Cyclo[2]benzimidazole: Luminescence turn-on sensing of anions

Article abstract of DOI:10.1039/c1cc10995b

Cyclo[2]benzimidazole is a new host for anions that turns on its luminescence up to 150 fold upon binding. Photoexcited cyclo[2]benzimidazole undergoes an efficient non-radiative deactivation through an excited-state intramolecular proton-transfer (ESIPT)

Full text of DOI:10.1039/c1cc10995b

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Cyclo[2]benzimidazole: luminescence turn-on sensing of anionswz
Yael Abraham,a Husein Salman,a Kinga Suwinskab and Yoav Eichen*a
Received 20th February 2011, Accepted 17th March 2011
DOI: 10.1039/c1cc10995b
Cyclo[2]benzimidazole is a new host for anions that turns on its
luminescence up to 150 fold upon binding. Photoexcited
cyclo[2]benzimidazole undergoes an efficient non-radiative
deactivation through an excited-state intramolecular proton-
transfer (ESIPT) mechanism. Upon binding an anion, the
ESIPT pathway is blocked, resulting in an increase in the
luminescence efficiency.
Many real world sensors require fast and easily detectable
means for reporting a molecular recognition event. Many
chemosensors rely on transducing the recognition event
through changes in the emission1 properties. The use of
luminescence for reporting a binding event has many advantages
as it provides high sensitivity and instantaneous signal
generation. Additionally, luminescence can be switched on
or off and these changes may often be detected even by the
naked eye. Binding dependent luminescence may rely on
different mechanisms such as intramolecular charge transfer
processes (ICT),2 photoinduced electron transfer processes
Scheme 1 (i) nitrobenzene, reflux 4 d, 70%; (ii) NaOH, MeOH/H2O,
(PET),3 metal-to-ligand charge transfer processes (MLCT),4
excimer/exciplex formation5 and guest induced changes in the
rigidity of the host.6
reflux 12 h, 95%; (iii) H2, Pd/C, MeOH, 3 h rt, 98%; (iv) BOP/NMM,
DCM, reflux, 6 d, 53%; (v) NaOH, MeOH/H2O, reflux 5 h, 80%;
(vi) H2, Pd/C, MeOH, 5 h rt, 98%; (vii) BOP/NMM, DCM, reflux,
6 d, 30%.9
Another, less common, report mechanism is the Excited-
State Intramolecular Proton Transfer (ESIPT) process.7
ESIPT occurs in systems having intramolecular hydrogen
bonds between hydrogen-bond donors such as –OH and –NH2
and hydrogen-bond acceptors such as N: and CQO.8 Upon
photoexcitation, the enol S0 ground state form transforms into
the enol S1 excited state which in turn transforms into the more
stable keto S1 photo excited tautomer. In most cases, this
species decays to the ground state in a radiative way. Due to
the tautomeric relaxation, the typical Stokes shifts of such a
process are in the range of 6000–12 000 cmꢀ1. In view of the
fact that anion binding is based on the formation of hydrogen
bonds, harnessing the ESIPT process for the development of
new anion binding signaling systems seems promising.
Here we report on a new cyclic amidophenyl benzimidazole-
based anion binding system, 1, that exhibits a dramatic
luminescence turn-on upon binding fluoride, bifluoride and
the oxo anions dihydrophosphate and benzoate, probably
through the inhibition of an ESIPT process.
Cyclo[2]benzimidazole, 1, that was prepared according to
Scheme 1, may exist in a wealth of possible tautomeric forms.
Electronic energy calculations (gas phase)10 predict that the
three tautomers 1-2Hin, 1-4Hin and 1-3Hin are the most stable
ones, Fig. 1. The calculations predict that the stability order is
1-2Hin > 1-3Hin > 1-4Hin. In the 1-2Hin conformer the
protons inside the cavity interact with the free nitrogen atoms
of the benzimidazole moieties, upon adding protons to the
cavity, forming 1-4Hin and 1-3Hin, these attractive interactions
are replaced by repulsive H–H interactions.
a Schulich Faculty of Chemistry, Technion – Israel Institute of
Technology, Technion City, 32000 Haifa, Israel.
E-mail: chryoav@tx.technion.ac.il; Fax: +927-4-8295307;
Tel: +927-4-8293708
The room temperature absorption and emission spectra of 1
in 0.1% H2O:DMSO reveal a large Stokes shift (Dl = 158 nm,
10 775 cmꢀ1), which is commonly observed in 2-aminophenyl
and 2-hydroxyphenyl benzimidazole derivatives and is
attributed to the significant structural reorganization upon
photoexcitation associated with an Excited State Proton
Transfer (ESPT) process.11
b Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52,
PL-01 224 Warszawa, Poland
w This article is part of a ChemComm ‘Supramolecular Chemistry’
web-based themed issue marking the International Year of Chemistry
2011.
z Electronic supplementary information (ESI) available. See DOI:
10.1039/c1cc10995b
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6087–6089 6087

Fig. 3 Absorption (dashed curves) and emission (lex = 322 nm)
(solid curves) spectra of solution of 1.5 M
10ꢀ5
a
ꢂ
Fig. 1 Energy diagram and structures of the low energy tautomeric
structures of 1 obtained from electronic energy calculations (Gaussian 03
suite programs,10 DFT, B3LYP, 6-311G*).
cyclo[2]benzimidazole, 1, in 0.1% H2O:DMSO before (blue) and after
(red) adding excess tetrabutyl ammonium fluoride (TBAF). Inset:
luminescence intensity of 1 as a function of added TBAF.
association model. NMR studies reveal that the complex
behavior originates from the effective transformation of
ꢀ 13
fluoride into bifluoride, HF2
.
ꢀ
The association of HF2 with 1 is characterized by a high
1 : 1 association constant of 100 000 ꢁ 10% Mꢀ1 and a 60 fold
turn-on of the luminescence. The affinity of cyclo[2]-
benzimidazole, 1, to other anions is much weaker. For
example, cyclo[2]benzimidazole, 1, forms a 1 : 1 complex with
tetrabutyl ammonium benzoate, TBABz, with an association
constant of only 100 ꢁ 10% Mꢀ1. Here too, the luminescence
turns on 130 fold upon complex formation, peaking at
saturation. Similar results were obtained for TBAH2PO4.
Table 1 depicts the association constants and luminescence
turn on factors for the association of 1 with different anions in
0.1% H2O:DMSO.z
Fig. 2 Proposed tautomeric forms and decay pathways of 1 in water
containing DMSO.
Low temperature fluorescence spectroscopy of 1 in 0.1%
H2O:DMSO reveals that upon cooling the dominant emission
is a new luminescence peak centred at B420 nm, while the
470 nm emission does not seem to vary in intensity. This type
of temperature dependent luminescence spectrum is also
indicative of an ESIPT process.
As can be seen in Fig. 3, the Stokes shift of complexed
cyclo[2]benzimidazole, 1, is significantly smaller than that of
the free one, indicating the inhibition of the ESIPT process.
The suggested mechanism for the fluorescence enhancement at
the anion recognition event is through the stabilization of the
1-4Hin tautomer due to the formation of four hydrogen bonds
between the N–H groups of the molecule and the anion. Fig. 4
presents the energy minimized structure of the complex
The probability of the ESIPT process decreases with the
temperature, through gradually blocking the tautomerism
relaxation pathways and enabling the relaxation of the S1 enol
tautomer to relax radiatively to the enol S0.12,13 Fig. 2 presents
the proposed tautomeric forms and decay pathways of 1.
The anion binding ability of cyclo[2]benzimidazole, 1, was
studied in 0.1% H2O:DMSO using spectrophotometric and
spectrofluorimetric titration experiments. Various anions were
added as their tetrabutyl ammonium (TBA) salts to solutions
of 1 in 0.1% H2O:DMSO and the relevant spectral changes
were monitored. The most significant changes were recorded
upon adding fluoride and basic oxo-anions.
Table 1 Association constants between cyclo[2]benzimidazole, 1, and
different anions in 0.1% H2O:DMSO
Lum. turn
on factor
Aniona
Fꢀ
Stoichiometry Kass
b
c
d
Fig. 3 presents the absorption and emission spectra
of cyclo[2]benzimidazole, 1, in the presence of different
concentrations of tetrabutyl ammonium fluoride in 0.1%
H2O:DMSO. Addition of fluoride induces the formation of a
new absorption peak at 340 nm, while the peak at 309 nm
decreases.
150
100 000 60
ꢀ
HF2
H2PO4
1 : 1
1 : 1
1 : 1
—
ꢀ
280
100
o5
137
130
No increase
C6H5COOꢀ
NO3ꢀ, Clꢀ, Brꢀ, Iꢀ, SCNꢀ,
HSO4ꢀ, p-toluene ꢀsulfonate,
PF6ꢀ, BF4ꢀ, BPh4
a
b
Furthermore, the emission peak of 1 at 412 nm, recorded
with excitation at the isosbestic point (322 nm), increases by
up to 150 times while the broad emission peak at 470 nm is
practically unchanged. The fluoride dependent spectral
changes do not fit a 1 : 1 complex model nor any other simple
Anion added as their tetrabutyl ammonium salts. Association
constants were calculated using a non-linear curve-fitting software
(see ESIz) and are based on the fluorescence titration data. Estimated
c
d
error o ꢁ 10%. Complex behaviour. Not determined due to
ꢀ
deprotonation and formation of HF2
.
c
6088 Chem. Commun., 2011, 47, 6087–6089
This journal is The Royal Society of Chemistry 2011

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the proposed mechanism of fluorescence regain upon binding
an anion by cyclo[2]benzimidazole, 1.
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1, was prepared and characterized. Compound 1 was shown to
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fluorescence increase. The molecule was found to undergo
an excited-state intramolecular proton transfer (ESIPT)
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6087–6089 6089