Spectroscopic evaluation of a novel multi-element sensitive fluorescent probe derived from 2-(2′-phenylbenzamide)benzimidazole: Selective discrimination of Al3?+ and Cd2?+ from their congeners

Article abstract of DOI:10.1016/j.inoche.2016.08.008

A novel fluorescent chemosensor 2 exhibiting excited-state intramolecular proton transfer (ESIPT) process has been synthesized by condensation approach, among which 2 could be used as a multielement sensitive fluorescent probe for both Al^3?+ and Cd^2?+. The emission wavelength of 2 underwent blue shift of 78?nm upon binding with these ions. The cation-induced inhibition of ESIPT phenomenon was responsible for these fluorescence changes. Moreover, the absorption spectra of 2 showed changes with F^? and AcO^? ions also. TD-DFT studies well displayed the blue shift in the emission wavelength of 2 upon binding with Al^3?+ ions.

Full text of DOI:10.1016/j.inoche.2016.08.008

Inorganic Chemistry Communications 72 (2016) 57–61
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Short communication
Spectroscopic evaluation of a novel multi-element sensitive fluorescent
probe derived from 2-(2′-phenylbenzamide)benzimidazole: Selective
discrimination of Al3+ and Cd2+ from their congeners
⁎
Gargi Dhaka, Navneet Kaur , Jasvinder Singh
Department of Chemistry, Panjab University, Chandigarh 160014, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 8 July 2016
Received in revised form 30 July 2016
Accepted 15 August 2016
Available online 16 August 2016
A novel fluorescent chemosensor 2 exhibiting excited-state intramolecular proton transfer (ESIPT) process has
been synthesized by condensation approach, among which 2 could be used as a multielement sensitive fluores-
cent probe for both Al3+ and Cd2+. The emission wavelength of 2 underwent blue shift of 78 nm upon binding
with these ions. The cation-induced inhibition of ESIPT phenomenon was responsible for these fluorescence
changes. Moreover, the absorption spectra of 2 showed changes with F− and AcO− ions also. TD-DFT studies
well displayed the blue shift in the emission wavelength of 2 upon binding with Al3+ ions.
© 2016 Elsevier B.V. All rights reserved.
Keywords:
Benzimidazole
Amide
ESIPT
Fluorescent probe
TD-DFT
Multielement fluorescent sensors are more important than a single-
element selective probe in analytical and bioanalytical determinations
in terms of economic viability [1–4]. A number of metal ions, such as
Na+, K+, Ca2+, Mg2+, Zn2+, Fe3+, and Ag+ play an important biological
role in human body [5], whereas other heavy metal ions, including
Cu2+, Cd2+, Cr3+, Hg2+ and Pb2+ can be harmful to living organisms
[6–7]. Al3+ is a competitive inhibitor in various biological processes in-
volving quite a few essential elements like Ca2+ (0.099 nm), Mg2+
(0.066 nm), and Fe3+ (0.064 nm) due to its similarities regarding atom-
ic size (0.051 nm) and charge (3+). Diseases such as rickets, anemia,
colic, speech problems, softening of bone, Parkinson's and Alzheimer's
can be caused by extreme Al3+ exposure [8–10]. It can be found in the
environment by various ways, namely from the water treatment plants,
medicines, food additives, aluminium cookware, antiacids, etc. [11–15].
Similarly, there have also been numerous reports on the toxicity of Cd2+
to nervous system, procreation, kidneys, and tissues, consequently
resulting in calcium metabolism disorders, renal dysfunction and an in-
creased incidence of certain forms of cancers [16]. Humans are exposed
to Cd2+ through the ingestion of contaminated food or water and the
inhalation of cigarette smoke [17]. Also, Cd2+ is widely used in many
processes such as electroplating, metallurgy, agriculture, war industry,
etc. Hence, developing an effective method to monitor such toxic Al3+
and Cd2+ is of paramount importance.
their rapidity, easy equipped procedure, high sensitivity and real-time
detection [18]. However, the recognition of Al3+ has always been exi-
gent because of its lack of spectroscopic characteristics and poor coordi-
nation skill, in comparison to transition metals. The majority of the
reported Al3+ sensors suffer from the interferences caused by Fe3+
and Cu2+, poor solubility and tedious synthetic methods of preparation
[19–23]. On the other hand, the greatest challenge for detecting Cd2+
comes from the interference of other transition-metal ions, in particular
Zn2+. They are in the same group of the periodic table and have compa-
rable properties. Therefore, similar fluorescence alterations including
the change of intensity and the shift of wavelengths are usually ob-
served when Zn2+ and Cd2+ are coordinated with fluorescent sensors
[24]. Thus, there is a great need for the design and easy synthesis of sim-
ple and economical on-type Al3+ and Cd2+ selective sensors that can
distinguish Cd2+ from Zn2+ with high sensitivity and selectivity. An or-
dinary fluorophore, benzimidazole, is oftenly used in coordination
chemistry for its capability to bind an array of d-and f-block elements
but it is rarely exploited as fluorescent probes for Al3+ and Cd2+ in lit-
erature [25].
Herein, we report a novel benzimidazole based fluorescent
chemosensor possessing the amide linkage as binding site for the detec-
tion of Al3+ and Cd2+ ions. The analyte-induced inhibition of ESIPT phe-
nomenon was responsible for fluorescence enhancement. Scheme 1
outlines the synthesis of amide possessing benzimidazole derivative,
2. The refluxing of 1 with benzoyl chloride, in dry CH3CN, gave deriva-
tive, 2, containing amide linkage. The desired product was explicitly
characterized by NMR spectra (1H NMR, 13C NMR), IR and Mass spec-
trum (Figs. S1–S3).
In real fact, a great number of fluorescent sensors have been de-
signed to detect different kinds of heavy toxic metal ions because of
⁎
Corresponding author.
E-mail addresses: neet_chem@yahoo.co.in, neet_chem@pu.ac.in (N. Kaur).
http://dx.doi.org/10.1016/j.inoche.2016.08.008
1387-7003/© 2016 Elsevier B.V. All rights reserved.

58
Short communication
O
C
H
N
Cl
H
N
H
N
SnCl2.2H2O
EtOH
N
Dry CH3CN, Reflux
HN
N
N
O2N
O
H2N
1
2
Scheme 1. Synthesis of chemosensor 2.
The binding capacity of 2 (20 μM) was primarily investigated to-
To determine the stoichiometry of 2 with Al3+/Cd2+ in the com-
wards various cations (100 equiv.) such as Na+, K+, Mg2+, Al3+
,
plexes, Job's plot method for the absorption changes was applied,
Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and anions (100 equiv.) such as
AcO−, F−, Cl−, Br−, I−, H2PO4−, HSO−4 ions. The UV–vis spectrum of 2
(20 μM) in CH3CN showed absorption bands at 271 and 321 nm
which underwent perturbation in the presence of Al3+, Cd2+, F− and
AcO− ions only (Fig. 1).
which pointed to 1:1 stoichiometric complex between 2 and Al3+
/
Cd2+. The association constants for 2 and Al3+/Cd2+ were calculated
using Benesi-Hildebrand equation [26] and were found to be
5.6 × 105 M−1 (LOD = 65 μM) and 1.3 × 104 M−1 (LOD = 76 μM),
respectively.
The quantitative estimation of Al3+ ions by 2 was performed via UV–
vis monitoring using 20 μM solution of 2 in CH3CN (Fig. 2). Upon addi-
tion of Al3+, the absorption intensity of both the absorption bands pres-
ent at 271 and 321 nm decreased along with formation of new band at
290 nm. The formation of clear isosbestic points at 286 and 300 nm in-
dicated the formation of well-defined 2-Al3+ complex. On the other
hand, upon incremental addition of Cd2+ (0–50 equiv.), only small de-
crease in absorption intensity of parental absorption bands was ob-
served (Fig. S4).
As the absorption spectra of 2 showed changes with F− and AcO−
ions, so, spectrophotometric titrations have also been carried out with
F
− and AcO− ions. Upon addition of F− ions, the absorption intensity
of band present at 271 nm decreased along with increased absorbance
of 321 nm band (Fig. 3). These changes were accompanied by the ap-
pearance of new band at 290 nm. Similar absorption changes were ob-
served with addition of AcO− ions to CH3CN solution of 2 (Fig. S5).
The association constants of 2 with F− and AcO− calculated for 1:1 stoi-
chiometry (Job's plot) through the Benesi-Hildebrand equation were
Fig. 1. UV–vis absorption responses of 2 (20 μM) with different (a) metal ions (100 equiv.) and (b) anions (100 equiv.) in CH3CN.
Fig. 2. The UV–vis absorption spectra of 2 (20 μM), in CH3CN, with the addition of
increasing equiv. of Al3+ (as perchlorate salt). Inset: Binding isotherm recorded at 271
and 321 nm with change of [Al3+] ions.
Fig. 3. The UV–vis absorption spectra of 2 (20 μM), in CH3CN, with the addition of
increasing equiv. of F− (as TBA salt). Inset: Binding isotherm recorded at 271 and
321 nm with change of [F−] ions.

Short communication
59
Fig. 4. (a) The fluorescence emission spectra of 2 (20 μM; λex 300 nm), in CH3CN, after addition of 100 equiv. of various cations and (b) bar representation of fluorescence emission spectra
recorded at 414 and 492 nm.
found to be 7.7 × 104 M−1 (LOD = 33 μM) and 1.5 × 104 M−1 (LOD =
47 μM), respectively.
Because of the almost complete lack of spectral overlap between ab-
sorption and emission spectra, the chemosensor 2 is found to be suitable
to serve as an ESIPT based fluorescence probe [27]. The emission spec-
trum of 2 in CH3CN (20 μM; λex 300 nm) displayed weak emission
band at 492 nm. As depicted in Fig. 4, among 100 equiv. of various bio-
logically important cations such as Na+, K+, Mg2+, Mn2+, Co2+, Ni2+
,
Cu2+ and Zn2+, only Al3+ and Cd2+ yielded a bright, blue-shifted fluo-
rescence emission when added to solution of 2. The quenching of the
fluorescence in the Cu2+ addition may be due to the paramagnetic na-
ture of Cu2+ ions [28]. However, due to weak fluorescence of 2, this
2.Al3+
2
LUMO (-1.38591 eV)
LUMO (-13.67076 eV)
2.08335 eV
4.35065 eV
HOMO (-5.73656 eV)
HOMO (-15.75412 eV)
2.Al3+
2
Fig. 5. The fluorescence emission spectra of 2 (20 μM), in CH3CN, with the addition of
increasing equiv. of Al3+ (as perchlorate salt). Binding isotherm recorded at 414 and
492 nm with change of [Al3+] ions.
Fig. 7. Frontier orbitals and their orbital energies for 2 and 2-Al3+ optimized by DFT
calculations at the B3LYP-6-31G (d,p) level of theory.
Fig. 6. Metal ion selectivity profile of 2 (20 μM) via changes observed in fluorescence emission spectra with addition of 50 equiv. of Zn2+ and 100 equiv. of other interfering cations at
414 nm.

60
Short communication
Table 1
Excitation for 2 and 2-Al3+ that contribute to the transitions in the 250–700 nm and their expansion coefficients in gaseous phase.
λ
abs (nm) (Theoretical value)
λabs (nm) (Experimental value)
Eexc (eV)
Oscillator strength (f)
Excited state
Percentage contribution
2
278
321
292
271
321
290
4.45636
3.8614
4.2420
0.2839
0.4552
0.4085
H-1 ➔ L
H ➔ L
H ➔ L + 1
64
100
71
2-Al3+
quenching could not be studied. On the contrary, upon addition of dif-
energy 3.8614 eV and oscillating strengths of 0.4552 and 0.2839, respec-
tively. As shown by the optimized structure, the coordination sphere of
Al3+ ion in 2-Al3+ complex consisted of two nitrogen atoms, one from
benzimidazole moiety and the other from amide linkage together with
an oxygen atom from carbonyl group of amide linkage and showed its
associated absorption band at 290 nm, which was well correlated with
the experimental values. The prominent transitions were due to
HOMO ➔ LUMO + 1, having excitation energy 4.2420 eV and oscillating
strength 0.4085 and. (Table 1).
Based on the above absorption, fluorescence and DFT results, a plau-
sible binding mechanism of 2-Al3+/Cd2+ complex has been proposed as
shown in Scheme 2. The sensor 2 has been expected to serve as an ESIPT
based probe, which involved the transfer of amide proton to neighbor-
ing imine nitrogen through a preexisting six-membered ring of hydro-
gen-bonding configuration, which led to very weak fluorescence [29].
Upon metal coordination, the ESIPT process got inhibited and a signifi-
cant emission enhancement could be observed.
ferent anions (100 equiv.) like AcO−, F−, Cl−, Br−, I−, H2PO4−, HSO−4
,
the fluorescence spectra of 2 remained unperturbed (Fig. S6).
Fig. 5 showed that initially 2 showed a low intensity fluorescence
peak at 492 nm. Upon addition of Al3+, firstly, a remarkable blue shift
(Δλ ~78 nm) from 492 nm to 414 nm was observed and after that
with increasing concentration of Al3+, a significant increase of the emis-
sion intensity around 414 nm were observed. When excited at 300 nm,
2, which contains an intramolecular hydrogen bond, underwent ESIPT
and yielded a highly Stoke's shifted low-intensity emission at 492 nm
from the proton-transfer tautomer. The coordination of Al3+ removed
the amide proton and disrupted the ESIPT, thus causing emission en-
hancement with a normal Stoke's shift at 414 nm. Similar fluorescence
behavior was observed with addition of Cd2+ metal ion to solution of
2 (Fig. S7). However, the emission enhancement observed in the case
of Al3+ was ~60%, which was much higher than observed in the case
of Cd2+ (~30%).
To reflect the high sensitivity, the detection limit was evaluated to be
39 μM and 85 μM for Al3+ and Cd2+ ions, respectively. The binding con-
stants for 1:1 stoichiometry between 2 and Al3+/Cd2+ were calculated
by the Benesi-Hildebrand nonlinear curve fitting method [26] and were
In conclusion, we have reported a benzamide benzimidazole conju-
gate 2 as an effective and specific chemosensor for Al3+ and Cd2+ metal
ions in CH3CN solution over other heavy and transition metal ions, espe-
cially Zn2+, with low detection limit. The assimilation of an amide link-
age provided supplementary synergic Al3+ and Cd2+ coordination sites,
which gave exclusively metal complex with a 1:1 stoichiometry. The re-
sults suggested that the blue-shifted “off-on” fluorescent recognition of
Al3+ and Cd2+ by 2 was due to the ESIPT inhibition and restricted intra-
molecular C\\C bond rotation between the phenylbenzamide and benz-
imidazole moieties after the complex formation as revealed by
fluorescence spectral titrations and molecular modeling study.
found to be 9.6 × 105 M−1 and 3.5 × 104 M−1 for Al3+ and Cd2+
respectively.
,
The selectivity and tolerance of 2 for Al3+ and Cd2+ ions over other
metal cations such as Na+, K+, Mg2+, Mn2+, Co2+, Ni2+, Cu2+ and Cd2+
were investigated (Fig. 6). The hypsochromic shift and fluorescent en-
hancement induced by both Al3+ and Cd2+ ions remained unperturbed
and did not get any interference by the coexistence of more than two
times higher concentration of other interfering cations, except the para-
magnetic Cu2+ ions, which coordinate to 2, and completely quench the
fluorescence emission due to non-radiative decay of the excited states [28].
Acknowledgements
To acquire more insight into binding strategies of 2 towards Al3+
,
The authors are greatly thankful to the SAIF, Panjab University,
Chandigarh for recording the NMR and Mass spectra and are grateful
to the DST (grant no. SR/FT/CS-36/2011); UGC (grant no. AB2/12/
3115) and DST PURSE-II (grant no. 48/RPC) for financial assistance.
structure of 2 was optimized in ground state through DFT and TD-DFT
calculation with B3LYP-6-31G (d,p) methods. In the optimized struc-
ture, the HOMO of 2 has been located completely on benzimidazole
ring; whereas LUMO has expanding π-nature orbital covering both
benzimidazole and benzamide moiety (Fig. 7). The TD-DFT calculations
for 2 showed that their respective absorption bands at 321 and 291 nm
are typically due to HOMO ➔ LUMO transitions, having excitation
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2016.08.008.
H
H
Mn+
References
N
N
H
[1] H.Y. Gong, Q.Y. Zheng, X.H. Zhang, D.X. Wang, M.X. Wang, Org. Lett. 8 (2006)
4895–4898.
N
N
N
N
n+ = Al3+, Cd2+
H
M
Mn+
O
O
[2] C. Lu, Z. Xu, J. Cui, R. Zhang, X. Qian, J. Org. Chem. 72 (2007) 3554–3557.
[3] Y. Mikata, M. Wakamatsu, S. Yano, Dalton Trans. (2005) 545–550.
[4] M.M. Henary, Y. Wu, C. Fahrni, J. Chem. Eur. 10 (2004) 3015–3025.
[5] F.F. Dang, K.W. Lei, W.S. Liu, J. Fluoresc. 18 (2008) 149–153.
[6] D.L. Liu, T. Pang, K.F. Ma, W. Jiang, RSC Adv. 4 (2014) 2563–2567.
[7] X.-J. Hu, C.-M. Li, X.-Y. Song, Y.-S. Li, Inorg. Chem. Commun. 14 (2011) 1632–1635.
[8] G.D. Fasman, Coord. Chem. Rev. 149 (1996) 125–165.
[9] G. Berthon, Coord. Chem. Rev. 228 (2002) 319–341.
λ
em = 492 nm
λem = 414 nm
max = 290 nm
λ
ESIPT
Mn+
[10] C.S. Cronan, W.J. Walker, P.R. Bloom, Nature 324 (1986) 140–143.
[11] E. Delhaize, P.R. Ryan, Plant Physiol. 107 (1995) 315–321.
[12] D.L. Godbold, E. Fritz, A. Huttermann, Proc. Natl. Acad. Sci. U. S. A. 85 (1988)
3888–3892.
H
H
N
Tautomerization
N
N
HN
N
N
[13] J. Barcelo, C. Poschenrieder, Environ. Exp. Bot. 48 (2002) 75–92.
[14] B. Valeur, I. Lerav, Coord. Chem. Rev. 205 (2000) 3–40.
[15] Z. Kreipcio, R.W.P.J. Wojciak, Environ. Stud. 11 (2002) 251–254.
[16] L. Friberg, C.G. Elinger, M.T. Kjellstro, Cadmium, World Health Organization, Geneva,
Switzerland, 1992.
2
O
OH
max = 271 nm, 321 nm
λ
λ
max = 271 nm, 321 nm
Scheme 2. Proposed binding mode of 2 with Al3+/Cd2+
.
[17] M.P. Waalkes, Mutat. Res. 533 (2003) 107–120.

Short communication
61
[18] Y.H. Lee, M.H. Lee, J.F. Zhang, J.S. Kim, J. Org. Chem. 75 (2010) 7159–7165.
[19] S.H. Kim, H.S. Choi, J. Kim, S.J. Lee, D.T. Quang, J.S. Kim, Org. Lett. 12 (2010) 560–563.
[20] J.L. Ren, J. Zhang, J.Q. Luo, X.K. Pei, Z.X. Jiang, Analyst 126 (2001) 698–702.
[21] M.L. Ramos, L.L.G. Justino, A.I.N. Salvador, A.R.E. de Sousa, P.E. Abreu, S.M. Fonseca,
H.D. Burrows, Dalton Trans. 41 (2012) 12478–12489.
[24] E.M. Nolan, J.W. Ryu, J. Jaworski, R.P. Feazell, M. Sheng, S. Lippard, J. Am. Chem. Soc.
128 (2006) 15517–15528.
[25] D.Y. Lee, N. Singh, D.O. Jang, Tetrahedron Lett. 51 (2010) 1103–1106.
[26] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703–2707.
[27] J.S. Wu, W.M. Liu, J.C. Ge, H.Y. Zhang, P.F. Wang, Chem. Soc. Rev. 40 (2011)
3483–3495.
[22] N. Chattopadhyay, A. Mallick, S. Sengupta, J. Photochem. Photobiol., A 177 (2006)
55–60.
[28] H. Li, S. Gao, Z. Xi, Inorg. Chem. Commun. 12 (2009) 300–303.
[29] Z.P. Liu, W.J. He, Z.J. Guo, Chem. Soc. Rev. 42 (2013) 1568–1600.
[23] S. Chen, Y.M. Fang, Q. Xiao, J. Li, S.B. Li, H.J. Chen, J.J. Sun, H.H. Yang, Analyst 137
(2012) 2021–2023.