Synthesis of new benzimidazolium salts with tunable emission intensities and their application as fluorescent probes for Fe3+ in pure aqueous media Dedicated to Professor Sakae Uemura on his 73rd birthday

Article abstract of DOI:10.1016/j.tetlet.2014.01.127

A series of novel, water-soluble benzimidazolium salts with common 'fluorophore-spacer-receptor' PET design has been synthesized. Despite the common PET scaffold these benzimidazolium salts displayed diverse emission intensities in pure aqueous solutions. The observed emission intensities were found to be influenced by the functionalized alkyl side arms present on the benzimidazolium ring. These benzimidazolium salts were also found to act as selective sensors for Fe³⁺ ions over other metal ions like Na ⁺, K⁺, Ca²⁺, Mg²⁺, Ba²⁺, Al³⁺, Cr³⁺, Co²⁺, Ni²⁺, Mn ²⁺, Zn²⁺, Pb²⁺, Ag⁺, Cu²⁺ and Hg²⁺ in pure aqueous media.

Full text of DOI:10.1016/j.tetlet.2014.01.127

Tetrahedron Letters 55 (2014) 1810–1814
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of new benzimidazolium salts with tunable emission
intensities and their application as fluorescent probes for Fe3+
in pure aqueous media
⇑
Amita Krishan Lal, Marilyn Daisy Milton
Department of Chemistry, University of Delhi, Delhi 110007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel, water-soluble benzimidazolium salts with common ‘fluorophore–spacer–receptor’ PET
design has been synthesized. Despite the common PET scaffold these benzimidazolium salts displayed
diverse emission intensities in pure aqueous solutions. The observed emission intensities were found
to be influenced by the functionalized alkyl side arms present on the benzimidazolium ring. These
benzimidazolium salts were also found to act as selective sensors for Fe3+ ions over other metal ions like
Na+, K+, Ca2+, Mg2+, Ba2+, Al3+, Cr3+, Co2+, Ni2+, Mn2+, Zn2+, Pb2+, Ag+, Cu2+ and Hg2+ in pure aqueous media.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 22 November 2013
Revised 25 January 2014
Accepted 27 January 2014
Available online 5 February 2014
Dedicated to Professor Sakae Uemura on his
73rd birthday
Keywords:
Photoinduced electron transfer (PET)
Benzimidazolium
Fluorescence
Fe3+ sensors in water
Photoinduced electron transfer (PET) is a long-range electron
transfer from the receptor to the fluorophore in the excited
state.1–3 The PET design includes a ‘fluorophore–spacer–receptor’
architecture, where the electron donor (receptor) and an electron
acceptor (fluorophore) are linked together by a flexible spacer. In
the excited state, electron transfer from the electron donor to the
electron acceptor may quench the fluorescence from the
fluorophore. The efficiency of the PET process strongly depends
upon a number of factors such as conformational changes, polarity
modulation, hydrogen bonding, distance between the fluorophore
and the receptor.4
During the past few years, the fluorescent chemosensors with
PET design have been used in metal ion detection due to their high
selectivity, sensitivity and precision.5,6 Amongst the various transi-
tion metals, iron is one of the most important elements for the
environment and living systems due to its significant role in vari-
ous metabolic and electron transfer processes of the cells.7,8 Thus,
considerable efforts have been made towards the development of
iron sensing probes. A variety of fluorescent iron sensing probes
based on coumarin,9 thiacalixarene,10 anthracene,11 fluoranth-
ene,12 rhodamine,13 1,8-naphthalimide,14 etc. have been devel-
oped. However, the poor water solubility of these fluorescent
probes limits their application under physiological conditions. Till
date, very few Fe3+ sensors in pure aqueous media have been
reported.15–18 Therefore, the development of sensors which can
work in pure aqueous media is a challenging task, and hence, needs
attention.
As part of our research work on the synthesis and applications
of benzimidazolium salts, we have designed
a series novel,
water-soluble benzimidazolium salts with a common ‘fluoro-
phore–spacer–receptor’ PET scaffold. The ligand design incorporates
the benzimidazolium fluorophore unit attached to a pyridine
receptor arm through a methylene spacer. Pyridine can act as a
receptor by transferring lone pair of electrons on nitrogen to the
benzimidazolium ring by PET mechanism. With an aim to fine-
tune the PET process, we have introduced functionalized alkyl
chains as side arms on the PET scaffold by N-quaternization of
the benzimidazole ring (Fig. 1). Herein, we report the synthesis
of new water-soluble benzimidazolium salts with tunable fluores-
cence intensities and their use as selective sensors for Fe3+ in pure
aqueous media.
Highly water-soluble benzimidazolium salts 2–6 were synthe-
sized in moderate to good yields by heating 2-(1H-benzimidazol-
1-ylmethyl)pyridine (1)19 with the corresponding alkyl halides in
a suitable solvent for 24–48 h (Scheme 1).
The structures of 2–4 were confirmed by X-ray crystallographic
⇑
Corresponding author. Tel.: +91 11 27667794 140.
E-mail address: mdmilton@chemistry.du.ac.in (M.D. Milton).
analysis.20 The molecular structures of 2–4 are shown in Figure S1.
http://dx.doi.org/10.1016/j.tetlet.2014.01.127
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

A. K. Lal, M. D. Milton / Tetrahedron Letters 55 (2014) 1810–1814
1811
benzimidazolium salts21–23 (Tables S2 and S3). However, the small
variation amongst these ligands was due to the difference in the
packing effects. A strong (C–H)+ꢀ ꢀ ꢀBrꢁ ionic hydrogen bonding
(2.93 Å) was observed between carbenic hydrogen and bromide
ion in 2 whereas, no such interactions were observed in 3 and 4.
Rather, the hydrogen bonding was observed between the hydroxyl
group and bromide ion in ligand 3 (Table S4). Also, many intermo-
lecular C–Hꢀ ꢀ ꢀBr contacts were observed for these ligands (2–4).
Moreover, in 2, the two benzimidazolium units present in the unit
cell dimerized together due to the antiparallel arrangement of the
benzimidazolium rings (Fig. S2). This also led to
interactions in the solid state (Fig. S3). However, in 3, despite the
offset arrangement of benzimidazolium rings, no and
ꢀ ꢀ ꢀ
interactions could be observed, due to improper orienta-
p
ꢀ ꢀ ꢀ
p
and C–Hꢀ ꢀ ꢀ
p
p
p
C–Hꢀ ꢀ ꢀ
p
Figure 1. Design of benzimidazolium based NHC ligands with common PET
scaffold.
tion of the benzimidazolium rings (Fig. S4). Similarly, in the crystal
packing of 4, two benzimidazolium units dimerized together due
to the C–Hꢀꢀꢀ
p
interactions but no
pꢀꢀꢀp interactions were seen
(Fig. S2).
The UV absorption maxima for 2–6 (5.0 ꢂ 10ꢁ5 mol dmꢁ3) in
water were observed at 262 nm with shoulder peaks at 257, 268
and 280 nm (Fig. S5). Despite the common PET scaffold, 2–6
showed diverse emission intensities ranging from low to very high
fluorescence intensities in 100% aqueous solutions with the
emission maxima around 373–375 nm with high values of Stokes
shift (Fig. 3 and Table S5). The emission intensities decreased in
the order 2 > 3 > 5 > 6 > 4.
(i)
(ii)
N
N
N N
N
N
1
n
OnH
Br
N
N
Br
Br
N
4:
5:
2:
3:
n = 2, 99%
n = 3, 65%
n = 2, 62%
n = 3, 62%
(iii)
N
N
Although, fluorescence studies were carried out in solution
phase, we reasoned that the single crystal X-ray diffraction studies
may provide an insight into the relative position of the fluorophore
and the receptor in these molecules (2–4). The crystal structures
revealed that the pyridine rings were inclined at an angle of
89.5°, 89.0° and 66.8° to the benzimidazolium rings in 2, 3 and 4,
respectively (Fig. 4). This conformational difference in the angle
of inclination may play a role in the fluorescence emission of these
ligands. In case of 2 and 3, pyridine rings were inclined nearly per-
pendicular (89.5° and 89.0°, respectively) to the plane of the benz-
NH3Br
N
Br
6:
55%
Scheme 1. Reagents and conditions: (i) 2-bromoethanol/3-bromopropanol, aceto-
nitrile, reflux, 48 h; (ii) 1,2-dibromoethane/1,3-dibromopropane, reflux, 24–48 h;
(iii) 2-bromoethylamine hydrobromide, acetonitrile, reflux, 48 h.
The pyridine side arm, attached to the benzimidazolium ring
through a methylene spacer, was common in 2–4. Whereas, the
other side arm had hydroxyethyl, hydroxypropyl and bromoethyl
groups in 2, 3 and 4 respectively. In all the structures, both the
pyridine and the functionalized alkyl side arms were positioned
on the same side of the benzimidazolium ring. The pyridine ring
in 2, 3 and 4 was inclined at an angle of 89.5°, 89.0° and 66.8°,
respectively to the benzimidazolium ring.
A symmetrical sheet-like packing was observed for these
benzimidazolium salts where the bromide ions were sandwiched
between the layers of the benzimidazolium units (Fig. 2). The key
parameters of 2–4 are summarized in Table S1; the bond
parameters were found to be similar to those reported in other
imidazolium ring. Thus, the p-orbitals of the benzimidazolium ring
and lone pair of electrons on pyridine nitrogen were spatially dis-
posed in the same plane. This reduced the rate of intersystem
crossing and hence, their effectiveness as electron transfer source
for the PET process.24 Thus, higher emission intensities were
observed with 2 and 3. However, in case of ligand 4, the angle of
inclination was 66.8° hence the possibility of partial PET process
increased. The heavy atom effect of bromine may also contribute
to the quenching of fluorescence in 4. The difference in the angle
of inclination between pyridine and benzimidazolium rings in
2–4 can be attributed to the influence of the functionalized alkyl
Figure 2. Symmetrical sheet-like packing of: (a) compound 2 when viewed along crystallographic axis c; (b) compound 3 when viewed along crystallographic axis b and (c)
compound 4 when viewed along crystallographic axis b.

1812
A. K. Lal, M. D. Milton / Tetrahedron Letters 55 (2014) 1810–1814
Br
NH3Br
CH3CN, reflux, 48 h
N
N
N
N
Br
8: 60%
Scheme 2. Synthetic route for 8.
NH3Br
7
Figure 3. Normalized fluorescence spectra of 2–6 and 8 (5.0 ꢂ 10ꢁ5 mol dmꢁ3) in
water with excitation and emission slit width of 5 nm.
side arm on the orientation of the pyridine ring w.r.t. the benzim-
idazolium ring.
In order to understand the role of the pyridine ring as receptor
in sensing mechanism, we synthesized 8, where the pyridine ring
was replaced by a benzene ring. Ligand (8) was synthesized in
60% yield by refluxing 1-benzylbenzimidazole (7)25 with 2-bromo-
ethylamine hydrobromide in acetonitrile solvent for 48 h
(Scheme 2).
Figure 5. A comparison of emission spectra of 6 and 8 (5.0 ꢂ 10ꢁ5 mol dmꢁ3) in
water with excitation and emission slit width of 5 nm.
The UV absorption maxima for 8 (5.0 ꢂ 10ꢁ5 mol dmꢁ3) in
water were observed at 272 nm with a shoulder peak at 280 nm
(Fig. S5). The emission maxima in water were observed at
374 nm. A comparison of the emission intensities of 6 and 8 re-
vealed that 8 showed much higher emission intensity as compared
to 6 (Fig. 5), in fact, 8 showed the highest emission intensity
amongst all the ligands (1–8). This could be attributed to the ab-
sence of receptor (pyridine ring) in 8, which removed the possibil-
ity of any PET process through the receptor as compared to 6 which
had a pyridine ring as receptor. However, in both 6 and 8, the
ammonium group present in the alkyl side chain might undergo
PET process up to some extent upon partial conversion to the ami-
no group on hydrolysis in aqueous solution.26
of the benzimidazolium salts. It was observed that addition of 33
equiv of Na+, K+, Ca2+, Mg2+, Ba2+, Cr3+, Mn2+, Ni2+, Zn2+, Cu2+
,
Hg2+, Co2+, Ag+, Al3+ and Pb2+ did not produce any major changes
in the emission spectra of 3 and 8. But the addition of 33 equiv
of Fe3+ quenched the emission intensity significantly (Figs. 6 and
S6). Thus, these benzimidazolium salts can act as fluorescent
probes for Fe3+ by ‘turn-off’ mechanism. Quenching was observed
due to the paramagnetic nature of Fe3+, which increased the rate
of the nonradiative pathways through energy transfer or electron
transfer when present in close vicinity of the probe.27 The quench-
ingꢁefficiency of Fe3+ was unaffected by the counter ion (Clꢁ and
NO3 ). The complexation studies with Fe3+ were carried out with
its nitrate salt. It is noteworthy that a substantial quenching was
Next, we studied the interaction of various metal ions with the
benzimidazolium salts 2–6 and 8 in aqueous media. All the synthe-
sized benzimidazolium salts 2–6 and 8 were capable of sensing
Fe3+. However, detailed studies were carried out with 3 and 8 by
the addition of various metal ion solutions to the aqueous solution
also observed with Fe2+
.
Further, the selectivity of the probes for Fe3+ was studied by the
addition of a solution of Fe3+ ions to the solution of probes 3 or 8
with other metal ions such as Na+, K+, Ca2+, Mg2+, Ba2+, Al3+, Cr3+
,
Figure 4. Molecular structure diagrams of (a) 2, (b) 3 and (c) 4 showing the angle of inclination between pyridine and benzimidazolium rings.

A. K. Lal, M. D. Milton / Tetrahedron Letters 55 (2014) 1810–1814
1813
for 3 and 8, respectively (Fig. S9). These data indicate that probe
3 has high sensitivity for Fe3+ detection as compared to 8.
The 1:1 stoichiometry of metal and ligand in probes 3 and 8 was
confirmed by Job’s plot (Fig. S10). Probe 3 can possibly interact
with Fe3+ through the hydroxyl group, carbenic carbon and pyri-
dine nitrogen. While probe 8 may interact with Fe3+ through the
carbenic carbon31 and the amino group (formed due to hydrolysis
of ammonium group in water). The availability of one additional
binding site in 3 suggests a stronger binding to Fe3+ as compared
to 8, which was further confirmed by the calculation of binding
constant using Benesi–Hildebrand equation.32
A good linear
relationship (R2 = 0.9983 and 0.9994) was obtained from the
graphs plotted between I0=I0 ꢁ I and 1/[Fe3+] for 3 and 8 and the
binding constants were calculated to be 3.12 and 2.14 ꢂ 103 Mꢁ1
for 3 and 8, respectively (Fig. S11).
In conclusion, a series of novel highly water-soluble benzimi-
dazolium salts with a common ‘fluorophore–spacer–receptor’ PET
design was synthesized with an aim to fine-tune the PET process
in these ligands. By varying the functionalized alkyl side arm
attached to the PET scaffold, we were successful in modulating
the emission intensities of these benzimidazolium salts in aqueous
solutions. Also, these benzimidazolium salts demonstrated high
selectivity for Fe3+ over 15 other metal ions in pure aqueous media.
Figure 6. Fluorescence spectra of 3 (1.0 ꢂ 10ꢁ4 mol dmꢁ3) on addition of 33 equiv
of different metal ions in water.
Fe2+, Co2+, Ni2+, Mn2+, Zn2+, Pb2+, Ag+, Cu2+ and Hg2+. It was found
that the quenching efficiency of Fe3+ remained unaffected in the
presence of these ions including the competitive metal ions like
Cu2+, Cr3+, Co2+ and Fe2+ (Figs. 7 and S7). Thus, it was concluded
that both probes 3 and 8 were highly selective for Fe3+ ions in pure
aqueous media.
Acknowledgments
We gratefully acknowledge financial support from the Univer-
sity of Delhi, India and the Department of Science and Technology,
India. A.K.L. also acknowledges the Council for Scientific and Indus-
trial Research, India for Junior and Senior Research Fellowships.
The authors are grateful to USIC-CIF, University of Delhi for NMR
spectral data and single crystal XRD data.
To determine the affinity of probes and the quantitative sensi-
tivity range, fluorescence titration experiments were performed
for Fe3+. During the fluorescence titration, the progressive addition
of Fe3+ diminished the emission intensity gradually (Fig. S8). Fur-
ther the detection limit28–30 was calculated from the titration data
by plotting a graph between (ðImax ꢁ IÞ=ðImax ꢁ IminÞ) and log[Fe3+].
Supplementary data
Supplementary data (1H, 13C NMR spectroscopic and HRMS
data) associated with this article can be found, in the online ver-
sion, at http://dx.doi.org/10.1016/j.tetlet.2014.01.127.
A linear regression curve in the range of 8.3 lM–1.6 mM afforded
an intercept of ꢁ5.02 and ꢁ4.74 at the ordinate axis, which corre-
sponds to the detection limit of 9.45 ꢂ 10ꢁ6 M and 1.80 ꢂ 10ꢁ5
M
Figure 7. Selectivity of 3 for Fe3+ on addition of different metal ions.

1814
A. K. Lal, M. D. Milton / Tetrahedron Letters 55 (2014) 1810–1814
20. Crystal data for 2: C15H16BrN3O, M = 334.22, triclinic, a = 8.8102(8),
b = 9.6686(10), c = 10.2553(10) Å, = 82.241(8), b = 70.803(9),
= 62.905(10)°, V = 732.19(12) Å3, T = 298(2) K, space group P-1, Z = 2, 6195
reflections measured, 2581 unique reflections (Rint = 0.0278). Final R1 = 0.0391
(I > 2
(I)), wR(F2) = 0.0799. CCDC 961641.
Crystal data for 3: 16H18BrN3O, M = 348.24, monoclinic, a = 9.1199(5),
b = 17.9089(8), c = 10.2501(5) Å, b = 106.985(6)°,
V = 1601.10(14) Å3,
T = 298(2) K, space group P21/n, Z = 4, 12,181 reflections measured, 3257
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