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Journal of Photochemistry & Photobiology A: Chemistry 376 (2019) 146–154

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Journal of Photochemistry & Photobiology A: Chemistry

journal homepage: www.elsevier.com/locate/jphotochem

A simple oxindole-based colorimetric HSO4¯ sensor: Naked-eye detection and spectroscopic analysis T ⁎ Sinan Bayindira, , Ferruh Lafzib a Department of Chemistry, Faculty of Sciences and Arts, Bingöl University, 12000, Bingöl, Turkey b Department of Chemistry, Faculty of Sciences, Atatürk University, 25240, Erzurum, Turkey

ARTICLE INFO ABSTRACT

Keywords: Detection of hydrogen sulfate from an aqueous organic solvent systems medium attracts to lots of interest be- Oxindole cause of could be in most environmental and biological systems. In this study, the simple receptors 5 and 6 Hydrogen sulfate containing oxindole core were synthesized, and the anion sensing properties were studied using colorimetric, Colorimetric fluorometric detection and 1H-NMR spectroscopy. The research indicated that the specific ligand affinity for 4,7-dihydroindole hydrogen sulfate ions results in drastic color and spectral changes. According to the data obtained, a new peak at 371 nm in the absorption spectrum of 5 and an increase in fluorescence intensity of 5 were observed in the Turn-on sensor ¯ presence of HSO4 ions. The binding ratio of 5 to HSO4¯ was calculated to be 1:1 according to Job's plot ex- − periments. The Ks value was found to be 1.21×105 M 1 using the Benesi-Hildebrand equation. The LOD value ¯ was calculated with value as low as 8.9 μM for HSO4 . Moreover, DFT calculations confirmed the nonplanar structures or propeller structures. As a result of all these studies, it can be said that 5 which is non-toxic, may be a

useful and selective candidate turn-on sensor for HSO4¯ sensing in the industrial wastewaters.

1. Introduction selectivity [18]. In recent years, oxindoles have received considerable attention from Ions play an essential role in many biological and chemical pro- synthetic organic chemists because of their technological properties cesses and the sensing of anions is often indispensable in environ- [19–21]. The researchers have focused on developing effective and mental, biological, and industrial research [1–3]. Therefore, the design innovative synthetic strategies because of their extraordinary biological and synthesis of chemosensors, which can selectively detect anions, are and photophysical properties [22–26]. One of these synthetic syntheses always of great interest in the research of anion sensing [4–6]. Syn- approach was developed by Bayindir and colleagues [27]. In this ap- thetic chemosensors, which selectively detect and bind some anions, proach the nucleophilic reactions of indole (2, Scheme S1) take place at especially F¯, AcO¯, HSO4¯ usually occur in hydrogen bonding units the C3 position, the nucleophilic reactions of 4,7-dihydro-1H-indole (3, such as indole, oxindole, , urea, amine, and phenol (Fig. 1 and Scheme S1) obtained from the reduction of indole take place at the C2 Scheme 1)[7–10]. In recent studies, researchers have focused on the position [28,29]. Moreover, to date, many researchers synthesized synthesis of fluorescent and colorimetric receptors that do not require oxindole-based organic ligands and investigated the ion-sensing prop- expensive instruments for anion sensing. Colorimetric sensor materials erties [30–36]. are better because the signaling can be detected by the naked eye In our previous studies, we developed a facile protocol for preparing [11,12]. For anions, the detection of hydrogen sulfate is of great in- oxindole derivatives with the addition of one or two equivalent 4,7- terest because of its widespread role in industrial and biological fields. dihydro-1H-indole (3) using isatine (1) as an electrophile followed by This compound is found in materials such as agricultural fertilizers, oxidation [27]. In this study, the obtained molecules were evaluated in nuclear fuel waste, and industrial wastage, and may have severe con- terms of their abilities in anion sensing and recognition. The skeletons sequences as a toxic pollutant when it contaminates the environment of the oxindole derivatives not only act as color-reporting groups, but [13–16]. An improved, highly selective method for detecting hydrogen also provide an acidic H-bond donor and basic H-bond acceptor moi- sulfate ions is an important goal in chemosensor studies [17]. In only a eties for ion-binding. few cases have chemosensors shown absorbance changes upon the in- troduction of hydrogen sulfate, and these have been of debatable

⁎ Corresponding author. E-mail address: [email protected] (S. Bayindir). https://doi.org/10.1016/j.jphotochem.2019.03.011 Received 13 June 2018; Received in revised form 5 March 2019; Accepted 6 March 2019 Available online 07 March 2019 1010-6030/ © 2019 Elsevier B.V. All rights reserved. S. Bayindir and F. Lafzi Journal of Photochemistry & Photobiology A: Chemistry 376 (2019) 146–154

Fig. 1. Structures of the oxindole derivatives 5 and 6.

Scheme 1. Plausible intermediates from the interaction between the receptor with hydrogen sulfate anions.

2. Experimental 2.2. Synthesis of oxindole-based probes

2.1. Material and apparatus The synthesis of 4,7-dihydro-1H-indole (3): The output compound 3 was prepared according to the literature method [10,27,37–39]. 1H-

All chemicals, reagents, and solvents were commercially available NMR (400 MHz, CDCl3): δ 7.70 (m, NH, 1 H), 6.72 (t, J = 2.5 Hz, =CH, from Sigma-Aldrich or Merck. The ethanol-4-(2-hydroxyethyl) piper- 1 H), 6.07 (t, J = 2.5 Hz, =CH, 1 H), 5.95 (bd, J = 10.1 Hz, =CH, 13 azine-1-ethanesulfonic acid (HEPES) buffer (pH range 6.8–8.2) was 1 H), 5.87 (bd, J = 10.1 Hz, =CH, 1 H), 3.30 (bs, CH2, 4 H); C-N MR prepared by dissolving 2.38 g of pure HEPES in deionized water (100 MHz, CDCl3): δ 128.0, 127.9, 125.98, 118.3, 115.9, 108.8, 27.1, (100 mL) and adding one NaOH pellet to raise the pH towards 7.4. The 26.0. pH is modulated by adding 75% HClO4 or NaOH solution. Melting point The synthesis of 3-(4,7-dihydro-1H-indol-2-yl)-3-hydroxyindolin-2-one was determined on a Buchi 539 capillary melting apparatus and are (5): Probe 5 was prepared according to the literature method [27]. 1H- uncorrected. Infrared spectra were recorded on a Mattson 1000 FT-IR NMR (400 MHz, DMSO-d6): δ 10.42 (bs, NH, 1 H), 10.21 (s, NH, 1 H), spectrophotometer. 1H NMR and 13C NMR spectra were recorded on a 7.38 (d, J =7.6 Hz, =CH, 1 H), 7.20 (t, J =7.6 Hz, =CH, 1 H), 6.97 (t, 400 (100)-MHz Varian and Bruker spectrometer and are reported in J =7.6 Hz, =CH, 1 H), 6.79 (d, J =7.6 Hz, =CH, 1 H), 7.31 (s, =CH, terms of chemical shift (δ, ppm) with SiMe4 as an internal standard. 1 H), 5.77 (m, =CH, 2 H), 5.27 (d, J = 2.5 Hz, OH, 1 H), 3.30-3.27 (m, 1 13 Data for H NMR are recorded as follows: chemical shift (δ, ppm), CH2, 2 H), 3.17-3.13 (m, CH2, 2 H); C-N MR (100 MHz, DMSO-d6): δ multiplicity (s: singlet, d: doublet, t: triplet, q: quartet, p: pentet, m: 178.3, 142.2, 133.1, 129.6, 129.4, 126.0, 125.7, 125.6, 124.2, 122.2, multiplet, bs: broad singlet, bd: broad doublet, qd: quasi doublet) and 112.3, 110.2, 105.5, 74.1, 25.1, 24.6. coupling constant (s) in Hz, integration. Elemental analyses were car- The synthesis of 1H,1”H-[2,3':3',2”-terindol]-2'(1'H)-one (6): Probe 6 ried out on a LECO CHNS-932 instrument. Column chromatography was prepared according to the literature method [27]. 1H-NMR was carried out on silica gel 60 (230–400 mesh ASTM). The reaction (400 MHz, CDCl3): δ 8.76 (bs, NH, 2 H), 8.30 (bs, NH, 1 H), 7.60 (d, J progress was monitored by thin-layer chromatography (TLC) (0.25- =7.6 Hz, =CH, 1 H), 7.52 (d, J =7.6 Hz, =CH, 2 H), 7.33-7.26 (m, mm-thick precoated silica plates: Merck Fertigplatten Kieselgel (60 =CH, 3 H), 7.20-7.13 (m, =CH, 3 H), 7.06 (t, J =7.6 Hz, =CH, 2 H), F254)). UV–vis absorption and fluorescence spectra of samples were 6.99 (d, J =7.6 Hz, =CH, 1 H), 6.42 (s, =CH, 2 H); 13 C-N MR recorded on a Shimadzu UV-3101PL UV–vis-NIR spectrometer and (100 MHz, CDCl3): δ 177.0, 140.3, 136.8, 135.1, 130.8, 129.4, 128.0, Perkin–Elmer (Model LS 55) Fluorescence Spectrophotometer, respec- 126.1, 123.6, 122.7, 120.9, 120.4, 111.4, 110.9, 102.4, 79.3. tively.

2.3. UV–vis and fluorescence studies of 5 with various anions

− The solution of oxindole derivative 5 (1 × 10 2 M) and anions

147 S. Bayindir and F. Lafzi Journal of Photochemistry & Photobiology A: Chemistry 376 (2019) 146–154

−2 (tetrabutylammonium salt, 1 × 10 M) were prepared in CH3CN/H2O (v/v: 7/3) with HEPES buffer solutions (pH 7.4). A solution of oxindole derivative 5 (5 × 10-6 M) was placed in a quartz cell and the UV–vis and fluorescence spectrums were recorded. After introduction of the solution of anions (1 equiv.), the changes in absorbance intensity were recorded at room temperature each time. As a result of pH studies, all measurements were carried out in CH3CN/H2O (v/v: 7:3) with HEPES buffer solutions (pH 7.4).

2.4. UV–vis and fluorescence titration of 5 with [Bu4N]HSO4

−2 −2 The solution of 5 (1 × 10 M) and [Bu4N]HSO4 (1 × 10 M) were prepared in CH3CN/H2O (v/v: 7/3). The concentration of 5 used in the experiments was 5 × 10-6 M. The UV–vis and fluorescence ti- tration spectra were recorded by adding corresponding concentration of

[Bu4N]HSO4 to a solution of 5 in CH3CN/H2O (v/v: 7:3) with HEPES buffer solutions (pH 7.4).

2.5. Job’s plot measurement

Probe 5 was dissolved in CH3CN/H2O (7:3/v:v) with HEPES buffer − solutions (pH 7.4) to make the concentration of 1 × 10 2 M. 5.00, 4.50, 4.00, 3.50, 3.00, 2.50, 2.00, 1.50, 1.00, 0.50 and 0.0 mL of the ligand solution were taken and transferred to vials. [Bu4N]HSO4 was dissolved in CH3CN/H2O (7:3/v:v) to make the concentration of − 1×10 2 M. 0.0, 0.50, 1.00, 1.50, 2.00, 2.50, 3.00, 3.50, 4.00, 4.50, and 5 mL of the [Bu4N]HSO4 solution were added to each ligand so- lution, and each vial had a total volume of 5 mL. After the samples were prepared, fluorescence measurements were performed at room tem- perature.

1 2.6. H-NMR titration of 5 with [Bu4N]HSO4

Increased quantities of concentrated solution of [Bu4N]HSO4 −2 (1 × 10 MinCD3CN) were added to a solution of oxindole derivative −2 5 (1 × 10 MinCD3CN). The chemical shift changes of 5 were Fig. 2. (A) UV-Vis spectrum of 5 (5 μMinCH3CN/H2O (v/v: 7/3) with HEPES monitored. buffer) with various anions. (B) Colorimetric screening of 5 (5 μMinCH3CN/

H2O (v/v: 7/3)) in the presence of 1 equiv. of anions. (C) Colorimetric screening −5 3. Results and discussion of 5 (1 × 10 MinCH3CN/H2O (v/v: 7/3 without HEPES buffer)) in the

presence of 5, 10 and 20 equiv. of [Bu4N]HSO4 (D) UV–vis spectrum of 5 (blue), ¯ 3.1. Chemistry [5-HSO4 ] complex (red and Mix (black). (E) Bar chart for absorbance response of various anions with 5 at 371 nm. The oxindole-based simple probes 5 and 6 were obtained through a synthesis scheme involving one or two easy steps as shown in CH3CN/water without HEPES buffer, the color dramatically reverted to Supplementary information (Scheme S2) [27]. Ion recognition and its original color (Fig. 2C). Structurally, 5 is the combination of one sensing have been the focus of many research groups. Indole derivatives oxindole and one 4,7-dihydro-1H-indole skeleton, supposing that it are commonly used for that purpose. To determine the interaction of possesses three H-bond donor sites, which may lead to double de-pro- indole derivatives with anions, researchers have carried out a wide tonation process in a certain condition. This was confirmed by the variety of studies using various solvent systems. Structurally, indole observation that the color changes drastically upon addition of [Bu4N] derivatives 5 and 6 could be considered a combination of two or three HSO4 on the solution of 5 in CH3CN/water (v/v: 7/3 with and without indole skeletons, having multiple hydrogen donor–acceptor groups HEPES buffer) [9]. Unfortunately, in studies performed at high con- ¯ (Fig. 1). centrations of HSO4 , the organic ligand 5 was found to be unstable. As ¯ a result of the addition of a large amount of HSO4 in the CH3CN/water 3.2. Colorimetric sensing, UV–vis and fluorescence spectral recognitions without HEPES buffer, it can be thought that this structure is degraded to the disintegrating products 4,7-dihydroindol and cores. Ac- The interaction of 5 and 6 with a wide range of anions, including cording to the experimental results, we proposed an elimination reac- ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ Cl ,F,I,Br, ClO4 ,CN, AcO and HSO4 (in the form of tetrabutyl tion mechanism as shown in Scheme S3. According to this, in the pre- ammonium salts), was studied. Unfortunately, studies on oxindole de- sence of extreme bisulfate ions, bisulfate ions act as a Lewis base and rivative 6 revealed that it has affinities toward multiple ions, which attacks the OH proton of 5. As a result of removing acidic proton of the makes it improper for chemosensor applications. Studies with the 1 hydroxyl group, 5 undergoes carbon-carbon bond cleavage to give 4,7- equivalent of the anions revealed that 5 responded selectively to an dihydroindol and isatin. In addition to observing the color change with increased concentration of hydrogen sulfate; a response characterized the naked eye, the researchers monitored the interaction of the oxindole by a distinct color change from pale yellow to purple, suggesting a red derivative 5 with the anions by using UV–vis spectroscopy (Fig. 2A and fi fi shift (Fig. 2B). This nding was con rmed through observation that the D). The maximum red shift was observed in the CH3CN-H2O solutions ¯ color changes strongly upon the addition of HSO4 . However, when 20 of the ligand-HSO4 complex at pH 7.4. The change in the electronic equivalents of [Bu4N]HSO4 were added to the solution of 5 in the

148 S. Bayindir and F. Lafzi Journal of Photochemistry & Photobiology A: Chemistry 376 (2019) 146–154 properties of 5 can also be probed by UV–vis spectroscopy whereby the UV–vis spectra of 5 at 5 μM concentration showed an absorbance band of probe 5 at 205 and 281 nm. After the addition of the 1 equivalent of the [Bu4N]HSO4 to 5 in the CH3CN/water (v/v: 7/3), the absorbance band corresponding to the bisindole chromophore underwent a hyp- sochromic shift from 205 and 281 nm to 200 and 276 nm, respectively

(Fig. 2D). Interestingly, the stepwise addition of the [Bu4N]HSO4 to 5 in the CH3CN/water of up to a 5 equivalent resulted in the formation of a new absorption band at 371 nm that reached its maximum when the 1 equivalent of the [Bu4N]HSO4 was added to the mixture and probed by titration with an increasing amount of [Bu4N]HSO4 (Figs. 2D and 3 A). To investigate the possibility that either the color changes or the new peaks resulted from the CH3CN-anion or 5-HEPES interactions, we carried out a series of control experiments with UV–vis spectroscopy. It was detected with both the naked eye and UV–vis that there was no color change in the anions in the solutions inside the CH3CN/water, and also there was no interaction of 5 with HEPES (Figure S7). The bar chart showing the response of the oxindole derivatives 5 in the presence of μ ff Fig. 4. Fluorescence spectra of 5 (5 M) in CH3CN/H2O (v/v: 7/3 with HEPES di erent anions and a mix of all the anions at a concentration of the ff λ − bu er) with various anions ( exc = 371 nm). 5.0 × 10 6 M, are shown in Fig. 2E. These results clearly indicate that the ligand selectively only binds with HSO ¯ over other anions (Fig. 2). 4 fl This coordination of HSO ¯with 5 was approved by the mass spectrum of 5 as a uorescent anion sensor (Fig. 4). Thus, to gain further insight 4 ¯ ¯ into the selective and sensitive HSO4 binding ability of 5 towards a of the solution of 5 with HSO4 as shown in Fig. S6, where the molecular ion peak of [5-HSO ¯+H+] complex could be found (calcd: 364.3710; series of anions were measured by observing the changes in their 4 fl found: 364.3920). uorescence emission spectra in CH3CN/H2O (7/3, v/v) with a HEPES ff ¯ The UV–vis titration experiments were performed to understand the bu er solution (pH 7.4). Upon the addition of small amounts of HSO4 binding rationale of 5 and the hydrogen sulfate ions (Fig. 3). With the to a solution of 5 in the CH3CN/H2O (7/3, v/v) a remarkable intensity gradual addition of [Bu N]HSO to 5 (5 μM) in CH CN/H O (v/v, 7/3) formation of the emission band was observed at 486 nm (Fig. 4). Si- 4 4 3 2 ¯ with the HEPES buffer solutions (pH 7.4), the UV–vis absorption peaked milarly, upon addition of F to a solution of 5 in the same conditions, a at 205 and at 281 nm decreased, and a new peak at 371 nm appeared. small intensity formation of emission bands was also observed at This change could be due to the formation of a hydrogen-bonded hy- 494 nm. When the amount of water in the solution (CH3CN/H2O; v/v, ¯ drogen sulfate complex with the oxindole derivative 5; the 371 nm peak 1/1) was increased, it was seen that the [5-HSO4 ] interaction peak corresponded to the hydrogen sulfate-ligand complexes. The increase of decreased and the 5-F¯ interaction peak disappeared. The analogs in- fl the peak at 371 nm began when the hydrogen sulfate concentration was vestigation in the uorescence were carried out with another series of greater than 1 equivalent, and it reached its saturation after the addi- anions. In all the situations, a very small quenching or increasing occurs tion of the 30 equivalents of the hydrogen sulfate. Interestingly, the on the addition of anions to the 5. Multiple H-bonding adduct formation ¯ between incoming guest HSO4 and host receptor 5 in such a polar experiments carried out in the in CH3CN/H2O (v/v, 7/3) without HEPES buffer, shown that decrease of the peak at 371 nm with the solvent system resulted in inhibition of C-N isomerization in the re- amount of increased [Bu N]HSO . These results showed that the ad- ceptor backbone. Thus after binding of oxindole derivative 5 with 4 4 ¯ HSO4 , receptor 5 becomes inflexible and as a result of this inhibits the dition of more than 15 equivalents of [Bu4N]HSO4 disrupted the structure of the molecule without HEPES buffer. rotational relaxation and vibrational modes of non-radiative decay. As a ‘ ’ fl UV–Vis studies have shown hydrogen bond formation between the 5 result of all of this, a turn on uorescence response at 486 nm was to HSO ¯. In addition to the UV–vis experiments, fluorescence spectro- observed. 4 fl scopy experiments were also performed in order to measure the ability After the uorescence studies of the ligand with anions, to study the sensitivity of 5 towards the bisulfate ion sensing, the fluorescence re- ¯ sponse to the interaction of 5 with the increasing HSO4 with an ex- citation at 371 nm in CH3CN/H2O (7/3, v/v) with the HEPES buffer solutions (pH 7.4) was investigated. As shown in Fig. 5, upon the ¯ progressive addition of HSO4 , the fluorescence intensity gradually in- ¯ creased. In the presence of the 15 equiv. of the HSO4 ions, the fluor- ¯ escence difference between the oxindole derivative 5 and [5-HSO4 ] was ∼22.4 times greater than that of other anions. While the first 0.25- 0.50 equiv. of the bisulfate ions induced a weak fluorescence response compared to the free probe 5, a noticeable fluorescence emission was ¯ observed after the addition of the 1 equiv. of the HSO4 ions. In the ¯ range of the 1–30 equiv. of the HSO4 , the emission intensity increased substantially, and after more additions of the 30 equivalent of the ¯ HSO4 , the rate of increase was reduced. On the other hand, one of the most important advantages of organic ligands should be photostability. ¯ For this purpose, in order to investigate the photostability of [5-HSO4 ] complex in CH3CN/H2O (7/3: v/v) with and without HEPES buffer, the relative photoluminescence intensity was monitored after continuous ¯ exposure of the [5-HSO4 ] complex to light for 1 h (Fig. S8). The pho- ¯ toluminescence intensity of [5-HSO4 ] complex with HEPES buffer was ¯ Fig. 3. UV–vis titration of 5 (5 μM) in CH3CN/H2O (v/v: 7/3 with HEPES almost unchanged, whereas that of [5-HSO4 ] complex without HEPES buffer) solution with [Bu4N]HSO4. buffer decreased remarkably from 142.74 to 123.42 arbitrary units.

149 S. Bayindir and F. Lafzi Journal of Photochemistry & Photobiology A: Chemistry 376 (2019) 146–154

The binding constant (KS) and limit of detection (LOD) values of the ¯ oxindole derivative 5 for the HSO4 ions were calculated by using the fluorescence and absorbance titration results and relevant equations. ¯ The Ks value of 5 with the HSO4 ions was obtained from the slope of the graph drawn using the data obtained from using the fluorescence titration and the following Benesi-Hildebrand Eq. (1). The fluorescence quantum yield (∅) of the probe 5 was also calculated using Parker-Rees ¯ Eq. (2) in the presence and absence of HSO4 ions. The quinine sulfate in 0.1 M H2SO4 was used as the reference solution. ∅ value of quinine sulfate in this solution is 0.54 [41,42]. As a result of studies by com- ¯ parison with a reference solution, the ∅ values of the 5 and [5-HSO4 ] were determined as 0.02 and 0.27, respectively. The increases in ∅ value in sample including hydrogen bisulfate anions approved that the ¯ HSO4 ions increase fluorescence property of the 5. 11 1 = + − n FF− 00 KFsmax()[]− FX Fmax − F 0 (1)

Fig. 5. (A) Fluorescence spectra of the 5 (5 μM) with the increasing con- η2 −ODr Ds ⎛ s ⎞⎛110− ⎞ λ ∅=∅sr⎛ ⎞⎜⎟⎜ ⎟ centration of [Bu4N]HSO4 ( exc = 371 nm). Dr η 2 110− −ODs ⎝ ⎠⎝ r ⎠⎝ ⎠ (2) More importantly, the plot of 1/F-F versus 1/[HSO ¯] was found to These results imply that the 5 in CH3CN/H2O (7/3: v/v) with HEPES 0 4 2 buffer exhibited an acceptable photostability. be linear (R = 0.9943) in this range (Fig. 7A), indicating that the 5 can ¯ fl To determine the binding stoichiometry between probe 5 and be used to determine the HSO4 concentration. Using the uorescence ¯ ¯ HSO , Job's plot experiments were performed. For this purpose, the titration data, the binding constant of 5 with HSO4 was calculated as 4 − ¯ 1.21 × 105 M 1. The LOD value was calculated with a value as low as fluorescence intensity of those mixtures of HSO4 and 5 in varying 8.9 μM for HSO ¯ (Fig. 7B). The K and LOD values of the 5 for the molar ratios (XHSO4¯ /X5; 1/9, 2/8, 3/7, 4/6, 5/5, 6/4, 7/3, 8/2 and 9/ 4 S 1) was measured and the results, which were obtained, showed that the ¯ interaction ratios of 5 with HSO4 ions were 1:1 (Fig. 6). Job's plot experiments are the most used method for understanding the stoi- chiometry of the interaction of the ligand with the probe. But, the traditional comments of Job’s plots have been limited to complex as- sociation equilibria, while little focus has been placed upon displace- ment type reactions (1:1 or 2:2), which can give Job’s plots that appear quite similar. Bühlmann and co-workers developed a novel method that allows the user to accurately distinguish between 1:1 and 2:2 complex association [40]. Another way to understand distinguish between 1:1 and 2:2 complex association is the use of mass spectroscopy. Upon ¯ understanding that the stoichiometry of the interaction of 5 and HSO4 ions from the Job’s plot experiments was 1:1, the mass spectra of the ¯ sample obtained from the overnight reaction of 1 equiv. of HSO4 ions with 1 equiv. of 5 were obtained. According to the mass spectrum, the peak (calcd: 364.3710; found: 364.3920) corresponding to the 1:1 complex formation of the ion by the ligand was observed, while the expected peak (calcd: 726.1321; not found) to the 2:2 binding was not observed (Scheme 1, Fig. S6). This result implies that the binding ¯ stoichiometry between HSO4 ions with 5 is 1:1.

Fig. 7. (A) Benesi-Hildebrand plot based on a 1:1 association stoichiometry ¯ between the 5 and HSO4 . (B) Change fluorescence intensity of the 5 with the ¯ Fig. 6. Job’s plot of the 5 with HSO4 in CH3CN/H2O (v/v: 7/3). increasing concentration of [Bu4N]HSO4 (λexc = 371 nm).

150 S. Bayindir and F. Lafzi Journal of Photochemistry & Photobiology A: Chemistry 376 (2019) 146–154

Table 1 ¯ Comparison of some HSO4 selective chemosensors.

− Ref. Binding Constant (M 1) Detection of Limit Sensing Ions

−6 [43] 2.17 × 10 M HSO4¯ 6 −7 [44] 4.13 × 10 5.50 × 10 M HSO4¯ 5 −6 3+ [45] 2.89 × 10 1.55 × 10 MCr, HSO4¯ 5 −6 2+ [46] 3.95 × 10 6.58 × 10 MHg, HSO4¯ 5 −6 This study 1.21 × 10 8.90 × 10 M HSO4¯

¯ HSO4 ions were also calculated by using the absorbance titration re- sults and relevant equations. Using the absorbance titration datas, while ¯ 4 −1 the Ks of 5 with HSO4 was calculated as 9.48 × 10 M , the LOD value was calculated as 9.2 μM (Fig S9A and S9B). The Ks and LOD values calculated using absorbance titration data were found to be close to the results obtained using fluorescence titration. A comparison of the applicability and analytical part of the present ¯ probe with some of the previous reports on the HSO4 sensors in terms of their Ks and LOD in the presence of other interfering ions was given in Table 1. It can be said that this values were acceptable values as 1 −2 Fig. 9. H NMR (400 MHz) spectra in CD3CN of the 5 (1 × 10 M) with much as the values obtained using for hydrogen bisulfate ion detection presence of [Bu N]HSO ; (a) 0 equiv. of [Bu N]HSO , (b) 1 equiv. of [Bu N] – 4 4 4 4 4 ligands in the literature [43 49]. HSO4, (c) 2 equiv. of [Bu4N]HSO4, (d) 5 equiv. of [Bu4N]HSO4, (e) 8 equiv. of

[Bu4N]HSO4, (f) 10 equiv. of [Bu4N]HSO4, (g) 15 equiv. of [Bu4N]HSO4, (h) 20 3.3. pH profiles of the probe 5 equiv. of [Bu4N]HSO4 and (i) 25 equiv. of [Bu4N]HSO4.

The effect of the different pH environment (a range of 2–12) was the 1 equivalent of the [Bu4N]HSO4 was added into a solution of 5. This studied for the practical application of the 5 (5 μM) in the absence and outcome indicated the formation of a strong hydrogen bond between ¯ presence of HSO4 (10 equiv.) and are described in Fig. 8 and S10. the hydrogen sulfate anion and the active NH groups. As the equiva- According to the studies done, the fluorescence intensity of 5 was not lents of the hydrogen sulfate anions increased, the signals of He,Hb,Hc, sensitive to a pH, except for pH = 2-4. However, upon the addition of Hd (oxindole) and the Hg of 5 showed up field shifts, whereas the signal ¯ fi – the HSO4 ion, it was identified that there was an evident increase in the of Hf (-OH) shifted down eld (Fig. 9a h). The interaction of the ligand emission bands of 5 at 486 nm between pH 2 and 9. Such broad pH with 25 equivalents of the hydrogen sulfate ions without HEPES buffer ranges in an aqueous environment can offer a great usage opportunity appeared to result in the disappearance of the Hf(OH) proton peak very useful in several applications, such as HSO4¯ detection in waste- (Fig. 9i). The disappearance of the OH peak and the formation of new water. proton resonance signals (*) are indicators that the ligand was dis- rupted under these conditions. According to the 1H-NMR results, we 3.4. Ion sensing mechanism: 1H NMR studies proposed an elimination reaction mechanism as in Scheme S3. Ac- cording to the proposed mechanism, in the presence of extreme bi- Furthermore, the mutual effect of 5 with hydrogen sulfate was sulfate ions, bisulfate ions attack the OH proton of 5, and removed the studied in detail using the 1H-NMR spectroscopic technique, and in- acidic proton of the hydroxyl group. As a result of this, 5 undergoes triguing spectral behaviors were observed as shown in Fig. 9. To obtain carbon-carbon bond cleavage to give 4,7-dihydro-1H-indole (3) and ¯1 isatin (1). The evidence above suggests that the hydrogen bonds were insight into the binding ability of the 5 with HSO4 H-NMR titration responsible for the observed chemical shifts of the NH–hydrogen sulfate experiments in CD3CN at 298 K were carried out. It was clear that the broad signal of the NH protons of the indole moieties disappeared when ion interactions. Thus, these interactions induce the polarization of the NeH bond where the partial positive charge creates a downfield shift. Consequently, the increasing electron density of the 4,7-dihydro-1H- indole ring promotes an upfield shift of the C-H (Hg) protons and especially in the C3-H proton in pyrrole moiety. There are a large number of hydrogen bond donors and acceptor groups in the structure of 5. The interaction of these groups with hy- drogen sulfate ions was seen both with the naked-eye detection of the color change and with spectroscopic studies such as UV–vis and 1H- NMR. Both the new interaction peak (Fig. 2A and D) in the UV spectrum and the upshifts or downshifts of the proton (H) signals in the NMR spectrum (Fig. 9a–g) are indicative of these interactions. It was de- ¯ termined that the stoichiometry of 5 with HSO4 was 1:1 from Job’s plots and mass spectrum. According to NMR spectrums, it was observed that the most chemical shift occurs in NH proton peaks. These results ¯ suggest that the interaction between 5 and HSO4 ions occur as shown ¯ in Scheme 1. However, it is observed that the complex [5-HSO4 ]-II, which forms a result of the direct interaction of bisulfate ions with NH groups in the structure of 5, appears to be in a highly unstable structure. On the other hand, it is seen that the complex [5-HSO ¯]-II, which is a μ μ 4 Fig. 8. Fluorescence (at 486 nm) of the 5 (5 M) and the probe 5 (5 M) + result of the interaction of bisulfate ions with NH and ketone groups in [Bu N]HSO (50 μM) at different pH (2–12), the pH is modulated by adding 4 4 the structure of 5, is more stable. Baeyer suggested that isatin exists two 75% HClO4 or NaOH solution.

151 S. Bayindir and F. Lafzi Journal of Photochemistry & Photobiology A: Chemistry 376 (2019) 146–154

¯ Fig. 10. DFT (B3LYP/6-311++G (d, p)) optimized structure of 5 (A) and [5-HSO4 ] (B).

¯ Fig. 11. Molecular orbital diagrams of 5 (A) and [5-HSO4 ] (B).

¯ ¯ tautomeric forms [50]. This information confirms that the [5-HSO4 ]-III carbonyl-O of 5, and between O-part of HSO4 ion and NH of 4,7-di- ¯ complex is present in two different tautomeric forms as [5-HSO4 ]-IIIA hydro-1H-indole side 5 (hydrogen bond length was 1.731°A and ¯ and [5-HSO4 ]-IIIB. This information explains NH shifts in isatin core 1.893°A respectively) (Fig. 10, Table S1). ¯ (Scheme 1). In addition to the geometries of 5 and its complex with HSO4 ion obtained at B3LYP/6-311++G(d,p) level were subjected to TD-DFT calculations for the study of absorption properties of 5 and its complex 3.5. Theoretical calculations ¯ with HSO4 ion. Calculated TD-DFT excitation results for 5 (320.74 nm, ¯ oscillatory strength f = 0.177) and [5-HSO4 ] (338.46 nm, f =0.090) In order to support the monitored photophysical changes, the den- are in considerable agreement with the experimental absorption re- sity functional theory (DFT) computation of 5 and its complex with ¯ ¯ corded. Furthermore, the frontier molecular orbitals of 5 and [5-HSO4 ] HSO4 ion were carried out by using the Gaussian 09 W program [51]. ¯ were examined. The highest occupied molecular orbital (HOMO) and The geometries of 5 and its complex with HSO4 ion were optimized at ¯ the lowest unoccupied molecular orbital (LUMO) of 5 and [5-HSO4 ] B3LYP/6-311++G(d,p) level of theory. The polarizable continuum was distributed mainly over the 4,7-dihydro-1H-indole side and 3-hy- ff model (PCM) is used to investigate the e ect of solvent (acetonitrile) on droxyindolin-2-one unit respectively with a band gap of 3.57 eV (probe molecule-complex interaction geometries and time-dependent density ¯ 5) and 3.51 eV ([5-HSO4 ]) (Fig. 11, Table S2). functional theory (TD-DFT). The optimized structure of probe [5- ¯ HSO4 ] indicates that the complex formed through multiple in- ¯ tramolecular hydrogen bonding between the HSO4 ions and the car- 4. Conclusion bonyl-O and 4,7-dihydro-1H-indole side-NH of 5. Relatively stronger ¯ hydrogen bonding interaction is seen between OH part of HSO4 ion and In conclusion, we have reported that the synthesis of oxindole

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