Structural Features of Eu3+ and Tb3+-Bipyridinedicarboxamide Complexes

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.


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 Eu 3+ and Tb 3+ 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.
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 Eu 3+ and Tb 3+ [11]. Previously, we designed PMCs of Eu 3+ and Tb 3+ -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 Eu 3+ , Tb 3+ , 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 Eu 3+ and Tb 3+ ions with N 6 ,N 6 '-diisopropyl-[2,2'-bipyridine]-6,6'-dicarboxamide (BDCA) organic ligand, (ii) establish their structure by singlecrystal X-ray diffraction (XRD), high-resolution electrospray ionization mass spectrometry (HRESIMS), 1 H, 13 C 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.
In the last stage, the complexation between BDCA and dry lanthanide(III) chlorides (EuCl 3 and TbCl 3 ) with metal-ligand molar ratio of 1:2 was conducted in a CH 3

Structure of Lanthanide Complexes
The synthesized complexes were then characterized by XRD, HRESIMS, FTIR, and UV-vis (Figures 1, 2, 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 Ln 3+ should be conducted via O,N,N,O-chelating moieties ligation. Thus, considering the coordination number 9 of Ln 3+ , the only possible conclusion in the metalligand 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.  13 C 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.

Structure of Lanthanide Complexes
The synthesized complexes were then characterized by XRD, HRESIMS, FTIR, and UV-vis (Figures 1-2 and S4-S5, Table S1). The single-crystal XRD data indicated that lanthanide(III) coordination with BDCA induces bonding between Ln-NBipy and Ln−O (Ln = Eu, Tb, Figure 1, for details, see Experimental section). The complexation between BDCA and Ln 3+ should be conducted via O,N,N,O-chelating moieties ligation. Thus, considering the coordination number 9 of Ln 3+ , 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.

Theoretical Calculations of HOMO-LUMO Energy Gaps
The energies for the coordination bonds Ln-O and Ln-NBipy (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

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,

Band Gap Estimation by UV-Vis Absorption Study
In the UV-vis spectra of [Eu(BDCA) 2 (H 2 O)]Cl 3 and [Tb(BDCA) 2 (H 2 O)]Cl 3 (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 (H 2 O)]Cl 3 and [Tb(BDCA) 2 (H 2 O)]Cl 3 was estimated from absorption spectral data using Tauc's relation (Equation 1), which is represented below: 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  As a result, optical band gap values gained from absorption spectral data using Tauc's plot (Eg = 4.0 eV) is c.a. two times lower than calculated EH-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 EH-L.
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 .
Emission  Figure S10) [11] were acquired ( Figure 4). Typical energy transitions of Tb 3+ and Eu 3+ were observed in PL emission spectra as signals of their characteristic wavelengths.  As a result, optical band gap values gained from absorption spectral data usin Tauc's plot (Eg = 4.0 eV) is c.a. two times lower than calculated EH-L (7.50 and 8.17 eV).
The calculated by the density-functional theory method (DFT) HOMO-LUMO ga gives only an approximation of the band gap and does not take into account the formatio of electron-hole semi-particle in the excited state [28]. Thus, the optical gap obtained b Tauc's relation from the UV-vis spectrum corresponds to the energy of the lowest ele tronic transition and could be substantially lower than EH-L.
The  3 in the emission spectra signal at 580 nm, which corresponds to the forbidden transition 5 D 0 → 7 F 0 , which disappears after encapsulation in PEG. The intensity increases for the signals 5 D 0 → 7 F 0 (592 nm) and 5 D 4 → 7 F 6 (485 nm), which correspond to energy transitions from excited state to ground state of Eu 3+ and Tb 3+ , respectively. More information about excitation spectra and PL lifetimes are described in the Supplementary Material (Section S5-"Luminescent properties", Figures S10 and S11).

Methods
The NMR spectra have been recorded on a Bruker Avance III 400 spectrometer (Germany) in DMSO-d 6 at r.t. (at 400 MHz for 1 H and 100 MHz for 13 C). 13 C NMR spectra were recorded with 1 H decoupling. The chemical shifts are given in δ-values [ppm] and refer to the residual signals of non-deuterated DMSO: δ 2.50 ( 1 H) and 39.5 ( 13 C). 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 CH 3 OH 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 (H 2 O)]Cl 3 and [Tb(BDCA) 2 (H 2 O)]Cl 3 ) 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 crystal-Polymers 2022, 14, 5540 8 of 12 lographic 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 . A freshly prepared 2,2'-bipyridine-6,6'-dicarbonyl dichloride (1.14 g, 4.06 mmol) solution in dry CH 2 Cl 2 (10 mL) was added to isopropylamine (2.39 g, 40.6 mmol) solution in dry CH 2 Cl 2 (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 Et 2 O to afford pure BDCA. Yield: 0.93 g (70%); beige powder. 1

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.

Conclusions
The red and green-emitting luminescent Eu 3+ ([Eu(BDCA) 2  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.
Supplementary Materials: The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/polym14245540/s1, Figure S1: 1 H (a) and 13 C NMR (b) spectrum of BDCA ligand; Figure S2: Molecular structures BDCA with thermal ellipsoids shown at the 50% probability level. The molecules of solvents are omitted for better representability; Table S1  Data Availability Statement: Data is contained within the article or supplementary material. The data presented in this study are available in https://www.mdpi.com/article/10.3390/polym14245 540/s1. All crystallographic data for this paper can be obtained free of charge via the Cambridge Crystallographic Database (CCDC numbers: 2215221, 2215232, and 2215233).