B. Chetia et al. / Journal of Molecular Structure 1042 (2013) 32–36
33
tained was a white powder (96%, 0.90 g) with a melting point in
excess of 250 °C.
2.2. Synthesis of solid bbp:urea and bbp:thiourea
bbp:urea complex: A solution of urea (0.020 g, 33.3 mmol) in
Fig. 1. 2,6-Bis(2-benzimidazolyl)pyridine (bbp) and thiourea.
methanol was added dropwise to a methanolic solution of bbp
0.104 g, 33.3 mmol) and allowed to stir for 25 min. The clear solu-
tion was evaporated to get the solid bbp:urea complex.
bbp:thiourea complex: solution of thiourea (0.020 g,
6.3 mmol) in methanol was added dropwise to a methanolic solu-
tion of bbp (0.082 g, 26.3 mmol) and allowed to stir for 25 min. The
clear solution was evaporated to get the solid bbp:thiourea
complex.
(
applicability. Receptors capable of recognizing chemical and bio-
logical guest molecules by supramolecular interactions has con-
tributed greatly in the development of host guest and senor
chemistry [9–12]. We present here the use of a simple tridentate
ligand 2,6-bis(2-benzimidazolyl)pyridine (bbp) for the recognition
of thiourea (Fig. 1). Moreover, recognition of thiourea by bbp is
compared with recognition of urea by the same receptor which
was reported earlier by our group [13]. Easy economic synthetic
method and use of simple spectroscopic techniques to follow the
recognition process possess bbp as a novel effective supramolecu-
lar receptor for thiourea. This detail systematic study supported by
structure optimization with DFT method, would significantly con-
tribute to optimize future synthetic receptors for similar biological
molecules.
Ligand bbp has two imine nitrogen lone pairs and two ANH
fragments in addition to the pyridine nitrogen lone pair that can
form hydrogen bonded adducts with guest molecules [14–16]. It
is structurally very simple, stable to heat/light and can be synthe-
sized in a single step from commercially cheap starting materials.
Additionally due to its solubility in several laboratory solvents, bbp
can be used as a host molecule for the recognition of thiourea by
hydrogen bonding interactions.
A
2
3. Results and discussion
Supramolecular complex formation between bbp and thiourea
was studied by optical techniques such as UV/visible and fluores-
cence spectroscopic titration in dry acetonitrile. The solution state
properties of receptor bbp was evaluated by careful addition of
0.1 equivalents of thiourea aliquots at regular intervals to a cuvette
containing bbp and the spectral changes were recorded by means
of UV/visible spectroscopy. Our observations suggest that on
increasing the concentration of thiourea, progressive decrease of
absorbance in the initial absorption band (Fig. 2a) having kmax at
327 nm occurred and a new peak at 249 nm with higher absor-
bance develops which is due to the formation of stable complex be-
tween bbp and thiourea. This group of spectra shows formation of
an isosbestic point at 263 nm indicating the presence of at least
one species at equilibrium.
Thus the gradual decrease in the band absorbance at 327 nm
with the formation of a new higher absorbance blue-shifted band
at 249 nm with a sharp isosbestic point confirms the formation
of hydrogen-bonded complex between bbp and thiourea. The
low concentrations at which these spectroscopic changes were ob-
served clearly reveal that bbp can be used for the recognition of
thiourea. The inset plot inserted in Fig. 2a shows that a limiting va-
lue is reached on forming a 1:1 adduct between bbp and thiourea.
Similar titration was performed by careful addition of 0.1 equiv-
alents of urea aliquots at regular intervals to bbp. On increasing the
concentration of urea, the initial absorption band (Fig. 2b) having a
2
. Materials and methods
All reagents were used as received without further purification
unless mentioned. These materials were of reagent grade or better.
Acetonitrile was distilled from calcium hydride. UV/visible spectra
were recorded on a Perkin Elmer Lambda-25 spectrophotometer.
Fluorescence spectra were recorded on a Varian Carry 25 spectro-
photometer. FT-IR spectra were taken on a Perkin Elmer spectro-
photometer with samples prepared as KBr pellets.
To study the binding of thiourea by the bbp receptor, the grad-
ual changes in the UV visible and fluorescence spectra of a solution
of bbp in acetonitrile is followed upon addition of thiourea solu-
tion up to 1.0 equivalents in increments of 0.1 equivalents.
UV/visible titration is performed as follows: A solution of bbp
kmax at 327 nm showed a marginal but progressive decrease in
absorbance with broadening and formation of a clear isosbestic
point at 277 nm, indicating the presence of at least one stable spe-
cies at equilibrium. The inset of Fig. 2b shows that a limiting value
is reached on forming a 1:1 adduct between bbp and urea.
The formation of supramolecular complex between bbp and
thiourea was also studied by the use of fluorescence titration
experiments. The changes observed in the fluorescence spectra of
a solution of bbp in acetonitrile on addition of up to 1.0 equivalents
thiourea is represented in (Fig. 3a). A large quenching (>54%) in
intensity of the 375 nm band was observed on the addition of
1.0 equivalents of thiourea indicating that on formation of the
hydrogen bonded complex between thiourea and bbp, the excited
state was modified considerably leading to the quenching of fluo-
rescence. Therefore, formation of supramolecular complex alters
the optical properties of bbp in solution and could be employed
for sensing essential guests such as thiourea having large atoms
such as sulfur.
(
0.128 mM) in acetonitrile was titrated with 20 lL aliquots of a
solution of thiourea (0.158 mM) in acetonitrile. The addition was
done stepwise, and after each step, the formation of [bbp:thiourea]
was monitored by UV/visible spectroscopy. Fluorescence titration
experiments were also performed in a similar manner.
2.1. Synthesis of ligand bbp
2
,6-Pyridinedicarboxylic acid (0.50 g) and o-phenylenediamine
3
(
0.70 g) were dispersed in orthophosphoric acid (10 cm ), and
heated for 4 h at 220 °C. Whilst still hot, the resulting solution
was poured into cold distilled water (1 dm ) with vigorous stirring.
The resulting blue precipitate was collected by filtration, then dis-
persed in 500 cm of hot 10% (w/v) Na
3
3
2
CO
3
(aq) and stirred for
Fluorescence titration was also carried out by the addition of
0.1 equivalents of urea to bbp where the 375 nm band in the spec-
trum was diminished by over 70% of the initial intensity (Fig. 3b). A
large quenching in intensity was observed up to the addition of
0.5 equivalents of urea after which the changes were minor.
3
0 min. The insoluble material was collected by filtration, dis-
3
persed in cold distilled water (800 cm ), and adjusted to pH 4 using
0% (v/v) HCl (aq) then recovered by filtration and recrystallized
from the minimum amount of methanol. The final product ob-
1
Chetia, Bolin
