Synthesis, structure and fluorescence of a novel zinc(II) polymer based on N-[(3-pyridine)-3-sulfonyl]-threonine

2.1 Materials and methods

The reagents utilized were of analytical grade and did not necessitate further purification. The ligand H₃L was synthesized according to a previously reported method with similar procedures. ²⁹ A mixture of acetonitrile, ethanol and aqueous solution at a ratio of 1:3:2 was added to N-[(3-pyridine)-3-sulfonyl]-threonine (0.4 mmol) and Zn(NO₃)₂·6H₂O (0.4 mmol). The pH of the mixture was adjusted to approximate 6 using triethylamine solution and stirred at 55 °C for 12 h. After filtration and evaporation of volatile residues at room temperature, colorless block-like crystals were obtained after 90 days with a yield of 8.3 % (based on Zn). Notably, the complex’s IR spectrum was obtained using KBr as the substrate on an FT-IR spectrometer (PE Spectrum One, USA), its thermal behavior examined by thermogravimetric analysis (Labsys Evo-TGA, France), and solid-state fluorescence characterization carried out using an HORIBA JOBIN JVON fluorescence spectrophotometer (FL3-P-TCSPC, France). IR spectrum (KBr, cm⁻¹): 3,424(vs), 2,395(w), 1,606(m), 1,384(vs), 1,171(s), 1,083(s), 1,026(m), 919(w), 825(m), 751(w), and 595(w). C₉H₂₃N₂O₁₁SZn (Mr = 432.72, %): calcd. C 24.98, H 1.16, N 6.47 %. Found C 22.38, H 1.26, N 7.27.

2.2 X-ray diffraction

The diffraction data were collected at 293 K using an Agilent Super-Nova diffractometer with Mo-Kα radiation (λ = 0.71073 Å) and analyzed using Olex2. ³⁰ The structure was solved through Direct Methods, and organic hydrogen atoms were generated based on their geometry. Additionally, anisotropic thermal parameters were employed to refine all non-hydrogen atoms, which were further refined using the full matrix least square method in the SHELXTL program package. ³¹ Tables 1 and 2 present the crystallographic data and selected bond distances and angles of synthetic polymer [Zn(HL)(H₂O)₂]_n·4H₂O, as well as the corresponding hydrogen bond geometries were depicted in Table 3.

3.1 Structural description

The as-formed coordination polymer 1 belongs to the trigonal crystal system corresponding to the space group P3₁21, based on single-crystal X-ray crystallography data. Additionally, only one chemically independent Zn atom was observed in the asymmetric unit (Figure 1). The Zn(II) ion forms coordination bonds with the nitrogen atom N1 and N2A located on the pyridine ring and the amino group respectively, along with the oxygen atom (O6A) from the carboxyl group and two water molecules (O1w and O2w). This leads to a trigonal bipyramidal coordination geometry for the central ion. Obviously, the coordinating atoms exhibit a monodentate coordination pattern, wherein the N2A atom on the ligand’s amino group and the O6A atom on the carboxyl group form a stable five-membered ring structure with the central metal atom. Moreover, the Zn–O bond lengths range from 2.023(3) to 2.110(3) Å, while the Zn–N bond lengths vary between 2.079(3) and 2.099(4) Å, which are comparable to previously reported results. ^{32 , 33 , 34}

Figure 1:

Coordination environment of Zn(II) ions in 1.

It is noteworthy that ligands bind to Zn(II) ion in a η¹:η²:μ₂ coordination pattern. Typically, the central metal ion forms a helical structure by bridging two different ligand-related atoms, with one ligand contributing the nitrogen atom on the pyridine ring and the other ligand providing the nitrogen atom on the amino group and an oxygen atom from the carboxyl group. This bridging pattern leads to the formation of an infinite helical chain structure along the a-axis (Figure 2). In addition, each individual chain of the complex exhibits 2-fold screw axis symmetry. In the presence of water molecules, extensive intermolecular hydrogen bonding interactions between chains give rise to a two-dimensional supramolecular structure. Notably, the chains are replicated and arranged in an identical helical manner among each other (Figure 3).

Figure 2:

The helical structure of 1. (A) Helical chain structure along the a-axis of for 1; (B) Schematic diagram illustrates the helix axis corresponding to the chain of 1.

Figure 3:

2D supramolecular network for 1 formed through the weak hydrogen bond interactions between 1D helical polymer chains.

Since the complex belongs to the space group P3₁21, it contains a 3₁ helical axis exists. By analyzing the two-dimensional structure of the complex, it can be observed that the polymer chains are replicated and arranged within a single layer. Therefore, an initial inference is made that the three-dimensional structure of the complex consists of three stacked planes (Figure 4). To further investigate how the one-dimensional chains in the complex are arranged in the three-dimensional space, a chain was selected from each layer and plotted, revealing a 3₁ helical arrangement between different layers along the c-axis direction. A careful examination of its three-dimensional stacking structure reveals an intriguing phenomenon: unlike common helical structures formed by minimal molecular units, this complex constructs its 3₁ helical axis based on chains as fundamental units. Thus, it can be concluded that countless units with 3₁ helical axes are orderly arranged to form the three-dimensional supramolecular structure.

Figure 4:

3D supramolecular structure of 1 and its representation of layered stacking mode along the 3₁ helical axis.

3.2 Thermogravimetric analysis

The thermogravimetric analysis was performed over a temperature range spanning from ambient conditions up to 1,000 °C, utilizing a heating rate of 278 K per minute and maintaining continuous N₂ flow. As depicted in Figure S5, an initial weight loss of [Zn(HL)(H₂O)₂]_n·4H₂O at around 148 °C is observed, which can be attributed to the release of coordinated and free water molecules. The observed weight loss percentage was approximately 8.4 %. Subsequently, upon further heating, the polymer framework underwent rapid decomposition with a significant weight reduction (observed weight loss percentage: 68 %), suggesting that this decline could be ascribed to the degradation of the sulfonyl threonine ligand. As the temperature continued to rise, the degradation rate of the polymer decreased, leading to the formation of a pyrolysis residue at approximately 1,000 °C.

3.3 Photoluminescence spectroscopy

The solid-state fluorescence spectra of the ligand, N-[(3-pyridyl)-3-sulfonyl]-threonine, and the resultant coordination polymer, [Zn(HL)(H₂O)₂]_n·4H₂O, were recorded at ambient temperature under identical experimental conditions. As illustrated in Figure 5, the emission signal of [Zn(HL)(H₂O)₂]_n·4H₂O reached its maximum at 478 nm while excited at a wavelength of 410 nm. Meanwhile, the coordination polymer exhibited a red shift of approximately 56 nm in its maximum emission wavelength compared to the ligand (the inset in Figure 5, λ_ex = 372 nm, λ_em = 422 nm). Furthermore, the amplitude-weighted average fluorescence lifetimes of both the ligand and the coordination polymer were examined, yielding fitted values of 1.3 and 1.4 ns, respectively (Figure S6). In fact, the Zn(II) ions, characterized by a d¹⁰ electron configuration, generally do not undergo significant electronic transitions involving changes in the d–d* configuration (i.e., transitions into excited states centered on the metal). However, when Zn(II) ions are coordinated with pyridine sulfonyl threonine ligands, the coordination interaction significantly alters the distribution of electron cloud density among these ligands. The observed phenomenon can be attributed to the coordination pattern of Zn(II) ions with the pyridine sulfonyl glycine ligand and solvent molecules. In particular, the redshift phenomenon is typically closely associated with the formation of hydrogen bonds between molecules, as these bonds can induce slight changes in electron energy levels, thereby impacting the emission wavelength of molecules. Moreover, when the rigidity of the as-formed coordination polymer is enhanced, it restricts rotational vibrational pathways that can improve the quantum yield of photoluminescence. Despite these influences, the decrease in fluorescence intensity may primarily result from the introduction of zinc atoms as a heavy metal center, which promotes stronger intersystem crossing (ISC) processes. Obviously, intersystem crossing refers to a non-radiative relaxation process that allows excited state molecules to release energy through non-radiative means and is particularly prevalent in molecules containing heavy atoms. We speculate that the addition of Zn(II) ions may significantly amplify this non-radiative relaxation process, resulting in decreased fluorescence intensity. In addition, the strength of hydrogen bonds exerts a substantial influence on the stability of excited states and fluorescent characteristics, particularly when hydrogen bond formation facilitates non-radiative relaxation processes. Consequently, the extensive hydrogen bond interactions observed within the as-formed polymer could potentially result in alterations to the fluorescence intensity. In fact, fluorescent mechanisms of coordination polymer are quite intricate and influenced by various factors including solvent effects, intersystem crossing processes, molecular rigidity and hydrogen bonding interactions among others. These factors collectively determine luminescent properties such as emission wavelength, fluorescence intensity and photoluminescence quantum yield of the polymers. ^{35 , 36 , 37 , 38} Therefore, further analysis and research are necessary for a more accurate understanding.

Figure 5:

Solid-state photoluminescence spectra of 1 and its ligand (the inset, λ_ex = 372 nm, λ_em = 422 nm) were measured at ambient temperature under identical experimental conditions. Noted: The red line indicating the excitation spectrum and the black line representing the emission spectrum of the coordination polymer 1.