Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 499–508

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Review Article

Anion recognition by simple chromogenic and chromo-fluorogenic salicylidene Schiff base or reduced-Schiff base receptors Sasanka Dalapati 1, Sankar Jana, Nikhil Guchhait ⇑ Department of Chemistry, University of Calcutta, 92, A.P.C. Road, Kolkata 700009, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Structurally simple Schiff bases or

reduced Schiff bases beneficial for selective anion recognition.  Receptor and anions weak interaction studied by simple colorimetry and spectroscopy.  Cheaper and user handling anion sensing test kits for useful practical applications.

a r t i c l e

i n f o

Article history: Received 22 October 2013 Received in revised form 3 March 2014 Accepted 18 March 2014 Available online 2 April 2014 Keywords: Anion sensors Test kits Schiff base Reduced Schiff base Chromogenic Chromo-fluorogenic

a b s t r a c t This review contains extensive application of anion sensing ability of salicylidene type Schiff bases and their reduced forms having various substituents with respect to phenolic AOH group. Some of these molecular systems behave as receptor for recognition or sensing of various anions in organic or aqueous–organic binary solvent mixture as well as in the solid supported test kits. Development of Schiff base or reduced Schiff base receptors for anion recognition event is commonly based on the theory of hydrogen bonding interaction or deprotonation of phenolic –OH group. The process of charge transfer (CT) or inhibition of excited proton transfer (ESIPT) or followed by photo-induced electron transfer (PET) lead to naked-eye color change, UV–vis spectral change, chemical shift in the NMR spectra and fluorescence spectral modifications. In this review we have tried to discuss about the anion sensing properties of Schiff base or reduced Schiff base receptors. Ó 2014 Elsevier B.V. All rights reserved.

Contents Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of anion receptors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chromogenic anion receptors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chromo-fluorogenic anion receptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Corresponding author. Tel.: +91 33 2350 8386; fax: +91 33 2351 9755. E-mail address: [email protected] (N. Guchhait). Present address: Institute for Molecular Science, National Institute of Natural Sciences, 5-1 Higashiyama, Myodaiji, Okazaki 444-8787, Japan. 1

http://dx.doi.org/10.1016/j.saa.2014.03.090 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

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Chemical structures of compound 1–45 used in this review. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A. supplementary material . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Introduction Design and synthesis of specific sensor for the recognition and sensing of anions is one of the most demanding areas of current chemical research because of their significant contribution in the field of chemical, biological, industrial, agricultural and environmental sciences [1–8]. Presently, designing of molecular sensors that can selectively recognize anions have great significance in supramolecular chemistry [9–12]. Over the past few years, a great deal of efforts have been dedicated for the development of fluorescent chemosensors for the detection of different anions such as F , Cl , I , AcO , H2PO4 , HSO3 , HCO3 , ClO4 , NO2 , NO3 , HSO4 and SO24 as fluorometric detection is cost-effective and highly sensitive. Regarding the development of fluorescent chemosensor quenching of fluorescence by interaction with anions is a rather commonly used technique. But, UV–vis absorption spectroscopy and colorimetric naked-eye detection techniques have gained comparatively faster attention than other techniques due to their simplicity, high sensitivity and cost effectiveness [12–15]. Currently, many excellent chemosensors have been reported for recognition and sensing of anions with high selectivity and sensitivity. But, there are disadvantages due to complicated structure of those chemosensors, required hard synthetic routes or troublesome purification process and poor yields. On the other hand, easy to synthesize with good yield are the most interesting and significant features of Schiff bases. Mainly, the Schiff bases derived from salicylaldehyde, i.e. salicylidene Schiff bases exist in a tautomerization equilibrium in solution due to the presence of AOAH


N@CA (Scheme S1) type of hydrogen bonding network [1,16–21]. Therefore, action of base (OH ) or strongly basic anions (such as F , AcO and Cl ) can easily deprotonate the AOH proton or promote the formation of keto-tautomer or inhibit the excited state proton transfer (ESIPT) phenomenon [2,22–25] and thereby modify the spectral characteristics and hence acts as an efficient sensor. Importantly, the acidity of the AOH proton also plays the key role for this H-bonding/acid–base type reaction. The acidity of AOH proton usually depends on the effect of substitution in the benzene ring, especially, with respect to the AOH group, in that case most electron withdrawing substituents (such as ANO2 and ACl) get preference. It is obvious that structurally reduced Schiff base loses the tautomerization equilibrium (as present in the Schiff base), but gain an extra function (ANHA proton, Scheme S2) which can help to form hydrogen bonded complex with anions, i.e. initially reduced Schiff base forms H-bonding complex with anions (with low conc.) and then produces a deprotonated species (at high conc. of anions) responsible for photophysical changes. The beneficial side of reduced Schiff bases is that these bases deserve to resist hydrolysis in water, but Schiff bases cannot. In this review, we have primarily emphasized two types of anion sensing Schiff bases and reduced Schiff bases chemosensors: (1) Chromogenic anion receptor, (2) Chromo-fluorogenic anion receptor. Chromogenic anion receptors selectively recognize various anions and the phenomena are well rationalized on the basis of their naked-eye color change, UV–vis spectral change and chemical shift in the 1H NMR spectra. On the other hand, chromo-fluorogenic anion receptors selectively detect various anions by means of naked-eye color change, UV–vis and 1H NMR spectral shift as well as fluorescence spectral change. The color change or spectral modification is due to the formation of

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506 507 507 507

hydrogen bonded complex (host–guest complex) between the receptor and the anion or may be due to the formation of deprotonated species. Host–guest interactions have been investigated by means of various spectral measurements and also with the assistance of simple theoretical model calculations. The summary of this review is complied in Tables 1 and 2. Types of anion receptors Chromogenic anion receptors There are various kinds of uniquely designed molecules which were reported to be as selective chromogenic anion receptors. Butcher et al. reported a simple but interesting Schiff base receptor 1 that exclusively exists in the solid state as keto-amine tautomeric form instead of phenol–imine tautomer due to the presence of highly acidic phenolic AOH proton and therefore enhances hydrogen bonding capability in presence of anions [26]. The receptor 1 shows yellow color in acetonitrile solvent and a broad absorption band was observed at 342 nm. After addition of considerable amount of F , AcO and H2PO4 anion (used their tetrabutylammonium (n-Bu4N+) salts) in acetonitrile medium the naked-eye color of the solution turns to deep yellow from light yellow and a new absorption peak at 450 nm appears in the expense of decreasing the initial peak at 342 nm. Under the same experimental circumstances, addition of large amount Cl , Br , ClO4 and HSO4 anions results in neither naked-eye color change nor significant UV–vis spectral changes. The naked-eye color and UV–vis spectral change are attributed to the formation of strong hydrogen bonding complex between the receptor 1 and the anions with 1:1 stoichiometry. The selectivity and sensitivity of the receptor clearly depends on the basicity and shape of the anions and definitely on the acidity of hydroxyl group which depends on the nature of the substituents in the benzene ring with respect to the AOH group, i.e. more electron withdrawing substituents makes it more acidic. The selectivity of the receptor 1 towards Y-shaped AcO anion with strong doubly H-bonding capability has been further proved by using 10% water–acetonitrile solution of 1, where only acetate anion is capable to change detectable naked-eye color as well as UV–vis spectral change, but there are hardly any changes that have been observed in case of fluoride and dihydrogenphosphate anions. Furthermore, the naked-eye color/UV–vis spectral change is reversible with respect to the addition of protic solvents, such as water and methanol. Because anions can exit in the highly solvated form in those solvents, which support the concept of host–guest H-bonding interactions. Another simple symmetrical azine Schiff base receptor 2 exhibits strong and broad absorption bands at 288 nm and 345 nm, respectively [12] in aqueous–acetonitrile (CH3CN:H2O: DMSO = 94:4:1) solvent. These bands are due to the transition between the localized p-orbitals on the azomethine group (AC@NA) and due to the existence of charge transfer processes within the whole molecule. Upon addition of increasing amount of F /AcO ion to the receptor 2 in aqueous acetonitrile solvent, peaks at 288 nm and 345 nm gradually decrease their initial intensity and new absorption peaks gradually appear at 397 nm and 455 nm. The appearance of a distinct isosbestic point at 365 nm during the titration process indicates host–guest complexation equilibrium. The titration results were found to be well fitted with

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S. Dalapati et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 499–508 Table 1 Some useful parameter from UV–vis spectral investigation on chromogenic receptors. Chromogenic receptors 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

Solvent ACN

DMSO

ACN

DMSO–H2O

kabs (nm) 342, 275 345, 288 298 345, 280 280–320 304 304 375, 435 320, 435 304, 262 344 382 301 348, 300 362, 301 550, 450 550, 390 400

DMSO

DMSO–H2O DMSO

450, 450, 550, 530, 350

350 350 375 412

kAabs (nm)

KA  10

L:A

a

3

b

c

Selective for

Color change Deep yellowa,b,c Yellowa,b Light yellowa,b,c – Light yellowa,b – Bright yellowa,b – – Bright yellowa,b,c Pale yellowa,b Violeta Bluish greena Orangea,b Fluorescent yellowa,b – Purpale – – Magentaa,yellowe Magentaa, yellowe – – – Yellowd Yellowa,b,c

450 397, 455 425, 359

1:1 1:2

20 , 22 , 9.6 39.8a, 480b 103.5a, 399b, 6.2c

F , AcO , H2PO4 F , AcO F , AcO , H2PO4

422, 360

1:1a, 1:2b

13.6a, 70.5b

F , AcO

403, 425 375, 435 435 355 464 441 411, 364 457, 369 417 550b, 444e 550b, 420e 420 500 410

1:1

12.7a, 11.2b 25.1a, 29.8b, 44.9c 13.4a, 5.73b, 9.25c >59.5a, 59.5b, 5.96c 22.9a, 10.4b 1.2 3.3 2.52a 6.20b

F , AcO F , AcO , H2PO4

350d, 345d, 374d, 374d, 469

F , AcO F F , AcO

2.27b

AcO

4.0b, 10.4e 2.68b, 10.0e

Cl , AcO Cl , AcO F , AcO , H2PO4

425–500a,b,c 450–500a,b,c 450–525a,b,c 540a,b,c

HSO4 F , AcO , H2PO4

Abbreviations used for receptors, anions and association constants as L, A and K, respectively. For 1:1 and 1:2 ratio the unit of K are M 105. kAabs indicates absorption band of receptors in presence of anions (A ). ACN is CH3CN. a Indicate F anion. b Indicate AcO anion. c Indicate H2PO4 anion d Indicate HSO4 anion e Indicate Cl anion.

1

and M

2

, respectively.  is a factor as

Table 2 Some useful parameters obtained from UV–vis and fluorescence spectral investigation on chromo-fluorogenic receptors. Chromo–fluorogenic receptors

Solvent

kabs (nm)

kAabs (nm)

kem (nm)

kAem (nm)

L:A

KA  10

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

ACN

336, 316 340, 318 355, 321 355

420

490, >500

475

1:1

487 370

2.787 5.684 25.3 48.6

THF

508, 500, 496, 342,

457 518 439 447

620 598 610 517

517, 478

326 326

447 425

517 463

517 475

1:1

DMSO DMSO

350

390

472

458

1:1

395, 335 386, 371

525 473, 443

495, 460

ACN

DMSO

316 315 316 326

472 485

1:1

3

5.785 9.2 19.0

23.1a, 17.7b

Selective for

Color change

F

Pale yellowa Fluorescent yellowa Orangea – – – Browna Dark pinka Golden yellowa Yellowa – – – Yellow–greena,b – – – Dark redc Dark redc

HSO4 HSO4 F

F F , AcO CN

117.0

kAem Indicates emission band of receptors in presence of anions (A ). ACN and DMSO indicates CH3CN:H2O = 1:1 and DMSO:H2O = 1:1, respectively. a Indicate F anion. b Indicate AcO anion. c Indicate CN anion.

Benesi–Hildebrand relation indicating the formation of stable 1:2 host–guest complex [12]. During this titration the initial colorless solution gradually turns to yellow color. However, similar kind of observation obtained from the UV–vis titration in presence of AcOH corresponding to 1:1 complexation ratio indicates the formation of H-bonding complex between the receptor and the anions (F and AcO ions) with different order of association constants

(Table 1). Hence, the observed 1:2 complexation ratio can only be explained on the basis of deprotonation (1 eqv.) and H-bonding (1 eqv.) interactions. The colorimetric test results and UV–vis spectral analysis were found to be well agreement with 1H NMR titration results in d6-DMSO solvent. It is obvious that the receptor 2 responses weakly to H2PO4 ion in absence and presence of AcOH due its poor basicity. Under the similar experimental condition

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other anions such as Cl , Br , I , ClO4 , HSO4 , NO2 , NO3 and HSO3 remain silent indicating no interaction or complexation. The ratiometric analysis of host–guest complexes as well as the guest induced deprotonation of highly acidic phenolic AOH corroborates well with the proposed theoretical model optimization at Density Functional Theory (DFT) level using Gaussian 03 suit [27]. A test kit has been prepared by dropping the acetonitrile solution of the receptor 2 on the filter paper (Whatman-40) and then dried it in air. To test with anions, acetonitrile solution of various anions has been dropped and it has been observed that only fluoride and acetate anions are capable for remarkable color change (colorless to yellow). Anion recognition ability of receptor 2 was found to be different in its reduced form (compound 3) [28]. Receptor 3 selectively detect F , AcO and H2PO4 anions, though the sensitivity of the receptor is less towards the phosphate ion, which was reported in the visual color change, UV–vis spectral change and in the observed 1:2 host–guest association constant in acetonitrile medium. Other anions such as Cl , Br , I , ClO4 , HSO4 , NO2 , NO3 and HSO3 were found to be insensitive towards this receptor. The colorless solution of receptor 3 was reported to be changed from deep yellow to light yellow in presence of F , AcO and H2PO4 anions. The UV–vis spectral investigation of receptor 3 in acetonitrile solvent shows absorption band at 298 nm and this band is found to be red shifted to 359 and 425 nm, respectively, in the presence of anions. However, the observed isosbestic points are different during H-bonding complexation reaction in case of F , AcO and H2PO4 anions. Based on their basicity and structure factors, these anions are associated with different stability order (Table 1). The binding order has been addressed as: AcO > F  H2PO4 . To examine the effect of electron withdrawing substituent (ANO2) on the hydroxyl group of salicylidene Schiff base, compounds 4 and 5 were synthesized [1]. Receptor 5 shows a broad absorption band from 298 nm to 329 nm with peak centered at 312 nm in acetonitrile–DMSO (95:5 v/v ratios) solvent mixture. It was reported that upon addition of increasing amount of F /AcO ions, two new absorption bands at 366 nm and 422 nm appear with decreasing the initial intensity of the broad band. During titration experiment an isosbestic point at 336 nm appears and the initial colorless solution gradually changes to yellow, indicating strong complexation equilibrium with 1:1 and 1:2 stoichiometries for F and AcO ions, respectively. Considering the size and structure of the anions (F and AcO ), the stoichiometry of the complexes has also been explained by theoretical model calculations. As the size of the fluoride anion is small, it is well fitted in the receptor cavity and can form 1:1 complex. But, due to larger size of acetate anion, it can be well fitted from opposite side of the receptor molecular plane and can form 1:2 complexes (Fig. S1). Under the similar experimental condition only H2PO4 was found to exhibit tiny spectral changes due to its poor basicity and tetrahedral shape. However, other anions such as Cl , Br , I and HSO3 remain silent in the similar experimental investigation due to their very poor basic property. Strongly basic F and AcO anions are only able to change color of acidic receptor 5 from colorless to yellow in acetonitrile solution, which can be easily detected by naked-eye. But receptor 4 neither exhibits naked-eye color change nor UV–vis spectral changes in the presence of F , AcO , Cl , Br , I and HSO3 anions, which is due to the absence of electron withdrawing signaling unit (ANO2) in 4, posses the less acidic phenolic AOH and unable to attend host–guest interaction event. To utilize the anion receptor properties for practical purpose, compounds 6 and 7 have been reported [13], which are the reduced form of compounds 4 and 5, respectively. It has been observed that the acetonitrile solution of compound 7 exhibits a

strong absorption band at 304 nm in the UV–vis spectral investigation. Upon addition of increasing concentrations of F ion to the acetonitrile solution of 7, the peak at 304 nm gradually decreases its intensity, whereupon a new absorption peak appears gradually at 403 nm. The appearance of new band at 403 nm is most likely due to the occurrence of charge transfer process between the phenolic oxygen to electron withdrawing nitro group by the formation of hydrogen bonded complex with the proton of phenolic group and the composition of the complex with fluoride ion was reported to be 1:1 stoichiometry. On addition of increasing amount of F ion, the peak at 403 nm gradually shifts to higher wavelength at 425 nm. The shifting of the absorbance band from 403 nm to 425 nm is probably due to deprotonation of the phenolic proton by means of simple acid–base type of reaction. In a consequence, the solution color changes from light yellow to deep yellow (Fig. S2). In contrast, while acetonitrile solution of 7 was titrated with OH ion, a red shifted band at 425 nm has been observed and the colorless solution became deep yellow, which is attributed to the deprotonation of the highly acidic phenolic AOH rather than the formation of a hydrogen bonded complex. Similar types of absorption spectral change has been reported upon addition of AcO anion, but addition of H2PO4 ion exhibits only a tiny spectral change. On the other hand, addition of Cl , Br , I , and HSO3 anions neither exhibits naked-eye color change nor UV–vis spectral change. The selectivity and sensitivity of receptor 7 towards the F , AcO , H2PO4 , Cl , Br , I , and HSO3 ions can be rationalized on the basis of their basicity and acidity of the nitro substituted reduced Schiff base 7. It is reported that receptor 6 neither exhibits any UV–vis absorption spectral change nor naked-eye detectable color upon addition of all the anions. On the basis of the above observation, the reduced Schiff base 7 can have real practical use. A test paper (Whatman-40) coated acetonitrile solution of 7 was prepared and then dried in air (Fig. 1). The aqueous solution of anions were dropped onto the test paper and dried in air. Interestingly, F and AcO ions exhibit a bright yellow color, but Cl , Br , I , HSO3 and H2PO4 do not show any detectable naked-eye color changes. The test paper can be reused many times without losing its originality when it is washed with 3% HCl solution and distilled water, respectively and is dried it in the air. Similar type of Schiff base and its reduced form (as that of compounds 5 and 7) of bipyridine derivatives 8 and 9 have been utilized for anion recognition process [29]. Compounds 8 and 9 exhibit almost similar response (except band position) in the UV–vis titration experiment in presence of F , AcO and H2PO4 anions in DMSO medium, but the magnitude of absorbance changes and the observed association constants have been reported to be different. With increasing the concentration of anions the absorption peak of 8 at 300 nm decreases gradually and the intensity of absorbance at 375 nm and 435 nm increase. On the other hand, the absorption band of receptor 9 at 320 nm decreases gradually and the intensity at 435 nm increases when anions are added to it. Addition of other anions such as Cl , Br and I remains almost silent when added to the DMSO solution of receptor 8 and 9. The binding ability of receptor 8 with various anions is higher than that of the receptor 9. The binding ability of receptor 8 with anions is in the order of H2PO4 > AcO > F  Cl , Br , I , where as this order for receptor 9 is of F > H2PO4 > AcO  Cl , Br , I . The differences

Fig. 1. Compound 7 used for re-usable test kit.

S. Dalapati et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 129 (2014) 499–508

in the structural matching of the host–guest complexation play a major role behind the utilizing of Schiff bases and reduced Schiff bases for the anion recognition events. Compounds 10 and 11 are some of the interesting examples where structural differences change their selectivity and sensitivity toward the anions detection event and exhibits unusual recognition behavior [30]. Receptor 10 can selectively detect F anion only with 1:1 stoichiometry in DMSO solvent, where as receptor 11 can selectively sense F and AcO ions with 1:1 stoichiometry. Bare compound 10 shows an ICT absorption band at 344 nm. Upon addition of increasing amount of F anion a new absorption band at 464 nm appears with a clear isosbestic point at 380 nm and the solution color changes from colorless to bright yellow. Similar spectral changes have been observed in case of AcO ion, but with much less intensity enhancement and unable to change the color of the solution. Other anions, such as Cl , Br , I , HSO4 and NO3 did not responses in the UV–vis measurement as well as in the colorimetric test. On the other hand, compound 11 shows two absorption peaks at 262 and 304 nm and a shoulder peak at 353 nm in DMSO solvent. Upon addition of increasing amount of F anion, a new absorption peak at 355 nm has been observed with decreasing and increasing the initial peak at 304 nm and 262 nm, respectively, and the solution color changes from colorless to pale yellow. Under the similar experimental setup, AcO ion shows similar color changes as that of the F ion, but H2PO4 ion causes moderate spectral change. In presence of Cl , Br , I , HSO4 and NO3 anions in DMSO solvent, compound 11 did not exhibit any spectral changes indicating no interaction between receptor 11 and these anions. The binding constant of 11 towards the anions follows the trend as F  AcO  H2PO4  Cl , Br , I , HSO4 , NO3 . The detailed theoretical structural optimization of compounds 10 and 11 explained the possible binding mode (Scheme 1) towards the anions and the big difference in their anion recognition behaviors. Dual responsive chemosensor 12 and 13 selective for F and Cu(II) ion was reported by Devaraj et al. [31]. Compound 12 shows three absorption band at 280, 320 and 380 nm, which can be assigned to p–electron transition of aromatic benzene ring, p–p transition of azomethine group (ACH@NA) and charge-transfer (CT) transition of the whole molecule, respectively. Upon addition of F ion, the peak at 380 nm disappears with the appearance of new peak at 441 nm and the spectrum shows two isosbestic points at 352 and 430 nm indicating the existence of equilibrium between

Scheme 1. Discrimination of anions by cooperative utilization of receptor 10 and 11.

503

the bare receptor 12 and its H-bonded complex formed with F anion. While receptor 13 shows an absorption band at 301 nm. With the addition of increasing amount of F anion, the band at 301 nm gradually decreases its intensity and two new peaks appear at 364 and 411 nm with an isosbestic point at 340 nm. Under the similar experimental condition, even in the presence of larger amount of other anions such as Cl , Br and I ions, these two receptors did not show any detectable spectral changes in the UV–vis spectral measurement. Reported colorimetric investigation of compounds 12 and 13 in CHCl3 and DMSO solvent towards anions shows that only F anion is capable of detectable color changes over the other anions (such as Cl , Br and I ) and the color changes from colorless to brown in CHCl3 and to yellow in DMSO solvent. Receptor 12 and 13 undergo different color change in acetonitrile solution from colorless to violet and bluish green, respectively, with adding increasing amount of Cu(II) ion and a bathochromic shift has been found in the UV–vis spectra. Simple Schiff base receptors 14 and 15 as phenol–hydrazone derivatives with nitro group as signaling unit and AOH and ANHA groups as a binding sites are highly responsible for colorimetric fluoride anion recognition process [32]. The FTIR and NMR spectral analysis suggest hydrogen bonding interaction of F anion with AOH and ANH proton. NMR studies also suggested deprotonation in presence of more than one equivalent of fluoride anion and the formation of HF2 complex in DMSO-d6 solvent. The colorimetric test and UV–vis spectral investigation of anions towards receptor 14 and 15 were performed in acetonitrile solvent. Color change has also been observed in CHCl3 and in DMSO solvent. The color of the acetonitrile solution of receptor 14 and 15 in presence of F anion turn to orange and fluorescent yellow, respectively. However, it has been reported that in presence of 100 equivalents Cl , Br and I anions, both receptors remain silent towards the visual color change. The anion binding capability of the above two receptors have been investigated by monitoring UV–vis spectral characterization in acetonitrile solution. Compound 14 exhibits three absorption bands at 232, 300 and 348 nm in acetonitrile solvent. During the addition of increasing concentration of fluoride anion, the initial absorbance of the receptor decreases its intensity with the appearance of two new bands at 369 and 457 nm. On the other hand, compound 15 initially shows three absorption bands at 233, 301 and 362 nm. Compound 15 generates a new absorption band at 417 nm with decreasing the initial absorbance by addition of fluoride anion. For both these compounds the absorbance of the new band increases stepwise manner and reaches maxima value after addition of 4 equivalents of fluoride anions. The color change and band shifting of the receptors can also be explained with addition of strong base (Bu4NOH) in acetonitrile solution. The absorption experiment of the receptors suggests the formation of HF2 complex when more than 1 equivalent fluoride anions are added. Similar UV–vis absorption properties or similar binding capability of both the receptors have been observed for AcO anion as that of the F anion, only a difference was reported in case of deprotonation, where acetate anion needs more concentration than fluoride anion. This is due to smaller size and stronger basic character of fluoride anion which shows stronger binding capability towards the receptors than larger size acetate anion. A series of Schiff base compounds 16–21 have been reported for anion recognition events by Zhang and his co-workers [33]. Colorimetric investigation of receptors 16–20 upon addition of various anions such as F , Cl , Br , I , H2PO4 , HSO4 and AcO in DMSO and DMSO–H2O binary solvent has been investigated. Only F or AcO anions were found to be able to change the color of the solution from orange to purple along with the generation of new

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bands at 500–550 nm in the UV–vis titration spectra. For H2PO4 ion, only sensor 21 shows color change from orange to purple in DMSO–H2O (9:1) binary solvent. Solution of receptors 16–20 did not show color change with rest of the anions. The anion sensing ability of all the above receptors was found to be varied with the volume ratio of water in DMSO–H2O binary solvent. Sensor 18 and 19 without having acidic AOH group, did not exhibit any anion sensing ability. It has been reported that the color of the solution of 20 and 21 turns from orange to yellow by the action of Cl ion in DMSO–H2O (9:1) mixed solvent. When water percentage was increased in DMSO (such as DMSO:H2O 7.5:2.5) the single-armed sensor 16 could not response towards any anions, but sensor 17 weakly interacts only with the AcO anion. Whereas an obvious color change from orange to magenta has been reported for twoarmed pincer type receptors 20 and 21. The selectivity and sensitivity of receptor 20 have also been reflected in their NMR studies, UV–vis titrations and in their association constants (Table 1). Zhang and his co-workers have established the hydrogen sulfate recognition properties of azo-salicylaldehyde Schiff base receptors 22–25 containing an acidic H-bond donor moiety [34]. The colorimetric and UV–vis spectral investigation of receptors 22–24 have been made in DMSO solvent and the receptor 25 in DMSO–H2O (95:5 v/v ratios) solvent. Addition of basic anions such as F , AcO and H2PO4 , the UV–vis spectral changes of receptors 22–25 were found to be similar, i.e. absorption band shifted from 350–550 nm to 450–550 nm in DMSO solvent. But no color change was observed for the receptors 22–24 in DMSO–H2O (95:5) binary solvent. On the other hand, colorimetric and UV–vis sensing ability of HSO4 ion towards receptors 24 and 25 is interesting. From the reported UV–vis spectral measurements it is clear that these two receptors selectively sense HSO4 ion and the absorption band is shifted from 530 to 374 nm. The solution (95%DMSO–5%H2O) color of receptor 25 has been changed from pale red to purple upon addition of basic anions F , AcO and H2PO4 and OH , indicating deprotonation of phenolic AOH rather H-bonding interaction. On the contrary, solution color of the receptor 25 has been changed from pale-red to yellow in presence of acidic guest HSO4 ion and in presence of percloric acid (HClO4) which indicates that color change occurs due to protonation of phenolic AOH rather than host–guest H-bonding interaction. A colorimetric azo-phenolic hydroxyl group containing Schiff base receptor 26 have been reported by Liu and Shao [35]. DMSO solution of this receptor undergoes naked-eye color change from colorless to yellow in presence of F , AcO and H2PO4 anions. UV–vis spectral change has been observed only in case of these anions in DMSO solution of the receptor. Compound 26 exhibits a strong absorption band at 350 nm, which is attributed to charge transfer band of the azo moiety. With addition of increasing concentration of fluoride anion the peak intensity at 350 nm gradually decreases and a new absorption band at 469 nm increases its intensity. Almost similar effects have been reported in case of acetate and phosphate anions. Other anions, such as Cl , Br and I anions remain silent towards color change as well as UV–vis spectral changes. The color change and UV–vis spectral changes of the receptor in DMSO solvent in presence of fluoride, acetate and phosphate anions have been addressed by their greater basicity compared to other anions. These three anions separately formed 1:1 complex with the receptor and the order of association constants are F > AcO  H2PO4 and the overall experimental investigation implies that the receptor binding capacity with anions are in the order: F > AcO  H2PO4  Cl , Br , I . Chromo-fluorogenic anion receptors A simple but potential colorimetric and fluorescence sensing F anion receptors 27–29 have been reported by Sivakumar and his

co-workers [36]. Colorimetric and UV–vis experiments of these receptors have been carried out in acetonitrile solvent, where in presence of fluoride anion the solution of receptors 27, 28 and 29 undergoes a naked-eye color change from colorless to pale yellow, fluorescent yellow and orange, respectively. Naked-eye color change has also been reported in CHCl3 and DMSO solvents. The UV–vis spectra of receptors 27–29 show their absorption band in the region of 207–355 nm (Table 2). All the three receptors 27, 28 and 29 generate new absorption peaks at 420, 420 and 487 nm, respectively, with decreasing the initial absorbance at 336, 340 and 355 nm, respectively, upon addition of increasing amount of fluoride anion. The color and UV–vis spectra of these three receptors remain silent even if with 100 equivalents of added Cl , Br and I anions. On the other hand, color intensity and spectral shift of receptor 29 was observed to be significant in presence of fluoride anion compared to two other receptors. Receptor 29 having most electron withdrawing nitro group favors stronger hydrogen bonding interaction with phenolic AOH group and hence it shows greater association constant (Table 2). Fluorescence experiments have also been carried out to know more about the sensing capability of these three receptors in acetonitrile solvent. The emission maxima of three receptors is around 490 nm in the order of 10 3 M concentration range, where a weak band exists above 500 nm in the order of 10 5 M concentration range. Upon addition of F anion, receptors show fluorescence enhancement at 472–475 nm. It was observed that other anions remain silent with this experiment. Fluorescence response Schiff base sensors selective for HSO4 anion have been synthesized and established by Kim and his coworkers [3] where they examined the action of anion on receptors 30–32 in 50% aqueous–acetonitrile (1:1) solution. Coumarin based receptor 31 shows an absorption band at 355 nm. Upon addition of HSO4 anion this absorption band is found to be shifted to 370 nm and a remarkable fluorescence enhancement (13-fold) at 485 nm is observed when excited at 355 nm. The fluorescence intensity of 31 is quenched upon addition of F , Cl , Br , I , AcO , NO3 , OH and H2PO4 anions through photoinduced electron transfer mechanism (PET). Compound 30, where phenolic AOH group is absent, shows non-selective nature towards anion detection. Similar response has been observed in case of compound 32 as that of 31 towards HSO4 ion. The phenolic AOH in compound 31 is intramolecularly hydrogen bonded with the imine ‘N’ atom which is the cause of initial fluorescence quenching of receptor and it blocked the intramolecular H-bond formation by means of 1:1 intermolecular H-bonded complex formation with bisulfate anion and this is the cause of selective fluorescence enhancement of receptor 31 and 32 only in presence of HSO4 anion. Theoretical calculations strongly recommended both the experimentally observed fluorescence ‘turn-on’ state due to H-bonding interaction with HSO4 anion and PET quenching for other anions. New anthraquinone–based Schiff base receptors 33–35 with dual chemosensing ability have been reported in DMSO, CHCl3 and CH3CN solvent by Devaraj et al. [37]. Visual sensing ability of those receptors have been investigated in DMSO solvent, where the color of the solutions of 33, 34 and 35 turn to brown, dark pink and golden yellow, respectively by addition of F anion. All receptors remain silent by the addition of large excess of Cl , Br and I anions. The UV–vis spectral investigation of three receptors in absence of anions show absorption band within the range of 235–508 nm (Table 2). By addition of fluoride anion to the DMSO solution of 33–35, absorption band shifts from 518 to 457, 508 to 518 and 496 to 439 nm, respectively with decreasing the initial absorbance. Investigation with other anions did not change the initial absorbance of these receptors. The fluorescence emission of three receptors 33–35 did change its intensity at 620, 598 and 610 nm, respectively. Other anions behave similar as that of the

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UV–vis experiments. The association constants for such host–guest complexation reaction had been observed and the association constant for receptor 35 was reported to be higher compared to others two. This may be due to the presence of most electron withdrawing nitro substituted in receptor 35 which causes stronger H-bonding interaction between phenolic AOH and fluoride anion. Effect of electron withdrawing nitro substitution has also been observed in the NMR shift of phenolic AOH group. The anion sensing capability of receptors 33–35 towards the F anion has been examined by cyclic-voltametry experiments which support selectivity of these sensors towards fluoride anion. Terphenyl based fluorescent chemosensors 36–39 for F ion were synthesized and reported by Bhalla and his co-workers [38]. They reported that the UV–vis spectral investigation of compound 36 in THF solvent shows absorption band at 265 and 342 nm. A new red shifted absorption band at 447 nm has been observed and side by side naked-eye color of the solution is changed from colorless to yellow by addition of measurable amount of F ion to the receptor solution. This phenomenon can be explained by considering complexation of fluoride ion with phenolic AOH or deprotonation of highly acidic proton, i.e. an intermolecular proton transfer takes place from phenolic oxygen to electronegative fluoride anion which facilitates charge transfer from electron reached phenolic oxygen to the electron withdrawing nitro subsistent. That is why the initial absorption band of compound 36 is red shifted and the color of the solution is changed with addition of fluoride ion. On the other hand, weakly acidic phenolic AOH group is present in compound 37. Receptor 36 remains silent with addition of other weakly basic anions (such as Cl , Br , I , HSO4 , NO3 and H2PO4 anions). The selectivity and sensitivity of the receptor have also been monitored by fluorescence titration experiments with fluoride anion. The THF solution of 36 shows an emission band at 517 nm, which has been assigned to the excited state proton transfer (ESIPT) band and that is why this band is absent in compound 38, where AOH is replaced by AOMe group. By addition of fluoride ion in THF solution of 36, the emission intensity of 517 nm band of the receptor is quenched, which is due to inhibition of proton transfer process. When the emission intensity of the receptor is quenched completely, on further addition of fluoride anion a new blue shifted band at 478 nm appears. The blue shifted band at 478 nm is due to the action of F anion on the AOTBS group, i.e. desilylation reaction or cleavage of SiAO bond. That is why by the action of fluoride ion the emission intensity of 517 nm of the receptor 39 (where AOTBS group has been replaced by crown-5 ring) is quenched without the appearance of new band at 478 nm. Interestingly, compound 36 also selectively recognizes Cu2+ ion over the other tested cations (Pb2+, Hg2+, Ba2+, Cd2+, Zn2+, Ni2+, Co2+, Mg2+, Ag+, K+, Na+ and Li+ ions). A cleft-shaped colorimetric F ion sensor 40 has been developed by Bao et al. [39]. Compound 41 has been synthesized as reference compound. The UV–vis spectral investigation of receptor 40 in DMSO solvent shows absorption peak at 326 nm. Upon addition of increasing amount of F anion a new absorption peak appears at 425 nm with gradual decreasing the initial absorbance and the color of the solution is changed from colorless to yellow– green. Similar spectral changes have been observed in case of AcO ion, but the color intensity is much weaker than F ion. H2PO4 ion induces only tiny spectral changes. Other anions Cl , Br , I , HSO4 and NO3 ions did not exhibit any change. Titration analysis shows 1:1 composition for the host–guest complexation reaction. The emission spectral investigation indicates that the

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emission maximum of receptor 40 is at 463 nm, where by the action of fluoride and acetate anion this band shifted to 475 nm with increasing emission intensity. Other anions, such as H2PO4 , Cl , Br , I , HSO4 and NO3 ions did not account any spectral change. The cause of color change and spectral changes have been rationalized on the basis of hydrogen bonding interaction between anions and both AOH and ANH group of the receptor. That is why compound 41 with the absence of AOH did not trigger the changes in neither the visual color nor the UV–vis spectra. The H-bonding interaction of the receptor has also been established through NMR titration experiments. Sun and coworkers [40,41] synthesized and reported some novel chromo-fluorogenic cyanide sensing receptors 42–45 in aqueous medium. When sodium salt of anions, such as CN , F , Cl , Br , I , ClO4 , AcO , N3 , HSO4 and NO3 were exposed into the 50% aqueous–DMSO solution of receptors 42–45 (except 43), only CN anion accounts for UV–vis and fluorescence spectral changes. Compound 42 exhibits a absorption band at 350 nm and it generates a red shifted band at 390 nm in presence of CN ion and the red shifting phenomena is invariant in case of receptors 44 and 45, but only the band position is at different position which is due to the presence of initial band at different position (Table 2). On the other hand, fluorescence investigation of receptor 42 shows an excited state CT emission band at 472 nm. Upon addition of cyanide anion, this emission band is blue shifted to 458 nm and the intensity of the emission is substantially increased. Spectral invariant of receptor 43 indicates that the presence of phenolic AOH in others receptor activates imine nitrogen and by the addition of cyanide anion it is preferentially attacked on imine group rather in the amide carbonyl. This causes intra-molecular proton transfer within the receptor and that is the cause of absorption and emission spectral changes. Due to stronger H-bonding capability and weak carbonyl affinity of F , AcO , Cl , etc., anions, these anions are hydrated in aqueous– DMSO binary solvent, where as weak H-bonding capability and strong carbonyl affinity of CN anion plays the key role for its selectivity. Besides these spectral investigations, compounds 44 and 45 have been used as colorimetric cyanide detection (color changed from pale yellow to dark red) as these systems contain azo dye unit. The cause of UV–vis and fluorescence change in presence of cyanide anion has also been proved by NMR titration experiments and theoretical calculations.

Summary and conclusions In the past few decades, salicylidene Schiff bases or reduced Schiff bases are commonly used for the formation of various metal complex recognition of ions (cations or anions), asymmetric synthesis, electrochemistry, magnetochemistry, epoxidation, molecular separation and biomedical application. A literature survey reveals that there are countable examples of Schiff bases which involve with strong hydrogen bonding interaction with the anion. Indeed, they have used for anion recognition due to their beautiful naked-eye color change in presence of different anions in solution or in solid supported test kits. The interactions can also be easily monitored by UV–vis, fluorescence and 1H NMR changes and make them useful in anion recognition chemistry and also outlined about their photophysical properties. In this review, we try to discuss how Schiff base or reduced Schiff base can be used for anion recognition with excellent selectivity and sensitivity.

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Chemical structures of compound 1–45 used in this review

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