Synthesis and photopysical properties of a polioxo ethylene chain alkaline earth metal fluorescent sensor with 2-aminoanthracene as terminal group

Article abstract of DOI:10.1021/jp200514u

A new symmetric polioxo ethylene chain fluorescent probe containing 2-aminoanthracene bichromophoric as the terminal group for the alkaline earth metal cation, 2,2′-[oxybis(3-oxapentamethyleneoxy)]-bis[N-(2-anthryl) benzamide)] (1), has been synthesized.

Full text of DOI:10.1021/jp200514u

ARTICLE
pubs.acs.org/JPCA
Synthesis and Photopysical Properties of a Polioxo Ethylene
Chain Alkaline Earth Metal Fluorescent Sensor with
2
-Aminoanthracene As Terminal Group
Chan Xu, Weisheng Liu, and Wenwu Qin*
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province and State Key Laboratory of Applied
Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, People's Republic of China
S Supporting Information
b
ABSTRACT: A new symmetric polioxo ethylene chain fluorescent probe
containing 2-aminoanthracene bichromophoric as the terminal group for
0
the alkaline earth metal cation, 2,2 -[oxybis(3-oxapentamethyleneoxy)]-
bis[N-(2-anthryl)benzamide)] (1), has been synthesized. The photo-
physical properties of 1 have been studied by means of absorption,
1
fluorescence spectroscopy, and H NMR. The difference in emission
spectra response to concentration of model compound 2-acetamido-
anthracene and 1 in acetonitrile implies that intermolecular excited
dimers is likely to occur. Fluorescence decay profiles of 2-acetamido-
anthracene can be described by a biexponential fit, while three lifetimes,
two of which are similar as those of 2-acetamido-anthracene, are found
for 1. The third lifetime might be attributed to intramolecular excited
dimers. Complex formation with alkaline earth metal ions are investi-
gated in acetonitrile as solvent via fluorimetric titrations. Fluorescence
2þ
intensity trend of the complex with Mg differed from those of other
2
þ
alkaline earth metal ions. The compound forms 1:2 (ligand/Mg
)
2
þ
2þ
2þ
complex with Mg while formed 1:1 complexes with Ca , Sr , and
Ba , producing large hypochromic shifts in the emission spectra and significant cation-induced fluorescence amplifications. On the
contrary, the addition of Ca , Sr , or Ba lead to a decrease in the fluorescence emission first, then an increase and blue shift in
2
þ
2
þ
2þ
2þ
emission could be found at the end.
1
. INTRODUCTION
oligo-oxyethylene compound, appending two quinoline units at
its terminals, can strongly bind K . Recently, the research group
þ
Fluorescence spectroscopy is an attractive analytical technique
of Nakamura synthesized novel fluorescent reagents that have
two fluorescent chromophores (anthracene, pyrene, fluorene,
etc.) symmetrically or asymmetrically placed at both terminals of
1,13-diamino-4,7,10-trioxatridecane, and their complexations
to measure intracellular concentrations of biologically important
ions and molecules. The design and development of fluorescent
chemosensors for the detection of analytically important species
1
ꢀ3
is still an expanding area of research.
1
0,11
with alkali and alkaline earth cations were examined.
Through
Crown ether derivatives incorporating a fluorescent moiety
are attractive tools for optical sensing of metal ions. Crown ethers
are well-known to bind alkali and alkaline-earth metal ions, as
well as other cations.
advantages in separation and determination of ions. The basic
molecular frame of ordinary polioxo ethylene chain molecules
2
þ
the complexation with Ca , the shape and the peak of the
fluorescence spectra of two anthracene fluorescent chromo-
phores significantly changed from a relatively structured band
with three peaks around 400 nm to a structureless band at
4
ꢀ6
Polioxo ethylene chain offers many
7
4
90 nm. The peak around 400 nm of uncomplexed species and
the broad peak at 490 nm of the complexed species could be
attributed to fluorescence from an anthracene monomer and
dimer, respectively. This phenomenon indicated that the struc-
ture of the ligand changed on complexation, and two anthracenes
approached each other and then stacked, whereas compounds
consists of a chain of several oxygen atoms connected by ꢀCH ꢀ
2
groups. When proper fluorophore groups are introduced onto
these ꢀCH ꢀ, they will serve as potential luminescent reagents.
2
The usual fluorophore is a planar aromatic molecule and tends to
stack each other. One can expect that fluorophores will show an
excellent probe and an attachment for stabilization of the
pseudocyclic structure of the complex.
Received: January 18, 2011
The early study on polioxo ethylene chains derivatives,
was carried out by V €o gtle et al. They demonstrated that the
Revised:
Published: April 11, 2011
March 27, 2011
8,9
r 2011 American Chemical Society
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|J. Phys. Chem. A 2011, 115, 4288–4298

The Journal of Physical Chemistry A
ARTICLE
where two fluorescent chromophores are pyrene or fluorene did not
show spectral changes upon addition of any metal salts. The same
research group also developed another polioxo ethylene chain with
two pyrenes linked at both ends of the chain show strong
intramolecular excimer formation. Addition of alkaline earth metal
ions leads to an increase in monomer emission at the expense of the
excimer band. The helical structure of the 1:1 complexes is
Scheme 1. Synthesis of Compound 1
1
12
supported by H NMR spectra. In addition to this, other polioxo
ethylene chains signaling of the binding event was achieved by a
1
3,14
twisted intramolecular charge transfer (TICT)
process and a
15
PET process between two terminal chromophore units. Due to
2þ
2þ
the difference in ionic radii between Mg and Ca , they can vary
the torsion angle in the complex structure. Conformational change
could be converted into physical signs, such as absorption and
fluorescence change; this will be more sensitive than metal ion
sensors. But some TICT processes show that the emission of
2þ
complexation with Mg was weak and gave no remarkable spectral
13
changes compared to those of other cations. So, recently, the
Nakamura group synthesized two chemosensors by substitution of
the anthrance group with n-propyl or ethyl benzene moieties;
complex results show that the peak shape and maximum wavelength
2
þ
of the emission of the complex with Mg differed from those of
2þ
14
Ca and other alkaline earth metal ions. These indicate polioxo
ethylene chain derivatives incorporating ionophoric functional
groups located at the terminals of molecules exhibit excellent
properties as carriers for metal ions.
uncorrected. All the materials for synthesis were purchased from
commercial suppliers and used without further purification. Deio-
nized water was used as solvents. Perchlorate salts of magnesium,
calcium, cesium, and barium were of spectroscopic grade and used
as received.
In this work we synthesized the fluorescent chemosensor 1
based on polioxo ethylene chain with 2-aminoanthracene bichro-
mophoric as the terminal group. The amino group was directly
linked to the anthrance fluorophore. The solvent-dependent photo-
physical characteristics of probe 1 were investigated by steady-state
absorption and emission spectroscopy, as well as time-resolved
fluorimetry. Furthermore, protonation and complex formation of 1
2.2. Syntheses. 2-Acetamido-anthracene. 2-Acetamido-
1
6
anthracene was synthesized following a literature procedure.
1
H NMR (DMSO-d , 400 MHz): δ = 2.13 (s, 3H, ꢀCH ), 7.47
6
3
(m, 3H), 8.01 (d, 3H), 8.45 (m, 3H), 10.20 (s, 1H, amide NH);
MS (ESI) m/z found 236.2 (M þ 1, 100%).
2þ
2þ
2þ
2þ
0
with alkaline earth metal (Mg , Ca , Sr , Ba ) ions was
2,2 -[Oxybis(3-oxapentamethyleneoxy)]-bis[N-(2-anthryl)-
spectroscopically studied. Finally, the measured ground-state dis-
benzamide)] (1). The synthetic pathway of compound 1 is shown
in Scheme 1. Compound 2 was synthesized following a literature
procedure.
sociation constants K for the formed complexes, the absorption and
d
17
emission spectral wavelengths in the absence and presence of ions,
the fluorescence quantum yields of the free and bound species of 1
are presented and discussed in this study.
A total of 20 mL of SOCl was added dropwise to the 0.43 g of
2
dicarboxylic acid (1 mmol) in an ice bath. The mixture was
The fluorophore of this molecule is the 2-aminoanthracene
group. On the other hand, because most studies on 2-aminoan-
thracene and 2-acetamido-anthracene are limited, reference
experiments on 2-aminoanthracene and 2-acetamido-anthracene
were performed here also.
stirred about 2 h at ꢀ5 °C. Excess SOCl was evaporated. Then
2
the corresponding acid chloride was obtained. A total of 0.41 g of
2-aminoanthracene (2.1 mmol) dissolved in 80 mL of acetoni-
trile, 2 mL of triethylamine, and a solution of the above acid
chloride in 30 mL of acetonitrile was added dropwise to this
2
1
-aminoanthracene in an ice bath. The mixture was stirred for
day at room temprature. The solvent was evaporated in
2
. EXPERIMENTAL SECTION
reduced pressure, and the residue was dissolved in 30 mL of
chloroform, then washed with 20 mL of water three times, and
1
13
2
.1. Instruments and Reagents. H and C NMR spectra
were taken on a Bruker DRX-400 spectrometer with TMS as an
dried over MgSO . The crude product was purifide by silica gel
4
internal standard and CDCl as solvent. Mass spectra were
column chromatography (eluent: chloroformꢀethyl acetateꢀ
3
1
recorded on a Bruker Daltonics esquire6000 mass spectrometer.
petroleum). Yield: 20.3%; mp 161ꢀ163 °C. H NMR (CDCl ,
3
1
In the case of H NMR measurements of metal complexes,
400 MHz): δ = 3.38 (ꢀCꢀCH ꢀO, q, 4H), 3.44 (ꢀCꢀ
2
ꢀ
2
3
concentrations of these reagents 1 ꢁ 10 mol/dm were taken
CH ꢀO, q, 4H), 3.65 (ꢀCꢀCH ꢀO, t, 4H), 3.95 (ꢀCꢀCH ꢀ
2
2
2
in acetonitrile-d . Metal ions were added in excess amount to
ensure complete formation of the complex. H NMR spectra
O, t, 4H), 6.67 (aromatic, d, 2H), 7.06 (aromatic, t, 2H), 7.23ꢀ
7.40 (aromatic, m, 6H), 7.47 (aromatic, d, 1H), 7.49 (aromatic, d,
1H), 7.81ꢀ7.91 (aromatic, m, 6H), 8.21 (aromatic, s, 2H), 8.27
(aromatic, t, 2H), 8.29 (aromatic, s, 2H), 8.67 (aromatic, s,
3
1
were measured by a AVANCE DRX- 200 at 20 °C. The pH was
measured using a FE20K pH meter (Mettler Toledo).
Chemical shift multiplicities are reported as s = singlet, d =
13
2H), 10.21 (NH, s, 2H). C NMR (CDCl , 100 MHz): δ 67.5,
3
13
doublet, and m = multiplet. C spectra were referenced to the
68.7, 70.0, 70.3, 112.5, 115.7, 121.7, 122.0, 124.8, 125.4, 125.6,
125.8, 127.8, 128.1, 128.6, 129.1, 130.9, 132.0, 132.1, 132.3,
133.0, 135.5, 156.3, 163.3. MS (ESI) m/z found 785.4 (M þ 1,
100%).
CDCl (77.67 ppm) signal. Mass spectra were recorded in E.I.
3
mode. Melting points were taken on an X-4 precise micro melting
point cryoscope (Beijing Fukai Instrument Co.) and are
4
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Figure 1. Normalized absorbance and emission spectra (λex = 340 nm) of (a) 2-aminoanthracene and (b) 2-acetamido-anthracene in various solvents.
2
.3. Steady-State UVꢀVis Absorption and Fluorescence
2.5. Determination of K via Direct Fluorometric Titration.
d
Spectroscopy. UVꢀvis absorption spectra were recorded on a
Varian UV-Cary100 spectrophotometer, and for the corrected
steady-state excitation and emission spectra, a Hitachi F-4500
spectrofluorometer was employed. Freshly prepared samples in
The ground-state dissociation constants K of the complexes between
d
1 and various ions were determined in CH CN solution at 20 °C by
3
direct fluorometric titration as a function of the ion concentration [X]
using the fluorescence excitation or emission spectra. Nonlinear
19
1
cm quartz cells were used to perform all UVꢀvis absorption and
fitting of eq 1 to the steady-state fluorescence data F recorded as
emission measurements. For the determination of the fluorescence
a function of [X] yields values of K , Fmin, Fmax, and n.
d
quantum yields, φ , of 1, only dilute solutions with an absorbance
below 0.1 at the excitation wavelength (λ = 340 nm) were used.
For the measurement of fluorescence emission spectra as the
function of concentration of 1, some of the absorbance higher than
.1. Anthrancene in cyclohexane (φ = 0.30) was used as fluores-
cence standard. The φ values reported in this work are the
averages of multiple (generally three) fully independent measure-
ments. The majority of the φ determinations were done using
undegassed samples. In all cases, correction for the solvent refractive
index was applied. All spectra were recorded at 20 °C using
undegassed samples. The titration experiments with alkaline earth
metals were carried out by adding small quantities of a stock solution
of alkaline earth metal perchlorate salts in MeCN to a much larger
volume (25 mL) of solutions of 1.
n
f
Fmax½Xꢂ þ FminKd
ex
F ¼
ð1Þ
n
K þ ½Xꢂ
d
In eq 1, F stands for the fluorescence signal at [X], whereas Fmin
and Fmax denote the fluorescence signals at minimal and maximal
X], respectively, and n is the number of ions bound per probe
0
f
18
f
[
molecule (i.e., stoichiometry of binding). Because the fits of eq 1 to
f
the fluorescence data F with n, K , Fmin, and Fmax as freely adjustable
d
parameters always gave values of n close to 1 or 2 (indicative of a 1:1
or 1:2 complex); n was kept fixed in the final curve fittings, from
which the estimated K values are reported here.
d
3
. RESULTS AND DISCUSSION
2
.4. Time-Resolved Fluorescence Spectroscopy. Fluores-
3.1. Steady-State Spectroscopic Behavior of 2-Aminoan-
cence lifetimes were measured by a FLS920 at room temperature.
thrancene and 2-Acetamido-anthracene. Figure 1a shows the
UV/vis absorption and fluorescence emission spectra of 2-amino-
anthrancene in several solvents. The absorption show typical
anthracene absorption spectra that have three sharp bands at
around 322, 344, and 354 nm. Furthermore, another strong and
broad absorption band could be found at about 400 nm in all the
solvents used. The absorption band maximum steadily shifts to
longer wavelengths in going from nonpolar to polar solvents
(from cyclohexane at 396 nm to acetonitrile at 407 nm).
The samples were dissolved in CH CN and the concentrations
3
were adjusted to have optical densities at the excitation wavelength
(340 nm) <0.1. Solutions were purged with nitrogen for 15 min
prior to analysis. The monitored wavelengths were 380, 400, 410,
and 430 nm.
Fluorescence decay histograms were obtained on an Edin-
burgh instrument FLS920 spectrometer equipped with a hydro-
gen flash lamp, using the time-correlated single photon counting
technique in 4096 channels. Histograms of the instrument
response functions (using a LUDOX scatterer) and sample
Moreover, fluorescence emission also shows a strong solvent
dependence. The common feature for 2-aminoanthrancene is the
emission have three narrow bands centered near 382, 402, and
425 nm in MeCN originating from the excited state of the
anthrancene (382, 402, and 425 nm in MeOH). Another red-
shifted broad band, apparent here. The broad emission band
maximum steadily shifts to longer wavelengths in going from
nonpolar to polar solvents (from cyclohexane 438 nm to acetonitrile
487 nm). The emission spectra in solvents are of similar shape as
3
decays were recorded until they typically reached 3 ꢁ 10 counts
in the peak channel. Obtained histograms were fitted as sums of
the exponential, using Gaussian-weighted nonlinear leastsquares
fitting based on MarquardtꢀLevenberg minimization implemen-
ted in the software package of the instrument. The fitting
parameters (decay times and preexponential factors) were de-
2
g
termined by minimizing the reduced chi-square χ . An additional
20
graphical method was used to judge the quality of the fit that
included plotsof surfaces (“carpets”) of the weighted residuals
those in the literature, whereas the total fluorescence quantum
yields are 0.28 in acetonitrile versus 0.44 in cyclohexane.
2
versus channel number. All curve fittings presented here had χ
The long wavelength absorption and emission bands can be
attributed to different phenomena. The amino group being a
values below 1.1.
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Table 1. Photophysical Properties of 2-Aminoanthrancene and 2-Acetamido-anthracene in Several Solvents
a
compound
solvent
λabs (nm)
λ(LE)em (nm)
λ(ICT)em (nm)
φ
f
2
-aminoanthrancene
cyclohexane
toluene
336, 396
336, 403
336, 407
372, 395
362, 380, 402
366, 385, 406
382, 405, 428
380, 401, 423, 447
408, 430, 450
414, 431
438
458
487
0.44
0.33
0.28
0.10
0.19
0.31
0.23
MeCN
b
2-acetamido-anthracene
cyclohexane
toluene
354, 375, 395
MeCN
346, 358, 375, 396
358, 377, 398
DMSO
380, 416, 437
a
b
Total fluorescence quantum yield (reference: anthrancene in cyclohexane, Φ = 0.30). 2-Acetamido-anthracene is moderately soluble in cyclohexane;
f
it is difficult to find all the absorption band because the absorband is very low in cylohexane.
excitation at 340 nm in acetonitrile solution. The emission spectra
are of similar shape as those of 2-acetylaminoanthracene in aqueous
22
buffer solution and foldamers incorporated with six decyl chains
23
and two anthracene units at the ends in n-octanol. As one can see,
the amino group acylation brings about a dramatic hypochromic
shift of the spectrum when compare with ICT band of 2-aminoan-
thrancene. All of these compounds (calix[4]pyrrole-aminoanthra-
cene, 2-acetylaminoanthracene, and compounds in refs 21ꢀ23) and
2-acetamido-anthracene have the same N-(2-anthryl)amide group
and have similar emission spectra. So the main chromophore should
be N-(2-anthryl)amide group. These absorption and emission band
are also similar as those of 9-cyanoanthracene (anthracene-9-
carbonitrile) in methanol or 9,10-diphenylanthracene (DPA) in
24
methanol or cyclohexane.
The anthracene molecule is well-known to form the relatively
25
stable excimer or exciplex. When the excimer or exciplex was form,
its fluorescence intensity will be decreased with increase concentra-
tion. Figure S1a shown the fluorescence spectra of 2-acetamido-
anthracene as the fuction of concentration in acetonitrile. Its
fluorescence intensity increasing with incerase concetration, then
decrease intensity with further incerase concetration. We can find
that fluorescence intensity at 292 μM was lower than that at 128
μM. Normalized their fluorescence spectra at longer band 431 nm
also shown that the shorter band at 414 nm slightly decrease with
increasing its concentration. The two dash curves in Figure S1b are
obtained from the normalized spectra (at 431 nm) at 292 and
1
28 mM subtract the spectra at concentration 3.2 μM. Because
2-acetamido-anthracene only contain one anthrance chromophore,
intramolecular excimers cannot be formed. So the different ratios of
two maximum bands (431 and 414 nm) of 2-acetamido-anthracene
in acetonitrile as the function of concentration should be the reason
that formed of the intermolecular excimer. With an increase in the
concentration, there have more dimers that can form; the intensity
of the longer maximum band will increase where the shorter bands
of the monomer will decrease. From the two dash curves of
Figure 1b, one can find that, with increasing the concentration,
the contribution from the excimers will become more and more.
Table 1 summarizes the photophysical data of 2-aminoan-
thrancene and 2-acetamido-anthracene in several solvents.
3.2. Absorption and Emission Spectra of 1. UV/vis absorp-
tion and fluorescence emission spectra of 1 in several solvents are
depicted in Figure S2 and Figure 2a, respectivity. The absorption
and fluorescence emission spectra of 1 are similar to those of
2-acetamido-anthracene in all the solvents used. Table 2 sum-
marizes the photophysical data of 1 in several solvents.
Figure 2. Normalized emission spectra of 1 (λex = 340 nm) in several
solvents (a) and in acetonitrile (b) at different concentrations.
good electron donor and the anthrancene being a reasonable
electron acceptor, the possibility of intramolecular charge trans-
fer (ICT) may be considered.
The absorption spectra (not shown) of 2-acetamido-anthracene
showed four narrow bands at around 346, 358, 375, and 396 nm in
acetonitrile. In other solvents, the absorption spectra are of similar
shape as in acetonitrile. No broad and red shift band could be found
for compound 2-acetamido-anthracene in all the solvents used.
The emission spectra in toluene shows three bands, with the
maximum at ∼408, ∼430, and ∼450 nm, respectively (Figure 1b).
The fluorescence emission spectra of 2-acetamido-anthracene in
toluene are quite similar as that of calix[4]pyrrole-aminoanthracene
2
1
in CH Cl . An emission maximum at ∼431 nm and another band
The anthracene molecule is well-known to form the relatively
stable excimer or exciplex. The excimer band of the anthracence
2
2
at the short wavelength side (∼414 nm) could be observed upon
4
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The Journal of Physical Chemistry A
ARTICLE
normally appears in the long wavelength area of about 490 or
considered the monomer emission of N-(2-anthryl)amide and
490 nm is therefore due to an excimer formed via an association
of two 2-aminoanthracene units situated at the ends of the
polyoxyethylene chain. This eximer is not so obvious in the
emission spectra of 1.
26
5
05 nm. In addition, similar emission spectra as 1 also appear in
the case of other compounds only containing a single anthracene
22,23
unit.
Because intermolecular excimer can be formed for 2-acet-
amido-anthracene, so what about 1? We measure the fluorescence
emission spectra of 1 in acetonitrile at different concentration and
shown the results in Figure S3 and Figure 2b. An emission
maximum at ∼433 nm, whereas another band at ∼415 nm, could
be observed at a high concentration of 1, this spectra is the same as
those in DMSO and toluene. The maximum of the fluorescence
emission band shifts hypochromic from 433 to 415 nm with an
decrease in intensity when gradual decreasing the concentration of
3.3. Complex Formation between 1 and Alkaline-Earth
Metal Ions in Acetonitrile Solution. Complex formation was
2
þ
2þ
2þ
investigated for 1 with alkaline-earth metal (Mg , Ca , Sr
,
2
þ
Ba ) in acetonitrile. The results are summarized in Table 3.
2
þ
Upon addition of Mg in the μM concentration range to
a mM solution of 1 in acetonitrile, a slight decrease in intensity
and hypochromic shift of the UV/vis absorption could be found.
2þ
1
and is accompanied by appear an new band at 380 nm. Figure 2b
The fluorescence emission spectra of 1 as a function of [Mg ]
shows the normalized fluorescence emission of 1 from Figure S3.
The spectra in different concentration of 1 cannot overlap. A closer
inspection shows the presence of a tail at the red edge of the
emission band (433 nm) with increasing in the quantity of 1, which
becomes more outspoken (about 500 nm) in higher concentrations.
This change in emission spectra of 1 not only shows the decrease in
emission intensity at 433 nm, but shifts slightly to shorter wave-
lengths with decreasing in the quantity of ligand in addition to the
appearance of isoemissiom points at 398 nm. These spectral changes
with the clean isosbestic point at 398 nm indicate that the present
bisanthracene systems exist in equilibrium, at least between two
states. The clearly different emission spectra response to concentra-
are also shown in Figure 3. In salt-free acetonitrile, the emission
spectrum of 1 displays one weak, broad band with a maximum
tion here implies that intermolecular excited dimers (1* 1) ob-
3
tained from the excited state of the 1* reaction with the group state
of 1 is likely to occur in acetonitrile.
In DMSO, the ration of two bands, 440/420 nm, changed with
concentration, which is also similar as in MeCN. So, intermole-
cular excited dimers can be formed in DMSO, also.
In addition, the molecule 1 has two of the same anthrance
chromophores that are linked by a flexible chain. Intramolecular
excimers may also be formed in the excited state. The intramo-
lecular anthracene dimer linked with ethyleneoxy units (bis(9,10-
anthracenediyl)coronand) has been reported to form a metal-
27
sensitive intramolecular excimer. One cannot exclude that
intramolecular excimers can also be formed for 1.
A Lorentz fit was applied to the fluorescence spectrum of 1 in
acetonitrile and three peaks were picked at 412, 436, and 490 nm,
respectively. In a fit of the spectra, the position, half-width,
and skewness of these two components were kept fixed. An
example of a fit is given in Figure S3. The band at 412 nm can be
Table 2. Photophysical Properties of 1 in Several Solvents
a
solvent
λabs (nm)
λem (nm)
φ
f
toluene
MeCN
DMSO
350, 377, 396
355, 372, 393
350, 377, 396
380, 416, 436
380, 415, 436
380, 420, 440
0.05
0.02
0.03
Figure 3. (a) Fluorescence emission (λex = 340 nm) spectra of 1 in
acetonitrile solution as a function of [Mg ]. (b) Best fit of eq 1 with n = 2
to the direct emission fluorimetric titration data of 1 obtained from the
spectra of (a): λex = 340 nm, λem = 381 nm.
a
2þ
a
High concentration of 1 (absorbance higher than 0.1).
a
Table 3. Spectral and Photophysical Characteristics of 1 in the Absence and Presence of Alkaline-Earth Metal Ions in Acetonitrile
2
8
complex
cation size (Å)
ctoichiometry ligand/metal
λem (nm)
log K
Φ
f
1
380, 415
0.020
0.088
0.020
0.060
0.059
2
þ
1
1
1
1
ꢀMg
1.44
2.12
2.52
2.94
1:2
1:1
1:1
1:1
366, 380, 401
366, 380, 401
366, 380, 401
366, 380, 401
8.11
3.58
4.60
4.17
2
þ
ꢀCa
2
þ
ꢀSr
ꢀBa
2þ
a
K is the average value determined from all direct analyses of fluorometric and spectrophotometric titrations.
d
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The Journal of Physical Chemistry A
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þ
Figure 4. ESIMS for the Mg adduct of 1 [(1 þ 2Mg þ H ): 834.5].
at ∼415 nm and a shoulder band at ∼380 nm. Upon increasing
less due to a decrease in the excited state of 1*; then the group
state of monomer of 1 will become more and more. So we can
observe the anthrancen-like emission spectra upon addition of
2
þ
[
Mg ], the emission band blue-shifted with a maximum at 366,
3
80, and 401 nm originating from anthrancene excited state
2
þ
increases its intensity. From the analyses of all the spectro-
photometric and fluorometric titrations of 1 with Mg ions, it is
Mg to 1. In addition to inhibiting the formation of intermo-
2
þ
2þ
lecular excited dimers (1* 1), coordination of the Mg poly-
3
clear that a 1:2 ligand/cation stoichiometry is found. The logK
oxyethylene and carbonyl oxygen atoms suppresses the required
bending movement of the polyoxyethylene chain for the associa-
tion of two fluorophores, so the intramolecular excimer can not
be formed, leading to a huge cation-induced fluorescence en-
hancement with spectral shift.
The formation of Mg2 1 adduct was further confirmed by
mass spectrometry. The electrospray ionization mass spectrum
d
2þ
(
Kd1 ꢁ K ) value for the 1ꢀMg complex, determined by
d2
direct fluorometric titration (eq 1) of emission spectral data F,
equals 8.11. The response of the absorption and fluorescence
spectra of 1 in acetonitrile upon addition of Mg is nearly the
2þ
þ
2þ
same as that found for 1 in the presence of H (Figure S6), only
3
þ
2þ
Φ of 1ꢀH is somewhat lower than Φ of 1ꢀMg . Because
f
f
direct fluorometric titrations emission spectra show that 1:1
of the Mg adducts for 1 showed a molecular mass of 834.5, which
þ
þ
stoichiometry between 1 and H , whereas there is a 1:2 ligand/
cation stoichiometry between 1 and Mg . One cannot exclude
correspond to the formula of [1 þ 2Mg þ H ] (Figure 4).
2
þ
2þ
Upon complexation by Ca in acetonitrile, the absorption
þ
þ
that with another H to 1ꢀH the emission intensity will
spectrum of 1 is blue-shifted combine with a decrease in the
2
þ
increase further until it is the same as that of 1ꢀ2Mg . With
absorption (Figure S7).
2
þ
2þ
an increase in the concentration of Mg , the free excited state of
Addition of Ca to a solution of 1 in acetonitrile shows a
2
þ
1
* will decrease due to the formation of the complex of Mg2 1.
decrease in the fluorescence emission intensity at ∼415 nm in
3
The intermolecular excited dimers (1* 1) will become less and
addition to the red tail at around 500 nm that slowly disappears
3
4
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The Journal of Physical Chemistry A
ARTICLE
the form of intermolecular excited dimers (1* 1) form, which
3
will lead to an increase in the monomer emission of anthrance.
2
þ
2þ
2þ
2þ
The stability constants measured for the Ca , Sr , or Ba
2
þ
2þ
cation complexes of 1 are in the order of Ca < Sr ≈ Ba
,
which can be explained by the cation size and the size of the
pseudocrown cavity. Binding affinity of macrocyclic crown ethers
mainly depends on the ionic size of the cation, whereas in
pseudocyclic systems, combined effects of the size of the
pseudocrown ether cavity, number of oxygen atoms, the charge
density, and the coordination number of the cations play a
2
þ
considerable role. The exceptional behavior of the Mg com-
plex showing 1:2 ligand/cation stoichiometry value can be
ascribed to the fact that the Mg ion is too small to only fit
2
þ
well one cation into the cavity (see Table 3).
3.4. Time Resolved Fluorescence Spectroscopy and Their
Complexes with Alkaline-Earth. To directly probe the excited
state dynamics of 2-acetamido-anthracene and 1, fluorescence decay
traces in different solvents were collected as a function of emission
wavelength λem by the single-photon timing technique. Fluores-
cence decay measurements reveal a more complex situation for 1.
Each fluorescence decay trace was analyzed individually as a sum of
mono, bi, or three exponential functions in terms of decay times τi
and associated pre-exponential factors a (i = 1ꢀ3). Details are given
i
in Tables 4 and S1.
The fluorescence decay of 2-acetamido-anthracene is fitted to
the bi- exponential profile with the lifetime of 10 and 3.0 ns in
acetonitrile in all of the fluorescence region. In DMSO, the
fluorescence decays also show two main components with time
constants of about 18.0 and 1.5 ns (Table S1). Contributions of
the faster decay times τ decrease with increasing the concentra-
1
tion of 2-acetamido-anthracene in acetonitrile, so fast decay
might be attributed to the lifetime of nonaggregated molecules,
while slow decay may be attributed to the lifetime of intermo-
lecular excited dimers.
In contrast, the decay profiles of 1 are more complicated than
those of 2-acetamido-anthracene. In acetonitrile, the emission is
triple-exponential (0.2, 3.8, and 10.5 ns). The contributions of
the two slower constant decay times τ (3.8 ns) and τ (10.5 ns)
Figure 5. (a) Fluorescence emission (λ = 340 nm) spectra of 1 in
ex
2
þ
acetonitrile solution as a function of [Ca ]. (b) Best fit of eq 1 with n =
to the direct emission fluorimetric titration data of 1 obtained from the
1
spectra of (a): λex = 340 nm, λem = 381 nm.
2
3
components are about 15 and 40%, respectively. The fluores-
cence decay is also a triple-exponential in DMSO. In DMSO, the
longest decay time has the largest contribution. The longer
component τ decreases from 19.0 ns at λ = 410 nm to 9.5
first (Figure 5a); then the fluorescence emission band hypochro-
mic shifts from 415 nm in ion-free acetonitrile to 366, 380, and
2
em
4
01 nm and increases its intensity with increasing the calcium ion
ns at λem = 430 nm. These may be due to the reason that oxygen
is easily quenched in this long lifetime. Solutions were purged
with nitrogen to elimate the effect on oxygen; when measured for
a long time, oxygen will quench the lifetime of the solution.
The fluorescence lifetimes of anthracene excimers strongly
depend on the environment, solvent polarity, and the substituents.
Lifetimes of 52ꢀ91 ns have recently been reported for the excimer
of a 9,10-dimethylene substituted anthracene chromophore in a
concentration. This mean that the presence of calcium at low
concentration will decrease emission of the monomer and dimer
2
þ
first. From the analyses of fluorometric titrations of 1 with Ca
ions, it is clear that a 1:1 ligand/cation stoichiometry is found.
2
þ
The log K value for the Ca complex with 1 determined by
d
direct fluorimetric titration (eq 1) of emission spectral data F
equals 3.58. The emission spectral changes of 1 in the presence of
2
þ
2þ
2þ
29
Sr or Ba (Figures S8 and S9) are similar to those of Ca
.
cyclodextrin-derived bianthracene system in water and an even
30
A closer inspection shows the fluorescence emission spectra
longer lifetime than 200 ns for the π-stacked dimer. In DMSO and
MeCN, the decay profile is analyzed by using three exponential
functions, where two of the parameters (10.0 and 3.0 ns for MeCN,
18.0 and 2.0 ns for DMSO) are similar to that obtained from the
emission analysis of 2-acetamido-anthracene. In addtion, contribu-
tions of the decay times 2ꢀ3 ns (about 10ꢀ15%) are almost the
same as 2-acetamido-anthracene. In DMSO, it is clearly indicated
that the third slowest one (140ꢀ160 ns) is mainly from the excimer
emission. The difference in the structure of 1 compared with
2-acetamido-anthracene is 1 has two of the same anthrance
chromophores, so the third lifetime in DMSO maybe could be
2
þ
2þ
2þ
change of 1 in the presence of Ca , Sr , or Ba at low
concentrations are completely the same as the fluorescence
emission spectra of 1 in acetonitrile at different concentrations
and accompanied by the appearance of an isoemissiom point at
2
þ
2þ
2þ
3
98 nm. This means that the cation (Ca , Sr , or Ba )
2
þ
binding model is not the same as Mg ; this should have a
relationship with the structure of the ligand 1. But after the
alkaline-earth metal completely complexes with 1, the rigid
structure of the complex may hinder intramolecular excimer
formation. At the same time, complexes formed can also suppress
4
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Table 4. Time-Resolved Emission Data of 1 and Complexes, Fitted Individually as a Sum of up to Three Exponential Functions
with Time Constants τ (ns) and Amplitudes R (i = 1ꢀ3)
i
i
compound
solvent
monitored wavelength
τ
1
(ns)
τ
2
(ns)
3
τ (ns)
1
MeCN
DMSO
410
3.82 (19%)
3.88 (13%)
3.22 (15%)
2.33 (14%)
0.74 (16%)
1.79 (23%)
1.58 (20%)
2.46 (30%)
0.91 (12%)
1.09 (9%)
10.97 (50%)
10.47 (40%)
9.99 (40%)
19.31 (13%)
11.16 (23%)
9.52 (19%)
6.64 (53%)
8.38 (17%)
4.63 (35%)
4.95 (44%)
6.56 (48%)
7.47 (60%)
6.89 (46%)
7.02 (61.2%)
0.22 (31%)
4
4
20
30
0.09 (47%)
0.26 (45%)
410
164.07 (73%)
135.36 (61%)
146.74 (58%)
27.42 (27%)
27.92 (53%)
16.88 (16.88%)
14.69 (47%)
18.31 (34%)
18.50 (23%)
21.51 (39%)
18.20 (28.3%)
4
4
20
30
2
þ
1
1
1
1
Mg
MeCN
MeCN
MeCN
MeCN
380
00
380
00
380
00
380
00
3
3
3
3
4
2
þ
Ca
4
2
þ
Sr
1.8 (18%)
4
2.62 (17%)
1.41 (15%)
1.13 (10.5%)
2
þ
Ba
4
1
CN: (a) 1ꢀMg2þ, (b) 1ꢀCa2þ, and (c) 1 alone.
Figure 6. Partial H NMR spectra of 1 in CD
3
assigned to intramolecular excimers. In acetonitrile, the fastest decay
of at least two different emitting electronic states and the likely
presence of inter- and intramolecular excimers since anthrance tend
to aggregate.
times τ of about 0.2 ns maybe also attributed to the lifetime of
1
intramolecular excimers. At all wavelengths, the shortest decay time
(
τ ≈ 0.2 ns) has the largest contribution (45%), which is the same
Fluorescence decay measurements also reveal a more complex
complexation behavior of 1. The fluorescence decay curves of all the
alkaline-earth metal ion complexes measured can be described by a
triple exponential fit. The lifetimes of all longer-lived and shorter-lived
emissions changed upon complexation. This result shows that the
environment around the compounds changes upon complexation.
On the contrary, as in the absence of alkaline-earth metal
cations in MeCN, most of the contribution are from two slows
decays (τ ≈ 6.6ꢀ8.4 ns and τ ≈ 27.5 ns) in the presence of
1
as the longest lifetime (>50%) in DMSO. Another possible reason
may be that the energy transfer from the excited anthracene
monomer to the anthracene dimer is facile in aggregrates; the
observation of short-lived components in MeCN is not unexpected.
The fluorescence decay of monoanthracene is well fitted to the
double exponential profile, the excimer decay profile is analyzed by
using three exponential functions that have been reported in
29
literature before. In general, the decay profiles of 1 are more
complicated than 2-acetamido-anthracene due to the involvement
2
3
2
þ
Mg . The longest lifetime can be attributed to the complex with
4
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ARTICLE
2þ
2þ
2þ
2þ
Figure 7. Schematic representation of the proposed conformational changes before and after binding with Mg and Ca , Sr , or Ba in acetonitrile.
alkaline-earth metal cations, where another slow decay may be
caused by the photoinduced decoordination of the cation. In case
yield. This suggests that the increase in the fluorescence lifetimes of
both components seems to be the direct consequence of the
complex formation in the ground state and the emission of both
excited complex conformers. The fluorescence decay times of the
31,32
of photodecoordination of the cation,
the fluorescence life-
times of the decoordinated complexes are similar for different
2þ
cations, which is the same as our measurements (τ ≈ 7.0 ns for
longest decay components of the Mg complex are longer than
that of the other alkaline-earth metal complexes, pointing to higher
2
2
þ
1
Mg and other alkaline-earth metals, except that τ ≈ 5.0 ns
3
2
2
þ
2þ
for 1 Ca ). The real nature of the third species remains unclear
fluorescence quantum yields of Mg compared to that of the other
3
at present. The short lifetimes from 1.0 to 2.5 ns may correspond
to the transition from complexation to photodecoordination.
This shows the existence of a slow exchange rate between free
and complexed species. As a result, three decay components were
observed. The absence of the red edge of the emission band when
in the presence of alkaline-earth metal cations does not support
that another slow decay can be attributed to the lifetime of the
aggregated intermolecule dimer.
alkaline-earth metal complexes.
1
3.5. H NMR Measurenment of the Complexes. To clarify
1
the structure of complexes in detail, a H NMR study was carried
out in the absence and in the presence of metal cations in
acetonitrile-d at 25 °C. The spectra of 1 before and after addition
3
2
þ
2þ
of Mg and Ca are shown in Figure 6.
The oxyethylene proton peaks of 1 became broad and shifted
2
þ
to low magnetic field in the presence of Mg . The reason is the
2þ
2þ
2þ
2þ
Complexes with Mg , Ca , Sr , and Ba reveal that
reduction of the electron density on the oxygen atoms by the
2
þ
fluorescence lifetimes increase with increasing values for quantum
coordinated cations. This also indicated that Mg was bound to
4
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The Journal of Physical Chemistry A
ARTICLE
oxyethylene parts. On the other hand, a high magnetic shift change
of the amide proton was observed, whereas the amide proton peak
Two new asymmetric fluorescent reagents were synthesized by
one of the anthracene aromatic amide moieties substituted by
2þ
0
14
of the complex Mg ꢀ2,2 -[oxybis(3-oxapentamethyleneoxy)]-
bis[N-(1-pyrenylmethyl)benzamide] shifted to a low magnetic
field. Protons of the anthracence ring and benzene moiety showed
n-propyl and ethyl benzene moieties. In the absence of metal
ions, the fluorescence emissions of these compounds were quite
weak, but their intensities were much greater in the presence of
alkaline earth metal ions. The peak shape and maximum wave-
12
the high-field shifts. This result can be explained by deshielding
2
þ
due to stacking of the benzene and pyrene rings. These observations
length of the emission of the complex with Mg differed from
2þ
2þ
indicated that Mg was mainly bound by the carbonyl group and
the oxygen atoms of oxyethylene moieties. Further details of the
assigned change of each proton of the spectra were too complicated
for complete assignments to be made.
those of Ca and other alkaline earth metal ions, but synthesis is
not so easy for these symmetric compounds. Complex alkaline-
earth metal cations with 1 not only increase the emission band,
but also the emission band hypochromic shift and shown maximum
at 366, 380, and 401 nm, which is the same as the locally excited
(LE) state of 2-aminoanthrance that could be observed. It formed a
1
2þ
The same trends in H NMR studies on 1 Ca were also
3
obtained. The protons of the anthracence ring and benzene
moiety show small high-field shifts more than 1 Mg ; the
complex 1 Ca had a much larger value of chemical shift
2
þ
2þ
2þ
2þ
2þ
1:2 complex with Mg , while 1:1 with Ca , Sr , or Ba , and
3
2
þ
2þ
the spectra change with Mg is not the same as those with other
3
2
þ
changes than those of 1 Mg
.
alkaline earth metal ions.
3
1
On the basis of fluorescence spectral and H NMR changes of
all the compounds, an expected structural change in 1 in the
presence of alkaline earth metal ions is depicted in Figure 7. Due
4. CONCLUSION
2
þ
We have synthesized a polioxo ethylene chain fluorescent dye
to the ionic radii of Mg , which is smaller than those of other
2
þ
2þ
2þ
1 with 2-aminoanthracene as the terminal group. Model com-
pound 2-acetamido-anthracene shows a dependence of fluores-
cence emission on the concentration in acetonitrile, which
indicates intermolecular excited dimers can be formed. Fluores-
cence decay profiles of 2-acetamido-anthracene can be described
by a biexponential fit, which may be attributed to nonaggregated
molecules and intermolecular excited dimers. Three lifetimes are
found of 1 in MeCN and DMSO, which implies that intramo-
lecular excited dimers are likely to occur also. The dye forms 1:2
earth-alkaline-metal cations (Ca or Sr or Ba ; Table 3), the
torsion angle varies in the complex structure, which will be different
for them. Conformational change will be reflected in the
fluorescence spectrum as a feature and a position of fluorescence
maximum. Before the formation of the complex, 1 shows weak
fluorescence emission of the N-(2-anthryl)amide group at ∼380,
∼
420, and/or ∼440 nm due to the intermolecular and intramo-
lecular excimers that can be formed from two molecules and two
anthrances of one of the same molecules, respectively. Com-
plexes formed will decrease the form of intermolecular dimers
2þ
complexes with Mg but 1:1 complexes with other earth-
2þ
2þ
2þ
2þ
alkaline-metal ions (Ca , Sr , Ba ). The Mg complexa-
tion-induced effects on the emission spectra led to a large blue
shift and a strong increase in the emission. The addition of other
earth-alkaline-metal cations (Ca , Sr , or Ba ) has a very
different effect on the emission than in the presence of Mg
(
1* 1) due to the decrease of 1*. On the other hand, after
3
2
þ
binding with Mg , the complex formed a rigid conformation
structure; two anthrance groups were separated. When com-
plexed with other alkaline earth metal ions, the two anthrance
groups approached each other first and lead to a decrease in the
monomer emission of 1. The complex can be further reduced in
conformational flexibility. These complexes can hinder excimer
formation and also coordinate with a carbonyl group, which
results in a fluorescence emission of a local excited state of
2þ
2þ
2þ
2þ
,
which leads to a decrease in the fluorescence emission first, then
an increase and blue shift in emission could be found at the end.
The new dye is an example of a very sensitive fluorescent probe
2þ
for Mg metal ions, displaying large fluorescence changes in
wavelength and intensity than those for other earth-alkaline-
2
-aminoanthrance enhancement at ∼360, ∼380, and ∼400 nm
metal cations. This difference change of spectra allowed a
(Table 1) .
2þ
discriminating detection between Mg and other earth-alka-
What are the different properties of 1 compared with other
line-metal cation by means of fluorescence spectroscopy.
0
polioxo ethylene chain probes? Linear polyether N,N -[oxybis-
(
3-oxapentamethylenoxy)-2-phenyl] bis(9-anthracenecarbonamide)
’
ASSOCIATED CONTENT
and its analogues have been synthesized, and fluorescence “Off-On”
behaviors for alkaline earth metal ions (Ca , Sr , and Ba
were reported by Hiroshi Nakamura. These compounds showed
almost no fluorescence emission (fluorescence quantum yield =
2þ
2þ
2þ
)
S
Supporting Information. Absorption and fluorescence
13
b
2þ
2þ
emission spectra of 1 in acetonitrile as a function of Sr or Ba
.
Protonation fluorescence emission spectra of 1 in acetonitrile.
0
.0003) because twisted intramolecular charge transfer (TICT)
1
13
H and C NMR spectra of 2-acetamido-anthracene and 1.
occurred between the carbonyl group of the amide bond and a
plane of the anthracene ring in the absence of a metal ion. The
fluorescence lifetime of these compounds and their calcium
complexes was too short and could not be determined. The same
This material is available free of charge via the Internet at http://
pubs.acs.org.
12
0
’ AUTHOR INFORMATION
group also reported 2,2 -[oxybis(3-oxapentamethyleneoxy)]-
bis-[N-(1-pyrenylmethyl)benzamide)] and its analogues have
two pyrenes at both terminals of polyoxyethylene compounds,
showing strong intramolecular excimer emissions around
Corresponding Author
*Tel.: þ86 ꢀ931-8912582. Fax: þ86-931-8912582. E-mail:
qinww@lzu.edu.cn
4
80 nm in the fluorescence spectra. On the complexation with
alkaline earth metal cations, the increase of monomer emission
around 400 nm accompanied by the disappearance of intramo-
lecular excimer emission of free reagents was observed. These
reagents formed a 1:1 complex with alkaline-earth metal cations.
’
ACKNOWLEDGMENT
W.Q. thanks the “Scientific Research Fund for Introducing
Talented Persons to Lanzhou University”. W.Q. is supported by
4
297
dx.doi.org/10.1021/jp200514u |J. Phys. Chem. A 2011, 115, 4288–4298

The Journal of Physical Chemistry A
ARTICLE
the “Program for New Century Excellent Talents in University”
(31) Martin, M. M.; Plaza, P.; Meyer, Y. H.; Badaoui, F.; Bourson, J.;
Lefevre, J.-P.; Valeur, B. J. Phys. Chem. 1996, 100, 6879.
(NCET-09-0444) and the “Fundamental Research Funds for the
(32) Mathevet, R.; Jonusauskas, G.; Rulli ꢁe re, C.; L ꢀe tard, J.-F.;
Central Universities” (lzujbky-2011-22). This study was sup-
ported in part by the “Key Program of National Natural Science
Foundation of China” (20931003).
Lapouyade, R. J. Phys. Chem. 1995, 99, 15709.
’
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