Ultrashield 400

Approximately 100 mg of fine crystalline sample was tightly packed into a zirconia rotor with the help of Teflon stick up to the cap Kel-F mark. A cross-polarization, magic angle spinning (CP-MAS) pulse sequence was used for spectral acquisition. Each sample was spun at a frequency of 5.0 ± 0.01 kHz and the magic angle setting was calibrated by the KBr method. Each data set was subjected to a 5.0 Hz line-broadening factor and subsequently Fourier transformed and phase corrected to produce a frequency domain spectrum. Solid-state ¹³C NMR spectra were obtained on a Bruker (Bruker BioSpin, Karlsruhe, Germany) Ultrashield 400 spectrometer utilizing a ¹³C resonant frequency of 100 MHz (magnetic field strength of 9.39 T). The chemical shifts were referenced to trimethylsilyl (TMS) using glycine (δ_glycine = 43.3 p.p.m.) as an external secondary standard. ¹⁵N CP-MAS spectra recorded at 400 MHz were referenced to glycine N and then the chemical shifts were recalculated to nitromethane (δ_glycine = −347.6 p.p.m.).

All starting materials, solvents, and TLC plates (TLC-silica gel-f 454 60 nm) were bought from Merck Co. The compounds were of reagent grade (98% purity or higher) and used without further purifications. Melting points are related to the purified products. All the products were known compounds, and their structures were confirmed by comparing between their melting point and spectral data (IR, ¹H-NMR, and ¹³C-NMR) with the reports. The melting points were recorded using Melt-temp apparatus with the end-capped capillary pyrex tube. FT-IR spectra were recorded in a JASCO instrument using a KBr pellet. ¹H- and ¹³C-NMR spectra were recorded on a Bruker Ultrashield 400 instrument (400 MHz for 1H-NMR and 100 MHz for 13C-NMR) using CDCl₃ as the solvent. The obtained FIDs were analyzed and interpreted using MestReNova software. The MIRA3TESCAN-XMU electronic microscope was used to obtain FESEM images and to perform EDS analyses. XRD patterns were obtained using a Philips X’PERT MPD instrument at 30 mA current and 40 kV electric potential. The XRD results were prepared with X’PERT High Score software.

Nuclear magnetic resonance (NMR) spectra were recorded on an ULTRASHIELD 400 (Bruker, Zürich, Switzerland) spectrometer at 400 MHz at room temperature. Electrospray ionization mass spectra ((ESI-MS)) were obtained by Micromass LCTTM system (Altringham, UK). The FT-IR spectra were carried out using KBr discs on an AVATAR 360 FT-IR spectrophotometer (Nicolet, Madison, WI, USA) at room temperature. UV absorption spectra were performed using a UV-1800 UV-Vis spectrophotometer (Shimadzu, Tokyo, Japan). The morphology of these samples was observed via scanning electron microscopy (FEI, QUANTA 450, Hillsboro, OR, USA) with an accelerating voltage of 20.0 kV. The SEM samples were prepared by placing a drop of gel on a flat surface of a cylindrical copper substrate. Then, the hydrogel was dried via freeze-drying for 3 h. Following that, the samples were coated with gold using a MSP-1S magnetron sputter (Japan) coater (Tokyo, Japan). Rheological properties were measured using a HAAKE RheoStress 6000 rheometer (Offenburg, Germany) with plate geometry (plate diameter, 35 mm). Frequency sweep measurements were carried out on freshly formed gels over a range from 0.01 to 10 Hz at 25 °C. Strain sweep was performed at a strain amplitude of 1%. Time sweep measurements were performed at a frequency of 6.28 rad·s⁻¹ and a strain of 30%.