Fluoride-triggered ESPT in the binding with sal(oph)en

Article abstract of DOI:10.1007/s10895-012-1063-z

In this paper, anion binding and sensing affinity of the simple and easy-to-make salen, a typical class of ligand used comprehensively in metal coordination, was investigated. Results indicated that salophen was both a colorimetric and fluorescent selecti

Full text of DOI:10.1007/s10895-012-1063-z

J Fluoresc (2012) 22:1231–1236
DOI 10.1007/s10895-012-1063-z
ORIGINAL PAPER
Fluoride-Triggered ESPT in the Binding with Sal(oph)en
Kai Liu & Jianzhong Huo & Bolin Zhu & Ran Huo
Received: 6 February 2012 /Accepted: 3 May 2012 /Published online: 16 May 2012
#
Springer Science+Business Media, LLC 2012
Abstract In this paper, anion binding and sensing affinity
of the simple and easy-to-make salen, a typical class of
ligand used comprehensively in metal coordination, was
investigated. Results indicated that salophen was both a
colorimetric and fluorescent selective chemosensor for
fluoride ion, which operated by the anion-induced confor-
mational changes and subsequently excited-state intramo-
lecular proton transfer (ESPT) process. The F--induced
quick response, as well as noticeable optical changes,
suggested that anion-sensing mechanism maybe help to
design and to synthesize the new preferential selective
probes for F-.
useful catalytic transformations [4, 5]. These came from
the intrinsic tetradentate with N2O2 set of donor atoms
capable of effective coordination in the planar fashion or
slightly distorted geometry [4, 6]. Additionally, the metal
complexes of salophens have been used comprehensively as
the functional materials [7–9] and modes for superoxide
dismutase [10–12]. Recently, sal(oph)en complexes of tran-
sition metal have been used as molecular sensors for anionic
and neutral guests [13, 14], along with the realization of
their vital importance in a wide range of biological, envi-
ronmental, and chemical process.[15, 16] In fact, many sal
(oph)en-metal complexes as receptors for anionic and neu-
tral subunits were reported after the seminal work of
Reinhoudt group in 1990s [17]. Uranyl-salophen complexes
with a well-defined preference for pentagonal bipyramidal
coordination and with the two oxygens in the apical positions
[18, 19], can effectively bind the anionic guest by substituting
the solvent molecules in the fifth equatorial site [20, 21]. Then,
various functionalized salophen ligands and their uranyl
complexes were prepared, and their anion recognition prop-
erties were investigated [13]. Additionally, zinc(II)-salophen
complexes with fluorescence gained an important position
in anion recognition and sensing [22, 23]. In these recep-
tors, zinc(II) metal center with five-coordinate square pyra-
midal geometry, possessed remarkable Lewis acidity. The
conformation allowed strong and suitable binding agents to
coordinate at the axial site [24]. Whatever, to the best of
our knowledge, there were few reports on the simple and
easy-to-make sal(oph)en themselves, containing binding
sites OH moiety and signal subunits C0N group, as recep-
tors (sensors) for charged species by ‘binding site-signaling
subunit approach’[25], maybe due to the stronger metal
complex of salens relative to the corresponding anion com-
plexes. Here, the anion binding abilities of sal(oph)en were
investigated.
.
.
.
Keywords ESPT Deprotonation Anion sensors
.
Supramolecular chemistry Fluorescent
Introduction
Sal(oph)ens, a well-known and oldest class of ‘privileged
ligands’ [1] used comprehensively after the work described
by Jacobsen [2] and Katsuki [3] in 1990, have constituted
one of the most fundamental building blocks for the stabi-
lization of different metals in various oxidation states, con-
trolling the performance of the metals in a great deal of
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-012-1063-z) contains supplementary material,
which is available to authorized users.
:
:
:
K. Liu (*) J. Huo B. Zhu R. Huo
Tianjin Key Laboratory of Structure and Performance
for Functional Molecule, College of Chemistry,
Tianjin Normal University,
Tianjin 300387, China
e-mail: hxxylk@mail.tjnu.edu.cn

1232
J Fluoresc (2012) 22:1231–1236
Specific anion recognition, binding and transportation by
Results and Disscussion
protein are carried out by H-bonds of NH and OH mainly in
biological systems [26, 27], therefore, the biomimetic of N
(O)-H…anion interaction is important. However, over the
past 30 years, only few systematic investigations involved
the O-H groups as a key component to obtain the anion
binding [28–31], and anion recognition based on amide and
(thio)ureas has been comprehensively studied. Furthermore,
the dramatic enhancement of the phenolic OH acidity
resulted from photoexcitation [32, 33], and therefore an
excited-state intermolecular proton transfer (ESPT) [29,
34, 35] channel might be opened due to anion binding. In
fact, the phenomenon has scarcely been exploited in anion
sensing [29, 35–37]. On the basis of above-mentioned back-
ground, herein we reported the anion sensing and binding
properties of sal(oph)en based on OH group as binding site
by UV–VIS and fluorescence spectroscopy in dilute solu-
tion. Results indicated that sal(oph)en has a high selectivity
for F− by ESPT [29, 34, 35], concomitant with color
changes.
Anion binding and sensing properties of sal(oph)en have been
studied by UV–VIS spectroscopic techniques. In the absence
of anions, receptor 2 (2×10−5 M) in CH3CN displayed three
strong absorption bands at ca. 329, 268, and 228 nm, resulting
from the intramolecular charge transfer from C0N and OH
group to the phenyl substituent. Upon gradually increasing the
concentration of F−, the absorption band at 329 nm decreased
slowly, and new red−shifted absorption maximum bands at
422 nm formed and developed (Fig. 1). As a result, the color
of the solution turned from colorless to yellow (Supporting
information Fig. S1), affording naked-eye detection of F−.
Considering the fact that F− is a strong Lewis base, and can
deprotonate proton of OH [29, 35, 41], these spectral changes
(bathochromic shift Δλ≈93 nm upon addition of F−) were
presumably ascribed to the occurrence of an “incipient” and
“frozen” proton-transfer process [42]. The OH proton may
undergo deprotonation, due to the strong basicity of F− and
high stability of complex [HF2]− [15]. This would imply the
occurrence of negatively charged PhO− of receptor 2, which
caused a significant increase in charge density, and followed
the enhancement in the push-pull effect of the intramolecular
charge transfer in the groud state. Furthermore, well-defined
isosbestic points (228, 288 and 339 nm) indicated a clean
conversion throughout the titration process. Additionally, the
Experimental
interaction of 2 with other anions (Cl−, Br−, I−, NO3 , AcO−,
−
−
H2PO4− and HSO4 ) was also investigated by UV–VIS spec-
troscopic titration methods (Supporting information Fig. S2).
No significant spectral changes were observed specifically for
−
−
Br−, I−, NO3 , and HSO4 , even at higher concentration, due
Receptor 1 and 2 were synthesized by condensation of
salicyaldehyde with corresponding diamine in ethanol. Aceto-
nitrile for spectroscopy was purchased from the J&K Scientific
LTD. All tetrabutylammonium salts were purchased from
Sinopharm Chemical Reagent Co., Ltd. The serial working
solutions provided with incremental multiples of tetraalkylam-
monium salts and the definite quantity hosts (2×10−5 M) in the
procedure were prepared using the acid-washed glass pipettes
and volumetric flasks to make appropriate solutions from the
stock solution and were stored in the room temperature for
0.5 h before used in the experiment. UV–VIS spectra and
fluorescence were performed on Varian Cary 300 spectropho-
tometer and Hitachi F-4500 spectrofluorometers, respectively.
For fluorescence measurements, the excitation and emission
slit were 3 nm, and scan speed was 100 nm/min. All NMR
spectra were measured on a buker spectrometer at 400 MHz
with DMSO-d6 as solvents.
to the weak coordination interaction. Furthermore, addition of
0.36
0.75
0.27
0.18
0.09
0.00
0.50
0.0
0.3
0.6
0.9
1.2
[F-] (mM)
0.25
0.00
240
300
360
420
480
540
Analysis of the solution phase anion-binding properties
was made using UV–VIS and fluorescent spectroscopic
titration techniques, respectively. Data analysis and stability
constant determinations were then made by the satisfactory
non-linear least-square analysis [38–40].
Wavelength (nm)
Fig. 1 UV–VIS spectral changes of receptor 2 (2×10−5 M) in MeCN
after the addition of 0–55 equiv TBAF. The inset displayed the absor-
bance at 422 nm vs [TBAF]; the line represented the fitting of the
experimental points assuming a 1:1 stoichiometry

J Fluoresc (2012) 22:1231–1236
1233
F− to those solutions of receptor 2, containing excess Cl−, Br−,
I−, NO3 , AcO−, H2PO4− and HSO4 , immediately produced
the expected optical response. These results suggested that the
simple and easy-to-make receptor 2 was preferential selective
chromogenic anion sensors for F− in contrast to the other
anions examined.
−
−
e)
d)
c)
17 16 15
17 16 15
Evidence for 1:1 complex formation was provided by
non-linear least-square analysis [43] for receptor 2 with
b)
a)
6
F−, Cl−, AcO− and H2PO4 . This was also proved by Job-
−
plot analysis. The binding constants were obtained from the
variation in the absorbance at the appropriate wavelength
[F− (422 nm), Cl−, AcO−, H2PO4− (329 nm)] by plotting the
A as a function of the [anion] (Fig. 2). The association
ppm
12
10
8
Fig. 3 Partial 1H NMR spectra (400 MHz) of receptor 2 in DMSO−d6
upon addition of: a 0, b 0.2, c 0.5, d 1.0, e 5.0 equiv. TBAF
−
constants for receptor 2 with F−, Cl−, AcO− and H2PO4
were determined to be 6.1×103, 91, 1.9×102, 1.1×102 M−1,
respectively. Moreover, few changes in UV–VIS spectra of
itself, the phenolic O-H signal of receptor 2 at 12.92 ppm
shifted dramatically to down field, suggesting a weak PhO-
H…N0C hydrogen bond. This implied that the conforma-
tional equilibrium of receptor 2 shifted more toward closing-
ring side [44–46] (Scheme 1). Furthermore, with the gradual
increase of F−, phenolic O-H signal of receptor 2 at
12.92 ppm decreased, and, upon addition of 0.5 equiv F−,
the phenolic O-H signal at 12.92 ppm disappeared com-
pletely, indicating the destruction of intramolecular
hydrogen-bond and the formation of hydrogen bond inter-
action between PhO-H and F−. Once again, with the further
gradual increase of the concentration of F−, the new triplet
resonance at 16.1 ppm, which pertained to the bifluoride
[HF2]− [47], occurred and developed. This implied the de-
struction of the initial O-H…F− hydrogen-bonding interac-
tion and the final proton transfer [42] from the PhOH of
receptor 2 to F−, along with the formation of the deprotona-
tion of receptor 2. Results also confirmed the previous
assumption. At the same time, the imine proton (CH0N)
and aromatic proton signals experienced continuous upfield
shifts, which resulted from the shielding effect of negative
electron delocalization on the π-conjugated framework
[48–50]. Similar results were also observed for receptor 1
(Supporting information Fig. S5). Such deprotonation was
related to the acidity of the binding sites, the basicity of the
analyte, and the corresponding stability of the conjugated
base [HF2]−. [15]
For further explore the anion-binding affinity of sal(oph)
en, spectrofluorimetric titration experiments were per-
formed. Figure 4 displayed the emission spectral changes
of receptor 2 in presence of various concentration of TBAF
in CH3CN. Receptor 2, with enol form of shiff base [44–46]
displayed very weak fluorescence with a short-wavelength
band at 431 nm, and a broad band centered at 475 nm, after
the excitation at the maximum absorption band (centered at
422 nm) of composited receptor 2 with F−. Moreover, upon
addition of F−, the emission peak at 475 nm increased
substantially by up to 150 times, at the same time the broad
receptor 2, induced by Br−, I−, NO3 and HSO4 , were not
enough to calculate the binding constants. Results showed
that receptor 2 bond F− more strongly than the other anions
examined, thus, explained the phenomenon observed in
anion sensing experiments.
−
−
Different from receptor 2, receptor 1, containing carbon-
carbon bond, exhibited strong UV–VIS response on the dif-
ferent concentrations of F− in MeCN. When increasing the
concentration of F−, a new band centered at 393 nm gradually
enhanced, and the absorption band at 315 nm significantly
reduced in intensity, with concomitant formation of isosbestic
point at ca. 337, 287 and 227 nm, respectively (Supporting
information Fig. S3). This was presumably attributed to pro-
ton transfer from the PhOH of receptor 1 to F−. In the case of
−
other anions examined (Cl−, AcO− and H2PO4 ), few spectral
changes were observed (Supporting information Fig. S4).
Furthermore, UV–VIS titration showed that the selectivity
trend of the binding affinity of receptor 1 was in the order of
F−>AcO−>H2PO4 >Cl−, and qualitatively accorded with the
−
basicity of these anions.
To further elucidate the interaction between sal(oph)en
1
and F−, H NMR titrations were carried out in DMSO-d6.
Figure 3 displayed the 1H NMR spectra of receptor 2 and its
complex with different F− concentration. Compared to O-H
-
Cl
0.425
-
AcO
-
H PO
0.420
0.415
0.410
2
4
0.00
0.25
0.50
0.75
1.00
Anion Concentration (mM)
Fig.2 UV–VIS titration curves for receptor 2 with TBA salts in MeCN

1234
J Fluoresc (2012) 22:1231–1236
Scheme 1 The proposed anion
binding mode of the receptor 2
in solution
N
N
N
H
N
H
N
H
N
H
O
O
OH
HO
O
O
F-
g
-rin
d
se
clo
g
in
n-r
e
op
F-
N
H
N
N
N
O
O
H
-O
F-
OH
peak at 431 nm enhanced slowly. Here we attributed the
fluorescence enhancement upon the anion addition to the
formation of two hydrogen bonds between the OH moiety of
receptor 2 and F−.[44–46] This implied the formation of new
geometrically restricted seven-membered intramolecular hy-
drogen bond ring transition state, as shown in Scheme 1. Thus,
the hydrogen-bond-induced π-delocalization [10] on the re-
ceptor 2 occurred and further resulted in the increase in
rigidity of conformational restriction, therefore, rendering
the non-radiative decay from the excited state less possible.
Therefore, the increase in intensity was observed. On the other
hand, the bound anions via phenolic OH, which played an
important role in anion binding [23, 29, 35], led to an increase
in local concentration of the anions. Subsequently, the inter-
molecular proton transfer in the excited state of receptor 2 to
weakly basic anions occurred, due to the enhanced acidity of
phenolic OH upon the photoexcitation [26, 27] and the solva-
tion of polar CH3CN solvents [51–53]. And the strong emis-
sion band at 475 nm was observed, due to the increase in
electron density of the deprotonation phenolic O−, compared
with phenolic OH linking F−, and its subsequent positive
effect on the efficiency of charge transfer. Furthermore, the
F−-dependent fluorescent intensity at 475 nm showed a good
linearity that can be expressed as I/I00117.63+33550.56 [F−]
(r00.986) in the F− concentration range of 0.2 to 11.0 mM.
The detection limit of fluoride, calculated as the concentration
corresponding to triple standard deviation of the back ground,
was 0.082 mM.
Additionally, no significant emission spectral changes
(Supporting information Fig. S6) were found for the other
−
−
−
anions (Cl−, Br−, I−, NO3 , AcO−, H2PO4 and HSO4 ),
even in large excess amount. Furthermore, the association
constants of receptor 2 with those anions were also deter-
mined by nonlinear regression methods [43] following the
changes of fluorescent intensity. These results were in line
with the equilibrium constants derived from absorption
spectral titrations. The higher binding ability of F−, as well
as the more efficient fluorescence enhancement by F−, com-
pared with other anions examined, was due to its smaller
size, higher electron density and the higher ratio between
charge density to size, which made it stronger hydrogen
bonding acceptor [54].
On the other hand, spectrofluorimetric titration technique
was also used to explore the binding properties of receptor 1
for F− (Fig. S7). Substitution of phenyl with carbon-carbon
bond, which hindered the formation of the large conjugated
system, displayed negative effect on the anion binding affin-
ities of receptor 1. Receptor 1 showed weak emission at
475 nm upon excitation at 393 nm. Upon continuous addition
of F−, the broad emission band at 475 nm enhanced essentially
up to 100 times. This, being similar to receptor 2 for F−, was
ascribed to the first hydrogen-bond interaction between F− and
receptor 1 and the followed ESPT [29, 34–36]. Once more,
500
500
400
300
400
200
100
0
300
0
10 20 30 40 50
equiv F-
60
addition of Cl−, Br−, I−, AcO−, H2PO4 , HSO4 and NO3 ,
even at higher concentration, induced no significant emission
changes, along with the irradiation at 393 nm.
−
−
−
200
100
0
Conclusions
450
500
550
600
Wavelength (nm)
In summary, the simple and easy-to-make sal(oph)en, as a new
kind of anion receptor, was proved to be both colorimetric and
fluorescent selective chemosensor for fluoride ion in CH3CN,
by means of the anion-dependent conformational changes [32,
Fig. 4 Emission spectral changes of receptor 1 (2×10-5 M) in MeCN
after the addition of 0–55 equvi. TBAF. The inset displayed the
emission intensity at 475 nm vs [TBAF]

J Fluoresc (2012) 22:1231–1236
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