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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 detectionand spectroscopic analysis

Sinan Bayindira,⁎, Ferruh Lafzib

a Department of Chemistry, Faculty of Sciences and Arts, Bingöl University, 12000, Bingöl, TurkeybDepartment of Chemistry, Faculty of Sciences, Atatürk University, 25240, Erzurum, Turkey

A R T I C L E I N F O

Keywords:OxindoleHydrogen sulfateColorimetric4,7-dihydroindoleIndoleTurn-on sensor

A B S T R A C T

Detection of hydrogen sulfate from an aqueous organic solvent systems medium attracts to lots of interest be-cause of could be in most environmental and biological systems. In this study, the simple receptors 5 and 6containing oxindole core were synthesized, and the anion sensing properties were studied using colorimetric,fluorometric detection and 1H-NMR spectroscopy. The research indicated that the specific ligand affinity forhydrogen sulfate ions results in drastic color and spectral changes. According to the data obtained, a new peak at371 nm in the absorption spectrum of 5 and an increase in fluorescence intensity of 5 were observed in thepresence 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 valuewas calculated with value as low as 8.9 μM for HSO4

¯. Moreover, DFT calculations confirmed the nonplanarstructures or propeller structures. As a result of all these studies, it can be said that 5 which is non-toxic, may be auseful and selective candidate turn-on sensor for HSO4¯ sensing in the industrial wastewaters.

1. Introduction

Ions play an essential role in many biological and chemical pro-cesses and the sensing of anions is often indispensable in environ-mental, biological, and industrial research [1–3]. Therefore, the designand synthesis of chemosensors, which can selectively detect anions, arealways of great interest in the research of anion sensing [4–6]. Syn-thetic chemosensors, which selectively detect and bind some anions,especially F¯, AcO¯, HSO4¯ usually occur in hydrogen bonding unitssuch as indole, oxindole, pyrrole, urea, amine, and phenol (Fig. 1 andScheme 1) [7–10]. In recent studies, researchers have focused on thesynthesis of fluorescent and colorimetric receptors that do not requireexpensive instruments for anion sensing. Colorimetric sensor materialsare better because the signaling can be detected by the naked eye[11,12]. For anions, the detection of hydrogen sulfate is of great in-terest because of its widespread role in industrial and biological fields.This compound is found in materials such as agricultural fertilizers,nuclear fuel waste, and industrial wastage, and may have severe con-sequences as a toxic pollutant when it contaminates the environment[13–16]. An improved, highly selective method for detecting hydrogensulfate ions is an important goal in chemosensor studies [17]. In only afew cases have chemosensors shown absorbance changes upon the in-troduction of hydrogen sulfate, and these have been of debatable

selectivity [18].In recent years, oxindoles have received considerable attention from

synthetic organic chemists because of their technological properties[19–21]. The researchers have focused on developing effective andinnovative synthetic strategies because of their extraordinary biologicaland photophysical properties [22–26]. One of these synthetic synthesesapproach was developed by Bayindir and colleagues [27]. In this ap-proach the nucleophilic reactions of indole (2, Scheme S1) take place atthe C3 position, the nucleophilic reactions of 4,7-dihydro-1H-indole (3,Scheme S1) obtained from the reduction of indole take place at the C2position [28,29]. Moreover, to date, many researchers synthesizedoxindole-based organic ligands and investigated the ion-sensing prop-erties [30–36].

In our previous studies, we developed a facile protocol for preparingoxindole derivatives with the addition of one or two equivalent 4,7-dihydro-1H-indole (3) using isatine (1) as an electrophile followed byoxidation [27]. In this study, the obtained molecules were evaluated interms of their abilities in anion sensing and recognition. The skeletonsof the oxindole derivatives not only act as color-reporting groups, butalso provide an acidic H-bond donor and basic H-bond acceptor moi-eties for ion-binding.

https://doi.org/10.1016/j.jphotochem.2019.03.011Received 13 June 2018; Received in revised form 5 March 2019; Accepted 6 March 2019

⁎ Corresponding author.E-mail address: sbayindir@bingol.edu.tr (S. Bayindir).

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

Available online 07 March 20191010-6030/ © 2019 Elsevier B.V. All rights reserved.

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2. Experimental

2.1. Material and apparatus

All chemicals, reagents, and solvents were commercially availablefrom Sigma-Aldrich or Merck. The ethanol-4-(2-hydroxyethyl) piper-azine-1-ethanesulfonic acid (HEPES) buffer (pH range 6.8–8.2) wasprepared by dissolving 2.38 g of pure HEPES in deionized water(100mL) and adding one NaOH pellet to raise the pH towards 7.4. ThepH is modulated by adding 75% HClO4 or NaOH solution. Melting pointwas determined on a Buchi 539 capillary melting apparatus and areuncorrected. Infrared spectra were recorded on a Mattson 1000 FT-IRspectrophotometer. 1H NMR and 13C NMR spectra were recorded on a400 (100)-MHz Varian and Bruker spectrometer and are reported interms of chemical shift (δ, ppm) with SiMe4 as an internal standard.Data for 1H NMR are recorded as follows: chemical shift (δ, ppm),multiplicity (s: singlet, d: doublet, t: triplet, q: quartet, p: pentet, m:multiplet, bs: broad singlet, bd: broad doublet, qd: quasi doublet) andcoupling constant (s) in Hz, integration. Elemental analyses were car-ried out on a LECO CHNS-932 instrument. Column chromatographywas carried out on silica gel 60 (230–400 mesh ASTM). The reactionprogress was monitored by thin-layer chromatography (TLC) (0.25-mm-thick precoated silica plates: Merck Fertigplatten Kieselgel (60F254)). UV–vis absorption and fluorescence spectra of samples wererecorded on a Shimadzu UV-3101PL UV–vis-NIR spectrometer andPerkin–Elmer (Model LS 55) Fluorescence Spectrophotometer, respec-tively.

2.2. Synthesis of oxindole-based probes

The synthesis of 4,7-dihydro-1H-indole (3): The output compound 3was prepared according to the literature method [10,27,37–39]. 1H-NMR (400MHz, CDCl3): δ 7.70 (m, NH, 1 H), 6.72 (t, J =2.5 Hz, =CH,1 H), 6.07 (t, J =2.5 Hz, =CH, 1 H), 5.95 (bd, J =10.1 Hz, =CH,1 H), 5.87 (bd, J =10.1 Hz, =CH, 1 H), 3.30 (bs, CH2, 4 H); 13 C-N MR(100MHz, CDCl3): δ 128.0, 127.9, 125.98, 118.3, 115.9, 108.8, 27.1,26.0.

The synthesis of 3-(4,7-dihydro-1H-indol-2-yl)-3-hydroxyindolin-2-one(5): Probe 5 was prepared according to the literature method [27]. 1H-NMR (400MHz, DMSO-d6): δ 10.42 (bs, NH, 1 H), 10.21 (s, NH, 1 H),7.38 (d, J =7.6 Hz, =CH, 1 H), 7.20 (t, J =7.6 Hz, =CH, 1 H), 6.97 (t,J =7.6 Hz, =CH, 1 H), 6.79 (d, J =7.6 Hz, =CH, 1 H), 7.31 (s, =CH,1 H), 5.77 (m, =CH, 2 H), 5.27 (d, J =2.5 Hz, OH, 1 H), 3.30-3.27 (m,CH2, 2 H), 3.17-3.13 (m, CH2, 2 H); 13 C-N MR (100MHz, DMSO-d6): δ178.3, 142.2, 133.1, 129.6, 129.4, 126.0, 125.7, 125.6, 124.2, 122.2,112.3, 110.2, 105.5, 74.1, 25.1, 24.6.

The synthesis of 1H,1”H-[2,3':3',2”-terindol]-2'(1'H)-one (6): Probe 6was prepared according to the literature method [27]. 1H-NMR(400MHz, CDCl3): δ 8.76 (bs, NH, 2 H), 8.30 (bs, NH, 1 H), 7.60 (d, J=7.6 Hz, =CH, 1 H), 7.52 (d, J =7.6 Hz, =CH, 2 H), 7.33-7.26 (m,=CH, 3 H), 7.20-7.13 (m, =CH, 3 H), 7.06 (t, J =7.6 Hz, =CH, 2 H),6.99 (d, J =7.6 Hz, =CH, 1 H), 6.42 (s, =CH, 2 H); 13 C-N MR(100MHz, CDCl3): δ 177.0, 140.3, 136.8, 135.1, 130.8, 129.4, 128.0,126.1, 123.6, 122.7, 120.9, 120.4, 111.4, 110.9, 102.4, 79.3.

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

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

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

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

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

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

The solution of 5 (1× 10−2 M) and [Bu4N]HSO4 (1×10−2 M)were prepared in CH3CN/H2O (v/v: 7/3). The concentration of 5 usedin 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 HEPESbuffer solutions (pH 7.4).

2.5. Job’s plot measurement

Probe 5 was dissolved in CH3CN/H2O (7:3/v:v) with HEPES buffersolutions (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 0.0mL of theligand solution were taken and transferred to vials. [Bu4N]HSO4 wasdissolved in CH3CN/H2O (7:3/v:v) to make the concentration of1× 10−2 M. 0.0, 0.50, 1.00, 1.50, 2.00, 2.50, 3.00, 3.50, 4.00, 4.50,and 5mL of the [Bu4N]HSO4 solution were added to each ligand so-lution, and each vial had a total volume of 5mL. After the samples wereprepared, fluorescence measurements were performed at room tem-perature.

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

Increased quantities of concentrated solution of [Bu4N]HSO4

(1× 10−2 M in CD3CN) were added to a solution of oxindole derivative5 (1× 10−2 M in CD3CN). The chemical shift changes of 5 weremonitored.

3. Results and discussion

3.1. Chemistry

The oxindole-based simple probes 5 and 6 were obtained through asynthesis scheme involving one or two easy steps as shown inSupplementary information (Scheme S2) [27]. Ion recognition andsensing have been the focus of many research groups. Indole derivativesare commonly used for that purpose. To determine the interaction ofindole derivatives with anions, researchers have carried out a widevariety of studies using various solvent systems. Structurally, indolederivatives 5 and 6 could be considered a combination of two or threeindole skeletons, having multiple hydrogen donor–acceptor groups(Fig. 1).

3.2. Colorimetric sensing, UV–vis and fluorescence spectral recognitions

The interaction of 5 and 6 with a wide range of anions, includingCl¯, F¯, I¯, Br¯, ClO4

¯, CN¯, AcO¯and HSO4¯ (in the form of tetrabutyl

ammonium salts), was studied. Unfortunately, studies on oxindole de-rivative 6 revealed that it has affinities toward multiple ions, whichmakes it improper for chemosensor applications. Studies with the 1equivalent of the anions revealed that 5 responded selectively to anincreased concentration of hydrogen sulfate; a response characterizedby a distinct color change from pale yellow to purple, suggesting a redshift (Fig. 2B). This finding was confirmed through observation that thecolor changes strongly upon the addition of HSO4

¯. However, when 20equivalents of [Bu4N]HSO4 were added to the solution of 5 in the

CH3CN/water without HEPES buffer, the color dramatically reverted toits original color (Fig. 2C). Structurally, 5 is the combination of oneoxindole and one 4,7-dihydro-1H-indole skeleton, supposing that itpossesses three H-bond donor sites, which may lead to double de-pro-tonation process in a certain condition. This was confirmed by theobservation that the color changes drastically upon addition of [Bu4N]HSO4 on the solution of 5 in CH3CN/water (v/v: 7/3 with and withoutHEPES buffer) [9]. Unfortunately, in studies performed at high con-centrations of HSO4

¯, the organic ligand 5 was found to be unstable. Asa result of the addition of a large amount of HSO4

¯ in the CH3CN/waterwithout HEPES buffer, it can be thought that this structure is degradedto the disintegrating products 4,7-dihydroindol and isatin cores. Ac-cording to the experimental results, we proposed an elimination reac-tion mechanism as shown in Scheme S3. According to this, in the pre-sence of extreme bisulfate ions, bisulfate ions act as a Lewis base andattacks the OH proton of 5. As a result of removing acidic proton of thehydroxyl group, 5 undergoes carbon-carbon bond cleavage to give 4,7-dihydroindol and isatin. In addition to observing the color change withthe naked eye, the researchers monitored the interaction of the oxindolederivative 5 with the anions by using UV–vis spectroscopy (Fig. 2A andD). The maximum red shift was observed in the CH3CN-H2O solutionsof the ligand-HSO4 complex at pH 7.4. The change in the electronic

Fig. 2. (A) UV-Vis spectrum of 5 (5 μM in CH3CN/H2O (v/v: 7/3) with HEPESbuffer) with various anions. (B) Colorimetric screening of 5 (5 μM in CH3CN/H2O (v/v: 7/3)) in the presence of 1 equiv. of anions. (C) Colorimetric screeningof 5 (1×10−5 M in CH3CN/H2O (v/v: 7/3 without HEPES buffer)) in thepresence of 5, 10 and 20 equiv. of [Bu4N]HSO4 (D) UV–vis spectrum of 5 (blue),[5-HSO4

¯] complex (red and Mix (black). (E) Bar chart for absorbance responseof various anions with 5 at 371 nm.

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properties of 5 can also be probed by UV–vis spectroscopy whereby theUV–vis spectra of 5 at 5 μM concentration showed an absorbance bandof probe 5 at 205 and 281 nm. After the addition of the 1 equivalent ofthe [Bu4N]HSO4 to 5 in the CH3CN/water (v/v: 7/3), the absorbanceband 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 inthe CH3CN/water of up to a 5 equivalent resulted in the formation of anew absorption band at 371 nm that reached its maximum when the 1equivalent of the [Bu4N]HSO4 was added to the mixture and probed bytitration with an increasing amount of [Bu4N]HSO4 (Figs. 2D and 3 A).To investigate the possibility that either the color changes or the newpeaks resulted from the CH3CN-anion or 5-HEPES interactions, wecarried out a series of control experiments with UV–vis spectroscopy. Itwas detected with both the naked eye and UV–vis that there was nocolor change in the anions in the solutions inside the CH3CN/water, andalso there was no interaction of 5with HEPES (Figure S7). The bar chartshowing the response of the oxindole derivatives 5 in the presence ofdifferent anions and a mix of all the anions at a concentration of the5.0×10−6 M, are shown in Fig. 2E. These results clearly indicate thatthe ligand selectively only binds with HSO4

¯ over other anions (Fig. 2).This coordination of HSO4

¯with 5 was approved by the mass spectrumof the solution of 5 with HSO4

¯ as shown in Fig. S6, where the molecularion peak of [5-HSO4

¯+H+] complex could be found (calcd: 364.3710;found: 364.3920).

The UV–vis titration experiments were performed to understand thebinding rationale of 5 and the hydrogen sulfate ions (Fig. 3). With thegradual addition of [Bu4N]HSO4 to 5 (5 μM) in CH3CN/H2O (v/v, 7/3)with the HEPES buffer solutions (pH 7.4), the UV–vis absorption peakedat 205 and at 281 nm decreased, and a new peak at 371 nm appeared.This change could be due to the formation of a hydrogen-bonded hy-drogen sulfate complex with the oxindole derivative 5; the 371 nm peakcorresponded to the hydrogen sulfate-ligand complexes. The increase ofthe peak at 371 nm began when the hydrogen sulfate concentration wasgreater than 1 equivalent, and it reached its saturation after the addi-tion of the 30 equivalents of the hydrogen sulfate. Interestingly, theexperiments carried out in the in CH3CN/H2O (v/v, 7/3) withoutHEPES buffer, shown that decrease of the peak at 371 nm with theamount of increased [Bu4N]HSO4. These results showed that the ad-dition of more than 15 equivalents of [Bu4N]HSO4 disrupted thestructure of the molecule without HEPES buffer.

UV–Vis studies have shown hydrogen bond formation between the 5to HSO4

¯. In addition to the UV–vis experiments, fluorescence spectro-scopy experiments were also performed in order to measure the ability

of 5 as a fluorescent anion sensor (Fig. 4). Thus, to gain further insightinto the selective and sensitive HSO4

¯ binding ability of 5 towards aseries of anions were measured by observing the changes in theirfluorescence emission spectra in CH3CN/H2O (7/3, v/v) with a HEPESbuffer solution (pH 7.4). Upon the addition of small amounts of HSO4

¯

to a solution of 5 in the CH3CN/H2O (7/3, v/v) a remarkable intensityformation of the emission band was observed at 486 nm (Fig. 4). Si-milarly, upon addition of F¯ to a solution of 5 in the same conditions, asmall intensity formation of emission bands was also observed at494 nm. When the amount of water in the solution (CH3CN/H2O; v/v,1/1) was increased, it was seen that the [5-HSO4

¯] interaction peakdecreased and the 5-F¯ interaction peak disappeared. The analogs in-vestigation in the fluorescence were carried out with another series ofanions. In all the situations, a very small quenching or increasing occurson the addition of anions to the 5. Multiple H-bonding adduct formationbetween incoming guest HSO4

¯ and host receptor 5 in such a polarsolvent system resulted in inhibition of C-N isomerization in the re-ceptor backbone. Thus after binding of oxindole derivative 5 withHSO4

¯, receptor 5 becomes inflexible and as a result of this inhibits therotational relaxation and vibrational modes of non-radiative decay. As aresult of all of this, a ‘turn on’ fluorescence response at 486 nm wasobserved.

After the fluorescence studies of the ligand with anions, to study thesensitivity 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 buffersolutions (pH 7.4) was investigated. As shown in Fig. 5, upon theprogressive 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 responsecompared to the free probe 5, a noticeable fluorescence emission wasobserved after the addition of the 1 equiv. of the HSO4

¯ ions. In therange of the 1–30 equiv. of the HSO4

¯, the emission intensity increasedsubstantially, and after more additions of the 30 equivalent of theHSO4

¯, the rate of increase was reduced. On the other hand, one of themost 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, therelative photoluminescence intensity was monitored after continuousexposure of the [5-HSO4

¯] complex to light for 1 h (Fig. S8). The pho-toluminescence intensity of [5-HSO4

¯] complex with HEPES buffer wasalmost unchanged, whereas that of [5-HSO4

¯] complex without HEPESbuffer decreased remarkably from 142.74 to 123.42 arbitrary units.

Fig. 3. UV–vis titration of 5 (5 μM) in CH3CN/H2O (v/v: 7/3 with HEPESbuffer) solution with [Bu4N]HSO4.

Fig. 4. Fluorescence spectra of 5 (5 μM) in CH3CN/H2O (v/v: 7/3 with HEPESbuffer) with various anions (λexc= 371 nm).

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These results imply that the 5 in CH3CN/H2O (7/3: v/v) with HEPESbuffer exhibited an acceptable photostability.

To determine the binding stoichiometry between probe 5 andHSO4

¯, Job's plot experiments were performed. For this purpose, thefluorescence intensity of those mixtures of HSO4

¯ and 5 in varyingmolar ratios (XHSO4¯ /X5; 1/9, 2/8, 3/7, 4/6, 5/5, 6/4, 7/3, 8/2 and 9/1) was measured and the results, which were obtained, showed that theinteraction ratios of 5 with HSO4

¯ ions were 1:1 (Fig. 6). Job's plotexperiments are the most used method for understanding the stoi-chiometry of the interaction of the ligand with the probe. But, thetraditional 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 appearquite similar. Bühlmann and co-workers developed a novel method thatallows the user to accurately distinguish between 1:1 and 2:2 complexassociation [40]. Another way to understand distinguish between 1:1and 2:2 complex association is the use of mass spectroscopy. Uponunderstanding that the stoichiometry of the interaction of 5 and HSO4

¯

ions from the Job’s plot experiments was 1:1, the mass spectra of thesample obtained from the overnight reaction of 1 equiv. of HSO4

¯ ionswith 1 equiv. of 5 were obtained. According to the mass spectrum, thepeak (calcd: 364.3710; found: 364.3920) corresponding to the 1:1complex formation of the ion by the ligand was observed, while theexpected peak (calcd: 726.1321; not found) to the 2:2 binding was notobserved (Scheme 1, Fig. S6). This result implies that the bindingstoichiometry between HSO4

¯ ions with 5 is 1:1.

The binding constant (KS) and limit of detection (LOD) values of theoxindole derivative 5 for the HSO4

¯ ions were calculated by using thefluorescence and absorbance titration results and relevant equations.The Ks value of 5 with the HSO4

¯ ions was obtained from the slope ofthe graph drawn using the data obtained from using the fluorescencetitration and the following Benesi-Hildebrand Eq. (1). The fluorescencequantum yield (∅) of the probe 5 was also calculated using Parker-ReesEq. (2) in the presence and absence of HSO4

¯ ions. The quinine sulfate in0.1 M H2SO4 was used as the reference solution. ∅ value of quininesulfate 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 theHSO4

¯ ions increase fluorescence property of the 5.

−=

−+

−−F F K F F X F F1 1

( )[ ]1

s maxn

max0 0 0 (1)

⎜ ⎟⎜ ⎟∅ = ∅ ⎛⎝

⎞⎠

⎛

⎝

⎞

⎠⎛⎝

−−

⎞⎠

−

−s r DsDr

ηη

1 101 10

s

r

ODr

ODs

2

2(2)

More importantly, the plot of 1/F-F0 versus 1/[HSO4¯] was found to

be linear (R2= 0.9943) in this range (Fig. 7A), indicating that the 5 canbe used to determine the HSO4

¯ concentration. Using the fluorescencetitration data, the binding constant of 5 with HSO4

¯ was calculated as1.21×105 M−1. The LOD value was calculated with a value as low as8.9 μM for HSO4

¯ (Fig. 7B). The KS and LOD values of the 5 for the

Fig. 5. (A) Fluorescence spectra of the 5 (5 μM) with the increasing con-centration of [Bu4N]HSO4 (λexc= 371 nm).

Fig. 6. Job’s plot of the 5 with HSO4¯ in CH3CN/H2O (v/v: 7/3).

Fig. 7. (A) Benesi-Hildebrand plot based on a 1:1 association stoichiometrybetween the 5 and HSO4

¯. (B) Change fluorescence intensity of the 5 with theincreasing concentration of [Bu4N]HSO4 (λexc= 371 nm).

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HSO4¯ ions were also calculated by using the absorbance titration re-

sults and relevant equations. Using the absorbance titration datas, whilethe Ks of 5 with HSO4

¯ was calculated as 9.48×104 M−1, the LODvalue was calculated as 9.2 μM (Fig S9A and S9B). The Ks and LODvalues calculated using absorbance titration data were found to be closeto the results obtained using fluorescence titration.

A comparison of the applicability and analytical part of the presentprobe with some of the previous reports on the HSO4

¯ sensors in termsof their Ks and LOD in the presence of other interfering ions was givenin Table 1. It can be said that this values were acceptable values asmuch as the values obtained using for hydrogen bisulfate ion detectionligands in the literature [43–49].

3.3. pH profiles of the probe 5

The effect of the different pH environment (a range of 2–12) wasstudied for the practical application of the 5 (5 μM) in the absence andpresence of HSO4

¯ (10 equiv.) and are described in Fig. 8 and S10.According to the studies done, the fluorescence intensity of 5 was notsensitive to a pH, except for pH=2-4. However, upon the addition ofthe HSO4

¯ ion, it was identified that there was an evident increase in theemission bands of 5 at 486 nm between pH 2 and 9. Such broad pHranges in an aqueous environment can offer a great usage opportunityvery useful in several applications, such as HSO4¯ detection in waste-water.

3.4. Ion sensing mechanism: 1H NMR studies

Furthermore, the mutual effect of 5 with hydrogen sulfate wasstudied in detail using the 1H-NMR spectroscopic technique, and in-triguing spectral behaviors were observed as shown in Fig. 9. To obtaininsight into the binding ability of the 5 with HSO4

¯ 1H-NMR titrationexperiments in CD3CN at 298 K were carried out. It was clear that thebroad signal of the NH protons of the indole moieties disappeared when

the 1 equivalent of the [Bu4N]HSO4 was added into a solution of 5. Thisoutcome indicated the formation of a strong hydrogen bond betweenthe hydrogen sulfate anion and the active NH groups. As the equiva-lents of the hydrogen sulfate anions increased, the signals of He, Hb, Hc,Hd (oxindole) and the Hg of 5 showed up field shifts, whereas the signalof Hf (-OH) shifted downfield (Fig. 9a–h). The interaction of the ligandwith 25 equivalents of the hydrogen sulfate ions without HEPES bufferappeared to result in the disappearance of the Hf (OH) proton peak(Fig. 9i). The disappearance of the OH peak and the formation of newproton resonance signals (*) are indicators that the ligand was dis-rupted under these conditions. According to the 1H-NMR results, weproposed an elimination reaction mechanism as in Scheme S3. Ac-cording to the proposed mechanism, in the presence of extreme bi-sulfate ions, bisulfate ions attack the OH proton of 5, and removed theacidic proton of the hydroxyl group. As a result of this, 5 undergoescarbon-carbon bond cleavage to give 4,7-dihydro-1H-indole (3) andisatin (1). The evidence above suggests that the hydrogen bonds wereresponsible for the observed chemical shifts of the NH–hydrogen sulfateion interactions. Thus, these interactions induce the polarization of theNeH 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 andespecially in the C3-H proton in pyrrole moiety.

There are a large number of hydrogen bond donors and acceptorgroups in the structure of 5. The interaction of these groups with hy-drogen sulfate ions was seen both with the naked-eye detection of thecolor change and with spectroscopic studies such as UV–vis and 1H-NMR. Both the new interaction peak (Fig. 2A and D) in the UV spectrumand the upshifts or downshifts of the proton (H) signals in the NMRspectrum (Fig. 9a–g) are indicative of these interactions. It was de-termined that the stoichiometry of 5 with HSO4

¯ was 1:1 from Job’splots and mass spectrum. According to NMR spectrums, it was observedthat the most chemical shift occurs in NH proton peaks. These resultssuggest that the interaction between 5 and HSO4

¯ ions occur as shownin Scheme 1. However, it is observed that the complex [5-HSO4

¯]-II,which forms a result of the direct interaction of bisulfate ions with NHgroups in the structure of 5, appears to be in a highly unstable structure.On the other hand, it is seen that the complex [5-HSO4

¯]-II, which is aresult of the interaction of bisulfate ions with NH and ketone groups inthe structure of 5, is more stable. Baeyer suggested that isatin exists two

Table 1Comparison of some HSO4

¯ selective chemosensors.

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

[43] 2.17× 10−6 M HSO4¯[44] 4.13× 106 5.50× 10−7 M HSO4¯[45] 2.89× 105 1.55× 10−6 M Cr3+, HSO4¯[46] 3.95× 105 6.58× 10−6 M Hg2+, HSO4¯This study 1.21× 105 8.90× 10−6 M HSO4¯

Fig. 8. Fluorescence (at 486 nm) of the 5 (5 μM) and the probe 5 (5 μM) +[Bu4N]HSO4 (50 μM) at different pH (2–12), the pH is modulated by adding75% HClO4 or NaOH solution.

Fig. 9. 1H NMR (400MHz) spectra in CD3CN of the 5 (1× 10−2 M) withpresence of [Bu4N]HSO4; (a) 0 equiv. of [Bu4N]HSO4, (b) 1 equiv. of [Bu4N]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) 20equiv. of [Bu4N]HSO4 and (i) 25 equiv. of [Bu4N]HSO4.

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tautomeric forms [50]. This information confirms that the [5-HSO4¯]-III

complex is present in two different tautomeric forms as [5-HSO4¯]-IIIA

and [5-HSO4¯]-IIIB. This information explains NH shifts in isatin core

(Scheme 1).

3.5. Theoretical calculations

In order to support the monitored photophysical changes, the den-sity functional theory (DFT) computation of 5 and its complex withHSO4

¯ ion were carried out by using the Gaussian 09W program [51].The geometries of 5 and its complex with HSO4

¯ ion were optimized atB3LYP/6-311++G(d,p) level of theory. The polarizable continuummodel (PCM) is used to investigate the effect of solvent (acetonitrile) onmolecule-complex interaction geometries and time-dependent densityfunctional 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-bonyl-O and 4,7-dihydro-1H-indole side-NH of 5. Relatively strongerhydrogen bonding interaction is seen between OH part of HSO4

¯ ion and

carbonyl-O of 5, and between O-part of HSO4¯ ion and NH of 4,7-di-

hydro-1H-indole side 5 (hydrogen bond length was 1.731°A and1.893°A respectively) (Fig. 10, Table S1).

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-DFTcalculations for the study of absorption properties of 5 and its complexwith 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)are in considerable agreement with the experimental absorption re-corded. Furthermore, the frontier molecular orbitals of 5 and [5-HSO4

¯]were examined. The highest occupied molecular orbital (HOMO) andthe lowest unoccupied molecular orbital (LUMO) of 5 and [5-HSO4

¯]was distributed mainly over the 4,7-dihydro-1H-indole side and 3-hy-droxyindolin-2-one unit respectively with a band gap of 3.57 eV (probe5) and 3.51 eV ([5-HSO4

¯]) (Fig. 11, Table S2).

4. Conclusion

In conclusion, we have reported that the synthesis of oxindole

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).

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derivatives 5 and 6 from the reaction of 4,7-dihydroindol (3) with isatin(1). In addition to synthesis, we carried out studies concerning thechemosensor properties of these oxindole derivatives. Oxindole deri-vative 5 showed hydrogen sulfate anion sensing capabilities via naked-eye detection of color changes, absorption and fluorescence signals. Theabsorption slider and fluorescence intensity increasing effects of theHSO4

¯ ions were in a concentration dependent manner. The interactionratio of oxindole 5 with HSO4

¯ was found by Job’s method as 1:1. UsingBenesi-Hildebrand equation, the binding constant of HSO4

¯ to 5 wasfound as 1.21× 105 M−1. LOD value were calculated as 8.9 μM. Inaddition to analytical studies, the interaction mechanism of 5 withHSO4¯ was also proposed by using 1H-NMR technique. Moreover, DFTcalculations 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¯ sen-sing in the industrial wastewaters.

Acknowledgement

Author is indebted to Department of Chemistry at Bingol University(BAP-209-324-2015) for their financial support for this study.

Appendix A. Supplementary data

Supplementary material related to this article can be found, in theonline version, at doi:https://doi.org/10.1016/j.jphotochem.2019.03.011.

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