Electrooptical characteristics and anion binding behaviour of organic receptors: Effect of substitution on colorimetric response

Accepted Manuscript Title: Electrooptical characteristics and anion binding behaviour of organic receptors: Effect of substitution on colorimetric response Authors: Srikala Pangannaya, Darshak R. Trivedi PII: DOI: Reference:

S0925-4005(17)30470-7 http://dx.doi.org/doi:10.1016/j.snb.2017.03.048 SNB 21960

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

29-11-2016 1-3-2017 11-3-2017

Please cite this article as: Srikala Pangannaya, Darshak R.Trivedi, Electrooptical characteristics and anion binding behaviour of organic receptors: Effect of substitution on colorimetric response, Sensors and Actuators B: Chemical http://dx.doi.org/10.1016/j.snb.2017.03.048 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Electrooptical characteristics and anion binding behaviour of organic receptors: Effect of substitution on colorimetric response Srikala Pangannaya, and Darshak R. Trivedi* *

Supramolecular Chemistry Laboratory, National Institute of Technology Karnataka (NITK), Surathkal575025, Karnataka, India

*

Author to whom correspondence should be addressed e-mail: [email protected],

Tel.: +91-824-2473205; fax: +91 824 2474033 Graphical Abstract 

  Highlights    

Receptors R1, R2, R3 and R4 exhibits electrochemical and optical response towards anions  Receptor R4 proved its selectivity towards acetate ion among other anions  1 H‐NMR titration studies throws light on the binding mechanism  1 

 

 

Band gap calculations prove the receptor‐anion complex formation  The role of positional substitution effect on the anion binding and selectivity is significant 

Abstract A series of four organic receptors, R1, R2, R3 and R4 possessing –NH unit as binding site with varied positional substitution of –OH and –NO2 functionalities have been synthesized and studied for their anion binding ability. Significant changes were observed in the UV-Vis spectra of receptors with the addition of AcO ‒ ion justifying the observed color changes visible to naked eye. Receptors R1, R2 and R3 showed mild color changes in the presence of F‒ and H2PO4‒ ions whereas

R4 exhibited selective response towards AcO

‒

ion. UV-Vis titration and cyclic

voltammetric studies performed with the incremental addition of AcO

‒

ion to the receptors

confirmed the anion binding process. The band gap value derived from the experimental results of UV-Vis spectroscopic and cyclic voltammetric studies support the formation of AcO ‒ ion-receptor complex. 1H-NMR titration studies performed with the addition of AcO‒ ion to receptor   R3 confirms the deprotonation of the –NH proton involved in the binding mechanism. Keywords: organic receptors; substitution effect; electrooptical; colorimetric sensor; anions

1. Introduction Supramolecular chemistry of host-guest interactions has seen progressive path unveiling the importance of ionic species in the biological system. Bioactive species such as fluoride (F ̶ ), acetate (AcO ̶) and phosphate (H2PO4 ̶ ) ions play vital role in promoting dental health 1,2, metabolic process3 and energy storage, signal transduction system4. Physiological and environmental relevance of anions necessitate the development of organic receptors which can detect anions in aqueous media. Detection of anions by means of optical or electrochemical output has been the standard paradigm in the recent years.5 Anion receptor chemistry has grown into an area of great 2   

interest for supramolecular chemists opening up new arena for precise design strategy. Primarily, binding interactions with anions being hydrogen bond or electrostatic force imposes vigilant insight for a constructive design unlike the simple metal-ligand coordination interactions involved in cation-receptor chemistry.6 Binding site-signalling unit approach involving covalent attachment of the chromophore and neutral receptor bearing hydrogen bond donor unit has been most commonly utilized in designing chromogenic receptors. Few receptors reported in the literature have successfully surpassed the challenges set by the anionic substrates such as similar basicity and surface charge density in the commonly encountered F ̶, AcO

̶

and H2PO4 ̶ ions. Despite these setbacks,

colorimetric receptors bearing chromophores such as indoles,7 bisindole,8 carbazole,9 nitrophenyl,10 quinone,11 nitrobenzene,12 azo groups and other electron withdrawing moieties9 have been developed. In the design of anion chemosensors, various non-covalent interactions such as hydrogen bonding, anion-π and reactions like hydrogen abstraction, electron transfer have been commonly encountered.13 Studies performed with molecules comprising of secondary amine groups possessing highly acidic proton are known to promote hydrogen bond interactions with anions.14 Furthermore, electroanalytical technique such as cyclic voltammetry could be a valuable tool to analyse the binding of anion by the receptors through redox process. Concurrently, band gap of the receptor and receptor-anion complex could be calculated. Previously, we presented a simple chemoreceptor R possessing –NH functionality as a potent binding site, which exhibited solvatochromic behaviour in the presence of AcO‒ ion.15 In addition, the presence of NO2 group as a signalling unit at position para to the NH functionality induced a strong colorimetric response visible to the naked eye. As an extension of our work, we have designed four organic receptors by 3   

modifying the functionalities on the aromatic ring to control the acidity of the H-bond sites, in turn fine tuning the selectivity and sensitivity of the sensor towards anions. The anion binding studies performed reveals significant colorimetric response visible to the naked eye paving way for the quantitative study through UV-Vis, 1HNMR spectroscopy and cyclic voltammetric studies. Besides, the nature of interaction by increasing the number of –NO2 groups and introduction of – OH functionality on the receptor molecule and its impact on anion binding has been studied.

2. Experimental 2.1 Materials and methods All the chemicals used in the present study were procured from Sigma-Aldrich and Alfa Aesar and were used as received without further purification. All the solvents were purchased from SD Fine, India, were of HPLC grade and used without further distillation. Melting point was measured on Stuart SMP3 melting-point apparatus in open capillaries. Infrared spectrum was recorded on Bruker Apex FTIR spectrometer. UV-Vis spectroscopy was performed with analytik jena Specord S600 spectrometer in standard 3.0 mL quartz cell with 1cm path length. The 1H NMR spectra were recorded on Bruker Ascend (400 MHz) instrument using TMS as internal reference and DMSO-d6 as solvent. Resonance multiplicities are described as s (singlet), d (doublet), t (triplet) and m (multiplet). Mass spectrum was recorded on Bruker Daltonics ESI Q TOF. Cyclic voltammogram was recorded on Ivium electrochemical workstation (Vertex) at a scan rate of 20 mV/s with the potential range 1.0 mV to -1.0 mV. The single-crystal X-ray diffraction (SCXRD) was performed on Bruker AXS APEX II system. 2.1.1 Synthesis of receptors R1, R2, R3 and R4

4   

Receptors R1, R2, R3 and R4 were prepared by simple Schiff base condensation reaction between 2,4-dinitrophenylhydrazine and different substituted aldehydes. Ethanolic solution of resulting mixtures were refluxed at 70 o C for 5 h in the presence of catalytic amount of acetic acid. The formation of the product was confirmed through TLC by the generation of single spot indicative of the disappearance of starting materials. After cooling to room temperature, the reaction mixture was filtered through filter paper, washed with ethanol to obtain pure product. R1: (E)-3-((2-(2,4-dinitrophenyl)hydrazono)methyl)-4-nitrophenol: Yield: 75%., m. p. 246 oC. 1H NMR (DMSO- d6, 400 MHz, ppm): δ 7.45 (s, Ar-H), 7.13-7.0 (m, Ar-H), 8.08-8.02 (m, 2Ar-H), 8.43 (d, Ar-H), 8.86 (s, -HC=N), 9.19 (s Ar-H), 11.16 (s, Ar-H), 11.95 (s, NH). FTIR (KBr)(cm1

): 3386 (OH), 3286 (Ar-CH), 3103 (NH), 1609 (CH=N), 1505 (C=C), 1330 (NO2), 1099 (C-H).

Mass (ESI): m/z Calculated: 347.05 Obtained: 331.13. R2: (E)-3-((2-(2,4-dinitrophenyl)hydrazono)methyl)benzene-1,2-diol: Yield: 70%., m. p. 246 oC. 1

H NMR (DMSO- d6, 400 MHz, ppm): δ 6.74 (s, Ar-H), 6.87 (s, Ar-H), 7.3 (s, Ar-H), 8.01 (d, Ar-

H), 8.37 (d, Ar-H), 8.87 (s, Ar-H), 8.97 (s, HC=N), 9.28 (s, Ar-H), 9.66 (s, Ar-H), 11.71 (s, NH). FTIR (KBr) (cm-1): 3458 (OH), 3252 (Ar-CH), 2922 (NH), 1616 (CH=N), 1338 (NO2), 1112 (CH). Mass (ESI): m/z Calculated: 318.06 Obtained: 319.13 (M+H)+. R3: (E)-1-(2,4-dinitrophenyl)-2-(2-nitrobenzylidene)hydrazine: Yield: 85%., m. p. 280 oC. 1H NMR (DMSO- d6, 400 MHz, ppm): δ 7.82-7.70 (d, Ar-H), 7.84 (d, Ar-H), 8.08-8.02 (d, 2Ar-H), 8.20 (d, Ar-H), 8.40 (d, Ar-H), 8.86 (s, HC=N), 9.08 (s, Ar-H), 11.96 (s, NH). FTIR (KBr)(cm-1): 3285(NH), 3087 (Ar-CH), 1615 (CH=N), 1502 (C=C), 1334 (NO2), 1138 (C-H). Mass (ESI): m/z Calculated: 331.06 Obtained: 332.13(M+H)+.

5   

R4: (E)-1-benzylidene-2-(2,4-dinitrophenyl)hydrazine: Yield: 85%., m. p. 230 oC. 1H NMR (DMSO- d6, 400 MHz, ppm): δ 7.4 (s, 3Ar-H), 7.8 (s, 2Ar-H), 8.13 (d, Ar-H), 8.38 (d, Ar-H), 8.72 (s, HC=N), 8.87 (s, Ar-H), 11.68 (s, NH). FTIR (KBr) (cm-1): 3282(NH), 3086(Ar-CH), 1610 (CH=N), 1505 (C=C), 1324 (NO2), 1128 (C-H).Mass (ESI): m/z Calculated: 286.07 Obtained: 287.13 (M+H)+.

3. Results and discussion The receptors were synthesized from commercially available chemicals by reacting 2,4-dinitrophenylhydrazine with 2-nitro,5-hydroxy benzaldehyde (R1), 2,3-dihydroxy benzaldehyde (R2), 2-nitrobenzaldehyde (R3) and benzaldehyde (R4) in a one-step reaction by refluxing in ethanol using acetic acid as catalyst at 70 oC for 5 h . The products R1, R2, R3 and R4 have been characterized by various spectral techniques such as FT-IR, 1

H-NMR and ESI-MS. The structure of new receptors R1, R2, R3 and R4 are represented

in Scheme 1. Further, the structure of R4 has been confirmed by single crystal X-ray diffraction study. Single crystals were obtained by slow evaporation of R4 from a binary solvent system comprising of acetonitrile and dimethylformamide. The ORTEP diagram of R4 is shown in Fig. 1 and the crystallographic data is presented in Table S1 (†ESI).

3.1 UV-Vis spectroscopic studies With a view to evaluate the effect of structural modification on the optical properties of phenylhydrazones, positional substitution of -OH and -NO2 groups on the phenyl moiety have been considered as part of the design strategy. UV-Vis spectroscopic study performed with 1x10– 4

M DMSO solution of the receptors R1, R2 and R4 displayed strong absorption band at 395 nm,

405 nm and 394 nm respectively whereas receptors R3 exhibited absorption band at ~413 nm.

6   

The absorption maxima for R1, R2, R3 and R4 could be assigned to the intramolecular charge transfer interactions in the presence of Ar-CH=N-NH conjugation. The electron donor nature of – NH and acceptor nature of –NO2 functionality are known to impart pale yellow coloration to the receptor in DMSO.

The interaction of anion with receptors R1, R2, R3 and R4 has been assessed by color changes visible to the naked eye and quantified by UV-Vis spectral studies. UV-Vis spectroscopic studies of receptors (1x10–4 M in DMSO) has been performed with the addition of tetrabutylammonium (TBA) salts of anions (1x10 –2 M in DMSO) such as F –, Cl –, Br –, I –, NO3 –, HSO4 –, H2PO4 – and AcO –. Receptors displayed significant colorimetric response with the addition of 1 equiv. of AcO –

ion as shown in Fig. 2. Receptors R1, R2 and R4 displayed red shift of original absorption band

to 545 nm, 497 nm and 495 nm respectively accompanied by distinct color change from pale yellow to blood red, orange red and red color visible to the naked eye(Fig. S13, Fig. S14, Fig S16 †ESI). On the contrary, receptor R3 exhibited a unique colorimetric response from pale yellow to purple with a significant red shift and appearance of new band at 570 nm (Fig. S15†ESI). However, receptors R1, R2 and R3 exhibited relatively similar but less intense colorimetric response towards F – and H2PO4 – ions owing to their properties such as similar basicity and charge to radius ratio. Gratifyingly, receptor R4 exhibited selective response towards AcO

–

ion. No

significant changes were observed with other anions in the present study which clearly indicates the absence of complex formation.

To obtain further insight into the interaction of receptors with TBA salt of AcO – ion, UV-Vis spectrophotometric titration has been performed. Fig. 3a shows the spectroscopic changes observed with the incremental addition of 0.1 equiv. of AcO – ion to R1. Two different processes 7   

were observed during the progression of titration. Until the addition of 0.7 equiv. of AcO – ion, the absorption band at 395 nm decreased in its intensity with a significant red shift differing by 150 units. With the addition of higher equiv. of AcO – ions, there was red shift of the original absorption band from 395 nm to 454 nm with the appearance of new band at 545 nm. The saturation point was reached with the addition of 2 equiv. of AcO – ion with complete diminution of the band at 395 nm. The first process corresponds to the strong hydrogen bond interaction between AcO – ---- H-O and H-N in the ground state. The second process provides an evidence of the deprotonation of the –OH moiety due to the interaction of hydrogen bond donor group -OH and AcO – ion through intermolecular proton transfer process.16

Proton abstraction from the –OH moiety induces a negative charge on the oxygen atom on the receptor R1 which further resulted in the enhancement of the intermolecular charge transfer transition. Presence of –NO2 chromophore para to the OH group led to a colorimetric response from pale yellow to red visible to the naked eye. The presence of clear isobestic point at 422 nm denotes the formation of R1---- AcO

–

ion complex. From the Benesi-Hildebrand (BH) plot,

linearity obtained with first power of concentration of AcO – ion confirmed the binding ratio to be 1:1 for R1---- AcO – ion complex as depicted in Fig. S17 †ESI. This clearly indicates the single step deprotonation process brought about by an AcO – ion resulting in a negative charge on the receptor upon proton abstraction yielding acetic acid (AcOH) as a product. Titration studies performed with incremental addition of AcO

–

ion to R2 exhibits sharp

changes in the absorption maxima accompanied by a red shift differing by 90 units and a clear isobestic point at 444 nm as displayed in Fig. 3b. Presence of two –OH and a NH substituents are known to promote strong ground state hydrogen bond interactions with AcO – ion. The OH group could withstand deprotonation process allowing abstraction of proton from NH group which is in 8   

close vicinity to electron withdrawing –NO2 group in Ar-CH=N-NH conjugation. From the B-H plot, binding ratio was found to be 1:1 between R2---- AcO – ion complex proving a single step deprotonation mechanism (Fig. S18 †ESI). R3 exhibited a substantial colorimetric response in the presence of AcO – ion inducing a color change from pale yellow to purple. The presence of –NO2 group at ortho position of the phenyl ring triggered strong changes in the anion detection mechanism. The increase in the number of chromphores is found to have a direct influence on the chromogenic response upon anion binding. Correspondingly, there was considerable shift in the absorption maxima differing by 165 units from the original absorption band as shown in Fig. 3c. The absence of –OH group was balanced by introduction of –NO2 substituent which enhanced the hydrogen bonding capability of the –NH group by rendering it acidic. This is justified by the 1:1 binding ratio between R3---- AcO – ion obtained from BH plot (Fig. S19 †ESI). Binding of AcO – ion is a one-step process involving an initial strong hydrogen bond interaction between AcO – --- HN group followed by deprotonation process aided by AcO – ion forming AcOH. R4, devoid of any substituent exhibited colorimetric response from pale yellow to orange red in the presence of AcO – ion. The nitro unit on the phenylhydrazine is potent enough to introduce acidity on the NH group rendering it active for proton abstraction by AcO – ion. Yet, the shift in absorption maxima differing by 96 units is very minute in comparison with other receptors R1 and R3 possessing –NO2 substituents. The occurrence of sharp isobestic point at 435 nm indicates the complex formation process. The titration spectra is shown in Fig. 3d. Binding ratio was found to be 1:1 between R4---- AcO – ion indicating a one-step deprotonation process (Fig. S20 †ESI). Detection of sodium salts of anions in aqueous media is gaining more interest with a view to investigate the solvent interferences in the binding process. In this regard, we have analysed the 9   

anion binding in aqueous media, 9:1, v/v DMSO: H2O with addition of sodium salt of AcO – ion and F – ion(10-2 M in distilled water). UV-Vis titration studies has been performed with incremental addition of NaOAc to solution of receptors of concentration 1 x 10 -4 M (9:1, DMSO: H2O, v/v). R1, R2, R3 and R4 showed a significant redshift which was comparable to the titration studies performed in 100% DMSO solvent (Fig. S21, Fig. S23, Fig. S25 and Fig. S27 †ESI). Appearance of clear isobestic point indicate the complex formation between receptor and anion surpassing the solvent interferences in the binding process. B-H plot represents 1:1 binding ratio between target anion and receptor. B-H plot of R1-F‒, R2- F‒, R3- AcO‒ and R4- AcO‒ are shown in Fig. S22, Fig. S24, Fig. S26 and Fig. S28 †ESI respectively. The binding constant and detection limit have been calculated and presented in Table 1.

3.2 Cyclic voltammetric studies The presence of –NO2, -OH and -NH functionalities in the receptor drives in the need to investigate the impact of electrochemical reaction in the anion binding process. Considering the aforementioned fact, cyclic voltammetric studies have been performed with the incremental addition of AcO‒ ion to the receptor solution. Receptor R1 exhibited substantial decrease in the original oxidation peak and in the presence of AcO‒ ion, there was appearance of new oxidation peak owing to the oxidation of –OH and NH functionality (Fig. 4a). R2 exhibited broad oxidation peak corresponding to the two OH functionalities and one NH group. There was no much shift observed in the presence of acetate ion owing to the strong hydrogen bond interaction of receptor with anion (Fig. 4b).

10   

In the presence of AcO – ion, R3 (Fig. 4c) and R4 (Fig. 4d) exhibited substantial shift of original oxidation and reduction peak along with the appearance of new oxidation peak in the case of R3 referring to the potent oxidation of the NH functionality leading to deprotonation. Reduction of – NO2 group is a kinetic driven process involving a slow step i.e, reduction of –NO2 to –NHOH and reduction of –NHOH to nitroso group (-NO) as a fast step.17 The shift of the reduction peak from a more negative to less negative potential indicates successful reduction of NH group leading to deprotonated receptor.

Optical absorption spectroscopy has been used to determine the optical band gap of the receptors and receptor-anion complexes. The Eg of the samples were estimated from the optical absorption edge and using the Tauc relation.18 Moreover, the onset potentials of oxidation and reduction of a material can be correlated to the ionization potential (Ip) and electron affinity (Ea) according to the empirical relationship proposed by Bredas and co-workers on the basis of a detailed comparison between valence effective Hamiltonian calculations and experimental electrochemical measurements.19 The band gap has been calculated for the free receptor and receptor-anion complex from the experimental data derived from cyclic voltammetric and UV-Vis studies. The values have been represented in Table 2.

3.3 1H NMR titration studies 11   

1

H NMR titration has been performed with 1x10-4 M of R3 in the presence of AcO – ion added as

TBA salt in DMSO-d6 as solvent. The color change was immediate upon addition of 0.1 eq. of AcO – ion which revealed changes in the 1H NMR spectra. With the incremental addition of 0.5, 1.0, 1.5 and 2.0 eq. of AcO – ion the –NH proton at δ 11.96 ppm of free receptor experienced simultaneous broadening with diminution of peak intensity referring to the initial hydrogen bond complex formation followed by deprotonation process. The signal corresponding to the aromatic protons and imine proton showed decrease in their intensity upon addition of higher eq. of AcO – ion. The titration spectra is represented in Fig. 5.

The foregoing results and discussion of spectral studies indicated that the addition of AcO‒ ion initially leads to the bifurcated hydrogen bond interaction with the –NH and imine functionality and a subsequent deprotonation. Deprotonation process introduces charge separation in the receptor which is a direct consequence of increased electron density on the receptor. The proposed binding mechanism is shown in Scheme 2.

4

Conclusions

In conclusion, the present findings serve to illustrate the role of positional substitution effect on the anion binding and selectivity. Receptors R1, R2, R3 and R4 proved their ability to act as colorimetric receptor for anions. Receptor R4 exhibited selective response towards AcO ̶ ion with a binding constant of the order 4.5 x 102 M-1. Electrooptical studies and 1H-NMR titrations of the receptors in solution phase provide full proof of the deprotonation involved in the binding

12   

mechanism. Consequently, colorimetric response of receptors in the organic and aqueous media signifies its practical utility as chemosensors. Acknowledgements Authors express their gratitude to the Director and the HOD (Department of Chemistry) NITK Surathkal for the providing the research infrastructure. SP is thankful to NITK for the research fellowship. We thank the Department of Science and Technology, Government of India for providing the SCXRD facility under the FIST program. SP thanks Mr. Bharath Kumar Momidi for the assistance in SCXRD analysis. We thank CDRI Lucknow for the mass analysis and MIT Manipal for the NMR analysis. Electronic supporting information: FT-IR, Mass spectra, 1H-NMR spectra, UV-Vis titration curves, B-H plot, B-H equation, crystallographic details are available. Notes and References Crystallographic data of structure R4 reported in this article are available in the Cambridge Crystallographic Data Centre as deposition No. CCDC 1468111.

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Ed. 41 (2002) 1416. (c) D.H. Lee, K.H. Lee, J.-I. Hong, An azophenol-based chromogenic anion sensor, Org. Lett. 3 (2001)5. Examples of electron-acceptor chromophores for anion sensing: (a) R. Nishiyabu, P. Jr. Anzenbacher, Sensing of antipyretic carboxylates by simple chromogenic calix [4] pyrroles, J. Am. Chem. Soc. 127 (2005) 8270. (b) D. EstebanGo´mez, L. Fabbrizzi, M.J. Licchelli, Why, on interaction of urea-based receptors with fluoride, beautiful colors develop, Org. Chem. 70 (2005) 5717. 13. P. D. Beer and E. J. Hayes, Transition metal and organometallic anion complexation agents, Coord. Chem. Rev. 240 (2003) 167; (b) T. Lazarides, T. A. Miller, J. C. Jeffery, T. K. Ronson, H. Adams and M. D. Ward, Luminescent complexes of Re (I) and Ru (II) with appended macrocycle groups derived from 5, 6-dihydroxyphenanthroline: cation and anion binding, Dalton Trans. (2005) 528; (c) Z. Lin, S. Ou, C. Duan, B. Zhang and Z. Bai, Nakedeye detection of fluoride ion in water: a remarkably selective easy-to-prepare test paper, Chem. Commun. (2006) 624; (d) T. Lin, C. Chen, Y. Wen and S. Sun, Synthesis, photophysical,

and

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functionalized receptors and their metal complexes, Inorg. Chem. 46 (2007) 9201; (e) E. Kim, H. J. Kim, D. R. Bae, S. J. Lee, E. J. Cho, M. R. Seo, J. S. Kim and J. H. Jung, Selective fluoride sensing using organic–inorganic hybrid nanomaterials containing anthraquinone, New J. Chem. 32 (2008) 1003; (f) S. J. Dickson, M. J. Paterson, C. E. Williams, K. M. Anderson and J. W. Steed, Anion binding and luminescent sensing using cationic ruthenium (II) aminopyridine complexes, Chem.–Eur. J. 14 (2008) 7296; (g) P. A. Gale and C. Caltagirone, Anion sensing by small molecules and molecular ensembles, Chem. Soc. Rev. 44 (2015) 4212-4227 (h) D. Sharma, S. K. Sahoo, S. Chaudhary, R. K. Bera and J. F. Callan, Fluorescence 'turn-on'sensor for F− derived from vitamin B6

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cofactor, Analyst 138 (2013) 3646; (i) M. R. Ajayakumar, P. Mukhopadhyay, S. Yadav and S. Ghosh, Single-electron transfer driven cyanide sensing: a new multimodal approach, Org. Lett. 12 (2010) 2646; (j) M. R. Ajayakumar, G. Hundal and P. Mukhopadhyay, A tetrastable naphthalenediimide: anion induced charge transfer, single and double electron transfer for combinational logic gates, Chem. Commun. 49 (2013) 7684. 14. G. Ravi, M. K. Kesharwani, A. Chakraborty, B. Ganguly and P. Paul, Dipicrylamine as a colorimetric sensor for anions: experimental and computational study, RSC Adv. 2014, 4, 53273–53281. 15. P. Srikala, K. Tarafder, A. N. Shetty and D. R. Trivedi, Insights into the electrooptical anion sensing properties of a new organic receptor: solvent dependent chromogenic response and DFT studies, RSC Adv. 6 (2016) 74649-74653. 16. M. A. Kaloo and J. Sankar, Exclusive fluoride ion recognition and fluorescence “turn-on” response with a label-free DMN Schiff base, Analyst 138 (2013) 4760 and references there in. 17. R. Sharma, S. K. Mittal and M. Chhibber, Voltammetric Sensor for Fluoride Ions Using Diphenylether Derivatives Supported by NMR and Theoretical Studies, J. Electrochem. Soc. 162 (2015) 9, B248-B255. 18. J. Tauc, R. Grigorovici, A. Vancu, Optical properties and electronic structure of amorphous germanium, Phys. Status Solid 15 (1966) 627–637. 19. J.L. Bredas, R. Silbey, D.X. Boudreaux, R.R. Chance, Chain-length dependence of electronic and electrochemical properties of conjugated systems: polyacetylene, polyphenylene, polythiophene, and polypyrrole, J. Am. Chem. Soc. 105 (1983) 6555– 6559.

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Biographies: Srikala P is a Ph.D. student at NITK Surathkal under the supervision of Dr. Darshak R. Trivedi. Her research area includes the design of organic receptors for the colorimetric detection of anions. Darshak R. Trivedi received his doctoral degree from CSIR-CSMCRI (Bhavnagar University) in 2006. He is a recipient of JSPS Postdoctoral Fellowship (Japan Society for Promotion of Science) at Kyushu University, Fukuoka JAPAN and Feinberg Postdoctoral Fellowship at Weizmann Institute of Science, Rehovot, ISRAEL. Currently he is an assistant professor of chemistry at NITK Surathkal. Research area includes crystal engineering, organic chemistry, green chemistry, supramolecular chemistry and materials chemistry.  

Fig. 1: ORTEP diagram of receptor R4  

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Fig. 2: Colour change of the receptors R1, R2, R3 and R4 with the addition of 1eq. of TBA salts of anions

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Fig. 3: UV-Vis titration spectra of receptors (10 -4 M in DMSO) with the incremental addition of TBAOAc (10 -2 M in DMSO); Inset plot representing the absorption isotherm: (a) Receptor R1 + TBAOAc, (b) Receptor R2 + TBAOAc; (c) Receptor R3 + TBAOAc; (d) Receptor R4 + TBAOAc

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Fig. 4: Cyclic voltammogram of receptors (5x10-5 M) with incremental addition of TBAAcO ion (0-1 equiv.) (a) Receptor R1 + TBAOAc, (b) Receptor R2 + TBAOAc; (c) Receptor R3 + TBAOAc; (d) Receptor R4 + TBAOAc

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Fig. 5: 1H NMR titration of receptor R3 with incremental addition of TBAAcO ion (0-2 eq.)  

 

Scheme 1: Structure of receptors R1, R2, R3 and R4

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Scheme 2: Proposed mode for AcO‒ ion induced deprotonation of receptor

Table 1: Calculation of binding constant and detection limit Receptor

Anion

Solvent system

R1

TBAOAc NaF

R2

TBAOAc NaF

R3

TBAOAc NaOAc

R4

TBAOAc NaOAc

DMSO DMSO:H2O (9:1 v/v) DMSO DMSO:H2O (9:1 v/v) DMSO DMSO:H2O (9:1 v/v) DMSO DMSO:H2O (9:1 v/v)

Binding constant (M-1) 2.33 x 102 1.46 x 102

Detection limit (ppm) 9 1

2.18 x 102 1.23 x 102

4.5 0.8

2.2 x 102 1.19 x 102

3.7 1

4.5 x 102 3.14 x 102

7.7 1.2

 

 

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