Simultaneous Detection of Cu2+ and Hg2+ via Two Mutually

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Simultaneous Detection of Cu2+ and Hg2+ via Two Mutually Independent Sensing Pathways of Biimidazole Push-Pull Dye Nilanjan Dey, Jiri Kulhanek, Filip Bures, and Santanu Bhattacharya J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b02591 • Publication Date (Web): 31 Dec 2018 Downloaded from http://pubs.acs.org on December 31, 2018

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The Journal of Organic Chemistry

Simultaneous Detection of Cu2+ and Hg2+ via Two Mutually Independent Sensing Pathways of Biimidazole Push-Pull Dye Nilanjan Dey,a Jiří Kulhánek,b Filip Bureš,* b and Santanu Bhattacharya*a, c a

Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012

b

Institute of Organic Chemistry and Technology, Faculty of Chemical Technology, University of Pardubice, Pardubice, CZ-53210, Czech Republic

c

Director’s Research Unit, Indian Association for Cultivation of Science, Kolkata 700032, India

KEYWORDS. Push-pull dye; Multiple metal ions; Orthogonal sensing; 6-Imino-5,6-dihydropyrrolo[3,4d]imidazole-4(3H)-one; Waste water management

ABSTRACT. An easy-to-synthesize, biimidazole push-pull dye has been designed, comprising two mutually independent analyte binding sites. It has been found that Hg2+ coordinates with the compound via thiophene residue and inhibits the charge-transfer (CT) process, which transforms the yellow-colored solution colorless. On the other hand, an unusually large bathochromic shift is observed in CT band upon addition of Cu2+, accompanied by a change in the color from yellow to red. A rather surprising observation is made from mechanistic studies, where it indicates that Cu2+ catalyzes the formation of 6-imino-5,6dihydropyrrolo[3,4-d]imidazole-4(3H)-one (IPIMO) derivative. This strongly affects the charge transfer state of the compound as well as its polarizability. Most importantly, this is the first report where IPIMO formation reaction has been exploited for sensing of a metal ion. Further, the system was employed for screening of both of these metal ions in wastewater samples. Recovery values ranging from 93.3 to 105.0 % confirms the suitability of the present method for estimating trace-level of metal ions in complex matrices. In addition, inexpensive on-site detection systems were developed using paper strips.

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INTRODUCTION Small organic molecule-based optical probes for detection of toxic metal ions or biologically relevant analytes have received considerable attention in the recent past. High sensitivity towards the target analyte, rapid response, wide possibility of structural variations, and simple detection strategy make these probes inexpensive as well as user-friendly in comparison to other traditional analytical techniques. 1 However, classical optical probes mostly work on one-to-one recognition strategy, where the binding of target analyte affects the optical behavior of the signaling unit. Till date, only a handful of systems have appeared in the literature, where a single compound can detect multiple metal ions simultaneously by producing distinguishable optical signals.2 Needless to say that these molecular sensors can effectively reduce the time and cost associated with the analysis of real-life samples, such as biological fluids or drinking water etc. In spite of their huge demand, the design of such molecular modules is challenging as one needs to incorporate more than one orthogonal interactive site in the same molecule. So that, the binding of one analyte does not affect the interaction with another analyte, not even by their sequence of addition. 3 Involving 'small chemical reactions' is one of the ‘smart’ strategies for developing such multi-responsive probes (Table S1).4 As these chemical reactions are highly analyte-specific, the possibility of receiving false positive result is likely to be less here. Herein we have designed a push-pull dye having 4,5-bis[4(N,N-dimethylamino)phenyl]imidazole as the donor moiety and 4,5-dicyanoimidazole as the acceptor unit (Fig. 1c). The polarizability of such D--A system depends on the chemical structure, particularly on the electronic characteristics of the donor and acceptor moiety, overall geometry, and the length of the -conjugated linker.5 Thus these compounds find tremendous applications as active parts of light-emitting diodes, optoelectronic devices, and organic photovoltaic cells etc. 6 Moreover, incorporation of heteroaromates, such as benzimidazole, thiazole, thiophene etc. into the chromophore backbone enhances the photostability of the dyes as well as their thermal and chemical robustness.7


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(a)

(b) Detection of multiple analytes

Reaction based probes for Cu2+ N H2N

N N

N

N

N

S

S O N

Interaction with A Interaction with B (c) Primary compound for investigation

O

O S NH NH2 O

OH

HO

N

π-linker N N

N

CN N

N

O N

CN

Donor

O S OH O

O O

Binding Site for B

Binding Site for A

N

Acceptor

S

1

N

Figure 1. (a) Examples of some reaction-based sensors (chemodosimetric probes) for Cu2+. (b) Schematic diagram shows the principle of multiple analyte sensing. (c) Structure of compound 1 involved in the present study. In this work, we are particularly interested in exploring the ion binding property of compound 1 in the aqueous environment. Among various transition metal ions, compound 1 showed selective responses towards Cu2+ and Hg2+. Metal ion Hg2+ coordinates via the thiophene residue of the π-linker and diminishes the extent of the intramolecular charge-transfer (ICT) interaction. As a result, the yellow-colored solution turned colorless in the presence of Hg2+. However, a completely distinct observation was made when compound 1 was added with Cu2+, where a remarkable red-shift of the CT band (accompanied by a distinct color change from yellow to red) was noticed due to the formation of 6-imino-5,6-dihydropyrrolo[3,4d]imidazole-4(3H)-one (IPIMO) derivative. Though there are several chemodosimetric probes known in the literature for the detection of Cu2+ prior to this report, no attempt has been made to employ such chemical transformation for sensing of Cu2+ (Fig. 1a).8 IPIMO is remotely related to the isoindolenine derivative, which is one of the major isolable intermediates formed during the metal ion-catalyzed phthalocyanine synthesis.9 It can also be found in a wide range of pharmaceutical drugs and natural products. Thus, such


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observations will not only help the researchers to devise new strategies for detection of copper ion but also to unravel new pathways for the screening of multiple analytes simultaneously (Fig. 1b). O B O

I

OHC

S

N

CN

N

CN

O B O

Heck olefination

N

Br

N O i.NH4OAC/ AcOH/ O ii. LHMDS/MeI

CN N

CN

S

N N

1

N N

N I

CN

N

Heck olefination

N

O

N

3

Br

+3

N H

S

N

N

CN

N

N

N

N

I

Direct C- Arylation

N

N

CN

N

N

CN

N

N

2

Scheme 1. Simple synthesis of compounds (1 and 2) involved in present study.

RESULTS AND DISCUSSION (a) Investigation of photophysical properties of compounds The biimidazole push-pull dyes engaged in the present work, 1 and 2, were prepared following the procedure reported in the literature by us (Scheme 1 & Fig. S25-S26).10 Both molecules utilize imidazole ring as the electron donor and acceptor, depending on the type of their 4,5-disubstitution, and differ in the -linker between the electron releasing and withdrawing parts. In contrast to compound 2 with simple 1,4-phenylene -linker, compound 1 showed low-energy lying charge-transfer band, narrowed HOMOLUMO gap, and higher polarizability due to presence of extended, planar, and thiophene‐appended -linker. Thus, in a semi-aqueous environment, compound 1 renders yellow colored solution with CT band situated at 420 nm, while for compound 2, the solution was colorless with no detectable absorbance beyond 400 nm (Fig. 2a). The UV-visible spectrum of compound 1 also showed presence of additional absorption bands at 274 nm ( = 3.50  104 M-1cm-1) and 340 nm ( = 2.42  104 M-1cm-1) as well. The band at 274 nm was probably originated from π-π* transition, while n-π* transition resulted in the 340 nm band. Most importantly, the red-shifted absorption maximum (at 415 nm) was found to be susceptible to changes in the ACS Paragon Plus Environment

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electronic environment of either donor or acceptor. As expected, red-shifts of ICT band were observed in polar solvents due to dipole-induced-dipole interactions with the solvent molecules (Fig. 2b & S1).11 Therefore, in the free probe, the HOMO was found to be localized on N,N-dimethylanilino fragments, whereas LUMO is spread over the 4,5-dicyanoimidazole unit (Fig. 7c).12 Most notably, due to effective conjugation, allowed by thiophene and styrene moieties, the frontier molecular orbitals are found to be distributed over the large section of molecular framework, rather than concentrating on a particular unit.

(a) 0.20

(b) 0.15

0.15

Absorbance

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Compound 1 Compound 2

0.10 0.05

In MeOH In DMF In THF In Acetonitrile In water

0.10

0.05

0.00

0.00 300

350

400

450

500

550

300

Wavelength (nm)

400

500

600

Wavelength (nm)

Figure 2. (a) UV-visible spectra of compound 1 and 2 (10 µM) in acetonitrile-water (1:1) mixture. (b) UVvisible spectra of 1 (10 µM) in different organic solvents. The fluorescence spectrum of 1 showed a broad emission band spread over a large wavelength range (Figure S6). On the blue side of the spectrum, we could observe a shoulder peak at 370 nm. However, the emission profile was dominated by three broad, red-shifted bands that appeared at ~412, 437 and 484 nm respectively. The excitation spectra for all the emission wavelengths were identical. Thus, one may conclude that various emission bands, including bands with large bathochromic shift were emerged exclusively due to excited state effects (single ground state species). 13 The origin of this excitation independent multiple emission bands can probably be explained by its rigid conformation. (b) Interaction of compound with metal ions When compound 1 was exposed to a wide range of transition metal ions, Cu2+ induced a change in solution color from yellow to red, while it became colorless in presence of Hg2+. The UV-visible spectra of 1 with Hg2+ showed a decrease in the absorbance value at 420 nm with concomitant increase at 340 nm. Titration studies with Hg2+ indicated ‘ratiometric’ variations in the absorbance values with isosbestic point


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at 405 nm (Fig. 3a & S2). Presence of well-defined isosbestic point during the titration studies indicates one-to-one equilibrium between the molecular probe and Hg2+ ion. On the contrary, titration with Cu2+ (01.0 equiv.) resulted in the formation of a new absorption band at ~520 nm region. In addition, we could notice a decrease in the absorbance values at both 274 and 420 nm band and an enhancement at 340 nm (Fig. 3c & S2). Thus, the presence of multiple isosbestic points was observed at 350, 405 and 472 nm regions. However, beyond 1.0 equiv. of Cu2+, there was a sharp change in the absorption spectrum. A new CT band was found to form at 545 nm (~30 nm red-shift from its original position). Appearance of new isosbestic points could be traced, located at 345 and 400 nm regions (Fig. 3d), but these points were not well-defined. This clearly indicates the differences in the mode of interactions between Hg2+ and Cu2+ with probe 1. Though the changes in absorbance were quite instantaneous in the presence of Hg2+, it is necessary to provide at least 2 min incubation time to see the desired change with Cu2+ (Fig. S3). The concentrationvariation studies also indicate that the present system can detect Hg 2+ and Cu2+ as low as 32.8 and 2.2 ppb, respectively. Since selectivity is one of the most desired criteria of colorimetric probes, we have investigated effects of other metal ions than Cu2+ and Hg2+ on the absorption spectra of 1 in acetonitrilewater (1:1) mixture medium. As can be seen from Figs. 3b & S4, none of the used metal ions has been able to induce significant changes in the absorption spectrum of 1. Considering the possibility of a time-delayed response, compound 1 was further incubated with these metal ions for 48 h. Though the time-evolution plot indicated increments in the absorbance value at 545 nm in the presence of some Lewis acid metal ions, e.g. Ca2+, Mg2+, and Zn2+, the extent of changes was sufficiently low compared to that observed with Cu2+. Hence, there are no detectable changes in the color of 1 in presence of these metal ions. Though the visible color changes were observed with both Hg2+ and Cu2+ ions, the cyan colored emission of 1 (seen under long UV lamp) was found to be quenched selectively in the presence of Cu2+ (Fig. S6). In spite of emission quenching (~8 fold), no significant change in the average lifetime was observed when time-dependent emission spectrum of 1 was recorded in the presence of Cu2+ (Figs. 4a & S7). Even upon addition of Cu2+,


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the compound showed multi-exponential decay. These observations suggest formation of a non-fluorescent complex formation in the presence of Cu2+ (static quenching).

0.12

0.09

Absorbance

(c) 0.12

Absorbance

(a) 0.16

0.08 0.04

300 0.20

350

400

450

500

0.03

300

550

Wavelength (nm)

400

500

600

700

Wavelength (nm)

(d) 0.15

0.16

Absorbance

(b)

0.06

0.00

0.00

Abs. at 545 nm

0.12 0.08 0.04 0.00

Bl an + k Ag + + C d 2+ + C o 2+ + C u 2+ + H g 2+ + Pb 2+ + N 2 + i + M g 2+ + M n 2+ + Zn 2 + + Al 3 + + Fe 3 + + C 3 r +

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.10

0.05

0.00 300

Metal Ions added (2 equiv.)

400

500

600

700

Wavelength (nm)

Figure 3. (a) UV-visible titration of compound 1 (10 µM) with Hg2+ (0-2 equiv.) in acetonitrile-water (1:1) mixture medium. (b) Change in absorbance of compound 1 (10 µM) at 545 nm in presence of different metal ions. UV-visible titrations of compound 1 (10 µM) with Cu2+ (c) 0-1.0 equiv. and (d) at 1.0-2.0 equiv. in acetonitrile-water (1:1) mixture medium. (all the data points were recorded after an incubation time of 10 min). Perchlorate salts of metal ions were used for sensing studies. (c) Mechanism of metal ion sensing: formation of IPIMO To gain insight into the mechanisms, stoichiometry of interaction between metal ions and probe was determined using continuous variation method (Job’s plot). We observed 1:1 interaction between 1 and Hg2+ ions with an inflection point at x = 0.5 (x = mol fraction of 1) (Fig. S8). However, 1 showed two inflection points at x = 0.33 and x = 0.55 in the presence of Cu2+ ions (Fig. 4b). This implies that Cu2+ can form two different types of complexes in the reaction medium, with 1:1 and 1:2 stoichiometry, presumably due to presence of two different nitrile functions. The binding constants of 1 with Hg2+ ion was calculated as 5.92 ± 0.01 (log scale) (Fig. S9). The addition of EDTA into a mixture of 1 and Hg2+ substantially recovered absorption peak at 442 nm, no such reversal of signal was observed for Cu 2+ (Fig. S10). Hence,


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we can conclude that probe 1 forms a reversible coordination complex with Hg2+, whereas its interaction with Cu2+ is dosimetric in nature. 1H NMR titration of 1 with Hg2+ was performed in DMSO-d6/D2O (5:1) mixture (Fig 4c). All the proton signals are down-fielded in the presence of Hg2+. In particular, the thiophene signals labeled as ‘a’ and ‘b’ experienced huge broadening and diminution upon coordinating the Hg2+ ions. These observations indicate that compound 1 is likely to coordinate Hg2+ through thiophene residue.

(a)

(b)

10000

Prompt 1 1 + 1 equ. Cu2+

1000

1 + 2 equ. Cu2+ 100

A x Cu2+

Intensity (counts)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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2.0 1.5 1.0

1:1 ratio

0.5

1:2 ratio

10

0.0 10

(c)

20

30

40

50

0.2

0.4

0.6

0.8

Mol Fraction of 1

Time (ns)

+ 2.0 equ. Hg2+ + 1.5 equ. Hg2+ + 1.0 equ. Hg2+

+ 0.5 equ. Hg2+

a

b

Comp alone

Compound 1 Figure 4. (a) Time-dependent fluorescence spectra of compound 1 (10 µM, ex = 340 nm) with Cu2+ (0-2 equiv.), monitored at 442 nm. (b) Job’s plot of compound 1 with Cu2+ in acetonitrile-water (1:1) mixture medium. (c) 1H-NMR titration of compound 1 (5 mM) with Hg2+(0-10 mM) in DMSO-d6/D2O (5:1) mixture medium. On the contrary, 1H-NMR titration of 1 with Cu2+ showed paramagnetic quenching of all peaks due to openshell electronic configuration of Cu2+ (d9 system) (Fig. S11). Therefore, to examine the binding mode of Cu2+, 1 and Cu2+ were allowed to react at room temperature in acetonitrile-water to afford red-colored precipitate, which was collected, thoroughly washed, and subjected to NMR analysis (DMSO-d6/D2O (5:1)


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(Fig.. S12-S13). The characteristic peaks of 1 were not traced in the 1H-NMR spectrum of the product, which indicates formation of a new species upon interaction of 1 with Cu2+. In general, the 1H-NMR signals are shifted downfield with a new signal arising at ~8.8 ppm, which can probably be assigned to an enamido proton. Another noticeable feature of the 1H- and

13

C-NMR spectra is the duplication of signals. This

indicates formation of two IPIMO isomers. This is especially supported by a new and easily distinguishable set of N-CH3 signals observed in the aliphatic region of the 1H-NMR spectra (Fig. S13) as well as by new peaks appearing at 178 and 150 ppm in 13C-NMR (Fig. S14). The latter signals would correspond to the newly formed carbonyl and imide carbons, respectively. However, a proper 13C-NMR analysis is hindered by a relatively low solubility of the sample. Formation of IPIMO positional isomers I and II (Fig. 5a) can be rationalized by a hydrolysis of two unequal CN groups of 1. Both regioisomers differ in the orientation of the NH and N-CH3 groups. We have already reported a similar formation of regioisomers upon N-functionalization of imidazolines.14 The HR-ESI-MS spectrum in positive mode confirmed the presence of a dosimetric adduct with m/z of 652.2734 Da (Fig. S15); the IPIMO calculated m/z is 652.2727 Da (∆m/z ~ 1 ppm). Further spectral evidence can be carried out by FT-IR spectrum that is free of the CN band of the original 1 appearing at 2231 cm-1 (Fig. S16-S17). A new peak at 1650 cm-1 may be attributed to the carbonyl group of the enamido unit, which is further supported by a significantly pronounced hydrogen bonding as indicated by the broad band appearing at 3410 cm-1. In contrast to reactivity towards Cu2+, 1 in the presence of Cu+ did not show a similar color change, most probably due to a different electrophilicity of Cu + and Cu2+ ions (Fig. S18). When the control biimidazole 2 was allowed to interact with Cu2+, similar spectral changes with redshifted CT band was encountered (Fig. 5b). This indicates the essential role of the nitrile groups in the formation of IPIMO derivative. On the contrary, no detectable change was observed when 2 was treated with Hg2+ (Fig. S19), which indicates that the presence of thiophene unit is essential for recognition of Hg2+. Further, to evaluate the deciding role of water in Cu 2+ sensing, the compound 1 was allowed to interact with Cu2+ in different water-acetonitrile mixtures with a varying water content. Initially, an increase in the absorbance value was observed at 545 nm with raising amount of water in mixture (< 50%), indicating direct involvement of water in the dosimetric interaction. After a certain percentage (>


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50%), a sudden fall in the absorbance value was observed (Fig. 5c). However, a monotonous decrease in the optical response with raising water content (0 to 100%) was recorded for Hg2+ (Fig. S20). This is expected observation for both Cu2+ and Hg2+ ions as the extent of interaction will decrease in water-rich medium due to poor solubility of 1 as well as high hydration of metal ions.15

Figure 5. (a) Mechanism of Cu2+-triggered IPIMO I and II formation from 4,5-dicyanoimidazole push-pull derivative. (b) UV-visible titration of compound 2 (10 µM) with Cu2+ (0-2 equiv.) in acetonitrile-water (1:1). (c) Effect of water content on interaction of compound 1 with Cu2+ (2 equiv.) in acetonitrile-water (1:1) mixture medium. (All the data points were recorded after an incubation time of 10 min). From the above shred of observations, we can conclude that Hg2+ simply binds to the thiophene moiety of compound 1, which inhibits the ICT process. Thus, the charge-transfer band shifts towards the higher energy region and the solution color turns from yellow to colorless. On the other hand, the interaction of 1 with Cu2+ is dosimetric in nature. Linear arrangement of cyano groups in 4,5-dicyanoimidazole (similarly to phtalonitrile) prevents their mutual intramolecular interaction. However, upon Cu 2+ coordination the linear geometry will change to trigonal, the carbon atom of the cyano group becomes more susceptible to a


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nucleophilic attack (H2O), and cyclization into IPIMO is highly facilitated (Fig. 6). 16 Since these two cyano groups are not identical, a formation of two IPIMO regioisomers is possible during hydrolysis. 1 alone

N

Π-linker N

N

S

N

N

CN N

N

CN N

Hg2+ ICT decreases

Donor

Acceptor

Donor

1 + Hg2+

N

Hg2+

S

N

N

CN

Acceptor

CN

N

Cu2+

Donor

ICT increases 2+ Acceptor 1 + Cu

Figure 6. Schematic diagram shows multiple metal ions sensing using compound 1 in acetonitrile-water (1:1). [The left-side vials are the images taken under visible light, whereas the images of the right-side vials were taken under UV irradiation] In principle, formation of IPIMO may also be induced by another metal ions such as Mg2+, Ca2+, Ag+ etc. However, the exceptionally high bond dissociation energy of C≡N-Cu (~ 0.90 eV) indicates preferential coordination of Cu2+ with nitrile groups.17 Moreover, high enthalpy of formation (∆H = −830 kJ. mol-1) associated with CuPc (copper phthalocyanine) also indicates that the cyclization is energetically favorable for Cu2+.18 An increase in the probe concentration (against a fixed amount of Cu2+) resulted in raised absorbance value at 545 nm, indicating that Cu2+ indeed acts as a catalyst of this chemical transformation (Fig. S21a). A similar and even pronounced interaction of 1 with Cu2+ has also been observed in MeOH: acetonitrile (1:1) mixture, which reflects higher nucleophilicity of MeOH in comparison to water (Fig. S21b). (d) Theoretical basis of metal ion induced color change In order to explain the large red-shift in charge transfer band (~125 nm) caused by Cu2+, we subjected both probe 1 and its IPIMO derivative for density functional analysis (using B3LYP/6-31G* level of theory) (Fig. 7c & Table S2).19 The optimized structure of 1 showed a dihedral angle of 45° between 4,5‐ dicyanoimidazole and rest of the molecule. However, upon interaction with Cu2+, the same dihedral angle


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in IPIMO derivative was reduced to ~2°. This could drastically enhance molecular dipole moment from 13.5 to 25.7 and make the ICT process energetically more favorable. (b)

0.16 1

0.12

1 + Hg2+ 1 + Hg2+ +Cu2+

0.08 0.04

0.10 0.05 0.00

0.00 300

(c)

1 1 + Cu2+ 1 +Cu2+ + Hg2+

0.15

Absorbance

(a) Absorbance

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400

500

300

600

Cu2+

500

600

Wavelength (nm)

Wavelength (nm)

IPIMO adduct

400

Comp 1

E = 2.35 eV

Hg2+

Comp 1 + Hg2+

E = 3.1 eV

E = 0.65 eV

Figure 7. (a) UV-visible spectra of compound 1 + Hg2+ ([1] = 10 µM, [Hg2+] = 20 M) in presence of Cu2+ in acetonitrile-water (1:1). (b) UV-visible spectra of compound 1 + Cu2+ ([1] = 10 µM, [Cu2+] = 20 M) in presence of Hg2+ in acetonitrile-water (1:1). (c) Energy-minimized structures of compound 1, IPIMO adduct and compound 1+ Hg2+ with frontier molecular orbital analysis (using B3LYP/6-31G* for C, H, N, O and LANL2DZ for Hg). The frontier molecular orbital analysis showed a decrease in the lowest energy band gap from 2.35 to 0.65 eV upon formation of the IPIMO derivative (Fig. 7c & Table S3). On the other hand, coordination of Hg2+ with the thiophene residue of 1 resulted in the decrease of molecular dipole moment from 13.5 to 8.0 and increase in the energy band gap from 2.35 to 3.10 eV. This, it, in turn, qualitatively supports the experimentally observed red/blue-shift in the absorption maxima during interaction with Cu2+/Hg2+. In order to ensure the exclusive detections of the target metal ions even in the mixture of others, we followed optical responses of 1 towards Cu2+ and Hg2+ in presence of other metal ions (in excess) under similar


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conditions. No interference was observed in the detection of Cu2+ and Hg2+ due to the presence of other metal ions. N CN N N N

S

N

N

+ Hg2+

N

CN

+ Cu2+

1 + Hg2+ CN N N

N

S

N

CN

N

1 + Cu2+ + Hg2+

1 + Cu2+

+ Hg2+

1 + Cu2+

Figure 8. Exclusive detections of Cu2+ and Hg2+ by probe molecule 1 in acetonitrile-water (1:1) mixture medium.

In order to check their mutual interferences, the absorption spectrum of 1 was recorded in the presence of Cu2+ with pre-added equimolar Hg2+. Even in the presence of Hg2+, the addition of Cu2+ resulted in the formation of a new absorption band at ~528 nm, which is accompanied by the color change from colorless to orange-red. Additionally, the blue shift in 420 nm band was quite visible even after Cu 2+ addition, which indicates that the compound is still attached to Hg2+ while interacting with Cu2+ (Fig. 7a). However, when Hg2+ was added to a solution containing both 1 + Cu2+, the blue-shift in the charge transfer band (at 420 nm) was not very well developed (Fig. 7b) and a slight color change from deep red to orange red was observed. In the presence of Cu2+, compound 1 is being transformed into a different entity (IPIMO) and thus its response towards Hg2+ is different to what is observed with 1. Thus, between Hg2+ and Cu2+, the detection mechanism of 1 appears to be more selective for Cu2+ (Fig. 8).

(e) Estimation of metal ions in natural water samples Though the metal ions at definite concentrations play vital roles in maintaining a wide range of physiological processes, their excess intake may cause severe health problems, including death. For example, excess of copper in the human body can cause gastrointestinal problems, cirrhosis of liver, and ACS Paragon Plus Environment

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several other neurodegenerative disorders such as Alzheimer's disease, Wilson's disease and Menkes' kinky hair syndrome etc.20 Moreover, in the free form, it can damage DNA by producing reactive oxygen species.21 On the other hand, exposure to excess Hg2+ causes renal failure, hypertension, and memory impairment in human.22 In addition, Hg2+ can irreversibly bind to the selenium-dependent enzymes and inhibit their natural activities.23 The international environmental protection agency (EPA) has defined the maximum allowable levels of these metal ions in drinking water, 1.3 ppm for Cu2+ and 2 ppb for Hg2+.24 It is essential to design low-cost strategies to detect the presence of toxic metal ions in drinking water samples above permissible limits. Considering this, we have applied compound 1 for estimation of Cu2+ and Hg2+ in real-life water samples collected from various natural sources such as tap, pool, and sea (Fig. 9a & Table S4). The changes in absorbance (∆A) values were monitored at 375 and 545 nm for Hg 2+ and Cu2+, respectively. In all cases, reliability of the present method was verified independently by atomic absorption spectroscopic (AAS) technique. 25 The quantitative estimations of metal ions were done following the regressive equation, Y = 2.51x + 1.55 (for Hg2+) and Y = 0.017x + 0.0015 (for Cu2+). In most cases, the recovery values varied from 90 to 105 % with relative standard deviations (RSDs) within the range of 1.1 to 3.8 %. Such small standard deviation values indicate high sensitivity of the present method. Moreover, considering the complexity of the real-life samples, we have also checked the interaction of 1 with Hg2+ (∆A at 375 nm) and Cu2+ (∆A at 545 nm) in the presence of excess of other metal ions. We were delighted that no interference was observed in such cross-reactivity studies (Fig. S22 and S23). (f) Fast-track detection of toxic metal ions Though probe 1 showed promising results in analyzing real-life samples, it is important to devise an alternative strategy of metal ion sensing using dye-coated paper strips. This kind of dip-stick method is beneficial over the traditional optical sensing, as it does not require any sophisticated laboratory facility or trained technicians. Hence, we have developed filter paper strips, soaked with probe 1 (test strips), for colorimetric detection of both Hg2+ and Cu2+.26 The dye-coated paper strips were dipped into solutions spiked with different amounts of toxic metal ions. Distinctive color changes from yellow to red and colorless was observed respectively, when the Cu2+ and Hg2+-containing solutions were subjected to


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analysis (Fig. 9b & S24). Further, the metal ions-treated test strips could be differentiated under UV lamp, where Cu2+ soaked paper strips specifically showed quenching of cyan colored emission. Therefore, this newly developed method can provide an alternative way to confirm nature of the metal ion (whether it is due to Cu2+ or Hg2+ contamination) as well as extent of its toxicity.

(a) Medium

In tap water

In pond water

In sea water

AAS method (µM)

Present method (in µM)

Average (in µM)

% Recovery

Exp-1

Exp-2

Exp-3

0.5

0.53

0.52

1

0.95

1.5

RSD

0.52

0.52

96

1.10

0.96

0.92

0.94

94

2.21

1.42

1.40

1.47

1.43

95.3

2.52

2

1.85

1.92

1.90

1.89

94.5

1.92

0.5

0.48

0.47

0.47

0.48

96

1.22

1

0.94

0.95

0.98

0.96

96

1.59

1.5

1.56

1.52

1.53

1.54

102.4

1.35

2

2.1

2.05

2.12

2.09

105.5

1.73

0.5

0.55

0.54

0.57

0.55

90

2.76

1

0.97

0.98

0.95

0.97

97

1.58

1.5

1.47

1.45

1.52

1.48

100

2.44

2

2.06

2.10

2.13

2.10

105

1.67

(b) Day light UV light 1

Ag+ Cd2+ Co2+ Cu2+ Mg2+ Hg2+ Ni2+ Pb2+ Zn2+

Figure 9. (a) Estimation of Cu2+ in different natural water samples using present method (verified by AAS method). (b) Detection of metal ions using inexpensive color strips (upper lane shows changes seen under daylight, down lane shows changes seen under UV light). CONCLUSION In conclusion, a highly conjugated biimidazole push-pull dye has been designed for simultaneous detection of Cu2+ and Hg2+ in aqueous environment. Hg2+ coordinates with the thiophene ring of the probe molecule and diminishes the extent of charge transfer (CT) process. This was evident by a change in solution color from yellow to colorless. However, the formation of highly red-shifted CT band was observed with Cu2+ accompanied by a color change from yellow to red. Unlike Hg2+, Cu2+ catalyzed the formation ACS Paragon Plus Environment

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of IPIMO derivative, which affect both the electronics of the acceptor residue as well as overall polarizability of the compound. This is a unique and unprecedented example, where metal ion-triggered IPIMO formation reaction has been exploited to design selective probe for Cu2+. Moreover, due to orthogonal nature of the interactions, we can independently determine presence of both these metal ions without much difficulty. Considering the toxic nature of these metal ions, the present system will also be useful for estimation of Cu2+ and Hg2+ in a wide-range of natural water samples. In addition, portable paper strips were developed for the on-site detection purpose. EXPERIMENTAL SECTION Materials and methods. All reagent materials and solvents were bought from the known commercial sources and were used without further purification. Solvents were distilled and dried prior to use. FT-IR spectra were recorded on a Perkin-Elmer FT-IR Spectrum BX system and were reported in wave numbers (cm−1). 1H-NMR and

13

C-NMR spectra were recorded with a Bruker Advance DRX 400 spectrometer

operating at 400 and 100 MHz for 1H and

13

C NMR spectroscopy respectively. Chemical shifts were

reported in ppm using the internal standard, tetramethylsilane (TMS). Mass spectra were recorded on Micro mass Q-TOF Micro TM spectrometer. Synthesis of compounds 1 and 2: The compounds 1 and 2 were prepared via Suzuki-Miyaura crosscoupling or direct C-arylation as reported in the literature.10 Characterization of compound 1: Prepared from the corresponding bromothiophene (120 mg, 0.25 mmol) and dioxaborolane (94 mg, 0.26 mmol).10 Yield 51 mg (32 %), red solid, Rf = 0.60 (SiO2; CH2Cl2/acetone 10:1); M. p. 274-277 °C; 1H NMR (400 MHz, CDCl3, 25 °C):  = 2.90 (s, 6H; N(CH3)2), 3.03 (s, 6H; N(CH3)2), 3.61 (s, 3H; NCH3), 3.86 (s, 3H; NCH3), 6.61 (d, 3J(H,H) = 9.0 Hz, 2H; DMA), 6.77-6.82 (m, 3H; DMA+CH), 7.21 (d, 3J(H,H) = 9.0 Hz, 2H; DMA), 7.33-7.38 (m, 2H; Th), 7.45 (d, 3J(H,H) = 9.0 Hz, 2H; DMA), 7.57 (d, 3J(H,H) = 8.4 Hz, 2H; 1,4-phenylene), 7.68 (d, 3J(H,H) = 8.4 Hz, 2H; 1,4-phenylene), 7.82 ppm (d, 3J(H,H) = 16.0 Hz, 1H; CH); 13C{1H} NMR (100 MHz, CDCl3, 25 °C):  = 32.8, 33.0, 40.6, 40.8, 108.7, 109.6, 110.9, 112.0, 112.5, 112.6, 112.9, 116.8, 118.3, 122.7, 124.5, 126.3, 126.4, 127.8, 128.5, 128.6, 130.5, 132.0, 133.9, 136.1, 140.1, 140.9, 143.4, 149.4, 150.5, 150.7 ppm; IR (neat):  = 2968, 2359 ACS Paragon Plus Environment

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(CN), 2340 (CN), 1738, 1614, 1504, 1361, 1226, 1216, 969, 946, 822, 794, 668 cm-1; MS (ESI): m/z (%): 635 [M++1]; Elemental analysis: calcd (%) for C38H34N8S (634.8): C 71.90, H 5.40, N 17.65, S 5.05; found C 71.96, H 5.42, N 17.71, S 5.07. Characterization of compound 2: Prepared from the corresponding iododerivative (146 mg, 0.28 mmol) and N-methylimidazole-4,5-dicarbonitrile (46 mg, 0.35 mmol). 10 Yield 25 mg (17 %), orange solid, Rf = 0.25 (SiO2; CH2Cl2/acetone 15:1); M. p. 174-177 °C; 1H NMR (400 MHz, CDCl3, 25 °C):  = 2.91 (s, 6H; N(CH3)2), 3.01 (s, 6H; N(CH3)2), 3.53 (s, 3H; NCH3), 3.83 (s, 3H; NCH3), 6.60 (d, 3J(H,H) = 9.0 Hz, 2H; DMA), 6.71 (d, 3J(H,H) = 9.0 Hz, 2H; DMA), 7.19 (d, 3J(H,H) = 9.0 Hz, 2H; DMA), 7.44 (d, 3J(H,H) = 9.0 Hz, 2H; DMA), 7.70 (d, 3J(H,H) = 8.4 Hz, 2H; 1,4-phenylene), 7.91 ppm (d, 3J(H,H) = 8.4 Hz, 2H; 1,4phenylene); 13C{1H} NMR (100 MHz, CDCl3, 25 °C):  = 29.9, 34.0, 40.4, 40.5, 110.9, 111.9, 112.3, 112.5, 114.5, 116.2, 121.1, 122.3, 127.9, 128.9, 129.8, 130.1, 131.5, 131.6, 131.8, 138.1, 146.2, 149.9, 150.7, 151.8 ppm; IR (neat):  = 2852, 2359 (CN), 2343 (CN), 1609, 1442, 1165, 942, 847, 821, 716 cm-1; MS (ESI): m/z (%): 527 [M++1]; Elemental analysis: calcd (%) for C32H30N8 (526.6): C 72.98, H 5.74, N 21.28; found C 73.00, H 5.77, N 21.29. Spectroscopic studies: The UV−vis and fluorescence spectra were recorded on a Shimadzu model 2100 UV-vis spectrometer and Cary Eclipse spectrofluorimeter respectively. Compounds (1 and 2) were dissolved in appropriate amount in DMSO to prepare the stock solutions (1 × 10 −3 M). Most of the spectroscopic studies were performed in acetonitrile-water (1:1) mixture medium. Perchlorate salts of different metal ions were used for sensing studies. To study effect of solvents, compounds were injected in different organic solvents. 1

H-NMR and FT-IR Studies: 1H-NMR titration with Hg2+ was performed upon dissolving 1 (5 mM) in

DMSO-d6/D2O (5:1) mixture medium and to that Hg2+ (0-10 mM) was added in step-wise manner. All the spectra were recorded using identical parameters. However, to characterize the IPIMO derivative, compound 1 (5 mM) was stirred at room temperature with equimolar amount of Cu 2+ in acetonitrile-water (1:1) mixture medium. The red precipitate was washed by aqueous solution of EDTA (to remove the


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adhered Cu2+ ions) and vacuum dried. 1H-NMR spectrum of this adduct was recorded in DMSO-d6/D2O (5:1) under the same conditions. FT-IR spectra of 1 and IPIMO derivative were recorded as neat. Fluorescence Decay Experiment: Fluorescence lifetime values were measured by using a time-correlated single photon counting fluorimeter (Horiba Jobin Yvon). The system was excited with 340 nm nano-LED of Horiba - Jobin Yvon with pulse duration of 1.2 ns (slit width of 5/5, monitored at 442 nm). Average fluorescence lifetimes (τav) for the exponential iterative fitting were calculated from the decay times (τi) and the relative amplitudes (ai) using equation, τav = (a1τ12 + a2τ22 + a3τ32) / (a1τ1 + a2τ2 + a3τ3)…….……...…………………………………….………...(1) where a1, a2, and a3 are the relative amplitudes and τ1, τ2, and τ3 are the lifetime values, respectively. For data fitting, a DAS6 analysis software version 6.2 was used. Preparation of ‘test-strips’ for rapid detection: We have prepared the thin films of 1 by dipping the paper discs in a MeOH solution of 1 (1 × 10-3 M). The films were then air-dried for overnight. The films exhibited bright yellow color with bright cyan-colored emission on illuminating under a UV-lamp. These pre-coated TLC plates were then used for checking different metal ions. ASSOCIATED CONTENT Supporting information file contains additional spectral data (UV-visible and fluorescence), 1H-NMR, FTIR studies related to metal ion sensing, spectral characterization and further data for 1 and 2 and the cartesian coordinates (computational data) of the compound 1, 1+ Hg2+ and IPIMO adduct. AUTHOR INFORMATION Corresponding Author: *Email: [email protected]; [email protected] ACKNOWLEDGMENT S.B. thanks DST (J. C. Bose Fellowship) and IACS for the financial support of this work. N.D. thanks IISc and IACS, Kolkata for the financial support of this work presented in this manuscript. REFERENCES 1. (a) Su, X.; Aprahamian, I. Hydrazone-based switches, metallo-assemblies and sensors. Chem. Soc. Rev. 2014, 43, 1963-1981. (b) Kaur, B.; Kaur, N.; Kumar, S. Colorimetric metal ion sensors – A comprehensive review of the years 2011–2016. Coord. Chem. Rev. 2018, 358, 13-69. ACS Paragon Plus Environment

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Graphical Abstract


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Simultaneous Detection of Cu2+ and Hg2+ via Two Mutually

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