Development of fluorescent sensing of anions under excited-state intermolecular proton transfer signaling mechanism

Article abstract of DOI:10.1021/ol034846u

(Matrix presented) A substantially red-shifted fluorescence emission in 3-hydroxyl-2-naphthanilide in acetonitrile was developed and drastically enhanced upon addition of anions such as F^-, AcO^-, and H₂PO₄^-, with the enhancement depending on anion basicity. Excited-state intermolecular proton transfer in the sensor-anion hydrogen-bonding complex was suggested to be the signaling mechanism.

Full text of DOI:10.1021/ol034846u

ORGANIC
LETTERS
2003
Vol. 5, No. 15
2667-2670
Development of Fluorescent Sensing of
Anions under Excited-State
Intermolecular Proton Transfer Signaling
Mechanism
,†
Xuan Zhang,† Lin Guo,† Fang-Ying Wu,†,‡ and Yun-Bao Jiang*
Department of Chemistry and the MOE Key Laboratory of Analytical Sciences,
Xiamen UniVersity, Xiamen 361005, China, and Department of Chemistry,
Nanchang UniVersity, Nanchang 330047, China
ybjiang@xmu.edu.cn
Received May 15, 2003
ABSTRACT
A substantially red-shifted fluorescence emission in 3-hydroxyl-2-naphthanilide in acetonitrile was developed and drastically enhanced upon
addition of anions such as F-, AcO-, and H PO -, with the enhancement depending on anion basicity. Excited-state intermolecular proton
2
4
transfer in the sensor−anion hydrogen-bonding complex was suggested to be the signaling mechanism.
Anion recognition and sensing via artificial receptors are of
current interest in supramolecular chemistry because of their
importance in biological and environmental assays.1 Of
particular interest in this regard are fluorescent sensors, as
they are both highly sensitive and easy to signal.2 The most
desirable property of an anion sensor based on fluorescence
is the ability to respond to applied perturbation in a highly
selective and sensitive manner by dramatic change in
emission color and/or intensity. For this purpose, many
fluorescent sensors for anions have been developed on the
basis of a variety of signaling mechanisms such as competi-
tive binding,3 photoinduced electron transfer (PET),4 metal-
to-ligand charge transfer (MLCT),5 excimer/exiplex,6 and
intramolecular charge transfer (ICT).7 It is surprising to note
† Xiamen University.
‡ Nanchang University.
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10.1021/ol034846u CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/03/2003

that, although the phenomenon of excited-state intra-/
intermolecular proton transfer (ESPT) has been well docu-
mented,8 it has scarcely been exploited in anion sensing.
Recently, Hamilton et al.9 reported a fluorescent macrocyclic
amide anion sensor containing coumarin fluorophore that
involves both the excited-state charge transfer and proton
transfer dual channels. We report here a simple fluorophore,
3-hydroxyl-2-naphthanilide (1, Figure 1), that is com-
Figure 2. Fluorescence spectra of 1 (10 µM) in CH3CN upon
addition of F- with an excitation wavelength of 377 nm (300 nm
for the dashed line). From bottom to top, [F-] (µM) increases from
0.0 to 20 at regular intervals of 1.0. Inset shows plots of fluorescence
intensity at 515 nm as a function of anion concentration.
Figure 1. Structures of sensor 1 and control compounds 2 and 3.
mercially available and can be used as a highly sensitive
fluorescent sensor for anions following an excited-state
intermolecular proton transfer signaling mechanism.
was noted in Figure 3 that, while the absorbance of 1 at 360
and 236 nm decreased with increasing F- concentration, two
new peaks appeared at 422 and 264 nm and four distinctive
isosbestic points were observed at 210, 248, 330, and 377
nm, respectively. The fact that the yellow solution of 1 and
F- mixture in acetontrile was returned to colorless when a
protic solvent such as methanol or water was introduced
suggested that the interaction between 1 and F- was
hydrogen bonding in nature, likely at the amide NH11 and
phenol OH12 sites. To confirm this assumption, NMR
titrations13 were carried out. It was found that the amide NH
proton signal was broadened and underwent a continuous
downfield shift from 10.377 to 10.882 to 11.584 ppm with
increasing F- concentration from 0 to 0.5 to 1.0 equiv,
whereas the OH proton signal (11.572 ppm) could not be
similarly followed as it completely disappeared upon addition
of 0.5 equivalents of fluoride ion (Figure 4). These observa-
The choice of 1 as the fluorescent sensor was mainly based
on the fact that (i) 1 contains both OH and amide NH groups
that are well-known to be involved in natural anion-binding
sites of peptides10 and have been extensively employed in
developing anion receptors and sensors11 and (ii) the acidities
of phenolic OH and aromatic amino NH protons are
drastically enhanced upon photoexcitation8 and therefore an
excited-state intermolecular ESPT channel might be opened
upon anion binding. Variations of the fluorescence spectra
of 1 in acetonitrile in the presence of anions such as F-,
AcO-, H2PO4 , Cl-, Br-, and ClO4 (added as tetrabuty-
lammonium salts) were followed. It was found that, whereas
1 emitted only a short-wavelength fluorescence at 392 nm
(λex ) 300 nm) with a very low fluorescence quantum yield
(Φ ≈ 10-4) in CH3CN, a new and substantially red-shifted
emission appeared at 515 nm upon addition of F- and was
dramatically enhanced with increasing F- concentration (see
Figure 2). Meanwhile, the absorption spectrum of 1 under-
went systematic variation when titrated by F-, and the
solution color turned from colorless to yellow (Figure 3). It
-
-
(4) (a) Vance, D. H.; Czarnik, A. W. J. Am. Chem. Soc. 1994, 116, 9397-
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Figure 3. Variation traces of the absorption spectrum of 1 (20
µM) upon titration by tetrabutylammonium fluoride in CH3CN
(step: 2.0 µM). Inset shows plots of absorbance at 422 nm as a
function of anion concentration.
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Org. Lett., Vol. 5, No. 15, 2003

Table 1. Association Constants (Kass, M-1) for Anions with 1
in Acetonitrile from Absorption (422 nm) and Fluorescence (λem
) 515 nm, λex ) 377 nm) Titrationsa
anion
Kass (abs.)
Kass (flu.)
-
Br-, ClO4
Cl-
H2PO4
AcO-
F-
nab
nab
(2.73 ( 1.14) × 103
(1.13 ( 0.06) × 104
(5.94 ( 0.39) × 105
>106 c
(1.00 ( 0.16) × 103
(1.20 ( 0.08) × 104
(4.43 ( 1.08) × 105
>106 c
-
a Anions exist as their tetrabutylammonium salts. b Not available because
of the minor spectral change. c Too high to be accurately determined.
whereas a 1:2 ratio was found between 1 and F-. A Hill
plot suggested that two F- ions bind sequentially to 1. The
association constants (Kass) between 1 and anions were fitted
from nonlinear regressions,15 and the data are listed in Table
1. The fact that 1 shows higher binding affinity to and more
efficient fluorescence enhancement by F- than other anions
is actually not surprising because of its high charge density
and small size, which enables it to be a strong hydrogen
bonding acceptor that shows interaction with phenol or amide
derivatives containing only a single hydrogen bonding donor
group.11g,12
Figure 4. 1H NMR (500 MHz) spectra of 1 in CD3CN + 10%
DMSO-d6 in the presence of (a) 0, (b) 0.5, and (c) 1.0 equiv of
tetrabutylammonium fluoride. Arrows point to the NH proton signal.
Complete 1H NMR spectra can be found in Figure 1SH in
Supporting Information.
To further identify the proton transfer site and possible
anion binding mode, two control compounds 2 and 316
(Figure 1) were synthesized, in which the amide proton and
hydroxyl group were removed, respectively. It was found
that with the N-methyl derivative 2, variations of absorption
and fluorescence spectra upon addition of F-, though similar
to those found with 1, were much less minor, giving
association constants of 103 M-1 in order of magnitude.17
With 3, however, addition of F- did not induce any
discernible spectral change. It is therefore obvious that proton
transfer occurs between the hydroxyl group of the sensor
and anions in the excited state, while both OH and amide
NH in 1 play an important role in its binding to anions. As
observed from Figures 3 and 4, 1 binds anion in the ground
state through hydrogen bonding interaction via amide NH
and phenolic OH.11,12 This leads to an increased local
concentration of the anion, and as a consequence, intermo-
lecular proton transfer in the excited state of 1 to weakly
basic anions occurs,9 with the formation of contact and/or
solvent-separated ion pairs in polar solvents such as CH3-
CN due to the enhanced acidity of the aromatic hydroxyl
tions clearly supported the hydrogen bonding interaction
between 1 and F- involving the amide NH and phenol OH
-
groups. Other anions such as AcO-, H2PO4 , and Cl- were
found to induce similar variations in both the absorption and
fluorescence spectra to extents that depended on the anion’s
basicity, whereas Br- and ClO4- hardly induced any spectral
changes.14 The Job plots for complexation of 1 with anions
obtained from both absorption and fluorescence titrations
pointed to the 1:1 stoichiometry between 1 and AcO-,
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(16) Compounds 2 and 3 were synthesized by reaction of 3-hydroxyl-
2-naphthoyl chloride and 2-naphthoyl chloride with N-methylaniline and
aniline, respectively. Compound 2: 1H NMR (500 MHz, CDCl3, ppm) δ
3.562 (s, 3H), 7.103-7.373 (m, 10H), 7.581 (d, 1H, J ) 8.0 Hz), 10.263
(s, 1H). Compound 3: 1H NMR (500 MHz, DMSO-d6, ppm) δ 7.122 (t,
1H, J ) 8.0 Hz), 7.380 (t, 2H, J ) 7.5 Hz), 7.614-7.666 (m, 2H), 7.830
(d, 2H, J ) 8.0 Hz), 8.012-8.068 (m, 3H), 8.095 (d, 1H, J ) 7.5 Hz),
8.582 (s, 1H), 10.427 (s, 1H).
(13) 1H NMR titrations were carried out in CD3CN + 10% DMSO-d6.
Titrations were also performed in DMSO-d6, and the chemical shifts of
NH and OH protons were found at 10.612 and 11.350 ppm in the absence
of fluoride ion, respectively. The peaks were broadened and shifted to 10.670
and 11.377 ppm in the presence of 0.1 equiv of fluoride ion, and while the
NH proton signal further shifted to 10.961 ppm, the OH proton signal
disappeared upon addition of 0.3 equiv of fluoride ion. Complete 1H NMR
spectra in DMSO-d6 can be found in Figure 2SH in Supporting Information.
(14) Absorption and fluorescence spectra of 1 in the presence of AcO-
-
(Figures 3SA and 3SF) and H2PO4 (Figures 4SA and 4SF) are provided
(17) Absorption and fluorescence spectra of 2-anion complex exhibit
maxima at 400 and ca. 580 nm, respectively.
in Supporting Information.
Org. Lett., Vol. 5, No. 15, 2003
2669

proton.8,18 Actually, similar spectral variations were observed
with 1 in CH3CN in the presence of an organic base
triethylamine. It should be noted that both the absorption
and fluorescence spectra of 2-naphthol, a model molecule
of 1, showed weak responses to F- (Kass ≈ 103 M-1) in CH3-
CN, and a weak fluorescence emission at ca. 420 nm was
observed upon introduction of F-, which is similar to the
observation made in water on the deprotonation of 2-naphthol
by anions where the new emission was assigned to the
naphtholate anion.19 These facts support the occurrence of
proton transfer between the hydroxyl group of sensor 1 and
anions in the excited state. In a recent report,20 it was shown
that the deprotonation of pyrrole NH group in the ground
state by F- induced substantial color change. It would
therefore be assumed that the intermolecular proton transfer
could serve as a new signaling mechanism in constructing
chemosensors for anions.
to form the red-shifted fluorescence emissive state, i.e., direct
excitation from the ground-state 1-anion binding complex
with λex of 422 nm, and the initial excitation of the sensor
moiety in the binding complex (λex ) 300 nm) that leads to
proton transfer from the excited-state sensor to anion.9 The
difference of the ground- and excited-state pKa values of 1
was calculated as 11 on the basis of the Fo¨rster cycle,8a,c,21
which indeed pointed to a dramatically enhanced acidity in
the excited state.
In conclusion, we presented a simple fluorescent sensor
for anions following the excited-state intermolecular proton
transfer signaling mechanism. Although detailed photophysi-
cal studies are required, this signaling mechanism would open
up a new horizon for developing fluorescent chemosensors
for anions.
Acknowledgment. This work was supported by NSFC
(29975023 and 20175020), the Ministry of Education (MOE)
of China, the NSF of Fujian Province (D0220001), and the
German VolkswagenStiftung (I/77 072).
In the presence of anion, the excitation spectrum of 1 (λem
) 515 nm) exhibits variations similar to those observed with
the absorption spectrum. This means that two pathways exist
Supporting Information Available: Complete 1H NMR
spectra of 1 in CD3CN + 10% DMSO-d6 and in DMSO-d6
(Figures 1SH and 2SH) in the presence of F- and absorption
and fluorescence spectra of 1 in acetonitrile in the presence
of AcO- (Figures 3SA and 3SF) and H2PO4- (Figures 4SA
and 4SF).
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OL034846U
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