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Article

Schiff Base Compounds Derived from 5-Methyl Salicylaldehyde as Turn-On Fluorescent Probes for Al3+ Detection: Experimental and DFT Calculations

by
Huan-Qing Li
,
Shi-Hang Yang
,
Yun Li
,
Wan-Xin Ye
,
Zi-Yu Liao
,
Jia-Qian Lu
and
Zhao-Yang Wang
*
School of Chemistry, South China Normal University, Guangzhou Key Laboratory of Analytical Chemistry for Biomedicine, GDMPA Key Laboratory for Process Control and Quality Evaluation of Chiral Pharmaceuticals, Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, Guangzhou 510006, China
*
Author to whom correspondence should be addressed.
Molecules 2025, 30(5), 1128; https://doi.org/10.3390/molecules30051128
Submission received: 7 February 2025 / Revised: 23 February 2025 / Accepted: 27 February 2025 / Published: 28 February 2025
(This article belongs to the Special Issue Theoretical Study on Luminescent Properties of Organic Materials)
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Abstract

:
Using 5-methyl salicylaldehyde (2) as a reactant to react with different amines, 2-aminobenzimidazole (1a), 2-aminobenzothiazole (1b), and 2-aminopyridine (1c), respectively, three types of Schiff base fluorescent probes 3a–3c were designed and synthesized for selective detection of Al3+ in aqueous media. The structure of the compounds was acquired by 1H NMR, 13C NMR, and X-ray single-crystal diffraction. Furthermore, their photochromic and fluorescent behaviors have been investigated systematically by fluorescence spectra. Compounds 3a–3c can exhibit high selectivity, sensitivity, and anti-interference properties towards Al3+ in aqueous media. Among them, the limit of detection (LOD) of probe 3b for Al3+ is 2.81 × 10−7 M. Notably, the response times of probes 3a–3c for Al3+ are 90 s, 80 s, and 80 s, respectively, which are much faster than most previously reported probes. The coordination stoichiometry between compounds 3a–3c and Al3+ has been verified to be 1:1 through the Job’s plot. After coordination with Al3+, the C=N isomerization of compounds 3a–3c is inhibited, leading to the closure of the excited state intramolecular proton transfer (ESIPT) effect. At the same time, the fluorescence intensity is significantly increased through chelation-enhanced fluorescence mechanism (CHEF), which is confirmed by density functional theory (DFT) calculations. In addition, probes 3a–3c can be potentially applied in the selective and high-precision detection of Al3+ in environmental systems.

1. Introduction

It is well known that aluminum is the most abundant metal element in the Earth’s crust [1] and is widely used in many fields, such as food packaging, building materials, and electronic devices [2]. Due to the widespread presence of aluminum products in people’s daily lives, the harm of aluminum to humans has attracted widespread attention in recent years [3]. Al3+ is a neurotoxin and a “silent killer” in the human body [4]. The World Health Organization (WHO) has listed Al3+ as a source of food contamination and set the maximum allowable concentration in drinking water at 7.41 μM [5]. Long-term excessive intake of Al3+ can lead to major diseases such as Parkinson’s disease, Alzheimer’s disease, and rickets [6]. In addition, high concentrations of Al3+ deposition can lead to soil acidification, which can seriously harm crops and soil ecosystems [7]. Therefore, developing a selective and effective method for detecting Al3+ is crucial for human health and ecological protection.
Traditional Al3+ analysis methods, such as inductively coupled plasma optical emission spectroscopy (ICP OES) [8], atomic absorption spectroscopy (AAS) [9], and capillary electrophoresis (CE) [10], can accurately determine the content of Al3+. However, these detection techniques still have many drawbacks, such as complex processes, high detection costs, the need for very professional personnel, and the need for a large number of sample pretreatment [11,12].
In recent years, fluorescence probe detection technology has become an effective method for detecting Al3+ due to its simple operation, good selectivity, and real-time detection [13], which has attracted widespread attention from researchers [14,15]. Due to the strong hydration ability of Al3+, its coordination with transition metal ions is relatively poor and lacks spectral characteristics. Therefore, many research groups have recently made new breakthroughs in the development of fluorescent probes to address this issue [16,17,18]. However, some fluorescent probes still have various other problems, including long response times, high detection limits, and poor anti-interference ability [19,20,21]. Therefore, fluorescence technology for detecting Al3+ still poses significant challenges [22].
Considering the Hard Soft Acid Base Theory (HSAB), Al3+ is a hard acid that tends to bind to coordination molecules with donor atoms such as N and O to form stable complexes [23,24]. Therefore, most reported Al3+ probes contain hard base donor sites, such as N and O atoms [25,26,27]. Among them, Schiff base compounds are widely used in the development of Al3+ fluorescent probes due to their advantages, such as simple synthesis routes, strong metal ion coordination ability, and fast response speed [28,29,30]. However, the structure-activity relationship of the obtained Schiff base Al3+ fluorescent probes based on different nitrogen heterocyclic compounds still needs to be deeply studied, especially the influence of different nitrogen heterocyclic fluorescent groups needs to be explored.
In view of this, based on our previous research on benzazole-based metal ion fluorescent probes [31,32], herein, we further designed three fluorescent probes containing benzimidazole group, benzothiazole group, and pyridine group (Scheme 1). Three Schiff base fluorescent probes 3a–3c were rapidly, efficiently, and environmentally synthesized through a simple one-step Schiff base condensation reaction by using 2-aminobenzimidazole (1a), 2-aminobenzothiazole (1b), 2-aminopyridine (1c), and 5-methyl salicylaldehyde (2) as starting materials.
The test results indicate that they can selectively detect Al3+ with a low detection limit and fast detection speed. Furthermore, we systematically studied the relationship between the structure and properties of fluorescent groups in Schiff base probes with different nitrogen heterocyclic structures using density functional theory (DFT) calculations, which can provide the guidance for the design and synthesis of novel Schiff base Al3+ fluorescent probes.

2. Results and Discussion

2.1. Design, Synthesis, and Structural Characterization of Compounds 3a–3c

Salicylaldehyde and its derivatives are commonly found bioactive elements in cosmetics and natural products [33]. They can also be used as raw materials to participate in the construction of conjugated metal–organic frameworks (c-MOF) materials [34], photochromic materials [35], nanomaterials [36], and fluorescent probes [37]. Primarily, due to their excellent photostability and ability to regulate fluorescence properties [38], salicylaldehyde and its derivatives have been widely used as fluorescent probes for detecting cations [39], anions [40], and monitoring pH changes in biological systems [41].
By using the reaction of the aldehyde group, salicylaldehyde often reacts with heterocyclic amine compounds to synthesize Schiff base probe molecules through a one-step Schiff base reaction [39,42]. In addition, the hydroxyl group of salicylaldehyde can also participate in coordination as a proton donor to detect metal ions [43]. The optical properties of Schiff base compounds can be easily adjusted by incorporating different fluorescent groups into their molecular skeleton [25], while N-heterocyclic compounds have good fluorescence properties and are often used to synthesize organic small molecule fluorescent probes [44,45]. Therefore, we designed three Schiff base fluorescent probes based on 5-methyl salicylaldehyde by introducing different nitrogen heterocyclic fluorescent groups, respectively.
At present, most Schiff base probe molecules are synthesized using anhydrous ethanol [46] or methanol [47] as solvent. In the synthesis process of compounds 3a–3c, we found that both compounds 3a and 3c can be synthesized by the above method with satisfactory yields. However, when compound 3b is synthesized using anhydrous ethanol as a solvent, the yield is relatively low. The reason for this may be related to the molecular structure of substrates 1a–1c (Scheme 2).
The imidazole structure in 1a and the pyridine structure in 1c make the entire molecule more alkaline, except that the substrates all have primary amino groups due to the electron-withdrawing property of benzothiazole, and 1b can stabilize the conjugated system of the entire molecule, making the alkalinity of 1b molecule weaker. Therefore, this allows compounds 3a and 3c to be synthesized directly in anhydrous ethanol, while the synthesis of compound 3b requires the addition of piperidine as a catalyst [48].
From the characterization spectra of 1H NMR and 13C NMR of the synthesized compounds, it can be observed that compounds 3a–3c can find the corresponding peaks at the expected chemical shifts. In addition, the HRMS and X-ray single crystal diffraction of compound 3a (Table S1) further confirmed its structure (Figure 1). In a word, various test characterizations indicate that the synthesized compounds 3a–3c are indeed the target compounds, as designed.

2.2. Theoretical Calculation Study for Compounds 3a–3c

As is well known, the excited-state intramolecular proton transfer (ESIPT) refers to the process of transferring protons from proton donor groups to nearby proton acceptor groups after a molecule transitions from the ground state to the excited state [49]. Salicylaldehyde Schiff base derivatives can use phenolic hydroxyl groups as proton donors and nitrogen atoms in imines as proton acceptors, thereby generating significant ESIPT driving forces [50].
Based on this, we designed and synthesized Schiff base probe molecules 3a–3c by using 5-methyl salicylaldehyde (2) and nitrogen-containing heterocycles (1a–1c) as the starting materials. When the probe coordinates with metal ions, the ESIPT effect is turned off, and the fluorescence of the probe changes, making it suitable for detecting metal ions. Compounds with the ESIPT effect often have ketone and enol isomers [49]. To demonstrate the ESIPT effect of probes 3a–3c, we calculated the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energies of the enol and ketone isomers of probes 3a–3c.
Based on the Gaussian 09 program D.01, time-dependent DFT (TD-DFT) calculations were performed on these synthesized compounds 3a–3c and their keto tautomers by using the B3LYP/6-31G(d,p) basis set, following the method described in references [51,52]. The aim is to analyze the relationship between structure and photophysical properties, as well as to investigate the ESIPT properties of compounds 3a–3c [53]. The optimized molecular structures and their corresponding frontier molecular orbitals are shown in Figure 2 and Figure 3 (enol form) and Figure 4 and Figure 5 (ketone form), respectively.
After geometry structural optimization, all compounds 3a–3c exhibit near-planar structure, as shown in Figure 2, which is favorable for molecular aggregation. Meanwhile, as shown in Figure 3, there is a large steric overlap between the HOMO and LUMO of compounds 3a–3c, which are widely distributed throughout the probe molecule. This indicates the good fluorescence properties of probes. However, the fast isomerization around the C=N bond weakens the fluorescence properties of compounds 3a–3c, which may be attributed to their ESIPT process [39].
As shown in Figure 4, the optimized geometric structures of the ketone isomers of compounds 3a–3c still exhibit near-planar structure. Furthermore, we calculated the energy states generated by the HOMO and LUMO orbitals of the ketone isomers of compounds 3a–3c (Figure 5). It can be seen that the LUMO energy level of the enol isomers is higher than that of the ketone isomers. Therefore, the enol isomers are unstable in the excited state, and the phenolic hydroxyl proton will transfer to the nitrogen atom of the Schiff base, forming ketone isomers. Meanwhile, the energy gaps of the ketone isomers of compounds 3a–3c (∆E’3a = 3.17 eV, ∆E’3b = 3.07 eV, ∆E’3c = 3.12 eV) are smaller than those of the enol isomers (∆E3a = 3.56 eV, ∆E3b = 3.48 eV, ∆E3c = 3.76 eV), laying the foundation for the formation of ESIPT effect [44].

2.3. Study on the Optical Properties of Compounds 3a–3c

2.3.1. Study on the Optical Properties of Compounds 3a–3c in Different Solvents

To better investigate the performance of probes, we studied the fluorescence emission spectra of compounds 3a–3c in different solvents (Figure S1). It can be seen that the compound has two emission peaks in lower polarity solvents (DCM, EA) for all three probes, belonging to the enol/ketone form, which is consistent with the ESIPT effect [54], and it is obvious that, in DMF, there is the best fluorescence signal. Therefore, in order to obtain a better luminescence intensity while taking into account the solubility problem, we chose to observe the fluorescence detection ability of compounds 3a–3c in DMF.

2.3.2. Study on the ACQ Properties of Compounds 3a–3c

We studied the fluorescence properties of compounds 3a–3c in solvent systems of DMF and water at different ratios using fluorescence emission spectroscopy. The fluorescence spectra are shown in Figures S2 and S3 and Figure 6. The compounds 3a–3c have similar structures, and their fluorescence emission peaks exhibit the same trend in solvent systems of DMF and water at different ratios.
It can be seen that compared with pure DMF solution, the fluorescence emission peak slightly red-shifts when water is added, and the fluorescence intensity is highest when the water ratio is 10%. Then, as the water ratio continues to increase, the fluorescence intensity decreases significantly. This may be due to the aggregation of compounds with increasing water ratio, resulting in an aggregation-caused quenching (ACQ) effect [55] and a decrease in fluorescence intensity.
Based on the above analysis, a DMF/H2O solvent system with a water content of 10% was selected for the subsequent performance research experiments.

2.4. Research on the Detection of Metal Ions with Compounds 3a–3c

2.4.1. Selective Study of Compounds 3a–3c with Metal Ions

The fluorescence emission spectra of compounds 3a–3c (10 μM) and different metal ions (100 μM) were studied in a solvent system of DMF/H2O (v/v = 9:1), including Al3+, K+, Na+, Mg2+, Ca2+, Zn2+, Cu2+, Mn2+, Co2+, Cr3+, Hg2+, Cd2+, Ni2+, Pb2+, Fe2+, and Fe3+. The fluorescence spectra are shown in Figure 7.
It can be observed that adding Al3+ to the solution of compound 3a enhances the fluorescence intensity of the emission peak at 508 nm, changing from weak light-yellow fluorescence to yellow-green fluorescence (Figure 7a). Similarly, adding Al3+ to the solution of compound 3b enhances the fluorescence intensity at 509 nm, changing from weak light red fluorescence to strong yellow-green fluorescence (Figure 7b). These may be attributed to the coordination of compounds 3a and 3b with Al3+, further enhancing their structural rigidity and inhibiting ESIPT and C=N isomerization processes. In addition, the fluorescence response of compounds 3a and 3b to other metal ions is almost negligible. Compared with blank probes, the test results indicate that both 3a and 3b can serve as a specific “turn-on” probe for Al3+.
Differently, both the addition of Al3+ and Zn2+ to compound 3c solution can enhance the fluorescence emission peak. Among them, the addition of Al3+ causes the solution to change from light purple fluorescence to yellow-green fluorescence while adding Zn2+ causes the solution to turn yellow fluorescence, and the fluorescence enhancement is more significant (Figure S4). Of course, the fluorescence response of compound 3c to other metal ions is almost negligible, also indicating that compound 3c can serve as a specific “turn-on” probe for Al3+ and Zn2+.

2.4.2. Anti-Interference Study of Compounds 3a–3c with Metal Ions

In order to further evaluate the selectivity of compounds 3a–3c towards target ions, competitive experiments were conducted in a solvent system of DMF/H2O (v/v = 9:1) to investigate the anti-interference of compounds 3a–3c.
From Figure 8, it can be seen that the fluorescence is enhanced when Al3+ is added to compounds 3a (Figure 8a) and 3b (Figure 8b). On the contrary, the fluorescence intensity of the solution does not change significantly when other metal ions are added. However, after continuing to add Al3+, the fluorescence is enhanced. These indicate that most metal ions do not significantly interfere with the detection of Al3+ for compounds 3a and 3b.
Similarly, as shown in Figure S5a, the fluorescence is enhanced when Al3+ is added to compound 3c, while the fluorescence intensity of the solution does not change significantly when other metal ions (excluding Zn2+) are added. However, after continuing to add Al3+, the fluorescence is enhanced. In addition, after adding Al3+ to compound 3c and continuing to add Zn2+, the fluorescence intensity did not change significantly, indicating that the complex formed between compound 3c and Al3+ is more stable. Thus, other metal ions (including Zn2+) have no significant interference with the detection of Al3+ for compound 3c.
Notably, as shown in Figure S5b, there is a difference in that the fluorescence is enhanced when Zn2+ is added to compound 3c, while the fluorescence intensity of the solution does not change significantly when other metal ions (excluding Al3+) are added. After continuing to add Zn2+, some solutions (previously added with K+, Na+, Mg2+, Ca2+, Cu2+, Mn2+, Co2+, Cd2+, Ni2+ solutions) show the enhanced fluorescence. However, after adding Zn2+ to the solution previously containing Cr3+, Hg2+, Pb2+, Fe2+, and Fe3+, there is no significant change in the fluorescence of the solution. These indicate that the above-mentioned metal ions can interfere with the detection of Zn2+ for compound 3c.

2.4.3. Quantitative Identification of Al3+ by Compounds 3a–3c

Probes 3a–3c exhibit excellent selectivity towards Al3+. Furthermore, the fluorescence titration experiments were conducted on 3a–3c for Al3+, and the changes in fluorescence emission curves were analyzed. Once the fitting curves of the maximum emission intensity and ion concentration of the probe are plotted, and the limit of detection (LOD) can be calculated using the formula LOD = 3δ/K [56].
As shown in Figure 9a, with the increase of Al3+ concentration from 0 to 15.0 eq., the fluorescence intensity at 509 nm increases with the increase of concentration. From Figure 9b, it can be observed that there is a good linear relationship between the fluorescence intensity of probe 3a and the concentration of Al3+ (R2 = 0.999), and the LOD of probe 3a for Al3+ is calculated to be 3.14 × 10−7 M.
Similarly, as shown in Figure 9c, as the concentration of Al3+ increases from 0 to 11.0 eq., the fluorescence intensity at 508 nm increases with the increase of the concentration. From Figure 9d, it can be observed that there is a good linear relationship between the fluorescence intensity of probe 3b and the concentration of Al3+ (R2 = 0.999), and the LOD of probe 3b for Al3+ is calculated to be 2.81 × 10−7 M.
For probe 3c, as shown in Figure S6a, when 0–12.0 eq. of Al3+ is added to the solution of probe 3c, there is a good linear relationship between the fluorescence intensity of probe 3c and the concentration of Al3+ (R2 = 0.996), and the LOD of probe 3c for Al3+ is calculated to be 2.86 × 10−7 M (Figure S6b).
Notably, the LOD values of probes 3a–3c are all lower than the maximum allowable concentration in drinking water specified by WHO [7]. In addition, when 0–1.0 eq. of Zn2+ is added to probe 3c, there is a good linear relationship between the fluorescence intensity and concentration of probe 3c (R2 = 0.991) (Figure S7a), and its LOD for Zn2+ is calculated to be 8.64 × 10−9 M (Figure S7b).
Although probe 3c can achieve nanomolar detection of Zn2+, its detection of Zn2+ is susceptible to interference, as mentioned above, so in the future, no further research will be conducted on the detection of Zn2+.

2.4.4. Study on the Time Response of Probes 3a–3c to Al3+

The response speed of the probe to metal ions is an important factor in determining the real-time performance of the proposed fluorescence sensor [57]. Therefore, by rapidly adding 10.0 eq. of Al3+ solution to the solution of probes 3a–3c and testing their fluorescence intensity at different times, the relationship between the maximum fluorescence intensity of the probe and time was studied.
From Figure 10, it can be observed that after adding Al3+ to the compound solution, the fluorescence intensity immediately increases and remains almost unchanged after 100 s. Meanwhile, the reaction times of probes 3b and 3c with Al3+ are both 80 s, slightly faster than probe 3a (90 s).
Delightedly, the response times of probes 3a–3c are shorter than most reported Al3+ small molecule fluorescent probes [20,21,23] (please see Table S2 for a more detailed comparison). Therefore, the rapid response of compounds 3a–3c to Al3+ indicates that these three probes can be used for actual sample detection of Al3+.

2.4.5. The Binding Ratio and Binding Constant of Probes 3a–3c Interacting with Al3+

In order to determine the binding ratio of Al3+ to probes 3a–3c, we conducted the continuous variation experiment to get Job’s plot as the reported method [58]: fixing the total concentration of Al3+ and ligands at 10 μM, changing the ratio of metal ions to ligands (0.1–0.9), and plotting the relationship between the maximum fluorescence intensity and the mole fraction of Al3+. The value of the inflection point corresponds to the situation where Al3+ is perfectly coordinated with the ligand.
As shown in Figure S8a, when the mole fraction of Al3+ is 0.5, the Job plot shows a turning point, indicating that the binding stoichiometric ratio of probe 3a to Al3+ forming a complex is 1:1. Furthermore, the fluorescence titration data are linearly fitted by using the Benesi–Hildebrand equation, and the Ka value can be calculated by using the intercept and slope of the line [59]. The binding constant between probe 3a and Al3+ is 6.53 × 103 M (Figure S8b). Similarly, as shown in Figure 11a, the maximum fluorescence intensity is achieved when the mole fraction of Al3+ is 0.5, indicating that the stoichiometric ratio of probe 3b to form a complex with Al3+ is 1:1. The binding constant between probe 3b and Al3+ is 1.57 × 104 M (Figure 11b), which is higher than most reported Al3+ small molecule fluorescent probes [20,29].
For probe 3c, as shown in Figure S9a, the stoichiometric ratio of compound 3c to Al3+ binding is 1:1. In addition, the binding constant between compound 3c and Al3+ is 7.67 × 103 M (Figure S9b).

2.4.6. Study on pH Tolerance of Probes 3a–3c in Interaction with Al3+

It is well known that pH has a significant impact on the selectivity and sensitivity of the probes for metal ions [39]. Therefore, we measured and compared the maximum fluorescence intensity before and after the interaction between probes 3a–3c solution and Al3+ under different pH environments (Figure 12).
From Figure 12, it can be seen that probes 3a and 3b both have weak fluorescence intensity in the acidic range and cannot detect Al3+. In other words, probes 3a and 3b can only detect Al3+ under neutral conditions, also indicating that they can effectively detect the target ion under physiological conditions.
Interestingly, it also can be found that under alkaline conditions, both probes 3a and 3b exhibit strong fluorescence. Especially when the pH is 8, the fluorescence intensity of probes 3a and 3b will suddenly increase, and it will be enhanced by the increase of alkalinity. This may be due to the hydrolysis of compounds 3a and 3b under alkaline conditions [57], and the hydrolysis product 5-methyl salicylaldehyde also exhibits strong fluorescence under alkaline conditions. Especially for probe 3b, there is also a noticeable color change under the naked eye (Figure S10). Therefore, probes 3a and 3b also can be considered as simple alkaline indicators with a mutation point pH of 8.
Differently, as shown in Figure S11, compound 3c can detect Al3+ under neutral and weakly alkaline conditions (no more than pH 8). Perhaps, due to the stronger alkalinity of probe 3c compared to compounds 3a and 3b, it will not be hydrolyzed under the weakly alkaline condition.
However, it also can be found that compound 3c exhibits strong fluorescence under strong alkaline conditions. When the pH is 9, the fluorescence intensity of compound 3c suddenly enhances, and it will increase with increasing alkalinity (Figure S11). Thus, considering that probe 3c also undergoes hydrolysis under strong alkaline conditions and exhibits strong fluorescence (Figure S10), it can also be considered a simple alkaline indicator with a mutation point pH of 9.
In addition, under acidic conditions, the fluorescence of probes 3a–3c did not show significant changes before and after the addition of Al3+, and all exhibited no fluorescence. This may be due to the weak fluorescence intensity of the probes caused by the hydrolysis of imine under acidic conditions.

2.5. Sensing Mechanism of Compounds 3a–3c to Al3+

2.5.1. 1H NMR and FT-IR Study on the Interaction Between Compounds 3a–3c to Al3+

From the previous Job’s plot, it has been known that the stoichiometric ratio for the formation of complexes between compounds 3a–3c and Al3+ are all 1:1. In order to further understand the recognition mechanism of probes 3a–3c for Al3+, the studies of 1H NMR and Fourier transform infrared (FT-IR) spectroscopy (liquid film, using DMF as the solvent) were conducted [29].
The 1H NMR spectra of compounds 3a–3c were tested in the absence and presence of Al3+ (its nitrate) in DMSO-d6. As shown in Figure 13, after the addition of Al3+ to compound 3a, the characteristic imine signal (Hc) at a chemical shift of 9.60 ppm is shifted to 10.20 ppm, and the amino proton (Ha) and phenolic hydroxyl proton (Hb) can not be observed, indicating that the N atom of the amino group, the O atom of the phenolic hydroxyl group, and the N atom of the imine all have participated in the coordination between compound 3a and Al3+ [39].
Similarly, the binding modes of compounds 3b and 3c with Al3+ can also be obtained through the 1H NMR titration experiments (as shown in Figures S12 and S14). Thus, the O atom of the phenolic hydroxyl group and the N atom of the imine in probes 3b and 3c can participate in the coordination of compounds 3b and 3c with Al3+.
In the FT-IR spectrum of compound 3a (Figure 14), there are peaks at 3430 cm−1 (-OH) and 3312 cm−1 (-NH), while they completely disappeared after the addition of Al3+, indicating that the O atom in the phenolic hydroxyl group and the N atom in the amino group are involved in the binding of compound 3a with Al3+. At the same time, the strongest peak of C=N vibration at 1580 cm−1 is weakened and shifted to 1695 cm−1, indicating that the N atom in the imine group also participates in the binding with Al3+ [16]. In addition, a new strong peak appears at 1344 cm−1, which represents the stretching frequency of the NO3− group [39].
As shown in Figures S13 and S15, the FT-IR spectra of compounds 3b and 3c also showed similar results.

2.5.2. DFT Study on the Interaction Between Compounds 3a–3c to Al3+

In order to further investigate the coordination interaction between probes 3a–3c and Al3+, DFT calculations were conducted on compounds 3a-Al3+, 3b-Al3+, and 3c-Al3+. The possible structures of compounds 3a–3c and Al3+ were geometrically optimized, and the LUMO-HOMO energy levels of 3a-Al3+, 3b-Al3+, and 3c-Al3+ were calculated [29].
As shown in Figure 15, probes 3a–3c coordinate with Al3+ in a stoichiometric ratio of 1:1. When probes 3a–3c coordinate with Al3+, the energy of HOMO is mainly distributed in the benzene ring and linker, while the energy of LUMO is mainly distributed near the aluminum atom (Figure 16).
After chelating with aluminum ions, probes 3a–3c exhibit enhanced fluorescence intensity and spectral changes, known as chelation-enhanced fluorescence effect (CHEF) [3]. The CHEF effect significantly increases the fluorescence intensity [17], emitting bright yellow-green fluorescence. The energy gaps of 3a-Al3+, 3b-Al3+, and 3c-Al3+ are 2.72 eV, 2.46 eV, and 2.61 eV, respectively (Figure 16), which are significantly smaller than the energy gaps of compounds 3a–3c (∆E3a = 3.56 eV, ∆E3b = 3.48 eV, ∆E3c = 3.76 eV). These indicate that compounds 3a–3c tend to bind with Al3+ and form more stable complexes [21,42,60]. The molecule’s stiffness decreases non-radiative decay processes, further verifying that compounds 3a–3c and Al3+ “turn on” fluorescence through the CHEF effect.
In addition, compared with 3a-Al3+ and 3c-Al3+, 3b-Al3+ has the lowest energy gaps (Figure 16). This outcome finely explains why probe 3b can recognize Al3+ with more obvious fluorescence changes and may correspond to the faster detection time [61].
At the same time, combining the 1H NMR and FT-IR tests has shown that when coordinating with Al3+, the O atom of the phenolic hydroxyl group and the N atom of the imine both participate in the reaction, inhibiting the C=N isomerization and enhancing the rigidity of the probes after complexation with Al3+, while the ESIPT effect was turned off and the CHEF effect was activated, resulting in fluorescence enhancement of the probe solution after the addition of Al3+ [39]. Therefore, taking probe 3b as an example with the best testing effect, the detection mechanism of probes 3a-3c can be shown as Scheme 3.

2.6. Practical Detection Application of Probes 3a–3c to Al3+

2.6.1. Application of Probes 3a–3c in Detecting Al3+ in Actual Water and Soil Samples

Al3+ is prone to accumulate in water and soil samples, posing a serious threat to human health [7]. In order to evaluate the feasibility of probes 3a–3c on Al3+ in actual samples, we took distilled water, tap water, river water (the Pearl River water), and three actual water samples as well as soil samples as research objects (the geographical coordinates of all samples: latitude 23.052594° North and longitude 113.380105° East), and used the spiking method to conduct fluorescence emission spectrum test and fluorescence analysis to calculate the content of Al3+ in the samples. The recovery rate and relative standard deviation (RSD) were determined through 3 parallel tests [60,62].
As shown in Figure S16, adding different concentrations (10 μM, 20 μM, 40 μM) of Al3+ to different samples will gradually increase the fluorescence intensity of probe 3a with the increase of Al3+ concentration. According to Table S3, the recovery range of probe 3a for low concentration Al3+ (10−5 M) is 95.5–107%, with RSD less than 3.16%. These results indicate that probe 3a exhibits high accuracy in identifying Al3+ in actual samples and is suitable for detecting Al3+ in actual water and soil samples.
Similarly, as shown in Figure 17, the fluorescence intensity of probe 3b gradually increases with the increase of Al3+ concentration. Meanwhile, according to Table 1, the recovery rate range of probe 3b for low concentration Al3+ (10−5 M) is 98–105%, with RSD less than 3.09%. Therefore, these results indicate that probe 3b is also suitable for detecting Al3+ in actual water and soil samples.
In addition, the fluorescence intensity of probe 3c is positively correlated with the concentration of Al3+ (Figure S17), and the recovery range of compound 3c for Al3+ (10−5 M) is 97–105%, with RSD less than 2.67% (Table S4). Therefore, these results indicate that probe 3c is also suitable for detecting Al3+ in actual water and soil samples.
In a word, compounds 3a–3c, as three probes, are practical for detecting Al3+ in actual water and soil samples.

2.6.2. Application of Test Strips Loaded with Compounds 3a–3c for the Detection of Al3+

The detection of metal ions through test strips has the advantages of convenience, simple operation, and low cost [44,63]. Therefore, in order to expand the practical applications of probes 3a–3c, strip experiments were conducted [64,65] by cutting the Whatman filter paper into appropriate sizes, soaking them in DMF solution (10−3 M) of compounds 3a–3c for 10 min, and drying the test strips in an oven to prepare convenient test strips loaded with probes 3a–3c.
Using one group as a blank control, different concentrations of Al3+ solution (10−2–10−6 M) were added dropwise to the other five test strips. After drying with a hair dryer, the test strips were observed under a 365 nm UV lamp, as shown in Figure 18.
Obviously with the increase of Al3+ concentration, the fluorescence of the test strips is changed significantly, from blue-green to blue fluorescence. Therefore, probes 3a-3c can be made into test strips for convenient and intuitive detection of Al3+.

3. Materials and Methods

3.1. Reagents and Instruments

2-Aminobenzimidazole (1a), 2-aminobenzothiazole (1b), 2-aminopyridine (1c), and 5-methyl salicylaldehyde (2) were all purchased from Anhui Ze Sheng Technology Co., Ltd. (Shanghai, China). Whatman filter paper (110 mm) was purchased from Shanghai Bitai Biotechnology Co., Ltd. (Shanghai, China). All reagents and solvents used in this study are AR grade.
All fluorescence spectra were performed on an F-4600 fluorescence spectrometer (HITACGI Corporation, Tokyo, Japan). FT-IR spectroscopy was obtained using a Fourier transform infrared spectrometer (German Platinum Elmer Spectrum Two model, Ettlingen, Germany). The 1H NMR and 13C NMR spectra were tested on a 600 MHz Fourier transform nuclear magnetic spectrometer (Bruker AVANCE NEO 600M model, Ettlingen, Germany). Melting points were obtained on the WRX-4 micro melting point instrument (Shanghai Yimenshan Instrument and Equipment Co., Ltd., Shanghai, China). Mass spectrometry was tested on a liquid chromatography high-resolution mass spectrometer (Thermos Fisher Q Exactive, Waltham, MA, USA). X-ray single-crystal diffraction was tested using an X-ray single-crystal diffractometer (Agilent Gemini Model E, Santa Clara, CA, USA).

3.2. Synthesis

3.2.1. Synthesis of Compound 3a

For Scheme 2a, using the method described in reference [66], 2 mmol of 2-amino- benzimidazole (1a) and 2 mmol of 5-methyl salicylaldehyde (2) were dissolved into 10 mL of anhydrous ethanol in a reaction flask. The reaction was carried out at 80 °C for 3 h. After the reaction was completed, the mixture was cooled to room temperature, giving solid precipitates. After filtration and washing with anhydrous ethanol, a solid crude product was obtained. Then, the further recrystallization with anhydrous ethanol gave the pure product 3a as a yellow solid with a yield of 91%; m.p.: 235.0–235.5 °C; 1H NMR (600 MHz, DMSO-d6), δ, ppm: 2.29 (s, 3H), 6.93 (d, J = 8.4 Hz 1H), 7.16–7.23 (m, 2H), 7.30 (d, J = 8.4 Hz, 1H), 7.46 (d, J = 8.4 Hz, 1H), 7.60 (d, J = 8.4 Hz, 1H), 7.66 (s, 1H), 9.60 (s, 1H), 11.84 (s, 1H), 12.73 (s, 1H), see Figure S18; 13C NMR (150 MHz, DMSO-d6), δ, ppm: 19.92, 111.22, 116.74, 118.60, 119.09, 121.94, 122.26, 128.28, 131.80, 134.02, 135.54, 142.36, 154.06, 158.45, 165.25, see Figure S19; ESI-MS, m/z (%): Calcd for C15H14N3O([M + H]+): 252.1131 (100), Found: 252.1126 (see Figure S20).

3.2.2. Synthesis of Compound 3b

For Scheme 2b, using the method described in reference [67], 2 mmol of 2-aminobenzothiazole (1b) and 2 mmol of 5-methyl salicylaldehyde (2) were dissolved into 5 mL of toluene in a reaction flask. After adding 30 μL of piperidine, the reaction was carried out at 110 °C for 3 h. Once the reaction was completed, the mixture was cooled to room temperature. After adding petroleum ether, there were solid precipitates. After filtration and washing with anhydrous ethanol, a solid crude product was obtained. Then, the further recrystallization with anhydrous ethanol gave the pure product 3b as a yellow solid with a yield of 85% (51% [68]); m.p.: 131.0–132.4 °C; 1H NMR (600 MHz, DMSO-d6), δ, ppm: 2.28 (s, 3H), 6.94 (d, J = 8.4 Hz, 1H), 7.33 (d, J = 8.4 Hz, 1H), 7.40–7.45 (m, 1H), 7.50–7.55 (m, 1H), 7.73 (s, 1H), 7.94 (d, J = 8.4 Hz, 1H), 8.07 (d, J = 8.4 Hz, 1H), 9.39 (s, 1H), 11.24 (s, 1H), see Figure S21; 13C NMR (150 MHz, DMSO-d6), δ, ppm: 19.90, 116.86, 119.31, 122.39, 122.56, 125.24, 126.70, 128.54, 130.41, 133.94, 136.59, 151.25, 158.57, 165.53, 170.62 (see Figure S22).

3.2.3. Synthesis of Compound 3c

As Scheme 2c, 2 mmol of 2-aminopyridine (1c) and 2 mmol of 5-methyl salicylaldehyde (2) were dissolved into 10 mL of anhydrous ethanol in a reaction flask. The reaction was carried out at 80 °C for 1 h. After the reaction was completed, the mixture was cooled to room temperature, giving solid precipitates. After filtration and washing with anhydrous ethanol, a solid crude product was obtained. Then, the further recrystallization with anhydrous ethanol gave the pure product 3c as an orange solid with a yield of 82% (63% [68]); m.p. 116.5–117.9 °C; 1H NMR (600 MHz, DMSO-d6), δ, ppm: 2.28 (s, 3H), 6.90 (d, J = 8.4 Hz, 1H), 7.27 (d, J = 8.4 Hz, 1H), 7.33–7.39 (m, 1H), 7.44 (d, J = 7.8 Hz, 1H), 7.57 (s, 1H), 7.89–7.95 (m, 1H), 8.53 (d, J = 6.0 Hz, 1H), 9.43 (s, 1H), 12.73 (s, 1H), see Figure S23; 13C NMR (150 MHz, DMSO-d6), δ, ppm: 19.91, 116.58, 118.71, 119.69, 122.84, 127.96, 132.80, 134.91, 139.01, 149.00, 157.69, 158.69, 164.26 (Figure S24).

3.3. Computational Details

DFT and TD-DFT calculations were performed using Gaussian 09 D.01, and the structures of probes 3a–3c before and after binding with Al3+ were optimized at the B3LYP/6-31G (d) level. Then, their HOMO and LUMO were calculated at the B3LYP/6-31G (d, p) level, and the bandgap value was finally calculated [51,52].

3.4. Spectrophotometric Experiments

Firstly, 1 mL of compound 3a–3c solution with a concentration of 1.0 × 10−2 M as the mother liquor was prepared and further diluted 1000 times during testing to prepare a test solution of compound 3a–3c with a concentration of 1.0 × 10−5 M. Then, serial 10 mL of metal nitrate solutions of Al3+, K+, Na+, Mg2+, Ca2+, Zn2+, Cu2+, Mn2+, Co2+, Cr3+, Hg2+, Cd2+, Ni2+, Pb2+, Fe2+, and Fe3+ with the concentration of 1.0 × 10−2 M were configured for use.
Fluorescence experiments were conducted to evaluate sensitivity by gradually increasing the concentration of target metal ions in the probe solution. In addition, in fluorescence measurement, the excitation wavelengths of compounds 3a–3c are 405, 400, and 400 nm, respectively. The slit widths of both the excitation and emission were 5.0 nm. All the experiments were performed at room temperature.

4. Conclusions

Starting with 5-methylsalicylaldehyde, we designed and synthesized three Schiff base compounds 3a–3c with different nitrogen heterocyclic fluorescent groups through a simple green reaction and characterized them by 1H NMR, 13C NMR, HRMS, and X-ray single crystal diffraction methods. Based on the coordination interaction between compounds 3a–3c and Al3+, compounds 3a–3c have good selectivity for Al3+ and can be used as fluorescent probes for detecting Al3+. Thus, the properties of three fluorescent probes for Al3+ were studied systematically for the first time by combining the theoretical calculations with the experimental observations.
Among them, probe 3b with benzothiazole as the fluorescent group is the best, with a detection limit of 2.81 × 10−7 M and a response time of 80 s. Notably, the LOD values of probes 3a–3c are all below the maximum allowable concentration (7.41 μM) specified by WHO for drinking water. Furthermore, the mechanism of detecting Al3+ with probes 3a-3c has been elucidated through 1H NMR, FT-IR spectroscopy, and DFT calculation analysis. The theoretical research indicates that the energy gap of compound 3b is the lowest of the three probes, resulting in its best results. Importantly, these probes 3a–3c can be successfully applied for the practical detection of Al3+ in water samples, soil samples, and test strips.

Supplementary Materials

The following supporting information can be downloaded at: www.mdpi.com/article/10.3390/molecules30051128/s1, Data of single-crystal X-ray analysis of compound 3a (Table S1); Study on the optical properties of compounds 3a–3c in different solvents (Figure S1); Study on the ACQ properties of compounds 3a and 3b (Figures S2 and S3); Selective study of compound 3c with metal ions (Figure S4); Anti-interference study of compound 3c with metal ions (Figure S5); Quantitative identification of Al3+ by probe 3c (Figures S6 and S7); Comparison of some fluorescent probes for Al3+ (Table S2); The binding ratio and binding constant of compounds 3a and 3c interacting with Al3+ (Figures S8 and S9); Study on pH tolerance of compounds 3a–3c in interaction with Al3+ (Figure S10 and S11); 1H NMR and FT-IR study on the interaction between compounds 3b and 3c to Al3+ (Figures S12–S15); Application of compounds 3a and 3c in the detection of Al3+ in actual samples (Figures S16 and S17, Tables S3 and S4); NMR Spectra and HRMS for compounds 3a–3c (Figures S18–S24); See [2,15,20,21,23,26,27].

Author Contributions

Conceptualization, Z.-Y.W.; methodology, H.-Q.L.; formal analysis, H.-Q.L. and S.-H.Y.; data curation, Y.L. and W.-X.Y.; writing-original draft preparation, H.-Q.L., Z.-Y.L. and J.-Q.L.; writing-review and editing, H.-Q.L. and Z.-Y.W.; project administration, Z.-Y.W.; funding acquisition, Z.-Y.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by NSFC (No. 20772035) and the Guangdong Basic and Applied Basic Research Foundation (No. 2021A1515012342).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Molecules 30 01128 sch001
Scheme 1. Design of Schiff base probes 3a–3c with nitrogen-containing heterocyclic structure for Al3+ detection.
Scheme 1. Design of Schiff base probes 3a–3c with nitrogen-containing heterocyclic structure for Al3+ detection.
Molecules 30 01128 sch001
Molecules 30 01128 sch002
Scheme 2. (a–c) Synthetic route of compounds 3a–3c.
Scheme 2. (a–c) Synthetic route of compounds 3a–3c.
Molecules 30 01128 sch002
Molecules 30 01128 g001
Figure 1. Crystal structure of compound 3a.
Figure 1. Crystal structure of compound 3a.
Molecules 30 01128 g001
Molecules 30 01128 g002
Figure 2. Optimized molecular structures of compounds 3a–3c (enol form).
Figure 2. Optimized molecular structures of compounds 3a–3c (enol form).
Molecules 30 01128 g002
Molecules 30 01128 g003
Figure 3. Frontline molecular orbitals and energy levels of compounds 3a–3c (enol form).
Figure 3. Frontline molecular orbitals and energy levels of compounds 3a–3c (enol form).
Molecules 30 01128 g003
Molecules 30 01128 g004
Figure 4. Optimized molecular structures of compounds 3a′–3c′ (keto form).
Figure 4. Optimized molecular structures of compounds 3a′–3c′ (keto form).
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Molecules 30 01128 g005
Figure 5. Frontline molecular orbitals and energy levels of compounds 3a′–3c′ (keto form).
Figure 5. Frontline molecular orbitals and energy levels of compounds 3a′–3c′ (keto form).
Molecules 30 01128 g005
Molecules 30 01128 g006
Figure 6. (a) The fluorescence spectra of compound 3c (10 μM) and (b) the plot of emission peak intensity in DMF/H2O systems with different water fractions (0–90% by volume).
Figure 6. (a) The fluorescence spectra of compound 3c (10 μM) and (b) the plot of emission peak intensity in DMF/H2O systems with different water fractions (0–90% by volume).
Molecules 30 01128 g006
Molecules 30 01128 g007
Figure 7. Fluorescence spectra of probe 3a (a) and 3b (b) (10 μM) with various metal ions in DMF/H2O (v/v = 9:1) (λex = 405 nm for 3a and λex = 400 nm for 3b).
Figure 7. Fluorescence spectra of probe 3a (a) and 3b (b) (10 μM) with various metal ions in DMF/H2O (v/v = 9:1) (λex = 405 nm for 3a and λex = 400 nm for 3b).
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Molecules 30 01128 g008
Figure 8. Fluorescence intensity of probe 3a (a) and 3b (b) (10 μM) with Al3+ (10 eq.) and solutions after adding Al3+ (10 eq.) ions upon the addition of various metal ions (10 eq.) in DMF/H2O (v/v = 9:1) (λex = 405 nm for 3a and λex = 400 nm for 3b).
Figure 8. Fluorescence intensity of probe 3a (a) and 3b (b) (10 μM) with Al3+ (10 eq.) and solutions after adding Al3+ (10 eq.) ions upon the addition of various metal ions (10 eq.) in DMF/H2O (v/v = 9:1) (λex = 405 nm for 3a and λex = 400 nm for 3b).
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Molecules 30 01128 g009
Figure 9. Fluorescence spectra of 3a (a) and 3b (c) in the presence of increasing amounts of Al3+; Plot of fluorescence intensities of 3a (b) and 3b (d) respectively.
Figure 9. Fluorescence spectra of 3a (a) and 3b (c) in the presence of increasing amounts of Al3+; Plot of fluorescence intensities of 3a (b) and 3b (d) respectively.
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Molecules 30 01128 g010
Figure 10. Time-dependent variation of maximum fluorescence intensity after the interaction between compound 3a–3c and Al3+.
Figure 10. Time-dependent variation of maximum fluorescence intensity after the interaction between compound 3a–3c and Al3+.
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Molecules 30 01128 g011
Figure 11. (a) The Job plot of 3b-Al3+; (b) the Benesi–Hildebrand plot of 3b-Al3+.
Figure 11. (a) The Job plot of 3b-Al3+; (b) the Benesi–Hildebrand plot of 3b-Al3+.
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Molecules 30 01128 g012
Figure 12. Different pH values of maximum fluorescence intensity before and after the interaction between compound 3a (a)/3b (b) and Al3+.
Figure 12. Different pH values of maximum fluorescence intensity before and after the interaction between compound 3a (a)/3b (b) and Al3+.
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Molecules 30 01128 g013
Figure 13. Hydrogen spectra of compound 3a and 3a + Al3+.
Figure 13. Hydrogen spectra of compound 3a and 3a + Al3+.
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Molecules 30 01128 g014
Figure 14. FT-IR spectra of compound 3a and 3a + Al3+.
Figure 14. FT-IR spectra of compound 3a and 3a + Al3+.
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Molecules 30 01128 g015
Figure 15. Optimized molecular structures of 3a-Al3+, 3b-Al3+, and 3c-Al3+.
Figure 15. Optimized molecular structures of 3a-Al3+, 3b-Al3+, and 3c-Al3+.
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Molecules 30 01128 g016
Figure 16. HOMO-LUMO energy level diagram of 3a-Al3+, 3b-Al3+, and 3c-Al3+.
Figure 16. HOMO-LUMO energy level diagram of 3a-Al3+, 3b-Al3+, and 3c-Al3+.
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Molecules 30 01128 sch003
Scheme 3. Proposed mechanism of compound 3b for detection of Al3+.
Scheme 3. Proposed mechanism of compound 3b for detection of Al3+.
Molecules 30 01128 sch003
Molecules 30 01128 g017
Figure 17. Relationship between the maximum fluorescence intensity of compound 3b in different water and soil samples with different concentrations of Al3+.
Figure 17. Relationship between the maximum fluorescence intensity of compound 3b in different water and soil samples with different concentrations of Al3+.
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Molecules 30 01128 g018
Figure 18. Application of probes 3a–3c in test papers.
Figure 18. Application of probes 3a–3c in test papers.
Molecules 30 01128 g018
Table 1. Determination of Al3+ in actual water and soil samples by compound 3b.
Table 1. Determination of Al3+ in actual water and soil samples by compound 3b.
SamplesAl3+ Added (10−5 M)Al3+ Found (10−5 M)RSD (%, n = 3)Recovery (%)
Distilled water10.980.1498
21.991.5399.5
43.991.6999.8
Tap water10.970.8897
21.970.1998.5
43.922.3498
River water11.051.22105
21.993.0999.5
44.022.62100.5
Soil11.010.81101
22.032.30101.5
44.101.35102.5
Equationy = 107.75x + 64.17
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Li, H.-Q.; Yang, S.-H.; Li, Y.; Ye, W.-X.; Liao, Z.-Y.; Lu, J.-Q.; Wang, Z.-Y. Schiff Base Compounds Derived from 5-Methyl Salicylaldehyde as Turn-On Fluorescent Probes for Al3+ Detection: Experimental and DFT Calculations. Molecules 2025, 30, 1128. https://doi.org/10.3390/molecules30051128

AMA Style

Li H-Q, Yang S-H, Li Y, Ye W-X, Liao Z-Y, Lu J-Q, Wang Z-Y. Schiff Base Compounds Derived from 5-Methyl Salicylaldehyde as Turn-On Fluorescent Probes for Al3+ Detection: Experimental and DFT Calculations. Molecules. 2025; 30(5):1128. https://doi.org/10.3390/molecules30051128

Chicago/Turabian Style

Li, Huan-Qing, Shi-Hang Yang, Yun Li, Wan-Xin Ye, Zi-Yu Liao, Jia-Qian Lu, and Zhao-Yang Wang. 2025. "Schiff Base Compounds Derived from 5-Methyl Salicylaldehyde as Turn-On Fluorescent Probes for Al3+ Detection: Experimental and DFT Calculations" Molecules 30, no. 5: 1128. https://doi.org/10.3390/molecules30051128

APA Style

Li, H.-Q., Yang, S.-H., Li, Y., Ye, W.-X., Liao, Z.-Y., Lu, J.-Q., & Wang, Z.-Y. (2025). Schiff Base Compounds Derived from 5-Methyl Salicylaldehyde as Turn-On Fluorescent Probes for Al3+ Detection: Experimental and DFT Calculations. Molecules, 30(5), 1128. https://doi.org/10.3390/molecules30051128

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