Insights into the electrooptical anion sensing properties of a new organic receptor: solvent dependent chromogenic response and DFT studies

Abstract

A highly selective hydrazine based electrooptical receptor featuring solvent based color transition properties in the presence of acetate ions has been developed. The AcO⁻ ion mediated color transition properties of a new organic receptor in solvents of varying polarity highlights the influence of dipole moment in stabilizing the excited state. Selective detection of acetate ions by a new organic receptor in the presence of DMSO : Tris HCl buffer (9 : 1, v/v) has been the pivotal concept of the present work. ¹H-NMR titration and DFT studies provide quantitative proof of the underlying detection mechanism. Solution and solid state sensing response of anions by the receptor reflects the practical utility in real time sample analysis.

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Anion receptor chemistry involving host–guest interaction has gained prime importance in recent years owing to the ubiquity of bioactive anions and their ecological importance.¹ Fluoride (F⁻), acetate (AcO⁻) and dihydrogen phosphate (H₂PO₄⁻) ions are known to play vital role in living system in promoting dental health, metabolic processes and ATP hydrolysis.² Inadequate amount of anions in living system are known to cause health impairments. In this regard, design strategies involving synthetic colorimetric receptors have seen a progressive path with a view to achieving the instantaneous detection of anionic species.^3,4 Researchers have explored a wide array of organic receptors bearing different functionalities for the selective and sensitive detection of anions.⁵ Selective detection of anions possessing similar basicity and surface charge density is considered to be a major challenge. Despite these setbacks, the burgeoning interest among researchers has resulted in few potent receptors active in aqueous media. In addition, electrochemical techniques have been studied for long with a vision to apply ion selective electrodes in the detection of anions. Incessant efforts have led to the development of F⁻ ion⁶ and AcO⁻ ion electrode⁷ for real sample analysis. As an added advantage, the choice of electrooptical techniques in the detection of anionic species are known to be helpful to arrive at the binding mechanism. This has been the driving force to utilize electrooptical method as an alternative approach in the detection of anions. With a viewpoint of designing anion receptors, researchers have considered various noncovalent interactions such as hydrogen-bonding, anion–π and reactions like hydrogen abstraction, electron transfer, etc.⁸ There are references in the literature wherein –NH containing receptors are capable of binding anions with single potential binding site. Researchers have worked on molecules containing amide and amine groups, particularly secondary amines having proton(s) with acidic character, which can effectively interact with anions and can function as anion sensors.⁹ To name a few, Paul and coworkers have studied the anion binding properties of dipicrylamine wherein –NH group solely acts as binding site.¹⁰ This suggests that the acidic nature of –NH group is sufficient enough to bind anion through single strong hydrogen bond interaction. Based on the above considerations, we have designed a new organic schiff base receptor comprising the hydrazine unit –NH as binding site and nitro group as redox active unit facilitating the binding site-signalling unit approach in the detection process. The present study has been focussed to evaluate the selectivity of receptor towards a specific ion of interest in the presence of buffer. Although known, solvatochromic behaviour of colorimetric receptor in the presence of AcO⁻ ion is an unexplored area. The receptor–anion complex formation has been supported by ¹H NMR titration and DFT calculations.

Receptor R has been synthesized by the condensation of 4-nitrobenzaldehyde and 2,4-dinitrophenylhydrazine in ethanol as solvent with a drop of acetic acid as catalyst (Scheme 1). The characterization of the receptor R through melting point, and standard spectroscopic techniques confirm the formation of desired product (ESI†).

Scheme 1 Structure of receptor R.

The electron donor nature of –NH and acceptor nature of –NO₂ functionality are known to impart pale yellow coloration to the receptor. DMSO solution of receptor (10⁻⁴ M) displayed an intense absorption band at 413 nm resulting from the n–π* transitions of the Ar–CH=N–NH conjugation. The UV-Vis absorption spectra was recorded with the addition of 1 eq. of anions (10⁻⁴ M in dry DMSO) such as halides, NO₃⁻, HSO₄⁻, H₂PO₄⁻ and AcO⁻ in the form of tetrabutylammonium (TBA) salts (Fig. S4, ESI†). More significant and instantaneous colour changes has been observed for F⁻, H₂PO₄⁻ and AcO⁻ ions from pale yellow to violet with a significant red shift of the original absorption band (Fig. S5, ESI†). Anion binding studies in buffer medium with receptor solution in DMSO/Tris HCl (10⁻⁴ M, 9 : 1, v/v, pH 7.4) showed excellent selectivity towards AcO⁻ ion (Fig. S6, ESI†) with greenish black coloration (Fig. 1). The preorganized geometry, conformational flexibility, basicity of the receptor and alkalescence of the anion could be accounted for the higher selectivity of receptor towards AcO⁻ ion. Titration studies performed upon gradual addition of standard TBAOAc (1 × 10⁻² M in dry DMSO) to receptor solution resulted in new absorption band centered at 572 nm with a clear isobestic point at 462 nm (Fig. S7, ESI†). The probable mechanism would be the anion–π charge transfer interactions existing between AcO⁻ ion and π-acidic receptor moiety. Diminution of the peak at 413 nm is a clear indication of deprotonation of the NH proton leading to a drastic color change. The 1 : 1 binding ratio between receptor R and AcO⁻ ion has been analysed with Benesi–Hildebrand (B–H) plot (Fig. S8, ESI†). Titration performed with incremental addition of TBAOH resulted in the similar titration profile confirming the deprotonation process (Fig. S28, ESI†). Similarly, incremental addition of 0.1 eq. of H₂PO₄⁻ and F⁻ ions in dry DMSO resulted in bathochromic shift differing by units of 159 nm and 165 nm in comparison with absorption band of the free receptor. Isobestic points centered at 462 and 464 nm each for H₂PO₄⁻ and F⁻ ions represent the complex formation (Fig. S9 and S11, ESI†). Resultant 1 : 1 binding ratio obtained with H₂PO₄⁻ and F⁻ ions indicate the deprotonation of receptor in dry DMSO (Fig. S10 and S12, ESI†).

Fig. 1 Colour change of the receptor R (10⁻⁴ M, DMSO/Tris HCl, 9 : 1, v/v) with the addition of 1 eq. of TBA salts of anions.

UV-Vis titration studies performed with the progressive addition of AcO⁻ ion to receptor in DMSO/Tris HCl (9 : 1, v/v, 10⁻⁴ M) resulted in a red shift of band by 156 units with a clear isobestic point centered at 465 nm (Fig. 2). B–H plot confirm the 1 : 1 binding ratio of R–AcO⁻ complex (Fig. S13, ESI†).

Fig. 2 UV-Vis titration spectra of receptor R DMSO/Tris HCl (9 : 1, v/v, 10⁻⁴ M) with incremental addition of standard solution of TBAAcO⁻ ion (1 × 10⁻² M in dry DMSO). Inset showing the absorption isotherm at 569 nm.

The study was extended to investigate the solvent dependent charge transfer interactions in the presence of active anions. Addition of AcO⁻ ion to the receptor induced optical signalling from pale yellow to purple, deep blue, maroon, pale violet and pale brown in various aprotic polar solvents such as ACN, THF, acetone, DCM and dioxane respectively (Fig. 3). UV-Vis spectra recorded with addition of 1 eq. of TBAOAc to receptor in solvents of varying polarity resulted in new peak with significant bathochromic shift (Table S1, ESI†). There was no any linear correlation observed between the solvent polarity and CT band due to the complex nature of solute solvent interactions dominated by the hydrogen bond accepting nature of the solvents. Among all the solvents, the shift in absorption maxima was greater in THF followed by DMSO, acetone and other aprotic solvents indicating the formation of a stabilized complex [R–AcO⁻]. However, with 1,4-dioxane there was minute shift in absorption maxima indicative of the minor interaction of the solvent molecules with complex [R–AcO⁻]¹¹ (Fig. S14, ESI†). Mechanistically, the occurrence of π–π* transitions in the receptor unit resulted in gradual increase of the dipole moment upon excitation. Concurrently, presence of polar solvent directed the stabilization of excited state more than the ground state. This reduced the separation between the two energy states which ensued in red shift of the absorption band. Magnitude of red shift was further influenced by the extent of variation of dipole moment in the excitation process. The occurrence of smaller magnitude of dipole moment in the ground state than in the excited state, led to a substantial shift of absorption band (termed as positive solvatochromism or bathochromic shift).¹²

Fig. 3 Solvatochromic effect observed with the addition of 1 eq. of TBAOAc to receptor solution (10⁻⁴ M) in various polar aprotic solvents. Top row: 10⁻⁴ M in different solvents; Bottom row: R + AcO⁻ ion.

It was interesting to find out the solvent dependent properties of receptor in the presence of H₂PO₄⁻ and F⁻ ions. THF solution of receptor could colorimetrically distinguish F⁻ and AcO⁻ ion in conjunction with H₂PO₄⁻ and AcO⁻ ion. Colour change from pale yellow to green with the addition of F⁻ and H₂PO₄⁻ ions and pale yellow to deep blue with addition of AcO⁻ ion clearly represents the selectivity of receptor for AcO⁻ ion (Fig. S15 and S16, ESI†). The solvent dependent properties of the receptor promoted optical signalling with less vivid colour change ranging from pale violet to brown with H₂PO₄⁻ and F⁻ ions. The colour change was restricted to solvents such as DMSO, THF and acetone (Fig. S17 and S18, ESI†).

Sodium salts of fluoride and acetate have been a major constituent of commercially available toothpaste, mouthwash and vinegar respectively. These have encroached into the household usage in the form of food, medicine and cosmetics. Beyond an optimum amount, anions can lead to health issues. In this regard, a real time monitoring system can be an immediate measure for on-field analysis. We have aimed to check the anion sensing ability in aqueous medium using sodium acetate and sodium fluoride. UV-Vis titration experiments with the successive addition of F⁻ and AcO⁻ ions as sodium salts displayed a bathochromic shift of 162 nm and 161 nm respectively (Fig. S19 and S21, ESI†). The receptor could effectively combat the solvent interferences in the presence of Na⁺ counter ion; which implies the binding ratio of 1 : 1 of R with F⁻ and AcO⁻ ions (Fig. S20 and S22, ESI†). Addition of a drop of commercially available mouthwash and vinegar induced violet coloration of the receptor. UV-Vis spectra of receptor in the presence of seawater, mouthwash and vinegar yielded similar charge transfer bands as observed in the case of standard AcO⁻ and F⁻ ions (Fig. S23 and S24, ESI†). With an attempt to envisage the solid state sensing property by grinding equimolar mixture of receptor with AcO⁻ ion, we observed a color change from yellow to greenish black (Fig. S25, ESI†). The binding constant, binding ratio and detection limit of receptor with active anions has been tabulated in Table S2 (ESI†).

Cyclic voltammetric studies of receptor (5 × 10⁻⁵ M) performed with three electrode cell in acetonitrile medium and (Bu)₄N(ClO₄​) as supporting electrolyte reveal the anodic peak at 0.38 V due to the oxidation of –NH group and cathodic peak at −0.41 V due to the reduction of the nitro group.¹³ 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.¹⁴ Addition of 1 eq. of AcO⁻ ion resulted in an increase in intensity of the original oxidation peak with slight shift to 0.51 V which is attributed to the abstraction of proton from –NH group by AcO⁻ ion leaving behind N⁻ species. It depicts the complex electrochemical mechanism involving both electrochemical and chemical reactions. The diminution of original reduction peak and appearance of a new peak centered at −0.5 V indicates the direct involvement of redox active –NO₂ moiety in the detection mechanism (Fig. S26, ESI†).

To arrive at the binding mechanism, ¹H-NMR titration studies has been performed with DMSO-d₆ solution of receptor upon addition of TBA salt of AcO⁻ ion. Disappearance of resonance signal ascribable to the –NH proton at 11.8 ppm upon successive addition of AcO⁻ ion is indicative of the deprotonation mechanism involved in the binding process.¹⁵ (Fig. S27, ESI†). The disappearance of the splitting pattern in the aromatic region indicates the formation of NH–AcO⁻ hydrogen bond followed by a deprotonation process.

Based on the UV-Vis and ¹H-NMR titration studies, the following binding mechanism has been proposed. The addition of strong basic anion (AcO⁻) initially leads to the bifurcated hydrogen bond interaction with the –NH and imine functionality with a subsequent deprotonation of –NH proton. Deprotonation process further tends to increase the electron density by introducing charge separation in the receptor. This facilitates ICT transition between electron deficient –NO₂ functionality at para position and electron rich N⁻ species resulting in the strong colorimetric response.¹⁶ The binding mechanism is represented in Scheme 2.

Scheme 2 Proposed binding mechanism of receptor R with AcO⁻ ion.

To support the AcO⁻ ion induced optical signalling event of the receptor in solvents of varying polarity, DFT calculations have been performed using B3LYP/6-31G (d,p) basis set. The optimized structure of receptor in gas phase (Fig. S29, ESI†) and the deprotonated form observed upon AcO⁻ ion binding in few selected solvents viz., acetone (Fig. S35, ESI†) and DCM have been derived. HOMO and LUMO of the deprotonated receptor in DCM is represented in Fig. 4. The band gap for optimized structure of receptor in gas phase is found to be 3.36 eV. Anion induced deprotonation of receptor in DCM results in lowering of the band gap value from 3.36 eV to 3.02 eV which is responsible for the red shift of the band observed in UV-Vis spectra. Significant redshift of the absorption band is the resultant of deprotonation of anion binding site.¹⁷ Similar observations were obtained for the receptor in acetone with variation in band gap from 3.36 to 3.0 eV. Structure of HOMO and LUMO of deprotonated receptor in acetone is given in Fig. S36 and S37.† Dipole moment calculated for the receptor and its deprotonated form reveals a change from 2.59 D to 9.29 D and 9.63 D in DCM and acetone respectively. The higher dipole moment of receptor in acetone in comparison with DCM indicates efficient charge transfer from receptor to solvent in case of acetone. With these values, the role of dipole moment in stabilizing the excited state more than the ground state is justified. The receptor in its deprotonated form in DCM and acetone exhibited absorption band at 564 and 569 nm respectively corroborating well with the experimentally observed UV-Vis spectral results (Fig. S38, ESI†).

Fig. 4 Schematic representation of (a) chemical structure of deprotonated receptor R (b) DFT derived optimized structure of the deprotonated receptor in DCM. Isosurface in (c) representing distribution of HOMO and (d) LUMO in deprotonated receptor.

In conclusion, the present findings serve to illustrate the selective detection of AcO⁻ ion in buffer. Lower detection limit of 0.8 ppm and 0.4 ppm observed in case of AcO⁻ ion and F⁻ ion which is relatively lower than the WHO guidelines highlights the efficiency of the sensor. Receptor–AcO⁻ ion complexation induced pronounced change in the position and intensity of absorption band emphasizing the effect of solvent polarity. The binding constant value of 5.28 × 10⁴ M⁻¹ with TBA⁺AcO⁻ ion and 4.2 × 10⁴ M⁻¹ with NaF reflect the stability of receptor–anion complex. Electrooptical studies, ¹H-NMR titrations and DFT calculations in solution phase provide full proof of the deprotonation involved in the binding mechanism. Colorimetric response of sensor towards AcO⁻ ion in the organic, aqueous and solid phase signifies its practical utility as a real time monitoring system.

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 Proteomics facility, MBU, IISc Bangalore for the mass analysis and MIT Manipal for the NMR analysis. KT wants to acknowledge the financial support received from SERB.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: UV-Vis titration spectra, ¹H-NMR titration spectra, cyclic voltammogram and DFT data. See DOI: 10.1039/c6ra16197a

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