Indian Journal of Chemistry Vol. 57B, February 2018, pp. 217-228

Imidazole and oxazole containing fluorescent dyad: Cu2+ induced fluorescence quenching and cyanide sensing “On-Off-On” via copper displacement approach

Arvind Misra* & Rashid Ali Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi 221 005, India

E-mail: arvindmisra2003@yahoo.com; amisra@bhu.ac.in

Received 6 January 2017

A new simple organic molecule 4 containing imidazole and oxazole moiety has been synthesized and utilized for Cu2+

and cyanide sensing via displacement approach and fluorescence On-Off-On mechanism with rapid response in either of two sequential sensing events. The photophysical behavior of probe 4 toward different metal ions and anions has been examined through the absorption and fluorescence spectroscopy in HEPES buffer. Probe 4 shows high selectivity for Cu2+ with fluorescence quenching (~93%; switch-Off). The in situ generated ensemble, 4+Cu2+ upon interaction with different anions shows high selectivity for CN− in which the fluorescence intensity revived (switched-On) due to the formation [Cu(CN)x]

1−x species in the medium. Further, the naked eye sensitive “On-Off-On” sensing behavior of 4 has been utilized for construction of a sequential logic circuit at the molecular level. The FT-IR, 1H NMR and ESI-MS spectroscopy and DFT support the proposed mechanism of interaction.

Keywords: Chemosensor, Cu2+, CN−, memory element

Currently scientific community has taken much interest to design and develop novel organic scaffolds that can be potentially utilized as a naked-eye sensitive chemosensors for the recognition of cations and anions. Both cations and anions play important roles in biological, environmental, and industrial processes1,2. Moreover, fluorescent based methods have advantages due to their high sensitivity, specificity, real time monitoring, and fast response time without resorting expensive and sophisticated instruments3-5.

Copper a soft transition metal ion, is a vital trace element, and is widely distributed in a variety of cells and tissues in different concentration level. Also it is an important catalytic cofactor of a variety of metalloenzymes like, superoxide dismutase, cyto-chrome-c oxidase, and tyrosinase6,7. The variation of copper ion concentration in neuronal cytoplasm is responsible for diseases like Alzheimer’s and Parkinson’s8. The U.S. Environmental Protection Agency (EPA) has set the limit of copper in drinking water around 1.3 ppm (~20 µM). Similarly, cyanide is widely used in different industrial and chemical processes such as, plastic, fibers, gold, dyes, electro-plating, chelating agents for water treatment and pharmaceuticals9. Being toxic, release of cyanide accidently may lead to a fearful environmental disaster and severe health hazard to human and living

being. As a potent inhibitor of some metallo-enzymes cyanide is responsible for diseases related to vascular, cardiac, visual, endocrine, central nervous and metabolic systems10. It inhibits the cellular respiration in mammalian cells by interacting with the active site of cytochrome a3

9. Cyanide has also been used as a chemical warfare agent and even as a terror material10,11. The World Health Organization (WHO) has recommended concentration of cyanide in drinking water to below 0.07 mg/l (2.27 mM) for a healthy life12.

Thus, the detection of Cu2+ and CN− is extremely important and the use of organic optical materials (chromo or fluorogenic) of good photophysical response, and naked-eye selectivity are better alternative than the conventional methods. In this context, significant progress has been made to develop good sensors for selective detection of Cu2+. However, cross sensitivity toward other metal ions, low water solubility, slow response, pH dependence, and a low quantum yield in the aqueous medium are certain limitation which limit the practical utility of such system13-15. Moreover, it is known that copper induces fluorescence quenching due to its paramagnetic nature16,17. Likewise, detection of cyanide through displacement method18-21 has received considerable interest among the scientific communities. The chemical

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displacement approach the high affinity of Cu2+ for cyanide is advantageous due to the formation of stable [Cu(CN)x]

1−x species in the medium10,20,21. Thus, the ensemble based demetallization of copper by cyanide can be utilized as a good strategy to detect the cyanide in the medium through enhanced fluorescence.

Keeping these facts in mind our ongoing research is currently paying attention toward the development of some efficient fluorescent organic scaffolds/motifs to detect anions sensitively in different media22-29. Through this contribution we present the design and synthesis of a new type of intramolecular charge transfer (ICT) based dyad, and its potential application to recognize Cu2+ and CN− through displacement approach. The objective of designing a selective and sensitive probe, 4 has been achieved by developing a conjugated π-electron system containing electron rich imidazole (the donor) and electron deficient oxazole (as the acceptor) units, linked through a phenyl ring. As expected, upon interaction with different cations in HEPES buffer (10 mM, pH 7.0; 20% aqueous THF) ensemble, 4-Cu2+ is generated and the emission of 4 diminished (Turn–Off) due to the chelation enhanced fluorescence quenching (CHEQ) process. However, 4-Cu2+ ensemble upon interaction with different anions showed enhanced fluorescence (Turn–On) with cyanide selectively, through the demetallation process. Moreover, the optical behavior of the probe in the absence and presence of both Cu2+ and CN− have been utilized to construct a memory device.

Results and Discussion Synthesis and Photophysical behavior of probe 4

Scheme 1 shows synthesis of imidazole and oxazole core containing molecular probe 4 and through a one pot multi-component reaction sequence30 in acetic acid involving hydroxybenzaldehyde (m, o, p

derivatives), benzil, aniline and ammonium acetate. The formylation m-substituted derivative, 1a by Reimer-Tiemann reaction31 gave desired products 2 in quantitative yield. Then the compound 2 and o-aminophenol were refluxed in ethanol with catalytic amount of iodine to get Schiff base derivative 3. Compound 3 was subjected to cyclization reaction in the presence of TBACN to yield compound 4 (Scheme 1). The compounds were characterized by 1H and 13C NMR, IR, HRMS and X-ray crystallography (Figures S1-S4, Supporting Information). pH studies

The preliminary optical behavior of 4 under different pH was investigated in HEPES buffer (10 mM, pH 7.0; 20% aqueous THF). The electronic transition spectrum of 4 (10 µM) displayed absorption band at 359 nm (ε = 2.11 × 104 M−1cm−1) and upon excitation at 359 nm 4 (1µM) displayed a strong emission band centered at 450 nm (Ф4 = 0.42; with respect to quinine sulfate32) with Stokes’ shift of 5633 cm−1 (Figure 1). In the acidic medium, pH 6 to 1 absorption spectra of 4 showed a blue shift of 9 nm and appeared at 350 (ε = 2.22 × 104 M−1cm−1) nm

OH

NN

CHO

O

NN

NHO

NH2

OH

ACN/I2/

2

3

CHO

NN

NH2OO

AcONH4

gl. AcOH/110 0C

CHCl3

3KOH

1a,b,c

HO

HO

1a

∆

H

NN

O N

OH

TBACN

∆∆∆∆, THF

Probe 4

Scheme 1 — Synthesis of probe 4.

MISRA & ALI: Cu2+ INDUCED FLUORESCENCE QUENCHING

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while in basic medium, pH 8 to 14, two new absorption band appeared at 312 nm (ε = 1.53 × 104 M−1cm−1) and 374 nm (ε = 1.44 × 104 M−1cm−1) along with two isosbestic point, at 332 nm and at 382 nm (Figure 1a). The formation of isosbestic points indicated about the existance of more than one species in the medium. Moreover, the emission behavior of 4 when examined in acidic medium (pH 6 to 1) displayed fluorescence quenching at 450 nm and a new band appeared at 502 nm (53 nm bathochromic shift), ratiometrically. The formation of an isosemissive point at 381 nm, with a shoulder at 440 nm showed existance of more than one species in the medium. While in the basic medium (pH 8 to 14) the emission band of 4 showed ~58% quenching (bathochromic shift, 22 nm) and appeared at 472 nm (Figure 1b). Such a typical behavior of 4 in acidic and basic medium has suggested about the the protonation and deprotonation as a result of which ICT get modulated due to inhibition in proton transfer as well as transformation of enol to keto form. Response of probe 4 toward different metal ions

The photophysical study of probe 4 in the absence and presence of different tested metal ions has been investigated in HEPES buffer (10 mM pH 7.0; 20% aqueous THF). Upon interaction with different cations (Na+, K+, Mg2+, Ca2+, Zn2+, Pb2+, Al3+, Ag+, Cd2+, Co2+, Ni2+, Cu2+ and Hg2+ as their nitrate salt) the absorption spectrum of probe 4 (10 µM) showed high selectivity for Cu2+ where the absorption band centered, at 359 nm disappeared completely and new broad band appeared at 385 nm (ε = 1.53 × 104 M−1 cm−1) (Figure 2a).

Similarly, the emission spectrum of 4 (1 µM) displayed significant fluorescence quenching with

Fe3+ (~31%) and Cu2+ (~93%) and a new emission band appeared at 440 nm with a blue shift of ~10 nm (Figure 2b). The bright blue-green color of the probe solution changed to a dark blue color with Cu2+. Rest of the cations failed to exhibit any significant change in absorption and emission spectra of 4. Further to confirm the selectivity of 4 toward Cu2+, interference study was performed by addition of other metal ions to the solution of 4+Cu2+ and reversibly Cu2+ to the solution of 4 containing tested metal ions. The interference experiment revealed insignificant change in the absorption and emission spectra of 4+Cu2+, even addition of higher concentration (20.0 equiv) of other metal ions, thus, suggesting about the high sensitivity of 4 toward Cu2+ selectively (Figure 2, bar diagram).

Further the absorption and emission titration experiments have been performed to examine the extent of binding of 4 with Cu2+ (Figure 3). Upon a sequential addition of Cu2+ (0-10.0 equiv) the absorption band centered, at 359 nm reduced gradually and new absorption band appeared at 385 nm, ratiometrically. The formation of an isosbestic point at 377 nm suggested the existence of more than one species in the medium (Figure 3a). Similarly, the emission titration experiment with Cu2+ (0-10.0 equiv) showed fluorescence quenching in which intensity of 4, centered at 450 nm reduced, ~93% and appeared at 440 nm (Figure 3b). Jobs plot analysis suggested about a 1:1 binding stoichiometry for a probe-Cu2+ interaction, consistently (Figure 3c). The binding constant based on emission titration spectral data has been estimated through Benesi-Hildebrand (B-H) method33, and was found to be Kass(em) = 5.16 × 105 /M (Figure 3d). The extent of fluorescence quenching was estimated quantitatively by obtaining an almost

Figure 1 — Change in (a) absorption spectra (10 µM) and (b) emission spectra (1µM) of 4 at different pH levels in HEPES buffer (10 mM pH 7.0; 20% aqueous THF)

390 455 520 585 6500

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300

450

600 (b)pH 7-1

Inte

nsi

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a.u

.)

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pH 7-14

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nsi

ty (

a.u

)

Wavelength (nm)

315 360 405 4500.00

0.08

0.16

0.24

pH 7-14

Ab

sorb

an

ce

Wavelength (nm)

315 360 405 4500.00

0.08

0.16

0.24 (a)

pH 7-1

Ab

sorb

an

ce

Wavelength (nm)

INDIAN J. CHEM., SEC B, FEBRUARY 2018

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linear Stern-Volmer (S-V) plot32 and was found to be Ks-v = 1.04 × 106 M−1 with a decrease in the quantum yield32, ~91% (Φ4+Cu

2+= 0.038) (Figure 3e). Moreover, fluorescence titration data have been acquired to estimate the detection limit of 4 for Cu2+ and was found to be 2.38 × 10−7 M (~14 ppb) (Figure 4a).

Furthermore, to realize that the process of complexation between probe 4 and Cu2+ is reversible a strong chelating agent, EDTA (10.0 equiv) was added to a solution of probable complex, 4−Cu2+. The fluorescence intensity revived and was found almost close to the intensity of 4 (Figure 4b). In contrast, when Cu2+ ions were added to a solution of probe 4 containing EDTA (in excess) insignificant change was observed probably due to the formation of a

strong EDTA-Cu2+ complex (Figure 4c). Thus, suggested about the reversible mode of complexation between 4 and Cu2+ and can be potentially utilized to detect copper in a repeated cycle. 1H NMR titration studies with Cu

2+

To have an insight about the mode of interaction between 4 and Cu2+ the 1H NMR titration experiments were performed in DMSO-d6. The 1H NMR spectrum of 4 (2.1 × 10−2 M) showed resonances at δ 7.95-7.93 (d), 7.84 (s), 7.52-7.17 (m) and 7.02 (s) ppm attributed to aromatic rings protons whereas, the phenolic, –OH proton appeared at δ 11.14 ppm (Figure 5 and S1). The observed significant downfield shift in phenolic proton (-OH) is attributable to the formation of a

Figure 2 — (a) Absorption (10 µM) and (b) emission spectra (1.0 µM) of probe 4 upon interaction of with various metal ions (10.0 equiv) in HEPES buffer. Bar diagram: shows change in absorption and emission intensities of 4 and 4+Cu2+ upon addition of tested metal ions

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stable six membered intramolecular H-bonding between the –OH and N atom of oxazole unit. Further, upon a sequential addition of Cu2+ (0-3.0 equiv) to a solution of probe 4, the phenolic protons shifted downfield and become broadened while the aromatic protons merged together and shifted upfield due to the through bond charge propagation. These result clearly suggested about the involvement of phenolic -OH and =N- atom of oxazolyl unit in the complexation with Cu2+ as shown in Figure 5. Additionally, the molecular ion peak [4+Cu+2NO3+H]+ appeared at m/z

693.2 in a negative ion ESI-MS spectrum confirmed the formation of a 1:1 complex, 4+Cu2+ (Figure S5). Theoretical Calculations

The geometry optimization and quantum chemical calculation data for 4 and 4.Cu2+ have been obtained using the density functional theory (DFT) method as implemented in Gaussian 03 suits of program34 employing basis set B3LYP/6-31G and B3LYP/LANL2DZ*, respectively (Figure 6). Relevant occupied molecular orbital HOMO (-8.5985 eV) of 4 is

Figure 3 — (a) Absorption (10 µM) and (b) emission (1.0 µM) titration spectra of 4 upon a gradual addition of Cu2+ (0-10.0 equiv) in HEPES buffer. (c) Job’s plot (d) Benesi-Hildebrand plot and (e) Stern-Volmer plot obtained from emission spectral data

Figure 4 — (a) Fluorescence intensity change of probe 4 (1 µM, λex = 359 nm) with Cu2+. Change in emission spectra of 4 upon addition of (b) EDTA to a solution of 4 + Cu2+ and (c) Cu2+ ions to solution of 4+EDTA in HEPES buffer

400 450 500 550 6000

150

300

450

600 (c)

Inte

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a.u

.)

Wavelength (nm)

4

4+EDTA

4+EDTA+Cu2+

-7.0 -6.5 -6.0 -5.5 -5.0 -4.50.0

0.2

0.4

0.6

0.8

1.0 (a)

Y = 0.4266x + 2.8258

R2 = 0.9955

(Im

in-I

) /

(Im

in-I

ma

x)

Log [Cu]400 450 500 550 600

0

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+EDTA

222

delocalized to the entire unit in which the electron density is more spread over the imidazole unit while unoccupied molecular orbital, LUMO (more localized to phenyl ring and Moreover, in 4.Cu2+ complex the electron density of the HOMO (-8.9969 eV) is localized on the oxazole and phenyl ring moiety while LUMO have maximum electron density on imidazole unit.suggested about possibility of the intramcharge transfer (ICT) and CHEQ phenomena in probe 4 and 4.Cu2+ complex, respectively.HOMO-LUMO energy difference (4.Cu2+ were found to be 2.59 and 0.3821(Figure 6). The decrease in HOMOgaps (from 2.59 and 0.3821 eV) depicted theof ICT phenomenon in the present sensing events, where the formation of 4.Cu2+ further polarization in electron density which is

Figure 5 — Stacked 1H NMR spectra of 4mechanism of interaction between 4 and Cu

Figure 6 — DFT optimized charge densities and HOMOdiagram for 4 (the thermal ellipsoids are drawn at the 30% ellipsoid probability)

INDIAN J. CHEM., SEC B, FEBRUARY 2018

delocalized to the entire unit in which the electron density is more spread over the imidazole unit while unoccupied molecular orbital, LUMO (-6.0085 eV) is more localized to phenyl ring and oxazole unit.

complex the electron density of is localized on the oxazole

and phenyl ring moiety while LUMO (-8.6148 eV) have maximum electron density on imidazole unit. It suggested about possibility of the intramolecular charge transfer (ICT) and CHEQ phenomena in probe

complex, respectively. Further, the LUMO energy difference (∆E) for 4 and

were found to be 2.59 and 0.3821 eV, respectively ). The decrease in HOMO-LUMO energy

depicted the involvement of ICT phenomenon in the present sensing events,

complex led to a further polarization in electron density which is

relatively more toward the imidazole unit due to metalto ligand charge transfer (MLCT) processes. Anion interaction studies

Cu2+ has strong affinity to interact with CNform stable [Cu(CN)x]

1−Therefore, to observe the change in photophysical property of the probe by displacement phenomenonfirst, ensemble 4+Cu2+ was examined in the presence of different class of anions such as F−, Cl−, Br−, I−, SCNSO4

2−, NO3− and S2− (as their nitrate salts) through the

absorption and emission spectroscopynoteworthy to mention that upon interaction with CNthe transition band of 4

revived and was almost similar to the absorption band of probe 4 (Figure 7a). Similarly, the observed quenched “turn-off” emission of

4 (2.0 × 10−2 M) upon addition of Cu2+ (0-3.0 equiv) in DMSOand Cu2+

DFT optimized charge densities and HOMO-LUMO energy gap. Minimum energy structures of (the thermal ellipsoids are drawn at the 30% ellipsoid probability)

relatively more toward the imidazole unit due to metal to ligand charge transfer (MLCT) processes.

Anion interaction studies has strong affinity to interact with CN− to

−x species in the medium. Therefore, to observe the change in photophysical

probe by displacement phenomenon was generated in situ and then

the presence of different class of anions , SCN−, AcO− CN−, CO3

2−, (as their nitrate salts) through the

absorption and emission spectroscopy. It is noteworthy to mention that upon interaction with CN−

4+Cu2+ centered, at 385 nm was almost similar to the absorption band

7a). Similarly, the observed off” emission of 4+Cu2+ “turn-on” to

3.0 equiv) in DMSO-d6. Scheme shows plausible

LUMO energy gap. Minimum energy structures of 4 and 4.Cu2+. ORTEP

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original emission of 4 at 450 nm (λex=359 nm) along with unique naked-eye sensitive colorimetric response wherein, the dark blue color of 4+Cu2+ upon interaction with CN− ion regenerated to intense blue-green color, under UV light (Figure 7b and Images). The rest of the anions failed to exhibit any significant change in absorption and emission intensity of 4+Cu2+ (Figure 8a).

Further the fluorescence titration data were utilized to estimate binding constant for ensemble 4+Cu2+ with CN− and was found to be 2.36 × 105 M−1 (Figure 8b). The high binding affinity of CN− with 4-Cu2+ clearly suggested about the demetallization of copper by the formation of stable [Cu(CN)x]

1−x species in the medium. Further the limit of detection for CN− was found to be 1.75 µM (~45.5 ppb) (Figure 8c), which is well below the recommended value of cyanide in drinking water10.

On-Off switching behavior of 4 and logic implication Recently, the logic gates have shown great

importance in molecular level computing devices and self-regulatory chemical systems35. Recently, our group7,19,20 and others36 showed the importance of sequential logic circuits in creating memory devices to store information and perform operation through feedback loops, in which one of the outputs of the device function as an input and is memorized as ‘memory element’. The observed photophysical behavior of present probe 4 correlate “On-Off-On” switching behavior with Cu2+ and CN− therefore, we tried to implement the output emissions of respective applied chemical inputs, In1 (Cu2+) and In2 (CN−) to construct the integrated and sequential logic circuits. To perform logic operation, 70% of maximum output

Figure 7 — (a) Absorption and (b) Emission-titration spectra of complex 4.Cu2+ (10 µM) with CN− (0-10 equiv) in HEPES buffer. Images: Illustrate fluorogenic (UV light; 365 nm) response of 4+Cu2+ in the presence tested anions

Figure 8 — (a) Bar diagram of change in emission intensities of 4+Cu2+ upon addition of tested anions and (b) Benesi-Hildebrand plots obtained from emission spectral data. (c) Fluorescence intensity change of 4+Cu2+ system with CN- ion.

CN F Cl Br I AcO s2- So4 no3 n3- po4 co3 Scn0

106

212

318

424

530

(a)

N 3

-

NO 3

-

SCN-

PO 4

3-

CO 3

2-

SO 4

2-

S2-

AcO

-

I-

Br-

Cl

-

F-

CN-

∆∆ ∆∆I

(4+Cu2+

)+Anions

(4+Cu2+

) CN-+Anions

-5.4 -5.1 -4.8 -4.5 -4.2 -3.9

0.0

0.2

0.4

0.6

0.8

1.0 (c)

Y = 0.6922x + 3.9855

R2 = 0.9932

(Im

in-I

) /

(Im

in-I

ma

x)

Log [CN-]

0.0 1.2 2.4 3.6 4.8 6.00.00

0.01

0.02

0.03

0.04

0.05

0.06 (b)

Y = 0.0089 x + 0.0021

R2 = 0.9993

1 /

∆∆ ∆∆I

1 / [CN-]

224

emission intensity (Rf ~ 0.7 at 570 au) has been considered as threshold value. The relative emission intensity above than threshold value has been assigned as “1” whereas, lower one as “0” and has been correlated to “On” and “Off

readout signals, respectively (FigureAccording to truth table (Figure

logic operations 4, 4+CN− and 4+Cuhigh output emission (at 580 au) whereas, low output emission was obtained when logic operations and 4+CN−+Cu2+ were performed, respectively. Thus, the equipped sequential logic circuit signifies the set/reset element, and corresponds to the memory device (Figure 9B) in which the two inputs: set (and reset (In1), based sequential logic operations mimic the function of a memory element. Moreover, the reversible and reconfigurable sequences of the set/reset logic operations in the feedback loop demonstrate a memory feature analogous to “writeread–erase–read” functions36 (Figurereset input (In1 = 1) leads to a fluorescence turnstate (at 570 au) and encoded information in the system as “read” and “erased” and the logic operation is saved as “Output = 0”. Further, the storedwithin the system sends signal “write” by the set input (In2 = 1) and the fluorescence device reaches to turnOn state (at 570 au) and the logic operation is saved as “Output = 1”.

Figure 9 — (A) Truth table (1 = On; 0 = Off) for the sequential logic circuit (BIn2) and one output (570 au), and (C) A schematic representation of the reversible logic operations for the element possessing ‘writeread-erase-read’ functions

INDIAN J. CHEM., SEC B, FEBRUARY 2018

~ 0.7 at 570 au) has been considered as threshold value. The relative emission intensity above than threshold value has been

signed as “1” whereas, lower one as “0” and has Off” states for the

readout signals, respectively (Figure 9 and Figure 10). According to truth table (Figure 9A) sequential

+Cu2++CN− resulted output emission (at 580 au) whereas, low output

emission was obtained when logic operations 4+Cu2+ were performed, respectively. Thus,

the equipped sequential logic circuit signifies the set/reset element, and corresponds to the memory

B) in which the two inputs: set (In2) 1), based sequential logic operations

mimic the function of a memory element. Moreover, the reversible and reconfigurable sequences of the set/reset logic operations in the feedback loop

rate a memory feature analogous to “write–(Figure 9C) in which the

1 = 1) leads to a fluorescence turn-Off state (at 570 au) and encoded information in the system as “read” and “erased” and the logic operation

saved as “Output = 0”. Further, the stored information within the system sends signal “write” by the set input

2 = 1) and the fluorescence device reaches to turn-state (at 570 au) and the logic operation is saved

Further, to examine the reproducibility of device, constructed on the basis of optical behavior of write-erase cycles were repeated by providing alternate chemical inputs of Cumonitored the fluorescence “behavior at 570 au. Figureaddition of Cu2+ (10 equiv.) to a solution of fluorescence quenching occurred (switched570 au) due to the formation of Next, addition of CN− (10 equiv.) led to fluorescence enhancement (switched–On

emission intensity was regained due to formation of [Cu(CN)x]

1−x species in the medium, and was almost close to intensity of 4. On repeating the process for around ten cycles probe switched –On/Off characteristics as well as visual fluorescence color changes that naked-eye.

Analytical application

Detection of Cu2+ and CN− on cellulose paper strip

In order to make sure the analytical probe 4, a paper strip test was performedpresence and absence of Cucellulose paper strips (Whatmanconcentration of probe 4

prepared (1.5 × 2.0 cm2) in dried in air. Stock solution concentrations (1 × 10−5,

Truth table (1 = On; 0 = Off) for the sequential logic circuit (B) which, displays memory unit with two inputs (and (C) A schematic representation of the reversible logic operations for the element possessing ‘write

Further, to examine the reproducibility of device, constructed on the basis of optical behavior of 4

erase cycles were repeated by providing alternate chemical inputs of Cu2+ and CN− ions and monitored the fluorescence “On-Off” switching behavior at 570 au. Figure 10 shows that after first

(10 equiv.) to a solution of 4 fluorescence quenching occurred (switched-Off; at 570 au) due to the formation of 4+Cu2+ complex.

(10 equiv.) led to fluorescence On) in which the observed

emission intensity was regained due to formation of in the medium, and was almost . On repeating the process for

probe 4 showed good rewritable characteristics as well as visual

fluorescence color changes that were sensitive to the

on cellulose paper strip

In order to make sure the analytical application of paper strip test was performed in the

presence and absence of Cu2+/CN− ions. Small cellulose paper strips (WhatmanTM) containing different

4 (4, 2, and 1.0 mM) were ) in 20% aqueous THF and

Stock solution of of three different , 1 × 10−6 and 1 × 10−7 M) of

which, displays memory unit with two inputs (In1 and and (C) A schematic representation of the reversible logic operations for the element possessing ‘write-

MISRA & ALI

copper nitrate was prepared in water.paper strips were dipped in a solutions different concentration of copper ion and dried in air. The observed change of paper strips from intense non fluorescent blue under UV light at 365 nmclearly demonstrated the potential application of probe 4 to detect copper ion on paper strip (Figure 11a,b,c). Test paper strips of 0.concentration of probe were also made. The paper strip could able to detect Cu2+ ions but the visibility of color on the strip was much better only up to range. Further, to confirm the potential sensitivity of resulted 4+Cu2+ complex towards cyanide ion onsurface of paper strip as it is shown in solution phase. The dried paper strips containing 4in a solution of CN− (5×10−5 M) fodried in air. Interestingly, the intense color of probe 4 was revived from non fluorescent blue color of 4+Cu2+ (Figure 11d).

Experimental Section Materials and Chemicals

All the reagents and solvents were purchased from Sigma-Aldrich Chemical Co. Pvt. Ltd. stored in a desiccator under vacuum containing self indicating

Figure 10 — (a) Change in fluorescence intensity of diagram represents input (In1 = Cu2+ and In

Figure 11 — Fluorescent paper strips of probe Cu2+ (a) 1 × 10−5, (b) 1 × 10−6, (c) 1 × 10florescent), (ii) 4+Cu2+ (1 × 10−5; weak blue florescent) and (iii)

0 2 40

130

260

390

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650CN

-

Cu2+

(3)

(2)

(1)

Cu2+

4

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MISRA & ALI: Cu2+ INDUCED FLUORESCENCE QUENCHING

prepared in water. The dried test solutions containing

of copper ion for 10-15 min and dried in air. The observed significant color

intense fluorescent blue to under UV light at 365 nm,

demonstrated the potential application of to detect copper ion on paper strip

Test paper strips of 0.5 and 0.05 mM concentration of probe were also made. The paper

ions but the visibility of color on the strip was much better only up to 1.0 mM

, to confirm the potential sensitivity of complex towards cyanide ion on solid

as it is shown in solution phase. 4+Cu2+ was dipped

M) for 10-15 min and intense fluorescent blue

was revived from non fluorescent

All the reagents and solvents were purchased from Aldrich Chemical Co. Pvt. Ltd. stored in a

desiccator under vacuum containing self indicating

silica, and used without any further purification. Solvents were purified prior to use. UVspectra were recorded on a Perkin Elmer LambdaUV-vis spectrophotometer using a quartz cuvette (path length = 1cm). Infrared (IR) spectra were recorded in potassium bromide (KBr) on FT-IR spectrometer. 1H NMR spectra (chemical shifts in δ ppm) were recorded on a JEOL AL 300 FT(300 MHz) spectrometer, using tetramethylsilane (TMS) as internal standard. Fluorescence spectra were recorded on Varian eclipse Carry spectrofluorometer using a quartz cuvette (path length = 1 cm) at 600 PMT voltage and slit width 5nm/5nm. All the spectroscopic experiments were carried out at room temperature.

Preparation of stock solutions and titration

experiments The stock solution of

prepared in THF. For each absorption and measurements 30 µL and 3taken and diluted to 3.0 mL to make the concentration of probe 10 µM and 1 µΜpH 7.0; 20% aqueous THFsolutions of different metal ions and anions (1×10were prepared by dissolving their nitrate salt in water.

(a) Change in fluorescence intensity of 4 (10 mM) upon addition of Cu2+ and CN− in HEPES buffer (In2 = CN−) and output emissions (at 570 au)

probe 4 (a) 4.0 mM, (b) 2.0 mM, (c) 1.0 mM before (intense blue florescent× 10−7 M (weak blue florescent); (d) Paper strips containing (i) probe

; weak blue florescent) and (iii) 4+Cu2++CN− (5 × 10−5; intense blue florescent)

4 6 8 10

(a)

CN-CN

-CN-

Cu2+

Cu2+

Cu2+

(10)

(9)

(8)

(7)

(6)

(5)

(4)

Cycles 2 4 60

150

300

450

600

(b)

CNCN-Cu

2+Cu

2+CN

-Cu

2+4

Inte

nsi

ty (

a.u

.)

INDUCED FLUORESCENCE QUENCHING 225

silica, and used without any further purification. Solvents were purified prior to use. UV-vis absorption spectra were recorded on a Perkin Elmer Lambda-35

vis spectrophotometer using a quartz cuvette (path length = 1cm). Infrared (IR) spectra were recorded in potassium bromide (KBr) on Varian-3100

NMR spectra (chemical shifts m) were recorded on a JEOL AL 300 FT-NMR

(300 MHz) spectrometer, using tetramethylsilane (TMS) as internal standard. Fluorescence spectra were recorded on Varian eclipse Carry spectrofluorometer using a quartz cuvette (path length = 1 cm) at 600

e and slit width 5nm/5nm. All the spectroscopic experiments were carried out at room

Preparation of stock solutions and titration

The stock solution of 4 (c = 1×10−3 mol L−1) was prepared in THF. For each absorption and fluorescence

and 3µL, of stock solution was taken and diluted to 3.0 mL to make the concentration

Μ in HEPES buffer (10 mM, 20% aqueous THF), respectively. The stock

solutions of different metal ions and anions (1×10−1 M) were prepared by dissolving their nitrate salt in water.

in HEPES buffer (λex = 359 nm). (b) Bar

intense blue florescent) and after addition of containing (i) probe 4 (4mM; intense blue

florescent)

8 10CN-

CN-Cu

2+Cu

2+

INDIAN J. CHEM., SEC B, FEBRUARY 2018

226

The cation interaction studies were performed by the addition of 10 equiv. of 1×10−1 M of different cations. The absorption and fluorescence titration experiment were performed by the gradual increase of concentration of Cu2+ (c =1×10−3). For 1H NMR titration experiment solution of probe 4 (1×10−2 M) and Cu(NO3)2 was prepared in DMSO-d6.

X-ray diffraction Single crystal of the compound 4 was obtained by

slow evaporation of saturated solution of compound 4 in dichloromethane coated with methanol over a period of few weeks at ambient temperature. Single crystal X-ray diffraction measurements were carried out on an Oxford Diffraction Xcalibur system with a Ruby CCD detector. All the determination of unit cell and intensity data were performed with graphitemono-chromated Mo-Kα radiation (λ = 0.71073 Å). Data were collected at the room temperature. Structure were solved by direct method, using Fourier techniques, and refined by full matrix least-squares on F

2 using the SHELXTL-97 program package.

Estimation of quantum yields The quantum yields of probe 4 and 4+Cu2+ were

estimated with respect to the quinine sulfate (Φ = 0.54) as standard in 0.1M H2SO4 solution, using equation (1) by secondary methods32.

Q = QR. I/IR. ODR/OD. n2/n2R … (1)

Where Q is the quantum yield, I is the integrated intensity, OD is the optical density, and n is the refractive index. The subscript R refers to the reference fluorophore of known quantum yield. Estimation of binding constant

The absorption and fluorescence experimental data were utilized to calculate association constants by Benesi-Hildebrand method33 (B-H method) employing equations (2) for 1:1 stoichiometries.

1/(I - Io) = 1/(I - If) + 1/K(I - If)[M] … (2)

Where K is the association constant, I is the absorbance/fluorescence intensity of the free probe 4, Io is the observed absorbance/fluorescence intensity of the 4+Cu2+

complex, and If is the absorbance/ fluorescence intensity at saturation level.

Estimation of quenching constant The quenching constant32 was estimated by using

Stern-Volmer relationship. I0/I = 1 + Ksv [Q] … (3)

Where I0 and I is the fluorescence intensity of probe before and after addition of quencher respectively, [Q] is the quencher concentration and Ksv is the Stern-Volmer constant.

Estimation of limit of detection The detection limit of probe for analyte (metal

ions/anions) was estimated from the respective fluorescence titration data based on a reported method37,38. According to the result of titration experiment, change in the fluorescence intensity of probe with analyte was normalized between the minimum intensity and the maximum intensity. A linear regression curve was obtained from the plot of these normalized fluorescence intensity of probe with analyte verses Log [analyte] and detection limit was calculated by using equation (4). 10 –[Slope / Intercept] …(4)

General procedure for synthesis of compound 1a,

1b, and 1c Benzil (1.05 g, 5 mmol) and aromatic aldehyde (m,

o and p-hydroxybenzaldehyde) (610 mg, 5 mmol) were dissolved in of glacial acetic acid (20 mL) at RT. To this solution aniline (0.7 mL, 7.5 mmol) was added dropwise. After the addition of ammonium acetate (2.0 g, 26 mmol) the reaction mixture was heated at 110°C for 4 hr and monitored the reaction on TLC. After completion of reaction, the reaction mixture was cooled to RT and poured into the ice-water. The precipitate was filtered, washed with cold water, air dried and recrystallized from ethylacetate to get desired compound 1a, 1b, and 1c.

1a: Yield 1.4 g (72%). Rf = 0.52 (Ethylacetate: DCM:: 2:8, v/v). 1H NMR (300 MHz, DMSO-d6): δ 9.52 (s, 1H, -OH), 7.48-7.62 (d, 2H, J = 7.2 Hz),

7.30-7.16 (m, 13H), 7.04-6.99 (t, 1H, J = 7.8 Hz, 7.8 Hz), 6.94 (s, 1H), 6.69-6.65 (t, 2H, J = 6.0 Hz, 6.3 Hz); FT-IR (KBr): 3051, 1597, 1582, 1497, 1482, 1443, 1397, 1376, 1300, 1213, 1178, 1076, 998, 970, 884, 766, 695 cm−1.

1b: Yield 1.7 g (85%). Rf = 0.6 (Ethylacetate: DCM:: 2:8, v/v). 1H NMR (300 MHz, DMSO-d6): δ 12.76 (s, 1H, -OH), 7.60-7.25 (m, 10H), 7.09-7.06 (d, 2H, J = 8.1 Hz), 6.93-6.90 (d, 2H, J = 7.2 Hz); FT-IR (KBr): 3329, 1595, 1588, 1516, 1495, 1435, 1381, 1336, 1272, 1168, 1146, 1107, 1076, 883, 764, 698 cm−1.

1c: Yield 1.6 g (80%). Rf = 0.46 (Ethylacetate: DCM:: 2:8, v/v). 1H NMR (300 MHz, DMSO-d6):

MISRA & ALI: Cu2+ INDUCED FLUORESCENCE QUENCHING

227

δ 9.70 (s, 1H, -OH), 7.90-7.87 (d, 2H, J = 8.4 Hz),

7.51-7.48 (d, 12H, J = 7.2 Hz), 7.34-7.26 (m, 3H), 7.85-6.83 (d, 2H, J = 8.7 Hz); FT-IR (KBr): 3061, 1597, 1520, 1475, 1341, 1233, 1179, 1107, 1071, 970, 854, 765, 696 cm−1. Synthesis of compound 2

Compound 1 (1.4 g, 3.65 mmol) was taken in dry ethanol (6 mL) and an aqueous solution (15 mL) of KOH (0.9 g, 16 mmol) was added. The reaction mixture was heated at 70-80°C and then added CHCl3 (1.43 g, 0.97 mL, 12 mmol) dropwise for 10-20 min. Once the color of reaction mixture became brine red stirring was continued for 2hr and then at RT for another 2-3hr. The excess of CHCl3 and EtOH were distilled off and the residue was treated with conc. HCl to make the pH of solution acidic (pH 2-3). After the addition of water (15 mL) precipitate was filtered, washed with water, air dried and purified by column chromatography using dichloromethane as eluent. The solvent was evaporated in vacuum to get compound 2. Yield 100 mg (7%). Rf = 0.56 (Ethylacetate:DCM:: 0.5:9.5, v/v). 1H NMR (300 MHz, DMSO-d6): δ 10.76 (s, 1H, -OH), 10.19 (s, 1H, -CHO), 7.49-7.47 (d, 2H, J = 6.3 Hz), 7.35-7.15 (m, 16H); 13C NMR (75 MHz, DMSO-d6): δ 190.89, 160.19, 144.60, 137.44, 137.27, 136.33, 134.06, 132.34, 131.09, 130.01, 129.30, 129, 128.80, 128.54, 128.21, 126.68, 126.38, 121.65, 118.99, 116.68; FT-IR (KBr): 3415, 3051, 2962, 2924, 2850, 1662, 1626, 1595, 1495, 1461, 1381, 1315, 1261, 1204, 1097, 1025, 966, 914, 874, 805, 701 cm−1; HRMS (micrOTOF-Q): m/z [2 + H]+ Calcd for C28H20N2O2: 417.1603. Found: 417.1598.

Synthesis of compound 3 Compound 2 (100 mg, 0.24 mmol) and 2-amino-

phenol (27 mg, 0.24 mmol) were taken in acetonitrile (10 mL) and stirred at RT for 15 min to get a clear solution. The reaction mixture was refluxed for 2 hr in the presence of iodine (3 mol % of reactant). The red colored precipitate so obtained was filtered, washed with acetonitrile and dried in air to get the desired compound 3 in good yield. Yield 90% (110 mg). Rf = 0.62 (Ethylacetate:DCM:: 0.5:9.5, v/v). 1H NMR (300 MHz, DMSO-d6): δ 13.79 (1H, -OH, Hb), 9.71 (1H, -OH, Ha), 8.89 (s, 1H, -HC=N), 7.50-7.47 (d, 2H, J = 6.3Hz), 7.35-7.08 (m, 17H), 7.02-6.99 (d, 1H, J = 7.8 Hz), 6.94-6.92 (d, 1H, J = 7.8 Hz), 6.85-6.83 (d, 1H, J = 6.3 Hz); 13C NMR (300 MHz, DMSO-d6): δ 160.75, 151.30, 145.25, 137.44, 136.71, 134.76, 134.28, 134.09, 132.22, 132.10, 131.30, 130.26,

129.48, 129.20, 128.68, 128.42, 126.92, 126.62, 119.87, 119.26, 118.77, 116.69, 116.27; FT-IR (KBr): 3430, 3061, 2958, 2924, 2852, 1615, 1515, 1506, 1497, 1465, 1352, 1285, 1224, 1163, 1119, 925, 722, 694 cm−1; HRMS (micrOTOF-Q): m/z [3 + H]+ Calcd for C34H25N3O2: 508.2025. Found: 508.2019.

Synthesis of compound 4 Compound 3 (75 mg, 0.15 mmol) and TBACN

(54 mg, 0.2 mmol) were stirred at RT for 5 min to get a clear solution in THF (10 mL). The reaction mixture was refluxed for 45 min. and monitored the reaction on TLC. After completion of reaction, the reaction mixture was cooled to RT, removed the excess solvent under reduced pressure and washed with little amount of cold methanol (3 × 0.5 mL). Yield 66% (50 mg). Rf = 0.5 (Ethylacetate:DCM:: 0.5:9.5, v/v). 1H NMR (300 MHz, DMSO-d6): δ 11.14 (1H, -OH), 7.95-7.93 (d, 2H, J = 8.1 Hz), 7.84 (s, 1H), 7.52-7.17 (m, 18H), 7.02 (s, 1H); 13C NMR (300 MHz, DMSO-d6): δ 162.83, 157.12, 148.89, 144.52, 139.53, 137.36, 136.47, 135.04, 134.12, 132.30, 131.92, 131.17, 130.07, 129.40, 129.14, 128.73, 128.54, 128.26, 127.57, 126.40, 125.98, 125.38, 119.75, 119.25, 116.13, 111.07, 110.08. FT-IR (KBr): 3433, 3055, 2963, 2921, 2951, 1632, 1573, 1573, 1517, 1494, 1477, 1452, 1410, 1245, 1226, 1072, 1046, 900, 805, 762, 741, 720, 696, 660 cm−1. HRMS (micrOTOF-Q): m/z [4 + H]+ Calcd for C34H23N3O2: 506.1868. Found: 506.1863. Conclusion

In summary, we have developed an efficient fluorescent probe comprising imidazole and oxazole moiety linked through phenyl ring. Probe showed a typical photophysical behavior (on-off-on fluorescence) in the presence and absence of Cu2+ and CN− ions in HEPES buffer. In the presence of Cu2+ significant fluorescence quenching (turn-off) was observed. The in situ generated 4+Cu2+ ensemble with CN− anion displayed unique colorimetric response along with revival of fluorescence intensity (turn-on) of probe. Additionally, the chemosensor 4 has shown fluorogenic response to detect Cu2+ and CN− ions in solution as well as on test paper strips. Further, the naked eye sensitive “On-Off-On” sensing behavior of probe 4

mimics the function of a sequential logic circuit at molecular level with inputs of Cu2+ and CN−. Thus, the present designing of probe further extend the possibility for the development of good chemosensor for the detection of copper and cyanide through the displacement approach.

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Supplementary Information Supplementary information is available in the website http://nopr.niscair.res.in/handle/123456789/60. Acknowledgement

The authors are thankful to the University Grants Commission (UGC) and the Council of Scientific and Industrial Research (CSIR), New Delhi (02(0199/14/EMR-II) for financial support and also thankful to Department of Science and Technology (DST-FIST, PURSE program) to enhance basic instrumentation and research facility in the department. References

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