J.-Y. Li et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 102 (2013) 66–70
67
Tetrazoles are an important class of 5-membered heterocyclic
compounds that are endowed with a wide range of pharma and
biological activity [4]. This functional group is regarded as biolog-
ically equivalent to the carboxylic acid, because it can provide ac-
tive proton of 1H-tetrazole [5]. We become attracted to the
chemosensors involving the tetrazole functional group, as this
moiety has the nature of low pKa (3–5) [6] value, lower than amide
or hydroxyl group. Accordingly, tetrazole-based chemosensors are
inclined to be deprotonated by fluoride and consequently improve
the sensitivity and equilibrium time in sensing fluoride. Coumarin
and its derivatives are used extensively as fluorescence labeling re-
agents for their excellent photophysical properties of high fluores-
cence quantum yield and efficient membrane permeability [7].
Therefore, we designed a chemosensor CMTZ for fluoride including
a coumarin chromophore and a tetrazole with an active proton,
which were combined via intramolecular hydrogen bond. Upon
interaction with fluoride ion, the intramolecular hydrogen bond
was broken and the tetrazole anion was formed, which induced
its optical changes from yellowish and weak fluorescence to color-
less and strong fluorescence. To the best of our knowledge, this
strategy has not yet been proposed for the development of fluoride
colorimetric or fluorescent chemosensor.
upon addition of FÀ. This strongly proves that the active proton of
1H-tetrazole plays an important role in the switch of the intramo-
lecular hydrogen bond structure to the tetrazole anion structure. In
the presence of 2 equiv of FÀ, the fluorescence difference between
CMTZ and CMTZ-FÀ is 5.6 times greater than that between
CMCOOH and CMCOOH-FÀ.
The UV/V is spectrum of CMTZ in CH3CN exhibits an absorption
maximum at 429 nm (
e
= 2.32 Â 104 MÀ1 cmÀ1), which can be as-
signed to the coumarin transition band (Fig. 2a). When FÀ ion
was gradually added to the solution of CMTZ, the maximum
absorption peak showed a 40 nm blue shift with an isosbestic point
at 402 nm. The new energy band (389 nm,
e
= 1.81 Â 104 MÀ1
cmÀ1) may be attributed to the formation a strong hydrogen bond
between FÀ ion and tetrazole proton, so that the proton is not
available for the intramolecular hydrogen bonding. The new en-
ergy band is responsible for the color change from yellowish to col-
orless, which is perceptible to the naked eye. Fig. 2c shows
photographs of 15
lM CMTZ in the presence of 0–16 lM TBAF.
No yellowish color in solution could be observed when adding
16 lM TBAF, which suggested that the stoichiometry between
CMTZ and FÀ anion was evaluated to be 1:1. The corroborative evi-
dence for the 1:1 stoichiometry was confirmed by the method of
Compound CMTZ was readily prepared in two steps (ESI ).
Knoevenagel reaction between 4-diethylamino-2-hydroxybenzal-
dehyde and ethyl cyanoacetate produced 3-cyano-7-diethylamino-
coumarin 2B and iminocoumarins 2A. CMTZ was prepared by the
NH4Cl-mediated click cycloaddition [8] of 2B with sodium azide
at 110 °C for 24 h (42% yield). CMCOOH was obtained in 76% yield
by synthetic modification of 2B. The photophysical properties of
CMTZ and CMCOOH were stable in CH3CN for over 2 weeks
(Figs. S1 and S2, ESI ). Due to the intramolecular hydrogen bonds
effect of the coumarin dyes, CMTZ in CH3CN displayed a relatively
short emission wavelength with a maximum at 470 nm. By
increasing the polarity of solvents, the strength of the intramolec-
ular hydrogen bonds between the coumarins and the tetrazole pro-
tons increased in the excited state, which leads to the red shift of
the fluorescence emission (Fig. S3, ESI ) [9].
continuous variation showing a break point at a [CMTZ]/
([CMTZ] + [FÀ]) mole fraction of 0.5 (Fig. S8, ESI ). Moreover, the
ratios of the absorbance at 429 and 389 nm showed sigmoid
dependence on the FÀ concentration accompanied with a 9.5-fold
enhancement (Fig. 2b). This curve can be served as the calibration
curve for quantitatively detecting FÀ by absorption ratiometry.
Next, we investigated the concentration-dependent changes in
the fluorescence spectra upon addition of CMTZ (15 lM) with
2.0 equiv FÀ. As shown in Fig. 3, free probe CMTZ showed weak
fluorescence at 471 nm (Uf = 0.08) due to the structure of intramo-
lecular hydrogen bond. However, the treatment with 2.0 equiv FÀ
induced an approximately 4.93-fold fluorescent enhancement with
a 16 nm blue-shift of the emission maximum (Uf = 0.42). The
chemosensor exhibited very efficient fluorescence responding,
and over 94.2% of the total fluorescence intensity increase was ob-
served with 1.0 equiv FÀ ion. The recognition interaction was com-
pleted immediately after the addition of TBAF within 1 min
(Fig. S4, ESI ), which was much faster than the early reported sili-
con-based sensors (needed 40 min to 2 h before detection). There-
fore, CMTZ could be used in real-time determination of FÀ in
To demonstrate the crucial role of the active proton of 1H-tetra-
zole in detection of the FÀ, we tested the photophysical properties
of CMCOOH bearing the similar intramolecular hydrogen bonding
on the coumarin dye. As seen in Fig. 1, unlike CMTZ, CMCOOH does
not show any distinct absorption or fluorescence spectral changes
Fig. 1. Structures of the chemosensor CMTZ and the control compound CMCOOH. Inset: changes in absorbance (A389/A429) and fluorescence (I 455) of chemosensor in the
presence of 2 equiv. of FÀ ion in CH3CN: [CMTZ] = 15.0
M, [CMCOOH] = 15.0 M. The samples were excited at 393 nm.
l
l