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Electronic Supplementary Information

A “Turn-on” Fluorescent Probe for the Detection of Permanganate in Aqueous Media

Genggongwo Shi, Mushtaq Ahmed Shahid, Muhammad Yousuf, Farzana Mahmood, Lubna Rasheed,* Christopher W. Bielawski, and Kwang S. Kim

Table of Contents

1. Additional Experimental Details

2. NMR Spectra

3. Fluorescent Quantum Yield Determination

4. NMR Studies

5. Absorption Studies

6. Electrochemical Studies

7. Calculations

8. References

Electronic Supplementary Material (ESI) for ChemComm.This journal is © The Royal Society of Chemistry 2019

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1. Experimental

All common reagents and solvents were purchased from commercial sources and used as received. Characterization was assisted through the use of 1H and 13C NMR spectroscopy (400 MHz FT-NMR Model 400-MR DD2 at 298 K) and low-resolution mass spectrometry (Bruker 1200 Series & HCT Basic System). Fluorescence studies were carried out on Shimadzu RF-5301 PC spectrofluorophotometer at 298 K. UV-Vis spectra were recorded using a Scinco S-3100 Photodiode Array UV-Vis spectrophotometer at 298 K. Electrochemical studies were carried out on BioLogic VSP Electrochemical Workstation.

N

COOHS

N

ONH

N

N

SN

N

IOHC

N

COOHOHC

N

I

2

DMF, POCl3120 C

CO, Pd(OAc)2, DMF, H2O

K2CO3, 60 C

SH

NH2

DMSO160 C

EDCI, NH2N

4-DMAP, DMF, RT

3 4

5P1

Scheme 1. Synthesis of P1.

Synthesis of 3-iodo-9-methylcarbazole (2). Iodomethane (0.75 mL, 12 mmol) was slowly added to a solution of 3-iodocarbazole (1, 2.93 g, 10.0 mmol) and potassium tert-butoxide (1.68 g, 15.0 mmol) in THF at 60 °C. After stirring the mixture at 60 °C for 1 h, the residual solvent was evaporated under reduced pressure. The addition of water facilitated the precipitation of solids, which were subsequently collected via filtration and washed with cold methanol. Yield: >99%. Spectroscopic data agreed with literature values.[1] 1H NMR (CDCl3, 400 MHz): 8.39 (s, 1H), 8.02-8.04 (dd, 1H), 7.70-7.73 (dd, 1H) 7.48-7.52 (t, 1H), 7.38-7.40 (d, 1H), 7.23-7.27 (t, 1H), 7.18-7.20 (d, 1H), 3.83 (s, 3H, -CH3).

Synthesis of 3-formyl-6-iodo-9-methylcarbazole (3). POCl3 (1.82 mL, 20.0 mmol) was slowly added to a solution of 2 (3.07 g, 10.0 mmol) in DMF (7.25 mL) at 0 °C. The resulting mixture was heated to 120 °C and then stirred for 3 h before being quenched with chilled water followed by extraction with EtOAc. The combined organic layers were washed with water followed by an aqueous solution saturated with Na2CO3, and then dried over anhydrous Na2SO4. The residue was subsequently purified by silica gel chromatography (eluent: hexane/EtOAc = 2/1 v/v) to afford a pale-yellow powder. Yield: 2.07 g, 62%. 1H NMR (CDCl3, 400 MHz): δ 10.09 (s, 1H, -CHO), 8.54 (s, 1H), 8.45 (s, 1H), 8.04-8.06 (d, 1H), 7.78-7.81 (d, 1H), 7.46-7.49 (d, 1H), 7.23-7.25 (d, 1H), 3.88 (s, 3H, -CH3). 13C NMR (CDCl3, 125.7 MHz): δ 191.5, 144.2, 140.8, 135.0, 129.5, 129.0,

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127.5, 125.2, 124.1, 121.6, 111.1, 109.0, 83.0, 29.5. LRMS (ESI+, acetone): m/z calcd. for [M+H]+ 358, found 358.

Synthesis of 6-formyl-9-methylcarbazole-3-carboxylic acid (4). Compound 3 (1.99 g, 5.94 mmol), K2CO3 (3.28 g, 23.7 mmol) and Pd(OAc)2 (13.3 mg, 59.4 μmol) were suspended in a mixture of 1 mL of DMF and 1 mL of water. The atmosphere was changed from air to nitrogen using a Schlenk line and then CO (from a balloon) was introduced. The resulting suspension was then stirred at 60 °C overnight. The insoluble materials that formed were removed by filtration and washed with an aqueous solution of NaOH (0.1 M). The filtrate was then combined with the mother liquor and then acidified to pH 1 with concentrated HCl. The precipitate that formed was collected via filtration, successively washed with 0.1 M HCl and water, and dried under vacuum to afford a grey solid. Yield: 1.45 g, 96%. 1H NMR (d6-DMSO, 400 MHz): δ 12.74 (bs, 1H, -COOH), 10.07 (s, 1H, -CHO), 8.89 (s, 1H), 8.88 (s, 1H), 8.11-8.14 (d, 1H), 8.03-8.05 (d, 1H), 7.80-7.82 (d, 1H), 7.73-7.76 (d, 1H), 3.97 (s, 3H, -CH3). 13C NMR (d6-DMSO, 125.7 MHz): δ 192.4, 168.2, 145.2, 144.4, 129.4, 128.3, 127.2, 125.1, 123.2, 123.0, 122.6, 122.4, 110.7, 110.2, 30.1. LRMS (ESI+, acetone): m/z calcd. for [M+Na]+ 276, found 276.

Synthesis of 6-(2-benzothiazolyl)-9-methylcarbazole-3-carboxylic acid (5). Compound 4 (127 mg, 0.500 mmol) and 2-aminobenzenethiol (57.0 μL, 0.750 mmol) were dissolved in 3 mL of DMSO, and the resulting mixture was stirred at 160 °C for 3 h. After cooling the mixture to ambient temperature, water was added. A precipitate formed, which was collected by filtration and successively washed with water and anhydrous ethanol. The solid was dried under vacuum to afford a green powder. Yield: >99%. 1H NMR (d6-DMSO, 400 MHz): δ 12.69 (bs, 1H, -COOH), 9.01-9.04 (dd, 2H), 8.29-8.32 (dd, 1H), 8.11-8.15 (t, 1H), 8.02-8.04 (d, 1H), 7.81-7.83 (d, 1H), 7.72-7.74 (d, 1H), 7.51-7.55 (t, 1H), 7.41-7.45 (t, 1H), 3.98 (s, 3H, -CH3). 13C NMR (d6-DMSO, 125.7 MHz): δ 168.8, 154.2, 144.3, 143.4, 127.0, 125.5, 125.3, 123.6, 122.8, 122.6, 122.5, 122.1, 120.8, 110.9, 110.0, 101.9, 30.0. LRMS (ESI+, acetone): m/z calcd. for [M+H]+ 359, found 359.

Synthesis of 6-(benzo[d]thiazol-2-yl)-N-(4-(dimethylamino)phenyl)-9-methyl-carbazole-3-carboxamide (P1). Compound 5 (89.6 mg, 0.250 mmol) was dissolved in 2 mL of DMF. To the solution was added N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride (EDCI) (71.9 mg, 0.375 mmol) and 4-dimethylaminopyridine (45.8 mg, 0.375 mmol). After stirring the resulting mixture at room temperature for 10 min, 4-(dimethylamino)aniline (31.4 mg, 0.250 mmol) was added. The mixture was stirred for 22 h, quenched with excess water and then the pH was adjusted to 2 with concentrated HCl. The precipitate was collected by centrifugation and successively washed with 0.01 M HCl, water, anhydrous ethanol and EtOAc to afford a green powder. Yield: 20.0 mg, 17%. 1H NMR (d6-acetone, 400 MHz): δ 9.44 (bs, 1H, -CONH-), 9.04 (d, 1H), 9.00 (d, 1H), 8.32-8.34 (dd, 1H), 8.25-8.27 (dd, 1H), 8.08-8.10 (d, 1H), 8.04-8.06 (d, 1H), 7.71-7.80 (m, 4H), 7.53-7.57 (t, 1H), 7.42-7.46 (t, 1H), 6.79-6.81 (d, 2H), 4.07 (s, 3H, -CH3), 2.94 (s, 6H, -CH3). 1H NMR (d6-DMSO, 400 MHz): δ 10.01 (bs, 1H, -CONH-), 9.07 (d, 1H), 9.00 (d, 1H), 8.24-8.27 (dd, 1H), 8.13-8.17 (t, 1H), 8.02-8.04 (d, 1H), 7.81-7.83 (d, 1H), 7.74-7.77 (d, 1H), 7.63-7.65 (t, 2H), 7.51-7.55 (t, 1H), 7.41-7.45 (t, 1H), 6.77-6.79 (d, 2H), 3.99 (s, 3H, -CH3), 2.88 (s, 6H, -CH3). 13C NMR (d6-DMSO, 125.7 MHz): δ 168.7, 165.4, 154.2, 143.4, 134.8, 127.0, 126.9, 126.0, 125.5, 125.0, 123.2, 122.8, 122.7, 122.3, 122.0, 120.1, 110.9, 109.9, 30.0. LRMS (ESI+, acetone): m/z calcd. for [M+Na]+ 499, found 499; calcd. for [M+H]+ 477, found 477.

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2. NMR Spectra

Figure S1-1. 1H NMR spectrum of P1 in d6-acetone.

Figure S1-2. 13C NMR spectrum of P1 in d6-acetone.

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Figure S1-3. 1H NMR spectrum of 3 in CDCl3.

Figure S1-4. 13C NMR spectrum of 3 in CDCl3.

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Figure S1-5. 1H NMR spectrum of 4 in d6-DMSO.

Figure S1-6. 13C NMR spectrum of 4 in d6-DMSO.

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Figure S1-7. 1H NMR spectrum of 5 in d6-DMSO.

Figure S1-8. 13C NMR spectrum of 5 in d6-DMSO.

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Figure S1-9. 13C NMR spectrum of P1 in d6-DMSO.

3. Fluorescent Studies and Quantum Yield Determination

Considering the solubility of P1 as well as the oxidizing potential of MnO4‾, stock solutions of P1 (10 mM) were independently prepared in aqueous H3PO4 and then diluted to 10 μM in 10 mM phosphate buffer at pH 7.4. Aliquots of 10 mM potassium permanganate, sodium hypochlorite, sodium chlorite, potassium chlorate, sodium perchlorate, sodium metabisulfite, potassium persulfate, potassium dichromate, sodium periodate, or sodium nitrite in water were then injected into the sample. For other species, the injection times were set to literature values.[2, 3] The solutions were vigorously shaken after each addition and then analyzed after a period of time. For the kinetic studies, samples were kept inside fluorescence cuvettes and a programmed procedure was used. Scanning was terminated after the change in fluorescence intensity became insignificant (saturated). The rate constants k were derived by plotting (Ffin-Fobs)/(Ffin-Fini), where Ffin: fluorescence intensity at saturated point; Fobs: observed; and Fini: initial (pure probe)) versus reaction time by assuming first-order kinetics. Emission spectra were measured at an excitation slit width of 5 nm and an emission slit width of 3 nm with an optical path length of 10 mm.

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Figure S3-1. Fluorescence spectra recorded for P1 (2 µM) upon the successive addition of (a) MnO4‾ (0, 0.2, 0.5, 0.7, 1, 2, 3, or 4 equiv.). Linear regression of fluorescence intensity at 440 nm for (b) P1 + MnO4‾.

Figure S3-2. Time-dependent fluorescence spectra recorded after adding oxidants to aqueous solution of P1 (2 μM), and their respective linear regressions. (a), (b): P1 + 5 equiv. of MnO4‾; (c), (d): P1 + 10 equiv. of MnO4‾. The measurements were recorded immediately after the introduction of the analyte.

Anthracene was used as a standard for the quantum yield measurement of P1. Typically, the quantum yield was obtained by integrating the photons emitted by P1 up to 600 nm and calculated using the following formula:[4]

Ф𝑢=Ф𝑠𝐴𝑠𝐹𝑢𝑛

2

𝐴𝑢𝐹𝑠𝑛20

where A refers to absorbance at the excitation wavelength, F refers to integrated emission area across the band, Ф refers to quantum yield, n is refraction index of the solvent containing the unknown (n0, standard), subscript u refers to “unknown”, and subscript s refers to “standard”.

4. NMR Studies

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10 mM solutions of P1 were independently prepared in mixtures of 10% D2O and d6-acetone. Afterward, known quantities of KMnO4 were added and kept for a specified length of time after vigorous shaking. The resulting mixtures were filtered to remove precipitated manganese species before data acquisition.

5. Absorption Studies

Samples were prepared in a similar manner as those described in the fluorescence titration section using an optical path length of 10 mm.

Figure S5-1. Absorption spectrum of P1 as recorded in a 10 mM phosphate buffer (pH 7.4).

Figure S5-2. Absorption spectrum of P1 after oxidation MnO4‾ in 10 mM phosphate buffer (pH 7.4).

6. Electrochemical Studies

Cyclic voltammetry (CV) of a 0.5 mM solution of P1 in a mixture of 75% acetone and 25% water was performed in the presence of 5 mM of tetrabutylammonium dihydrogen phosphate (TBAH2PO4) as the supporting electrolyte. Various scan rates were applied. A platinum wire was used as the working electrode, Ag/AgCl in 3 M NaCl (aq) was used as reference electrode, and carbon was used as the counter electrode.

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7. Calculations

The electronic structure calculations for P1 and their respective oxidation products were performed with the Gaussian09 program package.[5] The ground state geometries and electronic structures of the complexes were optimized at the CAM-B3LYP/6-311++G(d,p) level. Time-dependent density functional theory (TD-DFT) calculations were used to determine the optical properties of the complexes based on the S0 ground-state geometry. All Gaussian calculations were performed with polarizable continuum model (solvent = water) to account for solvent effects.

Table S7-1. Calculated singlet excitation energies, oscillator strengths, and molecular orbitals (MOs) involved in the excitation for P1.

Energy/eVa Wavelength/nm

Oscillator strength

MOs Coefficientb

2.64 469.2 0.0578 H LH L+1

0.699400.10854

3.24 444.0 0.4220H L

H L+10.108660.69682

3.24 381.7 0.700 H L+20.70032

a Only selected excited states were considered. b Configuration interaction (CI) coefficients represented as their absolute values.

Figure S7-1. Calculated molecular orbitals of P1 based on the ground state geometries.

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Table S7-2. Calculated singlet excitation energies, oscillator strengths, and molecular orbitals (MOs) involved in the excitation for [P2-p-2e]+.

Energy/eVa Wavelength/nm

Oscillator strength

MOs Coefficientb

1.47 843.4 0.0477

H LH-1 LH-2 LH-5 L

0.929430.184440.261480.92943

1.91 648.2 0.0495H-2 LH-1 L

0.824370.51623

a Only selected excited states were considered. b The CI coefficients are represented as their absolute values.

8. References

1 B. Alcaide, P. Almendros, J. M. Alonso, E. Busto, I. Fernández, M. P. Ruiz, and G. Xiaokaiti. Acs. Catal, 2015, 5, 3417-3421.

2 M. Kim, S. K. Ko, H. Kim, I. Shin and J. Tae, Chem. Commun., 2013, 49(72), 7959-7961.3 G. Shi, T. Yoon, S. Cha, S. Kim, M. Yousuf, N. Ahmad, D. Kim, H-W Kang and K. S.

Kim, ACS Sensors, 2018, 3(6), 1102-1108.4 D. F. Eaton, Pure Appl Chem., 1988, 60, 1107-1114. 5 Gaussian 09, Revision E.01, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;

Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian, Inc., Wallingford CT, 2009.