Selective quenching of benzimidazole derivatives by Cu2+ metal ion

Article abstract of DOI:10.1016/j.saa.2012.06.012

It is a very big challenge to develop a Cu²⁺ selective fluorescent sensor with the ability to exclude the interference of some metal ions such as Fe³⁺, Mg²⁺, Ag⁺, K⁺ and Na⁺. Herein, we rep

Full text of DOI:10.1016/j.saa.2012.06.012

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 384–387
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Selective quenching of benzimidazole derivatives by Cu2+ metal ion
⇑
J. Jayabharathi , V. Thanikachalam, K. Jayamoorthy, R. Sathishkumar
Department of Chemistry, Annamalai University, Annamalainagar 608 002, Tamil Nadu, India
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
"
"
"
"
The decrease in the fluorescence
intensity shows binding of copper
with benzimidazole derivatives.
FT IR spectra confirm the binding of
benzimidazole derivative with Cu2+
metal ion.
In the case of Fe3+, Mg2+, Ag+, K+, and
Na+ metal ions the fluorescence
quenching is very low.
Crystalline benzimidazole
derivatives (5) are triclinic crystals.
a r t i c l e i n f o
a b s t r a c t
It is a very big challenge to develop a Cu2+ selective fluorescent sensor with the ability to exclude the
interference of some metal ions such as Fe3+, Mg2+, Ag+, K+ and Na+. Herein, we report a fluorescence
quenching of some benzimidazole derivatives (1–6) with Cu2+ metal ion. These benzimidazole derivatives
have been shown to bind copper ions resulting in quenching of its fluorescence. The response to Cu2+ is
rapid, selective and reversible upon addition of a copper chelator. These benzimidazole derivatives were
characterized by 1H, 13C NMR mass and elemental analysis. XRD analysis was carried out for 1-(4-meth-
ylbenzyl)-2-p-tolyl-1H-benzo[d]imidazole.
Article history:
Received 2 March 2012
Received in revised form 5 June 2012
Accepted 6 June 2012
Available online 25 June 2012
Keywords:
Copper
Quenching
Chemosensor
Benzimidazole
XRD
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
An important property that makes imidazole derivatives more
attractive as a chelator is the appreciable change in its fluorescence
The imidazole ring system is the most important substructures
found in a large number of natural products and pharmacologically
active compounds. For example, the amino acid histidine, the hyp-
notic agent etomidate [1], the antiulcerative agent cimetidine [2]
the proton pump inhibitor omeprazole [3], the fungicide ketocon-
azole [4] and the benzodiazepine antagonist flumazenil [5] are
imidazole derivatives. In recent years, substituted imidazoles are
substantially used in ionic liquids [6] that have been given a new
approach to ‘Green Chemistry’. The imidazole compounds were
also used in photography as photosensitive compound [7]. Litera-
ture survey reveals the several methods for synthesizing them,
mainly using nitriles and esters [8–10] as the starting substrates.
upon metal binding. Therefore, imidazole derivatives have been
used to construct highly sensitive fluorescent chemisensors for
sensing and imaging of metal ions and its chelates in particular
those with Ir3+ are major components for organic light emitting
diodes [11,12] and are promising candidates for fluorescent chem-
isensors for metal ions.
The construction of fluorescent devices for the sensing and
reporting of chemical events is currently of significant importance
for both chemistry and biology [13–15]. Fluorescence proffers high
sensitivity among the many signal types available. Cu2+ is an
important co-factor of several enzymes and plays a significant role
in several cellular pathways and disease pathogenesis [16–18]. So
the chemosensors such as benzimidazole derivatives, which can be
targeted to specific organelles for detection of Cu2+, would prove
highly beneficial. In that regard, it is essential to understand the
⇑
Corresponding author. Tel.: +91 9443940735.
E-mail address: jtchalam2005@yahoo.co.in (J. Jayabharathi).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.06.012

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 384–387
385
binding of Cu2+ to benzimidazole derivatives. In this paper we re-
port binding effect of some novel benzimidazole derivatives with
Cu2+ metal ion by solution spectral studies.
due to binding of the benzimidazole derivatives on Cu2+ metal ion
(Scheme S1).
FT-IR characteristics of benzimidazole derivatives – Cu2+ metal ion
Experimental
Fig. 2 shows the FT-IR spectra of benzimidazole derivative 1 (so-
lid line) and benzimidazole derivative bound to the Cu2+ metal ion
(broken line). The spectrum of pure benzimidazole derivative
shows the >C@N stretching vibration at 1600 cmꢀ1. This band is
shifted in benzimidazole derivative bound to copper metal ion a
new band appears at 1619 cmꢀ1 in place of 1600 cmꢀ1. These
observations show that the benzimidazole derivative bind with
Cu2+ metal ion.
Spectral measurements
The ultraviolet–visible (UV–vis) spectra of the benzimidazole
derivatives were measured in an UV–vis spectrophotometer (Per-
kin Elmer Lambda 35) and corrected for background absorption
due to solvent. Photoluminescence (PL) spectra were recorded on
a (Perkin Elmer LS55) fluorescence spectrometer. NMR spectra
were recorded on Bruker 400 MHz NMR spectrometer. Mass spec-
tra were recorded on a Varian Saturn 2200 GCMS spectrometer.
Fluorescence quenching characteristics
Addition of Cu2+ metal ion to the solution of benzimidazole
derivatives resulted in the quenching of its fluorescence emission.
Fig. 3 displays the effect of increasing concentration of Cu2+ metal
ion on the emission spectrum of benzimidazole derivatives (1–6).
General procedure for the synthesis of ligands
A mixture of corresponding aldehyde (2 mmol), o-phenylenedi-
amine (1 mmol) and ammonium acetate (2.5 mmol) has been re-
fluxed at 80 °C in ethanol. The reaction was monitored by TLC
and purified by column chromatography using petroleum ether:
ethyl acetate (9:1) as the eluent.
Results and discussion
Absorption characteristics of benzimidazole derivatives – Cu2+
metal ion
The absorption spectra of benzimidazole derivatives in presence
of Cu2+ metal ion at different concentration and also in their ab-
sence are displayed in Fig. 1. The Cu2+ metal ion enhance the absor-
bance of benzimidazole derivatives remarkably without shifting its
absorption maximum. This indicates that the Cu2+ metal ion do not
modify the excitation process of the ligand. The enhanced absorp-
tion appeared around 290 nm observed with the Cu2+ metal ion are
Fig. 2. FT-IR spectra of benzimidazole derivative 1 (solid line) and benzimidazole
bound with Cu2+ metal ion (broken line).
Fig. 1. Absorption spectra of benzimidazole derivatives (1–6) in presence and absence of Cu2+ metal ion.

386
J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 384–387
fluorescence intensity of benzimidazole derivatives is reduced
(from I0 to I) by energy transfer to Cu2+ metal.
The apparent association constants (Kapp) have been obtained from
the fluorescence quenching data using to the following equation
According to Forster’s non-radiative energy transfer theory, the
energy transfer efficiency is related not only to the distance be-
tween the acceptor and donor (r0), but also to the critical energy
transfer distance (R0). That is
1=ðF0 ꢀ FÞ ¼ 1=ðF0 ꢀ FÞ þ 1=KappðF0 ꢀ FÞ ½Cu2þ metal ionꢁ
ð1Þ
where Kapp is the apparent association constant, F0 is the initial fluo-
rescence intensity of the benzimidazole derivatives, F is the fluores-
cence intensity of the benzimidazole derivatives bound with Cu2+
metal ion and F is the observed fluorescence intensity at its maxi-
mum. A good linear relationship between 1/(F0–F) and the recipro-
cal concentration of Cu2+ metal ion is seen. From the slope, the
values of apparent association constants (Kapp) have been assessed
for benzimidazole derivatives-Cu2+ metal ion.
E ¼ R60=ðR60 þ r60Þ
ð4Þ
where R0 is the critical distance when the transfer efficiency is
50%.
R60 ¼ 8:8 ꢂ 10ꢀ25 K2Nꢀ4u
J
ð5Þ
where K2 is the spatial orientation factor of the dipole, N is the
The fluorescence quenching behavior is usually described by
Stern–Volmer relation:
refractive index of the medium,
u is the fluorescence quantum yield
of the donor and J is the overlap integral of the fluorescence emis-
sion spectrum of the donor and the absorption spectrum of the
acceptor. The value of J can be calculated by using Eq. (7),
I0=I ¼ 1 þ KSV ½Qꢁ
ð2Þ
where I0 and I are the fluorescence intensities in the absence and
presence of quencher, KSV is the Stern–Volmer constant related to
the bimolecular quenching rate constant and Q is the quencher.
Addition of Cu2+ metal ion to a solution of benzimidazole deriva-
tives result in quenching of fluorescence emission. Typical linear
S–V plot for steady-state fluorescence quenching of benzimidazole
Z
J ¼ FðkÞ
e
ðkÞk4dk=FðkÞdk
ð6Þ
where F(k) is the fluorescence intensity of the donor and
e
(k) is molar
absorptivity of the acceptor. These experimental conditions, the
calculated values of E, R0 and r0 are ꢃ0.15–0.41, ꢃ1.69–2.08 and
ꢃ2.00–2.79 respectively. The literature values of K2 (=2/3) and N
(=1.3467) are used for the calculation [19,20] and the corresponding
derivatives by Cu2+ metal ion is shown in Fig. S1. In case of Fe3+
,
Mg2+, Ag+, K+ and Na+ metal ions the quenching property is very
low when compared to Cu2+ metal ion (Fig. S2).
u
values for 1–6 from the present study. Obviously, the calculated
value of R0 is in the range of maximal critical distance. This is in
accordance with the conditions of Forster’s energy transfer theory
[21] and suggests that electron transfer occurs between the Cu2+ me-
tal ion and benzimidazole derivatives (1–6) with high probability
[22].
Energy transfer between Cu2+ metal ion and benzimidazole derivatives
The decrease in fluorescence intensity is attributed to electron
transfer between benzimidazole derivative and the Cu2+ metal ion.
Energy transfer efficiency (E) is given by the following equation
Crystal structure
E ¼ 1 ꢀ ðI=I0Þ
ð3Þ
where I is the emission intensity of donor in the presence of accep-
tor and I0 is the emission intensity of the donor alone. From the
above results it is clear that, in presence of Cu2+ metal ion, the
Crystalline benzimidazole derivative (5) [23] are triclinic crys-
tal. It crystallizes in the space group P1ꢀ. The cell dimensions are
a = 9.6610 Å, b = 10.2900 Å, c = 17.7271 Å. ORTEP diagram of (5)
Fig. 3. Fluorescence quenching of benzimidazole derivatives (1–6) in the presence and absence of various concentrations Cu2+ metal ion.

J. Jayabharathi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 97 (2012) 384–387
387
Table 1
Selected bond lengths (Å), bond angles (°) and torsional angles (°) of 5.
Bond lengths (Å)
Experimental XRD (Å)
Bond angles (°)
Experimental XRD (°)
Torsional angles (°)
Experimental XRD (°)
N1A–C2A
N1A–C8A
N1A–C1A
N3A–C2A
N3A–C9A
N1B–C2B
N1B–C8B
N1B–C1B
N3B–C2B
N3B–C9B
C1A–C11A
C2A–C21A
C7A–C8A
C8A–C9A
C14A–C17A
C24A–C27A
1.3782(1.4972)
1.3833(1.4584)
1.4537(1.4700)
1.3163(1.3671)
1.3881(1.4606)
1.3795(1.4972)
1.3863(1.4584)
1.4550(1.4700)
1.3174(1.3671)
1.3866(1.4606)
1.5135(1.5400)
1.4729(1.5400)
1.3921(1.3862)
1.3999(1.4763)
1.5089(1.5400)
1.5078(1.5400)
C2A–N1A–C8A
C2A–N1A–C1A
C8A–N1A–C1A
C2A–N3A–C9A
C2B–N1B–C8B
C2B–N1B–C1B
C8B–N1B–C1B
C2B–N3B–C9B
N1A–C1A–C11A
N3A–C2A–N1A
N3A–C2A–C21A
N1A–C2A–C21A
N1A–C8A–C3A
N1A–C8A–C9A
N3A–C9A–C4A
N3A–C9A–C8A
106.23(101.69)
128.33(113.71)
124.79(113.33)
104.77(105.49)
106.27(101.69)
129.32(113.71)
123.98(113.33)
105.12(105.41)
115.07(109.47)
113.25(113.53)
123.44(123.23)
123.30(123.24)
131.76(130.72)
103.49(108.25)
129.91(130.69)
110.26(108.25)
C2A–N1A–C1A–C11A
C8A–N1A–C1A–C11A
C9A–N3A–C2A–N1A
C9A–N3A–C2A–C21A
C8A–N1A–C2A–N3A
C1A–N1A–C2A–N3A
C8A–N1A–C2A–C21A
C1A–N1A–C2A–C21A
C9A–C4A–C5A–C6A
C2A–N1A–C8A–C7A
C1A–N1A–C8A–C7A
C2A–N1A–C8A–C9A
C1A–N1A–C8A–C9A
C6A–C7A–C8A–N1A
C2B–N1B–C1B–C11B
C8B–N1B–C1B–C11B
C9B–N3B–C2B–N1B
C9B–N3B–C2B–C21B
C8B–N1B–C2B–N3B
C1B–N1B–C2B–N3B
C8B–N1B–C2B–C21B
C1B–N1B–C2B–C21B
C9B–C4B–C5B–C6B
C2B–N1B–C8B–C7B
C1B–N1B–C8B–C7B
C2B–N1B–C8B–C9B
C1B–N1B–C8B–C9B
C6B–C7B–C8B–N1B
109.45(ꢀ177.75)
ꢀ81.12(ꢀ62.25)
ꢀ0.02(ꢀ13.69)
ꢀ178.71(165.91)
0.59(17.54)
171.55(139.72)
179.28(ꢀ162.06)
ꢀ9.75(ꢀ39.88)
0.1(ꢀ0.1581)
178.45(166.15)
7.08(43.71)
ꢀ0.88(13.70)
ꢀ172.25(136.15)
179.75(176.19)
108.10(ꢀ177.75)
ꢀ79.66(ꢀ62.25)
ꢀ0.86(ꢀ13.69)
177.35(165.91)
1.24(17.54)
173.85(139.72)
ꢀ176.90(162.06)
ꢀ4.29(ꢀ39.88)
0.25(ꢀ0.1581)
176.39(166.15)
3.28(43.71)
ꢀ1.05(ꢀ13.70)
ꢀ174.15(ꢀ136.15)
177.76(176.19)
Values within the parenthesis corresponds to theoretical values.
(Fig. S3) shows that the benzimidazole ring is essentially planar.
The dihedral angles between the planes of the benzimidazole and
the benzene rings of the 4-methylbenzyl and the p-tolyl groups
are 76.64(3)° and 46.87(4)°, respectively, in molecule A. The cor-
responding values in molecule B are 86.31(2)° and 39.14(4)°. The
dihedral angle between the planes of the two benzene rings is
73.73(3)° and 80.69(4)° in molecules A and B, respectively. Opti-
mization of 5 have been performed by DFT at B3LYP/6-31G(d,p)
using Gaussian-03. All these XRD data are in good agreement
with the theoretical values (Table 1). However, from the theoret-
ical values it can be found that most of the optimized bond
lengths, bond angles and dihedral angles are slightly higher than
that of XRD values. These deviations can be attributed to the fact
that the theoretical calculations were aimed at the isolated
molecule in the gaseous phase and the XRD results were aimed
at the molecule in the solid state.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2012.06.012.
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Conclusions
We have developed a simple and sensitive fluorescent probe
for Cu2+ metal ion. Benzimidazole derivatives are bound with
Cu2+ metal ion through azomethine nitrogen. The decrease in
fluorescence intensity is attributed to electron transfer between
benzimidazole derivative and the Cu2+ metal ion. The Cu2+ metal
ion quenches the fluorescence intensity of benzimidazole deriva-
tives (1–6) more than ten times when compared to Fe3+, Mg2+
,
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One of the authors Prof. J. Jayabharathi is thankful to Depart-
ment of Science and Technology [No. SR/S1/IC-73/2010], University
Grants commission (F. No. 36-21/2008 (SR)) and Defence Research
and Development Organisation (DRDO) (NRB-213/MAT/10-11) for
providing funds to this research study.