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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Turn-On Fluorescent Sensors for the Selective Detection of Al3+ (and Ga3+) and PPi Ions Vijay Kumar,⊥ Pramod Kumar,⊥ Sushil Kumar,⊥ Divya Singhal, and Rajeev Gupta* Department of Chemistry, University of Delhi, New Delhi 110007, India

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S Supporting Information *

ABSTRACT: Rationally designed multiple hydroxyl-group-based chemosensors L1−L4 containing arene-based fluorophores are presented for the selective detection of Al3+ and Ga3+ ions. Changes in the absorption and emission spectra of L1−L4 in ethanol were easily observable upon the addition of Al3+ and Ga3+ ions. Competitive binding studies, detection limits, and binding constants illustrate significant sensing abilities of these chemosensors with L4, showing the best results. The interaction of Al3+/ Ga3+ ions with chemosensor L4 was investigated by fluorescence lifetime measurements, whereas Job’s plot, high-resolution mass spectrometry, and 1H NMR spectral titrations substantiated the stoichiometry between L4 and Al3+/ Ga3+ ions. The solution-generated [L-M3+] species further detected pyrophosphate ion (PPi) by exhibiting emission enhancement and a visible color change. The binding of Al3+/Ga3+ ions with chemosensor L4 was further supported by density functional theory studies. Reversibility for the detection of Al3+/Ga3+ ions was achieved by utilizing a suitable proton source. The multiionic response, reversibility, and optical visualization of the present chemosensors make them ideal for practical applications for real samples, which have been illustrated by paper-strip as well as polystyrene film-based detection.

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category of Schiff base based fluorescent chemosensors, offering hard aliphatic −OH as well as phenolic −OH donor groups, L1−L4 (Scheme 1). Such chemosensors also contain naphthalene, methylenenaphthalene, anthracene, and pyrene moieties as fluorophores. Chemosensors L1−L4 illustrate remarkable fluorescence enhancements with appreciable Stokes shifts for both Al3+ and Ga3+ ions. Importantly, in situ generated [L-M3+] species further demonstrate emissionenhancement-based unique sensing of pyrophosphate ion (PPi).

INTRODUCTION Aluminum is the third most abundant metal in Earth’s crust1 and has found wide applications in household devices, building materials, transportation, electronics, and sophisticated medical devices.2 Courtesy of its extensive use in daily life, aluminum also has very high exposure.3 High levels of aluminum ion in humans have been linked to several neurodegenerative diseases including Alzheimer’s, Parkinson’s, and dialysis encephalopathy.4 Aluminum ion is also widely acknowledged as a neurotoxic agent.4,5 Along these lines, the World Health Organization has limited its daily average intake between 3 and 10 mg/day with only a 7.4 μM permissible limit in drinking water.6 Because of aluminum’s significant adverse impact on human health, the development of effective sensors for selective and sensitive detection of Al3+ ion has received great interest.7 In this context, absorption- and emission-based sensors are considered to be pertinent for the detection of Al3+ ion because they offer convenient, selective, and sensitive detection.7,8 In particular, fluorescent “turn-on” chemosensors are not only desirable but also advantageous for the detection of Al3+ ion.9−12 Ga3+ ion, belonging to the same group IIIA, has also been identified as a potential health hazard.13 Selective detection of both Al3+ and Ga3+ ions by a single chemosensor is rare14 but would be more relevant.15,16 Because of their hard acid nature, both Al3+ and Ga3+ ions typically prefer to bind hard bases, e.g., oxygen atom.17 Realizing such a requirement, we have designed an interesting © XXXX American Chemical Society

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RESULTS AND DISCUSSION Synthesis and Characterization of Chemosensors L1−L4. Chemosensors L1−L4 were synthesized by the coupling of 2-hydroxy-3-(hydroxymethyl)-5-methylbenzaldehyde with 1-aminonaphthalene (L1), 1-(aminomethylene)naphthalene (L2), 2-aminoanthracene (L3), and 1-aminopyrene (L4), respectively. Fourier transform infrared (FTIR) spectra of chemosensors L1−L4 display a broad stretch together for aliphatic and phenolic −OH groups in the range of 3265−3322 cm−1, whereas CN stretches were noted between 1624 and 1635 cm−1 (Figures S1−S4).18 1H NMR spectra of L1−L4 displayed a sharp peak for the phenolic −OH group at 13.69−13.78 ppm, whereas the signal for the aliphatic −OH group was noted at 2.80 ppm (L1), 2.62 ppm Received: May 26, 2019

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DOI: 10.1021/acs.inorgchem.9b01550 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Scheme 1. (a) Chemical Drawings of Chemosensors L1−L4 and (b) Crystal Structure of Chemosensor L1, Where Thermal Ellipsoids Are Drawn at the 30% Probability Level While a Methylenehydroxy Oxygen Atom Was Positionally Disordered over Two Positions

Figure 1. Change in the emission spectra of chemosensors L1−L4 (5 μM) in the presence of 10 equiv of assorted metal ions in EtOH (containing 0.5% THF); (a) L1; (b) L2; (c) L3; (d) L4.

(L3), and 5.14 ppm (L4) (Figures S5−S8). The imine −CH proton for chemosensors L1−L4 was observed between 8.38 and 9.18 ppm. 13C NMR spectra of L1−L4 displayed signals for −CH3 and −CH2OH groups at 20.41−20.77 and 58.00− 61.99 ppm, respectively (Figures S9−S12). Positive-ion electrospray ionization mass spectrometry (ESI+-MS) spectra of chemosensors exhibited molecular ion peaks at m/z 292.1353, 306.1253, 342.1486, and 366.1503 for the [L + H]+ species for L1−L4, respectively (Figures S13−S16). The single-crystal structure of a representative chemosensor, L1, authenticated their chemical structures (Scheme 1b and Table S1). The structure illustrates the coplanar nature of two arene rings and the presence of a hydrogen bond between the phenolic −OH and imine N groups.

Detection of Al3+ and Ga3+ Ions by Fluorescent Spectral Studies. The detection abilities of photoresponsive chemosensors L1−L4 were investigated by fluorescence spectral titrations (Figure 1). The emission spectra of all four chemosensors, L1−L4, were measured at different wavelengths (280−370 nm); however, the best emission was noted after excitation at 280 nm compared to other excitation wavelengths (Figure S17). In particular, chemosensors L1−L4 (10 μM) exhibited emission at 425−480 nm upon excitation at 280 nm. For example, chemosensor L4 (the most successful one) illustrated the highest emission intensity at 429 nm when excited at 280 nm compared to other excitation wavelengths: 300, 320, 340, and 370 nm. All four chemosensors exhibited negligible changes in the emission spectra in the presence of 10 equiv of different metal B


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Figure 2. Bar diagram showing a relative enhancement in emission for chemosensors L1−L4 in the presence of 10 equiv of metal ions in EtOH (containing 0.5% THF); L1 at 425 nm (dark-blue pillars), L2 at 470 nm (green pillars), L3 at 481 nm (purple pillars), and L4 at 429 nm (red pillars).

Table 1. Stokes Shifts, Detection Limits, and Binding Constants (Kb) for Al3+ and Ga3+ Ions for Chemosensors L1−L4 Stokes shift (Δλ, nm) chemosensor L1 L2 L3 L4

Al3+ 68 121 107 47

Ga3+ 68 136 107 47

detection limit (μM) Al3+ 5.86 4.64 1.46 0.38

binding constant (Kb)

Ga3+

Al3+ (M−1)

Ga3+ (M−2)

5.96 3.40 1.25 1.17

1.44 × 10 7.68 × 103 6.66 × 103 16.2 × 103

1.25 1.27 1.43 1.56

3

× × × ×

102 102 102 102

Figure 3. Change in the absorption spectra of chemosensors L1−L4 (5 μM) in the presence of 10 equiv of assorted metal ions in EtOH (containing 0.5% THF): (a) L1; (b) L2; (c) L3; (d) L4.

ions such as Na+, K+, Mg2+, Ca2+, Cr3+, Co2+, Ni2+, Fe2+, Fe3+, Mn2+, Mn3+, Cu2+, Zn2+, Ag+, Hg2+, Pd2+, Cd2+, In3+, Pb2+, Cu+, and Au+. Remarkably, significant emission enhancement was noted upon the addition of Ga3+ ion and particularly Al3+ ion (Figure 2). A comparison by a bar diagram for four chemosensors, L1−L4, illustrated that L4 exhibits much larger

emission enhancement in the presence of both Al3+ and Ga3+ ions compared to the remaining three chemosensors (Figure 2). Because In3+ ion belongs to the same group, we investigated the effect of In3+ ion on the emission of all four chemosensors. As can be seen from Figure S18, none of the chemosensors exhibited any selectivity toward the In3+ ion. C


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Inorganic Chemistry

Figure 4. (a) Selectivity of chemosensor L4 toward the Al3+ ion in the presence of other metal ions: L4 + metal ions (red pillars) and L4 + metal ions + Al3+ ion (blue pillars). (b) Selectivity of chemosensor L4 toward Ga3+ ion in the presence of other metal ions: L4 + metal ions (purple pillars) and L4 + the metal ions + Ga3+ ion (green pillars).

Figure 5. 1H NMR spectral titrations of chemosensor L4 (2.0 mM, DMSO-d6) in the absence and presence of (a) Al3+ ion (2−10 equiv) and (b) Ga3+ ion (2−10 equiv). (c) Proposed structures of [L4-Al3+] and [L4-Ga3+] species generated during the ESI-MS spectrum recorded in CH3OH.

We then studied the effect of concentration variation of Al3+ and Ga3+ ions on the emission spectra of chemosensors L1−L4 (Figures S19 and S20). In all cases, both Al3+ and Ga3+ ions showed “turn-on” emission. From the concentration variation plots, the extent of binding was measured by determination of the detection limits19,20 and binding constants (Kb)20,21 (Table 1 and Figures S21−S24). All four chemosensors, L1−L4, illustrated impressive detection limits as well as binding constants (Kb) for both the Al3+ and Ga3+ ions (Table 1). The

detection limits of the present chemosensors were found to be better than that of several other chemosensors reported in the literature (cf. Table S2). Detection of Al3+ and Ga3+ Ions by the UV−Vis Spectral Studies. The sensing of both Al3+ and Ga3+ ions was further investigated by absorption spectroscopy. UV−vis spectra of chemosensors L1−L4 (5 μM) displayed intense transitions at 357 nm (ε = 147920 M−1 cm−1) and 334 nm (ε = 48900 M−1 cm−1) and multiple bands at 375 nm (ε = 34500 D


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Inorganic Chemistry M−1 cm−1) and 384 nm (ε = 147140 M−1 cm−1), respectively (Figure S25). We evaluated the effect of the addition of 10 equiv of the following metal ions on the absorption spectra: Na+, K+, Mg2+, Ca2+, Cr3+, Co2+, Ni2+, Fe2+, Fe3+, Mn2+, Mn3+, Cu2+, Cu+, Au+, Zn2+, Ag+, Hg2+, Pd2+, In3+, Cd2+, and Pb2+. Notably, absorption spectra of chemosensors L1−L4 showed perturbations only in the presence of Al3+ and Ga3+ ions (Figure 3). We then measured the effect of the concentration variation of Al3+ and Ga3+ ions on the absorption spectra of chemosensors L1−L4 (Figures S26 and S27). For all four chemosensors in the presence of 10 equiv of Al3+ or Ga3+ ions, π−π* band(s) showed “turn-off”, whereas n−π* band(s) exhibited “turn-on” behavior with several isosbestic points.7b,22 Schiff base based molecules are often hydrolytically sensitive and therefore unstable in the solution state, particularly in the presence of water (H2O).23 Therefore, to ascertain the hydrolytic stability of [L4-M3+] species, we measured the emission as well as absorption spectra of [L4-Al3+] and [L4Ga3+] species in a mixture of ethanol (EtOH)−H2O over a period of time (Figure S28). The results confirm that both [L4-Al3+] and [L4-Ga3+] species are hydrolytically stable.23 Selectivity Studies. Because chemosensor L4 displayed the highest affinity toward Al3+ and Ga3+ ions compared to the remaining three chemosensors L1−L3, L4 was studied for the selectivity studies for both the Al3+ and Ga3+ ions from a mixture of ions.24 The selectivity studies were performed by using competitive binding studies for the Al3+ and Ga3+ ions with chemosensor L4 in the presence of equimolar concentrations of other metal ions and measuring the emission intensity of chemosensor L4 at 429 nm. As shown in Figure 4, no significant change in the emission intensity of chemosensor L4 in the presence of an Al3+ or a Ga3+ ion was observed upon the addition of other metal ions, including the In3+ ion. We therefore conclude that the present chemosensors selectively detect Al3+ and Ga3+ ions even in the presence of other metal ions. Mode of Binding. Both absorption and emission spectral studies illustrated the interaction and/or binding of Al3+ and Ga3+ ions with the chemosensors L1−L4. All four chemosensors offer three types of functional groups: an aliphatic −OH, a phenolic −OH, and an imine −CHN. According to the hard−soft acid−base concept,25 both Al3+ and Ga3+ ions are hard acids and thus are likely to preferentially bind with hard donors such as the O atom of either an aliphatic −OH or a phenolic −OH group.26 Therefore, the binding of chemosensor L4 with Al3+ and Ga3+ ions was studied by 1H NMR spectral titrations (Figure 5). Upon the addition of 2 equiv of Al3+ or Ga3+ ions to a solution of L4, the aliphatic −OH proton (H1, δ = 5.14 ppm) completely disappeared. On the other hand, the phenolic −OH proton (H5, δ = 13.71 ppm) was not affected even in the presence of additional equivalents of Al3+ or Ga3+ ions.27 As expected, remote pyrene ring protons were least affected even after the addition of excess equivalents of Al3+ or Ga3+ ions. In the case of Al3+ ion (with 10 equiv), arene protons, H3 and H3′, are now fully separated, suggesting that Al coordination induced a difference in their chemical environment. Similarly, the imine proton, −CHN (H4, δ = 9.18 ppm), was significantly perturbed after potential coordination of the Al3+ ion. Notably, a singlet for the H4 proton was converted to a multiplet with a slight downfield shift. We propose that the phenolic −OH proton forms a hydrogen bond with the imine N atom, and such an interaction is responsible for splitting of the H4 signal [cf. density

functional theory (DFT) studies]. In contrast, unresolved broad peaks were noted both for the imine proton (H4) as well as for H3/H3′ protons in the case of the Ga3+ ion. Collectively, NMR spectral studies suggest the coordination of both Al3+ and Ga3+ ions through anionic aliphatic −O− groups only. Thus, 1H NMR spectral titrations not only established the coordination of Al3+ and Ga3+ ions to the chemosensor L4 but also explained the actual binding sites. To characterize the actual species formed in solution and responsible for the emission enhancement of chemosensor L4, high-resolution mass spectrometry (HRMS) spectra of solution-generated species, L4-Al3+ and L4-Ga3+, were recorded in methanol (CH3OH). L4-Al3+ showed a peak at m/z 494.5327, which is assigned to [(L4-H+) + Al3+ + 2CH3O− + H2O + Na+]+ (Figure S29). In the case of L4-Ga3+, a peak at m/z 847.2842 was attributed to [(L4-H+)2 + Ga3+ + CH3OH + H2O]+ (Figure S30). In both cases, the isotope distribution pattern was found to nicely match the simulated pattern.28 Because HRMS studies showed 1:1 and 2:1 complexation for L4/Al3+ and L4/Ga3+, Job’s plots29 were performed to establish the exact stoichiometry for the complexation of Al3+ and Ga3+ ions with chemosensor L4. In Job’s plot, a change in the emission intensity at 429 nm for L4 was plotted against the mole fractions of Al3+ or Ga3+ ions and the stoichiometry was indeed found to be 1:1 and 2:1 for Al3+ and Ga3+ ions, respectively (Figure S31). To further support binding of Al3+ ion to chemosensor L4, we performed cyclic voltammetric titration of chemosensor L4 after the incremental addition of Al(NO3)3 in EtOH (Figure S32). Chemosensor L4 exhibited a prominent reductive response at −0.90 V and a prominent oxidative one at 1.38 V (vs Fc/Fc+) in addition to a few redox features centered at 0.50 V. We tentatively propose that the reductive and oxidative responses are based on the imine and phenol groups, respectively.30 Importantly, after the incremental addition of Al(NO3)3, the oxidative response at 1.38 V started to decrease with a shift to 1.20 V. Similarly, the reductive response at −0.90 V disappeared, while a new reductive response appeared at −1.38 V. Moreover, new distinct redox responses were noted at 0.25 and 0.64 V, which were found to be coupled with a E1/2 value of 0.45 V and a peak-to-peak separation (ΔEp) of 390 mV. More importantly, the redox features that developed after the addition of Al(NO3)3 were stable, suggesting the stable nature of [L4-Al3+] species and therefore corroborating the NMR and MS spectral results. Further support was obtained from the FTIR spectra of compounds isolated from a reaction between chemosensor L4 and Al3+ as well as Ga3+ ions. In both cases, the shifting of several stretches confirmed metal complexation by the chemosensor L4 (Figure S33).31 Collectively, all of these studies assert coordination of both the Al3+ and Ga3+ ions to that of a chemosensor. Lifetime Measurements. It is well-established that the interaction between a chemosensor and an analyte can result in a notable change in the excitation energy.32 Therefore, fluorescence lifetime measurements were performed to evaluate the excited-state behavior of chemosensor L4 in the absence as well as the presence of the Al3+ as well as Ga3+ ion (Figure 6). The decay profile of chemosensor L4 displayed a biexponential decay with an average lifetime (τav) of 2.82 ns (Table S3). After the addition of Al3+ and Ga3+ ions, the τav values of chemosensor L4 were increased to 3.40 and 3.21 ns, respectively. These facts assert complexation of Al3+ and Ga3+ E


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Inorganic Chemistry

In quantitative experiments, the addition of 10 equiv of Al3+ and Ga3+ ions enhances the emission of chemosensor L4 (“turn-on”, 9-fold for the Al3+ and 3.8-fold for the Ga3+ ion), whereas the subsequent addition of 10 equiv of PPi further enhanced the emission of L4-Al3+ and L4-Ga3+ species 2.5-fold and 4.4-fold, respectively (Figure 7). However, the subsequent addition of PPi to this solution showed a negligible change. Importantly, no change in the absorption as well as emission spectra of chemosensor L4 was observed in the presence of only PPi (Figure S36). A similar enhancement in the emission of chemosensors L1−L3 was also observed in the presence of the Al3+/Ga3+ ion followed by the addition of PPi (Figure S37). It appears that the addition of PPi to [L4-M3+] species resulted in further deprotonation of the phenolic −OH group. Therefore, PPi potentially acted as a base to cause deprotonation of the phenolic −OH group. In order to prove such an assumption, we investigated the effect of NaH, as a base, on the emission of solution-generated L4-Al3+ and L4-Ga3+ species (Figure S38). Similar to PPi, the addition of NaH also resulted in enhancement of the emission of L4-Al3+ and L4-Ga3+ species. The change in the absorption spectra of L4-Al3+ and L4-Ga3+ species was also measured in the presence of PPi and compared to that of NaH (Figure S39). A similar change was observed either with PPi or with NaH, supporting deprotonation of the uncoordinated phenolic −OH group in the L4-Al3+ species (cf. 27Al NMR spectrum and DFT studies). The 1H NMR spectrum of chemosensor L4 in the presence of 10 equiv of Al3+ or Ga3+ ions showed a signal at δ = 13.71 ppm for the intact phenolic −OH group (Figure S40). However, the addition of 10 equiv of PPi to the solutiongenerated L4-M3+ species resulted in the disappearance of the phenolic −OH signal. From the earlier studies (vide supra), it is clear that both Al3+ and Ga3+ ions bind only with the anionic aliphatic −O− group and not through the phenolic −OH group of chemosensor L4. Hence, we propose that the presence of PPi deprotonates the phenolic −OH group, and M3+ ion now binds with both anionic aliphatic −O− and anionic phenolate −O− groups. Importantly, the presence of PPi resulted in a dark coloration (inset of Figure 7), presumably because of coordination of the phenolate −O group to the M3+ ion.36 Such a situation further enhanced the emission intensity of L4-Al3+ and L4-Ga3+ species. In the case

Figure 6. Lifetime profile for chemosensor L4 in the absence and presence of Al3+ and Ga3+ ions in EtOH (0.5% THF; λex = 370 nm; λem = 430 nm).

ions to the chemosensor L4 and further support absorption, emission, and NMR spectral and HRMS studies.33 Detection of PPi. To develop a reversible chemosensor for detection of the Al3+ and Ga3+ ions, we attempted to remove the chelated metal ion (Al3+ and Ga3+) by using assorted anions. Notably, the effect of various anions on the emission intensity of L4-Al3+ and L4-Ga3+ species was negligible (Figure S34), thus suggesting their inability to remove the chelated metal. In the literature, PPi has been effectively used to remove the Al3+ ion;34 therefore, PPi was attempted. Importantly, however, both solution-generated L4-Al3+ and L4-Ga3+ species showed further enhancement in the emission intensity after the addition of PPi (Figure S35).35 These results therefore suggest that the solution-generated L4-Al3+ and L4-Ga3+ species further interacted and/or reacted with PPi. Similar to L4, solution-generated Al3+ and Ga3+ species of the remaining chemosensors, L1−L3, also showed emission enhancement in the presence of PPi, thus suggesting a general nature of the interaction of PPi.

Figure 7. Emission spectra of chemosensor L4 (5 μM) in the presence of (a) Al3+ ion (10 equiv) and Al3+ ion (10 equiv) + PPi (10 equiv) in EtOH (0.5% THF) [inset: color change of chemosensor L4 after the addition of a Al3+ ion (10 equiv) followed by PPi (10 equiv) in EtOH (0.5% THF)] and (b) Ga3+ ion (10 equiv) and Ga3+ ion + PPi (10 equiv) in EtOH (0.5% THF) [inset: color change of chemosensor L4 after the addition of a Ga3+ ion (10 equiv) followed by PPi (10 equiv) in EtOH (0.5% THF)]. F


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Figure 8. DFT-optimized structures of chemosensor L4 and L4-Al3+ species [A] and [B]. See the text for details.

Figure 9. Contour plots of the HOMO, LUMO, and HOMO−LUMO energy band gaps for chemosensor L4 and L4-Al3+ species [A] and [B] calculated using the B3LYP/6-31G(d,p) method.

of L4-Al3+, a molecular ion peak was observed at m/z 475.3263 in HRMS after the addition of PPi, corresponding to [(L42H+) + Al3+ + NO3− + Na+]+ (Figure S41). Such a fact strongly supports our proposal that the M3+ ion is now ligated to both anionic aliphatic −O− and anionic phenolate −O− groups. 27 Al NMR spectrum of chemosensor L4 was recorded in DMSO-d6 after the addition of the Al3+ ion [as Al(NO3)3].37 Importantly, the standard signal at 3.0 ppm for Al(NO3)3 was changed to a broad one at 66.67 ppm after the addition of L4. Such a fact infers coordination of the Al3+ ion to chemosensor L4 (Figure S42).38 It is proposed that the broadness of the signal is due to the exchange of Al-coordinated solvent molecules by the NMR spectral study solvent.37 Importantly, the signal at 66.67 ppm was further shifted to 58.45 ppm after the addition of PPi. Such a fact suggests additional changes to the chemosensor L4, and we relate it to deprotonation of the phenolic −OH group followed by its coordination to the Al3+ ion. These results corroborate our 1H NMR spectral as well as MS spectral findings. Theoretical Studies. In order to support coordination of the Al3+ ion (as a representative case) with chemosensor L4, shed light on the proposed structure of L4-Al3+ species, and

evaluate the effect of further deprotonation of the phenolic −OH group on L4 mediated via PPi, DFT studies were performed. DFT calculations were carried out with the Gaussian 03 program using the B3LYP functional with 631G(d,p) and LANL2DZ basis sets39a−c for chemosensor L4 and L4-Al3+ species, respectively (Figure 8).39c Subsequently, chemosensor L4 and plausible structure(s) of L4-Al3+ species were optimized. In the optimized structure [A], the Al3+ ion was found to coordinate only with the anionic aliphatic −O− group with a Al···O bond distance of 1.75 Å. The remaining coordination sites were occupied by two η2-bidentate nitrate ions (bonding range: 1.96−2.05 Å). Interestingly, the phenolic −OH group, although intact, was noted to form an intramolecular hydrogen bond (1.47 Å) with the imine −N atom. It is worth mentioning that the 1H NMR spectral results also suggested the existence of such a hydrogen bond that caused splitting of the imine −H group. We then evaluated the possible structure of L4-Al3+ species after deprotonation of the phenolic −OH group mediated via PPi, [B]. Notably, in [B], the Al3+ ion was observed to be coordinated to anionic aliphatic −O− (1.69 Å) as well as anionic phenolic −O− (1.83 Å) groups. In addition, a nitrate ion was found to coordinate G


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Figure 10. Reversibility studies of (a) L4-Al3+ and (b) L4-Ga3+ species in the presence of a proton source (dilute HCl) in EtOH (0.5% THF).

Figure 11. Naked eye color change of chemosensor L4 with different metal ions (10 equiv) under visible light: (a) as a solution in EtOH (0.5% THF); (b) as filter paper test strips.

via a bidentate η2 mode (both at 1.93 Å). As a result, the Al3+ ion displayed a tetrahedral geometry. To further understand the sensing behavior, the highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) energy band gaps were calculated. Notably, the energy gap for HOMO and LUMO orbitals of chemosensor L4 was lowered upon complexation with the Al3+ ion.40−43 The calculated HOMO−LUMO energy gap for L4 (2.28 eV) was found to considerably decrease for two different Al3+ species: [A] = 1.75 eV and [B] = 0.81 eV (Figure 9). According to these results, the electronic transitions of chemosensor L4 and its aluminum complexes, L4-Al3+, originated from the promotion of electrons from HOMO to LUMO, illustrating a n−π* nature of the transitions.41 After coordination of the Al3+ ion in [A], electrons are delocalized over the entire molecule of L4, at both the ground and excited states.44 However, the situation is quite different for [B]. Herein, frontier molecular orbitals are mainly laid on the chemosensor rather than on the Al3+ ion, where HOMO is mostly localized over a substituted phenol ring, as expected for an electron-rich dianionic molecule, while LUMO spreads over the pyrene moiety. Collectively, these findings suggest the occurrence of a strong intramolecular charge transfer (ICT) due to the electron-withdrawing ability of the Al3+ ion. In addition, the coplanarity of the substituted phenol ring to that of the pyrene ring may also have enhanced ICT due to an improvement in the conjugation.45 Therefore, an increase in the conjugation and a lowering of the HOMO−LUMO energy gap for the doubly deprotonated species, [B], explains its emission enhancement behavior.46 Reversibility Studies. Investigation of the binding of chemosensor L4 with Al3+/Ga3+ ions and subsequently with PPi asserted the necessity of the deprotonation of aliphatic −OH and phenol −OH groups during the sensing event. Such

a fact suggested that the reversibility could be achieved by the involvement of a suitable proton source.47 Indeed, reversibility for the detection of Al3+ and Ga3+ ions was achieved by the addition of dilute HCl or CH3COOH as a proton source.48 Initially, both L4-Al3+ and L4-Ga3+ species were in situ generated by adding Al3+ and Ga3+ ions to an EtOH solution of chemosensor L4, followed by the addition of dilute HCl. As expected, the initial emission intensity of L4 at 429 nm, which was enhanced upon the addition of Al3+/Ga3+ ions, was restored after the addition of dilute HCl (Figure 10). Furthermore, using the sequential addition of Al3+/Ga3+ salt and dilute HCl, reversibility cycles were developed for chemosensor L4 (upper inset; Figure 10). In fact, both metalation and demetalation processes, in the presence of Al3+/Ga3+ ions and dilute HCl, respectively, were clearly noticeable with visible color changes of colorless−purple− colorless (lower inset; Figure 10). Such an exercise illustrated a good reversibility of chemosensor L4 during the detection of Al3+ and Ga3+ ions and their subsequent removal after the addition of a proton source. The reversibility was further confirmed by both UV−vis and 1 H NMR spectral studies. In the presence of a proton source, UV−vis spectra of L4-Al3+ and L4-Ga3+ species showed “turnon” at 385 nm and “turn-off” at 282 nm (Figure S43). 1H NMR spectra of solution-generated L4-Al3+ and L4-Ga3+ species showed coordination of the M3+ ion through anionic aliphatic −O− group(s), which reappeared (H1, δ = 5.14 ppm) upon the addition of only 2 equiv of dilute HCl (Figure S44). Such a fact confirms protonation of chemosensor L4 as well as demetalation of the M3+ ion. Along these lines, conversion of a multiplet to a singlet with an upfield shift of the imine (−CHN) proton (H4, δ = 9.18 ppm) further asserted demetalation of the Al3+ ion from the L4-Al3+ species. H


Article

Inorganic Chemistry Low-Cost Detection Methods. Because chemosensor L4 was found to show the highest affinity toward Al3+ and Ga3+ ions, it was utilized for developing a few simple and costeffective detection methods, such as colorimetric,24,49 paperstrip,24,49,50 and polystyrene-film-based sensing.19b,c i. Colorimetric Detection. The standard solutions of various metal ions (10 equiv) were added to an EtOH solution of chemosensor L4 (colorless), and color changes can be easily identified by the naked eye (Figure 11a). Notably, the presence of both Al3+ and Ga3+ ions resulted in an easily detectable purple color compared to the addition of other metal ions. ii. Paper Strips. For monitoring of the Al3+ and Ga3+ ions by paper strips, Whatman filter paper strips were immersed in an EtOH solution of chemosensor L4 followed by air drying to prepare test strips.24,49,50 Such test strips were used for on-site detection by immersion into the respective solution of a desirable metal ion (Figure 11b). Similar to colorimetric studies, the presence of Al3+ and Ga3+ ions produced purple test strips. iii. Polystyrene Films. We also fabricated peelable polystyrene films containing chemosensor L4 for detection of the Al3+ and Ga3+ ions from the potentially contaminated aqueous solutions (see the Supporting Information for details).19b,c Such polymeric films were used for detection of the Al3+ and Ga3+ ions by directly dipping them into an aqueous solutions (containing 5% EtOH) of Al(NO3)3·9H2O and GaCl3. As can be seen from Figure 12, such polystyrene

by DFT studies, whereas reversibility was achieved by utilizing a proton source. Multiionic response, reversibility, and optical visualization of the present chemosensors make them ideal for practical applications for real-sample monitoring that has been illustrated by distinct visible color change and filter-paperbased test strips as well as polystyrene-film-based detection strategies.

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EXPERIMENTAL SECTION

Materials and Methods. All reagents of analytical grade were purchased and used without further purification. 2-Hydroxy-3(hydroxymethyl)methylbenzaldehyde was synthesized according to a published procedure.51 High performance liquid chromatography grade solvents were used for absorption and emission spectral measurements. All stock solutions of Cd2+, Al3+, Ga3+, Na+, K+, Ag+, Hg2+, Pd2+, Cr3+, Co2+, Ni2+, Mn2+, Mn3+, Mg2+, Ca2+, Cu+, Cu2+, Fe2+, Fe3+, Au+, Zn2+, In3+, and Pb2+ ions (2 mM) were prepared in EtOH. A stock solution of all chemosensors L1−L4 (1 mM) was prepared in tetrahydrofuran (THF). All UV−vis and fluorescence spectral experiments were performed with a 1.0-cm-path-length cuvette at 25 °C in EtOH (containing 0.5% THF). Synthesis of Chemosensors L1−L4. (E)-2-(Hydroxymethyl)-4methyl-6-[(naphthalen-1-ylimino)methyl]phenol (L1). 2-Hydroxy3-(hydroxymethyl)-5-methylbenzaldehyde (0.30 g, 1 mmol) and 1naphthylamine (0.29 g, 1 mmol) were dissolved in 30 mL of EtOH, and 3 drops of acetic acid was added to this solution. The reaction mixture was refluxed for 4 h and subsequently filtered to remove a solid material. The filtrate was evaporated under reduced pressure to obtain a pale-yellow crude product. This product was purified by column chromatography over silica using trichloromethane, followed by the removal of solvents. Yield: 0.37 g (73%). Anal. Calcd for C19H17NO2: C, 78.33; H, 5.88; N, 4.81. Found: C, 78.28; H, 5.84; N, 4.76. FTIR (cm−1): 3265 (OH), 1624 (CN). UV−vis [MeOH, λmax, nm (ε, mol−1 cm−1)]: 357 (147920). 1H NMR (400 MHz, CDCl3): δ 13.69 (s, 1H, OH), 8.63 (s, 1H), 8.26 (dd, J = 8.1 and 4.6 Hz, 1H), 7.88 (dd, J = 6.5 and 2.8 Hz, 1H), 7.78 (d, J = 8.2 Hz, 1H), 7.59−7.52 (m, 2H), 7.48 (t, J = 7.9 Hz, 1H), 7.26 (d, J = 1.5 Hz, 1H), 7.19−7.10 (m, 2H), 4.83 (s, 2H), 2.80 (s, 1H, OH), 2.33 (s, 3H). 13C NMR (DMSO-d6, 100 MHz): δ 163.65, 157.28, 146.07, 134.67, 133.55, 131.79, 128.67, 128.31, 127.09, 126.81, 126.64, 126.05, 123.29, 118.95, 114.10, 61.79, 20.51. ESI+-MS (MeOH, m/z): 292.1353 for [L1 + H]+. (E)-2-(Hydroxymethyl)-4-methyl-6-[[(naphthalen-1-ylmethyl)imino]methyl]phenol (L2). Chemosensor L2 was synthesized using an identical procedure as discussed for L1 using following reagents: 2hydroxy-3-(hydroxymethyl)-5-methylbenzaldehyde (0.30 g, 1 mmol) and 1-naphthylmethylamine (0.28 g, 1 mmol). Yield: 0.37 g (70%). Anal. Calcd for C20H19NO2: C, 78.66; H, 6.27; N, 4.59. Found: C, 78.61; H, 6.22; N, 4.53. FTIR (cm−1): 3289 (OH), 1635 (CN). UV−vis [MeOH; λmax, nm (ε, mol−1 cm−1)]: 334 (48900). 1H NMR (400 MHz, CDCl3): δ 13.74 (s, 1H, OH), 8.38 (s, 1H), 8.06 (d, J = 8.2 Hz, 1H), 7.89 (d, J = 7.9 Hz, 1H), 7.82 (d, J = 7.1 Hz, 1H), 7.54 (dt, J = 14.5 and 6.7 Hz, 2H), 7.48−7.40 (m, 2H), 7.11 (s, 1H), 6.96 (s, 1H), 5.19 (s, 2H), 4.70 (s, 2H), 2.26 (s, 3H). 13C NMR (CDCl3, 100 MHz): δ 165.83, 157.47, 136.99, 133.89, 132.58, 131.42, 130.93, 128.95, 128.52, 128.41, 127.57, 126.67, 126.24, 126.05, 125.63, 123.51, 118.24, 62.01, 60.41, 20.41. ESI+-MS (MeOH, m/z): 306.1253 for [L2 + H]+. (E)-2-[(Anthracen-2-ylimino)methyl]-6-(hydroxymethyl)-4-methylphenol (L3). Chemosensor L3 was also synthesized using a procedure identical with that discussed for L1 using the following reagents: 2-hydroxy-3-(hydroxymethyl)-5-methylbenzaldehyde (0.30 g, 1 mmol) and 2-aminoanthracene (0.35 g, 1 mmol). Yield: 0.38 g (74%). Anal. Calcd for C23H19NO2: C, 80.83; H, 6.35; N, 2.38. Found: C, 80.79; H, 6.31; N, 2.33. FTIR (cm−1): 3314 (OH), 1631 (CN). UV−vis [MeOH; λmax, nm (ε, mol−1 cm−1)]: 375 (34500). 1 H NMR (400 MHz, CDCl3): δ 13.78 (s, 1H, OH), 8.79 (s, 1H), 8.43 (s, 2H), 8.06 (d, J = 9.0 Hz, 1H), 8.03−7.97 (m, 2H), 7.83 (s, 1H), 7.49 (m, 3H), 7.21 (d, J = 9.8 Hz, 2H), 4.80 (s, 2H), 2.62 (s,

Figure 12. Images of polystyrene films containing chemosensor L4 on a glass slide under (a) visible and (b) UV regions. In all cases, part i represents a film containing only chemosensor L4, whereas parts ii and iii are identical films after treatment with an EtOH solution of Al3+ and Ga3+ ions, respectively.

films were successfully able to detect both Al3+ and Ga3+ ions by exhibiting fluorescent enhancement under UV light. Importantly, these polystyrene films were stable in aqueous solution, thus creating practical detection opportunities.19b,c Hence, it is possible to detect both Al3+ and Ga3+ ions by using chemosensor L4 in solution as well as in the solid state, and such methods can be used for practical detection applications.

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CONCLUSIONS This report has illustrated two different hydroxyl-group-based chemosensors, L1−L4, containing arene-based fluorophores for selective detection of the Al3+ and Ga3+ ions with appreciable Stokes shifts. Absorption and emission spectral titrations, supported with competitive binding studies, detection limits, binding constants, and fluorescence lifetime measurements, illustrated noteworthy detection of the Al3+ and Ga3+ ions, with L4 showing the best results. Notably, solutiongenerated [L-M3+] species further detected PPi by exhibiting emission enhancement and visible color change. The binding of Al3+/Ga3+ ions with chemosensor L4 was further supported I


Article

Inorganic Chemistry 1H, OH), 2.34 (s, 3H). 13C NMR (CDCl3, 100 MHz): δ 162.18, 157.43, 144.99, 133.43, 132.31, 131.91, 131.84, 131.73, 130.59, 129.99, 128.61, 128.36, 128.24, 128.12, 126.61, 126.49, 125.97, 125.71, 120.39, 119.13, 118.79, 61.99, 20.52. ESI+-MS (MeOH, m/z): 342.1486 for [L3 + H]+. (E)-2-(Hydroxymethyl)-4-methyl-6-[(pyren-1-ylimino)methyl]phenol (L4). Chemosensor L4 was also synthesized using a procedure identical with that discussed for L1 using the following reagents: 2hydroxy-3-(hydroxymethyl)-5-methylbenzaldehyde (0.30 g, 1 mmol) and 1-aminopyrene (0.39 g, 1 mmol). Yield: 0.39 g (80%). Anal. Calcd for C25H19NO2: C, 82.17; H, 5.24; N, 3.83. Found: C, 82.11; H, 5.21; N, 3.78. FTIR (cm−1): 3322 (OH), 1624 (CN). UV−vis [MeOH; λmax, nm (ε, mol−1 cm−1)]: 384 (147140). 1H NMR (400 MHz, DMSO-d6): δ 13.71 (s, 1H), 9.18 (s, 1H), 8.39 (dd, J = 17.3 and 8.8 Hz, 2H), 8.29 (t, J = 7.9 Hz, 3H), 8.16 (dt, J = 17.6 and 8.7 Hz, 3H), 8.07 (t, J = 7.5 Hz, 1H), 7.41 (d, J = 6.7 Hz, 2H), 5.14 (s, 1H), 4.63 (d, J = 3.8 Hz, 2H), 2.31 (s, 3H). 13C NMR (DMSO-d6, 100 MHz): δ 165.73, 155.97, 142.19, 131.78, 131.67, 131.54, 131.22, 131.36, 131.20, 129.56, 128.71, 127.90, 127.85, 127.68, 127.55, 127.22, 126.60, 126.19, 125.90, 124.95, 124.82, 124.68, 124.46, 122.18, 119.09, 116.95, 58.00, 20.77. ESI+-MS (MeOH, m/z): 366.1503 for [L4 + H]+.

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instrumental facilities including crystallographic data collection.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01550. Experimental section, figures for FTIR, NMR, mass, UV−vis, absorption, and emission spectra, binding constants, detection limits, Benesi−Hildebrand and Job’s plots, and cyclic voltammetric titrations, and tables for X-ray data collection, literature examples of chemosensors, and fluorescence lifetime parameters (PDF) Accession Codes

CCDC 1895256 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Web site: http://people. du.ac.in/~rgupta. ORCID

Rajeev Gupta: 0000-0003-2454-6705 Author Contributions ⊥

These authors have contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.G. acknowledges the Council of Scientific and Industrial Research (CSIR), New Delhi, India [Grant 01(2841)/16/ EMR-II], for financial support. V.K. thanks UGC, New Delhi, India, for the award of a Senior Research Fellowship. P.K. and S.K. thank CSIR, New Delhi, India, for their SRA “Scientists’ Pool Scheme” (IA-27577) and RA, respectively. The authors thank Dr. Sunil Yadav for his assistance in electrochemical experiments and CIF-USIC at University of Delhi for the J


Article

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