Abstract
A new series of sensors SM-1 to SM-3 was designed and synthesized using indole carboxaldehydes (2a–2c) and 2,4-dinitrophenyl hydrazine. Accompanied by the synthesis, density functional theory investigation was also accomplished at the M06-2X/6-311G+(d,p) functional. A reduction in band gap (ΔE = 4.702–4.230 eV) along with a bathochromic shift (λmax = 433.223–471.584 nm) was seen in deprotonated chromophores than their neutral sensors. Further, significant charge transference from indole toward dinitrophenyl hydrazine was also examined. Global reactivity parameters also expressed the greater stability of sensors than that of their deprotonated form. SM-3 displayed high selectivity toward F ions as compared to SM-1 and SM-2, which respond to both F– and CN– ions. The electronic absorption spectrum was recorded in CH3CN. The sensor SM-3 showed high selectivity toward F– ions with a low detection limit (8.69 × 10–8), and the binding constant for SM-3 was determined as 7.7 × 105. The sensor displayed naked eye views as the color of solution changed from mustard to purple with a red shift of 96 nm. The mechanism suggests deprotonation from the NH group, which was confirmed by 1H NMR. The sensor is found to be useful for detection of F– ions in the real sample and for analytical application (test strip).
1. Introduction
Chemosensor schemes catch the substantial interest for monitoring cations and anions, because “guest–host” contact can alter the physical properties, such as fluorescence (fluorogenic sensors) or color (chromogenic sensors).1 Due to their high compassion, selectivity, and ability to achieve in situ and in vivo investigation, chemosensors are mainly proficient apparatus for distinguishing a variety of analytes. Also, chemosensors have simplicity and ease of understanding and are extensively used for the detection of the ionic composition of the medium in organic chemistry, biology, medicine, and environmental research.2 Due to their significant function in an extensive range of ecological, medical, chemical, and biological purposes, the identification and sensing of anions have gained noteworthy concentration.3 Among the anions, fluoride ions (F–) have gained much attention. Fluoride is the smallest and most significant electronegative atom having different chemical properties and also forms a strong hydrogen bond with target molecules.4 F– ions are determined to be valuable to avoid dental caries. Chronic ingestion of fluoride, even though below 1 mg/L, can cause insensitive dental and skeletal fluorosis. Accumulation of F– ions over a period can cause interference with liver and kidney functions, change in the DNA structure, lowering of IQ of children, and loss of mobility.5 With increasing industries of food products and cosmetic goods of fluorine products, the infection of fluorine turns out to be more serious. For these reasons, detection of fluoride ions is necessary.
Currently, UV–vis absorption spectroscopy and colorimetric naked-eye detection methods have engrossed extensive concentration because they can be advantageous and inexpensive equipment is required for detection. An anionic chemosensor can yield binding information by both behaviors (fluorescence or absorption spectra).6 The color differences are connected with structural or conformational changes after the addition of anions to receptors.7 These routes are based on N–H proton transfer from the receptor to an anion.8 The receptors involved for donation of protons include urea, amine, imidazole ions, phenol, hydroxyl groups, pyrrole, imine, thiourea, and hydrazones.9
Hydrazones of 2,4-dinitrophenyl hydrazines are very useful chemosensors used for the detection of anions, especially fluoride and cyanide ions. There was a strong alteration in color after the addition of anions to these hydrazones. This change was attributed to abstraction of protons from hydrazones. These hydrazones have strong acidic protons, which form hydrogen bonding with added anions.10 Herein, a series of novel hydrazones (SM-1 to SM-3) based on the indole carboxaldehyde derivative and 2,4-dinitrophenyl hydrazine moiety was synthesized in a single step. The synthesized compounds respond to F– and CN– ions, except SM-3, which displayed high selectivity toward F– ions. We predicted that our synthesized compounds possess acidic protons, which were abstracted by anions (F– and CN–), and as a result, fast and sensitive detection was observed.
2. Experimental Section
All the chemicals such as indole-4-carboxaldehyde, indole-5-carboxaldehyde, indole-6-carboxaldehyde, 2,4-dinitrophenyl hydrazine (DNPH), MeOH, acetic acid, petroleum ether, salts (tetrabutyl ammonium salts (bisulfate, F, chloride, iodide, bromide, perchlorate, thiocyanate, acetate, and cyanate)), and ethyl acetate were obtained from Sigma-Aldrich. The reaction was carried out in an oven-dried flask (50 mL). The 1H NMR spectrum was recorded on a Bruker MHz 400 spectrometer. The electronic absorption spectra were recorded on a Shimadzu UV-1800 spectrophotometer.
2.1. Computational Procedure
In the current study, quantum chemical investigation was performed for SM-1 to SM-3 sensors and their ionic forms: an1 (formed by deprotonation from the −N=NH group), an2 (formed by deprotonation from the indole group), and an3 (formed by double deprotonation from the −N=NH group and indole group). For this purpose, the Gaussian 09 package11 was utilized at the M06-2X/6-311G+(d,p) functional12 of density functional theory (DFT) in acetonitrile. Frontier molecular orbital analysis was performed to elucidate the charge transference and energy gap between the orbitals. From the findings of FMOs, global reactivity parameters were investigated to understand the reactivity and stability of these sensors. To investigate the excitations of sensors, UV–vis, hole–electron, and transition density matrix (TDM) analyses were also performed in acetonitrile media at the foresaid functional.
2.2. Synthesis of Chemosensors
The chemosensors were synthesized via a single-step condensation reaction of the indole-carboxaldehyde (0.1 g, 0.30 mmol) derivative with 2, 4-dinitrophenyl hydrazine (0.1 g, 0.30 mmol) in MeOH (15 mL) (Scheme 1). The reaction was refluxed at 70 °C and initiated by the addition of a catalyst (CH3COOH). After some time, precipitates were formed. The reaction was monitored by TLC and, after the completion of reaction, confirmed by TLC. The precipitates were filtered, washed with MeOH, and dried in air. The yield was calculated as 80%. The synthesized compounds were confirmed by the 1H NMR and 13C NMR spectra (Figures S1–S6). The reaction was monitored by TLC.
Scheme 1. Synthesis of Indole-Based Sensors (SM-1 to SM-3).
SM-1: (E)-4-((2-(2,4-dinitrophenyl)hydrazono)methyl)-1H-indole. m.p.: >300 °C. IR υmax (cm–1): 1175 (C=S), 1570 (C=N), 3125, 3311 (N–H); 1H NMR (DMSO-d6) δ (ppm): 11.60 (1 H, s), 11.35 (1 H, s), 8.81 (2 H, d, J = 62.4), 8.38 (1 H, d, J = 7.7), 8.08 (1 H, d, J = 8.2), 7.75 (1 H, s), 7.62 (1 H, d, J = 6.5), 7.52 (2 H, d, J = 34.8), 6.49 (1 H, s). 13C NMR δ (ppm): 101.91, 112.31, 116.75, 117.88, 120.69, 123.29, 126.82, 128.18, 129.14, 129.92, 135.91, 136.72; elemental analysis calculated for C15H11N5O4, 325.08; C, 55.39; H, 3.41; N, 21.53; found: C, 55.59; H, 3.19; N, 21.61.
SM-2: (Z)-5-((2-(2,4-dinitrophenyl)hydrazono)methyl)-1H-indole. m.p.: >300 °C. IR υmax (cm–1): 1188 (C=S), 1581 (C=N), 3129, 3323 (N–H); 1H NMR (DMSO-d6) δ (ppm): 11.57 (1 H, s), 11.36 (1 H, s), 8.78 (2 H, d, J = 62.4), 8.34 (1 H, s), 8.08 (1 H, s), 7.89 (1 H, s), 7.64 (1 H, s), 7.44 (2 H, d, J = 36.4), 6.53 (1 H, s). 13C NMR δ (ppm): 102.26, 112.30, 116.79, 119.85, 121.79, 123.25, 125.03, 126.83, 127.88, 128.96, 129.82, 136.58, 137.47, 141.63, 151.97; C15H11N5O4, 325.08; C, 55.39; H, 3.41; N, 21.53; found: C, 55.51; H, 3.35; N, 21.71.
SM-3: (Z)-6-((2-(2, 4-dinitrophenyl)hydrazono)methyl)-1H-indole. m.p.: >320 °C. IR υmax (cm–1): 1158 (C=S), 1579 (C=N), 3139, 3311 (N–H); 1H NMR (DMSO-d6) δ (ppm): 11.64 (1 H, s), 11.44 (1 H, s), 8.95 (1 H, s), 8.85 (1 H, s), 8.42 (1 H, d, J = 8.9), 8.05 (1 H, d, J = 9.1), 7.54 (2 H, s), 7.34 (1 H, d, J = 6.4), 7.19 (1 H, t, J = 6.8), 7.07 (1 H, s). 13C NMR δ (ppm): 101.93, 114.63, 116.57, 121.01, 122.45, 123.25, 124.62, 124.85, 127.46, 129.24, 130.16, 136.55, 136.82, 144.53, 151.74; C15H11N5O4, 325.08; C, 55.39; H, 3.41; N, 21.53; found: C, 55.49; H, 3.35; N, 21.55.
3. Results and Discussion
3.1. Solvatochromic Behavioral Studies of SM-3
To evaluate the solvatochromic effect, the UV–vis spectrum of SM-3 with F– ions in different solvents was recorded. The addition of 3 equiv of F– ions to the sensor SM-3 displayed changes in absorbance band, and the change in wavelength is negligible; thus, there is no effect of polarities of solvents on the sensor SM-3, except DCM and CH3Cl, as illustrated in Figure 1. We have used MeCN as a solvent for next further study.
Figure 1.
Effect of the solvent on absorbance.
3.2. Anion Tracking Study
The anion tracking ability of sensors SM-1 to SM-3 was checked by the naked-eye view after the addition of different anions such as F–, CN–, SCN–, Br–, Cl–, I–, HSO4–2, and ClO4– ions in CH3CN to the solution of sensors. Interestingly, the color of solutions SM-1 and SM-2 was changed from mustard to purple after the addition of F– and CN– ions. But, in the case of SM-3, the change was only observed with F ions as depicted in Figure 2. This study showed that the sensor SM-3 displayed high selectivity as compared to the other two sensors. We have also used NaF as a source of F ions and MeCN/water as a solvent for the detection of F ions by using SM-3, as water has a strong ability to donate its proton as compared to our synthesized sensor SM-3. The results are not satisfactory as shown in Figures S7–S9. For further confirmation of selectivity, the UV–vis selectivity of sensors was determined in CH3CN.
Figure 2.
Naked-eye views of sensors SM-1 to SM-3 in the presence of 3 equiv of different anions in CH3CN (A = SM, B = F–, and C = CN–).
When the solution of sensors was treated with different anions as depicted in Figure 3, there was a change in absorbance band with the addition of F– and CN– ions in the case of SM-1 and SM-2. But, in the case of SM-3, only the change was observed with F– ions. This result showed that SM-3 has high selectivity of detection of F– ions. This result also supports the naked-eye view results. These changes were attributed to the deprotonation of sensors or the formation of hydrogen bonding with anions and sensors. Due to the high selectivity of SM-3, further study was done on it.
Figure 3.
UV–vis spectrum of sensors SM-1 to SM-3 in CH3CN.
3.3. UV–vis Titration with TBAF
To check the ability of anion binding affinity of a sensor with F– ions, UV–vis titration was carried out. UV–vis titration was performed in CH3CN as shown in Figure 4. The sensor itself displayed an absorption band at 406 nm in the absence of F– ions. As addition of F– ions was continued (0 to 3 equiv), a new absorption band at 502 nm was observed with gradual enhancement in absorbance intensity along with a decrease in absorbance intensity at 406 nm. There was a red shift of 96 nm. The appearance of a single isosbestic point at 447 nm was an indication of stable complex formation. The decrease in absorbance band at 406 nm was an indication of deprotonation, which results in a change in color. Job plots indicate that the formation of a complex involved 1:1 stoichiometry (Figure S10). The stoichiometric ratio of the binding of a sensor with F– was calculated by the Benesi–Hildebrand (B–H) plot. The binding constant was calculated as 7.7 × 105, and the detection limit was given as 8.69 × 10–8 (Figures S11 and S12).
Figure 4.
UV–vis titration spectrum of the sensor SM-3 in CH3CN with 3 equiv of F– ions.
3.4. Competitive Study
From above results, it is noted that the F– ion has a high ability of forming hydrogen bonding/abstracting protons from the sensor SM-3. For further confirmation of this statement of high selectivity of F– ions, a competitive experiment was performed as illustrated in Figure 5. The solution of anions and SM-3 was treated with 3 equiv of fluoride ions, and it was noted that the competing anions have a negligible effect on the ability of F– ions to form hydrogen bonding/abstract protons from the sensor SM-3. This result showed that SM-3 has excellent selectivity toward F– ions.
Figure 5.
Competitive spectrum of the sensor SM-3 in CH3CN.
3.5. Reverse Reaction
One interesting thing is the reversibility of the action of F– ions with the help of a good proton donor such as methanol.13 Methanol has the ability to donate protons easily as compared to our synthesized sensor SM-3. This statement is justified in Figure 6 as SM-3 has an absorbance band at 406 nm, which disappeared, and a new band appeared at 502 nm after the addition of F– ions. When 0.1 mL of MeOH was added to this complex of SM-3 and F– ions, the band that appeared at 502 nm decreased and the original band of SM-3 reappeared at 406 nm. This showed that SM-3 displayed a reverse reaction upon addition of MeOH. We have to also try water as well as acids (HCl and acetic acid). The results showed that these solvents are easily deprotonated as compared to our synthesized sensors and a reverse response was observed after the addition of these solvents to the solution of complex of SM-3 and F– ions (Figure S13). To support our statement, we have to try other solvents such as ethyl acetate, DMSO, THF, DMF, and EtOH. We have noticed that only change was observed with EtOH (Figure S14). These results showed that solvents that have a strong ability to donate protons as compared to our synthesized sensor can reverse the action of the sensor SM-3.
Figure 6.
Reverse reaction of SM-3 with MeOH.
3.6. 1H NMR Titration
To confirm the binding interaction of SM-3 and F– ions, a 1H NMR titration experiment was recorded in DMSO-d6. The 1H NMR spectra of SM-3 displayed sharp peaks of proton at 11.44 and 11.64 ppm, which were related to N–H and the indole ring, respectively. When a solution of TBAF was added to this solution, the peaks at 11.44 and 11.64 ppm disappeared as illustrated in Figure 7, which confirms the abstraction of protons.
Figure 7.
1H NMR titration of SM-3 with and without TBAF.
3.7. Real-Life Application
The application of the sensor SM-3 was demonstrated in real samples like toothpaste. As reported,14 the toothpaste solution was prepared. When the solution of SM-3 was treated with the prepared solution of toothpaste, the absorbance band at 406 nm was decreased to some extent and a new band at 502 nm was observed as illustrated in Figure 8. Similarly, to fluoride ions, the color of solution is also changed from light yellow to purple. These results demonstrate that SM-3 could be used to detect F– ions in the toothpaste sample.
Figure 8.
Treatment of SM-3 with the toothpaste sample.
Furthermore, SM-3 can be used for analytical applications such as test strips. The Whatman filter paper is cut into the form of a test strip (two strips). First, these strips were dipped into the solution of SM-3. After that, these strips were air-dried. When these strips were fully dried, one of them was dipped into the solution of F– ions. The strip color changed from light yellow to peach as shown in Figure 9. This result shows that a test strip can be prepared for F ion detection by using SM-3.
Figure 9.
Test strip of the sensor SM-3 for F ion detection.
3.8. Logic Gate
For the logic gate, F– ions and MeOH were used as chemical input (1 and 2), while absorbance at 406 and 502 nm was used as chemical output (1 and 2). Binary codes “0” and “1” were used to confirm the presence of chemical inputs. The absorbance valve of 0.2 was used as the threshold valve for 406 and 502 nm. The absorbance valve below the 0.2 valve is given as “0”, and the absorbance valve higher than the 0.20 valve is given as “1”. This is connected to “off” and “on” positions. As illustrated in Figure 10, in the absence of F ions and MeOH (0, 0) or in the presence of MeOH (0, 1), output 2 (502 nm) is “0”, which represents the “off” state. But when input 1 (F– ions) is present and input 2 (MeOH) is absent, there is a strong absorbance at 502 nm, which shows that output 2 is “1”, representing the “on” position. This fabricates an INH gate at 502 nm. Similarly, an IMP gate was constructed, and in the absence of F– ions and MeOH (0, 0) or in the presence of MeOH (0, 1), the output 1 (406 nm) is “1”, which represents the “on” state. But when input 1 (F– ions) is present and input 2 (MeOH) is absent, the absorbance at 406 nm is quenched, which shows that output 1 is “0”, representing the “off” position. Similarly, in the presence of input 1 and input 2, the absorbance band at 406 nm is regained (output 1 is “1”), which represents the “on” state. The IMP logic function output is just complementary to INH logic. These results indicate that the sensor SM-3 has potential application in molecular devices.
Figure 10.
(A) Output signal of different inputs, (B) absorbance outcome at different chemical inputs, (C) truth table, and (D) representation of the MP/INH logic circuit.
3.9. Frontier Molecular Orbitals (FMOs)
The FMO investigation is a valuable tool to understand the reactivity, polarizability, and stability of a molecule.19 FMO is a powerful factor for determining the possibility of charge transfer among the compounds being studied.20 The FMO energy gap (Egap = ELUMO – EHOMO) is a representative signature to gain information about charge distribution and ICT rate between molecular orbitals.14,21 Herein, to investigate the energy gap and charge distribution between orbitals in SM-1 to SM-3 sensors and their ions, an FMO study was accomplished, and results are tabulated in Table 2.
Table 2. EHOMO, ELUMO, and Energy Gap (ELUMO – EHOMO) of Investigated Sensors and Their Ionic Formsa.
| comp. | HOMO | LUMO | energy gap |
|---|---|---|---|
| SM-1 | –6.889 | –1.912 | 4.977 |
| SM-1-an1 | –6.030 | –1.371 | 4.659 |
| SM-1-an2 | –6.064 | –1.834 | 4.230 |
| SM-1-an3 | –5.723 | –1.403 | 4.320 |
| SM-2 | –6.999 | –1.886 | 5.113 |
| SM-2-an1 | –6.092 | –1.390 | 4.702 |
| SM-2-an2 | –6.172 | –1.820 | 4.352 |
| SM-2-an3 | –5.707 | –1.250 | 4.457 |
| SM-3 | –6.899 | –1.895 | 5.004 |
| SM-3-an1 | –6.009 | –1.330 | 4.679 |
| SM-3-an2 | –6.101 | –1.820 | 4.281 |
| SM-3-an3 | –5.646 | –1.248 | 4.398 |
Units in eV.
The data tabulated in Table 1 shows that the energy gaps of sensors SM-1, SM-2, and SM-3 are noted as 4.977, 5.113, and 5.004 eV with HOMO/LUMO energies of −6.889/–1.912, −6.999/–1.886, and −6.899/–1.895 eV, respectively. Among all the studied chemosensors, the highest band gap is seen for SM-2, which indicated its higher stability than that of the other sensors. This might be due to the attachment of the 1H-indole group at the meta-position, which inhibits the charge transference from indole toward the other part of the molecule. The lowest band gap is examined in SM-1 as the 1H-indole group is attached at the ortho-position and improves the ICT all over the molecule. As alkyl groups are ortho- and para-directed by hyperconjugation, the nitrogen (−N) atoms push the electrons into the ring, which increases the electronic density at ortho- and para-positions due to hyperconjugation.22 The overall descending order of band gaps in studied chemosensors is noticed as follows: SM-2 > SM-3 > SM-1. The comparative study of foresaid chemosensors is done with their respective ions (an1 to an3), and a relative reduction in band gap is noted in ions as compared to their neutral structures (see Table 1). Interestingly, among all ions, greater diminishing is examined in an2, which is formed because of the deprotonation from the indole group. Nevertheless, among ions of all sensors, the lowest energy gap is found in SM-1-an2 (4.230 eV), which indicated its greater reactivity and higher response toward fluoride ions. Overall, the LUMO-HOMO band gap has the following descending order: SM-3-an1 > SM-3-an3 > SM-3-an2. From abovementioned results, it is concluded that deprotonation from indole facilitates the charge transference, hence resulting in the lowest band gaps of the sensors and improving sensing toward anions. The pictographic representation charge distribution of foresaid sensors along with their ions is illustrated in Figure 11. The charge density for HOMO is majorly concentrated over the indole part and minorly over the 1-(2,4-dinitrophenyl)-2-methylenehydrazine part of sensors. However, for LUMO, the major portion of the electronic cloud is located on 1-(2,4-dinitrophenyl)-2-methylenehydrazine and lower density is seen over the indole unit. These electronic pictures indicate the excellent charge transference from indole toward the higher electron acceptor region (1-(2,4-dinitrophenyl)-2-methylenehydrazine). A similar pattern is seen for an2. However, in the case of an1 and an3, the charge density for HOMO is concentrated over the entire molecule, whereas, for LUMO, the electronic charge is located on the 1-(2, 4-dinitrophenyl)-2-methylenehydrazine part of the sensor (see Figure 11). The charge transference in higher orbitals and their energies were also investigated, and their outcomes are described in Figure S15. Almost the same trend for energies and charge transference is seen in HOMO–1/LUMO+1 and HOMO–2/LUMO+2.
Table 1. Evaluation of Limits of Detection and Binding Constants of the Sensor SM-3 with Other Reported Sensors13a,14−18.
Figure 11.
Pictographic charge distribution representation of the HOMO/LUMO orbital of investigated sensors.
3.10. Global Reactivity Parameters (GRPs)
HOMO/LUMO energies are additionally employed to elucidate the reactivity as well as stability of the investigated systems by estimating the global reactivity parameters.23 The outcome of FMOs (Egap) is a dynamic aspect to calculate the GRPs like ionization potential (IP), electron affinity (EA), electronegativity (X), chemical potential (μ), hardness (η), electrophilicity index (ω), and softness (σ).24 GRPs for studied sensors were calculated at the M06-2X level of DFT by utilizing eqs S1–S7,25 and results are shown in Table 3.
Table 3. GRPs of Studied Sensors and Their Respective Ionsa.
| compound | IP | EA | X | μ | η | ω | σ |
|---|---|---|---|---|---|---|---|
| SM-1 | 6.889 | 1.912 | 4.401 | –4.401 | 2.489 | 3.891 | 0.201 |
| SM-1-an1 | 6.03 | 1.371 | 3.700 | –3.700 | 2.330 | 2.939 | 0.215 |
| SM-1-an2 | 6.064 | 1.834 | 3.949 | –3.949 | 2.115 | 3.687 | 0.236 |
| SM-1-an3 | 5.723 | 1.403 | 3.563 | –3.563 | 2.160 | 2.939 | 0.231 |
| SM-2 | 6.999 | 1.886 | 4.443 | –4.443 | 2.557 | 3.860 | 0.196 |
| SM-2-an1 | 6.092 | 1.390 | 3.741 | –3.741 | 2.351 | 2.977 | 0.213 |
| SM-2-an2 | 6.172 | 1.820 | 3.996 | –3.996 | 2.176 | 3.670 | 0.230 |
| SM-2-an3 | 5.707 | 1.250 | 3.479 | –3.479 | 2.229 | 2.715 | 0.224 |
| SM-3 | 6.899 | 1.895 | 4.397 | –4.397 | 2.502 | 3.864 | 0.199 |
| SM-3-an1 | 6.009 | 1.330 | 3.670 | –3.670 | 2.340 | 2.878 | 0.214 |
| SM-3-an2 | 6.101 | 1.820 | 3.961 | –3.961 | 2.141 | 3.664 | 0.234 |
| SM-3-an3 | 5.646 | 1.248 | 3.447 | –3.447 | 2.199 | 2.702 | 0.227 |
Units in eV.
Electronegativity, global hardness, and softness are the important qualitative chemical concepts and are extensively utilized to comprehend the different aspects of chemical reactivity. Results of Table 2 reveal that the global softness value is 0.201 eV for SM-1, which is the highest as compared to SM-2 and SM-3. This might be because of the attachment of 2-(2,4-dinitrophenyl)hydrazono)methyl at the ortho-position of the indole group. The nitrogen (−N) atom pushes the electrons into the ring, which increases the electronic density at −o and −p-positions due to hyperconjugation. Further, hardness values are found to be 2.489, 2.557, and 2.502 eV for SM-1, SM-2, and SM-3, respectively, and the lowest value of hardness shows that SM-1 is a kinetically least stable, more reactive, and soft chemosensor. Moreover, it can also be inferred from Table 2 that increased values of electronegativity (X) and decreased values of chemical potential (μ) are found for all the ionic forms as compared to their sensors (SM-1, SM-2, and SM-3). Furthermore, in ionic form, the higher softness is noticed for the an2 form as it is formed by the deprotonation from the indole group. It means that deprotonation from indole enhanced the sensing toward fluorides.
3.11. UV–Visible Analysis
To calculate the ground-state electronic transitions in SM-1 to SM-3 sensors, UV–vis investigation was performed via time-dependent density functional theory (TD-DFT). The UV–visible absorption spectra of foresaid sensors were simulated in acetonitrile solvent by utilizing the M062x/6-311+G(d,p) functional, and important results are tabulated in Table 4.
Table 4. Calculated Transition Energies (eV), Maximum Absorption Wavelengths (λmax), Oscillator Strengths (fos), and Transition Nature of Designed Compounds (SM-1 to SM-3) in Acetonitrilea.
| compound | λ (nm) | E (eV) | fos | MO contributions |
|---|---|---|---|---|
| SM-1 | 401.049 | 3.092 | 0.975 | H → L (61%), H → L + 1 (17%) |
| SM-1-an1 | 457.267 | 2.711 | 1.546 | H → L (81%), H → L + 1 (12%) |
| SM-1-an2 | 461.319 | 2.688 | 0.766 | H → L (38%), H → L + 1 (37%), H → L + 2 (16) |
| SM-1-an3 | 471.584 | 2.629 | 1.303 | H-1 → L (11%), H → L (69%), H → L + 1 (11%) |
| SM-2 | 391.599 | 3.166 | 0.785 | H → L (63%), H → L + 1 (16%), H-2 → L (8) |
| SM-2-an1 | 452.002 | 2.743 | 0.346 | H → L (87%), H-3 → L (2) |
| SM-2-an2 | 433.223 | 2.862 | 0.816 | H-1 → L (15%), H → L (33%), H → L + 1 (25) |
| SM-2-an3 | 470.957 | 2.633 | 0.391 | H → L (73%), H-2 → L (7) |
| SM-3 | 394.352 | 3.144 | 0.957 | H-2 → L (10%), H → L (61%), H → L + 1 (16%) |
| SM-3-an1 | 451.130 | 2.748 | 0.705 | H → L (81%), H-1 → L (3%), H → L + 1 (9) |
| SM-3-an2 | 439.084 | 2.824 | 0.956 | H → L (45%), H → L + 1 (29%), H-2 → L (9%) |
| SM-3-an3 | 471.226 | 2.631 | 0.439 | H → L (70%), H-2 → L (6%), H-1 → L (9%) |
MO = molecular orbital; fos= oscillator strength; H = HOMO; L = LUMO.
Data tabulated in the above table elucidates that the λmax values of sensors SM-1, SM-2, and SM-3 are observed as 401.049, 391.599, and 394.352 nm with transition energies of 3.092, 3.166, and 3.144 eV, respectively. Among all the chemosensors, the maximum absorbance is shown by SM-1 (401.049 nm) with the lowest energy gap (3.092 eV) and oscillator strength (0.975). This might be due to the attachment of the 1H-indole group at the ortho-position, which improves the ICT all over the sensor. Overall, the descending order of λmax is SM-1 > SM-3 > SM-2. Comprehensive analysis of abovementioned chemosensors is done with their respective ions (an1 to an3), and the λmax is found to be red-shifted compared to the neutral sensors (see Table 3). For example, in the SM-1 sensor, the λmax shift values are found as 457.267, 461.319, and 471.584 nm with low energy (2.688 and 2.629 eV) and fos (0.766 and 1.303) values. Correspondingly, the λmax values for SM-2 ions are estimated to be 433, 452, and 470 nm for SM-2-an2, SM-2-an1, and SM-2-an3, respectively. Similarly, SM-3-an3, SM-3-an1, and SM-3-an2 have λmax in decreasing order as 471, 451.130, and 439 nm, respectively. In all sensors, the maximum bathochromic shift is examined in ions when double deprotonation is done (an3). Overall, the decreasing order of λmax is SM-1-an3 > SM-3-an3 > SM-2-an3 > SM-1-an2 > SM-2-an1 > SM-3-an1 > SM-3-an2 > SM-2-an2 > SM-1 > SM-3 > SM-2. These simulated values of SM-1 to SM-3 showed agreement with experimental absorption values (see Figure 3), and among all sensors, SM-3 and its ions displayed a red shift, so they would express good sensing toward fluoride ions.
3.12. Hole–Electron Analysis
The hole–electron investigation demonstrates almost similar index values for the investigated sensors and their ions as shown in Table 5. From Table 5, compounds SM-2 and SM-3 possess high D index values at 1.95 and 2.10 Å, respectively, but SM-1 possesses a medium D index value of 1.82 Å, which implies that S0 → S1 excitations involve a medium distance between hole and electron distribution in SM-1 and a consequently medium ECoul value of 2.47 eV, which does not show a strong hole–electron interaction. However, in the comparative analysis of these chemosensors with the ions, the D index values of the ions are greater than their neutral counterparts except for SM-1-an1 and SM-3-an1, which exhibit less values at 0.932 and 2.04 Å, respectively. SM-2 has manifested a high ECoul value of 4.51 eV among the neutral molecules, but the highest ECoul value is demonstrated by its ion SM-2-an1 at 4.64 eV. SM-1 demonstrates the highest Sr index value of 0.68 among all the compounds.
Table 5. Indices of Hole–Electron Study Having the Highest fos of Compounds for S0 → S1 Excitation.
| compound | SM-1 | SM-1-an1 | SM-1-an2 | SM-1-an3 | SM-2 | SM-2-an1 | SM-2-an2 | SM-2-an3 | SM-3 | SM-3-an1 | SM-3-an2 | SM-3-an3 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| excitation | S0 → S1 | S0 → S1 | S0 → S1 | S0 → S1 | S0 → S1 | S0 → S1 | S0 → S1 | S0 → S1 | S0 → S1 | S0 → S1 | S0 → S1 | S0 → S1 |
| E (eV) | 3.09 | 2.71 | 2.69 | 2.63 | 3.17 | 2.74 | 2.86 | 2.63 | 3.14 | 2.75 | 2.82 | 2.63 |
| D (Å) | 1.82 | 0.932 | 3.14 | 6.37 | 1.95 | 2.17 | 3.23 | 2.39 | 2.10 | 2.04 | 3.68 | 2.53 |
| ECoul (eV) | 2.47 | 4.48 | 3.94 | 2.92 | 4.51 | 4.64 | 3.93 | 4.37 | 4.31 | 4.46 | 3.77 | 4.30 |
| Sr | 0.68 | 0.711 | 0.611 | 0.494 | 0.655 | 0.592 | 0.626 | 0.587 | 0.660 | 0.633 | 0.605 | 0.580 |
| H (Å) | 3.69 | 3.89 | 3.76 | 3.77 | 3.36 | 2.95 | 3.73 | 3.19 | 3.61 | 3.32 | 3.83 | 3.27 |
| t (Å) | –1.21 | –2.07 | –0.11 | 3.09 | –0.735 | 0.101 | 0.042 | 0.003 | –0.84 | –0.656 | 0.383 | 0.008 |
| HDI | 7.27 | 7.88 | 6.74 | 7.08 | 8.18 | 8.42 | 6.92 | 8.09 | 7.81 | 8.49 | 6.64 | 7.99 |
| EDI | 7.91 | 7.74 | 6.98 | 8.20 | 9.44 | 9.76 | 8.60 | 9.84 | 9.10 | 9.14 | 8.92 | 9.90 |
Compounds SM-1, SM-1-an1, SM-1-an2, SM-2, SM-2-an2, SM-3, SM-3-an1, and SM-3-an2 exhibit π–π* excitations owing to high Sr index values (>0.6), consequently with high H index values at 3.69, 3.89, 3.76, 3.36, 3.73, 3.61, 3.32, and 3.83 Å, respectively, indicating the presence of extended hole and electron distribution in these compounds. All the neutral compounds exhibit negative t values, indicating main excitation as local excitation (LE) with the ascending order of SM-1 < SM-3 < SM-2. Relative to their ions, two ions of SM-1 and only one ion of SM-3 exhibit a negative t index, i.e.,SM-1-an1 and SM-1-an2 as well as SM-3-an1, respectively, with main excitation being LE in both ions. Finally, the less and similar HDI and EDI values in SM-1-an1 and SM-1-an2 also manifest localized excitation. Moreover, the TDM graphs of all the compounds support diagonal charge transference. The charge transfers from nitrogen atoms to oxygen atoms owing to the greater electron-withdrawing capability of the oxygen atom (hydrazine unit). The benzene ring facilitates the charge transfer process from nitrogen toward oxygen as red spots are seen at atoms 15–17. In Figure 12, we showed heat maps of TDM of sensors (SM-1 to SM-3) and their an1, while the maps of other ions are shown in Figure S16.
Figure 12.
TDM heat map showing the charge transference in sensors and their ions.
4. Conclusions
In summary, we have synthesized three sensors, SM-1 to SM-3. These sensors are synthesized in a single step by the condensation of derivative of indole carboxaldehyde with 2,4-dinitrophenyl hydrazine in a good yield. DFT investigations have been also performed for the current study. Efficient charge transference within a molecule from HOMO toward LUMO is investigated through the FMO study. TDM maps further supported this significant transference from indole toward dinitrophenyl hydrazine. Interestingly, it is also investigated that the deprotonated form of sensors is more reactive than their neutral form as lower band gap and greater softness values are found for ions. The UV–vis finding indicated that among all the sensors, SM-3 showed a wider absorption spectrum. Additionally, the sensor SM-3 is found as more selective as compared to the other two sensors. SM-3 displayed high selectivity toward fluoride ions with a low detection limit (8.69 × 10–8), and the binding constant was determined as 7.7 × 105. The binding mode is proposed as 1:1 stoichiometry based on Job plots. The sensor is successfully used to detect F– ions in the real sample and used for preparation of strips. We hope that this work will be useful for the design of new sensors for F– ions.
Acknowledgments
This research work was funded by Institutional Fund Projects under grant no. IFPIP 487-665-1443. The authors gratefully acknowledge technical and financial support provided by the Ministry of Education and King Abdulaziz University, DSR, Jeddah, Saudi Arabia.
Supporting Information Available
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.3c00821.
1H NMR and 13C NMR spectra of SM-1 to SM-3; mixed solution of water/MeCN and SM-3 without and with addition of F ions; NaF response to SM-3; Job plot, B–H plot, and linear plot of SM-3; reverse reaction of SM-3 with water, acids, and other solvents; energy gap of HOMO–1/LUMO+1 and HOMO–2/LUMO+2 of SM-3; TDM heat map showing the charge transference in SM-3 (PDF)
Author Contributions
The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.
The authors declare no competing financial interest.
Supplementary Material
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