Designing Scintillating Coordination Polymers Using a Dual-Ligand Synthetic Approach

Herein we report the synthesis and structural characterization of three new europium containing coordination polymers: [Eu(terpy)(2,6-ndc)1.5]·H2O (Linus Pauling (LP) 1), [Eu(terpy)(1,4-ndc)(1,4-Hndc)] (LP-2), and [Eu(phen)(2,6-ndc)1.5]·DMF (LP-3).


Synthesis:
[Eu(terpy)(2,6-ndc) 1.5 ]•H 2 O (LP-1) was synthesized hydrothermally by combining 0.1 g of EuCl 3 •6H 2 O (0.27 mmol), 0.06 g of 2,6-ndc (0.27 mmol), 0.06 g of terpy (0.27 mmol) and 5 mL of ultrapure water (H 2 O) into a 23 mL Teflon-lined acid digestion vessel. The reaction contents were heated to 110℃ for 70 hours, and thereafter allowed to cool to room temperature. Clear, nearly colorless (faint green hue) rhombohedral-shaped crystals formed, Fig. S1. The crystals were washed three times with 5 mL of ethanol to remove excess ligand. Powder X-ray diffraction (PXRD) data collected on the bulk reaction product revealed an impurity that could not be identified, Fig. S5. The impurity is highlighted by the diffraction peak at 27° 2θ. The impurity was removed by washing the bulk reaction product with a mixture of ultrapure H 2 O and 30% NH 4 OH, followed by ethanol. The final yield of the reaction is 14.7%.
Electronic Supplementary Material (ESI) for CrystEngComm. This journal is © The Royal Society of Chemistry 2023 [Eu(terpy)(1,4-ndc)(1,4-Hndc)] (LP-2) was synthesized hydrothermally by combining 0.1 g of EuCl 3 •6H 2 O (0.27 mmol), 0.08 g of 1,4-ndc (0.37 mmol), 0.09 g of terpy (0.38 mmol), and 2.5 mL of ultrapure H 2 O in a 23 mL Teflon-lined acid digestion vessel. The reaction contents were heated to 110℃ for 70 hours, and thereafter allowed to cool to room temperature. Clear, nearly colorless, blade-like crystals with a faint green hue formed, Fig. S1. The crystals were washed three times with 5 mL of ethanol to remove any excess ligand. PXRD data collected on the bulk reaction product shows phase purity, Fig. S6. The final yield of the reaction is 37%.
[Eu(phen)(2,6-ndc) 1.5 ]•DMF (LP-3) was synthesized solvothermally by combining 0.1 g of EuCl 3 •6H 2 O (0.27 mmol), 0.06 g of 2,6-ndc (0.27 mmol), 0.1 g of phen (0.55 mmol) and 5 mL of DMF into a 23 mL Teflon-lined acid digestion vessel. The reaction contents were heated to 110 ℃ for 70 hours, and thereafter allowed to cool to room temperature. Clear, colorless blade-like and rhombohedral shaped crystals with a faint green color formed, Fig. S1. The crystals were washed three times with fresh DMF to remove any excess ligand. The crystals were unstable outside the mother liquor and prone to rapid degradation. Attempts to stabilize the material by solvent exchange with organic solvents (ethanol, acetone, isopropanol, etc.) were unsuccessful, and led to degradation.

Experimental and Relevant Data
Single Crystal X-ray Diffraction.
Reflection data of LP-1 and 3 were collected using 0.5° ω and φ scans at 100(2) K on a Bruker D8 Venture diffractometer equipped with a Photon 100 CMOS detector and a Mo Kα source with a triumph monochromator (Table S1). The APEX III software suite1 was used to integrate the data and apply an absorption correction (SADABS). 1 Reflection data of LP-2 were collected on a Rigaku Oxford Diffraction Synergy-S equipped with a PhotonJet-S Cu source (λ = 1.54178 Å) and HyPix-6000HE photon-counting detector. All the images were collected and processed using CrysAlisPro Version 40.21a, 40.53 and 40.81a (Rigaku Oxford Diffraction, 2018). 2 The reduced data were solved using direct methods via SHELXS3 and refined using SHELXL-153 within the WINGX software suite. 3 Publication materials were prepared using EnCifer6 4 and all figures of the title compounds were generated using CrystalMaker® (V10.7.1): a crystal and molecular structures program for Mac and Windows. CrystalMaker Software Ltd, Oxford, England (www.crystalmaker.com).
All the nonhydrogen atoms in LP-1, LP-2, and LP-3 were located on the difference Fourier maps and refined anisotropically. All H atoms associated with the carbon atoms were affixed to their parent atoms using a riding model. In LP-1, several of the C atoms of a 2,6-ndc ligand exhibit some signs of disorder (CheckCif Alert level B for Hirshfeld test) as well as some residual electron density surrounding the ring. This is caused by minor positional disorder of the ligand. We chose not to model this disorder as it was minor (no atoms were split or badly disordered). The lone solvent water molecule in LP-1 was disordered over two positions and was modeled using Part Instructions. The H atoms associated with the water molecules in LP-1 could not be modeled and refined with confidence (even after using heavy restraints) and were left unmodeled. In LP-2, the two . We have ruled out common modeling errors and are confident in the connectivity and motif modeled. Specific to LP-3, we note several unresolved electron density peaks that trigger several CheckCif A, B, and C level alerts. Errors in unit cell and space group assignment were ruled out as was the possibility of twinning. These errors are therefore attributed to the large crystal size and heavy atoms located within the crystal structure. Table S1. Selected crystallographic data for LP-1, LP-2, and LP-3 at 100(2) K. Secondary building unit (SBU) analysis: The analysis was performed with topcryst.com. 5 The RCSR three-letter codes 6 were used to designate the network topologies. The TTD collection 7 was used to determine the topological type of the crystal structure.     Thermogravimetric Analysis. Compounds LP-1 and LP-2 were analyzed using a SDT Q600 V20.9 Build 20 TGA-DSC. Roughly 10 mg of each powdered sample were heated from 25°C to 700°C at a heating rate of 10°C min -1 using either Ar (g) or air (21% O 2(g) ) as the carrier gases. Powder X-ray Diffraction. Data on LP-1 and LP-2 were collected using a Rigaku Ultima IV diffractometer equipped with a Cu sealed tube X-ray source (1.6 kW). The scans were collected in 0.02° count binning steps with a 1°/minute scan rate. The instrument featured a 10 mm divergence slit, 0.5 mm incident slit, 5° primary and receiving side solar slits, and a Ni foil filter to reduce contributions from Kβ.   Diffuse Reflectance. Solid-state DR data were collected on bulk material of LP-1 and LP-2 using a Cary-5000 UV-vis-NIR spectrometer with a DRA-2500 external DR accessory. Photoluminescence Characterization. Luminescence data of powdered samples of LP-1 and LP-2, and single crystals of LP-3 immersed in DMF were used for data collection. The samples were loaded into quartz NMR tubes and measurements were taken within a quartz dewar at 298 K using a Horiba Nano-log®-3 PL spectrophotometer outfitted with a 450W Xe excitation source. The data were collected at 90° from the excitation source and recorded using a UV-Visible PMT detector (185-850 nm). All data were processed using the FluorEssence software (V.3.9.0.1) and Origin (V.8.6001).    Compound LP-1 (left) and LP-2 (right) produce bright red emission when exposed to soft X-rays (Cu Kα, 8.04 keV).

Attenuated Total Reflectance Fourier Transform Infrared spectroscopy (ATR-FTIR)
was performed on samples of LP-1 and LP-2. The data were collected using a Bruker Lumos -FTIR spectrometer outfitted with an attenuated total reflection accessory and analyzed using the OPUS software (V.7.2).
Compound LP-1 and LP-2 display a series of vibrational bands between 1600-630 nm that correspond to stretching modes of C-C, C-O, C-H, and C-N bonds present in the organic ligands. [8][9][10][11][12][13][14][15] Between 3400-3600 nm the O-H stretch of the lattice and surface water molecules are observed. 16 Fig. S18. Bulk crystalline material of LP-1 (left) and LP-2 (right) were each analyzed via FTIR in three areas to demonstrate homogeneity.