Bis(pyrrole-benzimidazole) conjugates as novel colorimetric sensor for anions

Article abstract of DOI:10.1007/s12039-017-1272-8

Abstract: Novel bis(pyrrole-benzimidazole) (PYBI) conjugates were investigated as colorimetric anion recognition motif by employing multiple donor sites. In this regard, PYBI derivatives where pyrrole unit is connected to the C2 position of benzimidazole via its α-position were synthesized and their anion recognition behavior were evaluated by UV-Vis, fluorescence and ¹H NMR spectroscopy. PYBI selectively interacts with fluoride ion, whereas introduction of nitro group on the benzimidazole moiety enhances the binding affinity by at least one order, albeit at the expense of the selectivity. Bridging of two PYBIs led to interesting cooperative effect between the two subunits, which gets enhanced upon changing the spacer between the two from non-conjugating type (sp³-C) to conjugating one (quinoxaline). This also affects the way they interact with anions, with the latter moiety displaying very interesting stepwise double deprotonation of benzimidazole protons upon addition of fluoride ions with strong colorimetric as well as fluorometric response. Further, acidity of the H-bond donor sites was found to play pivotal role in the anion complexation mechanism and selectivity. Graphical abstract?: SYNOPSIS Novel bis(pyrrole-benzimidazole) (PYBI) conjugates where pyrrole units are connected to the C2 position of benzimidazole via its α-position were synthesized and their anion recognition behavior were evaluated. Anion selectivity and mode of interaction was found to depend on the nature of the substituent and the spacer between the two PYBI units. [Figure not available: see fulltext.].

Full text of DOI:10.1007/s12039-017-1272-8

J. Chem. Sci. Vol. 129, No. 5, May 2017, pp. 647–656. © Indian Academy of Sciences.
DOI 10.1007/s12039-017-1272-8
REGULAR ARTICLE
Bis(pyrrole-benzimidazole) conjugates as novel colorimetric sensor
for anions
SANJEEV PRAN MAHANTAa,b,∗ and PRADEEPTA KUMAR PANDAb,∗
aDepartment of Chemical Sciences, Tezpur University, Napaam, Assam 784 028, India
bSchool of Chemistry, University of Hyderabad, Hyderabad, Telengana 500 046, India
E-mail: spm@tezu.ernet.in; pkpsc@uohyd.ernet.in
MS received 24 January 2017; revised 16 March 2017; accepted 21 April 2017
Abstract. Novel bis(pyrrole-benzimidazole) (PYBI) conjugates were investigated as colorimetric anion
recognition motif by employing multiple donor sites. In this regard, PYBI derivatives where pyrrole unit is
connected to the C2 position of benzimidazole via its α-position were synthesized and their anion recognition
behavior were evaluated by UV-Vis, fluorescence and 1H NMR spectroscopy. PYBI selectively interacts with
fluoride ion, whereas introduction of nitro group on the benzimidazole moiety enhances the binding affinity by
at least one order, albeit at the expense of the selectivity. Bridging of two PYBIs led to interesting cooperative
effect between the two subunits, which gets enhanced upon changing the spacer between the two from non-
conjugating type (sp3-C) to conjugating one (quinoxaline). This also affects the way they interact with anions,
with the latter moiety displaying very interesting stepwise double deprotonation of benzimidazole protons upon
addition of fluoride ions with strong colorimetric as well as fluorometric response. Further, acidity of the H-bond
donor sites was found to play pivotal role in the anion complexation mechanism and selectivity.
Keywords. Pyrrole; benzimidazole; anion recognition; cooperative; proton migration.
1. Introduction
forms H-bonded complex, but in presence of excess
anion, deprotonation is favoured owing to the forma-
tion of stable H-bonded anion dimers.3 The mode of
interaction, i.e., H-bonding or deprotonation, generally
depends on the acidity of the H-bond donor site of the
receptor and basicity of the anion. In principle, increas-
ing the acidity of the H-bond donor moieties increases
both H-bond donor affinity as well as the deprotona-
tion tendency. Moreover, receptors where the negative
charge delocalization can be extended to the entire
molecular framework favours deprotonation.4 Depro-
tonation increases electron density at the receptor sites
and eventually decreases the H-bonding tendency of the
remaining H-bond donor sites of the receptor.
Pyrrole and benzimidazole are widely used as H-bond
donor components in the design of artificial receptors.5,6
While pyrrole acts as an H-bond donor, benzimidazole
endowed with both N-H and imine N functionalities
can respectively act as both H-bond donor and accep-
tor. The acidic hydrogen of benzimidazole is involved
in tautomerism and thereby shuttles between the two
N atoms. Therefore, benzimidazole binding sites could
be easily oriented on the demand of the complemen-
tary binding sites of the guest. Bordwell reported pKa
of benzimidazole as 16.4 in DMSO, which is lower
than that of pyrrole (pKa = 23.0), suggesting a rel-
Considering the important roles of anions in biological,
environmental and industrial processes, anion recog-
nition chemistry has developed into an independent
research area focused on the structure and binding
modes of anions.1 The goals of these studies have
been to understand specific binding behaviour, to derive
structure-function relationships, to search for new anion
binding motifs and, last but not the least, to find useful
application as sensors.2 In the last three decades, several
neutral receptors containing polarizable H-bond donor
units such as pyrrole, imidazole, phenol, amide, urea,
etc., and their conjugates such as amidopyrroles, ami-
doureas, pyrrole-ureas were reported.2,3 Anionic guest
interacts with these receptors either via H-bond for-
mation or via deprotonation mechanism.1 In case of
H-bonded system, the binding constant or the forma-
tion constant can be directly correlated to the stability
of the complex between the receptor and anion. How-
ever, deprotonation is a Brönsted acid-base reaction and
the equilibrium constant depends on the stability of
the corresponding conjugate base of the receptor. With
highly basic anions such as fluoride and acetate, recep-
tors with moderately acidic H-bond donor unit initially
*
For correspondence
647

648
Sanjeev Pran Mahanta and Pradeepta Kumar Panda
Figure 1. Structure of the synthesized pyrrole-benzimidazole (PYBI) derivatives.
atively stronger H-bonding as well as deprotonation ter colorimetric response for the recognition event, nitro
tendency of the former.7 Literature reports suggest that group is incorporated at the benzimidazole periphery. It
pyrrolic receptors interact with anions preferably via H- is assumed that insertion of nitro group will lead to facile
bonding with some exceptions.5 Gale and co-workers anion induced charge transfer from the binding domain
reported that diamidopyrrole derivatives with electron to the nitro group and yield a sharp color change.12 In
withdrawing groups interact with fluoride via deproto- addition to the basic conjugates, we have proposed two
nation mechanism.8 In 1999, Sessler et al.,9b reported different sets of dimers (bis-PYBI) containing two PYBI
that2,3-dipyrrol-2-ylquinoxalinereceptorinteractswith units, where both the units are bridged via two different
fluoride via H-bonding (N-H...F−) with subsequent spacer motifs - first, an electronically non-coupled sp3-
deprotonation, while other anions interact predomi- carbon(analogoustodipyrromethanes)andsecond, they
nantly via H-bonding.9 However, systems where ben- are coupled by a fluorescent quinoxaline moiety (analo-
zimidazole units are employed as H-bond donors are gous to quinoxaline bipyrrole). These dimeric motifs are
comparatively less common and mostly interact with expected to display cooperative binding effect towards
anions via the deprotonation mechanism.6 Caussey and anionic guests owing to varied interaction between the
Allen reported that benzimidazole diamides interact two PYBI units. This can be envisaged in view of their
with anions through H-bonding, and benzimidazole tau- difference in electronic communication and spatial dis-
tomerism affects the composition of the H-bonding position, which may be further aided by proton shuttling
array provided to the anion.10 Furthermore, Gale et al., ability of the two constituent benzimidazoles.
reported that benzimidazole appended ureas bind anions
through H-bonding, and the tautomeric H-bonding
structures of the anion complexes depend on the H-
bonding domain of the anions.11 These observations
clearly reflect that the mode of interaction of pyrrole
and benzimidazole units with anions basically depends
on the basicity of the anion and the nature of the organic
scaffold where it is incorporated.
Herein, we report new pyrrole-benzimidazole (PYBI)
conjugates where the pyrrole unit is directly linked to
the C2 position of the benzimidazole ring via its α car-
bon, along with their detailed anion binding behaviour
(Figure 1). These new motifs have two preorganized
heterocyclic N-H donor sites and represent larger, more
extended and flexible binding domain for anions (owing
to proton transfer between benzimidazole Nitrogens).
Further, we wish to explore the relative binding affin-
ity of the two N-H donor units, along with the role of
their acidity in anion selectivity. To increase the acidity
difference between the two NH units and to obtain bet-
2. Experimental
2.1 Materials and Methods
4-nitro-o-phenylenediamine and tetrabutylammonium salts
of anions employed for the study were purchased from
Sigma Aldrich and were used as received. Pyrrole, o-
phenylenediamine, phosphorous oxychloride, oxalyl chloride
and nitrobenzene were purchased from Sisco Research Labo-
ratories, India. Pyrrole, phosphorousoxychlorideandsolvents
used in the synthesis were dried and distilled prior to use.
Spectroscopic grade DMSO (Merck, India) was used for
recording all spectrophotometric data. Melting points were
determined with MR-Vis+ visual melting point range appa-
ratus from LABINDIA Instruments Pvt. Ltd. IR spectra were
recorded on a FT-IR spectrometer (JASCO FTIR model 5300
or NICOLET 5700). NMR spectra were obtained on Bruker
400 MHz FT-NMR spectrometer operating at ambient tem-
1
perature. TMS was used as internal standard for H NMR
spectra. LCMS spectra were recorded using Shimadzu-

Bis(pyrrole-benzimidazole) conjugates as novel colorimetric sensor for anions
649
LCMS-2010 mass spectrometer. Elemental analyses were to the NH tautomerism of benzimidazole moiety that
obtained using Thermo Finnigan Flash EA 1112 analyzer.
UV-Vis spectra were recorded on a Perkin Elmer Lambda
35 UV-Vis spectrophotometer. Fluorescence spectra were
recorded using Horiba Zobin Yovan Fluoromax-4 instrument.
becomes slower in presence of the nitro group. On the
other hand, proton shuttling between the two benzimi-
dazole nitrogens is probably too fast in the case of 1a, b
and 2a to be detected in the NMR time scale; therefore, it
appears as a broad signal (see SI, Figures S1, S3 and S5).
The absorption and emission spectra of compounds are
shown in Figure 2 (Table 1). UV-Vis spectrum of com-
pound 1a in DMSO revealed two absorption maxima
at 312 and 327 nm (not much resolved). In addition, it
shows fluorescence with emission maxima at 336 and
349 nm. On the other hand, insertion of nitro deriva-
tive (i.e., 1b) results in well-resolved absorption bands
with maxima at 299 and 387 nm indicating charge trans-
fer nature of the transition. However, this molecule
did not display any detectable fluorescence owing to
the intramolecular photoinduced electron transfer (PET)
fromtheexcitedfluorophoretotheelectronwithdrawing
nitro group (donor-excited PET; d-PET).14 Absorption
spectrum of quinoxaline-bridged dimeric conjugate 3, in
DMSO, exhibits two absorption bands with maxima at
318and402nmanditshowsarelativelylargeredshifted
emission at 560 nm. This indicates facile conjugation
between the two PYBI conjugates via quinoxaline moi-
ety both in the ground and the excited state.
3. Results and Discussion
3.1 Synthesis and characterization
Pyrrole-benzimidazole (PYBI) conjugates were conve-
niently synthesized by condensation of 1,2-phenylened-
iamine or m-nitro-1,2-phenylenediamine with the corre-
sponding pyrrole carboxaldehyde derivative in nitroben-
zene solvent at 120◦C (see Supplementary Informa-
tion).13 All compounds were characterized by standard
spectroscopic techniques including 1H, 13C NMR spec-
troscopy, mass and elemental analysis (in SI). 1H NMR
spectra recorded in DMSO-d6 at 298 K show two NH
signals above 12 ppm while pyrrole α- and β-Hs res-
onate around 6 and 7 ppm, respectively, for all the
compounds (see Supplementary Information: Figures
S1, S3, S5, S7 and S9). In the case of 2b, benzimidazole
NH signal splits into a doublet, which may be attributed
Figure 2. Normalized absorption (solid curves) and emission (dashed curves) spectra of the receptors in DMSO.
Table 1. Absorption and emission maxima along with log
( /M−1cm−1) and Stoke’s shift values for PYBI conjugates 1–3 in
ε
DMSO at 25◦C.
Receptors Absorption in log
Emission in
Stoke’s
DMSO λmax ( /M−1cm−1) DMSO λmax Shift (nm)
ε
(nm)
(nm)
1a
1b
2a
2b
3
312, 327
299, 387
313, 338
303, 398
318, 404
4.56, 4.49
4.35, 4.13
4.68, 4.58
4.55, 4.36
4.49, 4.27
337, 349
–
361 (broad)
–
10
–
23
–
563
159

650
Sanjeev Pran Mahanta and Pradeepta Kumar Panda
absorbance of the peak at 327 nm. These observations
3.2 Anion binding study
suggested the possible involvement of two equilibrium
process during the titration with varying concentration
of fluoride. It is envisaged that initially for lower effec-
tive [F−], H-bonding interaction (N-H. . .F−) could be
attributed for the initial set of spectral changes. Further
increase in [F−] might trigger the deprotonation of ben-
zimidazole moiety. Comparatively higher acidity of the
benzimidazole N-H and the high thermodynamic sta-
bility of the HF−2 ion have favoured the deprotonation
process. Deprotonation increases the electron density
in 1a and consequently reduces the strength of the H-
bonding interaction of the pyrrolic N-H with the fluoride
ion.16
3.2a Absorption and Emission Spectroscopy: Initial
anion binding studies of the receptors were carried out at
298 K, in DMSO, using UV-Vis spectroscopic titration
method. The studies were done with 20 μM solution
of the receptors. Titrations were performed by adding
known quantities of concentrated solution of the anions
to the receptor solution. The anion solutions used in the
titrations contained the receptor at the same concentra-
tion as the receptor solutions so as to nullify the dilution
effect. The absorption data were fitted in the Benesi-
Hildebrand plot of Connor equation (eq. 1) and from
the ratio of the slope and intercept, binding constant Ka
was evaluated.15
Anion screening experiment of receptor 1b displayed
significant changes in UV-Vis spectrum, upon addition
of F−, CH3COO− and H2PO−4 ions, and consequently
colour of the solution changed from very light yel-
low to dark yellow with varying intensities (Figure 4).
UV-Vis spectral titration revealed that with gradual
addition of fluoride and acetate ions, the intensities
of absorption bands at 299 and 387 nm decreased,
while concomitantly, two new peaks appeared at 305
and 439 nm (Figure 5). The titrations revealed two
clear isosbestic points at 324 and 404 nm. Whereas,
dihydrogenphosphate ion displayed relatively moder-
ate response with slight change in the peak maxima
(at 304 and 431 nm). The interaction of 1b with
anions resulted in increase in the electron density at
the PYBI moiety and, as a consequence, facile charge
transfer towards the acceptor –NO2 moiety stabilizes
the excited state and hence causes a bathochromic
shift in the absorption maxima, resulting in the change
in color.12 This clearly reflects that nitro substitu-
tion at the benzimidazole periphery led to the col-
orimetric change upon addition of anions, however,
1
1
1
=
+
St Kabꢀε [L] St bꢀε
(1)
[A − A0]
Here, A◦, A: Measured absorbance of the substrate
in absence and in presence of anion respectively; St :
Total receptor concentration (M); [L] : Anion concen-
ε
tration (M); Ka : Binding constant; ꢀ : Difference in
molar absorptivity of the free receptor solution and the
receptor-anion solution, and b: Path length.
Anion screening experiment of 1a showed an appre-
ciable change in the absorption spectrum for fluoride
ion only, among the tested anions. Addition of TBAF
resulted in ∼9 nm red shift of the two absorption bands
(Figure 3). In order to evaluate the affinity of fluoride
ion towards 1a, systematic spectrophotometric titration
was carried out in DMSO with varying concentration
of fluoride ion. During initial part of the titration (0–
10 mole equiv.) a decrease in intensity of the band at
327 nm was observed. However, on further increase
in F− concentration (>10 mole equiv.), a new band
appeared at 336 nm with concomitant decrease in the
Figure 3. Left: UV-Vis spectra of 1a (20 μM) in DMSO in the absence and presence of different anions;
right: UV-Vis spectra of 1a (20 μM) upon gradual addition of TBAF (0 to 15 mM).

Bis(pyrrole-benzimidazole) conjugates as novel colorimetric sensor for anions
651
Figure 4. Left: UV-Vis spectra of 1b (20 μM) in DMSO in absence and presence of different anions;
right: UV-Vis spectra of 1b (20 μM) in DMSO upon gradual addition of TBAF (0 to 0.35 mM).
Figure 5. Left: Evolution of UV-Vis spectra of 1b (20 μM) in DMSO upon gradual addition of TBA
(CH3COO) (4.3 × 10−4 M); right: UV-Vis spectra of 1b (20 μM) in DMSO upon gradual addition of
TBAH2PO4 (0 to 1.3 mM).
at the expense of reduced selectivity towards anions. sp3 carbon, it is enhanced by more than one order
Job’s plot analysis indicated the formation of 1:1 sto- when bridged through conjugating quinoxaline moiety,
ichiometric interaction, in the case of all the three upon complexation with fluoride ion. For example, com-
anions.
pared to 1a, Ka of fluoride complexation is increased
Anion binding study of the bis-PYBI receptors shows by more than two fold in the case of 2a; on the other
very interesting host-guest chemistry. For example, hand, in the case of 3, Ka is increased by more than
depending upon the basicity and coordination ability of twelve fold. Similar trend was also noticed between 1b
the anions, either positive or negative cooperative effect and 2b (i.e., Ka increased by three fold). This can be
between the two constituent PYBIs could be clearly attributed to lack of extended conjugation in the case of
noticed. Calculated equilibrium constants (Ka) value 2a and 2b, which could not stabilize the deprotonated
from the UV-Vis data are given in Table 2. For calcu- hosts much. On the other hand, the positive cooper-
lation of equilibrium constant, the reaction between 1a. ative effect between the two PYBIs during fluoride
X− and X− (where X− is anion) is considered. Although complexation by 3 could be ascribed to its extended con-
the values do not show any generalized trend w.r.t. basic- jugation that effectively stabilizes the negative charge
ity of the anions and the number of H-bond donor sites, a upondecomplexation. Second, incorporationofelectron
closer inspection reflects a few notable trends. First, the withdrawing nitro group in the benzimidazole periphery
presence of two PIBYs, while it marginally enhances (2b) and increased conjugation between the two PYBIs
the binding affinity when bridged through insulating (3) led to reduced anion selectivity with additional guest

652
Sanjeev Pran Mahanta and Pradeepta Kumar Panda
Table 2. Acid-base equilibrium constant,
ing of fluorescence of receptors 1a and 2a was observed
upon addition of F− ion, whereas no significant change
in the fluorescence intensity could be noticed in the case
of other anions (see Supplementary Information). This
observation further suggests interaction of receptors 1a
and 2a with fluoride ion only. In the case of receptor 3,
addition of F−, CH3COO− and H2PO−4 ion resulted in
almost complete quenching of its fluorescence but with
different affinity. This may be ascribed to intramolec-
ular photoinduced electron transfer (PET) and anion
complexation triggered the electron transfer from the
benzimidazole moiety to the ground state of the quinox-
aline moiety.
Ka (M−1) of the receptors towards the inter-
acting anions as determined by UV-Vis spec-
troscopy in DMSO at 25◦C.
Host
F−
CH3COO− H2PO−4
1a
1b
2a
2b
3
8.45 × 102
–
–
1.20 × 104 6.03 × 104 6.30 × 103
1.90 × 103
–
–
3.62 × 104 1.24 × 104 7.84 × 103
2.01 × 104 8.72 × 104 9.4 × 103
binding towards acetate and dihydrogen phosphate ions
by these hosts (Figures S37 and S49 in Supplementary 3.2b 1 H NMRstudy: Toobtainfurtherinsightregard-
Information). Surprisingly, complexation of acetate ion ing the nature of interaction involved in the complex-
1
with 2b resulted in reduced affinity compared to that ation process, H NMR titrations were carried out in
of 1b by about five times, whereas dihydrogen phos- DMSO-d6, with fluoride and acetate ions. In this exper-
phate ion complexation led to marginal increase in Ka iment, the change in the 1H NMR signals upon gradual
(ratio of 2b/1b ∼ 1.25). This type of negative cooper- addition of the anionic analyte was monitored.
ative effect between the two PYBIs could be explained
Addition of 0.17 equiv. of TBAF to 1a in DMSO
on the basis of competition between them to bind the revealed disappearance of both the NH signals and
bidentate acetate ion. Whereas, the relatively larger size instead a new broad peak arises at ∼12.1ppm, which
and presence of additional binding sites in the case of undergoes further broadening leading to complete dis-
H2PO−4 ion possibly reduced the negative cooperative appearance upon subsequent additions of TBAF (Fig-
effect.
ure 6). After addition of ∼1.2 equiv. of fluoride ion, a
Fluorescence spectroscopic titration studies were new broad triplet starts appearing at 16 ppm, indicating
also performed in order to check the ability of these the formation of HF−2 ion, which indicates deprotona-
receptors to function as fluorometric anion sensors. tion of benzimidazole NH at higher concentration of F−.
Among the receptors synthesized, nitro analogues are Aromatic proton signals displayed upfield shift upon
non-fluorescent (fluorescence is below detection limit). addition of fluoride ion, indicating an increase in the
Emission studies were carried out with 1 μM solution of electron density on both the pyrrole and benzimida-
the remaining three receptors; i.e., 1a, 2a and 3. Like the zole rings due to de−protonation. The strong electrostatic
UV-Vis spectroscopy observation, remarkable quench- repulsion from 1b may be preventing the F− ion to
Figure 6. Changes in the 1H NMR spectra of 1a (5 mM) in DMSO-d6 upon gradual addition of TBAF (0
to 12 mM).

Bis(pyrrole-benzimidazole) conjugates as novel colorimetric sensor for anions
653
Figure 7. Changes in the 1H NMR spectra of 1b (5 mM) in DMSO-d6 upon gradual addition of TBAF (0
to 30 mM).
Figure 8. Changes in the 1H NMR spectra of 1b (5 mM) in DMSO-d6 upon gradual addition of TBAOAc
(0 to 12 mM).
come closer to form H-bonding. The little downfield ance of the benzimidazole NH and downfield shift of
shift of the pyrrole NH proton indicates the involvement the pyrrole NH with concomitant upfield shift of the aro-
of pyrrole NH in weak H-bonding with the anion. How- matic protons (Figure 8). This observation indicates that
ever, broadening and subsequent disappearance of the more acidic benzimidazole N-H proton gets deproto-
signal upon increasing fluoride ion concentration might nated and pyrrole N-H is involved in a relatively stronger
indicate the fast shuttling of the NH proton between the H-bonding (than with fluoride ion) with the protonated
pyrrole and benzimidazole nitrogens, which is probably guest, possibly owing to its complementary bidentate
too fast to be detected in the NMR time scale (SI). On coordination mode (Figure S29, in SI). Furthermore,
the other hand, titration of 1b with fluoride ion showed titration of 2b with fluoride and acetate ions displayed
deprotonation of the benzimidazole NH, while pyrrolic similar binding patterns, as observed in the case of 1b
1
NH gradually broadens with slight upfield shift (Fig- (SI). H NMR titration of receptor 3 with fluoride ion
ure 7). The upfield shift of the pyrrolic NH is attributed showed two new peaks at 20.5 and 16 ppm, along with
to the increase in the electron density in the PYBI moi- a very broad peak between 12 and 16 ppm indicating
ety, owing to the deprotonation of the benzimidazole the very slow kinetics of the binding and deprotonation
NH. This probably reduces the hydrogen bonding inter- equilibrium (Figure 9). Interestingly, gradual addition
action between the pyrrole NH and fluoride ion. This of TBAF up to ∼2 equiv. resulted in sharpening of the
observation confirmed that the selection of the NH unit peaks at 20.5 and 16 ppm with concomitant loss of the
for interaction by anion is driven by the acidity of the the broad peak between 12 to 16 ppm. Further addition
NH. Titration of 1b with acetate ion revealed disappear- of TBAF resulted in upfield shift of the peak at 20.5 ppm

654
Sanjeev Pran Mahanta and Pradeepta Kumar Panda
Figure 9. Changes in the 1H NMR spectra of 3 (5 mM) in DMSO-d6 upon gradual addition of TBAF (0
to 25mM).
Figure 10. Probable H-bonding structures of mono- and di-deprotonated forms of 3.
to 19 ppm, while the broad peak at 16 ppm resolved changes in the β-CH pyrrolic region while no new sig-
into a triplet without undergoing shift. The peak at nal was observed in the downfield region, as seen in the
20.5 ppm emerging after addition of 1 equiv. of fluo- case offluoride ion binding (see SI). Upon increasing the
ride ion (with slight upfield shift of both uncomplexed amount of acetate ion, pyrrole NH peaks get broadened
1
benzimidazole and pyrrole signals), may possibly be without any noticeable shift. The results of H NMR
attributed to deprotonation of one of the benzimida- titration experiment corroborated the observations of the
zole moiety, followed by strong complexation of one UV-Visstudy, whichsuggestedthepresenceoftwoequi-
of the pyrrolic NH with fluoride ion and benzimidazole librium processes in the case of 1a and 2a, namely, the
Nitrogen (Figure 10, Complex A). On the other hand, H-bonded adduct formation for relatively lower F− con-
evolution of the signal corresponding to HF−2 ion indi- centration, whereas at higher concentration, Bronsted
cates the deprotonation of the benzimidazole protons, acid-base equilibrium prevails. However, in the case of
and the appearance of the broad peaks in between 12 nitro derivatives (1b and 2b), addition of fluoride ion
and 16 ppm signifies the intramolecular proton migra- leads to immediate deprotonation of the benzimidazole
tion in the dianionic 32− species. Possibly, the pyrrolic NH and subsequent charge transfer to the nitro group
NHs migrated towards the benzimidazole nitrogens and leading to color change. Incorporation of nitro group
trapped inside the dianionic binding domain, by forming in the benzimidazole periphery drastically enhances the
strong N-H…N H-bonding within the cavity resulting in acidity of the benzimidazole NH compared to the pyr-
the peak at 19 ppm (Figure 10, Complex B).17 Further, role NH, thereby increasing its deprotonation tendency.
titration of 3 with acetate ion showed similar type of Receptor 3 also follows the deprotonation mechanism.

Bis(pyrrole-benzimidazole) conjugates as novel colorimetric sensor for anions
655
4. Conclusions
perspectives Angew. Chem. Int. Ed. 40 486; (c) Gale
P A and Dehaen W 2010 In Anion Recognition in
Supramolecular Chemistry (Springer: Berlin); (d) Lang-
ton M J, Serpell C J and Beer P D 2016 Anion recognition
in water: Recent advances from a supramolecular and
macromolecular perspective Angew. Chem. Int. Ed. 55
1974;(e)ElmesRBPandJolliffeKA2015Anionrecog-
nition by cyclic peptides Chem. Commun. 51 4951; (f)
Ashton T D, Jolliffe K A and Pfeffer F M 2015 Lumines-
cent probes for the bioimaging of small anionic species
in vitro and in vivo Chem. Soc. Rev. 44 4547; (g) Gale
P A, Busschaert N, Haynes C J E, Karagiannidis L E
and Kirby I L 2014 Anion receptor chemistry: High-
lights from 2011 and 2012 Chem. Soc. Rev. 43 205; (h)
Ballester P 2010 Anion binding in covalent and self-
assembled molecular capsules Chem. Soc. Rev. 39 3810;
(i) Bregovic V, Basaric N and Mlinaric-Majerski K 2015
Anion binding with urea and thiourea derivatives Coord.
Chem. Rev. 295 80
Afamilyofcolorimetricchemosensorsbasedonpyrrole-
benzimidazole conjugates (PYBI) were synthesized and
their anion recognition behaviour was investigated in
detail. Receptor 1a and 2a act as colorimetric sensors
for fluoride ion only. The 1H NMR titration experiment
revealed that the binding of the fluoride ion is through
N-H. . .F− interaction. However, addition of more than
one equivalent of anion eventually led to the deproto-
nation. Further, we have observed that incorporation
of nitro group upon benzimidazole periphery induces
the colorimetric response (in case of 1b and 2b) but
at the expense of selectivity by exhibiting tendency
towards binding with acetate and dihydrogenphosphate
ions. This observation reveals that increasing the acidity
of the H-bonding proton without affecting the recep-
tor geometry sometimes leads to a drastic change in
anion selectivity. Addition of fluoride ion to 1b and
2b deprotonates benzimidazole NH, while reducing H-
bonding interaction with the pyrrolic NH. Acetate ion
deprotonates benzimidazole NH and the corresponding
conjugate acid; i.e., acetic acid forms strong H-bonding
with the receptor via complementary bidentate coor-
dination mode. Like 1b and 2b, receptor 3 interacts
with fluoride and acetate ions, and relatively weakly
with dihydrogenphosphate ion. Interestingly, fluoride
ions abstract, stepwise, both the benzimidazole NHs of
receptor 3 in a unique way and as a consequence, possi-
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domain, as indicated by the highly deshielded proton
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fluorometric response.
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Supplementary Information (SI)
Synthetic procedures, H and 13C NMR spectra, absorption
and emission spectra, anion binding data and computational
data are available in Supplementary Information at www.ias.
ac.in/chemsci.
1
Acknowledgements
This work is supported by Council of Scientific & Industrial
Research (CSIR), India project No. 01(2449)/10/EMR-II (P
K P) and the Department of Science & Technology (DST),
Government of India project No. SB/FT/CS-090-2014 (S. P.
M.).
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