Quinoline-functionalized norbornene for fluorescence recognition of metal ions

Article abstract of DOI:10.1016/j.jphotochem.2015.03.003

A quinoline-functionalized norbornene (1) and its homopolymer (P1) for metal ions recognition have been synthesized and characterized. In CH₃CN solution, 1 exhibited fluorescence enhancement response toward Hg²⁺, Zn²⁺ and Cu²⁺, with Hg²⁺ resulting in a green emission at 465 nm, and Zn²⁺/Cu²⁺ increasing 1's original emission at 406 nm. NMR titrations and DFT calculations results indicated that 1 bound Hg²⁺ in an imidic acid tautomeric form of the amide quinoline receptor, while Zn²⁺ and Cu²⁺ were bound to 1 in an amide tautomeric form. In addition, owing to the strong imidic acid tautomeric binding mode of 1-Hg²⁺ complex, 1 can recognized Hg²⁺ from Zn²⁺/Cu²⁺ and other competitive metal ions. In contrast, the homopolymer P1, which was obtained by ring opening metathesis polymerization (ROMP) method, exhibited only fluorescence enhancement sensing to Hg²⁺ and Cu²⁺ in an amide tautomeric binding mode with increasing emission at 416 nm.

Full text of DOI:10.1016/j.jphotochem.2015.03.003

Journal of Photochemistry and Photobiology A: Chemistry 305 (2015) 11–18
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Quinoline-functionalized norbornene for fluorescence recognition of
metal ions
Pei-Shui Yao, Qian-Yong Cao *, Rong-Peng Peng, Jing-Hua Liu
Department of Chemistry, Nanchang University, Nanchang 330031, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A quinoline-functionalized norbornene (1) and its homopolymer (P1) for metal ions recognition have
Received 5 December 2014
Received in revised form 27 February 2015
Accepted 3 March 2015
3
been synthesized and characterized. In CH CN solution, 1 exhibited fluorescence enhancement response
2+
2+
2+
2+
2+
2+
toward Hg , Zn and Cu , with Hg resulting in a green emission at 465 nm, and Zn /Cu increasing
’s original emission at 406 nm. NMR titrations and DFT calculations results indicated that 1 bound
1
Available online 7 March 2015
2
+
2+
2+
Hg in an imidic acid tautomeric form of the amide quinoline receptor, while Zn and Cu were bound
to 1 in an amide tautomeric form. In addition, owing to the strong imidic acid tautomeric binding mode of
Keywords:
Quinoline
Norbornene
Fluorescence recognition
Metal ions
2+
2+
2+
2+
1
-Hg complex, 1 can recognized Hg from Zn /Cu and other competitive metal ions. In contrast, the
homopolymer P1, which was obtained by ring opening metathesis polymerization (ROMP) method,
exhibited only fluorescence enhancement sensing to Hg2 and Cu in an amide tautomeric binding mode
+
2+
with increasing emission at 416 nm.
ã 2015 Elsevier B.V. All rights reserved.
1. Introduction
8-hydroxyquinoline and 8-aminoquinoline, are well-known fluo-
rogenic chelators for transition metal ions. Quinoline-base
In recent years, the design and synthesis of fluorescent sensors
for the recognition of heavy and transition metal ions have
attracted considerable attention, because these metal ions play
important roles in living systems and in the environment [1–7].
receptors for fluorescent recognition of metal ions have been
reported widely [24–29]. Among them, the carboxamidoquinoline
derivatives have attracted much attention for their tunable
bonding mode. According to the types of the auxiliary binding
sites and the center metal ions (Scheme 1), the carboxamidoquino-
line ligands can bind with metal ions in an amide tautomeric form
(type I)[30–32], an imidic acid tautomeric form (type II) [33,34],
and a deprotonation amide tautomeric form (type III) [35–38].
With tunable and suitable coordination structure, some sophisti-
cated carboxamidoquinoline-base sensors for recognition of
Especially, chemosensors that can detect Hg2 , Zn and Cu with
+
2+
2+
2
+
high sensitivity and selectivity are more important [8–17]. Hg is
considered as a prevalent toxic metal in the environment, because
it is not biodegradable, and hence can accumulate through the food
2
+
chain. When Hg is absorbed in the human body from the
environment, it can induce a great damage to the kidneys, brain
and central nervous system [18,19]. In contrast, Zn2 and Cu are
essential trace elements in the human body, and play an important
role in many fundamental physiological processes in organisms
+
2+
various metal ions, such as Zn , Cd and Cu , have been
2+
2+
2+
2
+
developed [30–40]. However, systems for recognition of Hg are
still unknown, to the best of our knowledge.
2
+
2+
[
20–23]. So far, a lot of excellent fluorescent sensors for Hg , Zn
It is worthy of noting that the reported quinoline-base
chemosensors are typically small molecular. The polymer an-
chored systems are still rare [41]. Compared to small organic
compounds, polymer based sensors display several important
advantages such as signal amplification and better water solubility
[42–44]. In this regard, we envision that the quinoline-function-
alized polymer such as polynorbornene may give some new sight
in ions recognition. Norbornene is a good monomer, which can be
polymerized by the ring opening metathesis polymerization
2
+
and Cu
have been designed. However, to the best of our
knowledge, examples of simultaneous sensing to these three metal
ions are still rare.
To construct a useful fluorescent chemosensors, it is very
important to select a good fluorophore, which undergo readily
discernible changes in its emission after binding with a given guest
species. These design criteria make systems based on quinoline
attractive.
Quinoline
and
its
derivatives,
particularly
(
ROMP), the vinyl-addition polymerization, and the cationic/
radical polymerization [45,46]. Some polynorbornenes for metal
ions recognition have been reported [47,48]. Herein, we report a
*
Corresponding author. Tel.: +86 791 83969514; fax: +86 791 83969386.
E-mail address: cqyong@ncu.edu.cn (Q.-Y. Cao).
quinoline-functionalized norbornene 1 (Scheme 2) and its
http://dx.doi.org/10.1016/j.jphotochem.2015.03.003
010-6030/ã 2015 Elsevier B.V. All rights reserved.
1

12
P.-S. Yao et al. / Journal of Photochemistry and Photobiology A: Chemistry 305 (2015) 11–18
The non-H atoms were refined anisotropically, using weighted full
2
matrix least-squares on F . Crystallographic data for 1:
,M = 345.35, orthorhombic, space group Pna2
N
N
N
C
20
15
H N
3
O
3
1
,
3
N
O
N
OH
N
O
a = 21.171(3) Å, b = 6.7176(9) Å, c = 23.957(3) Å, V = 3407.2(8) Å ,
H
ꢀ
1
ꢀ3
room temperature, Z = 8,
size 0.30 ꢁ 0.25 ꢁ 0.20 mm , 8385 reflections collected with
2
911 being independent (Rint = 0.0287); the final R and wR(F )
m= 0.093 mm , r= 1.347 g cm , crystal
3
tpye I
tpye II
tpye III
5
1
Scheme 1. Binding mode of carboxamidoquinoline ligands.
values were 0.0762 (all data) and 0.1254, respectively; goodness-
of-fit on F = 1.020. The CIF deposition number is CCDC-1026835.
2
.4. Synthesis
homopolymer P1, and investigate their metal ions sensing
1
properties by UV–vis, emission spectra, H NMR titrations and
2.4.1. Synthesis of the exo-norbornene-2,3-dicarboximide (2)
DFT calculations.
5-Norbornene-2,3-dicarboxylic anhydride (15.03 g, 91.4 mmol,
1
equiv) and aqueous ammonia (25%, v/v, 100 mL) were added in a
2. Experimental
round-bottom flask equipped with reflux condenser and drying
tube, and then heated to reflux for 48 h. Crystallization occurred
2
.1. Materials and instruments
over night, and the colorless crystal was filtered and washed with
1
hexane. Yield: 11.52 g (70.6 mmol), 77%. H NMR (400 MHz, CDCl
3
),
CH—
2
-bridge); 1.51
All the starting materials for synthesis were commercially
d
(ppm): 8.37 (brs, 1H, NH); 6.18 (s, 2H, C
CH); 3.29 (s, 2H, CO—CH); 1.72 (d, 1H, J = 8.0 Hz, CH
(d, 1H, J = 12.0 Hz, CH -bridge).
¼
CH); 3.36 (s, 2H,
¼
available and used as received. The solvents used for titration
1
measurements were purified by standard procedures. H NMR and
2
13
C NMR were measured on a Bruker AV-400 spectrometer with
chemical shifts in ppm (in DMSO-d or CDCl , TMS as internal
2.4.2. Synthesis of the 2-chloro-N-(quinol-8-yl)-acetamide (3)
6
3
standard). ’Absorption spectra were recorded with a Hitachi
U-4100 spectrophotometer (Hitachi Hi-Tech, Tokyo, Japan). Fluo-
rescence spectra measurements were performed on a Hitachi
F-7000 FL spectrophotometer (Hitachi Hi-Tech, Tokyo, Japan).
First, 500 mg (3.45 mmol) 8-aminoquinoline was dissolved in
ꢂ
30 mL dry dichloromethane at 0 C. Then 2 mL of dry Et
3
N and
0.5 mL of chloroacetyl chloride were added to the solution over a
few minutes. The resulting reaction mixture was stirred for 3 h.
After completion of the reaction, the resulting reaction mixture
was neutralized by saturated sodium bicarbonate solution
followed by addition of dichloromethane. The organic layer was
2.2. Preparation of UV–vis and emission titration solutions
Stock solutions of metal ions (0.01 M) were prepared in CH
3
CN
4
extracted and dried over MgSO . Pure 3 was obtained by column
solution. The concentration of 1 and P1 in the UV–vis and emission
titrations was 0.03 mM in CH CN solution. During the titration,
chromatography with ethylacetate/n-hexane (1/1, v/v) as the
1
eluent. Yield: 81%. H NMR (CDCl
3
, 400 MHz, TMS): d (ppm): 10.92
3
metal ion solution was added into a solution of 1 (2 mL) using a
micro injector, and the whole volume of the final system can be
considered constant because the volume of metal ion solution
added is negligible compared to that of 1’s solution.
(NHCO, s, 1H), 8.82 (d, J = 2.8 Hz, 1H), 8.76 (d, J = 6.4 Hz, 1H), 8.20 (d,
J = 8.4 Hz, 1H), 7.59 (m, 2H), 7.50 (m, 1H), 4.32 (COCH
2.4.3. Synthesis of 1
2
, s, 2H).
First, 3 (450 mg, 2.04 mmol), 2 (200 mg, 0.99 mmol), potassium
2.3. X-ray crystallographic analysis
carbonate (200 mg) and tetrabutylammonium bromide (20 mg)
were added to 50 mL CH
heated to reflux for 4 h. Then the solvent was removed under
reduced pressure and CH Cl (50 mL) was added. The organic layer
was washed with water, dried over anhydrous Na SO and
3
CN. The resulting mixture was stirred and
Single crystals of 1 were obtained by direct diffusion of hexane
into its CH
glass fiber, and diffraction measurements were taken with a Bruker
Smart APEX CCD-based diffractometer with Mo K graphite
2
Cl
2
solution. A suitable single crystal was mounted in a
2
2
2
4
a
evaporated under vacuum. The crude product was purified via
monochromated radiation. The structure was solved by direct
methods using the SHELXL-97 program. The refinement and all
further calculations were carried out using SHELXL-97 program.
column chromatography using methanol/dichloromethane
(2:98, v/v) as eluent to obtain a white solid product (300 mg,
1
52%). H NMR (CDCl
3
, 400 MHz, TMS):
d(ppm): 9.87 (NHCO, s, 1H),
O
a
O
O
NH
O
O
n
2
c
O
d
N
O
O
O
N
N
N
H
O
N
b
O
HN
1
N
O
N
NH
Cl
NH2
P1
3
ꢂ
3 2 3
Scheme 2. Synthesis route of 1 and P1. (a) aqueous ammonia, reflux for 48 h. (b) dry DCM, Et N, chloroacetyl chloride, 0 C, 3 h. (c) Dry acetonitrile, K CO , TBAB, reflux, 4 h. (d)
G1, dry DCM.

P.-S. Yao et al. / Journal of Photochemistry and Photobiology A: Chemistry 305 (2015) 11–18
13
Fig. 1. X-ray structure of 1. Hydrogen atoms except amide N–H are omitted for clarity.
8
.80 (d, J = 4.0 Hz, 1H), 8.67 (t, J = 8.0 Hz, 1H), 8.16 (d, J = 8.0 Hz, 1H),
CH); 4.35, (s, 2H, —H2C—
CH—CH, CO—CH); 1.78 (d,1H,
disappeared with appearing a new peak in 5.62 ppm, and the two
unequivalent bridged methylene protons (1.78 and 1.59 ppm)
merged to one signal (1.59 ppm in CDCl , and 1.50 ppm in DMSO).
3
The amide N–H proton of P1 was found in 9.93 ppm, which
indicated that the strong intramolecular hydrogen bond still
retains in the polymer.
The strong intramolecular hydrogen bond in 1 was also
confirmed by the X-ray crystal analysis (Fig. 1). Single crystal of
7.51 (d, 2H), 7.45 (m, 1H), 6.25 (s, 2H, C
¼
CO), 3.43 (s, 2H); 3.45 (s, 2H) (d, 4H,
¼
13
J = 8.0 Hz, CH
2
-bridge); 1.59 (d,1H, J = 12.0 Hz, CH
), (ppm) 176.96, 163.73, 148.19, 138.21, 136.34,
34.71, 133.78, 127.86, 127.27, 121.99, 121.65, 116.87, 52.25, 46.21,
2
-bridge). C NMR
(
1
4
100 MHz, CDCl
3
d
4.96, 42.09.
2.4.4. Synthesis of P1
2 2
1 was obtained by slow diffusion of hexane to its CH Cl solution.
First, 87 mg (0.25 mmol) 1 and 41.5 mg (0.05 mmol) first
There are two independent molecules in the cell. The X-ray
structure reveals that a strong intramolecular hydrogen bond
between the amide N–H donor and the quinoline ring N atom
generation Grubbs catalyst (G1) were dissolved in 3 mL dry
dichloromethane under nitrogen. The mixture was stirred at room
temperature for 24 h, and then 1 mL ethyl-vinyl ether was added to
quench the polymerization. The resulting polymer was isolated by
ꢂ
exists (N–Hꢃ ꢃ ꢃN, 2.26 Å, 2.659(4) Å, 109 ) to give a planar five-
membered ring. In addition, the two planar rings (the quinoline
ring and the dicarboximide ring) in the title compound are not
precipitating in cold ether and concentrated under vacuum.
1
ꢂ
(
36.9 mg, 42.5% yield). H NMR (DMSO, 400 MHz):
d10.5 (s, 1H),
coplanar, with the dihedral angle of 86.96(13) . This result may be
8
2
.9 (s, 1H), 8.1–8.6 (br, 2H), 7.3–7.7 (br, 2H), 7.2 (s, 1H), 5.2–5.6 (br,
H), 4.5 (s, 2H), 2.9–3.3 (br, 2H), 2.4–2.8 (br, 2H), 1.1–1.8 (br, 2H).
attributed to the weak emission of the title compound.
Gel permeation chromatography (GPC) measurements gave the
molecular weight of 27,000 with PDI = 1.8.
3.2. Cations sensing properties of 1
The response of 1 toward various metal ions (Hg2+, Cu2+, Cd2+
,
+
2+
2+
2+
2+
2+
2+
+
2+
Ag , Co , Ni , Zn , Ba , Mg , Ca , Na , Fe ) was firstly
investigated by UV–vis titration in CH CN solution. The free ligand
exhibits a typical quinoline-based absorption band centered at
3. Results and discussion
3
1
3.1. Synthesis and characterization
4
ꢀ1
ꢀ1
315 nm
(e= 2.1 ꢁ10 M cm ). Upon the addition of equiv
amount (5.0 equiv) of metal ions, 1 only shows UV–vis response
to Hg
Cu
The synthesis route of 1 and P1 was shown in Scheme 2. With
2+
2+
2+
,
Zn
and Cu
(Figs. S1–S3). The introduction of
commercial available 5-norbornene-endo-2,3-dicarboxylic anhy-
dride and 8-aminoquinoline as starting materials, we firstly
prepared the two key precursors exo-norbornene-2,3-dicarbox-
imide (2) and 2-chloro-N-(quinol-8-yl)-acetamide (3) according to
the literature methods [49]. The quinoline-appended norbornene
monomer 1 was then easily obtained by condensation of 2 and 3
2+
produces
a
new low-energy absorption at 373 nm
with K
in CH CN solution. Its structurewas confirmed by H NMR, C NMR,
ESI-MSspectraspectroscopyandX-raysinglecrystallography.Inthe
2 3
CO /TBAB (tetrabutyl ammonium bromide) as the catalysts
1
13
3
1
3
H NMR spectrum in CD Cl solution, the norbornene double bond
protons were located in 6.25 ppm, and the two bridged methylene
protons were unequivalent with the position at 1.78 and 1.59 ppm,
respectively, which is in accord with some reported results [47]. In
addition, the amide N–H proton of 1 was located in the far down-
filed position (9.87 ppm), which may be attributed to the strong
intramolecular hydrogen bond formation between the amide N–H
donor and the N acceptor of the quinoline ring.
After successful synthesis the pure monomer 1, polymerization
of this monomer was carried out by using the first generation
Fig. 2. The Bar charts of emission at 465 nm of 1 before and after addition 5 equiv of
cations (Hg , Cu , Cd , Ag , Co , Ni , Zn , Ba , Mg , Ca , Fe , Na ) in CH CN
3
solution. Inset shows fluorescence emission spectra of 1 upon the addition of
various of metal ions. (For interpretation of the references to color in text, the reader
is referred to the web version of this article.)
2
+
2+
2+
+
2+
2+
2+
2+
2+
2+
2+
+
2 2
Grubbs’ catalyst at room temperature in CH Cl solution. The
1
formation of polymer P1 can be confirmed by the H NMR
spectroscopy: the double bond protons of monomer 1 in 6.25 ppm

14
P.-S. Yao et al. / Journal of Photochemistry and Photobiology A: Chemistry 305 (2015) 11–18
3
ꢀ1
ꢀ1
(
2
e
= 6.4 ꢁ10 M cm ), with appearance two isosbestic points at
88 nm and 338 nm, respectively, which indicates a neat intercon-
version between the uncomplexed and complexed species.
2
+
2+
However, the addition of Hg and Zn to 1 leads to appearance
weak absorption band at 350–400 nm.
The fluorescent recognition abilities of 1 toward various metal
ions in CH CN solution are shown in Fig. 2. Compound 1 has a very
3
weak emission at about 406 nm with excitation at 315 nm, which
may be attributed to photo induced electron transfer (PET)
quenching effect from the quinoline ring to the norbornene motif
[50]. Upon the addition of equal amount (5 equiv) of metal ions, 1
exhibited selective and large fluorescent enhancements response
2
+
2+
2+
2+
to Hg , Zn and Cu with different manner. The addition of Zn
2
+
and Cu induced increasing the original emission in 1 without
changing its wavelength, while Hg caused a new low-energy
2
+
emission at 465 nm. This difference in response allows 1 to easily
Fig. 3. The emission titration spectra of 1 upon the additional of Hg2+. Inset shows
fluorescence with [Hg ] at 406 nm and 465 nm of 1.
2
+
2+
2+
2
+
distinguish Hg from Zn /Cu and other metal ions, since the
2
+
2+
2+
1-Hg
complex shows green emission, 1-Zn
and 1-Cu
complexes show blue emission, and other metal ions show no
emission. In contrast, little emission spectra changes were
Fig. 4. 1H NMR titration of 1 upon the additional of Hg2+ (a) and Zn2+ (b) in CD
3
2
CN solution. The ~ denotes the H O proton. The labeled protons are shown in Scheme 3.

P.-S. Yao et al. / Journal of Photochemistry and Photobiology A: Chemistry 305 (2015) 11–18
15
Scheme 3. The possible binding mechanisms of 1 and P1 with metal ions.
observed upon the addition of other metal ions, such as Ag , Co2+,
+
sites. To understand the role of norbornene anhydride moiety, the
fluorescence recognition ability of the precursor 3 to various metal
ions were also investigated. It revealed that 3 only showed
2
+
2+
2+
2+
2+
2+
+
Ni , Zn , Ba , Mg , Ca , Fe , Na , even at a large amount.
2+
2+
2+
The emission titrations of 1 with Hg , Zn and Cu were
2+
2+
2+
carried out (Fig. 3, Figs. S4–S5). The titration of Hg to 1 revealed a
two step responses: when 0–1.0 equiv of Hg2 was added, the
emission of 1 at 406 nm increased quickly at first, and then a new
peak at 465 nm appeared (>1.0 equiv Hg2 ) with the intensity
stronger than that at 406 nm at last. The appearance new longer
wavelength at 465 nm indicated that the quinoline N atom of 1
takes part in binding with Hg2 (vide infra), which enhances the ICT
fluorescent response to Cu and Hg (Figs. S12–S13). The addition
+
2+
of Cu induces the increasing emission at 416 nm, and the addition
2
+
of Hg leads a strong emission at 416 nm, and a shoulder emission
+
at about 470 nm. In addition, compared with 1, it need more
2
+
2+
amount of Cu /Hg ions to saturate 3’s emission. These results
indicate that the O atom of the norbornene anhydride group in 1
really participates in binding with metal ions.
+
1
(
intramolecular charge transfer) process of the heterocyclic
To obtain more information about the binding modes, H NMR
2
+
2+
quinoline ring. The emission changes of 1 were saturated with
the addition of about 5.0 equiv Hg2 , with the CHEF (chelation
enhanced fluorescence) factors of 12 at 406 nm, and 70 at 465 nm,
respectively. The binding stoichiometry of 1-Hg2 complex was
obtained by the Job’s plot based on the fluorescence titration at
titrations of 1 with Hg and Zn were also conducted in CD
3
CN
solution (Fig. 4). Upon the addition of Hg , the amide NH proton
(H ) at 9.8 ppm showed a little upfield shift and gradually
+
2+
1
+
2+
diminished at first (0–0.5 equiv). When more Hg was added
(>0.5 equiv), a new peak at 4.5 ppm, which can be attributed to the
4
0
06 nm. The maximal fluorescence intensity appears at about
1
imidic acid tautomer OH proton (H ), appeared and increased.
0
.5 mol fraction (Fig. S6), indicating that 1-Hg2 complex has a 1:1
+
These indicated that 1 binds Hg in the imidic acid tautomer form
as shown in Scheme 3. Similar imidic acid tautomer binding mode
of 8-amidoquinoline derivatives has been reported in literature
[33,34,54], which often shows a large up-field shift in the amide
proton. The amide to the imidic acid tautomer transformation
interrupts the intramolecular NH–N hydrogen bond, which makes
the protons in the quinoline ring a moderate down-field or up-field
2+
stoichiometry. Using the Benesi–Hildebrand equation [51,52], the
binding constant
of the 1-Hg2 was calculated to be
+
K
a
4
ꢀ1
4
.75 ꢁ10 M , and the detection limit [53] was also measured
ꢀ
6
2+
2+
to be 3.9 ꢁ10 M (Fig. S7). The titrations of Zn and Cu to
1
showed similar response with increasing the original emission in
2
+
2+
4
06 nm, but Cu
showed stronger binding than Zn . Job’s
2
+
2+
1
2
plot of Zn /Cu and 1 gives a 1:2 stoichiometry complexation
shift. In contrast, the H NMR changes of 1 upon the addition of Zn
2
+
2+
+
2+
(
Figs. S8–S9). The binding constants of 1
2
-Zn and 1
2
-Cu were
were marginal, which indicated that Zn was bound to 1 in the
8
ꢀ2
8
ꢀ2
calculated to be 1.43 ꢁ10 M
Figs. S10–S11).
Motivated by the unusual fluorescence response, we then
and 2.8 ꢁ 10 M , respectively
amide tautomer [33,34]. In this binding form, the amide O atom
(
and the norbornene anhydride O atom of 1 participate in binding
2
+
with bind Zn in a 2:1 stoichiometry (Scheme 3), with retaining
2
+
2+
wanted to make clear the binding modes between 1 and Hg , Zn
and Cu . In 1, the O atom of the norbornene anhydride, and the N
and O atoms of the carboxamidoquinoline are potential binding
the strong intramolecular N–Hꢃ ꢃ ꢃN hydrogen bond in 1. In addition,
2
+
1
+
2+
the H NMR titrations of 1 with Ag and Cd ions, which often
2
+
interfere in the Hg binding ability for the similar size and

16
P.-S. Yao et al. / Journal of Photochemistry and Photobiology A: Chemistry 305 (2015) 11–18
Fig. 5. (a) Optimized conformation of 1-Hg2+ by DFTcalculation. The red denotes O, blue N and gray for carbon respectively. Selected distances (Å) and angles ( ): Hg—O, 2.22,
ꢂ
2
+
Hg—N1, 2.27, Hg—N2, 2.38, N1—Hg—N2, 74.91, N2—Hg—O2, 103.78. (b) Optimized conformation of 1-Cu by DFT calculation. The red denotes O, blue N and gray for carbon
ꢂ
respectively. Selected distances (Å) and angles ( ): Cu—O1,1.95, Cu—O2, 2.01, O1—Cu—O2, 95.77, O1—Cu—O2A, 91.23. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
softness, were also investigated (Fig. S14). Little chemical shift
changes were observed upon the addition of these two metal ions,
which is accord with the emission titration results.
We could not get the 1H NMR titration of 1 with Cu for its
2+
2+
2+
paramagnetic effect. We assumed that 1 binds Cu and Zn in a
similar mechanism, owing to the similar fluorescence response of 1
with Cu2+ and Zn . Indeed, the deprotonation of amide NH group
2+
2+
does not happen during the interaction of 1 and Cu , because the
deprotonation of the NH group would cause a quenching effect in
emission spectra [32,33].
The proposed binding modes between 1 and Hg2+, Zn2+ and
Cu2+ were also confirmed by the density functional theory (DFT)
calculations. As shown in Fig. 5(a), the optimized configuration of
2
+
1
- Hg shows that 1 adopts an imidic acid tautomer binding mode
2
+
with Hg . The norbornene anhydride O atom and two amidoqui-
noline N atoms in 1 participate in coordination with the Hg
2
+
3
Fig. 6. The fluorescence responses of 1 (10 mM in CH CN solution) in the presence
2+
of various metal ions alone (50 mM, black) and interfering ions with Hg (50 mM,
red), respectively. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
center, with the bond lengths of O—Hg 2.22 Å, N1–Hg 2.27 Å, and
Hg–N2 2.385 Å, respectively. The optimized configurations of

P.-S. Yao et al. / Journal of Photochemistry and Photobiology A: Chemistry 305 (2015) 11–18
17
Cu2+ and Zn2+ by transformation of metal ions chelation from an
amide to an imidic acid tautomeric.
3
.3. Cations sensing properties of P1
The recognition abilities of P1 with metal ions were also
investigated. P1 showed similar quinoline-based absorption and
emission features as monomer 1, which indicated that the ground
and excited state of the quinoline in P1 is minimally affected by the
neighboring quinoline moieties. The UV–vis and emission titration
2
+
2+
results revealed that P1 shows only response to Hg and Cu , but
2
+
not to Zn and other metal ions (Figs. S15–S16). The emission
spectra of P1 upon the addition of various metal ions were shown
in Fig. 7(a), which show a large emission enhancement response of
P1 with Hg and Cu (Fig. 7(b) and Fig. S17). The emission
titrations of P1 with Cu exhibited that P1 shows similar emission
response to Cu as monomer 1 with increasing the original
emission in 416 nm. It is very interesting that the addition of Hg
also shows similar fluorescence response as Cu , which is large
different that of the monomer 1. These results revealed that both
Hg and Cu were bound to P1 in the amide tautomer (Scheme 3).
2
+
2+
2
+
2
+
2
+
2
+
2
+
2+
2
+
2+
The binding constants of P1 with Hg and Cu are calculated in a
9
ꢀ2
1
1
:2 binding mode, with the values of 1.11 ꢁ10 M
and
8
ꢀ2
.99 ꢁ10 M , respectively (Figs. S18–S19). These results exhib-
2+ 2+
ited that P1 shows stronger binding ability to Hg than Cu
.
4
. Conclusions
In conclusion, a quinoline-functionlized norbornene monomer
, and its homopolymer P1 prepared by ROMP method, have been
synthesized and characterized. The fluorescence, NMR and DFT
1
2
+
calculations studies indicated that 1 bind Hg in an imidic acid
tautomeric form in CH CN solution, which made 1 show green
emission at 465 nm. However, 1 binds Zn and Cu in an amide
3
2
+
2+
Fig. 7. (a) The emission spectra of P1 before and after addition 5 equiv of cations
2
+
2+
2+
+
2+
2+
2+
2+
2+
2+
2+
2+
(
(
3
Hg , Cu , Cd , Ag , Co , Ni , Zn , Ba , Mg , Ca , Fe , Pb ) in CH CN solution.
tautomeric form with blue emission at 406 nm. Owing to the
2
+
b) The emission titration spectra of P1 upon the additional of Hg . Inset shows
2+
2
+
strong imidic acid tautomeric binding mode of 1-Hg complex, 1
fluorescence with [Hg ] at 416 nm of P1.
2
+
2+
2+
can recognized Hg from Zn /Cu and other competitive metal
ions by the displacement approach. In contrast, the homopolymer
2
+
2+
- Zn2+ and 1
-Cu2 + complexes exhibited that the two O atoms of
2
P1 bind Hg and Cu in an amide tautomeric binding mode,
which makes P1 a large enhancement in its original emission at
1
1
2
participate in binding with the metal ions (Fig. 5(b) and Fig. S14),
416 nm.
with the two ligands in a head-to-tail conformation. The DFT
calculation results agree with our foregoing hypothesis.
The proposed binding modes in Scheme 3 can explain the
Acknowledgements
different fluorescence response of 1 with Hg2 , Zn and Cu
+
2+
2+
.
Owing to the noncoplanar geometry, free ligand 1 shows weak
This work was supported by the National Nature Science
Foundation of China (Nos. 21162017 and 21462027), Jiangxi
Province Nature Science Foundation (20132BAB203008), Jiangxi
Province Department of Education Science and Technology Project
(GJJ13111).
emission for the PET quenching from the quinoline ring to the
norbornene moiety. When binding with Cu2 or Zn , the PET
process is inhibited for increasing the coplanarity of the ligand and
reducing the loss of energy via non-radiative transitions. PET
mechanism was often proposed for the systems where it could only
change the fluorescence intensity. The turn-on fluorescence
response with red shift of 1 with Hg2 can be attributed to binary
effects of photoinduced electron transfer (PET) and intermolecular
charge transfer (ICT) mechanisms, because the binding quinoline N
+
2+
Appendix A. Supplementary data
+
Characterization data of compounds and additional spectra
data have been described in supporting information.
2
+
with Hg increases the electron-withdrawing ability to shift the
emission low energy [55].
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/j.
Competition experiments of 1 with Hg2+ in the presence of
other metal ions were also investigated (Fig. 6). The emission
jphotochem.2015.03.003.
2
+
profile of the 1-Hg complex was unperturbed in the presence of
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2
+
2+
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