3–(2–Pyridyl)pyrazole Based Luminescent 1D-Coordination Polymers and Polymorphic Complexes of Various Lanthanide Chlorides Including Orange-Emitting Cerium(III)

: A series of 18 lanthanide-containing 1D-coordination polymers 1 ∞ [Ln 2 (2–PyPzH) 4 Cl 6 ], Ln = La, Nd, Sm, dinuclear polymorphic complexes α –, β –[Ln 2 (2–PyPzH) 4 Cl 6 ], Ln = Sm, Eu, Gd, α – [Tb 2 (2–PyPzH) 4 Cl 6 ], and [Gd 2 (2–PyPzH) 3 (2–PyPz)Cl 5 ], mononuclear complexes [Ce(2–PyPzH) 3 Cl 3 ], [Ln(2–PyPzH) 2 Cl 3 ], Ln = Tb, Dy, Ho, and Er, and salt-like complexes [Gd 3 (2–PyPzH) 8 Cl 8 ]Cl and [PyH][Tb(2–PyPzH) 2 Cl 4 ] were obtained from the reaction of the respective lanthanide chloride with the 3–(2–pyridyl)pyrazole (2–PyPzH) ligand at different temperatures. An antenna effect through ligand-to-metal energy transfer was observed for several products, leading to the highest luminescence efﬁciency displayed by a quantum yield of 92% in [Tb(2–PyPzH) 2 Cl 3 ]. The Ce 3+ ion in the complex [Ce(2–PyPzH) 3 Cl 3 ] exhibits a bright and orange 5d-based broadband emission with a maximum at around 600 nm, marking an example of a strong reduction of the 5d-excited states of Ce(III). The absorption spectroscopy shows ion-speciﬁc 4f–4f transitions, which can be assigned to Nd 3+ , Sm 3+ , Eu 3+ , Dy 3+ , Ho 3+ , and Er 3+ in a wide spectral range from UV–VIS to the NIR region.

Recently, the investigation and determination of the luminescence properties of Ce 3+ -based CPs and complexes have increasingly become the focus of scientific interest after several reports in the literature of non-emissive Ce 3+ -based compounds due to luminescence quenching by linkers and/or solvent molecules [28].The reported Ce 3+centered luminescence is mainly in the near UV and blue region [22,29,30].Recently, the green/yellow [31][32][33][34] or even the unusual yellow emission [35] of some Ce 3+ -based doped materials as well as the red emission of Ce/Pr systems and their application in solid-state LEDs have been reported [36][37][38][39][40].  [2].An antenna effect through ligand-to-metal energy transfer was observed for several products for both ligands, resulting in the highest luminescence efficiency for Tb 3+ -based compounds, indicated by quantum yield reaching 76%.The reported results indicate the value of exploring new N-donor-based ligands and coordination compounds to achieve a wide variety of structures and PL for the lanthanides.Consequently, the objective of our work was the synthesis of new CPs and complexes along the lanthanide series with the tridentate ligand 3-(2-pyridyl)pyrazole (2-PyPzH) to develop a better understanding of the photophysical and thermal properties observed for the Ln series, as well as investigating the polymorphism, the ability of a pure compound to adopt more than one packing arrangement in the solid-state [41,42], of the studied compounds.
The highlight of this work is the strong bathochromic shift of the Ce 3+ -based emission towards the red region of the visible spectrum and the high luminescence efficiency for the Tb 3+ complex, reaching 92%.
Inorganics 2022, 10, x FOR PEER REVIEW 2 of 22 The organic ligand should also possess appropriate energy-donating states for efficient energy transfer [27].
Recently, the investigation and determination of the luminescence properties of Ce 3+based CPs and complexes have increasingly become the focus of scientific interest after several reports in the literature of non-emissive Ce 3+ -based compounds due to luminescence quenching by linkers and/or solvent molecules [28].The reported Ce 3+ -centered luminescence is mainly in the near UV and blue region [22,29,30].Recently, the green/yellow [31][32][33][34] or even the unusual yellow emission [35] of some Ce 3+ -based doped materials as well as the red emission of Ce/Pr systems and their application in solid-state LEDs have been reported [36][37][38][39][40].
Since 3-(3-pyridyl)pyrazole (3-PyPzH) and 3-(4-pyridyl)pyrazole (4-PyPzH) ligands have recently been used to obtain homoleptic and highly luminescent trivalent lanthanide 3D-CPs with the formula 3 ∞[Ln(3-PyPz)3] and 3 ∞[Ln(4-PyPz)3], Ln = Sm, Eu, Gd, Tb, Dy [7].In addition, 3-PyPzH has been further used to obtain a variety of 3D-frameworks and 2D-networks as well as complexes of Ln-trichlorides differing in constitution and structure: An antenna effect through ligand-to-metal energy transfer was observed for several products for both ligands, resulting in the highest luminescence efficiency for Tb 3+ -based compounds, indicated by quantum yield reaching 76%.The reported results indicate the value of exploring new N-donor-based ligands and coordination compounds to achieve a wide variety of structures and PL for the lanthanides.Consequently, the objective of our work was the synthesis of new CPs and complexes along the lanthanide series with the tridentate ligand 3-(2-pyridyl)pyrazole (2-PyPzH) to develop a better understanding of the photophysical and thermal properties observed for the Ln series, as well as investigating the polymorphism, the ability of a pure compound to adopt more than one packing arrangement in the solid-state [41,42], of the studied compounds.
The highlight of this work is the strong bathochromic shift of the Ce 3+ -based emission towards the red region of the visible spectrum and the high luminescence efficiency for the Tb 3+ complex, reaching 92%.11) crystallizes in the orthorhombic crystal system of higher symmetry with the space group Pbca, while the β-[Ln2(2-PyPzH)4Cl6], Ln = Sm (8), Eu (9), and Gd (10) crystallizes in the triclinic crystal system with the space group P‾ 1.

Photophysical Properties 2.2.1. UV-VIS-NIR Absorption Spectra
Electronic absorption spectra were recorded in the solid state at room temperature (RT) along with emission and excitation spectra to allow for detailed spectroscopic interpretations for 1   The absorption spectrum for 2-PyPzH (Figure S12) was as reported in the literature for the solid state and in acetonitrile solution (7.8 × 10 −5 mol L −1 ), with two characteristic K-band (ca.210~265 nm) and B-band regions (285~350 nm) observed corresponding to the π-π* transitions [55,56].An intense broad absorption band of the ligand in the UV range was detected for the compounds obtained (Figure 6).In addition, sharp and weak to medium bands originating from the respective f-f transitions (Table 1) in both the VIS and NIR regions for The absorption spectrum for 2-PyPzH (Figure S12) was as reported in the literature for the solid state and in acetonitrile solution (7.8 × 10 −5 mol L −1 ), with two characteristic K-band (ca.210~265 nm) and B-band regions (285~350 nm) observed corresponding to the π-π* transitions [55,56].An intense broad absorption band of the ligand in the UV range was detected for the compounds obtained (Figure 6).In addition, sharp and weak to medium bands originating from the respective f-f transitions (Table 1) in both the VIS and NIR regions for  11), the formation of a shoulder is observed at higher wavelengths due to the transition from 4f to 5d.

Emission and Excitation Spectra
The photoluminescence properties were recorded for all bulk products,  11) shows remarkable photoluminescence properties with Ce 3+ -centered light emission in the orange range of the visible spectrum, which can already be distinguishable by the eye under the UV lamp.The Ce 3+ orange emitter is an exception within other Ce 3+ -based emitters.Determinations via photoluminescence spectroscopy (Figure 7) revealed a broadband emission starting at 460 nm with a center at around 600 nm at 77 K and RT, indicating large crystal field splitting and a bathochromic shift for the emission wavelength.The excitation spectrum exhibits a shoulder at 370 nm, corresponding to the lowest energy levels of the crystal field splitting bands of the 5dexcited state of the Ce 3+ ion.The maximum excitation band is at 315 nm, correlated with the coordinated 2-PyPzH ligand.To the best of our knowledge, an orange-red emitting undoped cerium compound (11) has hardly been reported, only for doped systems such as Gd 3 Ga 5 O 12 doped with both Pr 3+ and Ce 3+ [40], Y 3 Al 5 O 12 :Ce nanophosphor doped with Pr 3+ [61], and the cerium-doped scandate [62,63].The nanosecond scale luminescence lifetime (τ) of 11 (2.83 ns) (Table 2) results from the parity-allowed nature of the 5d-4f transition.In contrast, the longer lifetime for the parity-forbidden 4f-4f transitions in [Tb(2-PyPzH ).These values decrease significantly for Sm 3+ (3), α-Sm 3+ (4), and β-Sm 3+ (8), reflecting an excellent antenna effect for Tb 3+ (12), where the ligand is mainly responsible for the excitation and a good antenna effect for α-Eu 3+ (5), β-Eu 3+ (9), and Dy 3+ (13), where additional weak direct 4f-4f excitation is present, indicated by a series of ion-specific sharp lines of low intensity and more distinguishable in Nd 3+ (2), Sm 3+ (3), α-Sm 3+ (4), β-Sm 3+ (8), and Er 3+ (15) (Figures 7 and 8).6) and characterized to the triplet state of 2-PyPzH with λ onset = 425 nm (23,529 cm −1 ).The energy differences (∆E) between the organic ligand triplet state and the energy position of Tb 3+ ( 5 D 4 = 20,500 cm −1 ) [57,59] considering the Latva's rule, ∆E = 3029 cm −1 , explain the long lifetime and the excellent quantum yield value of Tb 3+ .Emission lifetimes determined at RT. 2 Excitation and emission wavelengths for the emission lifetime at RT. 3 Emission lifetime determined at 77 K. 4 Excitation and emission wavelengths for the emission lifetime at 77 K. 5 Quantum yield. 6Excitation wavelength and emission range of QY determinations.
For  7), the highest intensity is found for the transitions 5 D 0 → 7 F 5 at 545 nm and 5 D 4 → 7 F 2 at 612 nm as expected for Tb 3+ and Eu 3+ ions [65,66], while multiple emission lines are Stark levels as a result of energy-level splitting due to the crystal field.
In summary, the 1D-coordination polymer 3 and the monomer complex 12 show the highest stability among the series up to 250 • C, while 5 and 9 are stable up to 230 • C. Further confirmation of the decomposition processes in 3, 5, 9, and 12 was the detection of a set of mass signals at the respective temperatures, which can be assigned to fragments of the ligand ( + m/z = 15).
Depictions of the crystal structures were created with Diamond [74].Structure overlays for polymorphs 5 and 9 were calculated with Mercury [75].
PXRD analyses of the investigated compounds were carried out on a Stoe Stadi P diffractometer (Darmstadt, Germany) with a focusing Ge(111) monochromator and a Dectris Mythen 1K strip detector in Debye-Scherrer geometry.All powder samples were ground in a mortar and filled into Lindemann glass capillaries with 0.3 mm diameter under an inert gas atmosphere.All samples were measured in transmission geometry with Cu-K α radiation (λ = 154.056pm).Data collection was done using the Stoe Powder Diffraction Software Package WinXPOW and Pawley fits on the data were performed using TOPAS Academic [76].The data are listed in Figures 5 and S6-S11.

Synthesis
A mixture of the respective LnCl 3 (76 µmol) and 2-PyPzH (158 µmol) in 0.3 mL MeCN was sealed in an evacuated Duran glass ampoule.The solvent was frozen using liquid nitrogen before a vacuum was applied to the ampoule and the ampoule was sealed.For 1 and 2, the ampoule was heated in a tubular furnace to 160 • C within 24 h.The temperature was held for 24 h and then lowered to 25 • C within another 72 h.For 3, the phase pure bulk was only achievable in a synthesis upon stirring using a Büchi oven.The furnace temperature was raised to 100 • C and held for 48 h until colorless crystals formed above the level of the solvent, followed by cooling to 25 • C. The obtained colorless crystalline bulk was washed with DCM before the characterization processes via SCXRD, PXRD, IR spectroscopy, and CHN analysis.A mixture of the respective LnCl 3 (138 µmol) and 2-PyPzH (276 µmol) in 0.3 mL MeCN in 5 and 6 or toluene in 4 was prepared and sealed in an evacuated Duran glass ampoule.For 4, the tubular furnace was heated to 120 • C within 24 h.The temperature was held for 72 h and then lowered to 25 • C within another 48 h.For 5 and 6, phase pure bulk was achieved by stirring while using a Büchi oven.The furnace temperature was raised to 100 • C and held for 24 h until colorless crystals formed above the level of the solvent, followed by cooling to 25  A mixture of the respective LnCl 3 (138 µmol) and 2-PyPzH (276 µmol) in 0.3 mL MeCN was sealed in an evacuated glass ampoule after freezing the solvent.A Büchi oven with a stirrer was used to raise the temperature of the ampoule to 160 • C and held for three days until colorless crystals formed above the solvent level, followed by cooling to 25  A mixture of CeCl 3 (77 µmol) and 2-PyPzH (241 µmol) in 0.3 mL MeCN was sealed in an evacuated glass ampoule after freezing the solvent using liquid nitrogen.The ampoule was heated to 90 • C in 1 h and then 160 • C within 24 h.The temperature was held for 24 h and then lowered to 25 • C within 72 h.The obtained colorless crystalline bulk was washed with DCM before the characterization process using SCXRD, PXRD, IR spectroscopy, and CHN analysis.A mixture of the respective LnCl 3 (80 µmol) and 2-PyPzH (175 µmol) in 0.6 mL MeCN was sealed in an evacuated Duran glass ampoule after freezing the solvent.The ampoule was heated in a tubular furnace to 160 • C within 48 h.The temperature was held for 72 h and then lowered to 25 A mixture of TbCl 3 (19 µmol) and 2-PyPzH (59 µmol) in 0.3 mL MeCN was sealed in an evacuated glass ampoule after freezing the solvent.The ampoule was heated in a furnace to 160 • C within 48 h.The temperature was held for 72 h and then lowered to 25 • C within another 96 h.A colorless single crystal of the product was selected for SCXRD measurement.A mixture of the respective LnCl 3 (138 µmol) and 2-PyPzH (276 µmol) in 0.3 mL MeCN was sealed in an evacuated glass ampoule after freezing the solvent.The ampoule was heated in a tubular furnace to 160 • C within 48 h.The temperature was held for 72 h and then lowered to 25 • C within another 24 h.A colorless single crystal of the product was selected for SCXRD measurement.

Single Crystals of [PyH][Tb(2-PyPzH) 2 Cl 4 ] (18)
A mixture of TbCl 3 (19 µmol) and 2-PyPzH (59 µmol) in 0.1 mL pyridine was sealed in an evacuated glass ampoule after freezing the solvent.The ampoule was heated in a tubular furnace to 100 • C within 72 h.The temperature was held for 72 h and then lowered to 25 • C within another 96 h.A highly reflective colorless single crystal of the product was selected for the SCXRD measurement.

Conclusions
A novel Ce 3+ -based orange-emitting material was synthesized from anhydrous CeCl 3 together with the ligand 3-(2-pyridyl)pyrazole (2-PyPzH).The obtained [Ce(2-PyPzH) 3 Cl 3 ] represents the first undoped Ce 3+ phosphor material to show intense orange emission based on 5d-4f transitions.This marks the presented compound an exception within other Ce 3+ -based emitters.[Tb(2-PyPzH) 2 Cl 3 ] exhibits high luminescence efficiency with a quantum yield of 92%, reflecting an excellent antenna effect through ligand-to-metal energy transfer.A great structural diversity has been observed along the lanthanide se-ries, from 1D-coordination polymers through dimers to monomer complexes, all of which have been synthesized and characterized that are all luminescent.Two polymorphs are found for each Sm 3+ , Eu 3+ , and Gd 3+ and the α-phase crystallizes at lower temperatures in the P2 1 /c, while the β-phase crystallizes in the P 1 space group.The Ln 3+ ions exhibit a change in coordination number from nine in Ce 3+ to seven in Tb 3+ , Dy 3+ , Ho 3+ , and Er 3+ ions.The characterization of the new compounds was achieved by SC and PXRD, elemental analysis, IR, photoluminescence spectroscopy, and thermal analysis.Overall, this shows the high potential of coordination polymers and complexes with a pyridylpyrazole ligand as the N-donor for the design of materials with versatile structures as well as photophysical properties.

Supplementary Materials:
The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics10120254/s1,additional experimental details; Tables S1 (12), Dy (13), Ho (14), Er (15).Reference [77] is cited in the Supplementary Materials.Funding: This research was funded by the Volkswagen Foundation within the project "Molecular materials-bridging magnetism and luminescence."Heba Youssef was awarded a PhD fellowship by the Egyptian Ministry of Higher Education (MoHE) and the German Academic Exchange Service (DAAD) within the German Egyptian Research Long-term Scholarship (GERLS) Program, 2017 (57311832), the funding agency is the German Academic Exchange Service Cairo.The synthesis of the studied ligand was funded by the Russian Science Foundation (project № 19-13-00272).

Conflicts of Interest:
The authors declare no conflict of interest.

Scheme 1 . 22 Scheme 1 .
Scheme 1. Structural diversity and polymorphism of the products from the reactions of respective anhydrous LnCl 3 with 2-PyPzH.The color changes indicate a different crystal structure.
• C within another 96 h.The obtained colorless crystalline bulk was washed with DCM before the characterization process using SCXRD, PXRD, IR spectroscopy, and CHN analysis.