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

Article abstract of DOI:10.1039/c6ra16197a

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 so

Full text of DOI:10.1039/c6ra16197a

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Insights into the electrooptical anion sensing properties of a new
organic receptor: Solvent dependent chromogenic response and
DFT studies
Received 00th January 20xx,
Accepted 00th January 20xx
Srikala P.,a Kartick Tarafdarb , A. Nityananda Shettyc and Darshak R. Trivedia*
DOI: 10.1039/x0xx00000x
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Highly selective hydrazine based electrooptical receptor featuring in few potent receptors active in aqueous media. In addition,
solvent based color transition property in the presence of acetate electrochemical techniques have been studied for long with a vision
ion has been developed. AcO‒ ion mediated color transition to apply ion selective electrodes in the detection of anions.
̶
6
̶
properties of a new organic receptor in solvents of varying polarity Incessant efforts have led to the development of F ion and AcO
highlights the influence of dipole moment in stabilizing the excited ion electrode7 for real sample analysis. As an added advantage, the
state. Selective detection of acetate ion by a new organic receptor choice of electrooptical techniques in the detection of anionic
in the presence of DMSO:Tris HCl buffer (9:1, v/v) has been the species are known to be helpful to arrive at the binding mechanism.
pivotal concept of the present work. 1H-NMR titration and DFT This has been the driving force to utilize electrooptical method as
studies provide quantitative proof of the underlying detection an alternative approach in the detection of anions. With
a
mechanism. Solution and solid state sensing response of anions by viewpoint of designing anion receptors, researchers have
the receptor reflects the practical utility in real time sample considered various noncovalent interactions such as hydrogen-
analysis.
bonding, anion pi 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.10This 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
Anion receptor chemistry involving host-guest interaction has
gained prime importance in the 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 the living system in promoting dental health,
metabolic process and ATP hydrolysis respectively.2 Inadequate
amount of anions in the living system are known to cause health
impairments. In this regard, design strategy involving synthetic
colorimetric receptors has seen progressive path with a view to
achieve 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 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
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H
N
NO2
H
C
N
O2N
NO2
Scheme 1: Structure of receptor R
̶
Fig. 1: Colour change of the receptor R (10 4M, DMSO/Tris HCl, 9:1,
v/v) with the addition of 2 eq. of TBA salts of anions
characterization of the receptor R through melting point, and
standard spectroscopic techniques confirm the formation of desired
Fig. 2: UV-Vis titration spectra of receptor R (DMSO/Tris HCl (9:1,
̶
v/v, 10 4M) with incremental addition of standard solution of
̶
̶
TBAAcO ion (1×10 2M in dry DMSO). Inset showing the absorption
product (†ESI).
isotherm at 569 nm
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—4M) 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
UV-Vis titration studies performed with the progressive
addition of AcO ion to receptor in DMSO/Tris HCl (9:1, v/v, 10 4M)
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
̶
̶
̶
̶
recorded with the addition of 1 eq. of anions (10 4 M in dry DMSO)
ratio of R--- AcO complex (Fig. S13, †ESI).
̶
̶
̶
̶
such as halides, NO3 , HSO4 , H2PO4 and AcO in the form of
The study was extended to investigate the solvent dependent
charge transfer interactions in the presence of active anions.
tetrabutylammonium (TBA) salts (Fig. S4, ESI). More significant
†
̶
̶
and instantaneous colour changes has been observed for F , H2PO4
̶
Addition of AcO ion to the receptor induced optical signalling from
̶
and AcO ions from pale yellow to violet with a significant red shift
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
of the original absorption band(Fig. S5,
in buffer medium with receptor solution in DMSO/Tris HCl (10 4 M,
†
ESI). Anion binding studies
̶
̶
̶
9:1, v/v, pH 7.4) showed excellent selectivity towards AcO ion
addition of 1 eq. of TBAAcO ion to receptor in solvents of varying
(Fig.S6
†ESI) with greenish black coloration (Fig. 1). The
polarity resulted in new peak with significant bathochromic shift
preorganized geometry, conformational flexibility, basicity of the
(Table S1,
†ESI). There was no any linear correlation observed
receptor and alkalescence of the anion could be accounted for the
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
̶
higher selectivity of receptor towards AcO ion. Titration studies
̶
performed upon gradual addition of standard TBAAcO (1×10 2M 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
stabilized complex [R---AcO ]. However, with 1, 4-dioxane there was
̶
transfer interactions existing between AcO ion and π- acidic minute shift in absorption maxima indicative of the minor
receptor moiety. Diminution of the peak at 413 nm is a clear interaction of the solvent molecules with complex [R--- AcO ]11 (Fig.
̶
indication of deprotonation of the NH proton leading to a drastic S14, †ESI). Mechanistically, the occurrence of π-π* transitions in the
̶
color change. The 1:1 binding ratio between receptor R and AcO
receptor unit resulted in gradual increase of the dipole moment
ion has been analysed with Benesi-Hildebrand (B-H) plot(Fig. S8, upon excitation. Concurrently, presence of polar solvent directed
the stabilization of excited state more than the ground state. This
†
ESI). Titration performed with incremental addition of TBAOH
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
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
̶
̶
a
substantial shift of absorption band (termed as positive
462 and 464 nm each for H2PO4 and F ions represent the complex
solvatochromism or bathochromic shift)12
.
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).
2 | J. Name., 2012, 00, 1-3
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̶
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).
Fig. 3: Solvatochromic effect observed with the addition of 1 eq. of
̶
TBAAcO
to receptor solution(10—4M) in various polar aprotic
solvents. Top row: 10—4M in different solvents; Bottom row: R +
̶
AcO ion
To arrive at the binding mechanism, 1H-NMR titration studies
has been performed with DMSO-d6 solution of receptor upon
It was interesting to find out the solvent dependent properties
̶
̶
of receptor in the presence of H2PO4 and F ions. THF solution of
̶
̶
̶
addition of TBA salt of AcO ion. Disappearance of resonance signal
receptor could colorimetrically distinguish F
and AcO
ion in
̶
̶
ascribable to the NH proton at 11.8 ppm upon successive addition
conjunction with H2PO4 and AcO ion. Colour change from pale
̶
̶
̶
of AcO ion is indicative of the deprotonation mechanism involved in
yellow to green with the addition of F and H2PO4 ions and pale
the binding process.15 (Fig. S27 †ESI). The disappearance of the
̶
yellow to deep blue with addition of AcO ion clearly represents the
̶
splitting pattern in the aromatic region indicates the formation of
selectivity of receptor for AcO ion (Fig. S15 and S16,†ESI). The
̶
NH--AcO hydrogen bond followed by a deprotonation process.
solvent dependent properties of the receptor promoted optical
signalling with less vivid colour change ranging from pale violet to
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.16The binding
mechanism is represented in Scheme 2.
̶
̶
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
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. S.36 & 37. 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
̶
̶
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 x 10 5 M) performed
with three electrode cell in acetonitrile medium and [Bu4N]-[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
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at 564 and 569 nm respectively corroborating well with the studies, 1H-NMR titrations and DFT calculations in solution phase
experimentally observed UV-Vis spectral results (Fig. S38,†ESI).
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.
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
.
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
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N
N
N
NO2
where X= AcO-
Scheme 2: Proposed binding mechanism of receptor R with AcO
ion
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 x 104 M-1 with TBA+AcO ion and 4.2x 104 M-1 with NaF
̶
reflect the stability of receptor—anion complex. Electrooptical
4 | J. Name., 2012, 00, 1-3
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AcO‒ ion mediated color transition properties of an organic receptor in solvents of varying polarity
highlighting the influence of dipole moment in stabilizing the excited state.