Novel Quinoline-Based Thiazole Derivatives for Selective Detection of Fe³⁺, Fe²⁺, and Cu²⁺ Ions

Abstract

New quinoline-based thiazole derivatives QPT and QBT were synthesized and characterized by various spectroscopic and single-crystal X-ray crystallographic studies. The metal-sensing properties of the probes were further examined by absorption and fluorescence spectrometry. The fluorescence intensity of QPT and QBT was remarkably quenched during the addition of Fe³⁺, Fe²⁺, and Cu²⁺ ions in THF/H₂O (1:1) at pH = 7.4 in HEPES buffer, while the addition of other metal ions did not affect the fluorescence intensity of the ligands. The detection ability of the probes QPT and QBT was further investigated by titration with various equivalents of metal ions, optimized pH ranges for detection, and reversibility with Na₂EDTA for biological applications.

Introduction

Heavy-metal ion detection has been given more attention in recent research because of its toxicity to the environment and living organisms.¹ In particular, iron and copper ions have an important role in enzyme activities and redox processes and are widely used in different fields like biological, agricultural, electronics, industries, etc.² Iron is the most essential biologically important element that is present in hemoglobin, myoglobin, and iron–sulfur protein.³ It also plays an important role in the formation of DNA and RNA in living organisms.⁴ Deficiency of Fe²⁺ and Fe³⁺ leads to low blood pressure, anemia, liver and kidney damage, etc.⁵ However, excess of the iron leads to the formation of reactive oxygen species and induces cell damage.⁶ Therefore, monitoring and detection of the level of iron plays an important role in the biological field.

Copper is an important trace element for essential physiological functions in human beings, animals, plants, and also for insects and microorganisms.⁷ It is a cofactor and structural element of various processes such as neurotransmission, enzyme and other protein-requiring metabolic processes, scavenging of free radicals, iron transportation, energy generation, and pigmentation.⁸ However, copper deficiency or the over-accumulation of copper in the human body can lead numerous disorders such as liver damage, kidney damage,⁹ neurotoxicity,¹⁰ Wilson’s disease,¹¹ Parkinson’s disease,¹² and Alzheimer’s disease.¹³ Furthermore, increased accumulation of copper can lead to severe toxic effects due to oxidative stress caused by redox cycling reactions between copper and cellular reduction-prone species such as biothiols.¹⁴

Hence, the detection of iron and copper ions with simple methods attracts interest in analytical chemistry. Different analytical instruments such as those of atomic absorption (AAS), emission spectroscopy (AES), inductively coupled plasma mass spectrometry (ICPMS), inductively coupled plasma atomic emission spectrometry (ICP-OES), electrochemical methods, and X-ray fluorescence spectroscopy (XRF) have been used.^1b15 However, fluorescent and colorimetric detection of metal ions is in demand due to simplicity, lesser time consumption, and cost-effectiveness. Therefore, the development of new Fe²⁺, Fe³⁺, and Cu²⁺ sensors with simple instruments is a recent trend in analytical chemistry.

In this context, we have focused on synthesizing a suitable chemosensor for the detection of Fe²⁺, Fe³⁺, and Cu²⁺ with several donor atoms like N and S for chelation. Among nitrogen and sulfur-containing heterocycles, quinoline and thiazole analogues were broadly used for the detection of various metal ions and used as intermediates for the formation of metal complexes.¹⁶ Herein, we have synthesized novel quinoline-based phenyl thiazole (QPT) and benzothiazole (QBT) derivatives. We have examined the detection ability of QPT and QBT chemosensors toward the metal cations Al³⁺, Ca²⁺, Cd²⁺, Co²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺, and Zn²⁺ in THF/H₂O (50:50, v/v buffered at pH 7.4). In the presence of Fe³⁺, Fe²⁺, and Cu²⁺, QPT and QBT have shown effective and selective fluorescence quenching with an abrupt color change from red to dark brown.

Results and Discussion

Initially, the starting precursor 2,3-dihydro-8-nitro-4-quinolone 1 was synthesized as per the reported procedure.¹⁷ In the next step, 2,3-dihydro-8-nitro-4-quinolone 1 was refluxed with 2-hydrazino-4-phenylthiazole 2 and 2-hydrazinobenzothiazole 3 in the presence of acetic acid in methanol to afford novel N-(8-nitro-2,3-dihydro-1H-quinoline-4-ylidene)-N′-(4-phenyl-thiazole-2-yl)-hydrazine (QPT) and (E)-2-(2-(8-nitro-2,3-dihydroquinolin-4(1H)-ylidene)hydrazinyl) benzo[d]thiazole (QBT) (Scheme 1). QPT remains stable in the atmosphere and soluble in acetone, DMF, DMSO, THF, methanol, ethanol, and acetonitrile.

Scheme 1. Synthesis of QPT and QBT.

The structures and purity of chemosensors QPT and QBT were confirmed by IR, ¹H and ¹³C NMR, HR mass spectroscopy methods, and X-ray single crystal analysis. In the IR spectrum of QPT, the carbonyl group of quinolone disappeared and characteristic absorption bands for N–H at 3389 and 3268 cm^–1, C=N at 2084 and 1944 cm^–1, and C–N at 1232, 1168, and 1123 cm^–1 had appeared. In the ¹H NMR spectrum of QPT, the proton signals of CH₂ protons for quinoline rings appear at δ 3.60–3.56 ppm as a multiplet and 2.83 ppm as a triplet, broad singlets at δ 9.14 for N–NH and δ 8.15 for Q-NH, and aromatic protons at δ 6.71 to 8.34 ppm. The ¹³C NMR spectra of QPT showed the disappearance of the carbonyl group and carbon assignments for thiazole N=C at δ 176.07, quinoline C=N at δ 170.91, thiazole C–N at δ 150.82, and aliphatic carbons at δ 38.92 and 23.59; all other signals are for aromatic carbons.

X-ray Crystallographic Studies

The structure of compounds QBT and QPT were further confirmed by single-crystal X-ray crystallographic analysis.

Molecular Geometry and Crystal Packing of QBT

The compound QBT crystallized in the centrosymmetric monoclinic unit cell with four molecular units. The adopted space group of the crystalline lattice is P2₁/n and the structure refinement is completed with the R factor of 5.78%. The nitro group attached to the 10-membered heterocyclic ring slightly deviated from the mean plane of the 10-membered ring with an angle of 1.2(1)°. According to Cremer and Pople conformational analysis, the six-membered heterocyclic ring (N2/C7/C8/C9/C10/C11) adopted an envelope conformation with the puckering coordinates of q₂ = 0.3461(3) Å, ϕ₂ = 252.56(1)°, and q₃ = −0.2252(3) Å. The crystal packing contains intermolecular classical N–H···O and N–H···N hydrogen bonds and two non-classical C–H···O interactions (Figure 2 and Table 1). One of the non-classical interactions (C2–H2A···O2^#4) leads to a ring R₂²(26) motif around the inversion center of the unit cell. A similar centro-symmetrically related ring R₂²(12) motif is observed through a N–H···O classical hydrogen bond. Further, the molecules are connected through a N–H···N hydrogen bond leading to a short-chain C(4) motif extending along the b axis of the unit cell in a zigzag fashion (Figure 3). This molecular aggregation is further supported through a C9–H9A···O1^#2 interaction leading to a chain C(7) motif.

Figure 2.

Packing of the molecules in the monoclinic unit cell is viewed along the b axis. H-bonds are shown as dashed lines.

Table 1. Hydrogen Bonding Geometry (Å, °)^a.

D–H···A	d(D–H)	d(H···A)	d(D···A)	<(DHA)
N2–H2···O1#1	0.86	2.34	2.993(3)	132.4
C9–H9A···O1#2	0.97	2.66	3.455(4)	140
N4–H4···N5#3	0.89(3)	2.36(3)	3.246(3)	175(3)
C2–H2A···O2#4	0.93	2.67	3.571(4)	164

^aSymmetry transformations used to generate equivalent atoms: #1 −x, −y – 1, −z + 1; #2 x, y + 1, z; and #3 −x + 1/2, y – 1/2, −z + 1/2.

Figure 3.

Molecular aggregations leading to the (a) ring R₂²(26) motif, (b) ring R₂²(12) motif, and (c) chain C(4) motif are shown. H-bonds are shown as dashed lines.

Molecular Geometry and Crystal Packing of QPT

The compound QPT crystallized in the centrosymmetric monoclinic unit cell with four molecular units. The adopted space group of the crystalline lattice is P2₁/c, and the structure refinement is completed with an R factor of 4.27%. The compound is almost flat with two heterocyclic rings and one phenyl ring. The nitro group attached to the quinoline heterocyclic ring has slightly deviated from the mean plane of the ring with an angle of 7.1(1)°. Similarly, the five-membered thiazole ring is making dihedral angles of 6.5(1)° with the mean plane of the 10-membered ring and 8.1(1)° with the phenyl ring. According to Cremer and Pople’s conformational analysis, the six-membered heterocycle ring (N4/C10/C11/C16/C17/C18) adopted an envelope conformation with the puckering coordinates of q₂ = 0.2702(3) Å, ϕ₂ = 284.07(1)°, and q₃ = 0.1930 Å. The molecular aggregations on the unit cell are almost parallel to the ab plane. The dihedral angle between the (001) plane and the mean plane of the molecule in the asymmetric part is 8.45(3)°.

The hydrogen bonds that are involved in the crystal packing are listed in Table 2, and the packing diagram of the compound is shown in Figure 4. Intermolecular interactions, especially classical and non-classical hydrogen bonds, are playing a crucial role in the formation of crystalline solids and their physiochemical properties. These hydrogen-bonding interactions can be classified and notated with graph-set nomenclature, which is useful in comparing the stability of molecular conformations and crystalline lattices between the similar molecules.¹⁸

Table 2. Hydrogen Bonding Geometry (Å, °)^a.

D–H···A	d(D–H)	d(H···A)	d(D···A)	<(DHA)
C17–H17B···N1#1	0.97	2.59	3.320(3)	132
C18–H18B···O1#2	0.97	2.59	3.133(2)	116
C3–H3···O1#3	0.93	2.56	3.429(2)	156
N4–H4N···O2	0.88(2)	1.96(2)	2.627(2)	132
N2–H2N···O2#2	0.88(2)	2.34(2)	3.181(2)	161

^aSymmetry transformations used to generate equivalent atoms: #1 −x, −y + 1, −z + 1; #2 −x, y + 1/2, −z + 3/2; and #3 x, −1 + y, z.

Figure 4.

Packing of the molecules showing flat aggregations along the ab plane in the monoclinic unit cell viewed along the a axis. H-bonds are shown as dashed lines.

The crystalline lattice features intra- and intermolecular classical N–H···O hydrogen bonds. Further, non-classical C–H···N and C–H···O interactions are supporting the crystalline assembly. The classical intramolecular hydrogen bond leads to a self-associated S(6) motif. Another classical intermolecular N–H···O hydrogen bond [N2–H2N···O2^#2; #2 −x, y + 1/2, −z + 3/2] leads to a zigzag chain C(9) motif extending along the b axis of the unit cell. A non-classical C–H···O interaction [C3–H3···O1^#3; #3 x, −1 + y, z] leads to a straight-chain C(15) motif extending along the b axis of the unit cell. These two infinite chain motifs intersect to form a ring R₃³(21) motif (Figure 5).

Figure 5.

Self-associated ring S(6), infinite chain C(9), and C(15) motifs along [010] and ring R₃³(21) motifs. H-bonds are shown as dashed lines.

UV–Visible and Fluorescence Studies

Detection of Fe³⁺, Fe²⁺, and Cu²⁺ by QPT and QBT

The absorption spectrum of the compound QPT showed intense bands at 279, 338, and 463 nm and the compound QBT showed absorption bands at 281, 338, and 464 nm in THF/H₂O (50:50, v/v buffered at pH 7.4). Initially, we have examined absorbance changes of QPT and QBT toward metal ions Al³⁺, Ca²⁺, Cd²⁺, Co²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺, and Zn²⁺ in THF/H₂O (50:50, v/v buffered at pH 7.4) at room temperature. Addition of Al³⁺, Ca²⁺, Cd²⁺, Co²⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺, and Zn²⁺ had no effect on the absorption spectrum. However, upon the addition of Fe³⁺, Fe²⁺, and Cu²⁺ both QPT and QBT showed significant enhancement at the range of 281–338 nm. Additionally, an immediate color change of the QPT and QBT solution from bright red to dark brown was obtained within 10 s upon the addition of Fe³⁺, Fe²⁺, and Cu²⁺ ions. The results suggest that QPT and QBT could serve as a potential detector of Fe³⁺, Fe²⁺, and Cu²⁺ ions (Figure 6).

Figure 6.

(a) UV–vis changes of QPT and (b) UV–vis changes of QBT (2 × 10^–4 M) with various metal ions (2 × 10^–4 M) in THF/H₂O (1:1) at pH = 7.4 in HEPES buffer.

Further, the fluorescence spectra of the compounds QPT and QBT were recorded at pH 7.4 [HEPES (10 μM)–THF with H₂O (50:50)], and the emission wavelength was observed at 310 nm. Initially, we examined the fluorescence changes of QPT and QBT toward Al³⁺, Ca²⁺, Cd²⁺, Co²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺, and Zn²⁺ in THF/H₂O (50:50, v/v buffered at pH 7.4) at room temperature. The fluorescence emission spectra of both QPT and QBT displayed maximum fluorescence quenching effects with Fe³⁺, Fe²⁺, and Cu²⁺. However, fluorescence intensities of chemosensors QPT and QBT did not show any remarkable changes with other metal cations (Figure 7).

Figure 7.

(a) Fluorescence changes of QPT and (b) fluorescence changes of QBT (2 × 10^–4 M, λ_ex = 310 nm) with various metal ions (2 × 10^–4 M) in THF/H₂O (1:1) at pH = 7.4 in HEPES buffer.

In the fluorescence titration experiments, Fe³⁺ (0–100 equiv), Fe²⁺ (0–100 equiv), and Cu²⁺ ions (0–75 equiv) were titrated with QPT and QBT in THF at room temperature. The fluorescence intensity was gradually quenched upon the addition of Fe³⁺, Fe²⁺, and Cu²⁺ (Figures 8 and 9). From the fluorescence titration data, the association constant K_a values of QPT and QBT with Fe³⁺, Fe²⁺, and Cu²⁺ were calculated by using the Benesi–Hildebrand equation (eq 1). The fitting curves showed a good linear pattern with high correlation coefficients. The calculated K_a’s of QPT with Fe³⁺, Fe²⁺, and Cu²⁺ are 2.5116 × 10⁴, 3.2578 × 10⁴, and 1.9990 M^–1. The K_a values of QBT with Fe³⁺, Fe²⁺, and Cu²⁺ are 1.9625 × 10⁴, 3.4606 × 10⁴, and 1.2405 × 10⁴ M^–1.

1

where F₀ is the intensity of fluorescence of the fluorophores without metal ions (M), F is the intensity with a particular concentration of metal ions (M), F^′ is the intensity at the maximum concentration of metal ions (M) used, and K is the binding constant.

Figure 8.

Fluorescent changes of QPT with different concentrations of (a) Fe³⁺ (100 equiv) with its (b) Benesi–Hildebrand plot, (c) Fe²⁺ (100 equiv) with its (d) Benesi–Hildebrand plot, and (e) Cu²⁺ (75 equiv) with its (f) Benesi–Hildebrand plot in THF/H₂O (1:1) at pH = 7.4 in HEPES buffer.

Figure 9.

Fluorescent changes of QBT with different concentrations of (a) Fe³⁺ (100 equiv) with its (b) Benesi–Hildebrand plot, (c) Fe²⁺ (100 equiv) with its (d) Benesi–Hildebrand plot, and (e) Cu²⁺ (75 equiv) with its (f) Benesi–Hildebrand plot in THF/H₂O (1:1) at pH = 7.4 in HEPES buffer.

The calibration curve between fluorescence intensities of QPT and QBT versus the concentrations of Fe³⁺, Fe²⁺_, and Cu²⁺ ions were plotted and showed a good linear relationship with good correlation coefficients. The detection limits of the compound were calculated using the formula 3σ/κ and were found to be 3.1201 × 10^–4 M for QPT-Fe³⁺, 4.4079 × 10^–4 M for QPT-Fe²⁺, 3.1740 × 10^–4 M for QPT-Cu²⁺, 2.9763 × 10^–4 M for QBT-Fe³⁺, 3.8395 × 10^–4 M for QBT-Fe²⁺, and 2.9169 × 10^–4 M for QBT-Cu²⁺.

The complexation stoichiometry of QPT and QBT was further confirmed by Job plot analysis. The values of QPT-Fe³⁺, QBT-Fe³⁺, QPT-Fe²⁺, QBT-Fe²⁺, QPT-Cu²⁺, and QBT-Cu²⁺ reached the maximum when the molar fraction was 0.5, which indicated that the complexation stoichiometry ratio was 1:1 (Figure 10).

Figure 10.

Job plots for the complexes of (a) QPT with Fe³⁺, (b) QPT with Fe²⁺, (c) QPT with Cu²⁺, (d) QBT with Fe³⁺, (e) QBT with Fe²⁺, and (f) QBT with Cu²⁺ ions in THF/H₂O (1:1) at pH = 7.4 in PBS buffer.

To examine the ability of QPT and QBT to resist the interference of other metal ions, the competition experiments were conducted where 100 equiv of metal cations Al³⁺, Ca²⁺, Cd²⁺, Co²⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺, and Zn²⁺ was mixed with QPT and QBT then 100 equiv of Fe³⁺, Fe²⁺, and Cu²⁺ was added to all the mixtures. The emission spectra of QPT-Fe³⁺, QBT-Fe³⁺, QPT-Fe²⁺, QBT-Fe²⁺, QPT-Cu²⁺, and QBT-Cu²⁺remain the same in the presence of other metal ions (Figure S1).

Additionally, to examine the reversibility of chemosensor QPT from its Fe³⁺, Fe²⁺, and Cu²⁺ complex, EDTA titration experiments were performed. Upon addition of Na₂EDTA (100 equiv) to the QPT-Fe³⁺, QBT-Fe³⁺, QPT-Fe²⁺, QBT-Fe²⁺, QPT-Cu²⁺, and QBT-Cu²⁺ mixtures, the fluorescence intensity was enhanced and reached the intensities of QPT and QBT (Figure S2). This cycle was repeated many times and the results suggest that the Fe³⁺, Fe²⁺ and Cu²⁺ recognition by QPT and QBT is a reversible process. The time dependence of the complexation experiment indicates the binding phenomena of chemosensors QPT and QBT completely binding with Fe³⁺, Fe²⁺, and Cu²⁺ions in 3 min.

For the biological applications, the suitable pH was investigated for QPT/QBT and its Fe³⁺, Fe²⁺, and Cu²⁺ complexes. The fluorescence intensities of QPT, QBT, QPT-Fe³⁺, QBT-Fe³⁺, QPT-Fe²⁺, QBT-Fe²⁺, QPT-Cu²⁺, and QBT-Cu²⁺ varied at different pH values, ranging from 1 to 12. However, the emission intensity remains almost the same in the pH range of 6–8 (Figure S3). Based on the results from the studies, a physiological pH value of 7.4 was chosen for all the fluorescence studies.

Conclusions

In conclusion, new quinoline-based thiazole derivatives QPT and QBT were successfully synthesized, which showed a fluorescence quenching effect with Fe³⁺, Fe²⁺, and Cu²⁺ at pH 7.4 in THF/H₂O. The Job plots demonstrated a 1:1 complex formation of QPT and QBT with Fe³⁺, Fe²⁺, and Cu²⁺ metal ions. The detection ability of QPT and QBT was further investigated by titration with various equivalents of metal ions and competitive experiments of Fe³⁺, Fe²⁺, and Cu²⁺ in the presence of other metal ions, and pH ranges for detection and reversibility with Na₂EDTA were investigated for biological applications.

Experimental Section

Chemicals and Instrumentation

All the chemicals and reagents were of analytical grade and purchased from Sigma-Aldrich and used without purification. A JASCO FT IR 4100 spectrometer was used to record the absorption frequencies for the compounds. Bruker Advance instruments (400 MHz for ¹H and 100 MHz for ¹³C) were used to record the ¹H and ¹³C NMR spectra using DMSO-d₆ as solvent. The reactions were monitored by using silica gel-precoated TLC F254 Merck plates. A Shimadzu UV-240 spectrophotometer and JASCO FP-8200 spectrofluorimeter were used to record absorption and fluorescence spectra of the compounds using standard quartz cuvettes of 1 cm in path length. The recorded excitation and emission slit width was 5.0 nm at 24 ± 1 °C temperature. A PerkinElmer 2400 series II Elemental CHNS analyzer was used to perform the elemental analysis. Mass spectra of the compounds were obtained on an HR mass spectrometer.

Synthesis of Chemosensors QPT and QBT

Equal mole ratios of 2,3-dihydro-8-nitro-quinolone (1 mmol) and 2-hydrazino phenylthiazole/hydrazino benzothiazole (1 mmol) were dissolved in 10 mL of methanol, and a catalytic amount of glacial acetic acid was added to the mixture. The reaction mixture was refluxed for 2–4 h at 80 °C. The reaction was monitored by TLC after completion of the reaction, and the mixture was cooled to room temperature; the pure products QPT and QBT were obtained as crystals.

N-(8-Nitro-2,3-dihydro-1H-quinolin-4-ylidene)-N′-(4-phenyl-thiazol-2-yl)-hydrazine QPT

Red solid, mp: 210–212 °C; IR(cm^–1): ¹H NMR (400 MHz, DMSO-d₆) δ: 11.42 (s, 1H, N-NH), 8.39 (s, 1H, Q-NH), 8.19 (d, 1H, J = 7.2 Hz, Ar-H), 8.08 (d, 1H, J = 8.4 Hz, Ar-H), 7.88 (d, 2H, J = 6.8 Hz, Ar-H), 7.41 (t, 2H, J = 7.2 Hz, Ar-H), 7.35 (m, 2H, Ar-H), 6.77 (t, 1H, J = 8 Hz, Ar-H), 3.51 (s, 2H, Q-CH₂), 2.84 (s, 2H, Q-CH₂); ¹³C NMR (100 MHz, DMSO-d₆) δ: 176.488, 156.960, 146.660, 144.782, 143.678, 140.004, 139.487, 138.635, 133.018, 131.413, 127.040, 122.980, 116.237, 115.766, 115.382, 108.671, 39.133, 23.134; HRMS: C₁₈H₁₅N₅O₂S m/z [M + H] 366.10350.

N-Benzothiazol-2-yl-N′-(8-nitro-2,3-dihydro-1H-quinolin-4-ylidene)-hydrazine QBT

Red solid, mp: 210–212 °C; IR (cm^–1): ¹H NMR (400 MHz, DMSO-d₆) δ: 11.80 (s, 1H, N-NH), 8.42 (s, 1H, Ar-H), 8.24 (d, 1H, J = 6.8 Hz, Ar-H), 8.10 (d, 1H, J = 8.0 Hz, Ar-H), 7.67 (brs, 1H, Q-NH), 7.26 (s, 2H, Ar-H), 7.07 (s, 1H, Ar-H), 6.76 (t, 1H, J = 8.4 Hz, Ar-H), 3.51 (s, 2H, Q-CH₂), 2.91 (s, 2H, Q-CH₂); ¹³C NMR (100 MHz, DMSO-d₆) δ: 176.488, 156.960, 146.660, 144.782, 143.678, 140.004, 139.487, 138.635, 133.018, 131.413, 127.040, 122.980, 116.237, 115.766, 115.382, 108.671, 39.133, 23.134; HRMS: C₁₆H₁₃N₅O₂S m/z [M + H] 340.08759.

Crystallographic Data Collection and Refinement

The slow solvent evaporation solution growth technique is adopted to get the X-ray quality crystals. Block-type single crystals were taken away from the crop, and the X-ray intensity data collection was done with an X-ray wavelength of 0.71073 Å at room temperature. A Bruker AXS KAPPA APEX-2 diffractometer equipped with a graphite monochromator was employed. The details of parameters regarding the data collection and structure solution are given in Table 3. The structure was solved by direct methods and refined by full-matrix least-squares calculations using SHELXL-2014.¹⁹ The ORTEP view of the molecule drawn in 50% probability thermal-displacement ellipsoids with the atom numbering scheme is shown in Figure 1.

Table 3. Crystallographic Details of the Compounds.

	QPT	QBT
molecular formula	C18H15N5O2S	C16H13N5O2S
formula weight	365.41	339.37
temperature	296 K	300 K
wavelength	0.71073 Å	0.71073 Å
crystal system	monoclinic	monoclinic
space group	P21/c	P21/c
unit cell dimensions	a = 12.8882(12) Å; α = 90°	a = 13.888(2) Å; α = 90°
	b = 16.8832(15) Å; β = 106.261(3)°	b = 4.7349(8) Å; β = 91.509°
	c = 7.8668(7) Å; γ = 90°	c = 23.274(4) Å; γ = 90°
volume	1643.3(3) Å3	1529.9(4)
Z	4	4
density (calculated)	1.477 mg/m3	1.473 mg/m3
absorption coefficient	0.222 mm–1	0.232 mm–1
F(000)	760	704.0
index ranges	h = 16, k = 21, l = 10	h = 19, k = 6, l = 2
CCDC no.	1957058	1961067

Figure 1.

ORTEP diagram of QBT and QPT.

Fluorescent Studies

Stock solutions of QPT and QBT (2 × 10^–4 M) were prepared in THF/H₂O (50:50 v/v) buffered at pH 7.4. The stock solutions of perchlorate and nitrate salts of Al³⁺, Ca²⁺, Cd²⁺, Co²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Hg²⁺, K⁺, Mg²⁺, Mn²⁺, Na⁺, Ni²⁺, Pb²⁺, and Zn²⁺ions (2 × 10^–4 M) were prepared in THF/H₂O (50:50 v/v) buffered at pH 7.4. The fluorescence test solutions for metal selectivity were prepared from 2 mL of QPT/QBT with 2 mL of each metal stock solution. The excitation wavelength was at 310 nm.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c03445.

• Interference studies of QPT and QBT with other metal ions, reversibility experiment of QPT and QBT, pH-dependent fluorescence changes of QPT and QBT, and IR ¹H and ¹³C NMR and HR mass analysis (PDF)

• Crystallographic data of C₁₈H₁₅N₅O₂S (CIF)

• Crystallographic data of C₁₆H₁₃N₅O₂S (CIF)

The authors declare no competing financial interest.