Fluorescence-enhanced chemosensor for metal cation detection based on pyridine and carbazole

Article abstract of DOI:10.1021/jo401808c

A series of donor-acceptor systems incorporating a carbazole moiety as the donating unit and pyridine moiety as the accepting unit have been designed and synthesized. The spectroscopic and electrochemical behaviors of the carbazole derivatives demonstrate that the carbazole unit interacts with the electron-accepting group through the π-conjugated spacer, thus leading to the intramolecular charge transfer (ICT). The pyridine-substituted carbazole derivatives show significant sensing and coordinating properties toward a wide range of metal cations. Compound S2 exhibits fluorescence enhancement upon association with transition metal cations, and compound V3 shows high selectivity for Cu²⁺ among this series of materials. DFT calculations indicate the different association abilities of the dyes and the enhancement of ICT upon addition of the metal cations.

Full text of DOI:10.1021/jo401808c

Article
pubs.acs.org/joc
Fluorescence-Enhanced Chemosensor for Metal Cation Detection
Based on Pyridine and Carbazole
Xin Jiang Feng,*,† Pin Zhan Tian,† Zheng Xu,*,† Shao Fu Chen,† and Man Shing Wong*,‡
†Key Laboratory of Organosilicon Chemistry and Material Technology of the Ministry of Education, Hangzhou Normal University,
Hangzhou 310012, P. R. China
‡Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR,
China
S
* Supporting Information
ABSTRACT: A series of donor−acceptor systems incorporat-
ing a carbazole moiety as the donating unit and pyridine
moiety as the accepting unit have been designed and
synthesized. The spectroscopic and electrochemical behaviors
of the carbazole derivatives demonstrate that the carbazole unit
interacts with the electron-accepting group through the π-
conjugated spacer, thus leading to the intramolecular charge
transfer (ICT). The pyridine-substituted carbazole derivatives
show significant sensing and coordinating properties toward a
wide range of metal cations. Compound S2 exhibits
fluorescence enhancement upon association with transition
metal cations, and compound V3 shows high selectivity for
Cu2+ among this series of materials. DFT calculations indicate the different association abilities of the dyes and the enhancement
of ICT upon addition of the metal cations.
derivatives exhibited multiphoton absorption properties and
interaction with biological systems in our previous research.28
We think that the intramolecular charge transfer (ICT) in
neutral compounds of this kind could be tuned by the
modification of the conjugated cores and the molecule
dimensions. Furthermore, changes in ICT induced by metal
cation binding could exhibit spectroscopic responses, which
could enable the use of these dyes as chemosensors.
In this work, the synthesis of carbazole−pyridine derivatives
by Heck coupling and the Wadsworth−Emmons reaction is
described. Elongating the bridging unit by inserting an aryl ring
or changing the molecule dimensions gives rise to a change in
ICT. The interaction of carbazole−pyridine derivatives with an
external stimulus of metal cations can tune the ICT, resulting in
remarkable changes in absorption spectra, fluorescence spectra,
1H NMR spectra, and electrochemical properties. Geometry
optimizations and orbital energies of the dyes and two metal-
associated species were calculated by density functional theory
(DFT).
INTRODUCTION
■
Heavy metal ion contamination poses significant risks to
environmental systems. As a result, the design and synthesis of
chemosensors with high selectivity and sensitivity for heavy
metal ions is a domain of wide interest.1−9 Among the
detection approaches, fluorescence spectroscopy is widely used
because of its high sensitivity, simple application, and low-cost
instrumentation. However, fluorescent sensors for heavy metal
ions have remained rare to date10−13 because heavy metal ions
are known to quench fluorescence emission via enhanced spin−
orbit coupling14 or energy or electron transfer.15 On the other
hand, communication between the donor (D) and acceptor (A)
in donor−acceptor (D−A) molecules has been studied
widely,1,4,16−19 and detection of heavy metal cations by
spectroscopic and electrochemical methods in such studies
has been reported.
Recently, carbazole derivatives have been shown to be
promising materials for two-photon applications20−22 and
molecular assembly.23 The carbazole moiety plays the role of
the donor in various D−A systems and can be easily
functionalized by classical reactions including Heck cross-
coupling, Sonogashira cross-coupling, and Wittig−Horner
reactions.24−27 Also, pyridine-related π-conjugated derivatives
with donor−acceptor properties involving the lone-pair
electrons on the pyridine nitrogen have been used as heavy
metal sensors.2 Pyridine can constitute π-conjugated donor−
acceptor molecules when coupled with carbazole by different π-
conjugated linkers. Methylated salts of carbazole and pyridine
RESULTS AND DISCUSSION
■
Synthesis. Heck coupling and the Wadsworth−Emmons
reaction were used to synthesize the carbazole−pyridine
derivatives. Their syntheses are outlined in Scheme 1.
Palladium-catalyzed Heck coupling of 1a and 1b with 4-
Received: August 24, 2013
Published: October 30, 2013
© 2013 American Chemical Society
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spectra are solvent-dependent, exhibiting less or no vibronic
structure and large red shifts with increasing solvent polarity.
For instance, the emission maxima of S3 exhibit solvatochromic
shifts of 33 and 37 nm when the solvent is changed from
toluene to chloroform and from chloroform to acetonitrile,
respectively (Figure 2). Furthermore, the solvatochromatic
effect is much larger for the chromophores with longer
conjugation lengths than for the lower homologues, indicating
that the electronic communication between the carbazole
moiety and the pyridine moiety can be optimized by varying
the conjugation length.
Scheme 1. Syntheses of S2, V2, S3, and V3
Electrochemical Properties. To probe the redox proper-
ties of the carbazole-based π-conjugated chromophores, cyclic
voltammetry (CV) was carried out in a three-electrode cell
setup with 0.1 M Bu4NPF6 as a supporting electrolyte in
CH2Cl2. The HOMO and LUMO energies of these
compounds were calculated from the oxidation and reduction
potentials and are listed in Table 1. Comparison of S2 with S3
and V2 with V3 showed that EHOMO increases and ELUMO
decreases with increasing conjugation length, indicating that the
energy gap of this series of molecules can be tuned by varying
the conjugation length. Such changes in the energy gap could
be attributed to the ICT, which in turn could tune the binding
affinity of the nitrogen on the pyridine ring to metal cations.
1
Metal Cation Association Study. H NMR, UV−vis, and
vinylpyridine using Pd(OAc)2/2P(o-tolyl)3 as a catalyst
afforded S2 and V2 in yields of 67% and 78%, respectively.
Wadsworth−Emmons reaction of 2a and 2b with diethyl 4-
bromobenzylphosphonate gave 3a and 3b in moderate to good
yields of 61% and 83%, respectively. S3 and V3 were prepared
via Heck coupling of 3a and 3b with 4-vinylpyridine in good
yields of 84% and 74%, respectively.
Photophysical Properties. The photophysical character-
istics of the new oligomers S2, V2, S3, and V3 were examined
by UV−vis and fluorescence spectroscopy. The absorption and
fluorescence spectra of these oligomers are depicted in Figure 1
and Figures 1S and 2S in the Supporting Information, and the
spectral data are summarized in Table 1. In view of the
absorption spectra obtained in toluene, chloroform, and
acetonitrile, all of the oligomers have similar absorption
spectral features, which are basically composed of two major
absorption bands. The shorter-wavelength absorption peak
appears around 300 nm and is assigned to the n → π*
transition of the carbazole moiety, while the lower-energy
absorption peaked at 338−399 nm arises from the π → π*
transition of the π-conjugated core. In general, the absorption
spectra of these oligomers do not exhibit significant
solvatochromic effects. On the contrary, the fluorescence
fluorescence spectroscopic titrations and cyclic voltammetry
were used to evaluate the binding properties of the compounds
toward various metal ions in acetonitrile.
1
1
Binding Properties: H NMR Titrations. H NMR titrations
were performed in 5 × 10−4 M acetonitrile-d3 with addition of a
5 × 10−2 M solution of Pb(ClO4)2 in acetonitrile-d3. The
chemical shifts of the resonances associated with the pyridyl
1
protons in the H NMR spectrum changed significantly upon
complexation with Pb2+ ion. The signals of all the protons in
the carbazole moiety shifted downfield, but the shifts were
small compared with that of the proton at the 3-position of the
pyridyl ring (Figure 3 and Figures S3 and S4 in the Supporting
Information). The maximum chemical shifts of H-3 on the
pyridyl ring were 0.531, 0.557, and 0.437 ppm for S2, S3, and
V2, respectively (V3 dissolved too poorly in acetonitrile for an
NMR titration to be performed). These observations confirm
that Pb2+ interacts with the nitrogen atom on the pyridyl group,
thus leading to the ICT in these molecules.
1
The variation of H NMR spectra upon titration with Pb2+
and the plots of normalized change of chemical shift versus
cation molar fraction (Figure S5 in the Supporting
Information) show that S2 and S3 exhibited saturation of the
Figure 1. (a) Absorption and (b) fluorescence spectra of chromophores S2, S3, V2, and V3 in CH3CN.
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Table 1. Summary of Linear Optical Measurements of Neutral Carbazole-Based Chromophores in Various Solvents
toluene
(λemmax) (nm)
chloroform
(λemmax) (nm)
acetonitrile
λ
(λemmax) (nm)
max
a
a
a
c
c
d
abs
max
abs
max
abs
λ
ΦFL
λ
ΦFL
ΦFL
EHOMO (eV)
ELUMO (eV)
Tdec (°C)
b
b
b
S2
S3
V2
V3
341 (412)
387 (468)
360 (408)
398 (450)
0.012
0.48
342 (433)
387 (501)
364 (436)
399 (499)
0.012
0.57
338 (445)
379 (538)
360 (455)
393 (545)
0.043
0.48
−5.32
−5.23
−5.31
−3.16
−3.18
−3.14
−3.25
−
439
436
436
b
b
b
0.086
0.26
0.055
0.093
0.20
0.46
−5.24
a
d
b
c
Using quinine sulfate monohydrate (Φ350 = 0.58) as a standard. Averages of two independent measurements. Estimated from CV measurements.
Determined using a thermogravimetric analyzer with a heating rate of 20 °C/min under N2.
⎛
⎜
⎞
⎟
δ0
δ0 − δ
δ0
nC0
δ0
1
=
+
nC0Ks ⎝ [M] ⎠
(1)
where δ and δ0 are the chemical shifts for proton in the
presence and absence of metal ion, respectively, n is the ratio of
the dye and Pb2+ ion, C0 is the dye concentration, Ks is the
binding constant, and [M] is the concentration of the Pb2+ ion.
The binding constants for S2 and S3 are 3.12 × 105 and 1.89 ×
105 M−1, respectively. The fact that the binding constant for S2
is larger than that for S3 indicates that S2 exhibits stronger
affinity than S3 when binding to Pb2+ ion. The result also shows
that ICT in carbazole−pyridine compounds can be tuned by
varying the conjugation length, thus leading to different
affinities for association with metal cations.
Figure 2. Solvatochromic effect of S3 in toluene, chloroform, and
Binding Properties: Cyclic Voltammetry. The progressive
addition of Pb(ClO4)2 to solutions of S2, S3, and V2 in
CH3CN in the presence of n-Bu4NPF6 caused significant
modification of the oxidation potentials in CVs, with substantial
positive shifts of the first oxidation potential from 647 to 872
mV, 519 to 620 mV, and 652 to 881 mV for S2, S3, and V2,
respectively (Figure 4 and Figure S7 in the Supporting
Information). However, data for V3 could not be obtained
because of its poor solubility in acetonitrile. The positive shifts
of the first oxidation potentials arise from the reduced electron
density located in the carbazole moiety because of increased
ICT upon binding of the pyridine nitrogen with Pb2+ ion. On
the other hand, S2 and S3 showed saturation of the positive
shift when 1 equiv of Pb2+ was added, while V2 showed
saturation of the positive shift when 2 equiv of Pb2+ was added.
The results also indicate that metal cation is complexed by only
the nitrogen atoms on pyridine rings and that one pyridyl
nitrogen associates with one metal cation in solution. The more
acetonitrile.
change in chemical shift when 1 equiv of Pb2+ ion was added,
while V2 exhibited saturation of the change in chemical shift
when 2 equiv of Pb2+ ion were added, which implies that every
nitrogen on a pyridyl ring in all three compounds associates
with one Pb2+ ion. Furthermore, Job’s plots of the change in
proton chemical shift upon molar equivalent addition of
Pb(ClO4)2 also indicated the 1:1 stoichiometries of the
complexes between S2 or S3 and Pb2+and the 1:2 stoichiometry
of the complex between V2 and Pb2+ (Figure S6 in the
Supporting Information)
The 1:1 stoichiometry for binding of Pb2+ ion by S2 and S3
was also evidenced by the close agreement of the experimental
data with the theoretical fit according to the Hildebrand
equation (eq 1):29
Figure 3. NMR titration of S2 with Pb(ClO4)2 in 0.5 mM acetonitrile-d3 solution.
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the Pb2+ cation by the pyridyl unit is therefore accompanied by
a spectacular red shift of the ICT band of 97 nm, illustrating
that the electron-accepting ability of the pyridyl moiety is
increased once it is coordinated to a cation, leading to a
lowering of the energy of the ICT band.
Binding Properties: Fluorescence Titrations. Since fluo-
rescence response is usually more sensitive than UV−vis
absorption, the binding and sensing properties of these dyes
were also investigated by fluorescence titrations of 1 μm
solutions in acetonitrile with various metal cations, including
Na+, Ag+, Ca2+, Mg2+, Zn2+, Cd2+, Cu2+, Hg2+, and Pb2+ (Figure
6 and Figures S9 and S10 in the Supporting Information).
Upon titration of S2 in acetonitrile with various cations, very
small or no significant spectral changes were observed for Na+,
Mg2+, Ca2+, Cd2+, and Ag+, while the fluorescence spectra
changed significantly for Zn2+, Cu2+, Hg2+, and Pb2+. Taking
Pb2+ for an example, the peak centered at 445 nm decreased
progressively and an obvious emission band slowly emerged at
around 575 nm. The peak around 445 nm arises from the ICT
of the neutral species. Upon the addition of metal ions, the
concentration of this species decreased, giving rise to the
decrease in fluorescence intensity around 445 nm. The
emission at 575 nm can be assigned to the emission of the
new species generated by the binding of S2 and Pb2+.
Moreover, the fluorescence intensity at 575 nm was stronger
than the original emission of the neutral species S2, which
implies that the new species is a stronger fluorescent
chromophore than the neutral one. As a result, S2 can be a
fluorescence-enhanced detector for heavy metal cations.
However, the dyes V2, S3, and V3 exhibited only fluorescence
quenching upon addition of metal ions. This could be
attributed to the fact that the new complex species of V2, S3,
and V3 upon association with metal cations exhibit no
fluorescence or very weak fluorescence. Both V2 and S3
show selectivity for Cu2+ and Hg2+ cations, while V3 exhibits
high selectivity for Cu2+ among the tested ions (Figure 7). The
four dyes show different affinities and selectivities for metal
cations, with S2 exhibiting the largest affinities for metal cations
and V3 showing the smallest affinities. The results indicate that
increasing the conjugation length and the dimensions of the
dyes decreases the binding affinity of the pyridine nitrogen to
metal cation. All of these facts suggest that these dyes can be
candidates for cation sensors for transition metals and that the
affinity can be tuned by varying the conjugation length and
dimension of the molecules.
Figure 4. CV titration of S2 in acetonitrile and (inset) plot of Ep vs
equivalents of metal.
highly conjugated compound (S3) shows less change in the
first oxidation potential (ΔE1ox) than do the less conjugated
compounds S2 and V2, indicating that the ICT in the dye
induced upon the stimulus of Pb2+ decreases with elongation of
the conjugated core. Also, V2 shows a ΔE1 value slightly
ox
larger than that for S2, which indicates that the binding of the
second Pb2+ results in further ICT in V2. However, the
influence on the first oxidation by the second binding of Pb2+ is
much smaller than that induced by the first binding of Pb2+.
These results also indicate that upon binding to Pb2+, the ICT
in these carbazole pyridine derivatives can be modified by
varying the conjugated core length or molecular dimensions.
Binding Properties: UV−Vis Titrations. To scan the metal
cation binding properties of these materials, UV−vis titrations
of carbazole derivative S2 with a wide range of cationic
solutions (Na+, Mg2+, Ca2+, Zn2+, Ag+, Cd2+, Hg2+, and Pb2+)
were performed by the progressive introduction of the
corresponding metal perchlorate solution into a 5 μm solution
of the 4-pyridyl derivative S2 in CH3CN (Figure 5 and Figure
S8 in the Supporting Information). As can be seen, among the
metal ions tested, no significant spectral changes were observed
for Na+, Mg2+, Ca2+, Ag+, and Cd2+, while moderate to
significant spectral changes were observed for Zn2+, Hg2+, and
Pb2+. Taking Pb2+ for an example, the resulting electronic
spectra clearly show the appearance of a new low-energy band
centered around 435 nm that increases with the progressive
addition of lead perchlorate at the expense of the initial ICT
band at 338 nm. This progressive change in the spectrum
revealed isosbestic points at 257, 278, and 379 nm, correlated
to the presence of only two species in equilibrium in solution,
namely, the free ligand S2 and the S2·Pb2+ complex. Binding of
Theoretical Calculations. In order to investigate the
relationship between the compounds’ optical and charge-
Figure 5. (a) Absorption spectra of S2 upon progressive addition of Pb2+ and (b) S2 absorbance intensity changes upon addition of 3 equiv of metal
ions in acetonitrile.
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Figure 6. Fluorescence spectra of S2, S3, V2, and V3 upon progressive addition of Pb2+ in acetonitrile.
Figure 7. (a) Fluorescence intensity changes of V3 upon addition of 3 equiv of metal ions in acetonitrile. (b) Fluorescence spectra of V3 before and
after addition of 3 equiv of Cu2+ in acetonitrile.
transfer properties and the molecular structure and complex-
ation state with metal cations, DFT and TD-DFT calculations
were carried out on the four dyes (S2, V2, S3, and V3) and two
metal complexes (S2·Cu2+ and V3·Cu2+) using the Gaussian 09
program.30 The B3LYP functional in combination with the 6-
31G(d,p) basis set for C, N, and H and the LANL2DZ basis set
for Cu was used for all of the calculations. It should be noted
that the alkyl chain on the carbazole nitrogen was simplified as
ethyl in all of the calculations.
The frontier molecular orbitals (HOMO and LUMO) for all
of the studied compounds, including the two metal complexes,
are shown in Figure 8. First, it can be seen that the HOMO of
S2 is delocalized throughout the backbone, including the
electron-withdrawing pyridine ring. When the molecular
structure is shifted from S2 to V2, S3, and V3, the contribution
from the pyridine ring gradually decreases in the series, and no
apparent contribution from the pyridine moiety is observed for
V3. This decreasing contribution of the pyridine ring to the
HOMO is supposed to be related to a decrease of the affinity of
the pyridine N lone pair to the metal cation, which accords well
with our experimental findings that the association ability of the
dyes decreases with increasing conjugation length and
molecular dimensions. Second, a brief comparison of the
HOMO and LUMO shapes of corresponding compounds
shows that the main ICT mode for the four dyes can be
considered to go from the carbazole moiety to the pyridine
moiety, and the complexation of the Cu2+ cation causes a
modification of the ICT mode: the Cu2+ plus pyridine moiety
becomes the accepting part for S2·Cu2+, while Cu2+ is the only
accepting part for V3·Cu2+. This may be why S2 exhibits
fluorescence enhancement and V3 shows fluorescence
quenching toward metal cations.
In addition, TD-DFT calculations based on the optimized
structures of S2, V2, S3, V3 were carried out to obtain the
wavelengths of the transitions for HOMO to LUMO
excitations together with the oscillator strengths (f, listed in
Figure 8). A gradual increase in the wavelength of the transition
for the HOMO to LUMO excitation was observed for the
series S2, V2, S3, and V3 with wavelengths of 390.8, 444.0,
470.1, and 510.4 nm, respectively, which is consistent with the
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Figure 8. Frontier molecular orbitals of S2, V2, S3, V3, S2·Cu2+, and V3·Cu2+.
evaporated in vacuum to afford a brown oil that was purified via silica
gel column chromatography using methanol/dichloromethane (0−
3%) as an eluent, affording 358 mg (67% yield) of S2 as a light-yellow
trend of the absorption maxima measured in UV−vis
measurements in solution.
1
viscous liquid. H NMR (400 MHz, CDCl3, δ): 8.53 (d, J = 5.6 Hz,
CONCLUSION
■
2H), 8.18 (d, J = 1.6 Hz, 1H), 8.08 (d, J = 7.6 Hz, 1H), 7.62 (dd, J =
8.4 Hz, 1.60 Hz, 1H), 7.47 (m, 4H), 7.33 (d, J = 6.0 Hz, 2H), 7.25 (m,
1H), 7.00 (d, J = 16.4 Hz, 1H), 4.45 (t, J = 6.0 Hz, 2H), 3.83 (t, J = 6.0
Hz, 2H), 3.48 (m, 2H), 3.39 (m, 2H), 3.29 (s, 3H). 13C NMR (100
MHz, CDCl3, δ): 149.9, 145.1, 140.8, 134.0, 127.3, 126.0, 124.6, 123.1,
122.9, 122.7, 120.4, 120.2, 119.4, 119.2, 109.2, 109.0, 71.8, 70.7, 69.1,
58.9, 43.1. HRMS (MALDI-TOF) m/z: [M]+ calcd for C24H24N2O2
372.1832, found 372.1835.
A series of carbazole derivatives conjugated with pyridine by a
π-conjugated core was synthesized via Heck coupling and the
Wadsworth−Emmons reaction. Spectroscopic, electrochemical,
and metal-cation-binding studies demonstrate that this series of
chromophores can communicate electrons between carbazole
and pyridine moieties through the π-conjugated cores, thus
leading to intramolecular charge transfer. The interaction of
carbazole derivatives with metal ions can significantly modulate
the ICT state, resulting in remarkable changes in UV−vis and
fluorescence spectra, proton chemical shifts, and cyclic
voltammograms. Among this series of materials, S2 exhibits
fluorescence enhancement upon association with the heavy
metal cations and V3 shows high selectivity for Cu2+. The
affinity of the pyridine N lone pair decreases with increasing
conjugation length and molecular dimensions. DFT calcu-
lations on S2, V2, S3, V3, S2·Cu2+, and V3·Cu2+ also indicate
the different association abilities of the dyes and the
enhancement of ICT upon addition of the metal cations.
Fluorescence enhancement of S2 and quenching of V3 are also
revealed by electronic structures of their metal complexes. Our
findings indicate that these materials can be candidates for
heavy metal ion detectors and that the affinity of these materials
upon association with metal cations can be tuned by varying the
conjugation length and molecular dimensions.
Compound V2. To a solution of compound 1b (500 mg, 1.17
mmol), palladium acetate (25 mg, 0.11 mmol), and tri-o-
tolylphosphine (67 mg, 0.22 mmol) in triethylamine (3 mL) and
DMF (9 mL) was added 4-vinylpyridine (0.5 mL, 6.68 mmol) under
N2. The mixture was stirred at 110 °C overnight. Water was added,
and then the reaction mixture was extracted with DCM. The
combined organic layers were washed with brine, dried over anhydrous
sodium sulfate, and evaporated to dryness. The residue was turned
into a brown solid under vacuum, which was further precipitated by
DCM and petroleum ether. The product was then purified by silica gel
column chromatography using methanol/dichloromethane (0−3%) as
1
an eluent, affording 434 mg (78% yield) of V2 as a yellow solid. H
NMR (400 MHz, CDCl3, δ): 8.56 (d, J = 6.4 Hz, 4H), 8.26 (d, J = 1.6
Hz, 2H), 7.68 (dd, J = 8.4 Hz, 1.6 Hz, 2H), 7.48 (m, 4H), 7.40 (dd, J =
4.8 Hz, 1.6 Hz, 4H), 7.06 (d, J = 16.0 Hz, 2H), 4.51 (t, J = 6.4 Hz,
2H), 3.88 (t, J = 6.0 Hz, 2H), 3.51 (m, 2H), 3.41 (m, 2H), 3.29 (s,
3H). 13C NMR (100 MHz, CDCl3, δ): 150.0, 145.1, 141.3, 133.8,
127.9, 125.2, 123.4, 123.2, 120.5, 119.3, 109.6, 71.9, 70.8, 69.2, 43.4.
HRMS (MALDI-TOF) m/z: [M]+ calcd for C31H29N3O2 474.2302,
found 474.2305. Anal. Calcd for C31H29N3O2: C, 78.29; H, 6.15; N,
8.84. Found: C, 78.49; H, 6.14; N, 8.71.
Compound 3a. To a solution of compound 2a (1.30 g, 4.2 mmol)
in dry THF was added sodium hydride (200 mg, 60% in mineral oil,
5.0 mmol). The resulting mixture was stirred for 40 min at 45 °C, and
diethyl 4-benzylphosphonate (0.84 g, 2.8 mmol) in THF was added
via syringe. The solution was then heated to reflux and stirred
overnight. The reaction mixture was cooled to room temperature and
quenched with aqueous ammonium chloride. The organic layer was
separated and dried over anhydrous sodium sulfate. The solvent was
removed in vacuo, and the residue was recrystallized from dichloro-
methane and hexane to give 600 mg (61% yield) of compound 3a as a
brown solid. 1H NMR (400 MHz, CDCl3, δ): 8.18 (d, J = 1.6 Hz, 1H),
8.09 (d, J = 7.6 Hz, 1H), 7.63 (dd, J = 8.4 Hz, 2.0 Hz, 1H), 7.38−7.47
EXPERIMENTAL SECTION
■
Materials and Reagents. Compounds 1a, 1b, 2a, and 2b were
prepared according to the procedures in our previous research.28
Tetrahydrofuran was used after refluxing for 2−3 h in the presence of
sodium. Other reagents were of chemical or analytical grade and were
used as received.
Compound S2. To a solution of compound 1a (0.50 g, 1.44 mmol),
palladium acetate (17 mg, 0.07 mmol) and tri-o-tolylphosphine (43
mg, 0.14 mmol) in triethylamine (2 mL) and DMF (6 mL) was added
4-vinylpyridine (0.31 mL, 2.87 mmol) under N2. The reaction mixture
was stirred at 120 °C overnight. Water was added, and then the
reaction mixture was extracted with DCM. The combined organic
layers were washed with brine, dried over sodium sulfate, and
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(m, 7H), 7.35 (d, J = 16.4 Hz, 1H), 7.22 (m, 1H), 7.05 (d, J = 16.4 Hz,
1H), 4.50 (t, J = 6.4 Hz, 2H), 3.85 (t, J = 6.4 Hz, 2H), 3.50 (m, 2H),
3.41 (m, 2H), 3.29 (s, 3H). 13C NMR (100 MHz, CDCl3, δ): 140.9,
140.5, 136.8, 131.6, 130.3, 128.3, 127.6, 125.9, 124.7, 124.4, 123.2,
122.8, 120.5, 120.3, 119.3, 118.6, 109.1, 109.0, 71.9, 70.8, 69.2, 59.0,
43.2. HRMS (MALDI-TOF) m/z: [M]+ calcd for C25H24BrNO2
449.0985, found 449.0978.
ASSOCIATED CONTENT
* Supporting Information
■
S
Absorption and fluorescence spectra of chromophores S2, S3,
V2, and V3 in toluene and CHCl3; NMR titration of S3 and
V2; plots of normalized change of chemical shift versus Pb2+
equivalents for S2, S3, and V2; Job’s plots of change of proton
chemical shifts; CV titrations; absorption spectra of S2 upon
addition of Na+ metal ions in acetonitrile; fluorescence spectra
of S2, S3, V2, and V3 upon addition of Na+ in acetonitrile;
fluorescence spectral changes for S2, S3, and V2 upon addition
Compound S3. A mixture of compound 3a (0.57 g, 1.27 mmol), 4-
vinylpyridine (0.27 mL, 2.53 mmol), palladium acetate (14 mg, 0.06
mmol), and tri-o-tolylphosphine (0.12 mmol, 36 mg) in triethylamine
(20 mL) and DMF (10 mL) was heated to 80 °C and stirred overnight
under N2. The reaction mixture was cooled to room temperature and
extracted with DCM. The organic layer was washed with brine, dried
over anhydrous sodium sulfate, and evaporated to dryness. The residue
was purified by silica gel column chromatography to give 0.74 g (84%
yield) of compound S3 as a yellow solid. 1H NMR (400 MHz, CDCl3,
δ): 8.55 (d, J = 5.6 Hz, 1H), 8.21 (s, 1H), 8.10 (d, J = 7.6 Hz, 1H),
7.67 (dd, J = 8.8 Hz, 1.6 Hz, 1H), 7.55 (d, J = 8.4 Hz, 4H), 7.45 (m,
3H), 7.38 (m, 2H), 7.31 (d, J = 16.8 Hz, 2H), 7.25 (m, 1H), 7.14 (d, J
= 16.0 Hz, 1H), 7.06 (d, J = 16.4 Hz, 1H), 4.51 (t, J = 6.4 Hz, 2H),
3.86 (t, J = 6.4 Hz, 2H), 3.51 (m, 2H), 3.42 (m, 2H), 3.30 (s, 3H). 13C
NMR (100 MHz, CDCl3, δ): 150.2, 144.7, 140.9, 140.5, 138.5, 134.7,
132.8, 130.3, 128.5, 127.4, 126.6, 125.9, 125.3, 124.5, 123.3, 122.9,
120.7, 120.3, 119.3, 118.7, 109.2, 109.0, 71.9, 70.8, 69.2, 59.0, 43.2.
HRMS (MALDI-TOF) m/z: [M]+ calcd for C32H30N2O2 475.2254,
found 475.2266. Anal. Calcd for C32H30N2O2: C, 80.98; H, 6.37; N,
5.90. Found: C, 80.76; H, 6.28; N, 5.82.
Compound 3b. To a solution of compound 2a (3.58 g, 11.7 mmol)
in dry THF was added sodium hydride (309 mg, 12.9 mmol). The
resulting mixture was stirred for 40 min at 45 °C and diethyl 4-
benzylphosphonate (1.27 g, 3.9 mmol) in THF was added via syringe.
The solution mixture was then heated to reflux and stirred overnight.
The reaction mixture was cooled to room temperature and quenched
with aqueous ammonium chloride. The organic layer was separated
and dried over anhydrous sodium sulfate. The solvent was removed in
vacuo, and the residue was recrystallized from dichloromethane and
hexane to give 2.05 g (83% yield) of compound 3b as a brown solid.
1H NMR (400 MHz, CDCl3, δ): 8.20 (s, 2H), 7.63 (dd, J = 8.4 Hz, 1.2
Hz, 2H), 7.47 (d, J = 8.4 Hz, 4H), 7.41 (m, 6H), 7.27 (d, J = 16.0 Hz,
2H), 7.06 (d, J = 16.0 Hz, 2H), 4.48 (t, J = 6.0 Hz, 2H), 3.86 (t, J = 6.4
Hz, 2H), 3.50 (m, 2H), 3.41 (m, 2H), 3.29 (s, 3H). 13C NMR (100
MHz, CDCl3, δ): 140.8, 136.7, 131.7, 130.1, 128.6, 127.6, 124.9, 124.7,
123.2, 120.6, 118.6, 109.3, 71.9, 70.8, 69.2, 59.0, 43.3. HRMS
(MALDI-TOF) m/z: [M]+ calcd for C33H29Br2NO2 629.0560, found
629.0569.
of metal ions in acetonitrile; and H and 13C NMR spectra of
S2, V2, 3a, S3, 3b, and V3. This material is available free of
charge via the Internet at http://pubs.acs.org.
1
AUTHOR INFORMATION
Corresponding Authors
*Fax: (+86)-571-28862271. E-mail: xjfeng@hznu.edu.cn.
*Fax: (+86)-571-28862271. E-mail: zhengxu@hznu.edu.cn.
*Fax: (+852)-34117348. E-mail: mswong@hkbu.edu.hk.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the General Research Fund (GRF)
(HKBU 202408), the Hong Kong Research Grant Council,
SAR Hong Kong, and the Zhejiang Provincial Natural Science
Foundation of China (ZJNSF LY13E030005). We are grateful
to the Computational Center for Molecular Design of
Organosilicon Compounds at Hangzhou Normal University
for provision of the SGI Altix 450 server.
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