref. [11]. Hence, the development of methods for the synthesis of luminescent complexes based on Eu³⁺, Tb³⁺, and BDCA, with high QY, is a challenging task.

Thus, the aims of the study are to (i) synthesize lanthanide complexes with a novel framework by complexation of Eu³⁺ and Tb³⁺ ions with N⁶,N⁶’-diisopropyl-[2,2′-bipyridine]-6,6′-dicarboxamide (BDCA) organic ligand, (ii) establish their structure by single-crystal X-ray diffraction (XRD), high-resolution electrospray ionization mass spectrometry (HRESIMS), ¹H, ¹³C NMR, UV-vis, and FTIR spectroscopy, (iii) study their photoluminescent properties, (iv) incorporation of Eu, Tb-complexes as fillers in polymer films (polyethylene glycol, PEG) for creating luminescent composites, and (v) study their thermal stability. All our experimental data and the corresponding discussion are detailed in the following sections.

2. Results

2.1. Synthesis of Lanthanide Complexes

Lanthanide(III) complexes [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ were synthesized by a three-stage procedure including the two-step synthesis of ligand and its complexation with lanthanide(III) chlorides (Scheme 1).

The BDCA ligand was obtained by the reaction between pre-prepared [2,2′-bipyridine]-6,6′-dicarbonyl dichloride (Scheme 1) and isopropylamine in CH₂Cl₂ at room temperature (r.t., 21 °C). The structure of BDCA ligand was established by ¹H NMR, indicating a ¹H NMR signal of amide groups (NHC(=O) at δ = 8.62 ppm, ¹³C NMR (spectra are illustrated in Figure S1 in the Supplementary Material), HRESIMS, and single-crystal XRD (Figures S2 and S3, Table S1).

In the last stage, the complexation between BDCA and dry lanthanide(III) chlorides (EuCl₃ and TbCl₃) with metal–ligand molar ratio of 1:2 was conducted in a CH₃OH solution at 40 °C for 24 h in order to obtain complexes [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃. The synthesized complexes are soluble in DMSO and alcohols, especially EtOH.

2.2. Structure of Lanthanide Complexes

The synthesized complexes were then characterized by XRD, HRESIMS, FTIR, and UV-vis (Figure 1 and Figure 2 and Figures S4 and S5, Table S1). The single-crystal XRD data indicated that lanthanide(III) coordination with BDCA induces bonding between Ln–N_Bipy and Ln−O (Ln = Eu, Tb, Figure 1, for details, see Experimental section). The complexation between BDCA and Ln³⁺ should be conducted via O,N,N,O-chelating moieties ligation. Thus, considering the coordination number 9 of Ln³⁺, the only possible conclusion in the metal–ligand molar ratio is 1:2. Europium and terbium complexes have the same geometry of the nine-vertex polyhedron with distorted pentagonal coordination formed by oxygen atoms in the equatorial position and nitrogen atoms along the tetragonal tetrahedron in apical positions. This coordination polyhedron contains crystallographic 2-fold axes passing through the metal atom and the coordinated water molecule.

In the HRESIMS spectra, the [Eu(BDCA)₂]³⁺ (m/3 = 268.4233) and [Tb(BDCA)₂]³⁺ ions (m/3 = 270.4241) were indicated with their characteristic isotopic distribution (Figures S4 and S5), respectively. In the FTIR spectra of [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃, the amide I ν(C=O) band was shifted from 1653 to 1631 cm⁻¹ in comparison with the BDCA (Figure 2a). According to the refs. [11,21,22,23], this band shift can be attributed to the involvement of the C=O group in the coordination.

As a result, [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ complexes were synthesized by a complexation of EuCl₃ and TbCl₃ with BDCA ligand. Considering all the spectral, XRD, and HRESIMS data, we conclude that the ligation of BDCA occurred via Ln–N_Bipy and Ln−O bonding. The complexes are heteroleptic, composed of Ln³⁺, two BDCA ligands, and one molecule of coordinated H₂O.

2.3. Theoretical Calculations of HOMO–LUMO Energy Gaps

The energies for the coordination bonds Ln–O and Ln–N_Bipy (Ln = Eu, Tb) were computed by appropriate quantum chemical calculations at the ωB97XD/DZP-DKH level of theory, followed by the topological analysis of the electron density distribution (for details, see the Computational Details section in the Supplementary Material). Our results are summarized in Table S2, while the model structures, the contour line diagrams of the Laplacian of electron density distribution ∇²ρ(r), bond paths, selected zero-flux surfaces, visualization of electron localization function (ELF), reduced density gradient analyses referring to coordination bonds Ln–O and Ln–N_Bipy, and Cartesian atomic coordinates of [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ model structures are shown in Figures S6−S8 and Table S3 (Supplementary Material). Thus, Ln–N coordination bonds exhibited a slightly longer bond length (c.a. 2.5 Å) and energy values (c.a. 11–12 kcal·mol⁻¹) lower than the Ln–O bonds (l = 2.4 Å, E_int = 15–16 kcal·mol⁻¹). Two BDCA ligands are similarly ligated to the Ln³⁺ center in terms of binding energies.

The calculated HOMO–LUMO energy gaps (E_H-L) in model structures [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ are 7.50 and 8.17 eV, respectively (theoretical calculations are presented in Section S4.2 in the Supplementary Material).

2.4. Band Gap Estimation by UV-Vis Absorption Study

In the UV-vis spectra of [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ (Figure 2b), a peak at 291 nm (with two shoulders at 283 and 302 nm) is observed, which corresponds to ligand-to-metal charge transfer (LMCT) (theoretical modeling of UV-vis absorption spectra is presented in Section S4.3 in the Supplementary Material, Figure S9). The UV-vis spectra of the complexes were similar to the spectral shape of the free ligand, since the π–π* state energy was probably not influenced by the coordination sphere of the metal [2,15,24,25,26].

Optical band gap of [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ was estimated from absorption spectral data using Tauc’s relation (Equation 1), which is represented below:

αE = A(E − E_g)^m,

(1)

where α, E, and m represent absorption coefficient, photon’s energy, and optical parameters, respectively [27]. Tauc’s profiles were plotted as (αhυ) ²–energy (E). Extrapolation of the tangent down to the x-axis provides the value of the band gap. Figure 3 shows the Tauc’s profiles of complexes, which demonstrate a similar band gap E_g = 4.0 eV of [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃.

As a result, optical band gap values gained from absorption spectral data using Tauc’s plot (E_g = 4.0 eV) is c.a. two times lower than calculated E_H-L (7.50 and 8.17 eV).

The calculated by the density-functional theory method (DFT) HOMO–LUMO gap gives only an approximation of the band gap and does not take into account the formation of electron-hole semi-particle in the excited state [28]. Thus, the optical gap obtained by Tauc’s relation from the UV-vis spectrum corresponds to the energy of the lowest electronic transition and could be substantially lower than E_H-L.

2.5. Luminescent Properties

The use of 2,2′-bipyridine-6,6′-dicarboxylate sensitizer provides efficient energy transfer from an excited state of ligand to Ln³⁺ excited state, which leads to energy transitions ⁵D₄→⁷F_J (J = 6–3) for Tb³⁺ and ⁵D₀→⁷F_J (J = 0–4) for Eu³⁺ [11,12]. As noticed in the ref. [29], 2,2′-bipyridine-6,6′-dicarboxylate is a highly efficient sensitizer for the “antenna effect”.

Emission spectra of [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ under UV light excitation (at excitation wavelength λ_ex = 340 nm, excitation spectra are illustrated in Figure S10) [11] were acquired (Figure 4). Typical energy transitions of Tb³⁺ and Eu³⁺ were observed in PL emission spectra as signals of their characteristic wavelengths.

The studied complexes demonstrate red and green phosphorescence according to acquired PL lifetimes 2.9 and 4.9 ms for [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃, respectively (Figure S11).

QY upon direct excitation of the Ln³⁺ ion is determined mainly by the probability of nonradiative processes. For free ions, the probability of nonradiative transitions is smaller, and the energy gap is larger between the resonant level of the Ln³⁺ ion and the first level of the main multiplet. QY values for [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ complexes are presented in

Table 1. QY values for [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ complexes compared to corresponding reported PMCs [11] at λ_ex = 340 nm.

Complex	QY, %	QY of Corresponding Reported PMCs [11], %	QY of Encapsulated Complexes in PEG, %
[Eu(BDCA)2(H2O)]Cl3	12.6	10.5	11.2
[Tb(BDCA)2(H2O)]Cl3	36.5	18.3	25.3

. According to the ref. [12], the difference in PL between complexes can be explained by ΔE = 12,300 cm⁻¹ (⁵D₀→⁷F₀ ) and 14,800 cm⁻¹ (⁵D₄→⁷F₆) for Eu³⁺ and Tb³⁺.

The QY values for [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ are expectedly higher than that of corresponding reported Eu³⁺ and Tb³⁺-incorporating PMCs [11] (Table 1) and some other bipyridine derivatives of Eu³⁺ and Tb³⁺ [2,14,15]. The increase in QY in complexes is associated with use of low-molecular-weight BDCA ligand in studied complexes instead of bipyridine-incorporating polymer ligand [11]. PMCs contain polymer ligands with the number-average molecular weight of 45,000–50,000 [21] compared to BDCA ligand with a molecular weight of 326, which leads to luminescence quenching [20,30].

The [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ can retain a high QY even after encapsulation in a semitransparent polymer matrix (Figure 5). Taking into account the good solubility of the complexes (Section 2.1. Synthesis of lanthanide complexes), soluble in EtOH PEG was chosen as the polymer matrix. The QY values of [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ encapsulated in PEG were 11.2 and 25.3%, respectively. This was compared to the pure [Eu(BDCA)₂(H₂O)]Cl₃ in the emission spectra signal at 580 nm, which corresponds to the forbidden transition ⁵D₀→⁷F₀, which disappears after encapsulation in PEG. The intensity increases for the signals ⁵D₀→⁷F₀ (592 nm) and ⁵D₄→⁷F₆ (485 nm), which correspond to energy transitions from excited state to ground state of Eu³⁺ and Tb³⁺, respectively. More information about excitation spectra and PL lifetimes are described in the Supplementary Material (Section S5—“Luminescent properties”, Figures S10 and S11).

2.6. Thermal Stability

TG and differential TG curves of [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ indicate some steps of mass loss (Figure 6): 5–7% mass loss at 100 °C (dehydration), 15–20% at 280 °C, and 40% at 480 °C. The final thermal destruction of coordination centers of [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ probably starts at 380 °C.

3. Materials and Methods

3.1. Materials

[2,2′-bipyridine]-6,6′-dicarboxylic acid (97%, BLD Pharm, Shanghai, China), Et₃N (99%, Abcr GmbH, Karlsruhe, Germany), and isopropylamine (99%, Abcr GmbH, Karlsruhe, Germany) were purchased from commercial suppliers, and their purity was checked by ¹H NMR. Ultradry EuCl₃ (99.99%) and TbCl₃ (99.99%) were acquired from ChemCraft Ltd. (Kaliningrad, Russia). SOCl₂ and CH₂Cl₂ (99%, Vecton, St. Petersburg, Russia) were distilled under argon (though the latter was also over P₂O₅). CH₃OH (99%, Vecton, St. Petersburg, Russia) was dried and distilled over (OCH₃)₂Mg prior to use. Et₂O (99%, Vecton, St. Petersburg, Russia) was freshly distilled over sodium/benzophenone under an argon atmosphere prior to use. DMSO (99%, Reachem, Moscow, Russia), EtOH (99%, Himprod, Ekaterinburg, Russia), and PEG polymer (M_n = 10 000; 1.2 g·mL; Merck KGaA, St. Louis, MO, USA) were used as received.

3.2. Methods

The NMR spectra have been recorded on a Bruker Avance III 400 spectrometer (Germany) in DMSO-d₆ at r.t. (at 400 MHz for ¹H and 100 MHz for ¹³C). ¹³C NMR spectra were recorded with ¹H decoupling. The chemical shifts are given in δ-values [ppm] and refer to the residual signals of non-deuterated DMSO: δ 2.50 (¹H) and 39.5 (¹³C). The following abbreviations were used to designate multiplicities: s = singlet, d = doublet, t = triplet, m = multiplet, br = broad, and dd = doublet of doublets. FTIR spectra were recorded on a Shimadzu IRAffinity-1 FTIR spectrophotometer (Kyoto, Japan) in KBr pellets. The measurements were carried out at RT in the wavenumber range of 400–4000 cm⁻¹. The following abbreviations of the absorption bands are used to designate intensity: s—strong, m—medium, and w—weak. UV-vis spectra were recorded on a Shimadzu UV-1800 spectrophotometer (Kyoto, Japan) in DMSO. The measurements have been carried out at RT using a quartz cell 1 cm wide in the wavelength range of 250–800 nm. HRESIMS was conducted on a Bruker Maxis HRMS ESI QTOF spectrometer equipped with an electrospray ionization source. The analyzed samples had been dissolved beforehand in pure CH₃OH for the HRESIMS measurements. The instrument has been operated at a positive ion mode using the m/z range of 50–400. The most intense peak in the isotopic pattern is noted.

Crystallographic data for all crystals were obtained using Rigaku Oxford Diffraction «SuperNova» (Tokyo, Japan) (in the case of BDCA) and Rigaku Oxford Diffraction «Synergy XtaLAB» (Tokyo, Japan) (in the case of [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃) diffractometers with monochromated micro-focus CuKα (λ = 1.54184) X-ray sources. All crystals were kept at 100 K during all data collection. Crystal structures were solved using ShelXT [31] structure solution program and refined by means using ShelXL [32] structure refinement program incorporated in the Olex2 program package [33]. All crystallographic data for this paper can be obtained free of charge via the Cambridge Crystallographic Database.

PL spectra were recorded on a Horiba Fliorolog-3 spectrofluorometer (Jobin Yvon Technology, Bensheim Germany) at r.t. QY measurements were performed using a Shimadzu RF-6000 spectrofluorometer (Kyoto, Japan) with an integrating sphere (101 mm in diameter). PL lifetime steady state measurements were performed via Horiba Fliorolog-3 spectrofluorometer with the impulse xenon lamp with a power of 150 W as an excitation source.

TG was carried out on a NETZSCH TG 209F1 Libra TGA209F1D0024 analyzer (Selb, Germany) in the air and inert (argon) atmosphere. The samples were heated from 50 to 800 °C at a heating rate of 10 °C∙min⁻¹.

3.3. Synthetic Procedures

3.3.1. [2,2′-Bipyridine]-6,6′-Dicarbonyl Dichloride

A round-bottom flask, which was purged with argon, and equipped with a reflux condenser, was charged with [2,2′-bipyridine]-6,6′-dicarboxylic acid (1.0 g, 4.1 mmol), 10 µL of Et₃N, and freshly distilled SOCl₂ (250 mL). The mixture had been refluxed for 2 h under a constant argon flow until complete dissolution of the dicarboxylic acid and then cooled to r.t. The excess SOCl₂ was removed at r.t. under reduced pressure for 3 h to obtain powdery dicarbonyl dichloride. Yield: 1.14 g (99%); beige crystals; mp 288 °C.

3.3.2. N⁶,N⁶’-Diisopropyl-[2,2′-Bipyridine]-6,6′-Dicarboxamide (BDCA)

A freshly prepared 2,2′-bipyridine-6,6′-dicarbonyl dichloride (1.14 g, 4.06 mmol) solution in dry CH₂Cl₂ (10 mL) was added to isopropylamine (2.39 g, 40.6 mmol) solution in dry CH₂Cl₂ (20 mL) by drops at 0 °C under an argon atmosphere. The reaction mixture was stirred at r.t. for 24 h and then filtered. Afterwards, it was filtered and washed with distilled water (3 × 100 mL). The residue was dried under vacuum at 65 °C and washed with Et₂O to afford pure BDCA. Yield: 0.93 g (70%); beige powder. ¹H NMR (DMSO-d₆, δ): 1.28 (d, J = 6.6 Hz, 12H, CH(CH₃)₂), 4.23 (m, J₁ = 6.6 Hz, J₂ = 8.5 Hz, 2H, CH(CH₃)₂), 8.13 (dd, J₁ = 7.7 Hz, J₂ = 1.3 Hz, 2H, 5,5′-H-Bipy), 8.19 (t, J₁ = 7.7 Hz, J₂ = 7.7 Hz, 2H, 4,4′-H-Bipy), 8.62 (d, J = 8.5 Hz, 2H, NHC(=O)), 8.98 (dd, J₁ = 7.7 Hz, J₂ = 1.3 Hz, 2H, 3,3′-H-Bipy). ¹³C NMR (DMSO-d₆, δ): 22.7 (CH(CH₃)₂), 41.4 (CH(CH₃)₂), 123.0 (3,3′-C_Bipy), 124.3 (5,5′-C_Bipy), 139.3 (4,4′-C_Bipy), 150.4 (6,6′-C_Bipy), 153.8 (2,2′-C_Bipy), 163.2 (C=O). FTIR (KBr, selected bands, ν, cm⁻¹): 3303 (s; ν(N–H)), 1653 (s; ν_{amide I} (C=O)), 1533 (s; ν_{amide II} (N–H)). UV-vis (DMSO, λ_max, nm): 302 (C=O, n→π*), 291 (Bipy, π→π*), and 282 nm (Bipy, π→π*). HRESIMS⁺: calculated for C₁₈H₂₂N₄O₂ 349.1640, found m/z 349.1630 [M+Na]⁺. Single crystals of BDCA were grown from CH₃OH solution over a one-week period for XRD. Crystal lattice parameters: a = 9.2043(6), b = 11.4313(5), c = 9.6843(6); α = 90°, β = 116.612(8)°, and γ = 90°; monoclinic, space group P2₁/c (14), temperature 100 K. CCDC number: 2215221.

3.3.3. [Eu(BDCA)₂(H₂O)]Cl₃

BDCA (465 mg, 1.42 mmol) was placed in a round-bottom flask and dissolved in anhydrous CH₃OH (20 mL). A solution of dry EuCl₃ (175 mg, 0.68 mmol) in 2 mL of anhydrous CH₃OH was slowly added dropwise to the resulting mixture. The solution was stirred at 40 °C for 24 h, and the solvent was removed by rotary evaporation at 60 °C. The obtained residue was then washed with Et₂O (3 × 50 mL). Yield: 608 mg (95%); beige crystals. FTIR (KBr, selected bands, ν, cm⁻¹): 3410 (s; ν(O–H)), 3220 (s; ν(N–H)), 1634 (s; ν(C = O--Eu)), 1558 (s; ν_{amide II} (N–H)), 1456 (m; ν(Py)). UV-vis (DMSO, λ_max, nm): 302 (C=O, n→π*), 291 (LMCT), 283 nm (LMCT). HRESIMS⁺: calculated for C₃₆H₄₄N₈EuO₄³⁺ 268.4233, found m/z 268.4232 [M–3Cl]³⁺. Single crystals of [Eu(BDCA)₂(H₂O)]Cl₃ were grown from EtOH solution over a two-week period for XRD. Crystal lattice parameters: a = 11.2713(2), b = 15.3222(2), and c = 14.2917(2); α = 90°, β = 106.511(2)°, and γ = 90°; monoclinic, space group P2/c (13), temperature 100 K. Selected bond lengths (Å): Eu1–O1 2.4137(19), Eu1–O2 2.4019(19), Eu1–O3(H₂O) 2.463(3), Eu1–N1 2.536(2), and Eu1–N3 2.541(2). Selected bond angles (°): O1–Eu1–N1 64.25(7), N1–Eu1–N3 62.37(7), O2–Eu1–N3 64.30(7), and O1–Eu1–O3(H₂O) 70.26(5). CCDC number: 2215232.

3.3.4. [Tb(BDCA)₂(H₂O)]Cl₃

BDCA (465 mg, 1.42 mmol) was placed in a round-bottom flask and dissolved in anhydrous CH₃OH (20 mL). A solution of dry TbCl₃ (180 mg, 0.68 mmol) in 2 mL of anhydrous CH₃OH was slowly added dropwise to the resulting mixture. The solution was stirred at 40 °C for 24 h, and the solvent was removed by rotary evaporation at 60 °C. The obtained residue was then washed with Et₂O (3 × 50 mL). Yield: 606 mg (94%); beige crystals. FTIR (KBr, selected bands, ν, cm⁻¹): 3416 (s; ν(O–H)), 3222 (s; ν(N–H)), 1634 (s; ν(C=O--Tb)), 1558 (s; ν_{amide II} (N–H)), 1457 (m; ν(Py)). UV-vis (DMSO, λ_max, nm): 302 (C=O, n→π*), 291 (LMCT), 283 nm (LMCT). HRESIMS⁺: calculated for C₃₆H₄₄N₈TbO₄³⁺ 270.4241, found m/z 268.4232 [M–3Cl]³⁺. Single crystals of [Eu(BDCA)₂(H₂O)]Cl₃ were grown from EtOH solution over a two-week period for XRD. Crystal lattice parameters: a = 11.2666(3), b = 15.2986(3), c = 14.2345(3); α = 90°, β = 106.335(3)°, and γ = 90°; monoclinic, space group P2/c (13), temperature 100 K. Selected bond lengths (Å): Tb1–O1 2.390(2), Tb1–O2 2.3802(19), Tb1–O3(H₂O) 2.445(3), Tb1–N1 2.512(2), and Tb1–N3 2.517(2). Selected bond angles (°): O1–Tb1–N1 64.76(7), N1–Tb1–N3 62.65(8), O2–Tb1–N3 64.88(7), and O1–Tb1–O3(H₂O) 70.18(5). CCDC number: 2215233.

3.4. Encapsulation of Lanthanide Complexes in a PEG Matrix

A solution of the corresponding lanthanide(III) chloride in EtOH (0.5 mL, concentration 2 mg·mL⁻¹) was mixed with a solution of PEG (1 g) in EtOH (2.0 mL). The resulting mixture was intensely stirred at r.t. for 10 min and then poured into a glass Petri dish and dried at 70 °C for 1 h in order to remove EtOH from the polymer product. For PL studies, thin films were peeled off the Petri dish via a razor blade.

4. Conclusions

The red and green-emitting luminescent Eu³⁺ ([Eu(BDCA)₂(H₂O)]Cl₃) and Tb³⁺ ([Tb(BDCA)₂(H₂O)]Cl₃) bipyridine complexes were synthesized in three steps using the pre-prepared N⁶,N⁶’-diisopropyl-[2,2′-bipyridine]-6,6′-dicarboxamide (BDCA) ligand and anhydrous LnCl₃ (Ln = Eu, Tb) as reactants.

The structure of [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ was established by XRD, HRESIMS, FTIR, and UV-vis spectroscopy. The XRD data indicated that Ln(III) coordination with BDCA induce bonding between Ln−N and Ln−O. Amide I ν(C=O) band displacements in FTIR spectra are related to the involvement of the C=O group in the coordination that provides the formation of coordinatively saturated complexes.

HOMO–LUMO energy gaps (E_g) estimated from crystallographic data by QTAIM analysis, are 7.50 and 8.17 eV for [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃, respectively. Optical band gap value of lanthanide complexes gained from absorption spectral data using Tauc’s plot was E_g = 4.0 eV.

Structural features of [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ provide strong PL of lanthanides in the red and green spectral range, respectively, with sharp lines of high intensity. The QY values of [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ complexes are 12.6% and 36.5% at λ_ex = 340 nm excitation, respectively, and are higher compared to previously reported PMCs [11].

We demonstrated that the complexes can be utilized as emitting fillers in polymer composites, especially in the PEG matrix. The QY values of [Eu(BDCA)₂(H₂O)]Cl₃ and [Tb(BDCA)₂(H₂O)]Cl₃ encapsulated in PEG are 11.2 and 25.3%, respectively.

Hence, we report on novel bipyridinic lanthanide complexes, which could be further employed as photoluminescent solid-state compounds and as emitting fillers in polymer photoluminescent materials for optoelectronic devices.